Design Principles for Efficient Hydrothermal Relithiation of Spent Lithium Iron Phosphate.

Direct regeneration, which involves replenishing lithium in spent cathode materials, is emerging as a promising recycling technique for spent lithium iron phosphate (s-LFP) cathodes. Unlike solid-state regeneration, the aqueous relithiation method consumes less energy, ensures even lithium replenishment, and significantly recovers the capacity of s-LFP. However, liquid-phase lithium replenishment formulations are generally less standardized. In this study, we propose designing principles for hydrothermal relithiation recipes to achieve efficient relithiation while ensuring a high yield of relithiated LFP products, assisted by various electrochemical techniques. This led to the discovery of an economical hydrothermal relithiation approach. Specifically, using sulfurous acid (HSO) as the reducing agent and LiOH as the lithium source in the hydrothermal precursor, we achieved complete relithiation at a mild hydrothermal temperature of 90 °C with a high yield (only 3.1% mass loss) of relithiated LFP products. The regenerated LFP recovers approximately 29% of its capacity and exhibits remarkable capacity retention (98.9%). This research highlights a significant advancement in the efficient hydrothermal regeneration of s-LFP, presenting a green and economically viable method for LFP recycling and setting a benchmark for sustainable battery recycling technologies.