Potassium ion batteries (PIBs) have shown great potential as a next-generation electrochemical energy storage system, due to the natural abundance of potassium and the relatively low redox potential of K ions. To accommodate the large ionic radius of K ions, conversion-type electrode materials are regarded as suitable candidates for K ion storage. However, the triggering mechanism of a conversion reaction in most anode materials of PIBs is unclear, which limits their further development. To reveal the mechanism, in this work, MoSe2, MoS2, and MoO2 were selected as model materials, guided by theoretical calculations, to investigate the K ion storage process. Through ex situ characterization, it was found that intercalation reactions preferentially occur in MoSe2 and MoS2, while an adsorption reaction preferentially occurs in MoO2. This is because of the larger interlayer spacing and lower K ion intercalation barrier in MoSe2 and MoS2 than in MoO2. The preferential intercalation reactions are able to induce a further conversion reaction by reducing the reaction barrier, thereby realizing high K ion storage capacities. As a result, the MoSe2-rGO and MoS2-rGO hybrids showed higher reversible capacities than the MoO2-rGO hybrid. By demonstrating a relationship between intercalation and the conversion reaction and understanding the mechanism, guidance is provided for selecting the electrode materials to obtain PIBs with high performance.
Keywords: crystal structure; heterogeneous nanostructure; ion storage mechanism; molybdenum dichalcogenides; potassium ion batteries.