Roles of surface structure and chemistry on electrochemical processes in lithium-rich layered oxide cathodes

Li-rich layered oxides (LLOs) are promising cathode candidates for next generation Li-ion batteries, as they exhibit a higher reversible capacity (> 250 mA h/g), enhanced safety and much lower cost. However, LLOs generally suffer from high first cycle irreversible capacity (IRC) loss, poor rate capability, and a substantial voltage decay over prolonged cycling. These major challenges are closely dependent on the surface structure and chemistry of LLO cathodes and, thus, different surfaces induce different irreversible reactions resulting in various levels of battery performance. This review presents the current understanding, as well as recent highlights, on the roles and fundamentals of surface structure in LLO cathodes, from a materials science perspective, concerning surface structural disorder in pristine LLO (antisites, composition segregation and crystallographic facets), the roles of surface structures on redox processes (oxygen evolution, cation activation and reversible anion redox reactions), surface structural evolution during the first cycle and long-term electrochemical operation, and surface modification strategies to stabilize the surface structure and to mitigate the performance degradation of LLOs. However, some fundamental problems remain yet ambiguous, especially with regard to characterization and understanding of the surface structure and chemistry in relation to synthesis conditions and composition, and charge transfer and ionic transport of the interfacial processes within LLOs. In order to exploit the potential of LLO cathodes, a clear understanding of these fundamental questions are essential to optimize the synthesis parameters and material properties.