What is the amount of free electrolyte in commercial power lithium batteries, do you know?

For lithium-ion batteries, the electrolyte is equivalent to human blood in a sense, which has a huge impact on battery performance and is essential. Regarding batteries of different chemical systems, there are differences in the amount of electrolyte injection and the retention coefficient. Specific to different power lithium battery manufacturers, due to different processes and understanding, the amount of liquid injection of their battery products is also different. Except that part of the electrolyte reacts with the negative electrode to produce SEI in the formation stage, the remaining electrolyte mainly exists in two forms: one part is infiltrated into the porous structure of the electrode, and another part is in the form of a free state. Among them, the amount of electrolyte infiltrated into the porous structure of the electrode is more difficult to measure, while the amount of free electrolyte is relatively easier to quantify through battery disassembly. In addition, the battery will gradually age after use, and theoretically the amount of electrolyte in the battery will also decrease after aging. So the question is, what is the amount of free electrolyte in the commercial power lithium battery? Natalia P. Lebedeva and others from the European Union Joint Research Center have a relatively special perspective. When most people are concerned about how to improve the electrolyte to improve battery performance, they first focus on the toxicity problems that the electrolyte may bring. [1]. At present, the commonly used electrolyte contains a variety of toxic solvents, and the use of LiPF6 may also cause HF. Once the electrolyte leaks in a confined space, a high concentration of toxic atmosphere is likely to cause harm to humans. In this work, Natalia P. Lebedeva et al. studied the amount of free electrolyte in commercial power lithium batteries. Since most of the batteries tested are from the vehicle end and include partially aging batteries, the results have high reference and practical value. The results were published on Journal of The Electrochemical Society with the title of AmountofFreeLiquidElectrolyteinCommercialLargeFormatPrismaticLi-IonBatteryCells. The specific information of the power lithium battery used in the experiment is shown in Table 1. The first and second groups are purchased bulk, fresh LFP system prismatic batteries with a battery capacity of 60Ah. The batteries of groups 3-6 are all disassembled from electric vehicles that have been running for several years. Among them, electric vehicles using batteries of groups 3, 4, and 6 rank among the top ten in the European sales list. Groups 3 and 4 are prismatic batteries of the NMC+LCO system with capacities of 38Ah and 63Ah respectively; group 5 is a soft pack battery of the NMC system with a battery capacity of 39Ah; group 6 is a soft pack battery of the LMO+LNO system , The battery capacity is 40Ah. The seventh group is a prismatic battery of the LFP system, which has been stored for three years before the experiment. The last group of batteries is of the same type as the third group. They are all disassembled from a PHEV. The only difference is that the batteries in this group are severely aging. The specific distribution of the batteries in the pack is shown in Figure 2. Before obtaining the free electrolyte, the author first performed CT observation on the battery to facilitate the determination of the appropriate drilling location. After the CT has determined the drilling position, drill holes on the surface of the square battery and soft-pack battery, and pour out part of the free electrolyte. The battery casing was then disassembled, and the remaining free electrolyte was taken out. The statistics of the amount of free electrolyte in different batteries are shown in Table 2. The first and second groups of fresh batteries are from the same company, and the two groups of batteries contain the highest amount of free electrolyte, reaching about 32g and 36g, respectively, which also indicates that the battery production company is not doing well in process control. The batteries in groups 3-6 are all disassembled from electric vehicles that have been in operation for several years. It can be seen that the amount of free electrolyte in the four groups of batteries varies greatly: the amount of free electrolyte in the sixth group of soft-packed batteries is 18.4g; Disassembling the 4 groups of prismatic batteries and the 5th group of soft-pack batteries can clearly observe that the pole pieces are in a wet state, but no electrolyte is in a free state; the free electrolyte of the 3rd group of prismatic batteries is only 3.6g. Regarding the aged battery, although the seventh battery has been stored for 3 years, the amount of free electrolyte is still 23.7g. Only one of the four batteries in the third group after aging on the vehicle side still has 0.8g of free electrolyte, and the remaining three have no free electrolyte, indicating that the aging degree of the battery at different positions of the pack is also different. . From the above results, it can also be seen that the manufacturing processes of different battery companies are different, and the amount of electrolyte used will also be different. Generally speaking, battery production companies rarely disclose the specific composition of the electrolyte used. From the small amount of information supplied by MSDS, it is understood that the solvents used in the first, second, fourth and seventh groups of batteries cover DMC, DEC, EMC, EC and EA, and all use LiPF6 as the lithium salt. In order to understand the battery electrolyte information of the 3rd and 6th groups, the author conducted a FITR analysis, and the results are shown in Figure 4. It can be roughly understood from the FITR chart that the solvents used in the two sets of batteries are the DMC, DEC, EMC, and EC systems that are also conventionally used, and PC and EA are not used.