Direct and Low-Temperature Regeneration of Degraded LiFePO₄ Cathodes at Ambient Conditions Using Green and Sustainable Deep Eutectic Solvent.

Abstract

The definite lifespan of lithium iron phosphate (LiFePO₄, LFP) batteries necessitates the advancement of cost-effective, nature-friendly, and productive recycling techniques for spent LFP batteries. In this study, ethylene glycol (C₂H₆O₂), a sustainable and economical small organic molecule, is employed as a multifunctional hydrogen-bonding donor, along with lithium chloride (LiCl), a readily accessible Li source and hydrogen-bonding acceptor. Together, they form a novel Li-salt deep eutectic solvent (DES) through hydrogen bonding interactions. This DES directly repairs and rejuvenates the spent cathode material (S-LFP) at 80 °C. The Li-salt DES not only replenishes the depleted Li in S-LFP and reduces the adverse effects of Li-Fe antisite defects but also establishes a reducing environment that facilitates the reversion of degraded Fe(III) species in S-LFP back to their original Fe(II) state. Consequently, the regenerated LFP exhibits remarkable electrochemical behavior, delivering an initial capacity of 155.6 mAh g^-1 at 0.1 C and retaining 93% of its initial capacity after 300 cycles at 1 C. This approach can be scaled up to treat large quantities of LFP cathode material recovered from fully retired batteries, presenting a practical pathway toward large-scale recycling of spent LFP batteries in the future.