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The influencing factors of the internal resistance of lithium-ion batteries include electrode and tab design, raw materials, manufacturing process, application and many other aspects. This article mainly discusses the impact on the internal resistance of the battery from the aspects of cell manufacturing process and process control.
Slurry mix process: The uniformity of the slurry dispersion when mixing the slurry affects whether the conductive agent can be evenly dispersed in the active material in close contact with it, which is related to the internal resistance of the battery. By increasing the high-speed dispersion, the uniformity of the slurry dispersion can be improved, and the internal resistance of the battery will be smaller. By adding a surfactant, the uniformity of the distribution of the conductive agent in the electrode can be improved, and the electrochemical polarization can be reduced and the median discharge voltage can be increased.
Coating process: Area density is one of the key parameters of battery design. When the battery capacity is constant, increasing the electrode area density will inevitably reduce the total length of the current collector and the separator, and the ohmic internal resistance of the battery will decrease accordingly. Within a certain range, the internal resistance of the battery decreases as the areal density increases. The migration and separation of solvent molecules during coating and drying is closely related to the temperature of the oven, which directly affects the distribution of binder and conductive agent in the electrode, and then affects the formation of the conductive grid inside the electrode. Therefore, the coating and drying process temperature is also an important process for optimizing battery performance.
Rolling process: To a certain extent, the internal resistance of the battery decreases with the increase of the compaction density, because the compaction density increases, the distance between the raw material particles decreases, the more the contact between the particles, the more the conductive bridges and channels, the lower the battery impedance. The control of compaction density is mainly achieved by rolling thickness. Different rolling thicknesses have a greater impact on the internal resistance of the battery. When the rolling thickness is large, the contact resistance between the active material and the current collector increases due to the failure of the active material to be rolled tightly, and the internal resistance of the battery increases. In addition, cracks are generated on the positive electrode surface of the battery with a large thickness after the battery is rolled, which will further increase the contact resistance between the surface active material of the electrode and the current collector.
Electrode turnaround time control: different shelf time of the positive electrode has a greater impact on the internal resistance of the battery. When the shelf time is short, the internal resistance of the battery is affected by the force of the carbon coating on the surface of the lithium iron phosphate and the lithium iron phosphate itself. The increase is slower. When the storage time is longer (more than 23h), the internal resistance of the battery increases significantly due to the combined effect of the reaction between lithium iron phosphate and water and the adhesion of the adhesive. Therefore, it is necessary to strictly control the turnaround time of electrode in actual production.
Liquid injection process: The ionic conductivity of the electrolyte determines the internal resistance and rate characteristics of the battery. The conductivity of the electrolyte is inversely proportional to the viscosity of the solvent, and is also affected by the concentration of lithium salt and the size of anions. In addition to the optimization research on the conductivity, the injection volume and the infiltration time after injection also directly affect the internal resistance of the battery. Small injection volume or insufficient infiltration time will cause the internal resistance of the battery to be too large, thereby affecting the battery capacity performance.
The 26650 battery cell designed and manufactured by Power Long Battery is subject to rigorous process design and strict manufacturing process control. The battery cell has obvious advantages of low internal resistance and good consistency.