What are the high and low temperature characteristics of lithium power batteries? Come and collect it!

Lithium power battery is a new type of high-energy battery successfully developed in the 20th century. The negative electrode of this battery is metallic lithium, and the positive electrode uses MnO2, SOCL2, (CFx)n, etc. It became practical in the 1970s. Because of its advantages of high energy, high battery voltage, wide operating temperature range, and long storage life, it has been widely used in military and civilian small electrical appliances.

The discharge capacity curves of lithium power batteries at different low temperatures are shown in Figure 1. Compared with room temperature of 20°C, the capacity attenuation at low temperatures of -20°C is already more obvious. At -30°C, the capacity loss is more, and at -40°C the capacity Not even half.

From an electrochemical perspective, the solution resistance and SEI film resistance change little over the entire temperature range, and have little impact on the low-temperature performance of lithium power batteries; the charge transfer resistance increases significantly as the temperature decreases, and increases over the entire temperature range. The temperature changes are significantly larger than the solution resistance and SEI film resistance. This is because as the temperature decreases, the ionic conductivity of the electrolyte decreases, and the SEI film resistance and electrochemical reaction resistance increase, resulting in an increase in ohmic polarization, concentration polarization, and electrochemical polarization at low temperatures. Large, the discharge curve of lithium power battery shows that the average voltage and discharge capacity decrease as the temperature decreases.

During the low-temperature charging process of lithium power batteries, the ohmic polarization, concentration polarization and electrochemical polarization will increase, leading to the deposition of metallic lithium and the decomposition of the electrolyte, which ultimately leads to the thickening of the SEI film on the electrode surface and the increase in SEI film resistance. The discharge curve shows a discharge plateau and a decrease in discharge capacity.

Under low temperature conditions, the chemical reaction activity of lithium power batteries decreases, and the migration of lithium ions slows down. The lithium ions on the surface of the negative electrode have been reduced to metallic lithium before they are embedded in the negative electrode, and precipitate out on the surface of the negative electrode to form lithium dendrites. This can easily pierce the separator and cause a short circuit within the battery, thereby damaging the battery and causing a safety accident.

(2) High temperature characteristics of lithium power battery

At a high temperature of 120°C for lithium power batteries, part of the positive electrode binder PVdF of the lithium power battery will migrate from the Part1 area to the positive electrode surface, causing the binder content in the Part1 area to decrease, that is, the binder in the active material is missing, resulting in battery failure. Decreased ability to react chemically. In the Part 2 area, since it is the main body of the positive electrode, the binder content is normal, the high temperature has little effect, and the active material can react normally.

When the lithium power battery is cycled at 85°C, solid electrolyte appears on the surface of the negative electrode of the lithium power battery, and the surface of the negative electrode is covered by the newly generated solid electrolyte. When the temperature rises to 120°C, more solid electrolytes are generated, and the surface of the negative electrode is covered with more solid electrolytes, which consumes more active lithium ions, causing the capacity of the lithium power battery to decrease.

2. Lithium power battery module temperature collection method

When designing the temperature collection point of the lithium power battery module, the temperature collection methods used are:

1) Directly collect the cell temperature, usually by placing an NTC thermistor on the surface of the lithium power battery module cell. When the characteristics of the lithium power battery module cells are relatively uniform, the NTC thermistor can be pasted on the surface of the lithium power battery module cells.

2) Indirectly connect the collector core temperature. A typical method is to embed NTC thermistors on the two end plates of the lithium power battery module, so that the head and tail can be accurately sensed. The temperature of the two power battery cells is calculated by collecting the temperatures of the first and last two power battery cells to calculate the temperature of the entire lithium power battery module cell.

3) Collect the temperature on the upper side of the power battery cell interconnection board, that is, embed the NTC thermistor into the internal interconnection board of the power battery cell to accurately sense the maximum temperature of the power battery cell.

4) Collect the temperature of the busbar of the lithium power battery module. There is a groove on the busbar of the lithium power battery module. The temperature sensor is fixed in the groove. There is a fixing glue for fixing the temperature sensor in the groove.

5) Collect the temperature on the surface of the lithium power battery module cover, and paste the NTC thermistor directly on the lithium power battery module cover.

When the NTC thermistor is connected to the power battery busbar and cell interconnection board or bonded to the surface of the lithium power battery module cell or cover plate, the impact of the operating process on the NTC thermistor needs to be considered. Improper operation during the fixing process may cause the NTC thermistor to be disconnected, short-circuited or the lead coating to be broken. Because the base ceramic inside the NTC thermistor is a fragile material, excessive pressure or impact cannot be applied during the connection or pasting process. Otherwise, the joint between the lead and the component will be disconnected, or the component will rupture. When connecting or pasting, it is also necessary to consider the expansion coefficients of different materials of the NTC thermistor in the entire temperature range, otherwise internal stress will occur and damage the NTC thermistor itself.