Dual-confinement and oxygen vacancy strategies for ultra-stable sodium storage and mechanisms in high-entropy spinel oxides

Developed SIBs anode materials based on the concept of high-entropy have shown excellent electrochemical performance due to their unique entropy effect. However, nanoscale high-entropy oxides (HEOs) face challenges such as particle agglomeration and side reactions induced by size effects. Furthermore, the functional relationship between the synergistic effects of multiple cations and their electrochemical properties remains to be determined. In this work, hollow nanospheres of spinel-structured high-entropy oxides were uniformly dispersed and encapsulated in carbon fibers, achieving a "dual-confined" structure with excellent cycling stability (304 mAh g- 1 at 200 mA g-1after 200 cycles) and rate capability (275.3, 266.2, 253.1, 242.4, 230.1, 207.4 mAh g- 1 at 100, 200, 500, 1000, 2000, 5000 mA g- 1, respectively, and a 266.6 mAh g- 1 reversible capacity is recovered at 200 mA g- 1). The unique nanoscale morphology, characterized by a low Young's modulus, mitigates the stress generated by the volume change during the sodiation/desodiation process, thereby reducing the continuous accumulation of stress and effectively enhancing structural stability. Furthermore, the oxygen vacancies significantly enhance Na+ adsorption and that the multi-element synergy in (FeMnNiCuZn)3O4 contributes to its near-metallic characteristics, with a notably reduced bandgap compared to CuMn2O4. Additionally, ex-situ XPS elucidates the redox mechanism of (FeMnNiCuZn)3O4, revealing the spontaneous formation of a more stable structure during cycling. At the same time, in-situ XRD reveals the gradual transformation of long-range ordered crystalline structures to nanocrystals during cycling, elucidating its reaction mechanism in SIBs. Research findings indicate that optimizing morphology, oxygen vacancies, and multi-element synergistic effects could be crucial strategies for developing advanced anode materials for SIBs.