Strategies in improving the initial coulombic efficiency of transition metal chalcogenides anode materials for sodium-ion batteries: A review

Sodium-ion batteries (SIBs) are gaining attention as a cost-effective option for energy storage systems, largely due to the widespread availability of sodium resources. Improvements in electrode materials and a better understanding of their working mechanisms have greatly enhanced the electrochemical performance of these batteries. Among the various options, transition metal chalcogenides (TMCs) have emerged as a focus of research because of their high capacity enabled by conversion or alloying reactions. However, these materials face challenges such as substantial volume changes and low electrical conductivity, which negatively affect their cycling stability and rate performance. The initial Coulombic efficiency (ICE) of anode materials plays a critical role in determining the energy density of both lithium-ion and sodium-ion batteries. In commercial lithium-ion batteries (LIBs), the low ICE of anodes is typically compensated by adding extra cathode material. However, because lithium-metal oxide cathodes have a lower specific capacity compared to anodes, a significant surplus of cathode material—about 10–15 % for graphite anodes—is required, which reduces the overall energy density. In sodium-ion batteries, the irreversible consumption of Na+ ions during the initial charge/discharge cycle, caused mainly by the formation of the solid electrolyte interphase (SEI) and other irreversible reactions, leads to a noticeable reduction in the capacity of the full cell. Improving the ICE of both anodes and cathodes is, therefore, essential for achieving high-performance sodium-ion batteries, particularly by enhancing the transport efficiency of Na+ ions. Low ICE remains a significant challenge in developing high-capacity anodes for practical sodium-ion batteries. This review examines the key factors behind low ICE in transition metal chalcogenides, including SEI formation due to electrolyte decomposition and the limited reversibility of the sodiation and desodiation processes. Then it will highlight innovative strategies to overcome these challenges, such as optimizing electrolytes and material structures, applying surface modifications and coatings, designing improved structures and morphologies, advancing surface engineering techniques, and developing polymetallic TMCs.