Factors affecting the internal resistance of lithium-ion batteries
With the use of lithium batteries, the performance of the battery is constantly attenuated, which is mainly manifested in capacity attenuation, increase in internal resistance, and decrease in power. Therefore, the factors affecting the internal resistance of the battery are expounded in combination with the battery structure design, the performance of raw materials, the process technology and the use conditions.
1.Structural Design Impact
In the battery structure design, in addition to the riveting and welding of the battery structure itself, the number, size, and position of the battery tabs directly affect the internal resistance of the battery. To a certain extent, increasing the number of tabs can effectively reduce the internal resistance of the battery. The position of the tab also affects the internal resistance of the battery. The internal resistance of the wound battery with the tab position at the head of the positive and negative pole pieces is the largest. Compared with the wound battery, the laminated battery is equivalent to dozens of small batteries connected in parallel. , its internal resistance is smaller.
2.Influence of raw material properties
(1)Positive and negative active materials –In lithium batteries, the positive electrode material is the lithium storage side, which determines the performance of the lithium battery. The positive electrode material mainly improves the electronic conductivity between particles through coating and doping. For example, the strength of the PO bond is enhanced after doping with Ni, the structure of LiFePO4/C is stabilized, and the unit cell volume is optimized, which can effectively reduce the charge transfer resistance of the cathode material. The large increase of activation polarization, especially the activation polarization of the negative electrode, is the main reason for the serious polarization. Reducing the particle size of the anode particles can effectively reduce the anode activation polarization. When the anode solid phase particle size is reduced by half, the activation polarization can be reduced by 45%. Therefore, in terms of battery design, research on the improvement of the positive and negative electrode materials themselves is also essential.
(2)Conductive agent –Graphite and carbon black are widely used in the field of lithium batteries because of their good properties. Compared with the graphite-based conductive agent, the battery with carbon black-based conductive agent added to the positive electrode has better rate performance, because the graphite-based conductive agent has a flaky particle morphology, which causes a large increase in the pore tortuosity at high rates, and is prone to Li liquid phase diffusion. Process limiting the phenomenon of discharge capacity. The internal resistance of the battery added with CNTs is lower, because the fibrous carbon nanotubes are in line contact with the active material compared to the point contact between graphite/carbon black and the active material, which can reduce the interface impedance of the battery.
(3)current collector –Reducing the interface resistance between the current collector and the active material and improving the bonding strength between the two are important means to improve the performance of lithium batteries. Coating conductive carbon coating on the surface of aluminum foil and corona treatment of aluminum foil can effectively reduce the interfacial impedance of the battery. Compared with ordinary aluminum foil, the use of carbon-coated aluminum foil can reduce the internal resistance of the battery by about 65%, and can reduce the increase in the internal resistance of the battery during use. The AC internal resistance of corona-treated aluminum foil can be reduced by about 20%. In the commonly used SOC range of 20% to 90%, the DC internal resistance is generally small and the increase is gradually smaller with the increase of the discharge depth.
Diaphragm–The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the pores of the separator. The liquid absorption and wetting ability of the separator is the key to the formation of a good ion flow channel. When the separator has a higher liquid absorption rate and porous structure, it can be improved. Conductivity reduces battery impedance and improves battery rate capability. Compared with ordinary base film, ceramic diaphragm and glued diaphragm can not only greatly improve the high temperature shrinkage resistance of the diaphragm, but also enhance the liquid absorption and wetting ability of the diaphragm. Adding SiO2 ceramic coating on the PP diaphragm can make the liquid absorption of the diaphragm. volume increased by 17%. Coating 1μm PVDF-HFP on the PP/PE composite separator, the liquid absorption rate of the separator increased from 70% to 82%, and the internal resistance of the cell decreased by more than 20%.
In terms of process technology and use conditions, the factors that affect the internal resistance of the battery mainly include:
3.the influence of process factors
(1)slurry dispersion during slurry mixing affects whether the conductive agent can be uniformly dispersed in the active material and is in close contact with it, which is related to the internal resistance of the battery. By increasing the high-speed dispersion, the uniformity of slurry dispersion can be improved, and the internal resistance of the battery is smaller. By adding surfactant, the distribution uniformity of the conductive agent in the electrode can be improved, and the electrochemical polarization can be reduced and the discharge median voltage can be improved.
