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

The energy density delivered by a Li-ion battery is a key parameter that needs to be significantly increased to address the global question of energy storage for the next 40 years. This quantity is directly proportional to the battery voltage (V) and the battery capacity (C) which are difficult to improve simultaneously when materials exhibit classical cationic redox activity. Recently, a cumulative cationic (M⁴⁺/M⁵⁺) and anionic (2O^2-/(O₂)^n-) redox activity has been demonstrated in the Li-rich Li₂MO₃ family of compounds, therefore enabling doubling of the energy density with respect to high-potential cathodes such as transition metal phosphates and sulfates. This paper aims to clarify the origin of this extra capacity by addressing some fundamental questions regarding reversible anionic redox in high-potential electrodes for Li-ion batteries. First, the ability of the system to stabilize the oxygen holes generated by Li-removal and to achieve a reversible oxo- to peroxo-like (2O^2-/(O₂)^n-) transformation is elucidated by means of a metal-driven reductive coupling mechanism. The penchant of the system for undergoing this reversible anionic redox or releasing O₂ gas is then discussed with regards to experimental results for 3d- and 4d-based Li₂MO₃ phases. Finally, robust indicators are built as tools to predict which materials in the Li-rich TM-oxide family will undergo efficient and reversible anionic redox. The present finding provides insights into new directions to be explored for the development of high-energy density materials for Li-ion batteries.