Metal fluorides have been widely studied as promising conversion-type cathode material for Li-ion batteries (LIBs) owing to their higher theoretical capacity and lower cost compared to typical intercalation materials. However, their practical utilization is limited by low round trip efficiency, significant voltage hysteresis and capacity fading during cycling, caused by irreversible structural reorganization during the conversion reaction. To overcome these challenges, cation substitution has been proposed to tune electrochemical properties of metal fluorides. [1]
Recently, high entropy stabilization has been introduced to achieve reversible conversion reactions for Li storage.[2] This concept involves stabilizing the single-phase structure by increasing its configurational entropy to overcome enthalpy gain associated with phase separation. Therefore, high-entropy materials offer the opportunity to tailor properties via the "cocktail effect" with optimal elemental selection. Following the concept, recently high entropy fluorides (HEF) were synthesized and investigated as a cathode for LIBs, showing improved electrochemical performance as number of cations increases.[3] In addition to the entropy stabilized effect, Mg²⁺ ion plays the pivotal role in keeping the fluoride structure stable for cycling as a “spectator effect”.
In this presentation, we will discuss the effect of elemental composition of HEF cathode by comparing Co_0.2Cu_0.2Ni_0.2Zn_0.2Fe_0.2F₂ (HEF-Fe) with Co_0.2Cu_0.2Ni_0.2Zn_0.2Mg_0.2F₂ (HEF-Mg). While the latter has the well-known transition metal composition including Mg as a spectator in previous studies, the former replaces Mg with Fe, an active participant in conversion reaction with higher energy density. Contrary to the role of Mg spectator in the literatures, we found that HEF-Fe showed more stable battery cycling and rate performance than HEF-Mg. This could be related that the former provides higher configurational entropy at the discharged state and electronic conductivity. Conversion reaction mechanism in HEF will be discussed with in situ X-ray absorption results. This work demonstrates the importance of tuning HEF composition, which can help guide the more rational design of high energy LIBs electrodes with the high entropy stabilization concept.

Jaejin Kim

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