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An Inorganic Electrolyte Li-O2 Battery with High Rate and Improved Performance Guruprakash Karkera, and Annigere S. Prakash ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00095 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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ACS Applied Energy Materials

An Inorganic Electrolyte Li-O2 Battery with High Rate and Improved Performance Guruprakash Karkera†, # and A. S. Prakash†, #,* †.

CSIR-Central Electrochemical Research Institute-Chennai unit, CSIR Madras Complex,

Taramani, Chennai 600113, India #

Academy of Scientific and Innovative Research (AcSIR), CSIR- Central Electrochemical

Research Institute-Chennai unit, CSIR Madras Complex, Chennai 600113, India. KEYWORDS: Li-O2 battery, Li-air battery, Solid electrolyte, Ionic liquids, Inorganic electrolyte, Molten salts, Nitrate melts, High rate performance, low overpotential, voltage hysteresis.

ABSTRACT: As an alternative to air intolerant non-aqueous electrolytes, LiNO3-KNO2-CsNO3 (37:39:24) eutectic salt mixture at 140 °C is investigated as molten electrolyte for Li-O2 batteries. The inorganic eutectic promotes highly reversible formation-decomposition mechanism of Li2O2, as a robust electrolyte in oxidative conditions. An in-situ formed shielding layer composed of Li2O and Li3N keeps the Li-anode intact during vigorous cell conditions providing faster Li-ion diffusion kinetics and stable cycling performance. We report an inorganic electrolyte Li-O2 battery that exhibits lowest charge-discharge overpotentials of 40 mV, high rate capabilities at 3 mA/cm2 and cycleable up to 200 times at restricted capacity of 500 mAhg-1carbon. The performance is achieved on a bare carbon electrode with round-trip energy efficiency close

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to 98%. This work emphasizes that the use of inorganic electrolyte would significantly eliminate the side reactions associated with charge-discharge reactions and boosts the promise of realizing next generation batteries.

INTRODUCTION Claiming to its exceptionally high energy density, lithium–air batteries are very promising candidates for electric vehicles1. However, numerous technical challenges are still to be solved before commercialization. The most reported non-aqueous Li-O2 batteries suffers a huge setback from the unstable organic electrolytes used in it. These electrolytes found to be degrading at the oxidative conditions on exposure to O2 and higher voltages of charge2, 3, 4, 5. Contamination of moisture and CO2 leads to the formation of undesired discharge products like Li2CO36, 7, LiOH8, which contributes to the sluggish kinetics of the charge and discharge. McCloskey et al. reported the evolution of CO2 as a bi-product due to the carbon reactivity with discharge product and solvents in non-aqueous Li-O2 cells 9. Hence, the operation of non-aqueous Li-air batteries leads to several unwanted side reactions resulting in lower round-trip energy efficiencies. Recent studies shows that LiNO3 is an effective additive for non-aqueous electrolytes, which helps in forming stable SEI (solid electrolyte interphase) on Li metal10. This strategy helps in using different solvents, which are unstable towards pure Li metal. Recently, increased attention has been received for room temperature molten salts (ionic liquids) as electrolytes in Li air batteries11,

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. The molten salts are known for wide voltage window, less volatility and

flammability. Various reports shows the usefulness of these electrolytes in effective reduction of charging overpotentials of Li-O2 batteries13,

14, 15

. Even after the implementations of various

novel strategies, high rate operation of Li-O2 batteries remained as a challenge. As the rate of


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reaction is directly proportional to temperature, operating a Li- O2 battery at elevated temperature would overcome the challenge of slower kinetics and avoid moisture contamination within the cell. In this context, eutectic mixtures of inorganic salts, which melts at intermediate temperatures (100-250 °C), are attractive as electrolytes for Li-O2 batteries.

Molten salt

(inorganic) electrochemistry has been the choice in the past decades for nuclear16, solar power17 and battery applications18. McManis et al reported Li-ion battery using molten electrolytes with improved energy densities19. Giordani et al have demonstrated a molten salt Li-O2 battery primarily with LiNO3-KNO3 as electrolyte at 150 °C20. Zambonin et al. have studied the oxygen redox mechanism in fused salts of nitrates21. Miles et al report that nitrate reduction readily happens in LiNO3 melts electrochemically and alongside forms stable Li2O22. Further, effect of different cations on the reduction behaviour of nitrates at intermediate temperature is also studied. In this article, we report LiNO3: KNO2: CsNO3 (37:39:24) eutectic salt mixtures at 140 °C as inorganic electrolyte and demonstrate a reversible, stable and efficient Li- O2 battery using molten salt electrolyte. Various electrochemical/analytical techniques such as cyclic voltammetry, Impedance spectroscopy, X-ray diffraction, Field emission scanning electron microscopy (FESEM) and galvanostatic cycling are used to evaluate the battery performance at different levels. We also propose the reaction mechanism of charge-discharge processes in molten salt electrolyte and interrogate the advantage of using them in Li-O2 batteries. EXPERIMENTAL SECTION Inorganic salts LiNO3, CsNO3 (Sigma Aldrich) and KNO2 (Across Organics) are prevacuum dried at 80 °C overnight and transferred to Argon filled Glovebox (MBraun, O2, H2O < 0.1 ppm) to maintain the high purity. LiNO3, KNO2 and CsNO3 are weighed in the mole ratio of 37:39:24 and ground thoroughly to homogenise. The schematic representation of Li-O2 battery is


