Effect Of Trace Water In Lithium Hexafluorophosphate Electrolyte On The Performance Of NCM/Graphite Battery

Lithium-ion batteries（LIBs）have attracted much attention because of their unique advantages such as long service life,less environmental pollution,and wide temperature range.So far their application fields are becoming more and more extensive,and higher requirements for the performance are pursued.Academia and industry have also been committed to realizing more excellent rate capability,wide temperature range operation,long cycle life,and other electrochemical performance of LIBs.As one of the major components of the lithium-ion battery,electrolyte plays a crucial role in the devices,However,the water content in electrolyte is a key factor that needs to be strictly controlled in the production process.The existence of trace water can lead to the decomposition of lithium salt in the electrolyte,and produces LIF and highly corrosive HF,which have the certain corrosion damage effect on the LIBs,This obviously leads to the reduction of electrochemical properties and safety of the battery,and causes irreversible loss to the capacity of the battery.Controlling the water content in electrolyte needs to consume a lot of energy and bring high production cost.Therefore,exploring the compatible range of trace water content in electrolyte to balance production cost and battery product quality has become the focus of current research.1.In this study,10 kinds of LiPF₆-EC/DEC electrolytes with different trace water contents are taken as the research object to explore the effects of trace water in the electrolyte upon the performances of LIBs,such as the rate capability,discharge capacity at different temperature and so on.The results show that the performances are closely related to the water contents in the electrolyte.When the water contents are in the range of 19.73-70.1 ppm,an appropriate amount of trace water helps to form a uniform and dense SEI film,reducing the embedding of electrolyte solvent into the negative electrode to occupy the Li⁺insertion vacancy,and improving the LIBs rate capability.When the content of trace water in electrolyte exceeds 70.1 ppm,it will consume too many lithium ions,increase the capacity loss and damage the rate capability of the battery.2.If the failure criteria is set as a capacity retention rate of 80%after 800 cycles of battery,the critical value of trace water in the electrolyte is 156 ppm at 30℃.At the same time,based on the previous researches on the modeling of the performances of LIBs,combined with a large number of experimental results obtained from electrochemical investigation,a semi-empirical model of battery capacity attenuation has been established in our work,The effects of trace water contents in electrolytes on the attenuation model of battery are systematically compared,and the attenuation activation energy of the battery is obtained with Arrhenius formula.The results show that when the content of trace water in the electrolyte is 70.1 ppm,the battery has the minimum capacity attenuation constants at 30 and 50℃,which are 0.77×10^-4and2.58×10^-4n^-1,respectively.This shows that even if a small amount of water is inevitably introduced into LIBs in the manufacturing process,it does not necessarily become a harmful factor in battery performances.If the water content in the electrolyte can be controlled to be about 70.1 ppm or less,that is,the dew point in the drying room of the lithium-ion battery manufacturer where the product line is located can be set at about-35℃to-40℃,which can reduce the production cost of controlling the water content and ensure the quality of battery products at the same time.