High-Entropy Spinel Oxide as Conversion Anodes for Li-Ion Batteries
Authors: Nam Ki-Hun , Yan Shan , Marschilok Amy C. , Takeuchi Esther S. , Takeuchi Kenneth J. , Doeff Marca M. 
Published: December 2024

The burgeoning sector of electric vehicles has significantly spurred the exploration of high-energy density and long-term cycling life electrode materials for advanced lithium-ion batteries (LIBs). Presently, the constrained capacity of commercialized graphite (LiC6: 372 mAh g–1) and Li4Ti5O12 (Li7Ti5O12: 175 mAh g–1) anodes falls short of meeting the heightened requirements for energy density in such applications. Transition metal oxides (TMOs, MxOy) with conversion reaction have been widely scrutinized as next-generation LIB anode materials owing to their resonable reversible capacity in comparison to conventional graphite anode. Unfortunately, the crystal structures of TMOs typically undergo severe degradation during rapid conversion/alloying reactions over continuous lithiation/delithiation process. This phenomenon leads to rapid capacity decay, poor reversibility, and consequently, a significant hindrance to their applicability as LIB anodes. In recent developments, high-entropy oxides (HEOs), analogous to high-entropy metallic alloys (HEMAs), have garnered considerable interest as an emerging class of solid solutions, involving a multitude of metalic cations in an equimolar ratio. Intriguingly, the high configurational entropy (Sconfig) present plays a crucial role in stabilizing their single-phase crystal structures. Pioneering this concept, Rost et al. introduced entropy-stabilization into TMOs, successfully creating the rocksalt HEO, (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O with the value of Sconfig ≥ 1.6R. Leveraging substantial compositional flexibility by modifying the stoichiometry and incorporating diverse cationic species, HEOs possess unforeseen and distinctive physicochemical properties. Consequently, they have been applied across diverse domains, encompassing catalysts, thermoelectrics, superionic conductors, and battery electrodes.

The initial investigation into HEOs as LIB anode materials featured the rocksalt-type composition (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O. Sarkar et al. demonstrated its remarkably reversible Li-storage properties, showcasing stable cycling performance with reversible capacities ranging from 500 to 700 mAh g−1 at current density of 200 mA g−1 even after 300 cycles. Subsequently, Patra et al. introduced the single-phase spinel HEO (Fe0.2Cu0.2Ni0.2Cr0.2Mn0.2)3O4 as a LIB anode, achieving a specific capacity of 640 mAh g−1 at current density of 500 mA g−1 over 400 cycles. Impressively, the specific capacity remained at 596 mAh g−1 at a high current density of 2.0 A g−1 after 1200 cycles, retaining 86.2%. In contrast to conventional TMOs, entropy-stabilized HEOs possess the ability to maintain partial stability even in a fully lithiated state, acting as a matrix to accommodate conversion reactions and greatly improve reversible cycling stability. Nevertheless, rocksalt HEOs encounter challenges stemming from inadequate active components, thereby affecting the reversible capacity during cycling. In contrast, the spinel structure of HEOs facilitates ionic diffusion through three-dimensional pathways. Furthermore, the induction of oxygen vacancies by multivalent metallic cations at Wyckoff positions (tetrahedral and octahedral) in the spinel serves to augment ionic conduction. Therefore, the imperative further development of new spinel HEO anodes necessitates the rational design of active metallic cation components within HEOs. Concerning the enhancement of Li-storage properties in HEO anodes for LIBs, a comprehensive understanding of their Li storage mechanisms during Li-insertion/extraction is critically important.

Herein, we present the development of a high-performance conversion-type anode comprising new multi-component HEOs with 5, 6, 7, and 8 cations for LIBs, synthesized through rapid techniques, specifically solution combustion synthesis (SCS). Additionally, we characterize their in-depth structural information using synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). With an increase in the number of cations, the HEO anodes exhibit improved Li-storage properties, including enhanced cycling performance and rate capability. To further evaluate the electrochemical performance of HEOs synthesized by different methods, we selected a specific composition based on its electrochemical performance and synthesized it using solvothermal synthesis. Subsequently, we assessed the electrochemical performance of these samples. Furthermore, we investigated the Li-storage mechanism of HEOs during lithiation/delithiation through ex-situ analytical techniques, encompassing XRD and XAS. The results collectively indicate that the highly reversible conversion reaction in cycled HEO anodes contributes to their outstanding Li-storage characteristics, facilitating stable cycling retention and fast rate capability. The findings of this study delve deeply into the highly reversible Li-storage in HEOs through conversion reactions, with potential implications extending to a broader class of HEO anodes, suggesting the promise of advanced LIBs exhibiting exceptional electrochemical performance.