Electrocatalysts for LithiumâAir Batteries: Current Status and

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Electrocatalysts for the Lithium air batteries: Current status and the challenges Awan Zahoor, Zafar Khan Ghouri, Saud Hashmi, Faizan Raza, Shagufta Ishtiaque, Saad Nadeem, Inayat Ullah, and Kee Suk Nahm ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06351 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 4, 2019

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ACS Sustainable Chemistry & Engineering

Electrocatalysts for the Lithium air batteries: Current status and the challenges

Awan Zahoor1*, Zafar Khan Ghouri2*, Saud Hashmi3, Faizan Raza1, Shagufta Ishtiaque4, Saad Nadeem1, Inayat Ullah1, Kee Suk Nahm5*

1 Department

of Chemical Engineering, NED University of Engineering and Technology, University Road, Karachi -75270, Pakistan

2 Chemical

Engineering Program, Texas A&M University at Qatar, P.O. 23874, Doha, Qatar

3Department

of Polymer and Petrochemical Engineering, NED University of Engineering and Technology, University Road, Karachi -75270, Pakistan

4 Department

of Chemical Engineering, University of Karachi, Main University Road, Karachi 75270, Pakistan

5Department

of Chemical Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do, 54896 Republic of Korea.

Corresponding authors: Zafar Khan Ghouri E-mail: [email protected]

[email protected] Awan Zahoor [email protected]

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: In past decade electrochemical energy storage gained undivided attention with the increase in electrical energy demand for the usage of new technology such as moveable electronics etc. LiIon battery (LIB) has been most successful energy storage system with their long-life cycle and efficiency, lower energy density and notably cost effective on small scale energy storage. However, on large scale energy storage and for long duration still need some work done to make LIB efficient on such scale as well. Recently Li-air battery has been suggested as a potential energy storage system that can provide the solution for large and long-term electrical energy storage. The Li-air battery utilizes the catalyst based redox reaction and still it’s not applicable commercially due to low current density, poor life cycle and energy efficiency. Generally, such problems are associated with the materials used an electrocatalyst and on the selection of electrolyte. Herein, we briefly review the current advancements in the field of electrocatalyst for Li-air battery which hinders its improvement towards commercial applications, and this review will provide an outlook for future Li-air battery systems.

Keywords: Li-air battery; Alloys nanoparticles; Electrocatalyst; Cyclic voltammetry; Oxygen evolution reaction.


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Introduction Consumption of worldwide fossil oil assets and natural concerns have been the subject themes of the world monetary and political circles over the previous decades. It has been accounted for that, a huge part (over 80%) of the absolute vitality supply originates from petroleum derivatives, for example, coal, oil and flammable gas, causing an emotional develop of ozone depleting substances in the air, and considerably more truly, it is unsustainable there are limited stores.1 On the other hand, significant reason for geopolitical unsteadiness is because of the CO2 discharge and the oil expending represents 40% of the complete CO2 outflow. Since most of oil is being utilized for car and light vehicle applications, a progress to an energized street transportation framework ought to be an objective of prime significance. Such a progress is in progress with an increasingly broad invasion of cross breed electric vehicles into the commercial center, and module half breed vehicles beginning to positively influence the most recent three years. Traditional rechargeable Li ion batteries are great contenders for EV uses because of their high cycle performance and energy efficiency.2-6 But it is identified that theoretically the gasoline energy density is 13,000 Wh/kg.7 Nonetheless, the vitality thickness of lithium-particle batteries is constrained by both the anode and cathode in light of the fact that their particular limits are confined by the heaviness of the dynamic materials which are commonly graphite (371mAhg−1) for the anode and metal oxide, for example, LiCoO2 (273mAhg-1) for the cathode. Thus, the vitality thickness of lithium-particle batteries is just somewhere in the range of 75 and 160Whkg-1. Great emphasize has been devoted to overcoming this problem for the development of metal-air batteries,8-10 because of their high theoretical energy density. A redox reaction between the metal and air (specifically oxygen) to produce current in metal-air batteries. An open cell structure, which allows the continues supply of the oxygen from air an infinite external source. Since the cathode oxygen is supplied externally



from air and not stored in the cell, which results in higher theoretical energy density for the metal– air batteries as compared with other batteries. According to the electrolytes, there can be two types of the metal air batteries. One is a battery utilizing a fluid electrolyte; such a framework isn't delicate to dampness. The other is a water-delicate framework utilizing an electrolyte with aprotic solvents. This framework is debased by dampness. In figure 1 a comparison of several available and presently under progress energy storage technologies with respect to their theoretical energy densities are shown.11, 12 For the aqueous system, metals like Fe, Cd, Ca, Zn, and Al are suitable among metal-air batteries. Among many metal battery architectures, Zn– air batteries have been considered for a long time since they have numerous points of interest, for example, a level release voltage level, high wellbeing and minimal effort, and long-time span of usability.11–13 While iron– air batteries have an extensive cycle life, it experiences the ill effects of low voltage and explicit vitality contrasted and other metal– air batteries.14 Aluminum– air batteries are attractive on the grounds that aluminum is one of the world's most bottomless assets and these batteries have high explicit vitality; anyway Al can be more effectively consumed than Zn in antacid solution,15 despite the fact that Al-air cells have an a lot more noteworthy vitality thickness than zinc-air cells. 16

However, the hypothetical explicit vitality thickness of Zn– air batteries is just 1084 W/kg,17

which is still much lower than gas and can't satisfy the necessities of some high-vitality uses of electric vehicles. Lithium is the lightest metal component and its hypothetical vitality thickness is around 11,680 Wh/Kg about comparable to gas. In this way, numerous endeavors have been dedicated to lithium– air battery examine. The anticipated realistic vitality thickness of a Li-air battery matches that of fuel for car applications 1700 Wh/Kg (considering that the hypothetical vitality thickness for a Li-air battery is around 11 000 Wh/Kg which is around multiple times that of best in class Li particle batteries). Lithium oxygen (Li-O2) batteries were proposed during the


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1970s for car transportation, however its encouraging was moderate, because of the bothersome response of lithium with water using watery electrolytes, until Abraham and Jiang displayed the Li– air framework with non-fluid electrolyte in 1996.18 In figure 2, a schematic design for an aprotic Li–air battery with Li (metal) anode, an electrolyte consisted of an aprotic solvent with dissolved lithium salt, a O2-breathing supported conducting porous carbon cathode and a conductive Li-ion membrane separating the anode and the cathode. On discharge, Li ions are formed at the anode are carried through the electrolyte into the porous cathode. O2 enters the cathode and dissolves in the electrolyte inside the pores. Subsequently, it is reduced at the porous carbon cathode surface by electrons from the outer circuit and incorporate with Li+ from the electrolyte, thus forming a solid Li2O2 as the final discharge product (the oxygen reduction reaction or ORR), which is stored within the voids of porous carbon. During the charge reaction, Li2O2 can be oxidized, and transformed back to O2 and Li.19 Current Challenges 16 years ago, a non-aqueous rechargeable Li-air battery was introduced. However, the existing lithium air battery system still presents various challenges.18 Based on differential electrochemical mass spectroscopy (DEMS) and in-depth studies, it was reviled and confirmed that the release of O2 is due to the oxidation of Li2O2,19, 20 which emerges again the profound enthusiasm for Li-O2 chemistry. An oxygen electrocatalyst, with the fundamental knowledge of Li−O2 reactions, is among the challenges in the development of lithium air batteries. Carbons act as ORR catalysts,21, 22 but are not as efficient for OER,23 Other significant challenges includes the incompatibility of the Li-metal anode with electrolyte and air, which results in formation of dendrites after few charge-discharge cycles,24 conductivity of electrolytes, instability in oxygenrich electrochemical environment, evaporation for nonaqueous electrolytes,25,26 and the oxygen



supply from air. Still huge challenges are present in oxygen electrocatalysts for Li–air batteries. In Li-air batteries electrocatalysts plays a key role in determining the power, energy density, and energy efficiency. Current Li–air batteries discharged–charged only at a rate of 0.1–0.5 mA/cm2 as compared with Li-ion battery which is >10 mA/cm2 and >1500 mA/cm2 for a polymer electrolyte membrane fuel cell. The potential difference between the charge and discharge is greater than 1.0 V, which results in a low potential with efficiency of 90% for a Li-ion battery).27 This results for the Li-air batteries can be largely ascribe to the poor performance of the O2-cathode due to its sluggish kinetics for oxygen reduction/evolution reactions. To improve the performance of Li-air batteries it is important to design an efficient O2cathode by understand the detail mechanism of ORR and OER at the electrode. Hence, the porous carbon O2-cathode is required to ensure a large electrolyte/electrode surface area and accommodate the insoluble Li2O2, the final discharge product, as well as to facilitate the transport of oxygen via diffusion to the porous reaction site through the cathode film. In addition, the porous carbon O2-cathode must have sufficient conductivity for the efficient transfer of electrons to the reaction site with low impedance. A homogenous distribution of a nano-catalyst is essential to enhance the performance by increasing the overall efficiency by decreasing the potential difference between charge and discharge cycle. According to the types of electrolyte, there are four class of Li-air batteries. Li salts-dissolved in i)

Non-aqueous (aprotic) solvents

ii)

Aqueous solvents

iii)

Hybrid (non-aqueous/aqueous) solvents

iv)

All solid-state electrolyte.


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All four systems use Li metal and oxygen gas as anode and cathode materials, respectively. The configuration of the non-aqueous electrolyte system is similar to that of conventional Li-ion batteries. Conventional Li-ion batteries use carbon or metal alloys as anodes, Li metal oxides or phosphates as cathodes, and Li salt dissolved in aprotic electrolytes. On the other hand, Li-air batteries use air (oxygen gas) as a cathode material and thus, porous carbon and catalyst composites must be added in the cathode to store the discharge product Li2O2. Also, Li metal must be used as the anode, because anodes play a role as the Li source in Li-air batteries. The major difference between the two systems is that an open system is required to for Li-air batteries, because oxygen is obtained from the air. In the non-aqueous system it has been proved that the reduction products can be reversed into the original reagents and is advantageous for the rechargeability. Moreover, the theoretical energy density of a non-aqueous Li–air battery system is higher than that of an aqueous Li–air battery system because of the water or acid being involved in the reactions in the aqueous system. To date, the non-aqueous configuration has attracted the most effort worldwide compared to other electrolyte systems in Li–air batteries. Therefore, in this progress report, we will focus on the most recent development of a wide variety of electrocatalysts for ORR and OER in aprotic Li-air batteries. So far, various Li2O2-oxidation mechanisms have been proposed and the major difference among these mechanisms lies in the type of the major charging reaction intermediate species, including off-stoichiometric or no reaction intermediates (directly evolving O2). It is imperative to resolve the issues which are responsible for the different observations since the type of reaction intermediates have critical implications on strategies in electrode/electrolyte design and selection. It has been reported that one major reason responsible for the differences observed in the literature originates from the donicity of the solvents, which



affects the stability of the reaction intermediates (e.g., O2) owing to different cation-solvent complex Li+(solvent)n. The influence of solvent donicity on discharge reaction pathways are well investigated but not the charge reaction pathways (Li2O2 oxidation). They investigated the Li2O2oxidation mechanism in four model solvents that exhibit distinct LiO2 (sol)-stabilizing ability,202 including two high-donicity solvents, 1-methylimidazole (Me-Im; DN = 4725) /dimethyl sulfoxide (DMSO; DN =29.851) and two low-donicity solvents, TEGDME/diethylene glycol dimethyl ether (DG; DN = 19.552). The experimental evidence of the formation of soluble LiO2(sol) upon Li2O2 oxidation which reveals a strong solvent-controlled Li2O2-oxidation reaction mechanism. The results indicate that Li2O2 oxidation in high-donicity solvents generates soluble LiO2(sol), which can disproportionate to form nanocrystalline Li2O2 and evolve O2. In contrast, no soluble LiO2(sol) was detected in low donicity glymes during Li2O2 oxidation. Preferential formation of soluble LiO2(sol) in the high-donicity solvents could account for low O2 evolution and severe CO2 evolution due to low chemical stability of soluble LiO2(sol) in the electrolyte. Li-air batteries: Electrochemical reactions The first true Li–air system with non-aqueous electrolyte was presented in 1996 by Abraham et al.18 Such Li–air battery anode (currently Li–metal is used), a non-aqueous electrolyte and an air cathode. Two generally possible energy-producing reactions are Li(s) + 1/2O2 → 1/2Li2O2

(i)

Li(s) + 1/42O2 → 1/2Li2O

(ii)

The reversible cell voltage is E01= 2.959V for the reaction (I) and E0II= 2.913V for reaction (II).28 In a real Li–air cell the reactions brake down into anode and cathode parts: During the


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discharge of an aprotic lithium–air battery, an oxidation reaction occurs at the anode. The electrons flow through an external circuit and the lithium ions generated from this reaction react with oxygen to form Li2O2 (and possibly Li2O) in the pores of the O2 breathing cathode.28 At the cathode, oxygen is reduced in either a two or four electron process as described by the following half-cell reactions (Figure 2):29 Li(s) ↔ Li+ + e-

(1)

(Anode)

