Lithium battery temperature is too high, more than 45℃ Lithium-ion batteries are more and more widely used in people's production and life, which makes its temperature environment become the main point of concern, relatively speaking, lithium batteries are easier to produce safety problems in high temperature environment, therefore, it is necessary to test the high temperature performance of lithium batteries, and compared with its normal temperature test data. When the lithium-ion battery is abused or misused, such as the use at high temperature or the charger control failure, it may cause a violent chemical reaction inside the battery, producing a lot of heat, if the heat is too late to dissipate and rapidly accumulate inside the battery, the battery may leak, vent, smoke and other phenomena, serious battery burning and explosion.

The chemical reactions that occur in batteries at high temperatures mainly include:

(1) Decomposition of SEI film: The protective film is metastable, and decomposition and heat release occur at 90-120 ° C.
(2) The reaction of embedded lithium and electrolyte: above 120 ° C, the membrane can not cut off the contact between the negative electrode and the electrolyte, and the lithium embedded in the negative electrode and the electrolyte exothermic reaction occurs.
(3) Electrolyte decomposition: decomposition occurs at higher than 200 ° C and heat release.
(4) Decomposition of positive active material: in the oxidation state, the positive material will exothermic decomposition and release oxygen, which will exothermic reaction with the electrolyte, or the positive material will react directly with the electrolyte.
(5) exothermic reaction between embedded lithium and fluoride binder.
The effect of high temperature on the performance of the 2Ah cylindrical battery (positive electrode material NCM, using PVdF binder, negative electrode material carbon, using CMC/SBR binder) was studied, and the conditions of the two batteries at different high temperatures were compared:
B2 battery - first cycle 2 times at 60 ° C, then cycle at 85 ° C

B3 battery - first cycle 2 times at 60 ° C, then cycle at 120 ° C

As can be seen from Figure 4, after 26 cycles at 85℃, the capacity loss of B2 battery is about 7.5%, and the battery impedance increases by 100%. After 25 cycles at 120 ° C, the B3 battery loses about 22% of its capacity and increases the battery impedance by up to 1115%.

The model shown in Figure 5 illustrates the changes of the positive electrode of the battery at a high temperature of 120℃. At 120℃, Part of the positive binder PVdF migrated from the Part 1 region to the surface of the positive electrode, which caused the binder content in the Part 1 region to decrease, and the active material NMC material decreased the ability of electrochemical reaction due to the lack of binder. In the Part 2 region, this part 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.

The effect of high temperature on the negative electrode can be seen by analyzing the negative electrode surface (Figure 6). FIG. 6a shows the initial state of the negative electrode. After cycling at 85℃, common solid electrolyte phases appear on the surface of the negative electrode (FIG. 6b the surface of the negative electrode is covered by newly generated substances, resulting in some small spherical substances different from the initial morphology. SEI: Solid Electrolyte Interface). When the temperature rises at 120 ° C, more SEI is generated (Figure 6c, the negative surface is covered with more particles), and more active lithium ions are consumed, resulting in a decrease in capacity.

FIG. 6 Morphological changes of the negative electrode surface

The effect of high temperature on battery life
The working temperature is too high: on the one hand, the anode reduction electrolyte at low potential for a long time causes the loss of active lithium ions, resulting in the decline of electrochemical performance; On the other hand, the high temperature leads to an increase in the side reaction of the anode reduction electrolyte, and the inorganic products of the reaction are deposited on the anode surface, which hinders the deimpingement of lithium ions and accelerates the aging of the battery. At high temperatures, the side reaction of the battery increases, such as the SEI film on the surface of the negative electrode will decompose, break or dissolve, etc., which leads to the continuous consumption of lithium ions during the cycle at high temperatures, and the capacity decreases rapidly.
Studies have shown that when the battery operating temperature exceeds 40 ° C, the cycle life of the battery will be halved for every 10 ° C increase. The battery pack is closely arranged in the battery compartment of the new energy vehicle, and the heat accumulation generated by the single battery causes the temperature difference inside the battery pack, resulting in different attenuation rates of the single battery, destroying the identity of the battery pack and reducing the performance of the battery pack.
The temperature of the battery is positively correlated with the charge and discharge current. When the charge and discharge of a small current are carried out, the highest temperature of the battery pack is in the position where heat exchange is not easy to occur with the outside world; when the charge and discharge of a large current or the structure design of the pole ear is unreasonable, the highest temperature of the battery pack is in the pole ear.
Therefore, the rational design of the battery cooling system according to the characteristics of the power battery and the working environment can not only improve the endurance performance of the vehicle, but also improve the safety and reliability of the vehicle.