Mechanisms of Degradation and Strategies for the Stabilization of

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Mechanisms of Degradation and Strategies for the Stabilization of Cathode−Electrolyte Interfaces in Li-Ion Batteries Published as part of the Accounts of Chemical Research special issue “Energy Storage: Complexities Among Materials and Interfaces at Multiple Length Scales”. Jordi Cabana,* Bob Jin Kwon, and Linhua Hu Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, United States

CONSPECTUS: Undesired reactions at the interface between a transition metal oxide cathode and a nonaqueous electrolyte bring about challenges to the performance of Li-ion batteries in the form of compromised durability. These challenges are especially severe in extreme conditions, such as above room temperature or at high potentials. The ongoing push to increase the energy density of Li-ion batteries to break through the existing barriers of application in electric vehicles creates a compelling need to address these inefficiencies. This goal requires a combination of deep knowledge of the mechanisms underpinning reactivity, and the ability to assemble multifunctional electrode systems where different components synergistically extend cycle life by imparting interfacial stability, while maintaining, or even increasing, capacity and potential of operation. The barriers toward energy storage at high density apply equally in Li-ion, the leading technology in the battery market, and in related, emerging concepts for high energy density, such as Na-ion and Mg-ion, because they also conceptually rely on electroactive transition metal oxides. Therefore, their relevance is broad and the quest for solutions inevitable. In this Account, we describe mechanisms of reaction that can degrade the interface between a Li-ion battery electrolyte and the cathode, based on an oxide with transition metals that can reach high formal oxidation states. The focus is placed on cathodes that deliver high capacity and operate at high potential because their development would enable Li-ion battery technologies with high capacity for energy storage. Electrode−electrolyte instabilities will be identified beyond the intrinsic potential windows of stability, by linking them to the electroactive transition metals present at the surface of the electrode. These instabilities result in irreversible transformations at these interfaces, with formation of insulating layers that impede transport or material loss due to corrosion. As a result, strategies that screen the reactive surface of the oxide, while reducing the transition metal content by introducing inactive ions emerge as a logical means toward interfacial stability. Yet they must be implemented in the form of thin passivating barriers to avoid unacceptable losses in storage capacity. This Account subsequently describes our current ability to build composite structures that include the active material and phases designed to address deleterious reactions. We will discuss emerging strategies that move beyond the application of such barriers on premade agglomerated powders of the material of interest. The need for these strategies will be rationalized by the goal to effectively passivate all interfaces while fully controlling the chemistry that results at the surface and its homogeneity. Such outcomes would successfully minimize interfacial losses, thereby leading to materials that exceed the charge storage and life capabilities possible today. Practically speaking, it would create opportunities to design batteries that break the existing barriers of energy density.

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INTRODUCTION The ongoing shift away from fossil fuels relies on the development of energy storage technology to power cars, compensate the intermittent nature of renewable sources, and increase the stability of the electrical grid.1 Despite an increase in announcements of new electric vehicles, they still constitute © XXXX American Chemical Society

a niche in the market, driven by issues of cost, range, and reliability of Li-ion batteries.2 Both cost and range depend on the energy density of the battery, motivating a major thrust in Received: October 5, 2017

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DOI: 10.1021/acs.accounts.7b00482 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. (a) Variation of the energy density of a battery by modifying only the capacity of the anode (red curve) or the cathode (blue and black curves). The effect of an increase of cell voltage is represented between blue and black curves. (b) Schematic depicting the relationship between the chemical potential of the anode (μA) and cathode (μC) and the LUMO−HOMO of the electrolyte. Voc represents the equilibrium potential of the battery reactions. Reproduced with permission from ref 12. Copyright 2013 American Chemical Society.

