Generally, battery internal resistance is divided into ohmic internal resistance and polarization internal resistance. Ohmic internal resistance consists of electrode material, electrolyte, diaphragm resistance and contact resistance of various parts. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reactions, including electrochemical polarization internal resistance and concentration polarization internal resistance. The ohmic internal resistance of the battery is determined by the total 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.

　　1. Ohmic internal resistance

　　Ohmic internal resistance is mainly divided into three parts, one is ionic resistance, the other is electronic resistance, and the third is contact resistance. We hope that the internal resistance of the lithium battery will be smaller and smaller, so we need to take specific measures to reduce the ohmic internal resistance for these three items.

　　1. Ionic impedance

　　Lithium battery ion impedance refers to the resistance to the transfer of lithium ions inside the battery. In lithium batteries, lithium ion migration speed and electron conduction speed play an equally important role. Ion resistance is mainly affected by positive and negative electrode materials, separators and electrolytes. To reduce ionic impedance, you need to do the following:

　　① Ensure that the positive and negative electrode materials and electrolyte have good wettability.

　　It is necessary to select an appropriate compaction density when designing the pole piece. If the compaction density is too high, the electrolyte will not be easily infiltrated and the ionic impedance will increase. For the negative electrode plate, if the SEI film formed on the surface of the active material during the first charge and discharge is too thick, it will also increase the ion resistance. At this time, the battery formation process needs to be adjusted to solve the problem.

　　②The influence of electrolyte

　　The electrolyte must have appropriate concentration, viscosity and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration between it and the positive and negative active materials. At the same time, the electrolyte also needs a lower concentration, and too high a concentration is also detrimental to its flow and infiltration. The conductivity of the electrolyte is the most important factor affecting ion impedance, which determines the migration of ions.

　　③The influence of diaphragm on ion impedance

　　The main influencing factors of the separator on ion impedance include: electrolyte distribution in the separator, separator area, thickness, pore size, porosity and tortuosity coefficient, etc. For ceramic diaphragms, it is also necessary to prevent ceramic particles from blocking the pores of the diaphragm, which is not conducive to the passage of ions. While ensuring that the electrolyte fully infiltrates the separator, no excess electrolyte can remain in it, which reduces the use efficiency of the electrolyte.

　　2. Electronic impedance

　　There are many factors affecting electronic impedance, which can be improved from aspects such as materials and processes.

　　①Positive and negative plates

　　The main factors that affect the electronic impedance of the positive and negative electrode plates include: the contact between the active material and the current collector, the factors of the active material itself, the plate parameters, etc. The active material must be in full contact with the current collector surface, which can be considered from the current collector copper foil, aluminum foil base material, and the adhesion of the positive and negative electrode slurries. The porosity of the active material itself, by-products on the particle surface, uneven mixing with the conductive agent, etc. will all cause changes in electronic impedance. Plate parameters such as active material density are too small and the particle gaps are large, which is not conducive to electron conduction.

　　②Diaphragm

　　The main factors that affect the electronic impedance of the separator are: separator thickness, porosity, and by-products during the charge and discharge process. The first two are easy to understand. After the battery core is disassembled, it is often found that the separator is stained with a thick layer of brown substance, which includes graphite anode and its reaction by-products, which will cause the separator pores to be clogged and reduce the battery life.

　　③Current collector base material

　　The material, thickness, width of the current collector and its contact with the tab all affect the electronic impedance. The current collector needs to choose a base material that is not oxidized and passivated, otherwise the impedance will be affected. Poor welding between the copper aluminum foil and the tabs will also affect the electronic impedance.

　　3. Contact resistance

　　Contact resistance is formed between the contact between the copper aluminum foil and the active material, and it is necessary to focus on the adhesion of the positive and negative electrode slurries.

　　2. Polarization internal resistance

　　When current passes through an electrode, the phenomenon that the electrode potential deviates from the equilibrium electrode potential is called the polarization of the electrode. Polarization includes ohmic polarization, electrochemical polarization and concentration polarization, as shown in Figure 1. Polarization resistance refers to the internal resistance caused by the polarization of the positive and negative electrodes of the battery during electrochemical reactions. It can reflect the internal consistency of the battery, but is not suitable for production due to the influence of operations and methods. The polarization internal resistance is not a constant and changes with time during the charge and discharge process. This is because the composition of the active material, the concentration and temperature of the electrolyte are constantly changing. Ohmic internal resistance obeys Ohm's law. Polarization internal resistance increases with the increase of current density, but the relationship is not linear. It often increases linearly with the logarithmic increase in current density.

