Comprehensively Understanding the Role of Anion Vacancies on K-Ion Storage: A Case Study of Se Vacancy Engineered VSe 2 .

Anion vacancy engineering (AVE) has been 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 is 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 VSe 2 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, we uncover the effects of Se vacancies on the redox potentials of K-ion insertion/extraction and the K-ion diffusions in electrodes upon cycling. In addition, we disclose the structural evolution of Se vacancies during potassiation/de-potassiation using various operando and ex-situ characterizations. Moreover, for the first time, we demonstrate that Se vacancies can facilitate the break 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. This article is protected by copyright. All rights reserved.