research in this field. However, these gains cannot come at the expense of reliability and, especially, safety. Batteries contain two electrodes, sites where reduction and oxidation (redox) reactions take place.3 In Li-ion batteries, the electrodes are solids capable of transporting both Li+ ions and electrons. Such transport must inevitably take place through various routes involving the active materials. At present, the energy storage capacity of a Li-ion battery is limited by the charge storage of the positive electrode (hereafter, cathode, following current conventions), which is, at best, roughly half that of the commercial graphite negative electrode (i.e., anode). Extending the voltage of operation of the battery also increases the amount of energy stored (Figure 1a). Again, the cathode is critical because a large percentage of current anodes already operate close to the potential of Li metal, the physical limit.4 Oxides with a layered structure, LiMO2 (M = Ni, Co, Mn, Al, etc.), are the leading family of candidates for the cathode. However, the reversible cycling of all Li is hindered by both intrinsic limitations and the inability to reach sufficiently oxidizing potentials. Undesired reactions occur at interfaces between the cathode and the nonaqueous electrolyte above 4.3 V vs Li+/Li0, which compromise device durability and safety, especially at moderate temperatures.5 Despite the intense recent efforts, issues with electrode−electrolyte interfacial stability continue to handicap battery technology. Interfacial reactions lead to layers that shut down charge transfer, and can induce cathode corrosion, anode contamination, and electrolyte depletion.6 Understanding them is essential to produce tailored solutions that enable high performance batteries. This statement naturally outlines the contents of this Account. Elements of this general topic have been covered to a varied extent by others in the literature.7−9 Here, we aim to present our integrated perspective, which is based on knowledge developed in recent years by us and others of the chemical drivers of reactions at cathode−electrolyte interfaces, to rationalize strategies to tackle undesired processes.

We will highlight our ability to assemble complex structures, and the challenges that lie ahead. Other strategies, involving the reformulation of nonaqueous electrolytes, are beyond the scope of this Account and have been reviewed elsewhere.10

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MECHANISMS OF DEGRADATION OF CATHODE−ELECTROLYTE INTERFACES

Thermodynamic Stability of the Electrode−Electrolyte Interface

The voltage of a battery is determined by the difference in redox potentials of the anode and cathode materials. These values are proportional to the change in free energy involved in each separate reaction, plus any overpotentials associated with kinetic barriers.11 Electrons are exchanged through the external circuit, with compensating flow of Li+ ions through an electrolyte, an electronically insulating medium typically made from solutions of a Li salt in alkylcarbonate solvents. The electrolyte is in contact with both electrodes, thereby being subject to their respective redox potentials. Therefore, the energy separation between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the bulk electrolyte provides a first parameter to establish the potential window of battery operation. In other words, if injection of electrons (and lithium) at the anode occurs at energy levels higher than the LUMO of the electrolyte, it will be reduced (Figure 1b).11,12 Likewise, oxidation of the electrolyte will occur if the extraction of electrons (and lithium) from the cathode involves energy levels lower than the HOMO of the electrolyte. This model is based on bulk potentials, which are further modulated at the interface by the possibility of local band bending and formation of a double layer when the electrode surface contacts the electrolyte.8 Side reactions can lead to solid films, which, in the case of the cathode, are most commonly both electronically and ionically insulating. They increase charge transfer resistance B


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Figure 2. Possible formation pathway of metal complexes upon oxidation of a solvent molecule at the LixNi0.5Mn1.5O4 surface. For details, the reader is referred to the text and ref 23. Reproduced with permission from ref 23. Copyright 2015 American Chemical Society.

and, eventually, shut down the cathode reaction.13 Therefore, it would be desirable to locate the potentials of oxidation of the cathodes within the thermodynamic window of potential of the electrolyte. When measured on inert electrodes such as Pt, the potentials of decomposition are measured above 5.0 V vs Li+/ Li0, supported by computational predictions.6 This upper limit provides ample room for high potential cathodes. In reality, when an active cathode is paired with the electrolyte, side reactions begin below this value, around 4.3 V.14,15 The origins of this discrepancy need to be explained.

