Role of the microstructure in the Li-storage performance of spinel-structured high-entropy (Mn,Fe,Co,Ni,Zn) oxide nanofibers

High-entropy oxides with spinel structure (SHEOs) are promising anode materials for next-generation lithium-ion batteries (LIBs). In this work, electrospun (Mn,Fe,Co,Ni,Zn) SHEO nanofibers produced under different conditions are evaluated as anode materials in LIBs and thoroughly characterised by a combination of analytical techniques. The variation of metal load (19.23 or 38.46 wt% relative to the polymer) in the precursor solution and of calcination conditions (700°C/0.5 h, or 700°C/2 h followed by 900°C/2 h) affects the morphology, microstructure, crystalline phase, and surface composition of the pristine SHEO nanofibers and the resulting electrochemical performance, whereas mechanism of Li storage does not substantially change. Causes of long-term (650 cycles) capacity fading are elucidated via ex situ synchrotron X-ray absorption spectroscopy. The results evidence that the larger amounts of Fe, Co, and Ni cations irreversibly reduced to the metallic form during cycling are responsible for faster capacity fading in nanofibers calcined under milder conditions. The microstructure of the active material plays a key role. Nanofibers composed by larger and better-crystallized grains, where a stable solid/electrolyte interphase forms, exhibit superior long-term stability (453 mAh g-1 after 550 cycles at 0.5 A g-1) and rate-capability (210 mAh g-1 at 2 A g-1).