Redox Mediator V⁵⁺ Doping for Effective Charge Compensation in the Li-Rich Mn-Based Layered Oxide Cathode of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂

Anion redox chemistry has emerged as a promising strategy for designing advanced cathode materials in lithium-ion batteries, especially for Li-rich Mn-based layered oxides (LRMOs). However, the associated oxygen evolution and irreversible structural rearrangements caused by oxygen redox lead to fatal degradation in performance, including voltage hysteresis, substantial irreversible capacity, and sluggish kinetics. Recently, pentavalent cation doping has been recognized as an effective approach to improving the reversible anion redox activities of layered oxides. However, most of the previous studies have predominantly emphasized the role of strong dopant-oxygen bonds in demonstrating the influence of pentavalent cation doping on LRMOs. Furthermore, most current reports lack a comprehensive understanding of the comparative effectiveness of pentavalent dopants. In this work, we have proposed novel aspects for the role of pentavalent cations in LRMO by comparatively investigating the effect of V 5+, Sb 5+, Ta 5+, and Nb 5+ doping on Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 from the crystallographic features and redox-behavior characteristics. In-depth crystallographic analyses by Rietveld refinement and synchrotron in situ X-ray diffraction were employed to provide insights into local structural properties, revealing oxygen sublattice distortion during electrochemical cycling with lithium ions. Furthermore, ex situ X-ray absorption near-edge structure and extended X-ray absorption fine structure were conducted to gain a more intuitive correlation between the transition-metal-redox behaviors and electrochemical properties, with a specific focus on the outstanding electrochemical properties, of V 5+ -doped LRMO. These findings suggest novel insights into the role of a high-valent dopant in oxygen redox chemistry, offering a guideline for modifying anion redox chemistry for high-energy cathode materials.