Lithium-ion batteries have numerous advantages, such as high working voltage (three times that of nickel-metal-hydride and nickel-cadmium batteries), large specific energy (up to 165Wh/kg, three times that of nickel-metal-hydride batteries), small size, light weight, long cycle life, low self-discharge, no memory effect, and no pollution. Lithium iron phosphate batteries are highly favored in the new energy industry. With a cycle life of around 3,000 times and stable discharge, they are widely used in power batteries and energy storage fields. However, the speed of its promotion and the breadth and depth of its application fields have not been satisfactory. In addition to factors such as price and batch consistency caused by the battery material itself, its temperature performance is also an important factor hindering its rapid promotion. Huaifeng Electronics investigated the impact of temperature on the performance of lithium iron phosphate batteries and also examined the charging and discharging conditions of battery packs under high and low temperature conditions. Now, Huifeng Electronics will discuss with everyone the influence of temperature on lithium iron phosphate batteries.

I. Summary of Normal Temperature Cycling for Individual Units (Modules)
The cycle life of batteries tested at room temperature shows the long-life advantage of lithium iron phosphate batteries. Currently, it has achieved 3,314 cycles, with a capacity retention rate still at 90%, and it may take about 4,000 cycles to reach the 80% end of its life.
Due to the processing technology of the battery cells and the assembly technology of the modules, the inconsistencies within the battery have already formed after the battery PACK is completed. The more sophisticated the technology, the smaller the internal resistance of the assembly, and the smaller the differences between the battery cells. The cycle life of the following modules is the basic data that most lithium iron phosphate batteries can achieve at present. Therefore, during the usage process, the BMS needs to regularly balance the battery pack to reduce the differences between cells and extend the service life.

Ii. The Impact of Low Temperature on Charge and Discharge Performance
At temperatures ranging from 0 to -20℃, the discharge capacity of the battery is respectively equivalent to 88.05%, 65.52% and 38.88% of the discharge capacity at 25℃. The average discharge voltages were 3.134, 2.963 V and 2.788 V respectively. The average discharge voltage at -20 ℃ was 0.431 V lower than that at 25℃. From the above analysis, it can be seen that as the temperature drops, both the average discharge voltage and the discharge capacity of lithium-ion batteries decrease. Especially when the temperature is -20℃, the discharge capacity and average discharge voltage of the battery drop rapidly.

From an electrochemical perspective, the solution resistance and SEI film resistance do not change much throughout the entire temperature range, and have a relatively small impact on the low-temperature performance of the battery. The charge transfer resistance increases significantly with the decrease of temperature, and its variation with temperature throughout the entire temperature range is significantly greater than that of the solution resistance and the SEI film resistance. This is because as the temperature drops, the ionic conductivity of the electrolyte decreases accordingly, while the resistance of the SEI film and the electrochemical reaction resistance increase. This leads to an increase in ohmic polarization, concentration polarization, and electrochemical polarization at low temperatures. On the battery's discharge curve, this is manifested as both the average voltage and discharge capacity decreasing with the drop in temperature.

After cycling 5 times at -20℃ and then another cycle at 25℃, both the battery capacity and the discharge platform will decrease. This is because as the temperature drops, the ionic conductivity of the electrolyte decreases, and the ohmic polarization, concentration polarization and electrochemical polarization during the low-temperature charging process increase, leading to the deposition of metallic lithium, causing the electrolyte to decompose. Eventually, this results in a thickening of the SEI film on the electrode surface and an increase in the SEI film resistance, which is manifested on the discharge curve as a decrease in the discharge platform and discharge capacity.

The influence of low temperature on cycling performance
The battery's capacity decaps rapidly in an environment of -10℃. After 100 cycles, its capacity drops to only 59mAh/g, with a decline of 47.8%. The batteries that have been discharged at low temperatures are subjected to charge and discharge tests at room temperature to observe the capacity recovery performance during the period. Its capacity has recovered to 70.8mAh/g, with a capacity loss of 68%. It can be seen from this that the low-temperature cycling of batteries has a significant impact on the recovery of battery capacity.

2. The impact of low temperature on safety performance
Lithium-ion battery charging is the process in which lithium ions are released from the positive electrode, migrate through the electrolyte and intercalate into the negative electrode material. Lithium ions aggregate towards the negative electrode, with one lithium ion captured by six carbon atoms. At low temperatures, the chemical reactivity decreases, and the migration of lithium ions slows down. Before the lithium ions on the surface of the negative electrode can be embedded into the negative electrode, they are already reduced to metallic lithium and precipitate on the surface of the negative electrode to form lithium dendrites. This can easily Pierce the separator and cause an internal short circuit in the battery, thereby damaging the battery and causing safety accidents.

From the above data, it can be concluded that lithium iron phosphate batteries are greatly affected by temperature. In the application field of power batteries and application environments where temperature is significantly influenced, thermal management of the battery (such as air cooling, liquid cooling, etc.) is required to improve the battery's usage efficiency and extend the service life of the battery system.