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shown in the figure 1. Li- O2 battery with the molten salt electrolyte is constructed in a Swagelok type cell inside the Glovebox. Lithium disc (Alfa-aesar) of 8 mm diameter is used as anode, glass micro fibre separator (whatman) and Vulcan carbon (~213 m2/g, ~30nm, Cabot corporation) coated on Toray paper (Fuel cell store) as cathode. Cathode fabrication method is as follows: Vulcan carbon is mixed with a binder solution of polyvinylidene difluoride (PVDF, Alfa aesar) using a solvent N-Methyl-2-pyrrolidone to form a uniform slurry. Slurry is uniformly coated on the toray paper (Coating comprises 90% of carbon and 10% of PVDF). Coated carbon paper is dried in oven at 80 °C. A 10 mm diameter sized coated film with a carbon loading of ≈1 mg/cm2 is used as cathode in this work. The homogeneous inorganic salt mixture (~263 mg) is placed in between separator and cathode.

High pure oxygen (INDOGAS) at a pressure of 1 bar is

supplied from the inlet placed above cathode. As constructed Li- O2 cell was then transferred to temperature chamber, where cell is maintained at 140 °C to melt the eutectic mixture and convert into an ionically conductive liquid (electrolyte). The cell was allowed to rest for 2 hours

Figure 1: Schematic representation of a molten electrolyte Li-O2 battery at discharged condition


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prior cycling to stabilize and to attain the open circuit voltage (~2.9 V). The Li-O2 cell with non-aqueous electrolyte (1M LiTFSi (Bis-trifluoromethane sulfonimide) lithium salt in emim (1-Ethyl-3-methylimidazolium (trifluoromethylsulfonyl) imide, a room temperature molten salt) is also constructed for comparison. Both LiTFSi and ionic liquid were procured from Sigma Aldrich. The cycled electrodes were retrieved from the cell within the Glovebox for post- mortem analysis and washed with NMA (N-Methyl Acetamide) to remove electrolyte residues from surfaces. X-Ray diffractograms of pristine, charged and discharged electrolytes are collected by extracting them from the batteries after cooling to room temperature. For cyclic voltammetry studies, Li metal is used as reference and counter electrode in molten salt electrolyte at 140 °C saturated with oxygen. In the present study, the capacity is expressed in terms of total carbon weight present in the cathode (i.e. mAh/gcarbon) and current density is expressed per area of cathode (i.e., mA/cm2). All the electrochemical experiments are performed using Biologic VMP3 workstation. The working electrode was characterised by X-ray diffraction (XRD) recorded on a Bruker D8 Advance diffractometer using Cu-Kα radiation (λ: 1.5438 Å, Ni filter) and FESEM performed with MIRA, TESCAN instruments. RESULT AND DISCUSSION Electrochemical studies Cyclic voltammetry (CV) experiments are carried out to understand the reversibility of redox processes in molten salt electrolyte. Figure 2a shows the CV of molten electrolyte Li- O2 battery. The area under the curve for cathodic peak (2.7 V), anodic (2.85 V) and peak currents (ipa/ipc ≈ 1) are almost equal suggesting same amount of charge transfer occurring during reduction and


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oxidation. A linear plot obtained from the ip vs. ν1/2 provides the evidence of a chemical reversible process (Figure S1a). The distinct peaks are result of generation of superoxide anion (O2.-) and its oxidation in reverse scan. The CV performed in the presence of N2 (red curve, figure.2b) resulting only in background currents whereas, it showed redox peaks in the presence of oxygen

Figure 2: a) Cyclic voltammograms obtained from Vulcan carbon:PVDF electrode molten electrolyte Li-O2 battery at 140 °C, b) CV in O2 saturated electrolyte (black) and N2 saturated electrolyte (red), Working electrode: Vulcan carbon (d=10mm), carbon loading ∼1 mg/cm2. Counter and reference electrodes: Li metal. Scan rate: 0.1 mV/s. Voltage window: 2.5−3.0 V vs Li/Li+. Current density expressed in mA per g of carbon. c) Charge-discharge profile of molten electrolyte Li-O2 battery, at limited capacity of 2000 mAh/g with current density of 1.6 mA/cm2, V=2.5-3.4 V, electrolyte weight=263 mg, d) Comparison of cycling profiles of Li-O2 batteries utilizing molten nitrate electrolyte (I=0.1 mA/cm2) and non-aqueous ionic liquid electrolyte (I=0.013 mA/cm2, V=2-4.6 V), 1M LiTFSi in emim.