O2 +2e- +2Li+ →Li2O2 (3.10 V)

(2)

(Cathode)

O2 +4e- +4Li+ →2Li2O (2.90V)

(3)

(Cathode)

The ORR procedure noticeable all around terminal of a lithium-air battery incorporates a few stages: oxygen dispersion from external climate to the impetus surface, oxygen assimilation on the impetus surface, exchange of electrons from the anode to oxygen particles, debilitating and breaking of oxygen security, and the expulsion of hydroxyl particle item from the impetus surface to the electrolyte (for non-fluid Li-air, strong item is framed). A run of the mill release charge circle is schematically appeared in Figure 3. The standard potential for the release response U0 is given by the thermodynamics of the response as U0=2.96 V, utilizing the Nernst condition. The working voltage of this phone amid release is fundamentally not exactly standard potential U0. The thing that matters is known as the release over potential ηdis. The release limit of the Li-O2 cell is additionally lower than its hypothetical limit because the released items hinder the dynamic surface zone of permeable cathode. This is because of the insolubility of release items in the nonfluid natural electrolyte, which causes a development of release items on the cathode surface, restraining the progression of reactants (O2, Li+, and electrons) to the dynamic surface. From the above conditions, Li2O2 and Li2O are both conceivable release results of the Li-O2 cell. Both Li2O2



and Li2O are considered as mass separators (Figure 2), bringing about genuine electrochemical obstruction yet it has been seen that Li2O2 is the response item for voltage shorts above 2.0V. 30-32 Lithium peroxide can electrochemically be disintegrated (Li2O2 → 2Li+ + 2e − + O2), or by means of superoxide (Li2O2 → Li+ + e − + LiO2) taking into account rechargeability of the non-fluid LiO2 cell. An ongoing report demonstrated that the steady surfaces of Li2O2 are half-metallic.33 The nearness of conductive surface pathways in released Li2O2 could counterbalance limit restrictions anticipated from constrained electron transport through the mass. Subsequently, electron transport through well-associated Li2O2 particles may not altogether obstruct execution in lithium– air batteries.34 There are not many disadvantages of development of lithium peroxide and superoxide as a release item as contrasted and the arrangement of lithium oxide. The reactivity of lithium peroxide has its influence in deterioration of natural electrolyte and furthermore bringing about erosion of carbon cathode.35-37 Lithium oxide is less receptive with natural solvents and have a high hypothetical vitality thickness and explicit vitality.38 Recently L. F. Nazar et.al, change the electrolyte framework to inorganic liquid braces to advance the development of Li2O rather than Li2O2 brought about close 100% coulomb proficiency and indicated half increment in hypothetical vitality stockpiling.39 However, the fundamental systems applicable to the exhibition of lithium– air batteries are not surely known. The structure, concoction piece, and electronic decencies of the essential release stages Li2O2 and Li2O are should have been additionally explored. Many challenges must be resolved before practical application of Li-O2 batteries. Li-O2 batteries faces several challenges, among these problems few are, high charging potentials, low round-trip efficiency, and limited cycle life, which have been attributed to the reactivity of Li-O2 discharge products,45-47 and poor oxidation kinetics of Li2O2 formed upon discharge.


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Various groups have investigated the kinetics of Li2O2 oxidation in Li-O2 batteries,40-44 which shows that the charging performance is strongly influenced by the morphology of the Li2O2 produced during discharge. For thin layers of Li2O2, McCloskey et al. have computed,45, 46 and experimentally measured low charging over potentials (< 0.2 V by cyclic voltammetry) to posit that electrocatalysis for the oxygen evolution reaction (OER) from Li2O2 oxidation may not be necessary.43,45 Similarly, Lu et al.42,47 have reported evidence showing that electrocatalysis is unnecessary during the removal of the first sub-nanometer of deposited Li2O2, where electrochemical oxidation of Li2O2 can proceed from first delithiation to form lithium-deficient Li2-O2 followed by oxygen evolution from Li2-O2. On the other hand, thicker deposits of Li2O2 (i.e. greater depth of discharge) have been shown to require greater overpotentials to oxidize, particularly on carbon electrodes.

46,48,47-50

This phenomenon is attributed to the formation of

byproducts during discharge that require a greater potential to oxidize,46-48, 53 and the insulating nature of Li2O2, which increases the potential needed to drive the oxidation reaction.63-66 One of the byproducts is carbonates such as Li2CO3, which is generally formed from electrolyte decomposition and/or from an interaction between Li2O2 and carbon electrodes.47,48 Various groups have reported high charging overpotentials (typically greater than 1 V) for a variety of carbon electrodes, from simple porous carbon48,50 to graphene,60, 67 and carbon nanofibers57 at moderate rates 50 to 100 mA.g-1carbon. It has been reported that the Li2O2 product morphology is a function of various factors which can affect Li-O2 cell performance.55 One major factor is the kinetics of precipitation which favors rapid dismutation and precipitation in the chemical case due to a high LiO2 concentration. In contrast, the electrochemical production of LiO2 is current limited which leads to slow nucleation via an electrochemical route to form crystalline Li2O2. Second factor is the strong cathodic



polarization of the carbon surface in the electrochemical cell which may diminish the binding of the generated O2−to the substrate. This would be expected to enhance diffusion of the solvated superoxide molecules away from the surface, and favor peroxide formation by disproportionation. Formation and Decomposition of Li2O2: Apart from the lithium anode and electrolyte, the reversible formation and decomposition of Li2O2 during cycling on the oxygen cathode is the key process also have a determining effect on the battery performance. Recent evidence suggests that Li2O2 formation and decomposition can be promoted through the use of catalysts. For example, Xin et al. used the nitrogen-doped LaNiO3 perovskite as bifunctional electrocatalysts for oxygen cathode in a rechargeable lithium-oxygen battery.56 It was observed that N doping significantly increased the Ni3+ contents and oxygen vacancies on the bulk surface of the perovskite, which helped to promote the oxygen reduction reaction and oxygen evolution reaction of the cathode and therefore, enabled reversible Li2O2 formation and decomposition on the cathode surface. As a result, the oxygen cathodes loaded with the N-doped LaNiO3 catalyst showed improved electrochemical performance in terms of discharge capacity and cycling stability to promise practical Li-O2 batteries. The improved performance is attributed to the introduction of N into the perovskite lattice introduces additional oxygen vacancies to facilitate the formation/decomposition of Li2O2 and enable the N-doped catalyst with improved ORR/OER catalytic activity over the nondoped one. Guo et al reported the decrease in charge overpotential to 0.55 V with the charge plateau lying at 3.5 V even in the case of a high discharge capacity (3450 mAh g-1).57 The decrease in charge overpotential is due to the introduction of the n-type Si coating layer on the CNTs surface, the Li2O2 formed by discharge changes from large toroidal particles (~300 nm) deposited on the pristine CNTs cathodes to nanoparticles (10-20nm). Such improvement is attributed to the fact that the introduction of the n


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type Si coating layer changes the surface properties of CNTs and guides the formation of nanosized amorphous-like lithium peroxides with plenty of defects. Similar results have been reported by Gu et al.58 Gold nanoparticles anchored to vertically aligned carbon nanotubes act as additional nucleation sites for the Li2O2 growth, leading to the decreased size while increased density of Li2O2 particles in process of discharge. Correspondingly, at the deep discharge to 2.0 V the batteries show increased specific capacity. Upon charge, the AuNPs exhibit promotion effect on the Li2O2 decomposition by improving the conduction property of the discharge-formed particles. It has been reported that the addition of quinone derivatives in electrolytes catalyze the aprotic ORR, resulting in the efficient Li2O2 formation.59 Another group reported that by incorporating WO3 nanowire array photocatalyst on cathode decomposition of Li2O2 can be enhanced in rechargeable Li-O2 battery.60 Notably, the charging potential was maintained at 3.55 V even after 100 cycles in this photo-assisted battery system, which is much lower than the charging potential of the traditional Li-O2 battery (4.4 V). The reason behind the stability of this battery is the abundant holes present in photocatalyst which are excited by visible light, the Li2O2 coated on WO3 nanowires can be efficiently oxidized during the charging process, resulting in the reduced charging potential and enhanced Li-O2 battery performance. These results demonstrate that cathode surface properties play an important role in formation and the decomposition of products formed during cycle, providing inspirations to design superior cathodes for Li-O2 cells. Structure of reasonable impetus for oxygen advancement response (OER) in nonaqueous lithium−oxygen (Li−O2) batteries has been upset because of enormous disparities in the viability and instrument of strong impetuses for the disintegration of strong lithium peroxide (Li2O2). Different reports have been distributed demonstrates the articulated impetus impact on improving



the oxidation energy of Li2O2. A considerable lot of them have credited the upgrade to the nearness of solvent species going about as mass-transport facilitators. Then again different scientists detailed that applying impetuses drives no improvement in the Li2O2 oxidation energy because of lacking dissolvable species got from Li2O2 oxidation. Infect strong impetuses viably elevate Li2O2 oxidation energy to advance O2 in a strong state condition, and nonaqueous OER is a synergist dynamic procedure and that impetuses improve the OER by means of solid−solid communication between the impetus and the release items. The central point for nonaqueous OER action lies in impetus' capacity to settle the strong response intermediates. So far, various reports have been published to check the effect of electrocatalysts in the cathode to reduce the charge overpotentials.61-65 It has been reported that by employing carbonate or mixed ether/carbonate based electrolytes the considerable electrocatalytic reduction of the overpotentials have been observed when metal or metal oxide nanoparticles are added to the cell cathode. Luntz et al. performed the coulometry (i.e., constant current discharge-charge cycles, cyclic voltammetry, etc.) and gas consumption and evolution data to elucidate the cathode catalysts’ role in fundamental Li-O2 electrochemistry.66 The results indicate that in Li-O2 cells employing dimethoxyethane (DME) as a solvent, where Li2O2 is the dominant discharge product, metal (platinum and gold supported on Vulcan XC72 carbon) and metal oxide (α-MnO2 nanowires mixed into XC72 carbon) catalysts do not lower the oxygen evolution reaction (OER) potential compared to pure carbon. Previous studies indicate that the addition of catalyst results in the lowering of the overpotential for CO2 evolution from carbonate decomposition products. A small voltage plateau was observed at 3.15 during the O2 evolution and this voltage plateau remains unaffected regardless of whether a catalyst is used


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or not. The basic reason behind the O2 evolution is the decomposition of Li2O2 in PC-DME because small amount of Li2O2 is generally formed during discharge in these solvents. The above-mentioned results indicate that metal or metal oxide nanoparticles act as conventional electrocatalysts during charging of carbonate-based cells (where Li alkyl carbonate formation is the dominant discharge process), but not during charging of ether-based cells (where Li2O2 formation dominates). Similar results have been reported by Plan of reasonable impetus for oxygen advancement response (OER) in nonaqueous lithium−oxygen (Li−O2) batteries has been prevented because of huge errors in the adequacy and instrument of strong impetuses for the decay of strong lithium peroxide (Li2O2). Different reports have been distributed demonstrates the articulated impetus effect on improving the oxidation energy of Li2O2. A significant number of them have credited the improvement to the nearness of dissolvable species going about as masstransport facilitators. McCloskey et al.67 that applying strong impetuses (e.g., gold, α-MnO2) does not prompt improvement in the oxygen advancement response (OER) in the Li−O2 batteries because of the absence of solvent species got from Li2O2 oxidation. The error may be come about because of the low OER exercises of the impetuses (e.g., gold, α-MnO2) utilized in their study.68 Different reports have been distributed which legitimize the adequacy of strong impetus toward strong Li2O2 disintegration.72,73 First, it has been recommended that solvent response middle of the road species (e.g., Li2−xO2) are produced amid the charging procedure (e.g., by means of delithiation).74 The solvent intermediates could then diffuse/relocate to the outside of the impetus; such procedure can empower consistent correspondence among Li2O2 and the catalysts.74 Second, it has been recommended that solvent species could be created by electrolyte decay or oxidation of side items (e.g., some dissolvable polluting influences) which fill in as redox go between or electron transports to get to and oxidize the Li2O2 particles.72,73 Lu et al. revealed that



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strong impetuses, for example, Ru, viably lessen the charge capability of Li−O2 batteries to advance O2.199 They demonstrated that strong impetuses advance the OER energy amid Li2O2 oxidation in a strong state condition. This unambiguously exhibits nonaqueous OER is a synergist dynamic procedure and that impetuses upgrade the oxygen assessment response by means of solid−solid communication between the impetus and the release items rather than fluid stage intercessions. To assess the impact of Ru and ongoing gas development of the Li-O2 cell charge response, online electrochemical mass spectrometer was utilized. It was seen that amid charging, the charge voltage of VC+Ru terminal is 450 mV lower than that of unadulterated VC cathode, over the whole charge process. It was seen that both VC and VC+Ru anodes display lower O2 advancement rate and indicated CO2 development around the finish of the charging procedure. The beginning capability of CO2 advancement on the VC+Ru cathode was lower than that on the unadulterated VC, proposing that the Ru impetus encourages the OER energy as well as decreases the overpotential expected to advance CO2. The wellspring of CO2 could be come about because of deteriorating carbonate-based side items as well as electrolyte decay. Ru impetus in fact advances the deterioration of the strong Li2O2 without the need of solvent intermediates or redox middle people. Based on these perceptions, it might be recommended that the job of the strong impetus for Li2O2 oxidation is to decrease the vitality of the response middle of the road states, for example, Li2−xO2 by shaping a steady moderate state for example Li2−xO2(solid)−Ru, from which to diminish the overpotential required for Li extraction from Li2O2. It is proposed that the Ru surfaces can be somewhat oxidized by the lithium-insufficient Li2O2 (or Li2−xO2), framing Li2−xO2 (solid)−Ru at the interface (Figure 5). Indeed, the solid holding proclivity of Ru metal toward the oxygenated species is settled in the literature.75 It is trusted that


