Enhanced Li storage performance of high-entropy spinels by electronic structure and crystallinity dual optimization

In this work, by one-step annealing in oxygen atmosphere, we turn the dealloyed, poorly crystallized nanoporous spinel-type high-entropy oxide (HEO, NiFe(AlCrMnV)O_x) with abundant oxygen vacancies and large specific surface area into well-crystallized single crystal particles with fewer oxygen vacancies and a significantly reduced surface area. Interestingly, we find that different from conventional wisdom, the suitably annealed well-crystallized spinel HEO perform much better for Li⁺ storage in terms of specific capacity (1621.3 mAh g⁻¹ at 0.1 A g⁻¹) and cycling stability even compared with the best result reported. We also reveal the effect of composition/element of HEO on the electrochemical performance. Specifically, the presence of V contributes to the capacity enhancement; Cr addition makes the HEO more inert and stable, but significantly reduces its conductivity and capacity; while further addition of Mn can optimize the electronic structure, resulting in enhanced conductivity and battery performance. Moreover, ex situ XRD, XPS and STEM characterizations reveal the irreversible reaction of Ni/Fe during cycling and their reduction/diffusion would form a uniform nanoporous structure within the HEO particles. The work shows the tunable electrochemical properties of HEO by composition and crystallinity modulation.