species on the surface are oxidized during charging, but only to Co(III), a reaction that is also partly reversible, in contrast to the bulk reaction. The introduction of electrochemical stimulus aggravates instabilities, because formal states appear that are known as unstable, such as Co(IV). These oxidized oxide surfaces create different pathways of decomposition of the electrolyte, involving mass transfer reactions that are not possible with nonoxide electrodes such as Pt,17 at redox potentials below 5.0 V vs Li+/Li0.14 Electrolyte decomposition thus occurs simultaneous to the redox activity of the cathode material.18 Aside from insulating layers from electrolyte decomposition, this reactivity also induces oxide surfaces that are more reduced than the bulk, often through atomic rearrangement to structures (e.g., disordered rock salt) that further impede lithium transfer.19 This phenomenon is widely documented for essentially all active materials of interest today.20,21 Mechanistic insight into these interfacial reactions involving oxide cathodes for high energy batteries has been sought by a number of authors in the literature.19,22,23 We focus here on the case of LiNi0.5Mn1.5O4 (Figure 2),23 which presents reactivity that is representative of other oxide cathodes. Concerted electron transfer reactions between the electron-rich alkylcarbonates and the highly oxidized metal−oxygen species were proposed, consistent with the formation of reduced surfaces during electrode charging. Possible products are diketonate anions that can easily coordinate the reduced transition metal

Active Role of the Transition Metal Species at the Oxide Surface

Many oxide surfaces containing a transition metal in a high oxidation state have been found to be unstable against common Li-ion electrolytes. An example can be found in LiCoO2, which undergoes a spontaneous reduction upon immersion in the electrolyte in open circuit.16 This process involves the reduction of the cathode surface and degradation of the electrolyte to form a native solid film even before battery charging. The specific mechanism of this interaction remains unclear, especially considering that analysis of the cathode upon controlled adsorption of organic molecules of the typical electrolyte provided evidence of chemical reactivity but no electron transfer between species.8 This observation suggests that other electrolyte components are involved and, perhaps, activate the reduction of the oxide surface. The resulting Co(II) C



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(Ni or Mn), weakening the bond with lattice oxygen and, ultimately, extracting it from the oxide surface. In the model for LiNi0.5Mn1.5O4 in Figure 2, these reactions are dominated by the transition metal center, with simultaneous loss of O in the form of H2O, but studies with other cathode materials, such as Li-rich layered oxides, have suggested an active involvement of surface O species depleted of electrons.22 In any case, the newly formed complexes present different degrees of solubility in the alkylcarbonate electrolyte, so varying ratios will redeposit on the electrode surface. Dissolution during cycling is widespread in Li-ion battery cathodes beyond LiNi0.5Mn1.5O4,24 suggesting that complex formation is common. The dissolved transition metal species can diffuse toward any other component. Indeed, deposition of transition metals on the anode has been widely observed.25 These transition metal species disrupt the solid electrolyte interphase that electronically passivates the graphite anode, reducing the stability of the full cell.26 Studies of materials subject to postsynthetic coatings provide further evidence of the importance of redox-active transition metals at the surface of active cathodes. In the case of LiNi0.5Mn1.5O4 coated with MgO, the presence of alkaline earth was associated with surfaces in charged electrodes that were significantly less reduced than in bare materials.21 The presence of these stabilized charged surfaces was correlated with an improved electrochemical performance. The introduction of Mg2+ should modify the electronic structure and chemical potential of the surface, possibly decreasing activity toward electron and mass transfer reactions described above. Similar effects were observed with Al2O3,25 where evidence was also found of decreased transition metal dissolution and redeposition on graphite anodes.

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STRATEGIES FOR THE STABILIZATION OF CATHODE−ELECTROLYTE INTERFACES

Rules of Design of Protective Layers

The application of solid passivating films on the surface of the cathode offers a means to increase the stability of its interfaces

Figure 3. (a) Schematics of transport of carriers in LiCoO2 composite electrodes when growth of Al2O3 by ALD is performed on the powder compared to the whole composite electrode. (b) Charge−discharge curves of materials with different surface treatments. Reproduced with permission from ref 46. Copyright 2010 Wiley-VCH.

