Anion-Driven Surface Reconstruction Enables Direct Regeneration of LiCoO2 Cathode Material.

The escalating volume of decommissioned lithium cobalt oxide (LCO) batteries necessitates recycling strategies capable of adding value by transforming spent cathodes into high-performance materials. While eutectic molten salt systems have emerged as a promising liquid-solid medium for direct regeneration, the selection of salts has been largely empirical, and the critical role of anions in governing the relithiation efficacy and interface chemistry remains underappreciated. Herein, we demonstrate that the strategic choice of anions in a binary molten salt flux is paramount to achieving structural repair and performance enhancement. We identify that anions with high adsorption energy to the CoO6 slabs and low dissociation energy for Li+, such as Cl-, are optimal. This anion-mediated process directs a preferential crystallographic reconstruction along the c-axis and fosters the development of a Li-O enriched, (104)-facetted surface. Consequently, the upcycled LCO cathode retained over 95% of its original capacity after 1000 cycles at 4.2 V. Even at an elevated voltage of 4.6 V, the upcycled LCO retains 85.5% of its capacity after 100 cycles at 0.5 C. This work underscores that understanding and engineering the anion in molten salts could be a powerful step toward the rational design of advanced upcycling protocols.