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 difficult to improve when materials exhibit a classical cationic redox activity.

Recently, a cumulative cationic (M4+/M5+) and anionic (2O2-/(O2)n-) redox activity has been demonstrated in the Li-rich Li2MO3 family of compounds, therefore enabling doubling the energy density with respect to high-potential cathodes such as transition  metal phosphates and sulfates.

This paper aims at clarifying 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 (2O2-/(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  in regards to experimental results  for 3d- and 4d-based Li2MO3 phases.

Robust indicators are built as tools to predict  which materials in the Li-rich TM-oxides 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.