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(black curve). Further, electrochemical stability of the O2 saturated LiNO3: KNO2: CsNO3 (37:39:24) electrolyte is analyzed using CV recorded at 0.1 mV/s scan rate in the voltage window of 2.5-3.6 V (figure S1c). It shows the presence of redox peaks corresponding to reversible Li2O2 formation below 3 V, further, non-faradic currents are observed until ~3.4 V. At potential greater than 3.4 V, faradic current related to decomposition of molten nitrate electrolyte is witnessed. The Li-O2 cell is subjected to long cycling by limiting the capacity in the voltage window of 2.5-3.4 V. Figure 2c shows the 50 charge-discharge cycles under the capacity limitation of 2000 mAh/g (I= 1.6 mA/cm2) displaying an overpotential of ~0.3 V. Figure S2a, S2c shows the charge-discharge cycling with a capacity limitation of 500 mAh/g at current densities of 0.8 and 1.6 mA/cm2 for 100 and 200 cycles respectively. The plot of cycle number vs. average potential of charge-discharge is shown in ESI (figure S2d). The cell is further tested at higher discharge capacity of 5000 mAh/g and it showed complete rechargeability (figure S2b). Figure S3 shows the voltage vs. time plot indicating the stable electrochemical performance up to 300 hrs. Referring to figure 2c & S2c, cycling profile of molten electrolyte Li-O2 battery shows a discharge plateau at ≈2.76 V, along with two-step charging curve in the initial cycles. First plateau, at ≈2.9 V and second plateau, at ~ 3.2 V, the onset of which depends on the discharge time. As the cycling continues, the onset of second plateau slowly shifts positively and finally merges with the first plateau. In line with this, a high voltage discharge plateau is observed in initial cycles (figure S2c). The reason for appearance of the second plateau will be discussed in later section. A decrease in the oxidation peak potential is seen in CV’s carried after different number of charge–discharge cycles (Figure S1b), which is in line with the above observation of reduction of charging voltage hysteresis upon cycling. The results displayed in figure 2d


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Figure 3: The rate performance of Libattery in LiNO3: KNO2: CsNO3 (37:39:24) electrolyte at 140 °C at current density ranges from 0.1 to 4.0 mA/cm2 in voltage window 2-3.4 V. Cathode: VC+PVDF (90:10), anode: Li metal, O2 pressure: 1bar. compares the Li-O2 cycling performance of room temperature molten electrolyte (ionic liquid), 1 M LiTFSi in emim (at I=0.013 mA/cm2) with molten nitrate electrolyte (at I=0.1 mA/cm2, T= 140 °C). At a low current density of 0.1 mA/cm2, the discharge and charge plateaus were at 2.8 V and 2.84 V respectively, giving rise to the voltage hysteresis, ∆V of only 40 mV, lowest ever recorded for any Li-O2 battery till date, to the best of our knowledge. The voltage hysteresis remains close to 2 V for ionic liquid emim based electrolyte. Thermodynamic potential for the formation-decomposition of Li2O2 in non-aqueous electrolytes is calculated to be 2.96 V at room temperature (30 °C). The reason for the observation of charging potential at 2.84 V in this study is the elevated temperature operation of the battery. When the cell is operated at 140 °C, the thermodynamic potential of Li2O2 is calculated to be ~2.83 V20. This is further supported by the anodic peak corresponding to Li2O2 oxidation in CV at 2.85 V (figure 2a). Figure 3 shows the rate performance of the Li-O2 cell at different current densities at a capacity limitation of 2000


