Li-Rich Layered Cathode Materials: Transition Metals in Transit

In the most recent issue of Nature Communications, William Gent, William Chueh, and co-authors examine in depth the electronic structure and redox processes that occur during cycling of Li1.17Ni0.21Co0.08Mn0.54O2 and reveal an important new insight that suggests oxygen redox is coupled with local transition metal migration. Aided by a suite of cutting-edge characterization methods, the authors formulate a clear and unprecedented view of both local and averaged structure evolution during cycling. In the most recent issue of Nature Communications, William Gent, William Chueh, and co-authors examine in depth the electronic structure and redox processes that occur during cycling of Li1.17Ni0.21Co0.08Mn0.54O2 and reveal an important new insight that suggests oxygen redox is coupled with local transition metal migration. Aided by a suite of cutting-edge characterization methods, the authors formulate a clear and unprecedented view of both local and averaged structure evolution during cycling. The rapid large-scale deployment of lithium ion (Li-ion) batteries is an important and encouraging success story playing out in the ongoing global transition to sustainable energy. Li-ion batteries power the majority of the world's mobile electronics and are responsible for the increasing rate of electrified vehicles (and most recently large trucks1Boudette, N. Tesla Unveils an Electric Rival to Semi Trucks. New York Times. November 16, 2017. https://www.nytimes.com/2017/11/16/business/tesla-electric-truck.html.Google Scholar) coming online. In the face of imminent planet-threatening climate change, however, the upward trend of improving Li-ion battery technology needs to continue for important applications like the grid, which have enormous greenhouse gas emission mitigation potential. Although the launch frequency of grid-scale Li-ion battery projects is certainly a positive signal that cannot be ignored,2Baidawi, A. Australia Powers Up the World's Biggest Battery – Courtesy of Elon Musk. New York Times. November 30, 2017. https://www.nytimes.com/2017/11/30/world/australia/elon-musk-south-australia-battery.html.Google Scholar total global grid energy storage demand is still many times the scale of the world's current Li-ion battery capacity. In this backdrop, the value of lowering the cost and increasing the energy density of Li-ion batteries is difficult to overstate. At the same time, there is a ceiling placed on achievable energy density that is locked in with the choice of active electrode materials. The most common cell configuration today involves graphitic carbon at the anode and a layered transition metal oxide at the cathode, and this currently represents the best, most optimized package as far as mass-produced Li-ion batteries. The energy density of a cell is determined by the product of the Li chemical potential difference (defining the voltage, V) and the number of Li that can be shuttled per cycle (defining the reversible capacity, reported in mAh/g) between the anode and cathode. Both anode and cathode materials in Li-ion batteries are "intercalation" compounds, meaning they can host and reversibly insert lithium in (and remove Li from) their crystal structure without significant atomic rearrangement of the lattice. This facilitates the reversible cycling capability and reasonable charge and discharge times we are accustomed to with Li-ion batteries. Compared to the anode material, modern Li-ion cathode materials typically exhibit lower capacity and thus limit the overall energy density of full cells. This clearly frames a prime research opportunity— there is a pressing need to push the boundaries of Li intercalation on the cathode side and systematically unlock additional capacity at high cell voltage. Unfortunately, wholesale increases in high-voltage capacity are infrequent and hard to come by, immediately apparent when noting that layered LiMO2 cathode materials (where M is a 3d transition metal such as Ni, Mn, Co) have represented the state-of-the-art since their introduction decades ago. The conventional view, only seriously challenged in recent years, has been that every Li ion intercalated into the cathode host structure is accompanied by an electron associated with a transition metal (M) redox center. However, by subtly tweaking the stoichiometry toward Li excess (from LiMO2 to Li1+xM1-xO2, for 0 < x < 1/3), there is a growing volume of evidence supporting the view that additional reversible oxygen redox is indeed possible and can contribute significant additional capacity. For an idea of the scale of potential improvement, LiNi0.8Co0.15Al0.05O2 (NCA) can achieve close to ∼200 mAh/g and represents the upper bound achievable with a traditional stoichiometric chemistry, while certain Li-excess chemistries can push the 300 mAh/g mark.3Hy S. Liu H. Zhang M. Qian D. Hwang B.-J. Meng Y.S. Performance and design considerations for lithium excess layered oxide positive electrode materials for lithium ion batteries.Energy Environ. Sci. 2016; 9: 1931-1954Crossref Google Scholar, 4Thackeray M.M. Kang S.