Peering into cathode layered oxides: The quest for better rechargeable batteries means finding ways to pack more energy into a smaller mass or volume. Lithium layered oxides are a promising class of materials that could double storage capacities. However, the design of safe and long-lasting batteries requires an understanding of the physical and chemical changes that occur during redox processes. McCalla et al. used a combination of experiments and calculations to understand the formation of O-O dimers, which are key to improving the properties of these cathode materials. Science , this issue p. 1516

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 (2O²⁻/(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 (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 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.

Oxygen lone-pairs (|O) are responsible for the extra-capacity observed in Li-rich Li ₂ MO ₃ electrodes and not in LiMO ₂ . M–O covalency is required to stabilize the oxidized O ⁻ species involved in the anionic process and to prevent O ₂ release.