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incomplete oxidation of the Ru impetus can balance out the very precarious peroxide/superoxide particles in the Li2−xO2 stage, from which to lessen the overpotential required for Li extraction from the Li2O2. It is proposed that the least response opposition is at the Li2O2−catalyst interface where the oxidation response is started, it is speculated that Li extraction begins at the Li2O2−catalyst interfaces and creates Li opening at the Li2O2−catalyst interfaces (Figure 5). At that point Li particles from the external piece of the molecule can enter the destinations emptied by Li by means of opening movement76-78 or surface dispersion, which at that point triggers oxygen misfortune from the external piece of the Li2O2 molecule (Figure 5). These discoveries feature that the key administering factor for nonaqueous OER action lies in impetus' capacity to connect/balance out the strong response intermediates. Comparison with Aqueous and Non-Aqueous Systems From the past reports and the response among lithium and watery electrolyte, an absolutely fluid Li-O2 framework isn't achievable as it would result in the unconstrained decrease (deterioration) of water in contact with metallic lithium.79 In 2004, Polyplus presented a defensive glass-earthenware layer for Li metal (LiSICON, LiM2(PO4)3); this layer empowers Li metal to stay stable in water.80 The layer is ionically conductive, and avoids energetic responses with water. Along these lines, Polyplus basically created Li-air batteries utilizing a watery electrolyte.81 The watery electrolyte framework is made out of the secured Li metal anode, the fluid electrolyte, and the air cathode. Be that as it may, considering the usage of LISICON-type films, there are two noteworthy issues to be tended to. To begin with, such materials are not steady enough in basic arrangement in a long-term use; 82-84 second, the materials are additionally not idle enough toward lithium metal and in this way, the LISICON/anode interface conductivity debases.85 Afterward, in 2007 the polyplus moved further one stage and presented water-stable lithium-particle leading clay



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electrolyte plate that empowered it to be utilized in fluid arrangements.86 The glass fired sort electrolyte likewise called as LTAP with creation of Li1+x+y Ti2−x AlxP3−y SiyO12, which involves a watery arrangement of acidic corrosive and lithium acetic acid derivation. The electrolyte had significant cushion capacity and had kept up causticity through the discharge, keeping pH < 4.87 Acidic corrosive is to be considered as a functioning material, and the cell vitality limit relies upon the measure of acidic corrosive in the electrolyte. Since LTAP is disintegrated in direct contact with Li metal, an ionically conductive, stable cushion is required between the lithium anode and the fired electrolyte. Polymer and inorganic electrolyte layers have been utilized. A few kinds of materials were recommended to intervene a contact among LISICON and lithium metal, for example, basic Li perfect non-watery battery electrolytes.88,

89

Lithium

directing strong inorganic electrolytes, for example, lithium phosphorousnitride (LiPON) that were sputtered onto the LTAP plate were utilized, related to Cu3N films. Despite the fact that the particle conductivity of these layers is high, the preparative strategy is troublesome and exorbitant for enormous scale planning; film thickness falls in the range somewhere in the range of 1 and 4µm, and in the range somewhere in the range of 0.2 and 1µm.72 In later alterations materials, for example, Li3N, Li3P, LiI, LiBr, LiCl, and LiF as poly (ethylene oxide) with salts, for example, Li(CF3SO2)N or LiTFSI were offered for intervening interlayer interface, too. This owes to their compound similarity with both Li and strong state electrolyte parts and their high ionic conductivity.80-82 The real bit of leeway of a watery electrolyte on the cathode side is the absence of insoluble lithium oxide item develop which happens amid release in the non-fluid cell.75 The strong LISICON electrolytes utilized are, lamentably, shaky in both solid acidic and solid essential arrangements which is a restriction for long haul use in a half breed Li-O2 cell.93 To beat this issue, the cross breed electrolyte framework was additionally presented by the Zhou bunch.94, 95 They


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likewise utilized the cross breed electrolyte, 1 M LiClO4 in ethylene carbonate + dimethyl carbonate and 1 M KOH watery arrangement, isolated by a LISICON film. Figure-4 demonstrates the voltage profiles of the initial two cycles of a carbonate-based non-fluid cell are contrasted with the voltage profiles for the main cycle of a cross breed cell at different current densities. The poor cycling and 1.4 V hole between the release and charge levels are ascribed to insoluble items shaped from electrochemical responses in the non-watery framework. In the half and half framework, the response items are solvent in fluid arrangements, and in this manner, a voltage hole of as meager as 0.3 V is seen at a low present thickness.96 An absolutely strong state lithium– air battery was shown by Kumar et al.,97,98 which utilized an exceptionally Li-particle conductive strong electrolyte layer cover manufactured from glass-and polymer-earthenware materials as the electrolyte. This electrolyte has a high soundness when presented to dampness, a wide electrochemical window, and a magnificent warm security, appearing great rechargeability in lithium– air battery frameworks at a wide worked temperature extend. The low electrochemical presentation coming about because of the low particle conductivity of the strong state electrolyte should be additionally improved. This strong state lithium– air battery framework with amazing reversibility merits further improvement and study. The majority of the above frameworks are viewed as promising, and much consideration has been pulled in by all types of Li-air batteries. Be that as it may, numerous issues still should be settled and commonsense issues should be tended to. Cathode catalysts for Lithium–air batteries One of the huge test for the lithium– air battery is the restricted electrical effectiveness which is because of the over potential or polarization misfortunes at the cathode amid release and charge. A high potential is required for charging (4.5 V) the permeable carbon terminal though the release



potential is around 2.5V; the enormous distinction between these two qualities prompts a low productivity. The improvement of this proficiency might be normal by applying successful impetuses. It is for the most part trusted that oxygen electrocatalysts are basic to improving the power thickness, cycling ability, and round-trip vitality proficiency of Li−air batteries. It has been demonstrated that the vitality stockpiling limit and power ability of Li−air batteries are resolved chiefly by the air cathode, which adds to most voltage drops of Li−air batteries (99,100). As of late, incredible endeavors have been made in the improvement of electrocatalysts for both essential and battery-powered metal– air batteries. Despite the fact that ongoing reports have scrutinized the genuine electrocatalytic impact in Li– air batteries.101,102 The exploration results can even now give some direction to future examination on oxygen electrocatalysts. Besides, because of comparative standards, the majority of the synergist materials appropriate to energy components could likewise serve in metal– air batteries, thus could the methodologies and systems to upgrade the cathode productivity. The electrocatalysts can be generally ordered into the accompanying five classes: 1. Carbon based materials, including carbon black, nanostructured carbon, functionalized carbon, including doped Carbons, and graphene which are shown in Table-1 2. Transition metal oxides 3. Metal–nitrogen complex 4. Polymer based materials 5. Precious metals. We will outline the uses of such impetuses in watery and non-fluid metal– air batteries, concentrating on the most as of late announced advancement. Through this survey, it will be stressed that one impetus can all the while assume reactant jobs in fluid and non-watery


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frameworks, and what is found out from fluid frameworks can be utilized to advance the improvement of non-watery frameworks. In a battery-powered Li– O2 battery, the carbon cathode go about as an ORR impetus, since it has an adequate ORR reactant movement, and the energy of the ORR procedure is a lot quicker than the partner OER process. An extra explicit ORR impetus may not be important in carbon based materials. The talk in this segment will be centered around the past and momentum inquire about patterns on carbon based impetuses. Non precious catalysts Non-valuable metal carbon-based impetuses for the ORR are promising contender for supplanting valuable metal impetuses in polymer electrolyte power devices and just as in metal air batteries. Their exhibition can be practically identical with Pt and Pt based impetuses. The dynamic locales are accepted to contain change metal cations composed by pyridinic nitrogen functionalities in the interstices of graphitic sheets inside micropores. Motivated by this achievement, the utilization of this sort of carbon bolstered progress metal composite was additionally endeavored in non-watery Li– O2 batteries. Carbon based catalysts When all is said in done, carbon isn't an electrocatalyst. In any case, carbon is the most broadly utilized material noticeable all around anode of Li-air batteries, either independent from anyone else or as an electrocatalyst support or as an electrically conductive added substances. It is notable that the porosity is critical for oxygen terminal. Carbon is the best material that can give the ideal porosity and electronic conductivity. Truth be told, carbon is the basic material for the engineering structure of an air terminal. There have been numerous examinations because of carbon properties on Li-air execution. It is by and large trusted that the pore volume, particularly the mesopores,103, 104 rather than explicit the surface territory is the most significant factor that



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decides the limit and rate capacity of a Li-air battery. Streamlining of mesopores is likewise significant on the grounds that excessively huge or too little pores lead to a less productive utilization of mesopore volumes. A few procedures were shown to develop a progressive structure of air cathode for Li-air batteries. As of late Guo and Zhou105 integrated three-dimensional arranged mesoporous/macroporous carbon circle exhibits (MMCSAs) (figure-5) and utilized as the impetus in Li-O2 batteries out of the blue. The arranged mesoporous channels and progressive mesoporous/macroporous structure of the carbon encouraged the electrolyte inundation and Li+ dispersion and gave a viable space to O2 dissemination and O2/Li2O2 transformation, which successfully

upgraded

the

electrochemical

presentation

of

the

Li-O2

batteries.

Mesoporous/macroporous structure of the carbon serves for the capacity of Li-O2 items and permits oxygen transport; the items do not obstruct the permeable carbon, and this verifies oxygen transport into the internal locales of the terminal and improves the usage effectiveness of the pores. As of late, the idea of double pore framework anodes has been exhibited by Xiao et al. in a Li-air battery utilizing a progressively permeable graphene air cathode.106 A colloidal microemulsion approach was utilized to develop this progressively permeable structure with functionalized graphene sheets (GNSs) that contains cross section deformities and surface functionalization by hydroxyl, epoxy and carboxyl gatherings. Various enormous passages built by the large-scale pores encouraged nonstop oxygen stream into the air terminal while other little "pores" gave perfect multi-stage areas to ORR (Figure-6). The progressively permeable graphene air cathode demonstrates a release limit of 15000 mAh/g carbon, which is the most noteworthy limit answered to date. This high limit is ascribed to the exceptional various leveled microporous channel structure of the GNS which encourages quick O2 dispersion and exceedingly associated nanoscale pores for a high thickness of receptive destinations. DFT estimations additionally


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uncovered that the deformities and useful gatherings on graphene support the development of confined nanosized Li2O2 particles and help counteract air obstructing noticeable all around anode. Wang et.al.107 proposed a novel procedure to boost the use of permeable carbon particles and the vehicle of reactants by developing a detached progressively permeable carbon (FHPC) by stacking a graphene oxide gel into a nickel froth. Fig. 7a demonstrates the immaculate nickel froth with macroporous skeletons. After the in situ blend of FHPC (Fig. 7b), on nickel froth, enormous interconnected passages all through the whole anode was made. High-amplification perception (Fig. 7c and d) of the carbon sheets uncovered that the sheets comprised of various little nanoscale pores. It gives a high explicit limit and superb rate ability Li– O2 battery. FHPC terminal shows a limit of 11060 mAhg-1 at a present thickness of 0.2 mAcm-2 and, a limit of 2020 mAhg-1 was gotten at high current thickness of 2 mA/cm2 (Fig. 7e). This amazing presentation is ascribed to the free pressing of carbon in the unsupported structure, which gives enough void volume to insoluble Li2O2 statement and builds the effective use of the carbon. In the interim, the progressively permeable structure, including macropores from the nickel froth, and mesopores and micropores from the carbon particles, encourages the O2 dissemination, wetting of the electrolyte, and mass transport of all reactants.108 Kang et al. created various leveled permeable carbon air anodes with controlled porosity from well-adjusted CNTs fibrils.109 The air cathode, with a controlled pore structure, was created by symmetrically utilizing individual sheets of adjusted multi walled nanotubes without the utilization of any cover or dissolvable (Fig.8). The remarkable permeable morphology of the air terminal fundamentally improved the cyclability of the Li-O2 battery without an impetus, even at an amazingly high current thickness (2A/g) and the battery keeps up 60 cycles with a cut-off limit of 1000 mAh/g. The elite was ascribed to the permeable system in the woven CNT anodes which empowers the viable formation– deterioration of lithium peroxide (figure-8 c



and d) by giving the effortless availability of oxygen to the inward side of the air cathode and averting the stopping up of pores by the release item. Another ongoing way to deal with create cover free permeable carbon air anode was shown by Shao-horn's gathering.97 Vertically adjusted varieties of empty carbon filaments with distance across on the request of 30 nm were saved on fired permeable substrate, which were utilized as an air terminal in Li– air batteries. These carbon-fiber (folio free) terminals conveyed gravimetric vitality densities up to 2500 Wh/kg at power densities up to 100 W/kg, meaning a vitality improvement multiple times that of the best in class lithium intercalation mixes, for example, LiCoO2 (600 Wh/Kg). The upgrade in electrochemical execution was ascribed to the more permeable structure than other carbon materials, which improved the usage productivity of the accessible carbon mass and void space for Li2O2 statement amid release. Moreover, such a nanofiber structure takes into consideration the unmistakable perception of Li2O2 development and morphological advancement amid release and its vanishing upon charge, which is a basic advance toward understanding the key procedures that limit the rate ability and result in the low roundtrip efficiencies of Li– O2 batteries. Notwithstanding the carbon porosity and structure, the carbon nature likewise influences the catalysis in lithium– air battery framework. The doping of the carbon with nitrogen particles has drawn much consideration since conjugation between the nitrogen solitary pair electrons and graphene pi-frameworks may make nanostructures with wanted properties, for example, improved oxygen decreases response action.110 In a strong state Li-air battery, Kichambare et al.110 revealed the improved presentation of nitrogen-doped enacted carbon with a high surface region displayed twice release limit of that for actuated carbon without nitrogen doping. This demonstrates the prevalent inborn movement from nitrogen doping. Li. et al.97 additionally reports that a Li-air


