Current state-of-the-art Li-ion batteries are reaching their theoretical limit with respect to the capacity, a property largely limited by cathode materials that have so far relied solely on cationic-redox of transition-metal ions (e.g., M^3+/4+ in LiMO₂ where M is Co, Ni, and Mn) for driving the electrochemical reactions. Recently, the introduction of new anion-redox cathode materials can lead to a doubling of capacity by accommodating multielectron redox chemistries that have gained research interest. (1) However, current anion-redox cathode materials based on Li-rich layered oxides (represented by the formula Li_1+xM_1-xO₂ where M is Co, Ni, and Mn) suffer from voltage fade, large hysteresis, and sluggish kinetics, which originate mysteriously from the anionic redox activity of oxygen ligand itself. (2) It is widely accepted that the covalency between transition metal – oxygen ligand in traditional cathode materials needs to be altered to take advantage of anion redox chemistry. Here, we present an alternative approach of incorporating improved metal – ligand covalency by less electronegative chalcogen sulfur ligands in the cathode structural framework where the metal d-band penetration into ligand p-band thereby utilizing mixed anionic and cationic redox chemistry. (3) Through this design strategy, we investigated anion redox activity in layered cathode material based on Li₂SnS_3, and their lithiation/delithiation properties were evaluated through in-depth electrochemical analysis. Further, the charge contributors during electrochemical reactions were identified by spectroscopy analysis, and morphological evolution due to mixed anionic and cationic redox reactions were evaluated using high-resolution transmission electron microscopy(HR-TEM) and high annular dark field-scanning transmission electron microscopy(HAADF) investigations.The results reveal the multi redox induced structural modifications and its surface amorphization with nanopore formation during cycling. Findings from this research will inspire Ni and Co free chalcogen cathode design and various functional materials in the pursuit of next generation cathode materials.

References

1. J. R. Croy, M. Balasubramanian, K. G. Gallagher, A. K. Burrell, Review of the US Department of Energy’s “Deep Dive” effort to understand voltage fade in Li-and Mn-rich cathodes. Accounts of chemical research 48, 2813-2821 (2015).
2. G. Assat, J. M. Tarascon, Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries. Nature Energy 3, 373-386 (2018).
3. S. Nagarajan, S. Hwang, M. Balasubramanian, N. K. Thangavel, L. M. R. Arava, Mixed Cationic and Anionic Redox in Ni and Co Free Chalcogen-Based Cathode Chemistry for Li-Ion Batteries. Journal of the American Chemical Society 143, 15732-15744 (2021).