　　Generally speaking, the DC internal resistance of a battery is equal to the sum of polarization internal resistance and ohmic internal resistance. The measurement of DC internal resistance is of great significance. There are many factors that affect polarization internal resistance, such as charge and discharge rate, ambient temperature, SOC status, electrolyte concentration, etc. Here is an example of the effect of temperature on the internal resistance of lithium iron phosphate batteries. If you need relevant literature, you can send a private message to the editor, as shown in the figure below:

　　3. Battery internal resistance measurement methods currently used in the industry

　　In industrial applications, accurate measurement of battery internal resistance is carried out through special equipment. Currently, there are two main battery internal resistance measurement methods used in the industry:

　　1. DC discharge internal resistance measurement method

　　According to the physical formula R=U/I, the test equipment forces the battery to pass through a large constant DC current in a short period of time (usually 2 to 3 seconds) (currently, a large current of 40A to 80A is generally used), and the battery is measured at this time. The voltage at both ends, and the current battery internal resistance is calculated according to the formula.

　　This measurement method has high accuracy. If controlled properly, the measurement accuracy error can be controlled within 0.1%. However, this method has obvious shortcomings:

　　(1) Only large-capacity batteries or accumulators can be measured. Small-capacity batteries cannot load a large current of 40A to 80A within 2 to 3 seconds;

　　(2) When the battery passes a large current, the electrodes inside the battery will be polarized, resulting in polarized internal resistance. Therefore, the measurement time must be very short, otherwise the measured internal resistance value will have a large error;

　　(3) Large current flowing through the battery will cause certain damage to the electrodes inside the battery.

　　2. AC voltage drop internal resistance measurement method

　　Because the battery is actually equivalent to an active resistor, we apply a fixed frequency and fixed current to the battery (currently generally using 1kHz frequency, 50mA small current), then sample its voltage, and undergo a series of processes such as rectification and filtering. Finally, the internal resistance value of the battery is calculated through the operational amplifier circuit. The battery measurement time of the AC voltage drop internal resistance measurement method is very short, generally around 100 milliseconds.

　　The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%.

　　Advantages and disadvantages of this method:

　　(1) Almost all batteries, including small-capacity batteries, can be measured using the AC voltage drop internal resistance measurement method. This method is generally used to measure the internal resistance of laptop battery cells.

　　(2) The measurement accuracy of the AC voltage drop measurement method is likely to be affected by ripple current, and there is also the possibility of harmonic current interference. This is a test for the anti-interference ability of the measuring instrument circuit.

　　(3) Using this method to measure will not cause much damage to the battery itself.

　　(4) The measurement accuracy of the AC voltage drop measurement method is not as good as the DC discharge internal resistance measurement method.

　　So what are the low-temperature characteristics of the NCM ternary cathode material commonly used in lithium batteries? There are many models of nickel-cobalt-manganese ternary cathode materials, including NCM111/NCM523/NCM622/NCM721/NCM811, etc. They are all layered structures. The layered structure not only has unparalleled rate performance of one-dimensional lithium ion diffusion channels, but also has the structural stability of three-dimensional channels. The earliest commercial cathode material was layered lithium cobalt oxide. Since then, it has been gradually doped and modified to evolve into what it is today. Commonly used NCM ternary materials.

　　Xie Xiaohua and others used LiCoO2/MCMB as the research object and tested its low-temperature charge and discharge characteristics. The results show that as the temperature decreases, its discharge platform drops from 3.762V (0℃) to 3.207V (–30℃); its total battery capacity also drops sharply from 78.98mA·h (0℃) to 68.55mA·h (–30°C). Similar phenomena are also seen in research reports by Chen Jitao et al.

　　Ternary materials are popular for their high capacity and low cost, and research on their low-temperature properties is also in the ascendant. Smart et al. studied the electrochemical properties of ternary lithium-rich materials (Li1+x(Co1/3Ni1/3Mn1/3)1-xO2) in low-temperature electrolytes and found that the capacities of low-temperature electrolytes with different compositions increased as the temperature decreased. Attenuation, and the lower the temperature, the more serious the trend of capacity attenuation. For example, for 1.0mol/LLiPF6/EC:EMC (20:80), the discharge capacity can reach 52% of room temperature at 0.2C and –40°C, but only 28% of room temperature at –50°C.

　　Not only the performance of the battery cathode material has a great impact on the low-temperature performance of lithium batteries, but the intrinsic impact of the negative electrode material is also very important. At the same time, it is also important to study the matching low-temperature electrolyte. In short, in order to ensure the low-temperature performance of lithium-ion batteries, the following points need to be done:

　　(1) Form a thin and dense SEI film;

　　(2) Ensure that Li+ has a large diffusion coefficient in the active material;

　　(3) The electrolyte has high ionic conductivity at low temperatures.

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