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battery with nitrogen-doped carbon nanotubes (N-CANT) show a higher release limit of 866mAh/g (about half improvement) and high release voltage, which was about 1.5 occasions higher than carbon nanotubes (CNTs) with a particular release limit of 590 mAh/g. Despite the fact that the BET surface region and pore volume (with a similar pore estimate) of N-CNT are lower than those of unblemished CNT. The improved electrocatalytic action can be ascribed to the progressions of the electronic structure amid doping of the carbon nanotubes. The consolidation of electron-tolerating nitrogen molecules in the conjugated nanotube carbon plane may deliver a generally high positive charge thickness on the adjoining carbon iotas. These show the advantage of the useful gatherings in improving the limit and oxygen response energy (i.e., the rate ability) in Li-air batteries. Comparative outcomes have been accounted for by Lin et.al.111 They orchestrated 3-dimensional fastener free nickel froth upheld nitrogen-doped carbon nanotubes by a coasting impetus synthetic vapor statement (FCCVD) strategy. The battery-powered Li/O2 cell exhibited a release limit of 18140 mAh/g at the present thickness of 0.05 mA/cm2, which is twofold than that of N-CNT cathode. The upgrade in cell execution is because of the free pressing 3measurement arrange structure which encourages the O2 dispersion in the inward cathode and gives enough void volume to the items statement amid release process. Sun bunch112 utilized nitrogen-doped graphene nanosheets (N-GNSs) as cathode materials in lithium oxygen battery, which conveyed a release limit of 11660 mAh/g. This limit is about 40% higher than that of perfect graphene nanosheets (GNSs). The electrocatalytic action of N-GNSs for oxygen decrease in the nonaqueous electrolyte is 2.5 occasions as that of GNSs. The great electrochemical exhibition of N-GNSs is credited to the deformities and practical gatherings as dynamic destinations presented by nitrogen doping. Zhou's gathering exhibited sans metal graphene sheet indicated great execution as an impetus for the air terminal in a half and half Li-air battery.98 GNSs and warmth



treated GNSs were utilized to plan air cathode for metal air batteries. At 0.5 mA/cm2, the GNSs demonstrated a high release voltage that was exceptionally near the 20 wt % Pt/carbon showing that the sans metal GNSs has great action for oxygen decrease in a half breed Li-air battery. The voltage distinction among release and charge is 0.56 V, which is lower than those revealed for different impetuses showing that GNSs are successful in bringing down the over possibilities of ORR and OER. This improvement in the release limit and the solidness came about because of expulsion of adsorbed useful gatherings and from crystallization of the GNS surface into a graphitic structure on warmth treatment. Li et al. likewise utilized graphene nanosheets (GNSs) as the cathode materials for non-fluid Li-air batteries.113,114 The creators analyzed the movement among graphene and different materials, including BP-2000 and Vulcan XC-72. The air terminal dependent on GNSs yielded a high release limit of 8700 mAh/g at a present thickness of 75 mA/g, which is a lot higher than other carbon materials. These outcomes credited to the remarkable structures of GNSs which structure a perfect 3D 3-stage electrochemical territory and the dispersion channels for the electrolyte and O2, which increment the proficiency of the impetus response. Furthermore, the dynamic locales at the edge altogether add to the predominant electrocatalytic movement towards ORR. Comparative outcomes were gotten by Sun et al.115 They thought about the reactant action of graphene and Vulcan XC-72 for non-fluid Li-air batteries. The graphene nanosheet terminals showed a vastly improved cycling strength and lower over potential than that of the Vulcan XC-72 carbon. As of late, Lin et.al presented cover free nickel froth bolstered nitrogen-doped carbon nanotubes (N-CNTs@Ni), which were combined by a coasting impetus substance vapor testimony (FCCVD) strategy (104). N-CNT utilized as an air cathode in the lithium-oxygen batteries and conveys 1814 mAh/g (standardized to the heaviness of the air anode) at the present thickness of 0.05 mA/cm2. Battery demonstrates a decent reversibility with


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a cut-off limit of 500mAh/g, the N-CNTs@Ni-based li-air battery showed great reversibility for 8 cycles, as the last voltage of each release fragment settles at 2.65-2.75 V. The improved contact between N-CNTs and the present gatherer Ni is useful to smother the volume extension and prompts less polarization just as great cycling execution. As of late Park et.al orchestrated the arranged mesoporous carbon (OMC).116 Requested mesoporous carbon (OMC) with exceptionally requested pore channels was utilized as an oxygen anode for a Li-O2 battery. At a present thickness of 200 mA/g, the OMC cathodes decreased polarization in the oxygen advancement response by 0.1 V contrasted with those comprising of traditional super P carbon terminal. The OMC anodes were likewise viable at high current densities (500 mA/g and 1000 mA/g). It was shown that the OMC channel constrained the Li2O2 estimate and released item saved inside the OMC pores instead of on the terminal surface as on account of the super P cathode. In outline the doping and functionalize the carbon material are successful procedures for the improvement of lithium air battery, however, the key components of functionalized carbon upgrading Li-air execution still need more examination so this methodology can be all the more adequately sent. Metal Oxides Transition metal oxides Transition-metal oxides represent a large family of oxygen electrocatalysts, including single metal oxides and mixed-metal oxides. Transition-metal oxides have many advantages such as high abundance, low-cost, nontoxic in nature and the environmental friendliness and so on and these are the best alternative to the noble metals. Transition metal elements possess multiple valences, resulting in a variety of oxides with different crystal structures. It has been proved in the past that variable valences and abundant structures, giving rise to rich redox electrochemistry. Transition metal oxides Fe2O3, Fe3O4, NiO, CuO, Co3O4, CoFe2O4 and MnOx have been employed in metal



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batteries by many researchers.117-119 These metal oxides have shown significant ORR and OER activities. Among these catalysts, Fe2O3 exhibits the highest initial discharge capacity, while Fe3O4, CuO and CoFe2O4 give the best capacity retention. Co3O4 shows the best compromise between the discharge capacity and the retention. On the other hand MnO2 and Cobalt oxide are promising and most suitable catalyst materials for high performance lithium air batteries.120-122 These oxides have their advantages of abundance, low cost, high activity in alkaline media and non–toxicity and can be applied as promising catalysts in air electrode for both alkaline fuel cells and metal–air batteries. Bruce et al.123 and few others,124, 125 have also compared all types of MnOx, such as α–MnO2, β– MnO2, bulk α-, β-, γ-, λ- MnO2 and commercial Mn2O3 and Mn3O4 and concluded that α–MnO2 nanowires are the most effective catalysts for lithium–air battery with their crystal structure and high surface area. Therefore, it is clear that the morphology and crystalline structure of MnO2 nanomaterials plays a main role in the catalytic property of the material. Recently, our group synthesized α-MnO2 urchin and δ-MnO2 flower structures and used as an air electrode in the lithium air batteries.126 Among the two phases and structures of MnO2 used, urchin shaped α– MnO2 exhibits very good catalytic activity with very high capacity and stable cycling than layered birnessite δ–MnO2 catalysts as shown in figure 10b-f. The catalytic performance of α-MnO2 is associated with the morphology and size of the particles, and also with their crystal structures with (2×2) tunnels (figure 10a). Recently Song et.al synthesized α-MnO2 nanowire catalyst and compared the discharge capacity with α-MnO2 nanotube.127 A very high capacity of 11000 mAh/g and high reversibility of Li-air batteries using α-MnO2 NWs catalyst was achieved. The highest capacity and the reversibility of α-MnO2 nanowire is attributed to the larger amount of Mn3+ exposed to surface (figure 11a-d). This result suggests that the surface oxidation states can be the dominant factor


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with impact on discharge products deposition mechanism and ORR/OER catalyst performance for lithium-air batteries. The reduced oxygen states of Mn ions enable the homogeneous distribution unique plate or lamellar shaped formation of discharge product. Beside the high ORR and OER activity of oxide based catalyst, the inherent low conductivity and aggregation of nanoparticles in oxide catalysts are the important drawbacks that limit their activity for the ORR and OER. To overcome this limitation the dispersion of the catalysts on a conductive substrate to form a composite is a common and useful strategy. As typical conducting substrates, different forms of carbons have been used for oxide catalysts. The synergistic coupling effect between catalyst and substrate results in excellent catalytic performance. Nazar et al. synthesized cobalt oxide nanoparticles supported on reduced graphene oxide (Co3O4/RGO) and mixed with Ketjen black (KB).128 The Co3O4/RGO on KB sample exhibited a charge plateau between 3.5 and 3.75 V, about 400 mV lower than that of KB at a current density of 140 mA/g carbon. This was attributed to the effect of Co3O4. The large discharge capacity may be a result of the porous structure of the cathodes, created by dibutyl phthalate (DBP). Very recently graphene was used as a substrate for the hybrid Co3O4-based catalyst.129 The use of graphene could enable the production of extremely small and well-dispersed Co3O4 particles as shown in figure-12. The well-dispersed nanoparticles on the thin graphene sheets provide a large active surface area of the catalyst. Hybrid Catalyst shows the improved performance as compared to the KB electrode. In the case of the KB + C + G electrode, however, the rechargeability of the Li–O2 cell has significantly improved (Fig. 12d). However, the first discharge and the charge capacity are 1400 and 1193 mAh/ g, and the tenth discharge and charge capacity are 1384 and 1140 mAh/g, respectively. The charge/discharge profile of the KB + C/G electrode (Fig. 12e) remained virtually unchanged during the 10 cycling processes. The enhanced cyclability is attributed to the use of nano Co3O4 as a catalyst, which may facilitate the



formation and decomposition of the discharge products in the air electrode. The enhanced performance is attributed to the nano catalyst directly grown on graphene, which have strong contact with the graphene support to maintain its high surface area over time. While the number of the triple-phase boundaries (TPBs: carbon-catalyst-product) for the charge/discharge reaction (Li2O2 or Li2CO3 formation and decomposition) is of importance, a mono-dispersion of the catalyst on graphene will secure the number of TPB sites. Yang et.al synthesized 3D mesoporous Co3O4 microspheres/Cu catalyst.130 The cobalt oxide-based catalysts show better performance during the discharging and charging processes at a current density of 0.05 mA/cm2 compared with that of the Vulcan XC-72(figure 13c). The round-trip efficiency (the ratio of discharge to charge voltages) of the cells with the porous Co3O4 microspheres and Co3O4 microspheres loaded with Cu nanoparticles as catalysts are 72.1% and 75.7%, respectively (figure 13c). These values were found higher than those of the Vulcan XC-72 (67.6%) and the commercial Co3O4 powder (63.8%). The enhancement in the round trip efficiency is due to the addition of Cu nanoparticles in Co3O4 catalyst. Copper nanoparticles may play a dual role in this case, copper nanoparticles improve the conductivity of the Co3O4 catalyst. On the other hand, the metallic copper could catalyze the electrochemical reduction of O2 at its surface. Mesoporous cobalt oxide (Co3O4) with different porosity mixed with carbon black was employed as a cathode with a carbon loading of 1.1 mg/cm2 in LiO2 batteries.131 The mesoporous Co3O4 demonstrated significantly improved round-trip efficiency and specific capacity over the bulk catalyst. It was found that the pore diameter, pore volume, and BET surface area of the Co3O4 depended on the aging temperature of the template KIT-6. A capacity close to 2250 mAh/ g carbon and a round-trip efficiency of 81.42% were obtained for the mesoporous Co3O4-KIT-6-40 catalyzed air electrode. This performance is


