1. At low temperatures, the viscosity of the electrolyte increases and its conductivity decreases;
2. Increased electrolyte/electrode interface film impedance and charge transfer impedance;
3. The migration rate of lithium ions in the active material decreases. This leads to increased electrode polarization at low temperatures and a decrease in charge and discharge capacity.
In general, the factors affecting the high and low temperatures of batteries can be summarized as: electrolyte conductivity, interfacial impedance, SEI film, etc. These factors work together to affect battery performance.
In addition, during low-temperature charging, especially during high-rate charging at low temperatures, lithium metal will be deposited and precipitated on the negative electrode. The deposited lithium metal is prone to irreversible reaction with the electrolyte, consuming a large amount of electrolyte. At the same time, it further increases the thickness of the SEI film, leading to a further increase in the impedance of the negative electrode surface film and further strengthening of battery polarization. This will greatly damage the battery's low-temperature performance, cycle life, and safety performance.
Under low temperature conditions, the effective discharge capacity and effective discharge energy of lithium-ion batteries will decrease significantly. At the same time, they are almost unrechargeable in environments below -10°C, which severely restricts the application of lithium-ion batteries.
Lithium-ion batteries are primarily composed of positive electrode materials, negative electrode materials, a separator, and an electrolyte. 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:
◆Positive electrode structure
The three-dimensional structure of the cathode material restricts the diffusion rate of lithium ions, with a particularly pronounced effect at low temperatures. Cathode materials for lithium-ion batteries include commercially available lithium iron phosphate, nickel-cobalt-manganese ternary materials, lithium manganese oxide, and lithium cobalt oxide, as well as high-voltage cathode materials under development such as lithium nickel manganese oxide, lithium iron manganese phosphate, and lithium vanadium phosphate. Different cathode materials have different three-dimensional structures. Currently, the most important cathode materials used in electric vehicle power lithium batteries are lithium iron phosphate, nickel-cobalt-manganese ternary materials, and lithium manganese oxide. Wu Wendi et al. studied the discharge performance of lithium iron phosphate batteries and nickel-cobalt-manganese ternary batteries at -20℃, finding that the discharge capacity of lithium iron phosphate batteries at -20℃ could only reach 67.38% of the room temperature capacity, while that of nickel-cobalt-manganese ternary batteries could reach 70.1%. Du Xiaoli et al. found that the discharge capacity of lithium manganese oxide batteries at -20℃ could reach 83% of the room temperature capacity.
◆High melting point solvents
Because the electrolyte mixture contains high-melting-point solvents, the viscosity of lithium-ion battery electrolytes increases at low temperatures. When the temperature is too low, the electrolyte will solidify, resulting in a decrease in the lithium-ion transport rate in the electrolyte.
◆ Lithium-ion diffusion rate
The diffusion rate of lithium ions in graphite anodes decreases at low temperatures. Xiang Yu systematically studied the impact of graphite anodes on the low-temperature discharge performance of lithium-ion batteries, proposing that the increased charge migration impedance of lithium-ion batteries at low temperatures, leading to a decrease in the diffusion rate of lithium ions in graphite anodes, is an important reason affecting the low-temperature performance of lithium-ion batteries.
◆SEI membrane
At low temperatures, the SEI film on the negative electrode of a 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 at low temperatures, reducing the charging and discharging efficiency.
◆Summary
Currently, many factors affect the low-temperature performance of lithium-ion batteries, such as the structure of the cathode, the migration rate of lithium ions in different parts of the battery, the thickness and chemical composition of the SEI film, and the selection of lithium salts and solvents in the electrolyte.
Low-temperature performance limits the application of lithium-ion batteries in electric vehicles, special fields, and extreme environments. Developing lithium-ion batteries with excellent low-temperature performance is an urgent market demand.
