Sustainable recovery of lithium from spent LiFePO4 via proton circulation

Excessive chemical consumption and its concomitant secondary pollution have become persistent issues in the wet chemical recycling process of spent LiFePO4 batteries (sLFP). In this study, a prospective recycling strategy is proposed based on sustainable proton circulation between organic leachants, extractants, and strippants, whereby expensive organic leachants can be regenerated and secondary pollutants are reduced. The leaching dependence of the sLFP powder on the structure of the organic leachants is determined. Monocarboxylic acids with low pKa and short carbon chains exhibit preferable lithium leaching performance (98.7 % for formic acid), and the leaching behavior of metal impurities (Al, Cu) are coupled with organic acid complexation and solution pH. The direct separation of lithium from organic leachates is explored with acidic extractants for the first time. Benefitting from the buffering capacity toward protons and the salt-out effect of organic acid anions, Li element could be successfully extracted with di-(2-ethylhexyl) phosphoric acid and the organic acids can be significantly regenerated by proton exchange equilibrium. Dilute H3PO4 is employed to selectively strip Li and contribute protons to regenerate the extractant, where the separation factors of Li/Cu and Li/Al are 148.9 and 54.1, respectively. Through two-stage counter-current extraction and stripping processes, the extraction and stripping efficiency of Li could enhance to 98.62 % and 99.38 %, respectively. Thus, the Li overall yield in the leaching-extraction-stripping processes is calculated as 96.73 %, exhibiting promising prospects for proposed proton circulation strategy.