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attributed to the porous structure of the Co3O4, which contribute to fast O2 transport and increase the utilization of the catalyst. Very recently, Lu eta al. reported the enhanced performance of Li-O2 OER activity by using Series of chromium oxide (Cr2O3) with varying fractions of high-energy facet.200 It was reported that the OER activity of Cr2O3 increases with increasing fraction of high-energy facet and demonstrates higher activity than the state-of-the-art precious metal Ru/Vulcan carbon catalyst. The average charging voltage of the TiCrOx-400 composite was lower than that of the state-ofthe-art precious metal Ru/VC by 120 mV, representing one of the most active charging catalysts reported for Li-O2 batteries.132,133 The high OER activity of the high-energy facets originates from their higher number of unsaturated surface atoms that can better stabilize (i.e., form stronger bonding) the charged intermediates (Li2-xO2), thereby reducing the reaction activation barrier. Spinel oxides For the reversibility of lithium air batteries an effective charging process reaction is quite important. Thus, a bi-functional catalyst with high catalytic behavior for both ORR and OER is vital. Spinel oxides with mixed valences exhibit electrical conductivity enabling their direct use as bi-functional catalysts, and the electron transfer takes place with relatively low activation energies between the cations of different valences by hopping processes. Metal oxides possessing the spinel structure show good performances in oxygen evolution and reduction in alkaline solutions. Sun et al.132 reported the synthesis of NiCo2O4 nanorods, which exhibited a superior catalytic activity, including low charge over-potential, high discharge capacity and high-rate capability as shown in figure 14c-f. The discharge capacity of 13,250 mAh/g of the NCO-NR electrode was obtained, which was much higher than the CB electrode (6240 mAh/g) at the current density of 200 mA/g. The specific capacity of the NCO-NR electrode is 6,625 mAh/g based on the



overall electrode weight. The NCO-NR electrode showed a better cycling performance than CB electrode, as evidenced by less increase of the over-potential on cycling (figure-14e). The high Crate capability of NCO-NR electrodes is attributed to the high ionic conductivity and good oxygen diffusion properties of the DMSO-based electrolyte and 1D nanorod nanostructure of the cathode catalyst, which provides more spaces to facilitate the electrolyte impregnation and oxygen diffusion. Zhang et al. reported mesoporous NiCo2O4 nanoflakes as electrocatalysts for rechargeable Li–O2 batteries.133 The as-prepared NiCo2O4 has a specific nanostructure with numerous catalytic active sites. The battery with a synthesized NiCo2O4- based cathode exhibited an improved performance, including a lower over potential than carbon cathode, specific capacity 1560 mAh/g with 10 stable cycles. The superior electrocatalytic behavior of NiCo2O4 towards both ORR and OER is attributed to its inherently electronic structure and favorable electronic transport capability. Moreover, the mesoporous and nanoflake structure also plays a crucial role in the electrochemical performance, which provides not only more electrocatalytic sites but also promotes mass transport (oxygen and ions) in the electrolyte, and eventually improves the capacity and cyclability. Dai group synthesized the MnCo2O4–graphene hybrid catalyst (figure-15a-b) and employed as oxygen electrode in non-aqueous lithium air battery.134 The synthesized catalyst outperforms the Pt/C, and exhibited the lower over potentials and longer cycle lives than all other catalysts (figure-15f). The cell showed good cycling ability over 40 cycles at a current density of 400 mA/g. This better performance is attributed to the direct nucleation and growth of MnCo2O4 nanoparticles on reduced graphene oxide, which controls the morphology, size and distribution of the oxide nanoparticles and renders strong covalent coupling between the oxide nanoparticles and the electrically conducting graphene substrate. As discussed earlier, that the porosity plays a vital role for the formation and storage of Li2O2 discharge product, and this characteristic endows the


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Li-O2 cells with a high specific capacity, good rate capability, and excellent cycle stability. Wang group reported the synthesis of multiparous MnCo2O4 microspheres, which demonstrate excellent bi-functional catalytic activity.135 The cell prepared with the MnCo2O4 exhibits discharge capacity of 6000mAh/g at 100mA/g current density. The MnCo2O4 microspheres possess lower over potential and larger discharge capacity than super P carbon. The porous structure of MnCo2O4 microspheres provides high electrocatalytic active sites and sufficient transmission paths for O2. Perovskite-type oxides Catalysts Perovskite-type oxides with the general formula ABO3 have been tried as bi-functional catalysts in alkaline media.136-140 The ideal perovskite structure is cubic although it may be somewhat distorted according to the type of the A and B cations involved. Typical A elements are rare earth, alkaline and alkaline earths while the B sites are usually occupied by transition metals. Generally, A-site substitution mainly affects the ability of adsorbed oxygen, whereas B-site substitution influences the activity of the adsorbed oxygen. The electrocatalytic properties of perovskite oxides which normally consist of several different transition metal cations vary depending on the metal selection and stoichiometry and thus, may also be tuned. The activity of the transition metal oxide catalysts can be correlated with the ability of the cations to adopt different valency states, particularly when they form redox couples at the potential of oxygen reduction/evolution. Therefore, different perovskite type oxides with various replacements have been conducted as bi-functional catalysts. Suntivich et al.141 reported the synthesis of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) catalyst the OER. The synthesized catalyst exhibits the better performance than iridium oxide catalyst in alkaline media (figure-16c). The results showed that the intrinsic OER activity also exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an eg symmetry of the surface transition metal cations in an oxide and the peak OER



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activity was predicted to be at an eg occupancy close to unity with high covalency of the transition metal–oxygen bonds. This work provides a promising strategy for the development of highly active non-precious-metal-containing oxide catalysts for oxygen reduction and oxygen evolution reactions by tuning the surface electronic structure features such as transition-metal eg-filling and covalence. More recently, Han et al. reported the synthesis of interconnected porous CaMnO3 nanostructures and the synthesized material was employed as cathode catalysts in rechargeable lithium–oxygen batteries.142 The CaMnO3-based cell displays lower over potential (voltage gap of 0.98V) than those of carbon-based cells with limited capacity of 500 mAh/gcarbon (Figure-17c) which is among the lowest reported values. The CaMnO3/C electrode attains good cyclability more than 80 cycles while the carbon-only electrode can merely sustain less than 25 cycles. The enhanced charge–discharge efficiency and cyclability is attributed to the intrinsically high catalytic capability of CaMnO3, which promotes the dual ORR/OER while the nanoporous structure provides more contact sites and larger space for Li2O2 deposition which benefits the transport of oxygen and electrolyte. Zhang group prepared perovskite-based porous La0.75Sr0.25MnO3 nanotubes and used as cathode catalyst in lithium oxygen batteries.106 With this novel electrocatalyst the charge voltage of the Li–O2 cells with PNT–LSM/KB was found to be much lower than that of KB, by about 200mV, with stable capacities of 9000-11000mAh/g for five cycles. The cell maintains 124 cycles with a capacity limit of 1000 mAh/g. The improved performance is attributed to the synergistic effect of the high ORR and OER catalytic activity and the porous hollow structure of the PNT–LSM. The porous tubular structure could offer more abundant oxygen and electrolyte transportation paths in the electrode, facilitating the formation and decomposition of the discharge product and thus improving the reversibility of the O2 electrode. The Zhang group also reported the synthesis of 3D ordered macroporous LaFeO3 (3DOM-LFO) (figure 18a) and


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employed as electrocatalyst in Li–O2 battery with TEGDME based electrolyte.143 The cell prepared with 3DOM-LFO/KB exhibits the discharge voltage lower than KB about 30mV, similarly charge voltage was found to be 150mV less than KB electrode. The enhanced ORR/OER kinetics could lead to improvements in the energy output, the recharging characteristic, and the round-trip efficiency of the Li–O2 cell. The cell maintains 124 cycles with 3DOM-LFO/SP electrode (figure 18e &f) and 75 stable cycles with NP-LFO catalyst (figure 18c and 18d). On the contrary, the operations of Li–O2 cells are limited to 43 or 75 cycles for Li–O2 batteries without catalyst. The enhanced performance is considered to stem from the synergistic effect of catalytic activity and porosity of the 3DOM-LFO catalysts. Jung et al. reported doped lanthanum nickelates (La2NiO4) with a layered perovskite structure and employed as a bi-functional catalyst in an aqueous alkaline electrolyte.107 Rechargeable lithium-air assembled with these catalysts exhibit remarkably reduced discharge-charge voltage gaps as well as high stability during cycling. The LSN catalyst reduces the over potentials for discharge and charge over the completely current range compared to that of the catalyst-free battery. The voltage gap of the battery with LSN is as small as 1.16 V at 2.0 mA/ cm, which is a 550 mV less than the catalyst-free battery (figure 19a and 19b). Yamamoto group reported the synthesis of La0.6Sr0.4Co0.2-Fe0.8O3 (LSCF) catalyst with and various carbons, such as Ketjen black (KB), acetylene black (AB and AB-S), Vulcan XC-72R (VX), and vapor grown carbon fiber (VGCF) and examined in an aqueous solution of saturated LiOH with LiCl in the different current density range.144 The best performance for oxygen reduction and evolution reactions was observed for the KB electrode and over-potential of 0.2 V was obtained for the oxygen reduction reaction using the KB electrode without the catalyst, while the over-potential was 0.15 V for KB with the LSCF catalyst at 2.0 mA/cm.



Metal–nitrogen composite catalysts In addition to metal oxides and carbon-based materials, other non-precious ORR catalysts such as transition metal nitrogen complex (transition-metal porphyrins, transition-metal phthalocyanines, etc.) and metal nitrides also show high activity for oxygen reduction in metal-air batteries. Transition metal N4-macrocycle complexes have long been known to be highly active for the catalytic reduction of oxygen. The heat-treated transition metal complexes have been considered as an excellent catalyst for the oxygen reduction in lithium–air batteries. For example, Heat-treated FeCu phthalocyanine (FeCuPc) complexes as the catalyst for oxygen reduction were investigated in non-aqueous Li-air batteries.145 FeCu/C catalyst reduces the polarization on discharge while simultaneously reducing the fraction of Li2O2 in the final discharged products. The Li/air cells with FeCu/C show at least 0.2V higher discharge voltage at 0.2mA/cm than those with pristine carbon. The impedance results indicated that the pyrolyzed FeCuPc catalyst can effectively reduce the apparent activation energy for the discharge of Li-air cells. Ren et.al reported the synthesis of CuFe catalyst and employed in the lithium oxygen batteries with different carbons.146 Higher discharge voltage and rate were achieved using pyrolyzed FeCuPc catalyst. The Li-air batteries with pyrolyzed CuFePC catalysts provided higher discharge voltage of 0.2 V with KB-carbon and 0.5 V with SP-carbon. The higher cell voltage was attributed to larger density of catalytic site and higher activity of pyrolyzed CuFePc catalysts than pristine carbons. Recently Fe/N/C composite was synthesized and its role in controlling the oxygen evolution reaction during Li-O2 battery charging was studied in ether-based electrolyte.147 Li-O2 cells using Fe/N/C as the cathode catalyst showed lower over potentials than α-MnO2/carbon catalyst and carbon-only material (figure 20a-c). The Li–O2 batteries with Fe/N/C as the catalyst also exhibited high cyclability (more than 50 cycles with excellent capacity retention) (figure 20d). The improved


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activity of the lithium air battery is due to the Fe/N/C active sites which are atomically dispersed in the carbon matrix with high surface density, which results in higher interfacial boundary with the lithium oxide precipitates, lowering both electron and mass transport barriers and thereby reducing the over potentials during charging.147 The catalytic ORR activity was strongly dependent on the NH3 heat-treatment temperature, the metal composition ratio and method of preparation. The structures of metal nitrides influence their catalytic activities toward the oxygen reduction reaction. Dong et al. reported the synthesis of Molybdenum nitride/nitrogen-doped graphene nanosheets (MoN/NGS) catalyst through a hydrothermal reaction and subsequent ammonia annealing, and employed the catalyst as air cathode in lithium oxygen battery.148 The hybrid nanocomposite exhibits a discharge plateau at around 3.1 V and a large specific capacity of 1050 mAh/g, the cell exhibits good reversibility for 7 cycles by restricting the capacity to 1100 mAh/g. These results demonstrate that the MoN electrocatalyst based hybrid nanostructure can be highly desirable as an alternative cathode for Li–O2 battery applications. The Cui’s group also reported the synthesis of Mesoporous Cobalt Molybdenum Nitride by coprecipitation method followed by ammonia annealing treatment. The catalyst was employed the in lithium oxygen batteries with LiTFSI/TEGDME electrolyte.149 By the use of Co3Mo3N based catalyst in Li-O2 cell, the lower over potential was observed as compared with carbon based Li-O2 cell. The discharge capacity of the Co3Mo3N based Li-O2 battery stabilizes above 1670 mAh/g on the subsequent 16 cycles at a deep discharge to 2.0 V. By contrast, the initial discharge capacity of Li-O2 battery with pure super P cathode is 1150 mAh/g, and it drops to 100 mAh/g after the 15th cycling. This promising performance should be attributed to the more active sites which were generated by the welldesigned mesoporous nanostructure, and the intrinsic electronic configuration lead to an excellent bi-functional electrocatalytic performance for the ORR/OER in non-aqueous Li–O2 cells,



delivering considerable specific capacity and alleviating polarization, indicating that Co3Mo3N is a promising cathode catalyst candidate for Li-O2 battery application. Later, the same group studied the charge-discharge and cycling performance of lithium oxygen batteries with molybdenum nitride/N-doped carbon nanospheres150 and used as cathode electrocatalyst in lithium air batteries. The cell with MoN/N-C nanospheres showed a 0.55 V lower voltage than that of super P catalyst. The discharge capacity of the MoN/N-C based Li- O2 battery stabilizes above 790 mAhg-1 on the subsequent 10 cycles at a deep discharge to 2.0 V at a current density of 0.1 mA/cm2, cell showed good cycling ability over 30 cycles under a controlled capacity of 400 mAh/g. By contrast, the initial discharge capacity of Li-O2 battery with pure super P cathode is 1150 mAh/g, and it drops to 280 mAh/g after the 10th cycling. These results show that MoN can clearly facilitate the OER and ORR processes by reducing the over potentials at the Li-O2 battery cathode. The superior performance of MoN catalyst is attributed to the generation of more triple-phase (solid-liquid-gas phases) regions required for electrocatalysis, which effectively lower both electron and mass transport barriers and reduce the over potential of discharge/charge process. Another successfully example is nitrogen doped graphene-rich catalysts (Co–N-MWNTs) derived from heteroatom polymers for oxygen reduction in non-aqueous Li–O2 battery cathodes (figure 21). The Co–NMWNTs was synthesized via graphitization of an aromatic heteroatom polymer, polyaniline under catalysis of cobalt species supported on multiwalled carbon nanotubes (MWNTs).151 The Co-NMWNT catalysts deliver the initial discharge capacity of ∼3700 mAh/gcat among the catalysts studied including the control catalysts and Co-N-KJ (∼2900 mAh/gcat). The cell showed very good cycling stability up to 20 cycles without significant capacity loss; further cycling to 30 and 50 cycle’s results capacity losses of 8.4 and 20.4%, respectively. Compared to the reported metalfree graphene catalysts, the addition of the Co species introduces a high level of quaternary and


