Delithiation and lithiation of LiFePO$_4$: Implications for direct Li extraction from synthetic solutions and geothermal brines

The demand for Li is and will be increasing in the future, and the development of a direct Li extraction (DLE) technology from unconventional resources, like geothermal brines, may contribute to a resilient supply in the future. This study investigates the deintercalation from and intercalation of Li in LiFePO$_4$ (LFP) at 25–80 °C, near neutral to acidic pH and the effect of high salinity on the Li extraction performance. The (de-)lithiation is a fully reversible redox process between triphylite and heterosite. Lithium is delithiated from LFP using 0.1 M Na$_2$S$_2$O$_8$ at 42–43 mg/g. The lithiation kinetics increase with temperature, but show a complex relationship to reducing agent (Na$_2$S$_2$O$_3$) concentration. The maximum re-intercalation is achieved in synthetic LiCl + 0.5 M Na$_2$S$_2$O$_3$ solution at 39 mg/g, 25 °C and 7 days, whereas 27 mg/g and 1.3 mg/g Li are intercalated to LFP within 3–4 h in experiments with Bruchsal and synthetic Neustadt-Glewe geothermal brines at 60 °C, respectively. At optimal parameters, >99 % Li are recovered from both geothermal brines in laboratory experiments. This shows that LFP can be used for DLE from geothermal brines under specific conditions in a purely chemical process.