-H. Johnson C.S. Vaughey J.T. Benedek R. Hackney S.A. Li2MnO3-stabilized LiMO2 (M= Mn, Ni, Co) electrodes for lithium-ion batteries.J. Mater. Chem. 2007; 17: 3112-3125Crossref Scopus (1725) Google Scholar The prospect of a significant capacity boost from anion redox would be a tremendous windfall in terms of both energy density and cost (as no additional transition metal is required) and represents a rare realistic opportunity for a step-change advance in Li-ion battery technology. Unfortunately, this class of cathode materials also comes with significant performance issues, including appreciable voltage decay and hysteresis upon cycling. The Li-ion battery research community is rallying around the topic and devoting significant effort to understand the fundamental mechanisms at play that give rise to reversible anion redox capacity,5Sathiya M. Rousse G. Ramesha K. Laisa C.P. Vezin H. Sougrati M.T. Doublet M.L. Foix D. Gonbeau D. Walker W. et al.Reversible anionic redox chemistry in high-capacity layered-oxide electrodes.Nat. Mater. 2013; 12: 827-835Crossref PubMed Scopus (1037) Google Scholar, 6Luo K. Roberts M.R. Hao R. 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Chem. 2016; 8: 692-697Crossref PubMed Scopus (817) Google Scholar with the ultimate goal of overcoming the remaining cycling deficiencies and charting a path toward commercial deployment. Characterizing the fundamental mechanism of oxygen redox in these materials is a formidable task that challenges researchers on several fronts. Not only does the notion of oxygen redox challenge the traditional understanding of the operation of Li-ion cathode materials, but the characterization methods required to isolate clear conclusions push against the boundaries of what is currently possible with current experimental and computational techniques. In the most recent issue of Nature Communications, a multi-institution research team led by William Gent and William Chueh employ a suite of cutting-edge characterization techniques to study the electronic structure and redox processes that occur during cycling of Li1.17Ni0.21Co0.08Mn0.54O2, revealing an important new insight that suggests oxygen redox is coupled with local transition metal migration.10Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Strong coupling between bulk oxygen redox and cation migration explains unusual electrochemistry in 3d Li-rich layered oxides.Nat. Commun. 2017; (Published online December 12, 2017)https://doi.org/10.1038/s41467-017-02041-xCrossref PubMed Scopus (370) Google Scholar Specifically, the authors combine scanning transmission X-ray microscopy and nanoscale XAS (STXM-XAS) with resonant inelastic X-ray scattering (RIXS), as well as other experimental structural probes and density functional theory (DFT) calculations, to formulate a clearer view of both local and averaged structure evolution during cycling. From putting the many characterization pieces together emerges a fascinating story that plays out over the course of battery cycling. In the first charge (where Li is removed electrochemically from the structure), there are two distinct redox mechanisms: first transition metal oxidation below 4.5 V, followed by oxygen oxidation at higher potentials. Reversible oxygen redox is shown to occur throughout the bulk of the active particles and persists even after 500 cycles, but it adopts a unique character. Surprisingly, upon discharge the transition metals reduce first, followed by oxygen reduction at lower potential (below 3.65 V), as shown in Figure 1. Image reproduced from Gent et al.,10Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Strong coupling between bulk oxygen redox and cation migration explains unusual electrochemistry in 3d Li-rich layered oxides.Nat. Commun. 2017; (Published online December 12, 2017)https://doi.org/10.1038/s41467-017-02041-xCrossref PubMed Scopus (370) Google Scholar which is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0, http://creativecommons.org/licenses/by/4.0/). This dynamic reshuffling of the energy levels cannot be explained solely by a static O2−/O− or 2O2−/O2n− redox couple mechanism as suggested in prior literature. Rather, Gent and co-workers uncover a dynamic structure-redox coupled process described by {O2− + M} → {O− + Mmig} + e− (where Mmig represents a coupled migrating transition metal) that alters the local oxygen coordination environment and modulates the electrochemical potential, in line with the observations. Moreover, the transition metal migration occurs "intra-cycle" between charge and discharge steps and appears to be partially reversible as well. Overall, this study highlights the importance of combining fundamental materials science theory (in electronic and crystal structure, for example) with state-of-the-art advanced characterization methods to produce deep insights into the performance of important technological materials. By uncovering the nature of oxygen redox and coupled transition metal migration, this work sets the stage for designing Li-rich layered oxides with improved electrochemical performance that takes full advantage of oxygen redox chemistry.