Electric potential-determined redox intermediates for effective recycling of spent lithium-ion batteries

Abstract

Recycling of spent lithium-ion batteries (LIBs) by hydrometallurgy faces a major problem of consuming an excessive amount of acid and requiring different redox additives for effective metal leaching. Reducing chemical consumption in the recycling process is highly desirable for the environmentally friendly and sustainable development of renewable energy. In this study, an electrochemical approach for analyzing electric potentials was developed to evaluate redox abilities. Based on the results, a salt leaching method was proposed using water-soluble NH₄Fe(SO₄)₂ as a redox intermediate for synergistic recovery of valuable metals from spent ternary lithium-ion batteries (NCM) and LiFePO₄ batteries (LFP). More than 97% of the Li, Mn, Co and Ni from the mixed cathode can be leached under mild conditions (50 °C, 30 min), with PO₄³⁻ being completely retained in residues. The first-step reaction between Fe³⁺ and LFP to release Fe²⁺ proceeded rapidly, whereas the following slower reaction between Fe²⁺ and NCM was the rate-controlling process. Thermodynamic analysis of leaching solutions was carried out systematically and shown to be feasible for designing a precipitate recovery process for both LFP and NCM battery systems, with the recovered products being used for regenerating new materials. The synergistic salt-leaching treatment of spent LFP and NCM batteries based on electrochemical principles helped achieve high efficiency and high selectivity with a great benefit to preserving the environment.