The role of tungsten-related elements for improving the electrochemical performances of cathode materials in lithium ion batteries

Lithium ion batteries using Ni–Co–Mn ternary oxide materials (NCMs) and Ni–Co–Al materials (NCAs) as the cathode materials are dominantly employed to power the electric vehicles (EVs). Increasing the driving range of EVs necessitates an increase of Ni content to improve the energy densities, which, however, degrades the cycle stability. Here we review the doping/coating of tungsten and related elements to improve the electrochemical performance of these cathodes especially the cycle stability. The selection of tungsten and related elements is based on their special properties including the high valence state, strong bonding with oxygen and the large ionic radius. The improvement of cycle stability mainly results from two features: (1) the enhancement of bulk structure stability upon doping (Mo, W, Ta, Nb) and (2) the resistance of side reactions of electrode/electrolyte by the surficial layer induced by direct coating (V, W, Nb) or bulk doping. For the recent high Ni materials, the formation of Ni2+ and its migration to the Li layer induced by these doped/coated tungsten-related elements, and the presence of spinel or rock-salt phase before cycling contributes to improving the cycle stability. The key challenges are the selection of an optimized additive concentration and the fundamental understanding of the reaction mechanism, which will provide insightful guidance for maximizing the electrochemical performance of the state-of-the-art lithium-ion batteries at minimal additional process costs.