New Horizons for Conventional Lithium Ion Battery Technology - The

Sep 11, 2014 - These challenges mark new horizons for Li ion battery technology, ..... (62) Since the discovery of this family of cathode materials by...
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New Horizons for Conventional Lithium Ion Battery Technology Evan M. Erickson,* Chandan Ghanty, and Doron Aurbach* Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel S Supporting Information *

ABSTRACT: Secondary lithium ion battery technology has made deliberate, incremental improvements over the past four decades, providing sufficient energy densities to sustain a significant mobile electronic device industry. Because current battery systems provide ∼100−150 km of driving distance per charge, ∼5-fold improvements are required to fully compete with internal combustion engines that provide >500 km range per tank. Despite expected improvements, the authors believe that lithium ion batteries are unlikely to replace combustion engines in fully electric vehicles. However, high fidelity and safe Li ion batteries can be used in full EVs plus range extenders (e.g., metal air batteries, generators with ICE or gas turbines). This perspective article describes advanced materials and directions that can take this technology further in terms of energy density, and aims at delineating realistic horizons for the next generations of Li ion batteries. This article concentrates on Li intercalation and Li alloying electrodes, relevant to the term Li ion batteries.

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electrolyte matrices, either polymeric10 (not gel) or ceramic11 (beyond the scope of this paper). In parallel to the efforts related to Li-metal based systems, highly reversible Li-carbon insertion anodes were explored, leading to the emergence of the Li ion battery revolution during the 1990s.12 Because the changes in the volume of graphite upon Li ion insertion (up to a stoichiometry of LiC6) is only a few percent, surface films formed on lithiated graphite at low potentials in a variety of Li salt solutions can accommodate these changes and provide a reasonable passivation for prolonged cycling of Li-graphite electrodes. It was found that electrolyte solutions based on alkyl carbonates, containing ethylene carbonate as a critically important component, form highly stable, passivating surface films on graphite electrodes13 (see details and mechanisms in SI Figures 4S−7S). LiPF6 was chosen as the main electrolyte for Li ion batteries because its solutions in alkyl carbonates are highly conductive and allow full passivation for the aluminum foils, used as current collectors for the positive electrodes.14 Alkyl carbonates/LiPF6 solutions demonstrate high anodic stability and enable the safe use of 4 V (vs Li) cathodes such as LiCoO2, LiMn2O4 spinel, and many kinds of layered LiMO2 cathode materials (M = a mixture of transition metal cations, including Ni, Mn, Co, Cr, and Al, with an overall stoichiometry of unity).15 Consequently, cells comprising graphite-LiCoO2 (denoted as LCO) with alkyl carbonates (including EC)/LiPF6 solutions (denoted as standard solutions) were commercialized as a major practical Li ion battery (around 150 Whkg−1 at the cell level). Lithiated transition metal cathodes also develop complicated surface chemistry in these standard solutions,16 which strongly affects

n continuation of previous perspective papers that were published recently in this Journal on topics related to Li ion battery technology,1−3 this Perspective article discusses the horizon of Li batteries. The commercialization of primary Li batteries in the 1970s stimulated the development of secondary, rechargeable Li batteries with Li metal anodes and Li insertion cathodes such as TiS 2 , 4 MoS 2 , 5 Li x MnO 2 6 and Li x VO y compounds7 during the 1980s and early 1990s. These systems could not however become fully practical because the cycling efficiency of Li metal anodes can never reach the high values necessary for rechargeable batteries (>99.5%). In addition, there are severe safety problems related to the formation of Li dendrites (see illustrations and images in Supporting Information (SI) Figure 1S) and pyrophoric Li surfaces (can be easily ignited upon accidental exposure to ambient air). In general, Li metal as well as many Li compounds react with all kinds of electrolyte solutions. The relevant playground of electrolyte solutions is described in SI Figures 2S and 3S. Li metal and many reducing Li compounds such as LixS and LixC are metastable in many polar aprotic solutions due to complicated passivation phenomena. All polar aprotic solvents are reduced by Li metal, Li−C, and Li−Si compounds to insoluble ionic Li compounds that precipitate to form surface films.8 These surface films are usually electronically insulating, thereby avoiding continuous reduction of solution species and providing passivation. Due to the presence of ionic Li compounds in them, these surface films behave like a solid electrolyte interphase (SEI) for Li ions.9 Such passivating surface films can never accommodate the pronounced morphological changes that Li metal electrodes undergo upon cycling so that Li metal electrodes cannot achieve appropriate protection and passivation upon cycling in any liquid electrolyte solution (SI Figure 1S). It should be noted, however, that Li metal anodes can be reversible in contact with solid © 2014 American Chemical Society

