Direct Regeneration of Spent Lithium Iron Phosphate via a Low-Temperature Molten Salt Process Coupled with a Reductive Environment

Abstract

A huge number of spent lithium-ion batteries (LIBs) have caused serious problems such as resource waste and environmental pollution. Lithium iron phosphate (LFP) is one of the major cathode materials in the spent LIBs. It is urgently needed to develop a safe, environmentally friendly, and cost competitive approach to regenerate the LFP cathode collected from the spent LIBs. The nitrate molten salt process has been utilized to regenerate layered cathode materials, which are nevertheless non-compatible with the LFP regeneration process. The LFP crystal lattice will be destroyed during the molten salt process by the oxidative environment, where Fe(II) is oxidized to Fe(III). A new approach is proposed in this work to tackle this issue, where a low-temperature molten salt process is coupled with a reductive environment to suppress oxidation of Fe(II). In detail, lithium nitrate is used as a molten salt medium and lithium source simultaneously. Sucrose is used as a carbon source to provide a reductive environment. Through a short molten-salt relithiation step at 300 °C and further annealing process at 650 °C, LFP particles with a lithium-deficient and damaged structure can be successfully recovered. The rapid lithium replenishment process exposes more (101) crystal planes facilitating lithium-ion transportation. As a result, the regenerated LFP delivers a specific capacity of 145 mAh g–1 at 0.5C, which is more than a 13% increase relative to the spent LFP and has a better rate performance than pristine LFP at 5C. In addition, we also point out that the LFP is converted to Li3PO4 with the increase in lithium source and the extension of treatment time. This work provides a new promising way to regenerate spent LFP cathodes.