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mAh/g. Starting from current densities of 0.1 mA/cm2 to 4 mA/cm2, the charge-discharge cycles shown corresponding voltage hysteresis varied from 40 mV- 0.85 V. The current density of 3 mA/cm2 is ~100 times higher than the current that is applied in non-aqueous Li-O2 batteries. The practical battery systems should exhibit round-trip energy efficiencies above 90 %. Overpotential of 40 mV corresponds to a round-trip energy efficiency of discharge-charge cycle close to 98 %. One of the major advantage of using molten electrolytes is the ability to overcome the problem of low rate performances that remained as a challenge in non-aqueous Li-O2 batteries. Discharge product analysis X-ray diffraction technique is used to probe the type of discharge product formed in molten electrolyte Li- O2 battery. Figure 4a shows the ex-situ diffraction patterns of discharged and charged electrodes compared with pristine electrode. The diffraction patterns indicate that discharge product is primarily Li2O2, which is not observed during charge, confirming reversibility of the charge-discharge process as suggested by the CV experiments. It is also to be noted that small traces of Li2CO3 are seen on cathode (marked in ‘*’) which probably due to the reaction of carbon in cathode with LixOy (x=1, 2 y=1, 2). Carbon corrosion is evidenced in nonaqueous Li- O2 batteries too23, 24. Molten electrolytes are not excluded from this problem and addressing these issues lies beyond the scope of this study. Ex-situ X-ray analysis is carried out on electrolyte at different stages. Figure 4b-d shows the diffractograms of physical salt mixture, pristine electrolyte at 140 °C, discharged and charged electrolytes respectively in molten Li-O2 batteries. Once the mixture is liquefied above eutectic temperature (pristine electrolyte), it appeared to form a solid solution with dominating CsNO3, KNO3 phases and disappearing LiNO3 reflections (figure 4b). There are low intense reflections marked in asterisks “*” in the 2 theta


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range 20-40, which do not match with any of electrolyte elements including LiNO3, Li2O or Li3N. The discharged electrolyte (figure 4c) additionally had Li2O reflections (PDF 010769237) which is not observed during charging (figure 4d). This indirectly suggests that Li2O has formed during discharge with the nitrate reduction process, which forms a protective layer on Li anode.

Figure 4: Ex-situ X-Ray diffractogramms of (a) Vulcan carbon cathode, (b) Pristine electrolyte compared with physical mixture, (c) discharged electrolyte, (d) charged electrolyte and (e) Anode after cycling in molten LiNO3: KNO2: CsNO3 (37:39:24) electrolyte Li-O2 battery.


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Decomposition of organic electrolyte in conventional non-aqueous Li- O2 batteries causes deterioration of both O2 electrode and Li metal. The major reason being reactive oxygen species cause auto oxidation of electrolyte and makes the passivating layer on the electrodes, which substantially increase the impedance and finally leads to the failure of the battery25,

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. In

contrast, the Li metal cycled in molten electrolyte (>350 hrs) rich in LiNO3 showed unchanged Li phase with additional reflections of Li2O, Li3N and KNO3 as evidenced in XRD shown in figure 4e. It is important to note that no traces of LiOH are observed, unlike with the nonaqueous electrolyte. This is in line with the recent report that concludes the higher concentration of LiNO3 in electrolyte minimises the probability of LiOH formation on anode25. KNO3 reflection seen in XRD are coming from electrolyte residues on lithium. Li3N could be formed from the electrochemical/chemical reaction of lithium with by-products of nitrate-nitrite interconversion. Li3N is good conductor of Li-ion with extremely high conductivity of 10-3 S cm1

at 25 °C and it is a potential solid electrolyte. It is reported that Li3N coating on Li, protects

from dendrite formations without affecting the diffusion kinetics of Li+ in Li- based batteries27-31. The Li metal anode, which was in contact with molten nitrate electrolyte , showing pronounced appearance of Li2O, Li3N suggests that LiNO3 containing electrolytes has stabilizing effect in prospective of Li metal protection, which is a much needed requirement in Li-O2 batteries. Further, in situ grown composite layer of Li3N, Li2O supress the undesired side reactions, moisture attack on lithium metal and enhance the cycling performance. The electrochemical impedance spectroscopy (EIS) measurements revealed the role of molten nitrate electrolytes in Li- O2 cell. The typical impedance spectra depicted in figure 5 indicate following contributions, a) resistance from electrolyte and connectivity cables, (Ze, blue shaded), b) small semicircle indicating the anodic contribution (Z1, green), this results from Li-


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electrolyte interphase at higher frequencies, which also comprises of a surface SEI layer on Li metal. c) bigger semicircle (Z2,pink) and inclined line Z3 (brown) comprises the cathodic contributions i.e., kinetic and mass transfer reactions32, 33. The impedance data has been analysed using the equivalent circuit Re(Q1R1)(Q2R2)(CR3) given in the figure 5a, where R implies the resistance contributions of different components of battery, C is the capacitance contributions, Q is the constant phase element (CPE). Warburg diffusion models cannot be applied to Li-O2 batteries as O2 concentrations tend to vary point to point within cell environments34. Thus, Z3 is attributed to the mass transfer of oxygen at cathode. EIS of pristine cell, after the first discharge and charge are shown in figure 5b. The electrolyte resistance of pristine cell is close to 10 Ω indicating the higher conductivity of molten electrolyte and remained same in all the three cells. During discharge, Z2 value increased to ≈200 Ω which is likely due to the formation of discharge products on cathode. During charge, Z2 reduced to ≈90 Ω close to the pristine cell resistance indicating the reversibility of charge-discharge process. Impedance of the cycled cell (after 200 cycles) is also measured and given in S5. It is observed that the Z2 value increased above 400 Ω, indicating higher cathodic resistance, probably due to the electrode passivation upon repeated

cell

Figure 5: Electrochemical Impedance Spectroscopy (EIS): Nyquist plots of Molten electrolyte Li-O2 batteries: a) pristine cell behavior, b) Comparison of pristine, discharged and charged cells.


