Impact Of Electrode Water Content On The Performance Of LiFePO₄ Based Lithium-ion Battery

Due to long cycle life and good security performance LiFePO₄ based lithium-ion batteries have been applied in electric vehicle power battery and realized commercial production.However the low consistency and poor cycle stability still exist.There are two ways to improve the cell consistency,one is to add the balance management for battery system and the other one is to optimize the technology of battery production to get the cell with the same resistance,voltage and capacity.In order to improve the cell consistency,the drying process of pole piece,the method to control the electrode water content and the suitable electrode water content have been researched.NanometerLiFePO₄ material and ordinary commercial LiFePO₄ material were used in the preparation of organic and aqueous pole pieces which were vacuum dried by 70?,100?,120?and test water content.The test results show that with the drying temperature increasing the water content of all the pole pieces drops.When the pole pieces were exposed to the air the water content increases or drops with the exposed time.100?is the ideal vacuum drying temperature for pole pieces.Compared to organic pole piece the aqueous pole piece is easier to absorb the moisture in the environment with higher water content.The application of nanometer LiFePO₄ material can improve lithium-ion battery's rate performance but the pole pieces are more easily to absorb moisture in the environment due to the larger specific surface area.So the nanometer LiFePO₄ material for pole pieces need stricter water content control technology.Cathode pole pieces with water contents of 300-700ppm are obtained under different expose time to air and measured by moisture test instrument.Then the cathode pole pieces are prepared to pouch lithium-ion battery and test electrochemical performance.The results show that cycle performance and rate performance are closely related to the water content of the electrode.When the water content is up to 600ppm both the cycle stability and rate performance fall.After 200 cycles both the LiFePO₄ based lithium-ion battery's internal resistance and the electrochemical reaction impedance increase.The pole pieces and powder material of pole pieces after running200 cycles are tested with XRD,SEM and particle size distribution.The results show that when the water content is up to 600ppm particle fracture phenomenon appears and the particle size increases,but the position of the main diffraction peak nearly keeps the same and the phase of cathode material hasn't changed.