Received: July 3, 2014 Accepted: September 11, 2014 Published: September 11, 2014 3313

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ion batteries, compared to existing systems is presented in SI Figure 9S. Options for promising anode and cathode materials are discussed separately below. Perspective for Anode Materials. Graphite flakes are the reference, commercially used anode material with a practical specific capacity around 350 mAhg−1 (out of 372 mAhg−1 theoretical value), relatively low initial irreversible capacity (no more than a few percent of the theoretical capacity) and redox potential between 250 and 50 mV vs Li. The mechanism of Li ion insertion into graphite is known and has been studied thoroughly.35 Graphite electrodes require excellent passivation because they undergo exfoliation very easily. Also, their low temperature performance (3000 mAhg−1), limiting the extracted capacity to more moderate values enables pronouncedly extended Si electrodes cycle life (up to thousands of cycles). In Figures 1 and 2, we show results related to two kinds of monolithic Si electrodes based on nanowires (prepared by CVD) 45 and nanocolumns (prepared by magnetron sputtering), respectively.46,47 The use of monolithic Si electrodes is highly important due to their expected superior specific gravimetric capacity. We stress the use of appropriate electrolyte

We stress the use of appropriate electrolyte solutions in which flexible surface films are formed that can provide passivation, despite the pronounced volume changes. 3315

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(corresponding to an exchange of 1 Li per Mn1.5Ni0.5O4 unit). Although there are a numbers of routes for the synthesis of each lithiated transition metal oxide, the choice of the right synthetic path may have a critical effect on the performance of these cathode materials. This is especially important for LMNO, whose practical capacity depends strongly on its synthesis, which can produce either a disordered (Fd3̅m) or ordered phase (P4332). A reversible specific capacity >140 mAhg−1 and a redox activity exclusively around 4.8 V, which relates to the transitions Ni2+ → Ni3+ → Ni4+ only (no involvement of the Mn3+/Mn4+ couple), indicates a “healthy” active material with good performance. This cathode material has been extensively explored during the last 15 years but these studies have not yet been concluded. There are intrinsic problems related to the high redox potential due to the limited anodic stability of the electrolyte solutions and the aluminum current collector. The availability of electrolyte solutions whose anodic stability is high enough to enable operation of 5 V Li ion batteries is questionable. There are indications that catalytic oxidation of alkyl carbonate solvents can occur at potentials below 4 V.57 Al foils, which are the only relevant positive current collectors for practical Li batteries, can also oxidize at low potentials and their apparent anodic stability is due to their passivation in alkyl-carbonates/LiPF6 solutions.58,59 Hence, it is clear that the apparent anodic stability of standard solutions (e.g., EC-DMC/LiPF6) up to 5 V is due to inhibition rather than intrinsic high oxidation potentials. Solutions based on ionic liquids can provide true anodic stability for high voltage Li batteries (see SI Figure 12S and related discussion). However, their poor low temperature ionic conductivity, high viscosity, wetting problems, and high cost raise serious questions regarding their practical use in batteries. Work is in progress by several prominent R&D groups to develop electrolyte solutions of high intrinsic anodic stability.60 It should be noted that solvents with high anodic stability may exhibit very poor cathodic stability and lack of sufficient passivation at low potentials (e.g., nitriles, sulfones). It was found that solutions based on fluorinated alkyl carbonates demonstrate high anodic stability.47 Another problem related to full cells (graphite anodes) with LiMn1.5Ni0.5O4 cathodes is a minor dissolution of transition metal cations from the cathode (not to an extent that changes its structure). These cations are reduced at the anode side (within the protecting surface films) to metallic clusters that worsen the anode’s passivation. It is possible to mitigate this detrimental phenomenon by a surface coating that forms a buffer layer on the active mass.61 Nevertheless, it has proved possible to elaborate and test prototypes of full cells that exhibit prolonged cycle life, high rate capability, and acceptable safety features. Figure 4 shows representative data of Si-LiMn1.5Ni0.5O4 cells, cycled hundreds of times at 100% DOD.47,58,59 This figure shows typical voltage profiles that reflect the high potential redox activity of the Ni2+ → Ni4+ reversible transition. Successful commercial development of such systems can increase the energy density of Li ion battery technology by around 15−20% compared to that of state-of-the-art commercial Li ion batteries. Another option for increasing the energy density of Li ion batteries is the use of Li and Mn rich lithiated mixed transition metal oxide layered cathode materials. Conventional synthesis of LixMnyNizCowO2 mixed metal oxides, where x + y + z + w = 2, x > 1, and y > 1/3, usually provides a mixture (on the nanometer scale) of Li2MnO3 electrochemically inactive phase and Li[MnNiCo]O2 electrochemically active phase (a conventional layered material with redox activity in the 3−4.2 V range). Polarization of these composite materials beyond 4.6 V leads to a

