Layered Na3/4(Ti1/5Mn1/5Fe1/5Co1/5Ni1/5)O2 high-entropy oxides as cathodes for Na-ion batteries: Interplay between microstructure and electrochemical performance”

Thank to the abundance of sodium on Earth and its similar chemistry to lithium, Na-ion batteries (NIBs) appear to be a sustainable altemative to traditional Li-ion batteries (LIBs). However, the lower performance compared to LIBs still strongly limits their broad market penetrati.on. \oVider ditfusion of SJB technology requires the development of new cathode materials with reversible Na+ conversion reaction. Layered oxides (NaxM02) are among the materials of choice due to their high theoretical capacities and competitive prices. Benefiting of the synergy between their multiple transition-mctal componcnts, laycrcd high-entropy oxidcs (LHEOs) show great potcntial as aùvanceù NIB cathode materials [l]. The engineering and investigation of LHEOs is the mai11 goal of the project "OPHELTA" (OPerando studies of High-Entropy oxide-based active materia!for Na-lon battery cathodes). In the fi·amework of this project, Na:v4(Tù i5 '1nlisFevsCov5Nivs)02 LHEOs with different morphology are prepared by spray p)Tolysis (SP) and sol-gel (SG) methods and evaluated as active NJB cathode materials. In spite of the same crystal structure and comparable Na content (0.52 and 0.57 for SP and SG, rcspcctivcly), thc SP- and SG-LHEOs cxhihit diffcrcnt clcctrochcmical performance. Aftcr 300 cyclcs at l C (200 mA g-1 ), the former ùelivers 45 m.A.h g- 1 specific capacity, with 35%) retention, whereas thc spccitìc capacity of thc l attcr is abovc 120 mAh g- 1 , with 69% capacity rctcntion. Thcsc capacity values well compare with those reported for LHEOs with .similar composition prepared by conventional solid-state reaction route [2]. Thc rcasons far thc diffcrcnt bchavior are clucidatcd via ex-situ synchrotron X-ray absorption spectroscopy (XAS). The results eviùence that SG-LHEOs are l:baracterized by a larger structural disorder compared to SP-LHEO, both before and af1er cycling. Also the LHEO microstructure plays a kcy rolc: thc bcst performance pcrtains to tbc activc matcrial with largcst crystal sizc (0.75 against 0.30 Lm). The Authors gratefully acknowleùge the Emopean Union tòr the finaneial support to this project through Next-Generation EU funds (National Recovery and Resilience Pian, PNRR - Mission 4 "Education and Rcscarch'' - C2 componcnt - Invcstmcnt LI, Fund for thc National Rcscarch Program anù Projects of Relevant Nationallnterest, PRlN).