Uncovering mechanism behind tungsten bulk/grain-boundary modification of Ni-rich cathode

Abstract

Introducing foreign elements is regarded as a promising strategy for realizing bulk doping/grain boundary (GB) coating to enhance structural/interfacial stabilities of Ni-rich cathodes. However, directionally achieving control over simultaneous bulk doping and GB coating dual-modification is difficult due to the unclear interdiffusion constant between foreign element and primary components (Ni, Co, and Mn). Herein, a novel mechanism for tungsten (W) diffusion into the interior of Ni-rich cathode has been elucidated, in which the interdiffusion coefficients between W⁶⁺ and transition metal cations have been firstly measured. Due to the fastest interdiffusivity of W⁶⁺/Mnⁿ⁺ (n = 3 and 4) couple proved by incorporating thermodynamic and dynamic results, the modification discrepancy foreign W element in the multi-component Ni-rich cathode has been successfully achieved by altering Mn content. It is found that single bulk W-doping has been obtained in LiNi_0.8Mn_0.2O₂ cathode. Encouragingly, when Mn proportion is decreased to 10 %, Li₆WO₆ GB coating and bulk W-doping have been achieved in LiNi_0.9Mn_0.1O₂ and LiNi_0.8Co_0.1Mn_0.1O₂ cathodes. Inspired by dual-modification, cyclic stabilities of W-modified LiNi_0.9Mn_0.1O₂ have been prominently improved. The work provides the in-depth understanding of W diffusion into Ni-rich cathodes, exploiting new approaches for engineering bulk/GB modification.