Figure 2. (A) Lateral and (B) cross sectional SEM micrographs of monolithic microcolumnar Si arrays electrodes, prepared by magnetron sputtering. (C) Typical photograph of these monolithic Si electrodes (1.3 mgcm−2) on Cu foil current collectors. Reprinted with permission from Elsevier.46 (D) Typical charge/discharge voltage profiles of these electrodes over 1500 cycles at 1C rate in FEC-DMC 1:4/LiPF6 solutions. SEM micrographs of the cycled electrodes in (E) ECDMC/LiPF6 solutions and in (F) FEC-DMC 1:4/LiPF6 solutions after 500 cycles. Reprinted from with permission from Elsevier.47

advanced Li ion batteries, we decided not to discuss high capacity LixVOy (insertion)50 and LixFeF3 (conversion)51 cathode materials because of their low average voltage (full capacity realization up to 250 mAhg−1 can be reached 140 mAhg−1) can be obtained at slower rates (C/8 not shown, see ref 57).58 Reproduced with permission from The Electrochemical Society.59

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Figure 5. Data related to Li and Mn rich Li2MnO3−LiMn0.33Ni0.33Co0.33O2 cathode materials.63 (A) dQ/dE vs E curve of the first activation process. (B) Typical voltage profiles demonstrating the high capacity and the down-shift in the potential during cycling. (C) dQ/dE vs E cyclic curves at different cycles showing voltage changes upon cycling (down shift) due to the formation of a Li−Mn−O spinel phase. (D) Electron diffraction data from high resolution TEM measurements showing the presence of both spinel (red, sp) and rhombohedral (rh) phases. (E) Raman shifts of an electrode after 50 cycles indicating the formation of the spinel phase.

electron diffraction data (from HRTEM measurements) that verify the L → S transition. Figure 6 provides data on full cells combining monolithic Si anodes and Li2MnO3−Li[MnNiCo]O2 cathodes with FEC-DMC/LiPF6 solutions.66 It should be noted that beyond the above-described structural changes upon cycling, impressive cycling performance at 100% cycling efficiency could be demonstrated. Intensive work on these cathode materials is in progress. There are reports on core−shell structures,1 particles with gradients of composition (from the center to outside) and on modified compounds that may demonstrate better stability.67 There is also impressive work on Ni rich cathode materials.68 Success in commercial implementation of the above-described high capacity layered transition metal oxide cathodes, and development of practical cells containing Si based anodes, can take Li ion battery 30% in terms of energy density, beyond the current state of the art.