Dissolution of Transition Metal Ions Induced by Acidic Impurities

Salts based on polyfluoroanions used in modern electrolytes can react with the few ppm of H2O present in all commercial formulations to form HF. In the case of LiPF6, the reaction is proposed to be27

with the electrolyte. While showing significant success, refinement of the features desired in these passivating layers is still needed, informed by knowledge of mechanisms of degradation. The primary feature is growth in as conformal a manner as possible, to effectively passivate all possible sites. This requirement is particularly challenging when using presynthesized aggregated powders as the active material because of the large number of buried interfaces that may not be reached by a precursor. A second critical feature is to block the transfer of electrons from electrolyte components to the oxidized cathode but with layers applied at very low thickness to avoid shutting down the electrode reaction.32 The most straightforward choices are compounds containing only elements in the s or p blocks of the periodic table, such as Al2O3, AlF3, or MgO.33 These phases also serve to eliminate transition metal species at the interface with the electrolyte, consequently addressing issues related to chemical pathways coupled with redox changes. However, clear rules on which phase is most appropriate for this function have yet to fully emerge. Arguments based on the Lewis acid−base considerations have been made, but no clear correlations with properties have been provided. The power to scavenge acidic impurities and, in particular, HF has also been considered as a selection rule.34 High throughput computational screenings have classified compounds according to their harvesting power.35 In general, gains are common irrespective of the phase chosen.33

LiPF6 ⇌ LiF + PF5 PF5 + H 2O ⇌ POF3 + 2HF

HF can react with native films of Li2CO3 on the positive electrodes, forming additional LiF films on the surface. This phase is both ionically and electronically insulating, as well as electrochemically inert, which undermines charge transfer kinetics at the cathode/electrolyte interface. HF is also corrosive toward transition metal oxide surfaces and, thus, leads to further dissolution, coupled with the deposition of insulating transition metal fluorides on the surface of the particles through metathesis. Unfortunately, HF can also be formed as a result of the electrolyte decomposition processes, since they can generate H+.28 The generation of H+ in itself also affects the surface of oxide cathodes by virtue of lowering the local pH. Many transition metal oxides are unstable against leaching of the cationic species in the presence of low pH.29 Perhaps the best known case in battery cathodes is that of Mn3+ in LiMn2O4,30 where dissolution can occur through a disproportionation reaction:31 2LiMn2O4 + 4H+ ⇌ Mn 2 + + 2Li+ + 3/2Mn2O4 + 2H 2O D


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Figure 4. (a) Schematic of an oxide particle containing a shell with different Ni, Mn, and Co ratios. (b) Scanning electron microscopy (SEM) and electron-probe X-ray microanalysis (EPMA) results showing compositional gradients. (c) Cycling performance of the core−shell material (blue) and the respective pure components. (d) Schematic of a particle showing a full concentration gradient between center and outer layer. (e) Verification of the compositional gradient by SEM and EPMA results. (f) Cycling performance of the material with full concentration gradient (FCG), compared to pure phases. Panels a−c reproduced with permission from ref 5. Copyright 2009 Nature Publishing Group. Panels d−f reproduced with the permission from ref 52. Copyright 2012 Nature Publishing Group.

kinetics compared to the use of Al2O3, which is an insulator,25 further confirming the benefit of high concentrations of Li at the cathode−electrolyte interface. An issue with many surface coatings is the potential dissimilarity of their crystal structure with the active oxide. This dissimilarity can lead to local atomic misfits at their interface, which result in strain or, in the extreme, poor adhesion of the protective layer. Both scenarios can induce blocking of transport channels on the electrode surface, affecting the cell capacity.37 The situation is aggravated when large changes in Li content during cycling impose commensurate distortions of the oxide lattice. Since the coating compound is inactive, this process increases misfits at the solid−solid interface, which further raises impedance, and can ultimately result in delamination. The solution to this challenge would come from engineering epitaxy between protective film

