With my country's increasing emphasis on energy conservation and environmental protection, more and more energy-saving and environmentally friendly equipment has been developed. In the battery market, both power lithium batteries and energy storage batteries have been gradually replaced by lithium batteries. Compared with traditional lead-acid batteries, lithium batteries have advantages such as a high number of charge-discharge cycles (1000 times, compared to 300 times for ordinary batteries), no memory effect, complete discharge capability, long service life, energy saving and environmental protection, and light weight (about one-third that of lead-acid batteries). Even though they are much more expensive than traditional lead-acid batteries, the advantage of longer service life more than makes up for it.
The operating environment of lithium-ion batteries has a significant impact on their cycle life. Ambient temperature is a particularly important factor; both excessively low and high temperatures can negatively affect the cycle life of lithium-ion batteries.
We investigated the charge-discharge performance of C/LiCoO2 lithium-ion batteries at -20℃. The results showed that the battery's discharge performance deteriorated at low temperatures; the 0.2C discharge capacity was only 77% of the capacity at room temperature, and the 1C discharge capacity was only 4% of the 0.2C discharge capacity. Constant voltage charging time increased at low temperatures, and charging performance also deteriorated significantly.
The main reasons for the decrease in discharge capacity of lithium-ion batteries at low temperatures include: decreased electrolyte conductivity, reduced wetting and/or permeability of the separator, slower lithium-ion migration, and a slower charge transfer rate at the electrode/electrolyte interface. Additionally, the impedance of the SEI film increases at low temperatures, further slowing the passage of lithium ions through the electrode/electrolyte interface. The increased SEI film impedance is due to the fact that lithium ions are easier to extract from the negative electrode but more difficult to insert at low temperatures. During charging, metallic lithium appears and reacts with the electrolyte, forming a new SEI film that covers the original SEI film, increasing the battery impedance and thus reducing battery capacity.
The same batch of lithium batteries underwent 300 charge-discharge cycle tests at 60℃ and room temperature. Initially, the batteries exhibited higher discharge capacity at 60℃. However, as the cycle progressed, the battery capacity decayed rapidly, cycle stability decreased, and some batteries even swelled in the later stages. The charge-discharge cycle of lithium-ion batteries is unstable at high temperatures. High temperatures intensify the electrochemical polarization of the battery electrodes and generate gas, causing swelling. Simultaneously, charge transport resistance increases, and ion transport kinetics deteriorates.
Currently, most lithium-ion batteries use LiPF6 as the electrolyte. However, due to impurities in the electrolyte or trace amounts of water catalyzing the decomposition of conductive salts, the electrolyte contains acidic substances such as HF. HF reacts with the main components of the SEI film, such as ROLi and ROCO2Li, to form LiF, which deposits on the negative electrode surface. The LiF-containing SEI film hinders lithium-ion migration. Simultaneously, the resulting high-resistivity material creates insulation between graphite particles. With continued high-temperature charging and discharging, the negative electrode performance gradually deteriorates, ultimately leading to battery failure.
Devices using lithium-ion batteries may be subjected to vibration, shock, and collision during transportation or normal operation. Some lithium batteries charge and discharge and receive data at a specific frequency when communicating with the system. The frequency of equipment vibration may interfere with the battery frequency, causing chip data errors or triggering protection circuits. Under strong vibration or shock, the lithium-ion battery tabs, external connections, terminals, solder joints, etc., may break or detach, and the active material on the battery electrodes may also peel off. All of these can affect battery life and even lead to dangerous situations.
