What are the factors that affect the internal resistance of lithium batteries?

Temperature and ambient temperature are important factors affecting various resistances. In the case of lithium batteries, temperature affects the activity of electrochemical materials and directly determines the speed of electrochemical reaction and the speed of ion movement. Current or load demand, on the one hand, the magnitude of the current is directly related to the internal polarization resistance. The general trend is that the greater the current, the greater the internal polarization resistance. On the other hand, the thermal effect of current has an impact on the activity of electrochemical materials. 3.2 The battery's own factors The degree of difficulty of the positive electrode material, negative electrode material, and lithium ion insertion and extraction determines the internal resistance of the material, which is part of the concentration polarization resistance. In the electrolyte, the movement rate of lithium ions in the electrolyte is affected by the conductivity of the electrolyte and is an important part of the electrochemical polarization resistance. The diaphragm, the diaphragm's own resistance, directly constitutes a part of the ohmic internal resistance, and at the same time it hinders the movement rate of lithium ions, and forms a part of the electrochemical polarization resistance. Current collector resistance, component connection resistance, is an important part of the battery's ohmic internal resistance. The level of craftsmanship, the production of pole pieces, whether the coating is average, and what is the compaction density, the level of craftsmanship in the production process of these batteries will also have a straight impact on the internal resistance of polarization. 4 Lithium battery internal resistance measurement Lithium battery internal resistance measurement methods are generally divided into two types: DC measurement method and AC measurement method. 4.1 The DC internal resistance measurement method uses a current source to apply a short pulse to the battery and measure the difference between its terminal voltage and the open circuit voltage. Dividing this difference by the detection current is regarded as the DC internal resistance of the battery. The polarization internal resistance of lithium batteries will be affected by the magnitude of the loading current. In order to avoid this factor as much as possible, the DC measurement of internal resistance has a relatively short energizing time and a relatively large loading current. Theoretically, the smaller the measurement current, the less polarization reaction will be caused and the interference of polarization resistance will be reduced. However, because the internal resistance of the battery itself is very small, in the order of milliohms, and the current is too small, the voltage test instrument is limited by the measurement accuracy, and the interference of the measurement error on the result cannot be ruled out. Therefore, people weigh the influence of instrument accuracy and polarization internal resistance to find a measured current value that balances the relationship between the two. For general battery cells, the measured current is generally around 5C-10C, which is very large. With the increase of cell capacity, or multiple cells in parallel, the internal resistance is reduced. Therefore, if there is no improvement in the accuracy of the instrument, it is difficult to reduce the measured current. 4.2 AC internal resistance measurement method Load an AC input with a small amplitude as an excitation to the battery to monitor the response of its terminal voltage. Use a specific program to decompose the data to get the AC internal resistance of the battery. The resistance value obtained by decomposition is only related to the characteristics of the battery itself, and has nothing to do with the size of the excitation signal used. Due to the existence of battery capacitance characteristics, the frequency of the excitation signal is different, and the measured resistance value is also different. The result of software decomposition can be represented by a set of complex numbers, the horizontal axis is the real part, and the vertical axis is the imaginary part. In this way, a map is formed, the so-called AC impedance spectrum, as shown in the figure above. Through further data decomposition, one can obtain the ohmic resistance of the battery, the diffusion resistance of the SEI film, the capacitance value of the SEI film, the equivalent capacitance value of the charge transfer in the electrolyte and the charge in the electrolyte from the AC impedance spectrum. Diffusion resistance value, and then draw the battery equivalent model for further research on battery performance. An equivalent battery model, the use of internal resistance in engineering practice, as one of the key characteristics of lithium batteries, the research results on it can be used in many areas such as engineering manufacturing. Internal resistance is closely related to battery charge, so it is used for SOC estimation in battery management systems; internal resistance directly reflects the degree of battery aging. Some people use cell internal resistance as the basis for evaluating battery health SOH; monomer internal resistance consistency It directly affects the module capacity and service life after grouping, so it is widely used as a static indicator of battery cell sorting and matching group; internal resistance is also an important indicator of battery failure. In the fault diagnosis system of power lithium-ion battery packs, It is researched and used; the internal resistance is matched with the capacity loss and other indicators, and it can also be used to judge whether the battery has the phenomenon of lithium evolution. It is used in the category of echelon utilization decommissioned batteries. Disclaimer: Some pictures and content of the articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: Analysis of common terms for lithium batteries