Lowering Lattice Strain in High Entropy Co-Free Cathode Enabled by Introduction of Covalency Modulation for Sodium-Ion Batteries

Li-ion batteries (LIBs) are already playing a key role in the increased electrification of modern societies [1]. However, the increasing demand for electricity storage systems and the declining lithium resources have pushed the researchers to explore its alternative [2]. Na-ion batteries owing to the high abundancy of sodium resources have recently emerged as a promising alternative to Li-based batteries [3]. The present work aims to achieve a cobalt-free high capacity cathode for Na-ion batteries using high entropy approach. High entropy approach comprises a cocktail effect as it implicates the mixing of more than five elements in a single phase. The challenge involve with this approach is to create a balance between different elements to get the desired properties. The optimized high entropy cathode delivers a high capacity and appreciable cycle life of 200 cycles at a voltage range of 2-4V. Further, diffraction studies both <i>insitu</i> and <i>exsitu</i> has revealed that the high entropy strategy is effective in suppressing the O3’ phase during charging/discharging at 2-4V. However, increasing the voltage window beyond 4V for 200 cycle shows the significant lattice strain on the structure which is further tackled by increasing the covalent character of M-O bond. The presented outcomes strongly motivate to pursue introduction of covalency modulation in high entropy approach in developing efficient cathode for Na-ion batteries.<br/><br/>Keywords: High Configuration entropy, Covalency modulation, Cobalt-free cathode, , O3 layered structure, Na-ion battery<br/>References<br/>[1] J.W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nat. Rev. Mater. 1 (2016) 16013.<br/>[2] I. Hasa, D. Buchholz, S. Passerini, J. Hassoun, A comparative study of layered transition metal oxide cathodes for application in sodium-ion battery, ACS Appl. Mater. Interfaces 7 (2015) 5206–5212.<br/>[3] J.Y. Hwang, S.T. Myung, Y.K. Sun, Sodium-ion batteries: present and future, Chem. Soc. Rev. 46 (2017) 3529–3614.