The intriguing question of anionic redox in high-energy density cathodes for Li-ion batteries

International audienceThe energy density delivered by a Li-ion battery is a key parameter that needs to be significantlyincreased to address the global question of energy storage for the next 40 years. This quantity is directlyproportional to the battery voltage (V) and the battery capacity (C) which are difficult to improvesimultaneously when materials exhibit classical cationic redox activity. Recently, a cumulative cationic4+ 5+ 2À nÀ(M /M ) and anionic (2O /(O2) ) redox activity has been demonstrated in the Li-rich Li2 MO3 familyof compounds, therefore enabling doubling of the energy density with respect to high-potentialcathodes such as transition metal phosphates and sulfates. This paper aims to clarify the origin of thisextra capacity by addressing some fundamental questions regarding reversible anionic redox inhigh-potential electrodes for Li-ion batteries. First, the ability of the system to stabilize the oxygen holes2À nÀgenerated by Li-removal and to achieve a reversible oxo- to peroxo-like (2O /(O2) ) transformation iselucidated by means of a metal-driven reductive coupling mechanism. The penchant of the system forundergoing this reversible anionic redox or releasing O2 gas is then discussed with regards toexperimental results for 3d- and 4d-based Li2MO3 phases. Finally, robust indicators are built as tools topredict which materials in the Li-rich TM-oxide family will undergo efficient and reversible anionicredox. The present finding provides insights into new directions to be explored for the development ofhigh-energy density materials for Li-ion batteries