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 (H₂SO₃) 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.