Lithium iron phosphate (LFP) batteries experience capacity degradation at low temperatures (0 to -20°C). LFP batteries are significantly affected by temperature, necessitating thermal management to improve their efficiency. At temperatures between 0 and -20°C, the discharge capacity of LFP batteries is equivalent to 88.05%, 65.52%, and 38.88% of their discharge capacity at 25°C, respectively.
Low temperatures affect the positive and negative electrodes, electrolyte, and binders of lithium iron phosphate batteries. For example, the positive electrode of a lithium iron phosphate battery has relatively poor electronic conductivity and is prone to polarization at low temperatures, thus reducing battery capacity. Low temperatures also slow down the lithium intercalation rate in graphite, making it easier for metallic lithium to precipitate on the surface of the negative electrode. If the battery is put into use without sufficient resting time after charging, not all the metallic lithium can be re-intercalated into the graphite, leaving some on the surface of the negative electrode. This can potentially form lithium dendrites, affecting battery safety.
At low temperatures, electrolyte viscosity increases, leading to a corresponding increase in lithium-ion migration resistance. Furthermore, binders are a crucial factor in the manufacturing process of lithium iron phosphate batteries, and low temperatures significantly impact their performance. Moreover, during low-temperature charging, lithium ions may not have enough time to embed into the graphite anode, resulting in the precipitation of metallic lithium dendrites on the anode surface. This reaction consumes the battery's rechargeable lithium ions and drastically reduces battery capacity. The precipitated metallic lithium dendrites may also puncture the separator, compromising safety performance.
The low-temperature characteristics of lithium iron phosphate batteries are determined by their material properties and are difficult to change. Numerous experiments have shown that different lithium battery materials exhibit varying low-temperature performance. Currently, the most popular lithium iron phosphate battery has the worst low-temperature performance, with its capacity dropping to 89% of its maximum at -10°C, which is relatively high in the industry. At 55°C, it can reach 95% of its maximum capacity, indicating relatively little capacity degradation at lower temperatures.
For lithium iron phosphate batteries, both over-discharging and over-charging can damage their capacity. The correct way to use lithium batteries is to charge them when they are not fully charged, avoid completely depleting them before charging, and avoid overcharging. The optimal operating temperature for lithium batteries is 0 to 35 degrees Celsius. Low temperatures reduce the activity of lithium ions, weakening the battery's discharge capacity and shortening its lifespan. If a lithium battery is exposed to low temperatures for a short period, this damage is temporary and will not affect its capacity. Performance will recover once the temperature rises.
However, if lithium iron phosphate batteries operate and are charged and discharged at low temperatures for extended periods, metallic lithium will precipitate on the anode surface. This process is irreversible and will cause permanent damage to the battery capacity.
