A review and outlook on anionic and cationic redox in Ni-, Li- and Mn-rich layered oxides LiMeO2 (Me = Li, Ni, Co, Mn)

TL;DR

This review discusses charge compensation mechanisms in Ni-rich layered cathodes, highlighting how different redox processes at various states of charge influence electrochemical performance and stability.

Contribution

It provides a comprehensive analysis of oxygen and transition metal redox mechanisms in Ni-, Li-, and Mn-rich layered oxides, linking electronic structure changes to electrochemical behavior.

Findings

Low/medium SOCs involve oxygen redox and Ni electron shifts.

High SOCs involve oxygen release and additional redox processes.

Performance depends on composition and redox mechanisms.

Abstract

The present work reviews the charge compensation in Ni based layered oxides (LiNi1-xMexO2 with x <= 0.2, Me = Co, Mn, space group R-3m) relating performance parameters to changes in the electronic and crystallographic structure of the cathode materials. Upon charge and discharge two fundamentally different redox mechanisms are observed: At low and medium states of charge (SOCs) charge compensation takes mainly place at oxygen sites while electron density is shifted from the oxygen lattice to nickel (formation of sigma bonds). At high SOCs the shift of electron density from the transition metals to oxygen (formation of pi bonds) enables an additional redox process but also oxygen release from the transition metal host structure and subsequent detrimental reactions. Depending on the Ni:Co:Mn content, both processes lead to characteristic features in the voltage profile of the cathode materials and performance parameters like the capacity, the cycling stability and the open cell voltage become a function of the composition.