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cycling. Figure 6 (a-c) shows the FESEM images of pristine, discharged and charged electrodes. Abundant appearance of hexagonal and octahedron shaped (≈5 microns) crystals are observed in discharged electrode. These crystals did not appear on charged electrode. Figure 6 (d, e, f) shows the discharged electrodes at higher magnifications. Several theoretical studies based on the Wulff’s construction predicts that Li2O2 has a hexagonal shape and Li2O has octahedron shape35, 36

. In coherence with the theoretical estimates, cathode surface is filled with hexagonal and

O

Figure 6: Field emission SEM images of VC: PVDF O2 cathode: Low magnification images of a) pristine, b) discharged, c) charged electrodes. Higher resolution images of discharged cathode (d-f). g-h) Elemental mapping of oxygen centered on Li2O2 crystal, i) EDS spectrum of Li2O2 crystal. Magnification ranges: 2000x-30,000x.


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octahedral crystals. However, X-ray diffraction studies confirms the presence of only Li2O2 particles on cathode surface. The Energy Dispersive X-ray spectroscopy (EDS) is used to obtain the information of local chemical composition of discharge products. EDS focused on discharged cathode, in specific; a single lithium oxide crystal confirms the presence of oxygen rich surface (figure 6i). As lithium has very low energy of characteristic radiation, EDS cannot detect it and hence only oxygen reflection is shown in the spectrum. Elemental mapping (green) and line spectra (pink lines) focused on the single particle in a discharged Vulcan carbon cathode is shown in figure 6h. It indicates the presence of abundant oxygen on grown hexagonal shaped Li2O2 particles. Figure S4 in ESI shows the FESEM images of cycled Li anode in molten nitrate electrolyte. The lithium metal surface is found to be covered with layer of particles. The EDS reveals the presence of O (78.9 atomic %), N (20.14) and K (0.94) on the cycled Li metal surface (table S1 in SI). This is in line with the X-ray diffraction results that indicated the presence of Li2O, Li3N on anode and KNO3 residues from the electrolyte. Possible mechanism: Mechanism of molten electrolyte Li- O2 batteries are not well understood due to unavailability of sufficient literature on molten nitrate intermediate reactions in presence of O2. Nitrate ions undergo reduction as in equilibrium reaction (1) and released oxygen combines with Li+ ions to form Li2O as in equation (2).

X-ray diffraction studies confirm the presence of Li2O in

electrolyte during discharge (figure 4c). Due to a gradient in oxidation states, surface species with lowest oxidation states (Li2O, Li3N etc.) are tend to be moved to environs of Li37. Thus, Li2O will remain as a passivating layer on Li, which protects it from further destabilization. This layer allows Li-ions to move across but not electrons during the cell reactions. X-ray results confirms the presence of Li2O on Li metal surface as SEI. Additionally semicircle at higher


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frequency side in impedance spectroscopy supports presence of a surface film on anode. The corresponding resistance value of 3 Ω also suggests that the protective film is an excellent conductor of lithium ions. On cathode, Li-ions combines with external oxygen to form Li2O2 as given in equation (3). It is noteworthy that Li2O2 can be formed only by direct oxygen reduction and not by nitrate reduction. Discharge:

LiNO3 ↔LiNO2+ ½ O2

(1)

2Li+ + 2e- + ½ O2 → Li2O (2) 2Li++O2 + 2e- → Li2O2

(3)

On charge, LiNO3 will be regenerated from LiNO2 as in equation 4. In Li- O2 batteries that utilizes LiNO3 as a salt in electrolyte, the reaction 4 is thermodynamically feasible under the influence of positive currents as O2 availability within the cell favours the process. The nitrite (NO2-) ions react with external oxygen and forms nitrate (NO3-) ions. The external oxygen supplied into the cell compensates the oxygen consumed from the electrolyte during discharge. Thus, LiNO3 concentration stays composed throughout the cell operation and therefore keeps the surface layer intact on Li metal. We believe that the initial high voltage plateau during discharge and second plateau observed in charging cycle (figure 2c) are related to the reaction LiNO3+2Li+ + 2e-↔LiNO2+ Li2O. The reaction of nitrate ions with the lithium metal will rapidly decline when Li metal will be completely covered with the Li2O layer resulting from the nitrate reduction. Once the saturated layer of Li2O is formed on the Li metal, no more Li is accessible for the reaction with LiNO3, thus, there will be no nitrate reduction and nitrite oxidation. In this line, we anticipate that the second plateau is due to the nitrite oxidation to nitrate. The plateau disappears on prolonged cycling when a protective layer covers Lithium, giving rise to a flat charging curve. Further, extensive in-situ studies may be crucial in revealing the overall mechanism of molten nitrate based Li-O2 batteries.


