Understanding and improving the initial Coulombic efficiency of high-capacity anode materials for practical sodium ion batteries

Sodium ion batteries have emerged as a potential low-cost candidate for energy storage systems due to the earth abundance and availability of Na resource. With the exploitation of high-performance electrode materials and in-depth mechanism investigation, the electrochemical properties of sodium ion batteries have been greatly improved. However, there are still various challenges need to be overcome before its commercial application. Among them, the low initial Coulombic efficiency of high-capacity anode is a critical issue that seriously limits the practical energy density of sodium ion full battery. This review firstly highlights the underlying mechanisms of the low initial Coulombic efficiency, such as the formation of SEI film from the decomposition of electrolyte, poor reversibility of sodiation/desodiation process, etc., and then summarizes the emerging strategies including electrolyte optimization, structure/morphology design, surface engineering, binder optimization and so on, to overcome them. Finally, some Na compensation strategies like pre-sodiated anode, sacrificial additives and sodium-rich cathode for practical application are discussed. This review provides pertinent insights into the challenges of initial Coulombic efficiency for the development of high energy sodium ion battery, which will benefit to its practical application.