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pyridinic N in the graphene composite catalysts and significantly improves the catalytic activity for the ORR. At the same time, beneficial mass and electron transport, specific interactions between active site and MWNTs, as well as high corrosion resistance can all improve the cathode performance. Zhou’s group reported the synthesis of TiN nanoparticles supported on Vulcan XC72.152 As the cathode catalyst, TiN nanoparticles supported on Vulcan XC-72 (n-TiN/VC) exhibited an onset potential for the OER at 2.9 V, contrasting with a mixture of micro-sized TiN and VC (m-TiN–VC), and VC, both at about 3.1 V in a nonaqueous Li–O2 battery. The discharge– recharge voltage gap of n-TiN/VC was estimated to be 1.05 V, which is 390 and 450 mV smaller than that of m-TiN/VC and VC, respectively, at 50 mA/g carbon. These indicate that n-TiN/VC can function as both an active ORR catalyst during discharge and an efficient OER catalyst during recharge. N-TiN–VC exhibited a capacity of 6407 mAh/g in comparison with m-TiN–VC and VC. The enhanced performance can be ascribed to the high catalytic activity of TiN nanoparticles and the intrinsic contact between them and VC. Polymer based air cathode Catalyst Organic conductive polymers such as polypyrrole (PPy), polyaniline (PANI), polythiophen (PTh), are very much attractive materials in oxygen electrocatalysis. The cathode materials can be well combined with electrical and ionic conducting agent, such as the electro-polymerized conductive polyaniline (PANI) and polypyrrole (PPY), which would improve the battery’s rechargeability.153,154 The polypyrrole (PPy), a very attractive polymer, has many advantages, such as high electric conductivity, high chemical and electrochemical stability, stable three-dimensional structure, ease of synthesis, good adhesion, especially the higher polarity than the carbon materials expressed as higher hydrophilic property, qualifying it as a very promising candidate of support for Li–O2 batteries. Cui et al. was the first to introduce a tubular structured conducting polymer



polypyrrole (TPPy) as an alternate support material for the air electrode of Li–O2 batteries.155 The results showed that the tubular PPy supported air electrode exhibited a higher reversible capacity, round-trip efficiency, and significantly better cycle stability and rate capability than the conventional carbon (acetylene carbon black, AB) supported cathodes as shown in figure 22. The results showed that tubular PPy composite enhanced the cycle life and promotes the reaction ORR and OER efficiently with reduced over potential in comparison with granular PPy supported cell and AB supported cells. The excellent performance of the tubular PPy based cell can be attributed to improved oxygen diffusion kinetics owing to the hydrophilic property and the special tubular structure with hollow channels of PPy. The hydrophilic properties and additional open hollow channels help to transfer oxygen to and from the inner air electrodes of Li–O2 batteries, which greatly improves the reversible capacity, round-trip efficiency, cycling stability and especially the rate capability of the batteries, demonstrating the effectiveness of the tubular conductive polymer based design. Later Wang group reported the synthesis of low cost water dispersed conducting polyaniline nanofibers doped with phosphate ester, and investigate the electrochemical performance in lithium air battery.156 The experimental result showed us that this low cost and easily produced material could catalyze the discharge reaction independently, and after an initial degradation from 3260 to 2320 mAh/g PANI during the first three cycles at current density of 0.05 mA/cm2, its discharge capacity kept relatively stable in the next 27 cycles with only a 4% loss (figure 23d). Zhang et al. was the first one to use the polypyrole (one of the most commonly used conducting polymers) with Cl- and ClO4- as dopants and applied as catalysts in non-aqueous LiO2 batteries in alkyl carbonate electrolyte.157 The discharge capacities of 6208 mAh/g and 5164 mAh/g were observed for PPy-Cl and PPy-ClO4 electrodes respectively. The carbon black electrode delivered a lower capacity of 1365 mAh/g. On the other hand conducting polymer


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electrodes shows the reduced over potential in comparison with carbon black electrodes, this indicated that PPy has better ORR and OER activity. PPy-Cl and PPy-ClO4 electrodes exhibited much better cycling stability than carbon black electrodes in the first five cycles. Furthermore, PPy doped with Cl- showed better cycling stability than the PPy-ClO4 electrode. The improved performance is due to formation of LiCl layer which interact with O2- and stabilize the superoxide radical. This process may lower the barrier of the reaction and provide a reduced over-potential. The presence of LiCl layer on the cathode could lead to better cycling performance and lower overpotential. Recently a polyethylenimine (PEI) supported anthraquinone (AQ) catalyst which has been used as a ORR catalyst for the Li-O2 battery.158 The addition of the anthraquinone-based catalyst improved the cycleability of the Li-O2 battery when cycled in a tetraethylene glycol dimethyl ether electrolyte. With the PEI-AQ polymer, the discharge voltage of the cell increased to 2.67 V, which is ∼70 mV higher than cells with the unmodified PEI polymer/SP or SP alone. The charge voltage profiles of both PEI-AQ/SP and SP present a plateau at 4.25 V. The improved cycling performance of the PEI-AQ/SP cathode can be ascribed to formation of Li2O2 as the dominant discharge product. Calculations based on density functional theory also indicate that the enthalpy of formation of Li2O2 is found to be more exothermic in the presence of anthraquinone than anthracene (nocarbonyl groups). Precious metal catalysts: Noble metal catalysts Research efforts toward better understanding of the oxygen reduction reaction (ORR) mechanism and the design principles of highly active ORR catalysts are critical to improve the discharge performance, which directly affects the deliverable gravimetric energy and power of Liair batteries. To date, Pt is still the most active catalyst for oxygen reduction reaction (ORR). Pt is often chosen as a benchmark material in current studies of alternative catalysts. However, the



scarcity and cost of Pt make it necessary to maximize the activity of a Pt-based catalyst by engineering its morphology and composition. During the last few decades, a number of strategies have been proposed for improving the performance of Pt-based catalysts. For example, tuning the size and morphology to achieve a small/dispersive size, high surface areas and desired highly active facets has been proven as an efficient route to improve the ORR properties on a mass basis. Another most viable strategy to simultaneously enhance the performance and lower the cost is alloying or modifying Pt with other appropriate noble metals or early transition metals that are less expensive. Significant improvements in ORR catalysts have been reported where transition metals, including Ni, Co, Fe, Cu, and Pd have been incorporated into Pt-based multimetallic electrocatalysts.159-161 To further decrease the amount of expensive Pt required in ORR catalysts, numerous investigations of Metal-core/Pt-shell structures have been reported, and enhanced ORR activities of the core/shell catalysts have been validated using a d-band model. Interest in Pt-Au bimetallic catalysts has recently increased because of their OH-repulsive properties and excellent electrochemical stability in acidic media. The Pt-Au bimetallic catalysts are expected to exhibit enhanced ORR performance with lower surface coverage of OH, whereas the surface of Pt can be more extensively poisoned through strong OH binding at high potentials. However, the ORR performance of Pt-Au catalysts has been mostly reported to be lower than or comparable to that of Pt catalysts.162-164 The promising stability of Pt-Au catalysts is attributed to the stability of Au itself and to the improved stability of Pt via the prevention of the place-exchange mechanism.165 ShaoHorn et al. was the first to introduce the bimetallic Pt-Au catalyst in lithium air batteries. In the initial reports, they found that Au/C promotes the discharge process (ORR) and Pt/C promotes the charge process (OER) in a Li-air battery.166,167 In order to exert both advantages, they combined Au and Pt onto the surfaces of individual PtAu nanoparticles and examined ORR and OER activity


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of such particles supported on carbon in Li–O2 cells. They demonstrated that the Pt–Au alloy particles can serve as bi-functional catalysts, leading to a high round-trip efficiency from 57 to 77%.The discharge voltage of electrode with PtAu/C is higher than that of Vulcan XC-72 carbon electrode, while the average charge voltage of PtAu/C is 3.6V which is 900 mV lower than that of carbon (4.5 V). The electrocatalysts for the ORR and OER must be different for a Li-air battery, which is consistent with the different mechanisms for ORR and OER. For example, Pt is an excellent electrocatalyst for oxygen reduction, but not good for oxygen evolution; in contrast, iridium and iridium oxide exhibit poor electrocatalytic activity for oxygen reduction but outstanding activity for oxygen evolution.167 Keeping this idea in mind, later they designed a bifunctional electrocatalyst Pt-Au alloy for rechargeable Li-air batteries.168 The new bi-functional Pt-Au electrocatalyst significantly decreases the overvoltage, especially for the charge process, thus increasing the round-trip efficiency of the Li-air battery. The charging voltage on their PtAu/C is considerably lower than many reported catalyst as shown in figure 24. In order to reduce the cost of noble metals Tatsumi et al. investigated the Pd with other metal oxide catalysts.169,170 It was observed that by adding MnO2 to the electrode, the discharge plateau of the battery increases to 2.9–2.7V, while the charge potential decreases to 3.6V, leading to a high specific energy efficiency of 82%. Shao-Horn group further investigated the catalytic activity trends of the ORR for four noble metals (Pd, Pt, Ru and Au) in a 0.1 M LiClO4 1, 2dimethoxyethane electrolyte which is more stable than other electrolytes.171 It was found that the Li+-ORR activity of these surfaces primarily correlates to the oxygen adsorption energy, forming a ‘‘volcano-type’’ trend. The Li+-ORR activity was found to be Pd > Pt > Ru ≈ Au > GC. It was shown that the activity trend found on the polycrystalline surfaces was in good agreement with the trend in the discharge voltage of Li–O2 cells catalyzed by nanoparticle catalysts. Zhou group



reported the electrocatalytic activity of platinum, iridium, and platinum-iridium alloy in an oxygen electrode.172 Chronoamperometry analysis shows that Pt-Ir/C is much effective for OER than Pt/C. Pt-Ir/C showed the 0.6V charge potential which was lowest over potential compared to other samples. The potential difference between the discharge and charge potentials of the Ir/C, Pt/C and Pt-Ir/C electrodes are 0.875, 0.639, and 0.727 V, respectively (figure 25). The discharge potentials for all three catalysts were found to be similar, ∼2.81 V vs. Li/Li+, and the discharge over potential (∼0.15 V) is very low. Huang et.al. reported the synthesis of bi-functional catalyst composed of Pt and IrO2 supported on carbon nanotubes.173 The synthesized material was used as a cathode catalyst in lithium air battery using sulfuric acid as catholyte. The specially designed and synthesized bi-functional catalyst showed significant over potential reduction and achieved a round trip energy efficiency of 81% after 10 cycles, higher than many achieved in aprotic Li–O2 batteries. The hybrid Li–air battery was discharged and recharged for 20 cycles at 0.2 mA/cm2, showing a fairly stable cell performance. A specific capacity of 306 mAh/g and a specific energy of 1110 Wh/kg were obtained for the hybrid Li–air battery in terms of acid weight. This performance is attributed to the formation of thinner oxide layer by Pt which forms at lower OER over potentials, because Pt suffer less oxidation, which may reduce the barrier to access active Pt surface for ORR. Consequently, the cell exhibited a much improved discharge performance. As an OER catalyst, IrO2 not only decreases the charging over potential, but also protects Pt from over-oxidation. It has been widely reported that the Ru, CrOx, mesoporous pyrochlore or promoters can effectively decrease the Li−O2 charge/Li2O2 oxidation potential compared to the uncatalyzed carbon over the entire charge capacity. The large Li2O2 particles (hundreds of nanometers to micrometers) were found to shrink and disappeared completely upon charging.67-71


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Redox Mediator As discussed earlier that in Li–O2 batteries, electrolyte solutions decomposition during both discharge and charge was observed and determined to be a major obstacle in the development of practical Li–O2 batteries. To circumvent this issue, various attempts were made to improve the energy efficiency and suppress decomposition of electrolyte solutions components, for example, changing the composition and the structure of the cathodes, using different kinds of catalysts, and explore more stable electrolyte solutions.174-178 In order to increase the stability and durability of Li–O2 batteries, multiple approaches have been used, among these approaches, the use of soluble catalysts that facilitate oxygen reduction and evolution reactions (ORR and OER, respectively) was found to be the most effective to lower the overpotential of ORR and OER.179-181 Highly important are studies of redox mediators in solutions, which are catalysts that are being oxidized fast during charging and then they oxidize very effectively the Li-peroxide formed by oxygen reduction. The redox mediators have been found very effective for the decomposition of discharge product because redox mediators can freely move and homogeneously dissolved in the electrolyte and have wet contact with solid discharge products, thus maximizing the interaction areas for the catalysis. This approach was also beneficial in charging the discharge products that are detached or isolated from the electrode, which led to a marked increase in the cycle life and efficiency of Li-O2 batteries.182-184 The redox mediator can participate in the decomposition of Li2O2, irrespective of the size and the structure of Li2O2. The redox mediators for Li-O2 cells can be selected according to their unique specific redox reaction potentials of the cells. In general, the use of RMs in Li-O2 cells exhibits more effective and reproducible results than the use of solid catalysts.