(2)Coating surface density is one of the key parameters of battery design. When the battery capacity is constant, increasing the electrode surface 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. Therefore, within a certain range , the internal resistance of the battery decreases as the areal density increases. The migration and detachment of solvent molecules during coating and drying are closely related to the temperature of the oven, which directly affects the distribution of the binder and conductive agent in the pole piece, which in turn affects the formation of the conductive grid inside the pole piece. Temperature is also an important process for optimizing battery performance.
(3)Rolling 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 contacts between the particles, the more conductive bridges and channels, the more the battery Impedance decreases. The control of the compaction density is mainly achieved by rolling the 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. And after the battery is cycled, the surface of the positive electrode of the battery with a larger rolling thickness will have cracks, which will further increase the contact resistance between the active material on the surface of the pole piece and the current collector.
(4)Pole Piece Turnaround Time Different shelving time of the positive electrode sheet has a great influence on the internal resistance of the battery. When the shelving time is short, the internal resistance of the battery increases slowly due to the interaction between the carbon coating on the surface of the lithium iron phosphate and the lithium iron phosphate. When the shelving 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 bonding effect of the binder. Therefore, the turnaround time of the pole piece needs to be strictly controlled in actual production.
Injection 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 range of the solvent, and is also affected by the concentration of lithium salt and the size of the anion. In addition to the optimization study of conductivity, the amount of liquid injection and the soaking time after liquid injection also directly affect the internal resistance of the battery. A small amount of liquid injection or insufficient soaking time will cause the internal resistance of the battery to be too large, thus affecting the battery capacity play.
4.Conditions of use affect
(1)Temperature The effect of temperature on the internal resistance is obvious. The lower the temperature, the slower the ion transmission inside the battery, and the greater the internal resistance of the battery. The battery impedance can be divided into bulk impedance, SEI membrane impedance and charge transfer impedance. The bulk impedance and SEI membrane impedance are mainly affected by the ionic conductivity of the electrolyte, and the change trend at low temperature is consistent with the change trend of the electrolyte conductivity. Compared with the increase of bulk impedance and SEI film resistance at low temperature, the charge reaction impedance increases more significantly with decreasing temperature. Below -20°C, the charge reaction impedance accounts for almost 100% of the total internal resistance of the battery.
(2)SOC When the battery is in different SOC, its internal resistance is also different, especially the DC internal resistance directly affects the power performance of the battery, which in turn reflects the battery performance of the battery in the actual state: the DC internal resistance of the lithium battery varies with the depth of discharge DOD of the battery In the 10%~80% discharge range, the internal resistance is basically unchanged, and generally the internal resistance increases significantly at a deeper discharge depth.
(3)Storage As the storage time of the lithium-ion battery increases, the battery continues to age, and its internal resistance continues to increase. Different types of lithium batteries have different degrees of internal resistance change. The internal resistance increase rate of LFP battery is higher than that of NCA and NCM batteries after a long storage period of 9-10 months. The rate of increase in internal resistance is related to storage time, storage temperature and storage SOC.
(4)Cycling Whether it is storage or cycling, the effect of temperature on the internal resistance of the battery is consistent. The higher the cycle temperature, the greater the increase rate of internal resistance. Different cycle intervals have different effects on the internal resistance of the battery. The internal resistance of the battery increases rapidly with the increase of the depth of charge and discharge, and the increase of the internal resistance is proportional to the increase of the depth of charge and discharge. In addition to the influence of the depth of charge and discharge in the cycle, the charge cut-off voltage also has an influence: too low or too high upper limit of the charge voltage will increase the interface impedance of the electrode, and the passivation film cannot be formed well under the too low upper limit voltage, and Too high upper voltage limit will lead to the oxidative decomposition of electrolyte on the surface of LiFePO4 electrode to form products with low conductivity.
(5)Other Vehicle lithium batteries will inevitably experience poor road conditions in practical applications, but the study found that the vibration environment of lithium batteries during application has little effect on the internal resistance of lithium batteries.
Outlook
Internal resistance is an important parameter to measure lithium-ion power performance and evaluate battery life. The larger the internal resistance, the worse the rate performance of the battery, and the faster it increases during storage and cycling. The internal resistance is related to the battery structure, battery material properties, and manufacturing process, and changes with ambient temperature and state of charge. Therefore, the development of low internal resistance batteries is the key to improving the power performance of batteries, and it is of great practical significance for battery life prediction to grasp the change law of battery internal resistance.