On March 1, four ministries issued the "Action Plan for Promoting the Development of the Automotive Power Battery Industry" notice also includes a temperature target for the use environment of lithium-ion power batteries: ".... The use of the environment up to -30 ℃ to 55 ℃...." . Here the temperature requirements of the power battery is proposed: the battery can be used at low temperature -30℃ and high temperature 55℃, but it is not clearly stated whether it is a battery monomer, module or pack/system, nor how to use the battery in this temperature range (or the performance requirements in this temperature range are not proposed), especially the requirements of low temperature -30℃ (for example, the power capacity requirements under this temperature, power requirements, etc.).
On the requirements of power batteries at high or low temperatures, first look at how the relevant regulatory standards provide for.
1.QC/T743-2006 Lithium-ion battery for electric vehicles. This is the old battery standard previously implemented, and the requirements related to high and low temperature are mainly for single cells.
-20±2℃ C/3 discharge capacity is not less than 70% of the rated value
Under 55±2℃, the C/3 discharge capacity is not less than 95% of the rated value
Under 55±2℃ 100% SOC storage after 7 days, the charge retention rate is not less than 80% of the rated value, and the capacity recovery is not less than 90% of the rated value
2.GB/T 31486-2015 Electric performance requirements and test requirements for power batteries for electric vehicles. This is the latest national standard requirements for single cells and modules, which are about the performance requirements of battery modules at high and low temperatures as follows
The 1C discharge capacity at -20±2℃ is not less than 70% of the initial capacity
1C discharge capacity at 55±2℃ is not less than 90% of the initial capacity
After 7 days of 100% SOC storage at 55±2℃, the charge retention rate is not less than 85% of the initial capacity, and the capacity recovery should be not less than 90% of the initial capacity
3.GB/T 31467.1/2-2015 Lithium-ion dynamic batteries and systems for electric vehicles Part 1/2: Test protocol for high power/high energy applications. This standard series is about the requirements of the battery pack/system and only provides the test method, not the specific requirements. The requirements related to high and low temperature are
Capacity and energy test (this is 1C continuous discharge ) the maximum and minimum temperature: 40 ℃ and -20 ℃
Power and internal resistance test (short time high current discharge) of the maximum and minimum temperature: 40 ℃ and -20 ℃
Capacity loss test without attached load, the maximum temperature is 40℃
Capacity loss in storage test, the maximum temperature is 45℃
High and low temperature start-up power test, the maximum temperature and minimum temperature is: 40℃ and -20℃
Energy efficiency test, the maximum temperature, the minimum temperature is: 40 ℃ and -20 ℃
Taking the maximum and minimum values, it can be seen that the current standard requirements for temperature are
Battery monomer and module: -20 ~ 55℃
Battery pack / battery system: -20 ~ 45℃
Compare the objectives of the Action Plan for Promoting the Development of the Automotive Power Battery Industry can be seen as follows
1. Battery monomer/module
High temperature target is consistent with the current high temperature of the monomer/module
Low temperature target is 10℃ lower than the current standard, reaching -30℃
2. Battery pack/system
High temperature target is 10°C higher than the current pack/system temperature, reaching 55°C
Low temperature target 10℃ lower than the current standard to -30℃
Figure 1 is a schematic of the discharge capacity curve of lithium-ion batteries at different low temperatures (used here to indicate the general trend of change). Compared with the room temperature of 20 ℃, low temperature -20 ℃ capacity decay has been more obvious, to -30 ℃ is more capacity loss, -40 ℃ capacity of even less than half.
Here is a look at the factors that affect low temperature performance. By comparing the capacity and electrolyte conductivity relationship (Figure 2), we can see that the lower the temperature, the lower the conductivity of the battery electrolyte. When the conductivity decreases, the ability of the solution to conduct active ions decreases, which is expressed as an increase in the resistance of the reaction inside the battery (this resistance is expressed as impedance inside electrochemistry), resulting in a decrease in discharge capacity, i.e., a decrease in capacity. Further, the effect of each part on the battery impedance can be seen by measuring the impedance of each part inside the battery (positive electrode, negative electrode, electrolyte) (Figure 3). When the temperature is <-10℃, the interface impedance of positive and negative electrodes (graphite is used as an example in the figure) increases rapidly, while the impedance of electrolyte rises rapidly after about -20℃, and the combined result of these impedances shows that the battery impedance rises rapidly around <-10℃ (represented by Li-ion cell in the figure).
Saft, a famous French battery company, has studied the effect of high temperature on battery performance by using 2Ah cylindrical cells (positive material NCM with PVdF binder and negative material carbon with CMC/SBR binder), comparing the two cells at different temperatures.
- B2 cell - first 2 cycles at 60°C, then at 85°C
- B3 cell - first 2 cycles at 60°C, then at 120°C
As can be seen in Figure 4, the B2 battery loses about 7.5% capacity and increases 100% impedance after 26 cycles at 85°C. The B3 battery loses about 22% capacity and increases up to 1115% impedance after 25 cycles at 120°C.
The model in Figure 5 is used to illustrate the changes of the positive electrode of the battery at high temperature 120℃. At 120°C, part of the cathode binder PVdF migrates from the Part 1 region to the cathode surface, which causes a decrease in the binder content in the Part 1 region, and the active material NMC material causes a decrease in the electrochemical reaction due to the absence of the binder. In the Part 2 region, which is the main part of the anode, the binder content is normal, the high temperature does not affect much, and the active material can react normally.
The effect of high temperature on the negative electrode can be seen by analyzing the surface of the negative electrode (Fig. 6). Figure 6a shows the initial state of the negative electrode, and after cycling at 85°C, the common solid electrolyte phase appears on the surface of the negative electrode (Figure 6b the surface of the negative electrode is covered with newly generated material, causing the surface morphology to be different from the initial morphology, with some small spherical material. 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), which consumes more active lithium ions and causes a decrease in capacity.
In general, the factors affecting the high and low temperature of the battery can be summarized as follows: conductivity of the electrolyte, interfacial impedance, SEI film, etc. These factors combine to affect the performance of the battery. Generally speaking, improving the conductivity or electrical conductivity of each component of the battery (including choosing active materials with better electrical conductivity, optimizing electrolyte composition, improving the composition of the negative SEI film, inhibiting the dissolution of substances on the surface of the positive electrode, etc.), thus reducing the overall impedance of the battery, is helpful for improving the high and low temperature performance. The adaptation of lithium-ion battery to temperature is just like the human body, too high or too low temperature is not conducive to its maximum function. Selecting the right material, optimizing the structural design and customizing the appropriate use conditions can give full play to its performance.