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Charge:

LiNO2+ ½ O2 → LiNO3

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(4)

On other hand, lithium peroxide on cathode is decomposed into Li and oxygen (equation 5). Xray diffraction and FESEM studies confirms this process on charged cathode. Li2O2→ 2Li+ + O2 + 2e-

(5)

Thus, molten nitrate Li-O2 batteries involve a reversible nitrate-nitrite interconversion linked to the electrolyte and formation-decomposition of lithium oxides (discharged product) on the cathode during a complete charge-discharge cycle. CONCLUSIONS In conclusion, molten salt electrochemistry of alkali nitrates are utilized to address one of the longstanding problem of electrolyte stability in Li- O2 batteries. The inorganic salt mixture LiNO3: KNO2: CsNO3 (37:39:24) at 140 °C is found to address the issues associated with charge-discharge overpotentials of Li- O2 battery. Molten nitrate Li-O2 batteries involves slightly different reaction mechanism than the conventional non-aqueous batteries. At first step, it involves a reversible nitrate-nitrite interconversion process to form Li2O surface layer on Li metal. Secondly, the formation, growth and consequent decomposition of Li2O2 particles during the complete cycle operation. Excellent cycling stability is achieved with low overpotential of 0.3 V after 200 cycles at restricted capacity of 500 mAhg-1. The reason for the significant reduction in overpotential is attributed to the faster charge transfer kinetics of molten electrolyte and better solubility of discharge product. LiNO3 based molten electrolyte creates an in situ protection layer containing Li2O and Li3N on Li metal anode to sustain a very stable Li-O2 battery cycling at lowest ever overpotentials. Major advantage being the higher Li-ion conductivity, Li-O2 battery with molten nitrate electrolyte shows remarkable performances under


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the high load operations up to 3 mA/cm2. Intermediate temperature operation also terminates the possibilities of moisture intake to cell. Thus, they remain chemically and thermally stable towards anodic and cathodic reactions. Nitrate salts are economical, readily available and nonflammable unlike non-aqueous electrolytes. They greatly increases the reaction rates of oxygen cathode, which in turn minimises the requirement of expensive catalysts in Li-O2 batteries. Finding a porous, anticorrosive cathode compatible to molten nitrate environment and heat inputs to liquefy the eutectic mixture, would permit to realize a practical Li-O2 battery. For the present Li-O2 battery technology that suffers from various challenges, this work would act as a catalyst towards the development of next generation advanced batteries specifically for stationary applications and electric vehicles. ASSOCIATED CONTENT Supporting Information. Additional charge –discharge curves, cyclic voltammetry curves, FESEM images of cycled Li metal, PEIS of cycled Li-O2 cell are available in supporting information. AUTHOR INFORMATION Corresponding Author Email: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Guruprakash K. greatly acknowledges Council of Scientific and Industrial Research (CSIR) for granting Senior Research Fellowship.


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REFERENCES 1

Gallagher, K. G.; Goebel, S.; Greszler, T.; Mathias, M.; Oelerich, W.; Erogluab, D.; Srinivasan, V. Quantifying the Promise of Lithium‒Air Batteries for Electric Vehicles. Energy Environ. Sci. 2014, 7, 1555.

2

Freunberger, S. A.; Chen, Y.; Drewett, N. E.; Hardwick, L. J.; Bard, F.; Bruce, P. G. The Lithium–Oxygen Battery with Ether-Based Electrolytes. Angew. Chem. Int. Ed. 2011, 50, 8609 –8613.

3

Thotiyl, M.M.O.; Freunberger, S.A.; Peng, Z.; Bruce, P.G. The Carbon Electrode in Nonaqueous Li−O2 Cells. J. Am. Chem. Soc., 2013, 135 (1), pp 494–500.

4

Chen, Y.; Freunberger, S.A.; Peng, Z.; Bardé, F.; Bruce, P.G. Li−O2 Battery with a Dimethyl formamide Electrolyte. J. Am. Chem. Soc. 2012, 134, 7952−7957.