complicated activation process (its mechanism remains inconclusive and is still under investigation) that involves delithiation and changes in the oxidation state of the anionic framework (the oxygen atoms) that may involve also some molecular oxygen evolution. Then, the entire active mass becomes electrochemically active, showing for optimized compositions a specific capacity approaching the maximal value for these compounds, ∼300 mAhg−1.62 Since the discovery of this family of cathode materials by groups at the Argonne National Laboratory, they have been studied extensively.63,64 Optimized cathodes demonstrated prolonged cycling at reasonable stable capacity and high rate capabilities. However, it appears that they suffer from a decrease in their average voltage upon cycling due to a gradual layered to spinel (L → S) phase transition.65 Figure 5 provides a collection of data that show the capacity/potential response of the activation process, typical voltage profiles, a demonstration of the down-shift in the potential upon cycling, Raman shifts and 3318

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These nucleophilic reactions are regenerative because they replace P−O by P−F bonds, forming water in solution due to the presence of protons, which in turn form more HF by reaction with the PF6− anions.70,71 Thereby, LiCoPO4 cathodes do not work in standard electrolyte solutions. They work reasonably well in FEC-DMC/LiPF6 solutions (Figure 7) because of the formation of protecting surface films by FEC oxidation (mechanistic studies are in progress). The use of trace water and HF scavenger additives such as trimethylboroxine, helps further (Figure 7). As presented in this Figure, it was possible to test full Si-LiCoPO4 cells. However, because of the limited capacity that can be extracted from this material even in stabilized electrolyte solutions, these cells are inferior compared to the above-described LMNO high voltage cathode material. Hence, for developing high voltage Li ion cells, LMNO cathodes seem to be the only relevant option. It is worth mentioning interesting attempts to develop Li2MSiO4,72 LiMSO4F,73 and Li2MPO4F74 cathodes. The relevant transition metal was mostly Fe. With their relevant voltage (up to around 4 V) and capacity (averagely 150 mAhg−1), these systems do not mark a breakthrough in the field. On the use of nanomaterials in Li ion batteries. In recent years, nanotechnology and the preparation and study of nanomaterials have become among the most important topics in materials science and chemistry. It is important to examine the relevance of nanomaterials to Li ion batteries. In general, the use of nanomaterials means very short diffusion length for the Li ions insertion reactions and high surface area for interfacial charge transfer, which may lead to very fast kinetics for electrode reactions. In turn, the use of nanomaterials means low density, low volumetric specific capacity, and possible problems of interparticle electrical contact. In Li ion battery systems, the high specific surface area of nanoparticles causes severe parasitic side reactions. All Li battery systems are thermodynamically unstable. The apparent meta-stability that enables operation of these systems is achieved as a result of complicated surface chemistry that leads to passivation due to precipitation of surface films. The high surface area of nanoparticles may catalyze and promote side reactions. Lithiated transition metal oxides include anionic framework based on nucleophilic and basic oxygen anions that readily react with electrophilic alkyl carbonate molecules and acidic and protic contaminants (e.g., HF, PF5, H2O, CO2). Thereby, lithiated transition metal oxide cathode materials comprising nanoparticles usually demonstrate poor performance. Due to their intensive surface reactivity, they develop high impedance and, hence, slow kinetics.75 This is demonstrated well in SI Figure 13S, which compares performance of LiMn0.33Ni0.33Co0.33O2 electrodes comprising nano- and submicrometric particles. The superior performance of the latter electrodes is demonstrated.75 The use of nanometric carbonaceous materials, particles, CNT or graphene as anode materials is irrelevant as well, due to the super-reactivity of all kind of lithium−carbon compounds (strong reducing agents) with all relevant polar-aprotic systems.76 We discussed above conversion reactions and Li−Si based anodes, in which the use of nanomaterials is critically important: for the former systems, the nanosize of the MXy particles is crucial for the reversible reduction and interaction with Li ions. For the silicon anodes, nanostructures accommodate the huge periodic volume changes. However, the nanosize requires especially effective passivation for these systems as discussed above. The use of nanoparticles is important for olivine, LiMPO4 cathodes due to their poor electronic and ionic transport properties (the Li ions transport in

