Talking about the recycling and utilization of lithium iron phosphate batteries

by:CTECHi     2021-07-31

Lithium iron phosphate has good cycle performance, low price, good safety, and has the potential for fast charging. Therefore, with the rapid development of the domestic electric vehicle industry, the demand for lithium iron phosphate batteries has also increased rapidly. At present, electric buses These types of cars with higher safety requirements are basically lithium iron phosphate batteries. As these lithium iron phosphate batteries enter the end of life, we have to face a thorny problem-the recycling of used batteries. Lithium iron phosphate battery recycling has its own characteristics. Compared with other layered structural materials, lithium iron phosphate material has a more stable olivine structure, so it is very stable, even if all Li+ is removed from the lithium iron phosphate material 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 the cycle is generally not caused by the loss of positive and negative active materials. Neelima Paul, Technical University of Munich, Germany He and his team used the method of neutron diffraction to study the long-term cycling lithium iron phosphate battery (LFP/MCMB) that the main factor causing the life decline of the lithium iron phosphate battery is the reconstruction and growth of the SEI film during the cycle. Li consumption caused by [1]. NeelimaPaul used the method of neutron diffraction to analyze the battery after 4750 cycles of 1C cycle and 2 years (20%SoC) stored at 23℃, and found that even after the battery is fully discharged (the positive electrode is in the state of lithium insertion, the negative electrode is Lithium state), but a considerable proportion of FePO4 is still observed in the diffraction peak of the positive electrode. The ratio of LFP:FP is 67:33 in the battery cycled 4750 times, and the ratio of LFP:FP is 75: 25, while the diffraction peak of LiC6 is not observed in the diffraction peak of the negative electrode. This result shows that a considerable proportion of Li+ in lithium iron phosphate batteries 'disappears out of thin air' during cycling and storage. It also shows that both the positive and negative active materials can participate in the charge and discharge reaction during the cycle, and there is no active material. Therefore, the main reason for the degradation of lithium iron phosphate batteries is the loss of Li during the cycle. Since the LFP material can maintain the stability of the crystal structure during the battery cycle, for the recycling of waste LFP batteries, we only need to supplement the appropriate Li to regain good performance LFP materials, which can greatly reduce The production cost of LFP material reduces environmental pollution. XueleiLi et al. [2] of Tianjin University of Technology 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 realizes low-cost, high-efficiency and environmentally friendly recycling based on the characteristics of the lithium iron phosphate material. From the flow chart, we can see that this process not only realizes the recovery and regeneration of the positive electrode LFP material and the negative electrode graphite material, but also recovers the electrolyte and other materials that are difficult to recover. XueleiLi and others first discharged and dismantled the discarded lithium iron phosphate battery. The remaining electrolyte was treated with low-concentration NaOH. According to the different density, solubility and boiling point of the solvent in the electrolyte, the physical characteristics of the DMC were realized. For the separation of, DEC and EC, the solvent salt LiPF6 will decompose in the aqueous solution, as shown in the following formula, and then it can be recovered by filtration. After the positive electrode LFP material separated in this process is mixed with a certain amount of Li2CO3, the regenerated LFP material can be obtained by performing heat treatment at different temperatures in an Ar/H2 atmosphere. In order to ensure that the recycled and recycled LFP materials can have good performance, XueleiLi conducted LFP regeneration experiments at 600, 650, 700, 750 and 800 degrees Celsius respectively, and performed performance tests using button half-cells. The results are shown in the following table. From the table, we can see that the capacity of the LFP material without regeneration treatment is about 143mAh/g, and the capacity of the LFP material after 650 degrees Celsius treatment has increased to 147mAh/g, but after other temperature treatments, LFP On the contrary, the capacity of materials has decreased to varying degrees. At the same time, we have also noticed that the first time efficiency of the regenerated material is significantly lower than that of the unregenerated LFP material. This is mainly caused by the presence of impurity phases in the regenerated LFP. XueleiLi's research shows that the heat treatment time can be appropriately extended to improve The first efficiency of LFP material. Research on the electrochemical performance of recycled LFP shows that heat treatment can significantly improve the cycle performance of LFP materials (as shown in figure a below), while heat treatment also significantly improves the rate performance of LFP materials (as shown in figure b below). XueleiLi’s recycling method for lithium iron phosphate batteries combines the structural stability of lithium iron phosphate materials. It does not use traditional methods such as acid treatment and recovery of valuable elements. Instead, it directly regenerates them, with low utilization. The cost of obtaining high-performance recycled LFP materials, and the process also realizes the recovery of electrolyte and other materials, which greatly reduces the environmental pollution during the recycling process of lithium iron phosphate batteries. With a large number of lithium iron phosphate power batteries being scrapped and entering the recycling stage, the battery recycling market has shown explosive growth. In order to avoid secondary pollution to the environment during the recycling process, we need to adopt more green and environmentally friendly recycling methods. XueleiLi’s research provides us with useful reference

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