Lithium iron phosphate batteries have their own characteristics. Compared with other layered structure materials, lithium iron phosphate materials have a more stable olivine structure, so they are very stable. Even if all Li+ is extracted from the inside of lithium iron phosphate materials during charging, lithium iron phosphate materials can still maintain the FePO4 structure without structural collapse and transformation. Therefore, the degradation of lithium iron phosphate batteries during cycling is generally not caused by the loss of positive and negative electrode active materials. Neelima Paul and his team at the Technical University of Munich, Germany, used neutron diffraction to study long-term cycled lithium iron phosphate batteries (LFP/MCMB) and believed that the important factor causing the degradation of lithium iron phosphate battery life is the Li consumption caused by SEI film reconstruction and growth during cycling [1].
Neelima Paul analyzed batteries after 4750 cycles at 1C and 2 years of storage at 23°C (20% SoC) using neutron diffraction. He found that even after complete discharge (with the positive electrode in a lithium-intercalated state and the negative electrode in a lithium-deintercalated state), a significant proportion of FePO4 was still observed in the diffraction peaks of the positive electrode. In the battery with 4750 cycles, the LFP:FP ratio was 67:33, and in the battery stored for 2 years, the LFP:FP ratio was 75:25. However, no LiC6 diffraction peaks were observed in the negative electrode. This result indicates that a significant proportion of Li+ ions disappears during cycling and storage in lithium iron phosphate batteries. It also suggests that the active materials in both the positive and negative electrodes participate in the charge-discharge reaction during cycling, without any loss of active materials. Therefore, a major reason for the degradation of lithium iron phosphate batteries is the loss of Li+ ions during cycling.
Since LFP materials can maintain the stability of their crystal structure during battery cycling, we can obtain high-performance LFP materials again by simply adding appropriate amounts of Li to recycle waste LFP batteries. This can greatly reduce the production cost of LFP materials and reduce environmental pollution. Xuelei Li et al. of Tianjin University of Technology [2] designed a green and environmentally friendly process for recycling waste lithium iron phosphate batteries. The specific process steps are shown in the figure below. The biggest feature of this step is that it achieves low-cost, high-efficiency and environmentally friendly recycling based on the characteristics of lithium iron phosphate materials. As we can see from the flowchart, this process not only realizes the recycling and regeneration of positive electrode LFP materials and negative electrode graphite materials, but also recycles materials that are difficult to recycle, such as electrolytes.
Xuelei Li et al. first discharged and disassembled the waste lithium iron phosphate batteries. The residual electrolyte was treated with low-concentration NaOH. Based on the different physical characteristics of the solvents in the electrolyte, such as density, solubility and boiling point, DMC, DEC and EC were separated. The solvent salt LiPF6 decomposed in the aqueous solution, as shown in the following formula, and could then be recovered by filtration.
