The last step in the production of lithium battery is to grade and screen the lithium battery to ensure the consistency of the battery module and the excellent performance of the battery module. As is known to all, modules composed of batteries with high consistency have longer service life, while modules with poor consistency are prone to over-charge and over-discharge due to bucket effect, and their battery life attenuation is accelerated. For example, different battery capacities may cause different discharge depths of each battery string. The batteries with small capacity and poor performance will reach the full charge state in advance. As a result, the batteries with large capacity and good performance cannot reach the full charge state. Inconsistent battery voltages cause each battery in a parallel string to charge each other. The battery with a higher voltage charges the battery with a lower voltage, which speeds up battery performance degradation and consumes energy of the entire battery string. A battery with a high self-discharge rate has a large capacity loss. Inconsistent self-discharge rates cause differences in the charged status and voltage of batteries, affecting the performance of battery strings. And so these battery differences, long-term use will affect the life of the entire module.

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FIG. 1.OCV- operating voltage – polarization voltage diagram

The battery classification and screening is to avoid the discharge of inconsistent batteries at the same time. The battery internal resistance and self-discharge test is a must. Generally speaking, battery internal resistance is divided into ohm internal resistance and polarization internal resistance. Ohm internal resistance consists of electrode material, electrolyte, diaphragm resistance and contact resistance of each part, including electronic impedance, ionic impedance and contact impedance. 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 resistance of the battery is determined by the total conductivity of the battery, and the polarization resistance of the battery is determined by the solid phase diffusion coefficient of lithium ion in the electrode active material. In general, the internal resistance of lithium batteries is inseparable from the process design, the material itself, the environment and other aspects, which will be analyzed and interpreted below.

First, process design

(1) The positive and negative electrode formulations have low content of conductive agent, resulting in large electronic transmission impedance between the material and the collector, that is, high electronic impedance. Lithium batteries heat up faster. However, this is determined by the design of the battery, for example, the power battery to take into account the rate performance, it requires a higher proportion of conductive agent, suitable for large rate charge and discharge. Capacity battery is a bit more capacity, positive and negative material proportion will be a bit higher. These decisions are made at the beginning of the battery’s design and cannot be easily changed.

(2) there is too much binder in the positive and negative electrode formula. The binder is generally a polymer material (PVDF, SBR, CMC, etc.) with strong insulation performance. Although the higher proportion of binder in the original ratio is beneficial to improve the stripping strength of the poles, it is disadvantageous to the internal resistance. In the battery design to coordinate the relationship between binder and binder dosage, which will focus on the dispersion of binder, that is, slurry preparation process, as far as possible to ensure the dispersion of binder.

(3) The ingredients are not evenly dispersed, the conductive agent is not fully dispersed, and a good conductive network structure is not formed. As shown in Figure 2, A is the case of poor dispersion of conductive agent, and B is the case of good dispersion. When the amount of conductive agent is the same, the change of stirring process will affect the dispersion of conductive agent and the internal resistance of the battery.

Figure 2. Poor dispersion of conductive agent (A) Uniform dispersion of conductive agent (B)

(4) The binder is not completely dissolved, and some micelle particles exist, resulting in high internal resistance of the battery. No matter dry mixing, semi-dry mixing or wet mixing process, it is required that the binder powder is completely dissolved. We cannot pursue efficiency too much and ignore the objective requirement that the binder needs a certain time to be fully dissolved.

(5) The electrode compaction density will affect the internal resistance of the battery. The compact density of the electrode plate is small, and the porosity between the particles inside the electrode plate is high, which is not conducive to the transmission of electrons, and the internal resistance of the battery is high. When the electrode sheet is compacted too much, the electrode powder particles may be overcrushed, and the electron transmission path becomes longer after crushing, which is not conducive to the charge and discharge performance of the battery. It is important to choose the right compaction density.

(6) Bad welding between positive and negative electrode lug and fluid collector, virtual welding, high battery resistance. Appropriate welding parameters should be selected during welding, and welding parameters such as welding power, amplitude and time should be optimized through DOE, and the quality of welding should be judged by welding strength and appearance.

(7) poor winding or poor lamination, the gap between the diaphragm, positive plate and negative plate is large, and the ion impedance is large.

(8) The battery electrolyte is not fully infiltrated into the positive and negative electrodes and diaphragm, and the electrolyte design allowance is insufficient, which will also lead to large ionic impedance of the battery.

(9) The formation process is poor, the graphite anode surface SEI is unstable, affecting the internal resistance of the battery.

(10) Others, such as poor packaging, poor welding of pole ears, battery leakage and high moisture content, have a great impact on the internal resistance of lithium batteries.

Second, materials

(1) The resistance of anode and anode materials is large.

(2) Influence of diaphragm material. Such as diaphragm thickness, porosity size, pore size and so on. Thickness is related to internal resistance, the thinner the internal resistance is smaller, so as to achieve high power charge and discharge. As small as possible under a certain mechanical strength, the thicker the puncture strength is the better. The pore size and pore size of diaphragm are related to the impedance of ion transport. If the pore size is too small, it will increase the ion impedance. If the pore size is too large, it may not be able to completely isolate the fine positive and negative powder, which will easily lead to short circuit or be pierced by lithium dendrite.

(3) Influence of electrolyte material. The ionic conductivity and viscosity of the electrolyte are related to the ionic impedance. The greater the ionic transfer impedance, the greater the internal resistance of the battery, and the more serious the polarization in the charging and discharging process.

(4) Influence of positive PVDF material. A high proportion of PVDF or high molecular weight will also lead to high internal resistance of lithium battery.

(5) Influence of positive conductive material. The selection of the type of conductive agent is also key, such as SP, KS, conductive graphite, CNT, graphene, etc., due to the different morphology, the conductivity performance of lithium battery is relatively different, it is very important to select the conductive agent with high conductivity and suitable for use.

(6) the influence of positive and negative pole ear materials. The thickness of the pole ear is thin, the conductivity is poor, the purity of the material used is not high, the conductivity is poor, and the internal resistance of the battery is high.

(7) the copper foil is oxidized and welded badly, and the aluminum foil material has poor conductivity or oxide on the surface, which will also lead to high internal resistance of the battery.

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(1) Internal resistance test instrument deviation. The instrument should be checked regularly to prevent inaccurate test results caused by inaccurate instrument.

(2) Abnormal battery internal resistance caused by improper operation.

(3) Poor production environment, such as loose control of dust and moisture. Workshop dust exceeds the standard, will lead to the increase of internal resistance of the battery, self-discharge aggravated. Workshop moisture is high, will also be detrimental to lithium battery performance.