What factors affect the low-temperature performance of lithium-ion batteries?
Lithium-ion batteries are mainly composed of positive electrode materials, negative electrode materials, separators, and electrolytes. Lithium-ion batteries operating at low temperatures exhibit characteristics such as a decrease in discharge voltage plateau, low discharge capacity, rapid capacity decay, and poor rate performance. The main factors limiting the low-temperature performance of lithium-ion batteries are as follows:
1. Cathode material: The three-dimensional structure of the cathode material restricts the diffusion rate of lithium ions, and the effect is particularly obvious at low temperatures.
2. Electrolyte: The material and physicochemical parameters of the electrolyte have a significant impact on the low-temperature performance of the battery. Increased electrolyte viscosity, slower ion conduction speed, mismatch with the electron migration speed of the external circuit, and a sharp decrease in charge and discharge capacity. Especially under low-temperature charging conditions, lithium ions can easily form lithium dendrites on the surface of the negative electrode, leading to battery failure.
3. Lithium-ion diffusion rate: The diffusion rate of lithium ions in the graphite anode decreases at low temperatures. The increased charge migration impedance of lithium-ion batteries leads to a decrease in the diffusion rate of lithium ions in the graphite anode, which is an important reason affecting the low-temperature performance of lithium-ion batteries.
4. SEI film: Under low temperature conditions, the SEI film of the negative electrode of lithium-ion battery thickens, and the increased impedance of the SEI film leads to a decrease in the conduction rate of lithium ions in the SEI film. Ultimately, the lithium-ion battery forms polarization during charging and discharging under low temperature conditions, reducing the charging and discharging efficiency.
cathode materials
The cathode material, as the power source, is one of the key parameters affecting the low-temperature performance of lithium-ion batteries. Currently, the mainstream material systems on the market are ternary materials and lithium iron phosphate materials. Compared with the latter, ternary materials have better low-temperature performance. The poor low-temperature performance of lithium iron phosphate is mainly due to its insulator nature, low electronic conductivity, and poor lithium-ion diffusion. This poor conductivity at low temperatures increases the battery's internal resistance, makes it more susceptible to polarization, and hinders battery charging and discharging, thus resulting in unsatisfactory low-temperature performance. The insertion/extraction of lithium ions between the positive and negative electrodes at low temperatures is greatly affected by the material. Ternary materials, with their layered structure and high diffusion coefficient, are more conducive to lithium-ion insertion/extraction.
The structure, particle size, and type of materials have a significant impact on the low-temperature performance of batteries. Smaller particle size and larger specific surface area of the cathode material are beneficial for low-temperature performance. Smaller particle size results in a shorter lithium-ion diffusion path and less polarization. At the same time, the electrolyte can easily adhere to the surface of the original particles, reducing concentration polarization. Larger particle size results in a longer lithium-ion diffusion path. During battery operation and discharge, the diffusion of lithium ions from the negative electrode to the positive electrode cannot compensate for the electrons flowing from the negative electrode to the positive electrode, resulting in an excess of electrons in the positive electrode. This causes a negative shift in electrode potential and a lower discharge voltage plateau.
Besides the inherent properties of the materials themselves, the dispersion and adhesion properties of the conductive agent in the positive electrode slurry, as well as parameters such as electrode areal density and active material density, also significantly affect low-temperature performance. Uniform dispersion of the conductive agent, without agglomeration, can improve the conductivity of the lithium-ion battery, reduce its ohmic internal resistance, and thus improve its charge-discharge performance. Higher areal density increases the ion diffusion distance and the resistance encountered. This increases the distance from the solid-liquid interface between the electrode surface and the electrolyte to the current collector. During lithium-ion insertion/extraction, the resistance to electron transfer between the two surfaces to maintain electrode charge balance also increases, leading to a greater deviation between the electrode potential and the equilibrium potential. This increases battery polarization, naturally resulting in poorer low-temperature performance.
electrolyte
The material and physicochemical parameters of the electrolyte have a significant impact on the low-temperature performance of a battery. The challenges batteries face during low-temperature cycling include increased electrolyte viscosity, slower ion conduction speed, a mismatch with the electron migration speed in the external circuit, severe battery polarization, and a sharp decrease in charge/discharge capacity. Especially during low-temperature charging, lithium ions easily form lithium dendrites on the negative electrode surface, leading to battery failure.
The low-temperature performance of an electrolyte is closely related to its conductivity. Higher conductivity results in faster ion transport and allows for greater capacity utilization at low temperatures. The more lithium salts dissociate in the electrolyte, the more ions migrate, leading to higher conductivity. Higher conductivity means faster ion conduction, less polarization, and better battery performance at low temperatures. Therefore, high conductivity is a necessary condition for achieving good low-temperature performance in lithium-ion batteries.
The conductivity of an electrolyte is related to its composition, and reducing the viscosity of the solvent is one way to improve its conductivity. Good solvent flowability at low temperatures is essential for ion transport, while the solid electrolyte film formed at the negative electrode at low temperatures is also crucial for lithium-ion conduction. Therefore, improving the conductivity of the electrolyte at low temperatures can be achieved through two approaches: 1. Low freezing point of the solution; 2. A low internal resistance SEI film.
diaphragm
The impact of the separator on the low-temperature performance of lithium-ion batteries is largely determined by the effect of its resistance at different temperatures. Pore size directly affects battery performance; too small a pore size increases internal resistance, while too large a pore size can lead to direct contact between the positive and negative electrodes or allow lithium dendrites to puncture the battery, causing a short circuit. Appropriate porosity is crucial for the performance of both the separator and the battery: too small a porosity results in poor gas permeability, weak electrolyte adsorption, and low conductivity. While too high a porosity significantly improves gas permeability and electrolyte adsorption, it correspondingly reduces shrinkage and puncture resistance.
Besides the materials listed above, factors affecting the low-temperature performance of lithium-ion batteries include battery manufacturing processes, formation processes, and aging processes. Improving the low-temperature cycle performance of lithium-ion batteries can be achieved by reducing internal resistance and minimizing low-temperature polarization. At the same time, we would like to remind car owners that charging in low temperatures during winter is not advisable. If possible, charge the battery in your garage. For batteries that are not frequently used, charge them halfway and leave them unused. Avoid overcharging and over-discharging the battery.