The compounds most conventionally used as protective layers (e.g., Al2O3) also block the diffusion of Li because they contain no ionic carriers, and even if they could be introduced, they are predicted to show low mobility.36 There is ample evidence that, in reality, the identity of the actual layer does not correspond to the nominal formulation. Subsequent annealings are required as part of this process, which induce reaction with the surface of the oxide electrode.21,37,38 This reaction likely results in surface layers that contain both the passivating ion and Li, thus creating charge carriers while maintaining stability. Comparison of the performance of coated materials with and without annealing supports this conclusion.38 Therefore, an ideal coating would combine low electronic and high ionic conductivity, that is, would be a solid electrolyte.39 As an example, the use of LiAlO2, which conducts ions, as coating led to comparably higher cycling stability and faster reaction E


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Figure 5. (a) Scheme for the preparation of core−epitaxial shell (C-ES) nanocrystals with Li1+xMn2−xO4. (b) EDX mapping and line-scans, (c) highresolution images, highlighting structural epitaxy, and (d) cycling performance of C-ES nanocrystals. Reproduced with permission from ref 40. Copyright 2015 American Chemical Society.

and active oxide.40 Improvements were reported when protective layers show favorable epitaxial relationships compared to conventional formulations.40−42

Sophisticated deposition techniques exist that deliver desired compositions in gas form in order to reach surfaces buried inside aggregates, thus providing control of thickness and conformality. Sputtering has been used to build passivating layers of LiPON, a popular Li conductor.39 Atomic layer deposition (ALD) has attracted even more widespread interest. It relies on the discrete delivery, in vapor form, of chemical precursors to the surface of the electrode. Pioneered by Lee and co-workers for battery applications,32 ALD coatings of the conventional protective phases (e.g., Al2O3) are extremely common today. The mildness of the process allows application on both premade particle aggregates and full composite electrodes, that is, containing conducting and binding additives46 (Figure 3), enabling insightful comparisons. The variety of phases that can be deposited is rather rich, which enables deposition of Li conductors.25 Nonetheless, the complexity of several battery materials, with up to three transition metals, remains to be fully demonstrated.47 Studies of individual particles have revealed a highly conformal character in many cases, but evidence of deficiencies in coverage uniformity also exists.48 Since ALD is carried out at low temperature, the deposited phases are often amorphous, precluding any epitaxy with the underlying oxide without postdeposition annealing,49 as in the case of traditional coating methods.

Synthesis and Deposition of Protective Layers

To summarize, ideally, protective layers to stabilize the cathode/electrolyte interface would (i) be conformal and epitaxial, (ii) enable electronic insulation, (iii) protect against acidic impurities, and (iv) preserve sufficiently high Li-ion diffusion. These demands impose challenges on the growth of solid layers on rough substrates. The most common process involves a postsynthetic treatment to apply solid layers containing redox-inactive ions on aggregated powder of active oxides. In a typical procedure,43 the premade powder is immersed in a solution of the desired inactive ion, followed by removal of the solvent (e.g., by evaporation). Alternatively, nanoparticles of the inactive phase can be milled with large particles of the active material.44 In all cases, the resulting modified powder is subsequently heat-treated to form the final coated material. Despite some evidence of coverage of interfaces buried inside aggregates,42 these methods lack control of film growth, in both thickness and homogeneity,21 as well as composition.45 This deficiency limits the ability to finely tailor the structure of the coating to enhance functionality. The complexity of these coated materials also complicates efforts to define the specific characteristics that determine function. F


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Figure 6. Representative TEM images and elemental line scans (see yellow arrows) of nanocrystals of LiCoO2 with aluminum oxide layers, annealed at 400 °C (a, d), 500 °C (b, e), and 600 °C (c, f). The scale bars correspond to 10 nm (a), and 100 nm (b, c). The Al and Co signals are represented in red and green, respectively. (g) Schematic of the surface-modified LiCoO2 nanoplates, showing the chemical structures at different temperatures. Reproduced with permission from ref 41. Copyright 2017 American Chemical Society.

