Factors affecting the low temperature performance of lithium batteries
1. Cathode material
Cathode material as a power source is one of the main parameters that affect the low-temperature performance of lithium batteries. At present, the mainstream material systems on the market are ternary materials and lithium iron phosphate materials. The two materials have better low-temperature performance than ternary materials. The poor low-temperature performance of lithium iron phosphate is mainly due to the material itself is an insulator, low electronic conductivity, poor lithium ion diffusivity, and poor conductivity at low temperatures, which increases the internal resistance of the battery, greatly affects the polarization, and hinders the charge and discharge of the battery, so the low temperature Performance is not ideal. The insertion/extraction of lithium ions between the positive and negative electrodes at low temperatures is greatly affected by the material. The ternary material has a layered structure and the material has a high diffusion coefficient, which is more conducive to the insertion/extraction of lithium ions.
The structure, particle size and type of the material have a greater impact on the low temperature performance of the battery. The small particle size and large specific surface area of the positive electrode material are conducive to the performance of low temperature performance. The small particle size means the corresponding lithium ion diffusion path is short and the polarization received is small. At the same time, the electrolyte is also easy to adhere to the surface of the original particles to reduce the concentration difference. When the particle size is large, the diffusion path of lithium ions is long. When the battery is working and discharging, the diffusion of lithium ions from the negative electrode to the positive electrode is too late to compensate for the electrons flowing from the negative electrode into the positive electrode, which causes excessive electrons in the positive electrode, causing the electrode potential to shift negatively, resulting in discharge voltage The platform becomes lower.
In addition to the properties of the material itself, the dispersion of the conductive agent in the positive electrode slurry, the adhesion performance, the surface density of the pole piece, the density of the active material and other parameters also have an important impact on the low temperature performance. The conductive agent is evenly dispersed without agglomeration, can improve the conductivity of the lithium ion battery, reduce the ohmic internal resistance of the lithium ion battery, and help improve the charge and discharge performance of the battery. The greater the areal density, the greater the ion diffusion distance and resistance. The distance between the solid-liquid interface of the electrode surface and the electrolyte and the current collector will increase. When lithium ions are deintercalated, the electrons that migrate while maintaining the charge balance of the electrode will increase. The resistance transmitted between the two also increases, which makes the difference between the electrode potential and the equilibrium potential greater, and the polarization of the battery increases, and its low temperature performance is naturally not very good.
The material and physical and chemical parameters of the electrolyte have an important influence on the low temperature performance of the battery. The problem of battery cycling at low temperatures is that the viscosity of the electrolyte increases, the ion conduction speed becomes slow, and the electron migration speed of the external circuit does not match, the battery becomes severely polarized, and the charge and discharge capacity decreases sharply. Lithium battery manufacturers pointed out that, especially in the case of low-temperature charging, lithium ions can easily form lithium dendrites on the surface of the negative electrode, causing battery failure.
The low temperature performance of the electrolyte is closely related to the electrical conductivity of the electrolyte itself. The higher conductivity transports ions faster, and can exert more capacity at low temperatures. The more the lithium salt in the electrolyte is dissociated, the greater the number of migration and the higher the conductivity. The higher the electrical conductivity, the faster the ion conductivity, the smaller the polarization, and the better the performance of the battery at low temperatures. Therefore, higher electrical conductivity is a necessary condition for achieving good low-temperature performance of lithium-ion batteries.
The conductivity of the electrolyte is related to the composition of the electrolyte, and reducing the viscosity of the solvent is one of the ways to improve the conductivity of the electrolyte. The good fluidity of the solvent at low temperature is the guarantee of ion transport, and the solid electrolyte membrane formed by the electrolyte on the negative electrode at low temperature is also the key to affecting lithium ion conduction.
Lithium battery manufacturers pointed out that improving the conductivity of the electrolyte at low temperatures can start from the following two points: 1. The solution has a low freezing point; 2. The SEI film with low internal resistance.
Third, the diaphragm
The impact of the diaphragm on the low-temperature performance of lithium batteries mainly considers the impact of its resistance on the battery performance at different temperatures. The size of the pore size has a direct impact on the battery performance. Too small a pore size will increase the internal resistance of the battery, while a too large pore size will easily cause the positive and negative electrodes to directly contact or be easily pierced by lithium dendrites and cause short circuit of the battery. Proper porosity is particularly important for the performance of the diaphragm and battery: if the porosity of the diaphragm is too small, the diaphragm has poor air permeability, weak electrolyte adsorption capacity, and low conductivity. If the porosity is too high, although the air permeability and electrolyte adsorption capacity are obviously improved, the corresponding shrinkage rate and puncture resistance become worse.
In addition to the materials listed above, the factors that affect the low-temperature performance of lithium batteries include battery production technology, formation system, and aging system. Starting from reducing internal resistance and reducing low-temperature polarization of lithium batteries can improve the low-temperature cycle performance of lithium batteries.