Abstract

Transition-metal high-entropy oxides show superior electrochemical properties and excellent long-term cycling stability in secondary batteries due to their entropy stabilization effect. However, the effects on storage mechanisms of Na+ batteries remains an open question, especially considering its larger ionic radius, sluggish kinetics movement, and serious volumetric expansion. In this work, ultra-small size spinel-structured (FeCuCrMnNi)3O4 composition of hollow porous microspheres is dispersed on the reduced graphene oxide net work. The architecture can effectively suppress the continuous volume expansion and maintain valid interface contact between electrode and electrolyte, thus achieving highly reversible sodium storage. Moreover, oxygen vacancies promote the desorption of Na+ during the charge process due to weak chemical bonding and migration barrier, improving cycling stability. Furthermore, highly reversible structural transformation of spinel to monometallic oxides and then to rock salt structure is observed by in situ XRD. The results provide an in-depth understanding of Na+ storage mechanism in HEOs with self-supporting electrode materials.

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