The Effect Of Low Temperature On Lithium Batteries
(Last Updated On: 06/02/2024)
The Effect Of Low Temperature On Lithium Batteries
The use of lithium batteries is limited in low battery temperature environments. In addition to a significant decrease in discharge capacity, lithium batteries cannot be charged even at low battery temperatures. During low-temperature charging of batteries, the insertion of lithium ions into the graphite electrode and the lithium plating reaction coexist and compete with each other.
Under low battery temperature conditions, the diffusion of lithium ions in graphite is inhibited, and the conductivity of the electrolyte decreases, resulting in a decrease in the insertion rate, and it is easier for a lithium plating reaction to occur on the graphite surface. The main reason for the decrease in lifespan of lithium-ion batteries when used at low battery temperatures, is due to the increase in internal resistance and capacity loss caused by lithium ion plating.
1. The impact of battery low temperature on battery discharge capacity
Capacity is one of the most important parameters of lithium batteries, and its size changes with temperature. For lithium iron phosphate batteries, the charge end voltage is 3.65±0.05V and the discharge end voltage is 2±0.05V. The two curves are the temperature capacity curves obtained by discharging the battery at 0.1C and 0.3C at different temperatures.
It is obvious that as the temperature increases, the capacity gradually increases. The capacity at -20℃ is only equivalent to about 60% of the capacity at 15℃. In addition to capacity, battery open circuit voltage also decreases as temperature increases. We all know that the energy contained in a battery is the product of capacity and terminal voltage. When both multiples decrease, the energy in the battery must be the superposition of the decreasing effects of both.
When the battery temperature is low, the activity of the cathode material decreases, which reduces the number of lithium ions that can move and bring discharge current. This is the fundamental reason for the decrease in capacity.
2. The impact of low battery temperature on battery internal resistance
The relationship between temperature and resistance of lithium batteries is shown in the following figure. Different curves represent different charging levels of the battery itself. In any charging situation, the internal resistance of the battery will significantly increase with the decrease of temperature. The lower the charge, the greater the internal resistance, and this trend remains unchanged with temperature changes.
When the battery temperature is low, the diffusion and movement abilities of charged ions in the cathode and anode electrode materials decreases, making it difficult to pass through the passivation film between the electrode and the electrolyte. The transfer rate in the electrolyte is also reduced and a large amount of additional heat is generated during the transfer process.
After lithium ions reach the anode electrode, the diffusion inside the anode electrode material also becomes unsmooth. Throughout the process, the movement of charged ions becomes very difficult. From the outside, it means that the internal resistance of the battery cell has increased.
3. The impact of low battery temperature on battery charging and discharging efficiency
The curve below is the charging efficiency as a function of temperature. We can observe that the charging efficiency at -20℃ is only 65% of that at 15℃.
The low battery temperature brings various changes in electrochemical performance mentioned above, and the internal resistance significantly increases. During the discharge process, a large amount of electrical energy is consumed on the internal resistance, generating heat.
4. Internal side reactions of lithium-ion batteries at low battery temperatures
The performance of lithium-ion batteries will seriously decrease when the battery temperature is low, and some side reactions will occur during the charging and discharging process of lithium-ion batteries. These side reactions are mainly irreversible reactions between lithium ions and electrolyte, which will lead to a decrease in lithium battery capacity and further deteriorate battery performance.
Depletion of conductive active material results in capacity fading. Considering the potentials of the cathode and anode electrodes in the battery, these side reactions are more likely to occur on the anode side than on the cathode side. Since the potential of the anode electrode material is much lower than that of the cathode electrode material, side reaction deposits of ions and electrolyte solvents are deposited on the electrode surface to form an SEI film. The impedance of the SEI film is one of the factors causing the anode electrode reaction overpotential.
When the battery undergoes further cycling and aging, the continuous insertion and removal of lithium ions on the anode electrode during continuous cycling can cause electrode expansion and contraction, leading to the rupture of the SEI film. The cracks after the rupture of the SEI film provide a direct contact channel between the electrolyte and the electrode, forming a new SEI film to fill the cracks and increase the thickness of the SEI film.
These reaction processes are constantly repeated with the continuous charging and discharging of the battery, causing the lithium ions to be continuously reduced during the reaction, resulting in a decrease in the discharge capacity of the lithium-ion battery.
During charging, deposits will form on the surface of the active material, increasing resistance. The effective surface area of the active particles decreases and the ionic resistance increases. The available capacity and energy of lithium batteries decrease at the same time. Lithium batteries are more prone to side reactions during charging.
At the beginning of lithium battery charging, lithium ions move toward the anode electrode through the electrolyte, so the potential difference between the electrode and the electrolyte decreases, making it easier for lithium ions to have irreversible side reactions with substances in the electrolyte. Different lithium-ion battery electrode materials have different relationship curves between the potential of the electrode material and the lithium concentration fraction.
Low temperature preheating technology for lithium batteries
Faced with the limitations of using lithium batteries at low battery temperatures, technicians have found solutions for charging and preheating. Although it is a temporary measure, it has a significant effect on improving the discharge capacity and long-term lifespan of lithium batteries.
Before charging or using lithium batteries in environments with low battery temperatures, the battery must be preheated. The battery management system (BMS) can roughly be divided into two types of heating methods for batteries: external heating and internal heating.
Compared with external heating methods, internal heating avoids long path heat conduction and the formation of local hotspots near the heating device. Therefore, internal heating can heat the battery more evenly, achieving better heating with higher efficiency and easier implementation.
At present, most research on internal AC preheating schemes focuses on heating speed and efficiency, and heating strategies to prevent side reactions such as lithium deposition are rarely explicitly considered.
To prevent lithium deposition during preheating, it is necessary for the BMS to estimate and control the conditions for lithium deposition in real time. To achieve the above functions, a model-based low battery temperature control battery heating technology is required.
With the development of new energy, the use of power lithium batteries is also increasing day by day. There is an urgent need to solve the problem of battery preheating when using lithium batteries at low battery temperatures. This is an area that is very close to practical applications. In addition, issues such as AC heating, mobilizing electrochemicals to produce motion, and the impact on battery life have not yet been seen. This is also an issue that deserves continued attention.
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