With the use of lithium-ion batteries, the battery performance continues to decline, mainly manifested as capacity decay, internal resistance increase, power drop, etc. The change of battery internal resistance is affected by various conditions such as temperature and discharge depth. Therefore, this paper mainly explains the factors affecting the internal resistance of batteries from the aspects of battery structure design and raw material performance.
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 tabs can also affect the internal resistance of the battery. The internal resistance of the coiled battery with the tab position at the head of the positive and negative electrodes is the largest. Compared with the coiled battery, the laminated battery is equivalent to dozens of small batteries. Connected in parallel, the internal resistance is smaller.
1. Positive and negative active materials
The positive electrode materials in lithium batteries are transition metal oxides and phosphides containing lithium, such as LiCoO2, LiFePO4, etc., which determine the performance of lithium batteries. The positive electrode materials mainly improve the electronic conductivity between particles by coating and doping. For example, after doping Ni, the strength of the P-O bond is enhanced, the structure of LiFePO4/C is stabilized, the volume of the unit cell is optimized, and the charge transfer resistance of the positive electrode material can be effectively reduced.
According to the simulation analysis of the electrochemical thermal coupling model, under the condition of high-rate discharge, the activation polarization, especially the significant increase of the activation polarization of the negative electrode, is the main reason for the severe polarization. Reducing the particle size of the negative electrode can effectively reduce the activation polarization of the negative electrode. When the particle size of the negative electrode solid phase is reduced by half, the activation polarization can be reduced by 45%. Therefore, as far as the design of lithium-ion batteries is concerned, 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-ion batteries due to their good properties. Compared with graphite-based conductive agents, the battery rate performance of adding carbon black-based conductive agents to the positive electrode is better, because graphite-based conductive agents have a flaky particle shape, which causes a large increase in the pore tortuosity coefficient under high magnification, and is prone to Li liquid phase diffusion. The phenomenon is that the process limits the discharge capacity.
The internal resistance of the battery added with CNTs is smaller because compared to the point contact between graphite/carbon black and the active material, the fibrous carbon nanotube and the active material are in line contact, 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 lithium-ion battery during use.
The AC internal resistance of the corona-treated aluminum foil can be reduced by about 20%. In the commonly used range of 20% to 90% SOC, the overall DC internal resistance is relatively small and the increase is gradually smaller with the increase of the discharge depth.
4. Battery separator
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 forming a good ion flow channel. When the separator has a higher liquid absorption rate and porous structure, it can be improved. The conductivity reduces the battery impedance and improves the rate performance of the battery.
Compared with ordinary base separators, ceramic separators, and rubber-coated separators can not only greatly improve the high-temperature shrinkage resistance of the separator, but also enhance the liquid absorption and wetting ability of the separator. Adding SiO2 ceramic coating to the PP separator can make the separator absorb liquid Volume increased by 17%. Coating 1μm PVDF-HFP on the PP/PE composite separator increases the liquid absorption rate of the separator from 70% to 82%, and the internal resistance of the cell decreases by more than 20%.