The increase in internal resistance of lithium batteries at low temperatures is the result of a combination of factors. The underlying mechanism involves changes in electrolyte properties, reduced electrode material activity, increased polarization, and increased SEI film impedance. These factors, combined, lead to decreased charge and discharge efficiency, capacity decay, and even irreversible damage.
Changes in the physical properties of the electrolyte at low temperatures are a direct cause of increased internal resistance. The viscosity of organic solvents increases significantly with decreasing temperature, increasing the resistance to lithium ion migration in the electrolyte and sharply decreasing conductivity. Simultaneously, the solubility of lithium salts decreases at low temperatures, further diminishing the electrolyte's ionic conductivity. This dual effect hinders lithium ion transport between the positive and negative electrodes, significantly increasing ohmic internal resistance.
Decreased electrode material activity is a key internal factor contributing to low-temperature performance degradation. At low temperatures, the rates of lithium ion insertion and extraction reactions in the positive and negative electrode materials decrease significantly. The crystal structure of the positive electrode material stabilizes at low temperatures, narrowing the lithium ion diffusion channels. Meanwhile, lithium ion migration is hindered on the surface of the negative electrode material, leading to charge accumulation and increased electrode polarization. This decrease in activity directly manifests as a decrease in the lithium battery's charge and discharge efficiency and a significant increase in the polarization component of the internal resistance.
The intensification of polarization at low temperatures is a dynamic manifestation of increased internal resistance. At low temperatures, a lithium battery's ohmic polarization and electrochemical polarization increase simultaneously. Ohmic polarization arises from increased electrolyte viscosity and contact resistance, while electrochemical polarization is exacerbated by reduced electrode reaction rates. The combined effect of these factors leads to greater deviations in the lithium battery's terminal voltage during charge and discharge, significantly reducing its actual usable capacity.
The low-temperature increase in SEI membrane impedance is a structural factor contributing to the increase in internal resistance. As a key pathway for lithium ion transport, the SEI membrane's impedance increases significantly at low temperatures. Low temperatures densify the SEI membrane structure, lengthening the lithium ion diffusion path. Furthermore, increased electrolyte viscosity impairs interfacial contact between the SEI membrane and the electrolyte, further increasing transport resistance. This structural change significantly reduces the lithium battery's charge and discharge efficiency at low temperatures.
To address the issue of increased internal resistance at low temperatures, electrolyte optimization is the primary technical approach. By adjusting the solvent composition and reducing the proportion of low-melting-point, low-viscosity solvents, the low-temperature conductivity of the electrolyte can be effectively improved. For example, blending linear carboxylate solvents with cyclic carbonates ensures both low-temperature fluidity and cycling stability. Furthermore, developing new lithium salts and additives can improve the low-temperature ionic conductivity of the SEI film and reduce film formation resistance.
Electrode material modification is a key approach to improving low-temperature performance. Positive electrode materials can be coated with a conductive layer or doped with bulk elements to enhance interfacial conductivity and lithium ion diffusion rate. Negative electrode materials can be treated with surface treatment, coating, or particle size reduction to shorten lithium ion migration paths and enhance low-temperature lithium insertion capacity. For example, carbon or metal coatings prevent direct contact between the negative electrode and the electrolyte, reducing irreversible capacity loss.
System-level solutions provide comprehensive support for low-temperature applications. Lithium battery preheating technology uses an external heater to raise the initial temperature of the lithium battery to activate electrode activity; thermal management systems utilize insulation materials and air duct designs to maintain a stable operating temperature. Furthermore, improved charging strategies, such as lowering charging current and voltage at low temperatures, can reduce polarization and improve charging efficiency and safety. These combined measures can significantly improve lithium battery performance in low-temperature environments.
