Charge Storage Mechanism in Electrospun Spinel‐Structured High‐Entropy (Mn_0.2Fe_0.2Co_0.2Ni_0.2Zn_0.2)₃O₄ Oxide Nanofibers as Anode Material for Li‐Ion Batteries

Abstract

High‐entropy oxides (HEOs) have emerged as promising anode materials for next‐generation lithium‐ion batteries (LIBs). Among them, spinel HEOs with vacant lattice sites allowing for lithium insertion and diffusion seem particularly attractive. In this work, electrospun oxygen‐deficient (Mn,Fe,Co,Ni,Zn) HEO nanofibers are produced under environmentally friendly calcination conditions and evaluated as anode active material in LIBs. A thorough investigation of the material properties and Li+ storage mechanism is carried out by several analytical techniques, including ex situ synchrotron X‐ray absorption spectroscopy. The lithiation process is elucidated in terms of lithium insertion, cation migration, and metal‐forming conversion reaction. The process is not fully reversible and the reduction of cations to the metallic form is not complete. In particular, iron, cobalt, and nickel, initially present mainly as Fe3+, Co3+/Co2+, and Ni2+, undergo reduction to Fe0, Co0, and Ni0 to different extent (Fe < Co < Ni). Manganese undergoes partial reduction to Mn3+/Mn2+ and, upon re‐oxidation, does not revert to the pristine oxidation state (+4). Zn2+ cations do not electrochemically participate in the conversion reaction, but migrating from tetrahedral to octahedral positions, they facilitate Li‐ion transport within lattice channels opened by their migration. Partially reversible crystal phase transitions are observed.

Recent advances and understanding of high-entropy materials for lithium-ion batteries

“…The HEO nanofibers present a granular architecture are comprised of densely packed oxide grains. Furthermore, they offer a narrow inner channel which is beneficial to Li + diffusion [69]. Triolo et al [122] have demonstrated that the morphologies of HEMs are influenced by the synthesis methods.…”

Porous High-Entropy Oxide Anode Materials for Li-Ion Batteries: Preparation, Characterization, and Applications

“…Currently, the preparation techniques of high-entropy oxides can be categorized according to the initial feedstock as well as the application. As shown in Figure 2, the preparation methods of HEOs include solid-state reaction method (high-temperature solid state reaction) [22,23], wet chemical methods (spray pyrolysis, co-precipitation, and solution combustion synthesis) [24][25][26][27][28][29][30], epitaxial growth of films (pulsed laser deposition and magnetron sputtering deposition) [31,32], and so on. This section focuses on the specific flow and characteristics of the process for the preparation of porous HEOs.…”

High-entropy oxides: an emerging anode material for lithium-ion batteries

“…In the same time, Cu becomes an inert element after initial lithiation. In chromium-free (MnFeCoNiZn) 3 O 4 , Santangelo et al 83 revealed that the reaction mechanism of Fe 3+ , Co 3+ /Co 2+ , and (Ni 3+ )/Ni 2+ is a single/multi-step redox behavior. Manganese is a partially reversible component with Mn 4+ /Mn 3+ present in the original HEOs and only Mn 3+ /Mn 2+ present after charge.…”