Metal Sulfide-Based Potassium-Ion Battery Anodes: Storage Mechanisms and Synthesis Strategies

Rechargeable potassium-ion batteries (PIBs), with their low cost and the abundant K reserves, have been promising candidates for energy storage and conversion. Among all anode materials for PIBs, metal sulfides (MSs) show superiority owing to their high theoretical capacity and variety of material species. Nevertheless, the battery performance of MSs is hindered by many factors such as poor conductivity, low ion diffusivity, sluggish interfacial/surface transfer kinetics, and drastic volume changes. In this review, the electrochemical reaction mechanisms, challenges, and synthesis methods of MSs for PIBs are summarized and discussed. In particular, the most common synthesis methods of MSs for PIBs are highlighted, including template synthesis, hydro/solvothermal synthesis, solid-phase chemical synthesis, electrospinning synthesis, and ion-exchange synthesis. During the potassium storage process, the two-dimensional layered MSs follow the intercalation/extraction mechanism, and the MSs with inactive metal undergo the conversion reaction, whereas the metal-active MSs follow the conversion-alloying reaction mechanism. Given the inherent properties of MSs and the reactions they undergo during cycling, when used as anodes for PIBs, such materials experience a series of problems, including poor ion-/electron-transport kinetics, structural instability, and loss of active material caused by the dissolution of discharged polysulfide products and the occurrence of side reactions. These problems can be solved by optimizing the methods for synthesizing MSs with an ideal composition and structure. The template method can precisely prepare porous or hollow-structured materials, the hydro/solvothermal method can alter the thickness or size of the material by adjusting certain synthesis parameters, and the one-dimensional-structured material obtained via electrospinning often has a large specific surface area, all of which can shorten the transport pathway for potassium ions, thereby improving the performance of the battery. The ion-exchange method affords difficult-to-synthesize MSs via anion- or cation-exchange, in which the product inherits the structure of the starting material. The solid-phase synthesis method makes it possible to combine MSs with other materials. Combinations with materials such as carbon or other MSs helps to provide sufficient buffer space for the volume expansion of MSs during cycling, while promoting electron transport and improving the potassium-storage properties of the anodes. Therefore, this review aims to highlight the current defects of MS anodes and explore the construction of their ideal architecture for high-performance PIBs by optimizing the synthesis methods. Ultimately, we propose the possible future advancement of MSs for PIBs.