The challenges faced by postsynthetic techniques to grow conformal, epitaxial protective layers, while lowering the concentration of transition metals at the surface, has led to the development of synthetic routes toward heterostructures. In these architectures, individual particles show variations in the concentration of metals from their interior to their exterior but share an underlying crystalline framework. The most common example would be core−shell arrangements, where there is a sharp change in concentration between these regions. These architectures are common at the secondary particle level (Figure 4a−c).5 In this case, the individual primary particles are mostly homogeneous but vary in composition from the inside to the outside of the secondary aggregate. A precursor

containing the desired heterogeneity of transition metal concentrations within a microparticle is synthesized first, most often in water.5 The precursor is subsequently reacted with a Li salt at temperatures above 700 °C. This step alters the distribution of elements within the particle,50 but, overall, heterogeneity is preserved at greater control than with postsynthetic coatings. These synthetic methods are compatible with existing protocols in industry, providing rapid impact on application. High packing density is achieved, and most exposed surfaces would be largely protected by a shell. Most studies of core−shell secondary architectures focus on varying the contents of transition metal with a single secondary particle, but examples exist that introduce redox-inactive ions at G


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Accounts of Chemical Research surfaces.51 The compositional control of these architectures has been refined to where full nanoscale gradients between interior and exterior are accomplished (Figure 4d−f).52 It is even possible to control the rate of change in these gradients.34 In all cases, comparative advantages with single-phase products have been observed (Figure 4c, f),5,52 even in full cell configurations.51 The changes in crystal structure undergone by battery cathodes upon cycling generate mechanical stresses that result in fracture of secondary aggregates,53 especially at boundaries between primary particles.54 Fracture generates voids where the electrolyte can contact previously unreachable surfaces, thus triggering degradation. This mechanism would not be addressed by secondary heterostructures because the particles in the interior are protected by those in the exterior as long as internal porosity is very low. Instead, the ultimate solution would be the design of heterogeneity at the level of primary particles, creating a fascinating synthetic challenge. Borrowing lessons from nanotechnology, passivating shells containing Al3+ were grown on individual nanocrystals of Li1+xMn2−xO4.40 The distinguishing feature of this method is the synthesis nanocrystals in the presence of a surfactant that prevents aggregation, thus exposing all surfaces to the source of Al, which is subsequently added to the solution (Figure 5a). After reaction with a Li salt at moderate temperature, the resulting nanocrystals presented a conformal 1−2 nm shell, which contained both Mn and Al but was richer in the latter. The shell also presented an epitaxial relationship with the spinel core (Figure 5c). The resulting core−epitaxial shell (C-ES) materials showed improved cycle life compared to bare counterparts (Figure 5d), as well as reduced sensitivity to acidic environments. Rate capability measurements revealed that the kinetics of the electrode were not handicapped by the Al-rich shell, consistent with the formation of semiconducting surface structures containing Li. A variation of this approach was recently demonstrated for LiCoO2. In this case, LiCoO2 nanoplates were grown hydrothermally in the presence of a surfactant that ensures colloidal dispersibility. A second hydrothermal treatment with Al3+ was performed while in the dispersed state, followed by annealing at low temperatures to induce surface crystallinity while avoiding coarsening.41 The result was a precise control of the degrees of Al−Co interdiffusion while maintaining conformality (Figure 6). The resulting architectures varied with temperature, from LiCoO2−Al2O3 (amorphous) core− shell nanoplates to the formation of LiAlxCo1−xO2 solid solutions where the Al/Co ratio decreased from the surface to the interior within a single nanocrystal. While all samples presented improved electrochemical performance compared to the bare material, the LiAlxCo1−xO2 gradient heterostructure presented the greatest advantage (Figure 6c).

