Entropy stabilization effect and oxygen vacancy in spinel high-entropy oxide promoting sodium ion storage

Rapid capacity decay is a crucial issue for conversion reaction anodes of sodium ion batteries (SIBs). Here, we introduced the concept of “high entropy” into the conventional transition metal oxides, successfully preparing spinel high-entropy oxide [(FeCoNiMn)1-xNax]3O4 (x = 0.05, 0.08, 0.12) (named after HEO 0.12, HEO 0.08, HEO 0.05) by spray pyrolysis. Due to the “entropy stabilization effect”, an electrode material exhibits respectable stability. Simultaneously, the precise adjustment of oxygen vacancy accomplished by ion substitution demonstrates that oxygen vacancy naturally in HEO not only provides additional active sites for Na+ accommodation and lowers the sodiation energy barrier enabling reversible Na+ intercalation, but also effectively propels charge transfer. As a result, the cooperation of the “entropy stabilization effect” and oxygen vacancy makes HEO 0.12 competitive in terms of cycling performance (average capacity decay of 0.069% cycle−1 at 1000 mA g−1), reversible specific capacity, coulombic efficiency and rate performance compared to those of counterparts. Significantly, the evolution of composition and structure during cycling suggests that the key inducing factors for the satisfactory cycling stability of HEO are the excellent reaction reversibility and structure durability, excavating the nature of the “entropy stabilization effect”. This work sheds novel insights into the development of anodes for SIBs.