5 Freunberger, S.A.; Chen, Y.; Peng, Z.; Griffin, J. M.; Hardwick, L. J.; Bard, F.; Novak, P.; Bruce, P.G.; Reactions in the Rechargeable Lithium-O2 Battery with Alkyl Carbonate Electrolytes. J. Am. Chem. Soc. 2011, 133, 8040–8047. 6

Lim, H.K.; Lim, H.D; Park, K.Y.; Seo, D.H.; Gwon, H.; Hong, J.; Goddard, W. A.; Kim, H.; Kang, K. Toward a lithium-‘air’ battery: The Effect of CO2 on the Chemistry of a Lithium-Oxygen Cell. J. Am. Chem. Soc. 2013, 135, 26, 9733–9742.


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Page 19 of 24 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


7 Gowda, S. R.; Brunet, A.; Wallraff, G. M.; McCloskey, B. D. Implications of CO2 Contamination in Rechargeable Nonaqueous Li-O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 276−279. 8

Cho, M.H.; Trottier, J.; Gagnon, C.; Hovington, P.; Clement, D.; Vijh, A.; Kim, C. S.; Guerfi, A.; Black, R.; Nazar, L; Zaghib, K. The Effects of Moisture Contamination in the Li-O2 Battery. J. Power Sources. 2014, 268, 5, 565-574.

9

McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar, G.; Luntz, A. C. Solvents’ Critical Role in Nonaqueous Lithium-Oxygen Battery Electrochemistry. J. Phys. Chem. Lett. 2011, 2, 1161–1166.

10 Uddin, J.; Bryantsev, V. S.; Giordani, V.; Walker, W.; Chase, G. V.; Addison, D. Lithium Nitrate As Regenerable SEI Stabilizing Agent for Rechargeable Li/O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 3760−3765. 11 Monaco, S.; Soavi, F.; Mastragostino, M. J. Phys. Chem. Lett. 2013, 4, 1379−1382. 12 MacFarlane, D. R.; Forsyth, M.; Howlett, P. C.; Kar, M.; Passerini, S.; Pringle, J. M.; Ohno, H.; Watanabe, M.; Yan, F.; Zheng, W.; Zhang, S.; Zhang, J., Ionic liquids and their solid-state analogues as materials for energy generation and storage. Nature Reviews Materials 2016, 1, 15005. 13 Elia, G. A.; Hassoun, J.; Kwak, W. J.; Sun, Y. K.; Scrosati, B.; Mueller, F.; Bresser, D.; Passerini, S.; Oberhumer, P.; Tsiouvaras, N.; Reiter, J., An Advanced Lithium–Air Battery Exploiting an Ionic Liquid-Based Electrolyte. Nano Letters 2014, 14 (11), 6572-6577.


19


Page 20 of 24

14 Ulissi, U.; Elia, G. A.; Jeong, S.; Mueller, F.; Reiter, J.; Tsiouvaras, N.; Sun, Y.-K.; Scrosati, B.; Passerini, S.; Hassoun, J., Low-Polarization Lithium–Oxygen Battery Using [DEME][TFSI] Ionic Liquid Electrolyte. ChemSusChem, 2017, 10, 1 – 9. 15 Monaco, S.; Arangio, A. M.; Soavi, F.; Mastragostino, M.; Paillard, E.; Passerini, S., An electrochemical study of oxygen reduction in pyrrolidinium-based ionic liquids for lithium/oxygen batteries. Electrochimica Acta 2012, 83 (Supplement C), 94-104. 16

Delpecha, S.; Cabetb, C.; Slima, C.; Picard, G. S. Molten Fluorides for Nuclear Applications Mater. Today. 2010, 13, 34-41.

17 Gabisa, E. W.; Aman, A. Characterization and Experimental Investigation of NaNO3: KNO3 as Solar Thermal Energy Storage for Potential Cooking Application. J. Solar Energy. 2016, 6, 2405094. 18 Miles, M. H. The Effect of Passivating Films Involving Lithium Anode in Thionyl Chloride, Bromine Trifluoride, Molten Nitrates and Molten Perchlorates. Proceedings of the Thirty-Second Intersociety Energy Conversion Engineering Conference. 1997, 1, 6467. 19 McManis, G. E.; Fletcher, A. N.; Miles, M. H. Factors Affecting the Discharge Lifetime of Lithium-Molten Nitrate Thermal Battery Cells Using Soluble Cathode Materials. J. Appl. Electrochem. 1986, 16, 636-642.


