The environment in which the polymer lithium battery is used is also very important in influencing its cycle life. Among them, the ambient temperature is a very important factor. Too low or too high ambient temperature can affect the cycle life of Li-polymer batteries. In power battery applications and applications where temperature is a major influence, thermal management of Li-polymer batteries is required to improve the efficiency of the battery.

Causes of internal temperature change of Li-polymer battery pack

For Li-polymer batteries, the internal heat generation is reaction heat, polarization heat and Joule heat. One of the main reasons for the temperature increase of Li-polymer battery is the temperature increase caused by the internal resistance of the battery. In addition, due to the dense placement of the heated cell body, the middle region is bound to gather more heat, and the edge region is less, which increases the temperature imbalance between the individual cells in the Li-polymer battery.

Polymer lithium battery temperature regulation methods

1. Internal adjustment

The temperature sensor will be placed in the most representative, the largest temperature change in the location, especially the highest and lowest temperature, as well as the center of the polymer lithium battery heat accumulation more powerful area.

1. External regulation

Cooling regulation: At present, considering the complexity of the thermal management structure of Li-polymer batteries, most of them adopt the simple structure of air-cooling method. And considering the uniformity of heat dissipation, most of them adopt the parallel ventilation method.

1. Temperature regulation: the simplest heating structure is to add heating plates on the top and bottom of the Li-polymer battery to implement heating, there is a heating line before and after each Li-polymer battery or the use of heating film wrapped around the Li-polymer battery for heating.

The main reasons for the reduction in the capacity of lithium polymer batteries at low temperatures

1. Poor electrolyte conductivity, poor wetting and/or permeability of the diaphragm, slower migration of lithium ions, slower charge transfer rate at the electrode/electrolyte interface, etc.

2. In addition, the impedance of the SEI membrane increases at low temperatures, slowing down the rate of lithium ions passing through the electrode/electrolyte interface. One of the reasons for the increase in impedance of SEI film is that it is easier for lithium ions to come off from the negative electrode at low temperatures and more difficult to embed.

3. When charging, lithium metal will appear and react with the electrolyte to form a new SEI film to cover the original SEI film, which increases the impedance of the battery thus causing the capacity of the battery to decrease.

Low temperature on the performance of lithium polymer batteries

1. low temperature on the charge and discharge performance

As the temperature decreases, the average discharge voltage and discharge capacity of lithium polymer batteries are reduced, especially when the temperature is -20 ℃, the battery discharge capacity and average discharge voltage decreases faster.

2. Low temperature on the cycle performance

The capacity of the battery decays faster at -10℃, and the capacity only remains 59mAh/g after 100 cycles, with 47.8% capacity decay; the battery discharged at low temperature is tested at room temperature for charging and discharging, and the capacity recovery performance is examined in the period. Its capacity recovered to 70.8mAh/g, with a capacity loss of 68%. This shows that the low-temperature cycle of the battery has a greater impact on the recovery of battery capacity.

3. Low temperature impact on safety performance

Polymer lithium battery charging is the process of lithium ions coming off from the positive electrode through the electrolyte migration embedded in the negative material, lithium ions to the negative electrode polymerization, by six carbon atoms capture a lithium ion. At low temperatures, the chemical reaction activity is reduced, while the migration of lithium ions becomes slower, the lithium ions on the surface of the negative electrode has not been embedded in the negative electrode has been reduced to lithium metal, and precipitation precipitation on the surface of the negative electrode to form lithium dendrites, which can easily pierce the diaphragm causing a short circuit in the battery, which can damage the battery and cause safety accidents.

Finally, we still want to remind you that lithium polymer batteries are best not charged in the winter at low temperatures, due to low temperatures, the lithium ions nested on the negative electrode will produce ion crystals, directly piercing the diaphragm, which generally causes a micro-short circuit affects the life and performance, serious direct explosion. So some people reflect the winter polymer lithium battery charging can not be charged, this is due to part with a battery management system is due to the product protection.