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Intercalation chemistry has dominated electrochemical energy storage for decades, and storage capacity worldwide has now reached the terawatt-hour level. State-of-the-art intercalation cathodes for Li-ion batteries operate within the limits of transition metal cation electrochemistry, but the discovery of anion-redox processes in recent decades suggests rich opportunities for substantially increasing stored energy densities. The diversity of compounds that exhibit anion redox in the solid state has inspired the exploration of new materials for next-generation cathodes. In this Review, we outline the mechanisms proposed to contribute to anion redox and the accompanying kinetic pathways that can occur in layered transition metal oxides. We discuss the crucial role of structural changes at both the atomic and mesoscopic scales with an emphasis on their impact on electrochemical performance. We emphasize the need for an integrated approach to studying the evolution of both the bulk structure and electrode–electrolyte interphase by combining characterization with computation. Building on the fundamental understanding of electrochemical reaction mechanisms, we discuss engineering strategies such as composition design, surface protection and structural control to achieve stable anion redox for next-generation energy storage devices.

Epitaxial growth is a powerful tool for synthesizing heterostructures and integrating multiple functionalities. However, interfacial mixing can readily occur and significantly modify the properties of layered structures, particularly for those containing energy storage materials with smaller cations. Here, we show a two-step sequence involving the growth of an epitaxial LiCoO

The electrode-electrolyte interfaces (EEIs) play important roles in lithium ion batteries, often determining the capacity retention, safety, and lifetime. The EEI of the positive electrode is far less understood than its negative counterpart. In addition to possible electrolyte oxidation products, layered cathodes often exhibit pronounced surface phase transformations that can also limit the performance of the battery. For example, recent experiments have correlated charge-transfer impedance growth with surface phase transformations at Li1-xNi0.80Co0.15Al0.05O2(NCA) surfaces, which result from surface oxygen loss. [1] Electrolyte reactions with the active material play a role in surface oxygen loss as coating layers [2] and electrolyte additives [3] can mitigate impedance growth and capacity fade. Determining reaction pathways that promote surface oxygen loss is essential to optimizing the selection of coating layers or additives that further improve capacity retention. Here we report on our surface studies of NCA, to examine the evolution of the cathode-electrolyte interface (CEI) for stressed NCA electrodes held at high voltages (≥4.5V). Using a combination of surface sensitive x-ray absorption spectroscopy and x-ray photoelectron spectroscopy, we correlate transition metal reduction with electrolyte decomposition species at the cathode surface. From comparison of NCA electrodes held under various constant voltage conditions, we examine the influence of voltage and temperature on the formation of electrolyte decomposition species. Our work provides insight into possible reaction mechanisms that influence changes of the CEI layer for electrodes under thermal and electrochemical stress. This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583 [1] S. Sallis et al., App. Phys. Lett. 108 (2016) 263902 [2] I. D. Scott et al., Nano Letts. 11 (2011) 414 [3] R. Wagner et al., Appl. Mater. Interfaces. (2016) 30871 Figure 1