Figure 6. Full cell cycling data over 200 cycles of monolithic Si anodes (top image, see Figure 2) and Li2MnO3−LiMn0.33Ni0.33Co0.33O2 cathodes (insert) with FEC-DMC/LiPF6 solutions. Voltage profiles are displayed in the inset on the right. Reprinted under the Creative Commons License from Materials Today.64

Another high voltage (4.8 V) cathode material is the LiCoPO4 olivine compound. Figure 7 provides data related to the behavior of LiCoPO4 cathodes. It is impossible to fully delithiate this material electrochemically and to realize its full capacity.69 Furthermore, when delithiated this material is very sensitive to nucleophilic attacks of F− on the phosphate center. Hence, the presence of trace HF in solutions is detrimental for stable cycling.

Figure 7. (A) Discharge capacity vs cycle number of the LiCoPO4 cathodes in EC based electrolyte solutions (pink and blue) compared to FEC containing electrolyte solutions (green, red, and black). A greater amount of electrolyte (volume/mass ratio written in color) results in lower performance due to greater relative amount of water and HF present, which attack and destroy the cathode material. Utilizing a water and HF scavenger such as trimethyl boroxine increases cycling stability (black). Reprinted with permission from Elsevier.70 (B) Full SiLiCoPO4 cells cycled in FEC containing solutions, with and without the HF scavenger TMB. Reprinted from ref 71. 3319

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Figure 8. Cycling performance of LiMn0.8Fe0.2PO4/Li4Ti5O12 in cells in 1 M LiPF6, EC-DMC-EMC 1:1:1 electrolyte solution. (A) Typical slow scan rate cyclic voltammograms of the Li4Ti5O12 anodes (Ti4+/Ti3+ redox couple, black) and LiMn0.8Fe0.2PO4 cathodes (Fe3+/Fe2+ and Mn3+/Mn2+ redox couples, blue) compared with the cycling profile of a passivated glassy carbon electrode in EC-DMC-EMC 1:1:1/1 M LiPF6 solutions. Reproduced with permission from The Royal Society of Chemistry.48 (B) Cycling profiles of full cells showing high rate capability (the C rates are indicated). (C) Cycling these cells at 30 and 60 °C shows capacity fading at high temperatures, due to consumption of active lithium in side reactions at the anode side. (D) When the LTO anodes are prepassivated, the capacity fading observed in (C) is avoided. Reprinted with permission from The Electrochemical Society.79

should be noted that although lithiated LTO does not react with solvents and most of Li salt anions, its redox potential is low enough to react with trace oxygen, water, HF, and PF5, which often exist as contaminants in standard electrolyte solutions. The latter moiety may be formed at elevated temperatures by the thermal decomposition of LiPF6. The major capacity fading mechanism in Li ion batteries is not destruction of the electrodes but rather depletion of active Li ions from the battery system due to their consumption by irreversible surface reactions. This capacity fading process also occurs in LTO based cells, at elevated temperatures76 Thus, prepassivation of LTO anodes may be crucially important for cycling LTO-LMFP cells at high temperatures (see Figure 8). Other possible contributions of Li electrochemistry to load leveling applications are the development of aqueous Li ion batteries81 and nonaqueous flow batteries based on Li insertion reactions.82 At present, it is difficult to estimate the importance of these directions, so that they are just mentioned briefly here. Basic science aspects. The complicated nature of Li ion batteries, due to their thermodynamic instability, pronounced surface reactions, delicate structural aspects of Li intercalation electrodes, and dangerous thermal reactivity, has necessitated over the years intensive and extensive basic studies, without which they would never have become a commercial reality. Hence, the field of Li ion batteries attracts many prominent research groups throughout the world due to scientific challenges, including mechanistic research, surface chemical aspects, in situ spectroscopic and microscopic studies, the use of all kinds of spectroscopic, microscopic and diffractometric tools, in conjunction with electrochemical measurements. Therefore, the horizons of this field include development of new synthetic routes, challenges such as in situ high resolution structural studies by electron microscopy,83 rigorous mechanistic studies of phase-