The products of the reaction typically form insulating layers that impede transport. Overall, degradation of the performance of the battery is induced. These issues are not unique to Li-ion, but they will be an issue in any technology that relies on electroactive transition metal oxides to achieve high energy density. Reducing the transition metal content at the surface by introducing inactive ions within protective layers improves interfacial stability. The main challenge is to minimize their thickness on each individual particle, to minimize losses in capacity, while ensuring conformality to passivate all interfaces. Recent research leads to the conclusion that (i) the presence of Li in a protective layer will ensure the presence of ion carriers that prevent losses in charge transfer kinetics and (ii) matching the shell structure and composition to the core introduces epitaxy, which benefits properties. Designing materials meeting all the rules of design presented here requires moving beyond the application of the coating of interest in premade agglomerated powders. Significant advances in the synthesis of heterostructured powders have occurred, which create opportunities to supersede these limitations. While they are mature at the level of secondary particles, the ability to build heterogeneity at the level of a single (nano)crystal is much more incipient. We have reviewed studies that prove both the possibilities ahead and their potential. Despite the performance improvements brought about by the increasingly refined synthetic control, the ultimate strategy that eliminates interfacial degradation has not yet emerged. Further improvements will demand profound understanding of decomposition processes on surface-modified cathode materials. A strategy that successfully eliminates interfacial losses would result in materials that exceed the charge storage and life capabilities possible today, creating roadmaps to batteries breaking barriers of energy density.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jordi Cabana: 0000-0002-2353-5986 Funding

This work is supported by the National Science Foundation under Grant No. CBET-1605126. Notes

The authors declare no competing financial interest. Biographies Jordi Cabana is an Associate Professor in the Department of Chemistry at the University of Illinois at Chicago. Prior to his appointment at UIC, he was a Research Scientist at Lawrence Berkeley National Laboratory (U.S.A.), from 2008 until 2013. Prof. Cabana completed his Ph.D. in Materials Science at the Institut de Ciència de Materials de Barcelona (Spain) in 2004 and worked in the Department of Chemistry at Stony Brook University (U.S.A.) as a postdoctoral associate. His groups focuses on the physical and inorganic chemistry of materials, with emphasis on their redox and transport properties and applications in electrochemical energy storage. Members of his group combine approaches from solid state chemistry with nanoscience to design novel materials directed by insight from reaction mechanisms.

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CONCLUSION The use of surface-passivating layers has steadily improved the performance of battery cathodes for Li-ion batteries. These improvements have relied on increasing insight into the underlying mechanisms of degradation at cathode−electrolyte interfaces. The current view is that undesired interfacial reactions are driven by not only the redox potential of the cathode but also secondary chemical interactions between electrolyte molecules and oxidized transition metal ions. The result is electrolyte decomposition, often accompanied by metal dissolution and redeposition in different battery components.

Bob Jin Kwon is a Ph.D. graduate student at University of Illinois at Chicago. He received B.S and M.S. from Donnguk University in 2008 H


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and 2010. His current research is focused on the stabilization of electrode−electrolyte interfaces in high energy cathode materials for Li-ion batteries by introducing passivation layers. Linhua Hu is a Visiting Research Assistant Professor at the Department of Chemistry of the University of Illinois at Chicago. He received his Ph.D. in Chemical Engineering at Beijing University of Chemical Technology in 2007. He was a postdoctoral associate at Northwestern University with Prof. Kenneth Poeppelmeier from 2009 to 2015 and at Tsinghua University with Prof. Yadong Li from 2007 to 2009. His general research interests are synthesis, structure, and applications of catalytic and energy materials, with current focus on developing new cathode materials of Li and Mg batteries.

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