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The energy efficiency of the Li–O2 battery during cycling can be optimized if the redox mediator is chosen according to the suitability of their redox potential to the desirable charging potential of the cells, so any parasitic oxidation reaction can be fully avoided.185-187 So far, various redox mediators have been reported in the literature and can be classified into three types: organic, organometallic, and halide redox mediators. It has been found that the use of redox mediator LiO2 cells exhibits more effective and reproducible results than the use of solid catalysts. Organic redox mediators are molecules with double bonds and/ or aromatic nature which carry out red-ox reactions by using their noncovalent/resonance structure for the electrons exchange. Tetrathiafulvalene (TTF),188-190 5,10‐dihydro‐5,10‐dimethylphenazine (DMPZ),191,192 (2,2,6,6tetramethylpiperidin-1-yl) oxidanyl (TEMPO) and 4,N,N-trimethylaniline(TMA)193 are some of the organic RM used in Li-O2 batteries. Recently 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) has been successfully used as a RM which shows enhanced performance in the in Li-O2 batteries.194 The role of DBBQ differs from other RMs which reduce O2 to Li2O2 by transferring electrons from electrode surface rather DBBQ stabilizes Li ions, forming stable LiO2− ions in solution phase. The mechanism of DBBQ is as follows.195 DBBQ(sol) + Li+(sol) + e− → LiDBBQ(sol)

(iii)

LiDBBQ(sol) + O2(sol) ↔ LiDBBQO2(sol)

(iv)

2LiDBBQO2(sol) → Li2O2(s) + O2(sol) + 2DBBQ(sol)

(v)

LiDBBQ(sol) + LiDBBQO2(sol) → Li2O2(s) +2DBBQ(sol)

(vi)

Organometallic redox mediators are a combination of active transition metal cations and organic ligands that participate in the redox reaction by changing the oxidation state of the central metal ions.196 Iron(II) phthalocyanine (FePc),197,198 copper(II) tetra-tert-butyl-phthalocyanine(tbCoPc),199 cobalt bis(terpyridine) and (Co(Terp)2)200 are some of the substantial organometallic


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RMs used in Li-O2 batteries. Halide redox mediators, such as LiI and LiBr201 promotes the decomposition of Li2O2 by the redox reaction of halide ions. During discharge LiI with H2O facilitate the production of LiOH. More than 10-fold increase in cycle life compared to conventional dual-mediator cells was achieved by manipulating the fluxes of reduced discharge mediator (rDM) and O2, thereby creating a steady-state rDM-rich region which completely covers the cathode.202 This rDM-rich region acts as a dynamic O2 shield and isolates the cathode from oxygen reactions. This strategy was also helpful to reduce the amount of by-product deposited on the cathode compared with a conventional dual mediator cell and only 1.6% of Li2O2 deposited on the cathode and 98.4% on the deposition layer in the protected cell. Conclusion and future outlook Aprotic lithium–air batteries are perceived to the next generation battery technology because they have potential to meet the ever increasing demands of electrical energy storage for many emerging applications such as electric vehicles and smart grids. Although this technology has the potential to provide higher energy density than current lithium-ion system, but it is still in the developmental stages, and before its practical implementation, many challenges yet to be solved. Understanding of this novel electrochemical energy system is still largely incomplete. Catalysis is defined as a lowering of the activation barriers in electrochemical reactions in non-aqueous Li–O2 batteries, and accordingly catalysts are applied to reduce the over potentials on either discharge or charge. Only a large discharge capacity without a reduction in the over potential cannot be assigned to catalysis. The large discharge capacity is more related to a large pore volume in the cathodes than catalysis itself when the discharge current is very low. Several types of non-precious and precious catalysts have showed very promising catalytic activity and stability, including metal



oxides, carbonaceous materials, metal nitrides, and polymer based materials, perovskite material and the precious metals. Among them, metal oxides and the carbon based catalysts are the most widely studied as non-precious catalysts in Li-air batteries. In particular, manganese oxide based catalysts have been intensely investigated for ORR in Li-air batteries. The combination of crystalline structure and morphology determines the activity for ORR. For carbonaceous materials, proper dopant and the doping technique plays a critical role in tailoring the catalytic activity; the morphology and pore size are important as well. In addition, metal–nitrogen complexes are an important alternative catalyst for ORR. The choice and the optimization of the metal precursors are the key to produce high performance catalysts. The molecular structure of transition metal complexes should be carefully designed. On the other hand, perovskite and the polymer based catalysts have been studied relatively less but shows superior performance among many other nonprecious catalysts. Precious metals and alloys generally possess virtues of both high activity and favorable stability but disadvantages of cost and scarcity. Table 3 summarizes the characteristics of major ORR catalysts studied for Li-air batteries. While many non-precious catalysts have showed promising results for li-air batteries, difficulties still remain to develop a practical lithiumair battery with performance better than the existing Li-ion batteries. Based on our current understanding of the existing catalysts and results obtained in our laboratory (Figure 27), we believe that the following research directions are important to the development of highly efficient non-precious catalysts for lithium-air batteries. 1. The deposition of reduction products during discharge has a direct relationship with the porosity of the electrode. The reduction products are not soluble in a non-aqueous electrolyte, hence deposits are formed on the cathode surface and they often block the pores available for oxygen diffusion. This ultimately starves the discharge reaction, which in turn


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leads to a lower specific capacity. Therefore the pore size should not be less than 10nm to sustain oxygen diffusion and accommodate the reduction products during discharge. 2. A further important factor to consider is the relationship that forms between the porosity and the carbon content of the cathode. As the carbon content increases, the porosity of the cathode decreases and leads to a decrease in oxygen diffusion length, which in return reduces the reaction kinetics and results in a lower specific capacity. However, it is important to optimize the carbon content of the cathode to retain good electronic conductivity while maintaining the oxygen diffusion length and volume available for the deposition of the reduction products. 3. Pore diameter should be optimized because a large pore diameter corresponds to a larger volume available for good oxygen diffusion and the necessary space to accommodate the deposition of reduction products during discharge. 4. Since the cathode reactions take place on the surface of the catalyst/carbon cathode, a larger surface area corresponds to more area for reactions to take place leading to higher electrochemical performance. 5. The air electrode or catalyst materials should have highly electrical conductivity for efficient current collection. Insufficient electrical conductivity of catalysts must be mitigated by the use of highly-conductive current collectors/substrates. Further, the contact between catalysts and current collectors must be adequate to ensure easy electron transfer across the interfaces. 6. Design of cathode materials/structures with improved and optimized porosity for facilitating high oxygen diffusivity and Li ion transport toward the active surface of the electrode is critical for the overall electrochemical reaction and the resulting



discharge/charge capacity of the battery. Further, since the solubility of oxygen in liquid electrolytes is relatively low, it is necessary to control hydrophobicity and porosity of the electrodes and catalysts to avoid flooding of the active sites. One strategy is to increase the partial pressure of oxygen which enhances the diffusivity and concentration of oxygen in the cathode, which corresponds to higher specific capacities. 7. It is important to control the operational conditions of the lithium–air batteries during discharge. The performance is sensitive to the magnitude of the current density and the depth of discharge. At low current density (typically 0.05–0.1mA/cm3), the lithium air batteries exhibit high discharge capacity. However, in a practical battery, high operational current density is desired to achieve high power. Therefore, one key aim in the development of high-performance lithium–air batteries is to maximize the electrode reaction kinetics so that higher current densities can be withdrawn from the lithium–air battery while maintaining a large and stable specific capacity. 8. Conventional electrocatalysis concept and theory might not work for Li2O2 electrochemistry. The product Li2O2 forms a nonconductive layer on air electrode. This significantly decreases the conductivity of electrode; most importantly, it covers the electrocatalytically active sites and deactivates them. Material selection and design for an air cathode needs a comprehensive vision that combines its activity for lithium air battery reaction, its inertness for electrolytes and reaction intermediates/ products, and its catalytic role in the formation of desired Li2O2 structure and morphology.

Although there are still many problems to overcome with the research and development of lithium–air batteries, it is an exciting and challenging field that encompasses the knowledge and


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expertise of researchers from various disciplines. The multidisciplinary nature of research in this field has the potential to bridge the knowledge gaps in current research projects which will enhance the development of this technology into a commercially available product.

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Figure Captions: Figure1: The gravimetric energy densities (Wh/kg) for various types of rechargeable batteries compared to gasoline. Reprinted with permission from ref no. 10. Copyright 2010 American Chemical Society. Figure 2: A schematic of a Lithium-air battery composed of a lithium metal as the anode and a porous air electrode as the cathode. Reprinted with permission from ref no. 10. Copyright 2010 American Chemical Society. Figure 3: A typical discharge-charge loop for metal air battery. Reprinted with permission from ref no. 100. Copyright 2014 Royal Society of Chemistry Figure 4: Voltage profiles of first two cycles and cycling performance of non-aqueous electrolyte lithium-air cell charge and discharged at 0.2 mA/cm2 (b) Voltage profiles at different current density of rechargeable lithium-air cell with hybrid electrolyte. Reprinted with permission from ref no.82. Copyright 2011 The Electrochemical Society Figure 5: (a) Schematic illustration of O2 /Li2O2 conversion in an ordered hierarchical mesoporous/macro porous carbon catalyst (b) SEM image and c–d) TEM images with different magnifications. Reprinted with permission from ref no. 99. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Figure 6: (a) A schematic structure of graphene sheets. (b) Electrochemical performance of hierarchical graphene sheets in a non-aqueous Li-air battery (c) Discharged air electrode using FGS with C/O = 100. Reprinted with permission from ref no. 31. Copyright 2011 American Chemical Society. Figure 7: SEM images of the pristine nickel foam (a) and different magnifications of the FHPC electrode (b–d). (e) Discharge curves at different current densities ranging from 0.2 mA cm2 to 2 mA cm2 (f) Cycle performance with a restriction of the capacity to 2000 mAh/ g at a current density of 0.5 mA/cm2. Reprinted with permission from ref no. 100. Copyright 2014 Royal Society of Chemistry Figure 8: SEM images of the CNT fibril at (a) low magnification (inset: large area image of the air electrode), (b) high magnification c) TEM image of CNT fibrils after the first discharge (inset: high magnification TEM image), d) SEM image of the CNT fibrils after 100 cycles Discharge– charge profiles, (e) cyclabilities (f) of the Li–O2 cells based on the air electrode of the woven CNT. Reprinted with permission from ref no. 101. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Figure 9: (a) Voltage profiles of GNSs and N-GNSs electrodes at various current densities; (b) CVs of GNSs and N-GNSs electrodes, inset is the CVs in Ar-saturated solution. (c) TEM image of N-GNSs; (d) N2 adsorption–desorption isotherms, inset is the pore size distribution Reprinted with permission from ref no. 96. Copyright 2012 Elsevier.