them is one-directional, in tunnels). The anionic framework of olivines, in which oxygen atoms are bound to phosphorus atoms of high oxidation state, is much less nucleophilic and basic, compared to the oxygen atoms of lithiated transition metal oxides.77 Thereby, the use of nano-LiMPO4 particles does not promote pronounced surface reactions with solution species. Consequently, all kinds of olivine cathodes comprise nanoparticles, which facilitate their good rate capability. Li ion batteries for storage of sustainable energy. Li4Ti5O12 (LTO) with a spinel structure is a very fast and stable Li ion insertion anode material that intercalates fully reversibly three Li ions per formula (corresponding to 150 mAhg−1) at potentials around 1.5 V vs Li.78 This relatively high redox potential ensures very few if any side reactions because the cathodic stability limit of most relevant polar-aprotic solvents and relevant Li salt anions is below 1.5 V vs Li. Consequently, LTO electrodes exhibit prolonged cycle life, low thermal reactivity and very fast kinetics, even at low temperatures (down to −50 °C). Combining LTO anodes and olivine cathodes leads to Li ion batteries that may have ideal properties and characteristics for load leveling applications: very prolonged cycle life, high rates, wide temperature range of operation, low hysteresis upon discharge−charge cycling, excellent safety features, abundant raw materials, and reasonably low cost. The 2 V LTO-LFP battery systems for load leveling applications have already been discussed and demonstrated. We proposed recently, as described in Figure 8, full cells comprising LTO anodes and LiMn0.8Fe0.2PO4 (LMFP) cathodes as interesting 2.5 V Li ion batteries for sustainable energy storage and conversion.79 LMFP cathodes can realize most of the theoretical specific capacity of olivine compounds (165 mAhg−1). They are very stable, safe and fast. As exhibited in Figure 8, their stepwise voltage profile reflects both the Mn3+/Mn2+ and Fe3+/Fe2+ redox activities.80 It 3320

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transition reactions,84 development of precise electrochemical and calorimetric85 tools for the study of parasitic side reactions, rigorous−predictive thermal studies,86,87 design of new electrode materials by computational tools,88 studying and modeling complicated transport phenomena,89 and more. In conclusion, the field promotes first rate studies at the frontier of materials, surface, and electrochemical science. Summary. Li ion battery technology can be considered to be the major success of modern electrochemistry and an important field in materials science and engineering. The easy and reliable use of all kinds of mobile electronic equipment, with its wide-ranging influence on modern life is enabled by the high fidelity production of billions of very reliable, rechargeable, and safe Li ion batteries per year. These achievements result from intensive and careful research throughout the world during recent decades. It is important to preserve this prestige, by understanding the real horizons and possible true future directions, while avoiding directions based on unsubstantiated promises.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Evan M. Erickson received his B.S. in 2006 from North Dakota State University and his Ph.D. in 2012 from the University of Illinois at Urbana−Champaign working under Prof. Ralph Nuzzo, collaborating with Prof. Andrew Gewirth and Prof. Anatoly Frenkel (Yeshiva Univ.) on oxygen reduction electrocatalysis. He is now a postdoctoral fellow at Bar-Ilan University in Prof. Doron Aurbach’s group. He works in the lithium ion battery cathode materials subsection where he is interested in transition metal oxide positive electrode degradation mechanisms. Chandan Ghanty did his Ph.D. in Materials Science from Indian Institute of Technology, Kharagpur, India, in 2013. His doctoral research covers the synthesis and characterization of lithium- and manganese-rich integrated cathode materials for lithium rechargeable batteries. Presently, he is working as a postdoctoral fellow in Prof. Doron Aurbach’s group at Bar-Ilan University, Israel. His present research interest is on the lithium transition metal oxide based cathode materials for lithium ion batteries.

It should be noted that increasing the energy density of battery systems leads to the concomitant increase in their potential safety risks.

