Layered oxides for Na‐ion batteries containing Fe have attracted strong interest mainly due to their low cost. However, full oxidation of Fe³⁺ to Fe⁴⁺ is rarely seen before O‐redox sets in and is typically accompanied by voltage and capacity fade on cycling. On charging P2‐Na_0.67[Fe_0.5Mn_0.5]O₂, Fe³⁺ is oxidized to only ≈Fe^3.3+ before the onset of O‐redox. O‐redox occurs when the Na content is sufficiently low (Na ≈0.3) to permit the transition from P‐type to O‐type stacking, thus enabling Fe³⁺ migration to the Na layer. Fe³⁺ migration generates cation vacancies in the transition metal layer, forming □‐O‐□ configurations, which trigger the onset of O‐redox. In contrast, doping this material with Mg²⁺ to form P2‐Na_0.67[Fe_0.25Mn_0.6Mg_0.15]O₂ allows full oxidation of Fe³⁺ to Fe⁴⁺ before the Na content is low enough to favor O‐type stacking. During O‐redox, Mg²⁺ is displaced into the Na layers instead of Fe. Mg substitution enables greater reversibility of the Fe³⁺/Fe⁴⁺ redox couple and significantly suppresses Fe migration, which is responsible for the voltage and capacity fade observed for P2‐Na_0.67Fe_0.5Mn_0.5O₂.

Lithium‐rich transition metal cathodes can deliver higher capacities than stoichiometric materials by exploiting redox reactions on oxygen. However, oxidation of O²⁻ on charging often results in loss of oxygen from the lattice. In the case of Li₂MnO₃ all the capacity arises from oxygen loss, whereas doping with Ni and/or Co leads to the archetypal O‐redox cathodes Li[Li_0.2Ni_0.2Mn_0.6]O₂ and Li[Li_0.2Ni_0.13Co_0.13Mn_0.54]O₂, which exhibit much reduced oxygen loss. Understanding the factors that determine the degree of reversible O‐redox versus irreversible O‐loss is important if Li‐rich cathodes are to be exploited in next generation lithium‐ion batteries. Here it is shown that the almost complete eradication of O‐loss with Ni substitution is due to the presence of a less Li‐rich, more Ni‐rich (nearer stoichiometric) rocksalt shell at the surface of the particles compared with the bulk, which acts as a self‐protecting layer against O‐loss. In the case of Ni and Co co‐substitution, a thinner rocksalt shell forms, and the O‐loss is more abundant. In contrast, Co doping does not result in a surface shell yet it still suppresses O‐loss, although less so than Ni and Ni/Co doping, indicating that doping without shell formation is effective and that two mechanisms exist for O‐loss suppression.

In a recent study, we showed by solid-state NMR that LiVPO₄F, which is a promising material as positive electrode for Li-ion batteries, often exhibits some defects that may affect its electrochemical behavior. In this paper, we use DFT calculations based on the projector augmented-wave (PAW) method in order to model possible defects in this (paramagnetic) material and to compute the Fermi contact shifts expected for Li nuclei located in their proximity. The advantage of the PAW approach versus FP-LAPW we have been previously using is that it allows considering large supercells suitable to model a diluted defect. In the first part of this paper, we aim to validate the Fermi contact shifts calculation using the PAW approach within the VASP code. Then we apply this strategy for modeling possible defects in LiVPO₄F. By analogy with the already existing homeotypic LiVOPO₄ phase, we first replace one fluoride ion, along the VO₂F₄ chains, by an oxygen one and consider, in a second step, an association with a lithium vacancy. As a result, the agreement between the calculated NMR spectra and the experimental one is satisfying. In both cases, the local electronic structure and the spin transfer mechanisms from V³⁺ or V⁴⁺ ions to the Li nuclei are analyzed.

A new Tavorite-like NaVPO₄F type material with limited electrochemical performance versus its lithium analog.