What Factors Affect the Internal Resistance of the Battery in the Production Process of Lithium Batteries?
The final process in the production of lithium batteries is to classify and screen the lithium batteries to ensure the consistency of the batteries constituting the battery modules and the excellent performance of the battery modules. As we all know, the use of modules with high consistency batteries has a longer service life, and battery modules with poor consistency are prone to overcharge and overdischarge due to the bucket effect, and battery life attenuation is accelerated. For example, inconsistent battery capacity will cause inconsistent discharge depths of individual cells in the battery pack. A battery with a small capacity and poor performance will reach a fully charged state in advance, causing a battery with a large capacity and a good performance to fail to reach a fully charged state. The inconsistency of the battery voltage will lead to the mutual charging of the single cells in the parallel battery pack, and the battery with the higher voltage will charge the battery with the lower voltage, which will accelerate the degradation of the battery performance and consume the energy of the entire battery pack. The capacity loss of a battery with a large self-discharge rate is large, and the inconsistency of the battery self-discharge rate will cause a difference in the state of charge and voltage of the battery and affect the performance of the battery pack. Waiting for the differences between these batteries, long-term use will affect the service life of the entire module.
The classification and screening of the battery is to prevent the inconsistent batteries from flowing out at the same time. Among them, battery internal resistance and self-discharge test are mandatory items. Generally speaking, battery internal resistance is divided into ohmic internal resistance and polarization internal resistance. The ohmic internal resistance is composed of electrode materials, electrolyte, diaphragm resistance, and contact resistance of various parts, including electronic resistance, ionic resistance, and contact resistance. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including electrochemical polarization internal resistance and concentration polarization internal resistance. The ohmic internal resistance of the battery is determined by the total electrical conductivity of the battery, and the polarization internal resistance of the battery is determined by the solid phase diffusion coefficient of lithium ions in the electrode active material. In general, the internal resistance of lithium batteries cannot be separated from several major aspects such as process design, materials themselves, and environment, which will be analyzed and interpreted below.
1. Process design
(1) The content of conductive agent in the positive and negative electrode formulations is low, resulting in a large transmission impedance of electrons between the material and the current collector, that is, a high electronic impedance. Lithium batteries will heat up quickly during use. However, this is determined by the design of the battery. For example, to consider the rate performance of a power battery, a higher proportion of conductive agent is required, which is suitable for large rate charge and discharge. Capacity batteries consider a little more capacity, and the ratio of positive and negative materials will be higher. It was decided at the beginning of the battery design and cannot be easily changed.
(2) There are too many binders in the positive and negative electrode formulations. The binders are generally high-molecular materials (PVDF, SBR, CMC, etc.) with strong insulation properties. The higher proportion of binder in the original ratio is beneficial to improve the peel strength of the pole piece, but it is not good for internal resistance. When designing the battery, try to coordinate the relationship between the adhesiveness and the amount of binder. This should focus on the dispersion of the binder, that is, the slurry preparation process, as far as possible to ensure that the binder is evenly dispersed.
(3) The ingredients are unevenly dispersed, the conductive agent is not sufficiently dispersed, and a good conductive network structure is not formed. As shown in FIG. 2, A is a case where the conductive agent is poorly dispersed, and B is a case where the dispersion is good. When the amount of conductive agent added is the same, the change of the stirring process will affect the dispersion of the conductive agent, which has a greater impact on the internal resistance of the battery.
(4) The binder is not completely dissolved, and some micelle particles are present, resulting in high internal resistance of the battery. Whether it is dry blending, semi-dry blending or wet blending process, the binder powder is required to be completely dissolved, and efficiency cannot be excessively pursued, and the objective requirement that the binder needs a certain time to fully dissolve is ignored.
(5) The compact density of the pole piece will affect the internal resistance of the battery. The compact density of the pole piece is small, and the porosity between the particles inside the pole piece is high, which is not conducive to the transmission of electrons, and the internal resistance of the battery is high. When the pole piece is compacted too much, it may cause the electrode powder particles to be crushed by overpressure. After crushing, the electron transmission path becomes longer, which is not conducive to battery charge and discharge performance. It is important to choose the right compaction density.
(6) The positive and negative electrode lugs are poorly welded to the current collector, virtual welding occurs, and the internal resistance of the battery is high. Welding parameters should be selected when welding, and the welding parameters such as welding power, amplitude, time, etc. should be optimized through DOE, and the welding strength and appearance should be used to determine the welding quality.
(7) Poor winding or lamination failure. The gap between the separator, positive electrode, and negative electrode is large, and the ion resistance is large.
(8) The battery electrolyte does not fully infiltrate the positive and negative pole pieces and the separator, and the insufficient design margin of the electrolyte will also cause a large battery ion resistance.
(9) The formation process is poor, and the SEI of the graphite negative electrode surface is unstable, which affects the internal resistance of the battery.
(10) Others, such as poor packaging, poor soldering of the pole and pole posts, battery leakage, and high moisture content, have a greater impact on the internal resistance of the lithium battery.
(1) The positive and negative electrode materials themselves have large resistance.
(2) Influence of diaphragm material. For example, membrane thickness, porosity, pore size, etc. The thickness is related to the internal resistance, and the thinner the internal resistance is, the smaller the internal resistance is, thereby realizing high-power charge and discharge. It is as small as possible under a certain mechanical strength, and the thicker the puncture strength, the better. Diaphragm porosity and pore size are related to the impedance of ion transmission. If the pore size is too small, the ion impedance will increase. If the pore size is too large, the fine positive and negative electrode powders may not be completely isolated, which may easily cause a short circuit or be punctured by lithium dendrites.
(3) Influence of electrolyte material. The ion conductivity and viscosity of the electrolyte are related to the ion impedance. The greater the ion transmission impedance, the greater the internal resistance of the battery and the more serious the polarization during charging and discharging.
(4) Influence of positive PVDF material. When the addition ratio of PVDF is high or the molecular weight is large, the internal resistance of the lithium battery will also be high.
(5) Influence of positive electrode conductive material. The selection of the type of conductive agent is also more critical. For example, SP, KS, conductive graphite, CNT, graphene, etc. due to different morphologies, the performance difference used in lithium batteries is relatively large. is very important.
(6) The influence of the positive and negative pole lug materials. The thickness of the tab is poor, the conductivity is poor, the material used is not high in purity, the conductivity is also poor, and the internal resistance of the battery is high.
(7) The copper foil is poorly oxidized and welded, and the aluminum foil material has poor conductivity or oxide on the surface, which can also cause high internal resistance of the battery.
3. Other aspects
(1) Deviation of internal resistance test instrument. The instrument should be calibrated regularly to prevent inaccurate test results caused by inaccurate instruments.
(2) The internal resistance of the battery is abnormal due to improper operation.
(3) Poor production environment, such as dust and moisture control are not strict. Excessive dust in the workshop will increase the internal resistance of the battery and increase self-discharge. High moisture in the workshop will also be detrimental to the performance of lithium batteries.