Doron Aurbach is a full Professor in the Department of Chemistry at Bar-Ilan University (BIU), BIU Senate member, the Director of the Nano Cleantech Center at the Bar-Ilan Institute of Nanotechnology and Advanced Materials (BINA), the leader of INREP − Israel National Research center for Electrochemical Propulsion and the chairman of Israel national laboratories accreditation authority. He did his Ph.D. in physico-organic chemistry at BIU (1983). Presently, his group (40 people) is working in the fields of basic, nonaqueous, and spectroelectrochemistry, Li ion batteries (all aspects), secondary Mg batteries, supercapacitors, electronically conducting polymers, engineering of new carbonaceous materials, lithium-oxygen and lithium-sulfur batteries, development of devices for storage and conversion of sustainable energy, lead acid batteries, and water desalination by electrochemical means. He serves as an associate editor in the three electrochemistry journals, JES, EEL, and JOSSEC. Website: http://ch.biu.ac.il/aurbach

It should be noted that increasing the energy density of battery systems leads to the concomitant increase in their potential safety risks. The state-of-the-art Li ion batteries comprise mostly modified graphite anodes, LiCoO2, Li[MnNiCo]O2, LiFePO4, or LiMn2O4 3−4.2 V cathodes and standard electrolyte solutions based on alkyl carbonate solvents and LiPF6. They can deliver an average energy density around 150 Wh/kg at the single cell level. Reasonable advancement of this technology may include the replacement of graphite by high capacity silicon content anodes (a 2-fold gain in specific capacity), introduction of 4.8 V LiMn1.5Ni0.5O2, high capacity Li and Mn or Li and Ni rich high capacity Li1+xMnyNizCowO2 cathodes with suitable electrolyte solutions that enable development of appropriate electrode passivation, especially at elevated temperatures. With successful implementation of these advances, one can expect a gain of no more than 30% of energy density of Li ion batteries. Although this prediction seems to promise only a moderate advancement in this technology, it does not delay the EV revolution, as electromobility is already a commercial reality. Finally, it is worth mentioning that the success and progress of Li ion batteries promoted the development of several post Li ion battery technologies including rechargeable Li-sulfur, magnesium, and sodium ion batteries. Li-S systems may take the battery field further in terms of energy density. Mg and Na batteries should be inferior to Li ion systems in term of energy density but may be relevant for load leveling applications. An ultimate competitor to ICE propulsion may Li-oxygen batteries; however, at present it is not clear at all that it will be possible to develop practical Li-O2 systems. Discussion on these systems is far beyond the scope of this paper.



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ACKNOWLEDGMENTS The work described herein was carried out by support of GM (U.S.A.), BASF (Germany) and the Israel Science Foundation (ISF, INREP project).



REFERENCES

(1) Myung, S.-T.; Noh, H.-J.; Yoon, S.-J.; Lee, E.-J.; Sun, Y.-K. Progress in High-Capacity Core−Shell Cathode Materials for Rechargeable Lithium Batteries. J. Phys. Chem. Lett. 2014, 5, 671−679. (2) Yu, H.; Zhou, H. High-Energy Cathode Materials (Li2MnO3− LiMO2) for Lithium-Ion Batteries. J. Phys. Chem. Lett. 2013, 4, 1268− 1280. (3) Manthiram, A. Materials Challenges and Opportunities of Lithium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, 176−184. (4) Whittingham, M. S. Electrical Energy Storage and Intercalation Chemistry. Science 1976, 192, 1126−1127. (5) Stephenson, T.; Li, Z.; Olsen, B.; Mitlin, D. Lithium Ion Battery Applications of Molybdenum Disulfide (MoS2) Nanocomposites. Energy Environ. Sci. 2014, 7, 209−231. (6) Dan, P.; Mengeritski, E.; Geronov, Y.; Aurbach, D.; Weisman, I. Performances and Safety Behaviour of Rechargeable AA-size Li/ LixMnO2 Cell. J. Power Sources 1995, 54, 143−145.

ASSOCIATED CONTENT

S Supporting Information *

Additional information about anode background, the full EV option, anode vs cathode development. This material is available free of charge via the Internet at http://pubs.acs.org. 3321

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