Unleashing the Potential of Fast Charging Batteries: Leveraging Anion Redox Chemistry in Ni- and Co-Free Cathodes

Designing Li-ion battery cathodes free of critical raw materials such as Co and Ni has a huge technological and societal impact. Although anion redox-based Li-rich oxide cathodes allow for designing Co- and Ni-free cathode compositions, the Li-rich oxides demonstrated voltage fade, voltage hysteresis, and irreversible oxygen release despite their high capacity. Conversely, anion redox through highly covalent chalcogenides (S/Se) is emerging due to the improved covalency between metal d and ligand p bands. Here, we investigate the tuning of multichalcogen (S/Se) p band and redox-active metal d band in a model Li-rich chalcogen composition, Li1.13Ti0.57Fe0.3S2–ySey (y = 0–1), through in-depth electrochemical, X-ray spectroscopy, and DFT-based electronic structure investigations. Introducing the appropriate amount of Se p band character in anion redox sulfides increases the interlayer distance and metal–ligand covalency without modifying the original crystal structure, promoting significant electrochemical reversibility through mixed anionic (Se2–/Sen–, S2–/Sn–, wherein n < 2) and cationic (Fe2+/Fe3+) redox reactions. We show the detailed Fe, S, and Se redox contributions during Li insertion–extraction through X-ray absorption (XAS) and hard X-ray photoemission spectroscopy (HAXPES) measurements. The orbital tuning approach improves the rate capability for more than 10C charge–discharge rate, exhibiting more than 50% of its original capacity obtained at the C/20 rate. The buffer cation in the lattice (Ti4+) remains electrochemically inactive even after significant Se p band introduction in the sulfide framework. Overall, this work takes advantage of multianion redox chemistry to uncover practically demanding fast charging–discharging characteristics in intercalation cathodes. The obtained knowledge of this design can be extended to other oxide and chalcogen cathodes for high-performance Li-ion batteries.