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 because the material itself is not an insulator, low electronic conductivity, poor lithium ion diffusivity, and poor conductivity at low temperatures, which increases the internal resistance of the battery, which is greatly affected by polarization, and the charge and discharge of the battery are blocked, 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 area 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, which 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 is also increased, making the deviation of the electrode potential and the equilibrium potential greater, 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 problems faced by the battery cycle at low temperatures are that the viscosity of the electrolyte increases, the ion conduction speed becomes slower, and the electron migration speed of the external circuit does not match, the battery appears 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, leading to battery failure.