Comprehensively Understanding the Role of Anion Vacancies on K-Ion Storage: A Case Study of Se-Vacancy-Engineered VSe₂.

Abstract

Anion vacancy engineering (AVE) is widely used to improve the Li-ion and Na-ion storage of conversion-type anode materials. However, AVE is still an emerging strategy in K-ion batteries, which are promising for large-scale energy storage. In addition, the role of anion vacancies on ion storage is far from clear, despite several proposed explanations. Herein, by employing VSe₂ as a model conversion-type anode material, Se vacancies are intentionally introduced (labeled as P-VSe_2-x ) to investigate their effect on K⁺ storage. The P-VSe_2-x shows excellent cyclability in half cells (143 mA h g^-1 at 3.0 A g^-1 after 1000 cycles) and high energy density in coin-type full cells (206.8 Wh kg^-1 ). By applying various electrochemical techniques, the effects of Se vacancies on the redox potentials of K-ion insertion/extraction and the K-ion diffusion in electrodes upon cycling are uncovered. In addition, the structural evolution of Se vacancies during potassiation/de-potassiation using various operando and ex characterizations is revealed. Moreover, it is demonstrated that Se vacancies can facilitate the breaking of VSe bonds upon the P-VSe_2-x conversion using theoretical calculations. This work comprehensively explains the role of anion vacancies in ion storage for developing high-performance conversion-type anode materials.