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Mixed Cationic and Anionic Redox in Ni and Co Free Chalcogen-Based Cathode Chemistry for Li-Ion Batteries.

Mixed cationic and anionic redox cathode chemistry is emerging as the conventional cationic redox centers of transition-metal-based layered oxides are reaching their theoretical capacity limit. However, these anionic redox reactions in transition metal oxide-based cathodes attained by taking excess lithium ions have resulted in stability issues due to weak metal-oxygen ligand covalency. Here, we present an alternative approach of improving metal-ligand covalency by introducing a less electronegative chalcogen ligand (sulfur) in the cathode structural framework where the metal d band penetrates into the ligand p band, thereby utilizing reversible mixed anionic and cationic redox chemistry. Through this design strategy, we report the possibility of developing a new family of layered cathode materials when partially filled d orbital redox couples like Fe2+/3+ are introduced in the Li-ion conducting phase (Li2SnS3). Further, the electron energy loss spectroscopy and X-ray absorption near-edge structure analyses are used to qualitatively identify the charge contributors at the metal and ligand sites during Li+ extraction. The detailed high-resolution transmission electron microscopy and high annular dark field-scanning transmission electron microscopy investigations reveal the multi-redox induced structural modifications and its surface amorphization with nanopore formation during cycling. Findings from this study will shed light on designing Ni and Co free chalcogen cathodes and various functional materials in the chalcogen-based dual anionic and cationic redox cathode avenue.