20

Page 21 of 24 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


20 Giordani, V.; Tozier, D.; Tan, H.; Burke, C. M.; Gallant, B. M.; Uddin, J.; Greer, J. R.; McCloskey, B. D.; Chase, G. V.; Addison, D. A Molten Salt Lithium-Oxygen Battery. J. Am. Chem. Soc. 2016, 138, 2656−2663. 21 Zambonin, P. G.; Jordanz, J.; Redox Chemistry of the System 02-02--022—O2- in Fused Salts. J. Am. Chem. Soc. 1969, 91:9, 2225-2228. 22 Miles, M. H.; Fletcher, A. N. Cation Effects on the Electrode Reduction of Molten Nitrates J. Electrochem. Soc. 1980, 127, 8, 1761-1766. 23 McCloskey, B. D.; Speidel, A.; Scheffler, R.; Miller, D. C.; Viswanathan, V.; Hummelsh, J. S.; Nørskov, J. K.; Luntz, A. C. Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li−O2 Batteries. J. Phys. Chem. Lett. 2012, 3, 997−1001. 24 Leskes, M.; Moore, A. J.; Goward, G. R.; Grey, C. P. Monitoring the Electrochemical Processes in the Lithium−Air Battery by Solid State NMR Spectroscopy. J. Phys. Chem. C. 2013, 117, 26929−26939. 25 Giordani, V.; Uddin, J.; Bryantsev, V. S.; Chase, G. V; Addison, D. High Concentration Lithium Nitrate/Dimethyl acetamide Electrolytes for Lithium/Oxygen Cells. J. Electrochem. Soc. 2016, 163(13), 2673-2678. 26 Yao, X.; Dong, Q.; Cheng, Q.; Wang, D. Why Do Lithium–Oxygen Batteries Fail: Parasitic Chemical Reactions and Their Synergistic Effect. Angew.Chem.Int. Ed. 2016, 55, 11344 –11353.


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Page 22 of 24

27 Ma, G.; Wen, Z.; Wu, M.; Shen, C.; Wang, Q.; Jin, J.; Wu, X. Lithium Anode Protection Guided Highly-Stable Lithium-Sulfur Battery. Chem. Commun. 2014, 50, 14209-14212. 28 Guo, J.; Wen, Z.; Wu, M.; Jin, J.; Liu, Y. Vinylene carbonate–LiNO3: A Hybrid Additive in Carbonic Ester Electrolytes for SEI Modification on Li Metal Anode. Electrochem. communs. 2015, 51, 59–63. 29 Jaumann, T.; Balach, J.; Klose, M.; Oswald, S.; Eckert, J; Giebeler, L. Role of 1,3Dioxolane and LiNO3 Addition on the Long Term Stability of Nanostructured silicon/Carbon Anodes for Rechargeable Lithium Batteries. J. Electrochem. Soc. 2016, 163(3), 557-564. 30 Xiong, S.; Xie, K.; Diao, Y; Hong, X. Properties of Surface Film on Lithium Anode with LiNO3 as Lithium Salt in Electrolyte Solution for Lithium–Sulfur Batteries. Electrochim. Acta. 2012, 83, 78– 86. 31 Zhang, A.; Fang, X.; Shen, C.; Liu, Y; Zhou, C. A Carbon Nanofiber Network for Stable Lithium Metal Anodes with High Coulombic Efficiency and Long Cycle Life. Nano.Res. 2016, 9, 11, 3428–3436. 32 Adams, J.; Karulkar, M.; Anandan, V. Evaluation and Electrochemical Analyses of Cathodes for Lithium-Air Batteries. J. Power Sources. 2013, 239, 132-143. 33 Hojberg, J.; McCloskey, B. D.; Hjelm, J.; Vegge, T.; Johansen, K.; Norby, P.; Luntz, A. C. An Electrochemical Impedance Spectroscopy Investigation of the Overpotentials in Li−O2 Batteries. ACS Appl. Mater. Interfaces. 2015, 7, 4039−4047.


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34 Mehta, M.; Mixon, G.; Zheng, J .P; Andrei, P. Analytical Electrochemical Impedance Modeling of Li-Air Batteries under D.C. Discharge. J. Electrochem. Soc. 2013, 160(11), 2033-2045. 35 Mo, Y.; Ong, S.P.; Ceder, G. First-Principles Study of the Oxygen Evolution Reaction of Lithium Peroxide in the Lithium-Air Battery. Phys. Rev. B. 2011, 84, 205446. 36 Radin, M. D.; Rodriguez, J. F.; Tian, F.; Siegel, D. J. Lithium Peroxide Surfaces Are Metallic, While Lithium Oxide Surfaces Are Not. J. Am. Chem.Soc. 2012, 134, 1093−1103. 37 Aurbach, D.; Pollak, E.; Elazari, R.; Salitra, G.; Kelley, C. S.; Affinito, J. On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li–Sulfur Batteries. J. Electrochem. Soc. 2009, 156 (8) 694-702.


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“For Table of Contents Only”. The schematic representation of molten electrolyte Li-air cell. Reversible formationdecomposition of hexagonal shaped Li2O2 (wulff) particles upon cycling with lowest overpotential of 40 mV.


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An Inorganic Electrolyte Li-O2 Battery... (PDF Download Available)

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