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Figure 10: (a) Crystal structures of α– and δ– MnO2 nanomaterials (b) Charge discharge profiles of α– and δ– MnO2 nanomaterials(c-f) Cycle life efficiency of α– and δ– MnO2 nanomaterials Reprinted with permission from ref no.109. Copyright 2014 Royal Society of Chemistry. Figure 11: (a) Mn 2p2/3 partial spectra of α-MnO2 NWs and α-MnO2 NTs(b) The initial galvanostatic discharge/charge curves of lithium-air cells using MnO2 nanomaterials obtained at 200 mA/g(carbon) (c) 15 Discharging potential profiles of α-MnO2 nanomaterials (d) the cyclabilities obtained at the current rate of 200 mA/g(carbon). Reprinted with permission from ref no.114. Copyright 2014 Royal Society of Chemistry. Figure-12: (a) Illustration of the strategy of the hybrid graphene/Co3O4 catalyst in the Li–O2 battery system, While partial agglomeration of the catalyst particles easily arise(b) the graphenesupported catalyst maintains its well-dispersed feature of individual catalyst particles on graphene, resulting in enhanced catalytic activity and cyclability.(c) Charge/discharge profiles of the KB, (d) KB+C+G, and (e) KB+C/G electrodes between 2.5–4.4 V vs. Li/Li+ at a current density of 0.2 mA/cm2. Reprinted with permission from ref no. 116. Copyright 2012 Elsevier. Figure 13: (a) Co3O4 products, (b) high-magnification SEM image of an individual Co3O4 microsphere (c) Comparison of the first charge and discharge curves of the prepared lithium–air batteries with various catalysts at a current density of 0.05 Ma/ cm2 (d) the discharge voltage profiles at different current densities of the rechargeable lithium–air batteries with the porous Co3O4 microspheres/Cu nanoparticles catalyst. Reprinted with permission from ref no. 117. Copyright 2013, Elsevier. Figure 14: (a) HRTEM images of NiCo2O4 nanorods (b) The corresponding SAED pattern (c) Charge/discharge voltage curves of NiCo2O4 nanorod electrodes (solid line) and carbon black electrodes (dash line) at different current densities (d) The initial charge/ discharge curves of NiCo2O4 nanorod and carbon black electrodes at a current density of 200 Ma/g carbon. Cycling performances of (e) of NiCo2O4 nanorod electrode (f) carbon black electrode with the curtailing capacity of 1000 mAh/g carbon at a current density of 400 Ma/g. Reprinted with permission from ref no. 119. Copyright 2013 Elsevier. Figure 15: Schematic structure of the Li–O2 cell catalyzed by the MnCo2O4–graphene hybrid (b) Schematic structure of the MnCo2O4–graphene hybrid material (c) Charging and discharging voltage profiles of the cell at various current densities. (d) Charging and discharging voltage profiles of the cell at various cycle numbers at a current density of 400 mA/g (e) Specific discharge capacity of the cell over 40 cycles at 400 mA/g (f) Charging and discharging voltage profiles of Li–O2 cells catalyzed by different catalysts at a current density of 100 mA/g. Reprinted with permission from ref no.121. Copyright 2012 Royal Society of Chemistry. Figure16: (a) Exemplary OER currents of La1–xCaxCoO3 and LaCoO3 thin films on GCE in O2saturated 0.1 M KOH at 10 mv/s scan rate at 1600 rpm. The contributions from AB and binder (Nafion) in the thin film and GCE are shown for comparison. The inset shows the structure of perovskite ABO3 (where A is an alkali or a rare earth, yellow; B is a transition metal, blue; and O is oxygen, red) (b) The relation between the OER catalytic activity, defined by the overpotentials at 50 Ma/ cm2 ox of OER current, and the occupancy of the eg-symmetry electron of the transition



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metal (B in ABO3) Data symbols vary with type of B ions (Cr, red; Mn, orange; Fe, beige; Co, green; Ni, blue; mixed compounds, purple),where x=0,0.25, and 0.5 for Fe. Error bars represent SDs of at least three independent measurements. The dashed volcano lines are shown for guidance only. (c) OER specific activity (in logarithmic scale) of BSCF (squares) and IrO2 (circles) as a function of potential error bars represent standard deviations of at least three independent measurements. Reprinted with permission from ref no. 125. Copyright 2011 AAAS. Figure 17: (a) Crystal structure and (b) SEM image of porous CaMnO3 (c) Discharge–charge profiles of Li–O2 batteries with CaMnO3/C or pure carbon electrodes at a current density of 50 mA/g carbon (d) discharge voltage versus cycle number with and without the CaMnO3 catalyst at 100 Ma/g carbon. Reprinted with permission from ref no.124. Copyright 2014 Royal Society of Chemistry. Figure18: (a) Crystal structure and (b) SEM image of porous CaMnO3 (c) Discharge–charge profiles of Li–O2 batteries with CaMnO3/C or pure carbon electrodes at a current density of 50 mA/ g carbon (d) discharge voltage versus cycle number with and without the CaMnO3 catalyst at 100 mA/ g carbon. Reprinted with permission from ref no.123. Copyright 2014 Royal Society of Chemistry. Figure19: (a) Polarization curves and (b) cycling performance of the hybrid Li-air batteries assembled with the catalyst-free and La1.7Sr0.3NiO4 (LSN- 03)-containing cathodes. Reprinted with permission from ref no. 129. Copyright 2013 American Chemical Society. Figure20: (a) Discharge/charge voltage profiles of Li−O2 cells using α-MnO2/XC-72 and Fe/N/C as cathode catalysts (b) and (c) Load profiles of cells at first and 18th discharge− charge cycles. Fe/N/C and BP were used as cathode materials. Current was 0.05 mA with duration of 5 h, corresponding to capacity of ∼500 mAh/g (d) Cycling performance of cells with catalysts Fe/N/C and carbon black (BP) as cathode catalysts current was 0.05 mA with duration of 5 h. Reprinted with permission from ref no. 133. Copyright 2013 American Chemical Society. Figure 21: Scheme of the formation for nitrogen-doped graphene sheets derived from polyaniline and Co precursors using MWNTs as a template. Reprinted with permission from ref no. 137. Copyright 2012 American Chemical Society. Figure 22: Schematic representations of the organic electrolyte and oxygen distributions on hydrophilic PPy nanotubes (a), oxygen gradient across the flooded porous cathode (b) and the schematic discharge–charge process (c). Reprinted with permission from ref no.139. Copyright 2012 Royal Society of Chemistry. Figure23:(a and b) discharge curves of polyaniline electrodes in lithium−oxygen batteries (c) Charge−discharge curves and (d) discharge capacities vs cycle number for the lithium−oxygen battery based on polyaniline nanofibers cathode at current density of 0.05 mA/cm2. Reprinted with permission from ref no. 140. Copyright 2012 American Chemical Society. Figure 24: (a) Cyclic voltammograms of PtAu/C collected in Ar-saturated 0.5M H2SO4 between 0.05 and 1.7V vs RHE (room temperature and 50 mV/s), (b) Li-O2 cell discharge/charge


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profiles of carbon (black, 85mA/g carbon), PtAu/C (red, 100 mA/g carbon) in the third cycle at 0.04mA/cm2 electrode, (c) background measurement during charging at 100mA/g carbon of Arand O2-filled cells (charging first) for PtAu/C, (d) Li-O2 cell first discharge/charge profiles of carbon at 85mA/g carbon and of Au/C, Pt/C, and PtAu/C at100mA/g carbon and (e)Li-O2 cell discharge/charge profiles (first cycle) of PtAu/C at 50, 100, and 250 mA/g carbon. Reprinted with permission from ref no. 40. Copyright 2010 American Chemical Society. Figure-25: Initial discharge profiles of Li-O2 cells of Pd/C, Pt/C, Ru/C, Au/C, and VC at 100 mA/g carbon. Reprinted with permission from ref no. 156. Copyright 2011 American Chemical Society. Figure-26: Discharge/charge potential profiles (first cycle) of Ketjen black EC600JD carbon and Pt/C (a), Ir/C (b), and PtIr/C (c and d). Cycling was carried out at a current density of 0.05 mA cm2 in 100 psi O2 at room temperature. Reprinted with permission from ref no. 157. Copyright 2012 Elsevier. Table 1: Summary of carbon-based catalysts used in Li-air batteries. Table 2: Summary of non-precious catalysts used in Li-air batteries. Table 3: Characteristic summary of the catalysts in lithium air batteries.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Figure 7


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Figure 8



Figure 9


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Figure 10



Figure 11


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Figure 12



Figure 13


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Figure 14



Figure 15


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Figure 16



Figure 17


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Figure 18



Figure 19


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Figure 20



Figure 21


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Figure 22



Figure 23


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Figure 24



Figure 25


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Figure 26



Figure 27


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Table 1



Table 2


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Table 3



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Dr. Zahoor ul Hussain received his Ph.D Degree in Chemical Engineering from Chonbuk National University South Korea in 2014. His research focuses on the development of transition metal oxide with various nanostructures and application in Lithium Air Batteries. So far, he has published twenty three (23) research articles in well-reputed peer reviewed journals. Currently he is serving as Associate Professor in the Department of Chemical Engineering N.E.D. University of Engineering & Technology Karachi Pakistan.


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Dr. Zafar Khan Ghouri has been working as an Assistant Research Scientist in the Department of Chemical Engineering at Texas A&M University, Qatar and serving as an editorial board member for PLOS ONE journal. His research interests include synthesis, characterizations, processing and application of functional nanomaterials; some specific research areas are synthesis of various class of carbon materials (CNFs, Gr, CNT, GQD, CQD and etc.) and modification of 2D materials for energy (fuel cells, supercapacitors, dyes synthesized solar cells and batteries) and water treatment applications. He has enormous experience in the study of the structures and chemical properties of various materials, including metals, alloys, ceramics, semiconductors and polymers, to develop new products or to enhance the properties of existing products. He did his Ph.D. in Engineering from the Department of BIN Fusion Engineering, Chonbuk National University, Republic of Korea, in 2017, under the prestigious Brain Korea (BK21) Fellowship and received two postdoctoral training in materials science and chemical engineering at Qatar University and Texas A&M University at Qatar, respectively. So far, he has published about 40 research papers in top quality international peer-reviewed journals, along with two book chapters.



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Dr. Saud Hashmi is an associate professor at the department of Polymer & Petrochemical Engineering at NED University of engineering & technology. He received his PhD in chemical engineering from chonbuk national university, South Korea in 2014 with advisor Prof. Dr Florian J.Stadler. His research focuses on polymer solutions and gels with covalent and supramolecular interactions. Of special interest to him are advanced functional polymers enabling sensing and stimuli responsiveness. Furthermore, he is also involved in functional nanomaterials synthesis, especially for energy storage applications. So far saud hashmi has published 21 academic research publications on various aspects of advanced functional materials in reputable journals like Hydrogen Energy, Macromolecules, Macromolecules rapid communication, Soft matter, Polymer Chemistry and Chemical Communication


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Dr. Faizan Raza specializes in semiconductor nanomatertials for energy conservation and storage applications. He received his Bachelors of Engineering from University of Karachi in Chemical Engineering and PhD in Fusion Chemical Engineering from Hanyang University, South Korea under advisor Prof. Jong-Ho Kim (PhD) in August 2017. His research is related to synthesis of nanomaterials applied as photocatalysts for organic reactions had been published in high impact factor peer reviewed journals. He is currently serving as an Assistant Professor in Chemical Engineering department at NED University of Engineering and technology Karachi



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Dr. Shagufta Ishteyaque received her PhD in Chemical Engineering from University of Karachi, Pakistan. Her research areas included synthesis and applications of nanomaterials for energy applications. She is serving as an assistant professor in the Department of Chemical Engineering, University of Karachi. She has 24 research publications in the journals of national and international repute.


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Saad Nadeem is working as an assistant professor at the department of Chemical Engineering at NED University of engineering & technology. He is currently pursuing his PhD in chemical engineering from Universiti Teknologi PETRONAS, Malaysia. His research focuses on photocatalysis especially on CO2 conversion to valuable hydrocarbons. He has also worked on Metal organic frameworks and zeolites for waste water treatment and Nano-fluid based enhanced oil recovery using metal oxide nano-crystals. He has also worked with synthesis of visible light active organic macro-cyclic ligands and their successful interaction with mesoporous silicas for efficient charge transfer.



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Prof. Dr. Inayatullah Memon received his PhD Degree in Chemical Engineering with Specialization in Fuel and Energy from University of Leeds, England, in December 1992. His research focuses on the Energy Storage and Combustion & Gasification of Coal. So far he has published twelve (12) research articles in well reputed peer reviewed journals. Currently he is working as professor in the Department of Chemical Engineering N.E.D. University of Engineering & Technology Karachi Pakistan.


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Prof. Nahm obtained his doctorate degree in 1986 from Seoul National University and currently he is working as Professor Emeritus in the Department Chemical Engineering Conbuk National University South Korea. He was Director of R&D Education Center for Fuel Cell Materials and Systems at Chonbuk National University. Prof. Nahm was in charge of The President of Korea Electrochemical Society in 2014-2015. Nahm’s research interests embrace energy materials and devices including H2 Storage, fuel cells, and Li-battery, especially, the synthesis and characterization of novel and nano-sized materials. Recent efforts have focused on the understanding of nano-engineering applications for energy materials. He has been an invited speaker at many international conferences and is author of more than 304 scientific publications, 8 books and chapters in books, and 13 patents. He was elected to the regular member of the Korean Academy of Science and Technology in 2011.



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Synopsis: A comprehensive review on electrocatalyst for Li-air battery for the development of sustainable technologies for energy and the environment is presented.


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Electrocatalysts for LithiumâAir Batteries: Current Status and

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