The performance of lithium-ion battery at high and low temperature

The "Action Plan for the Development of the Automobile Power Battery Industry" issued by the four ministries also includes a temperature target for the use environment of lithium-ion power cells: ". And ... And ... The use environment reaches -30 °C to 55 °C. And ... And ... ". The temperature requirements for the power cell are proposed here: The battery can be used at a low temperature of -30 °C and a high temperature of 55 °C, but it is not explicitly stated that it is a battery monomer, module or battery pack/system. There is also no explanation of how the battery is used in this temperature range(or the performance requirements in this temperature range are not stated), especially the low temperature -30 °C requirements(such as discharge capacity requirements at this temperature, power requirements, etc.).

Regarding the requirements of power cells at high or low temperatures, let's first look at how the relevant regulatory standards are stipulated:

1. QC/T743 -2006 Lithium-ion batteries for electric vehicles. This is the old battery standard that was previously implemented. The requirements related to high temperature and low temperature are mainly for monomer batteries:

· 20 ± 2 °C C/3 discharge capacity not less than 70 % of the rating

· 55 ± 2 °C C / 3 discharge capacity not less than 95 % of the rating

· 55 ± 2 °C 100 % SOC storage 7 days after the charge retention rate is not less than the rating 80 %, capacity recovery is not less than 90 %

2. GB/T31486-2015 electric storage battery electrical performance requirements and test requirements for electric vehicles. This is the latest national standard requirement for monomer batteries and modules. The performance requirements for battery modules at high and low temperatures are:

Discharge capacity of 1C at -20 ± 2 °C not less than 70 % of initial capacity

Discharge capacity of 1C at 55 ± 2 °C not less than 90 % of initial capacity

After 7 days of 100 % SOC storage at 55 ± 2 °C, the charge retention rate shall not be less than 85 % of the initial capacity, and the capacity recovery shall not be less than 90 % of the initial capacity.

3. GB/T31467.1 / 2 -2015 Lithium-ion active batteries and systems for electric vehicles part 1/2: high power/high energy application testing procedures. The standard series is a requirement for battery packs/systems, providing only test methods and not specific requirements. The requirements related to high and low temperatures are:

:: Maximum and minimum temperature for capacity and energy tests(this is a continuous discharge of 1C): 40 °C and -20 °C

:: Maximum and minimum temperatures for power and internal resistance tests(short periods of high current discharge): 40 °C and -20 °C

:: No attached capacity loss test with a maximum temperature of 40 °C

:: Test for loss of capacity in storage with a maximum temperature of 45 °C

· High and low temperature starting power test, maximum temperature, minimum temperature: 40 °C and -20 °C

:: Energy efficiency test, maximum temperature, minimum temperature: 40 °C and -20 °C

Taking the maximum and minimum values, you can see that the current standard's temperature requirements are:

Battery monomers and modules: -20 ~ 55 °C

Battery pack/battery system: -20 ~ 45 °C

Comparing the objectives of the Action Plan for Promoting the Development of Automotive Power Battery Industry, we can see:

1. Battery monomer / module

The high temperature target is consistent with the current monomer / module high temperature

:: The low temperature target is 10 °C lower than the current standard, reaching -30 °C

2. Battery pack / system

The high temperature target is 10 °C higher than the current battery pack/system temperature, reaching 55 °C

:: The low temperature target is 10 °C lower than the current standard, reaching -30 °C

Compared with room temperature 20 °C, the capacity attenuation at -20 °C is already relatively obvious. At -30 °C, the capacity loss is more, and at -40 °C, the capacity is less than half.

The lower the temperature, the lower the conductivity of the battery electrolyte. When the conductivity drops, the ability of the solution to conduct active ions decreases, as the resistance of the internal reaction of the battery increases (this resistance is expressed by impedance in the electrochemical), causing a decrease in discharge capacity, that is, a decrease in capacity. Furthermore, by measuring the impedance of various parts inside the battery (positive, negative, and electrolyte), the influence of each part on the impedance of the battery can be seen. When the temperature is about <-10 °C, the interface impedance of the positive electrode and the negative electrode (in the figure, graphite) increases rapidly, and the impedance of the electrolyte rises rapidly after about -20 °C. The integrated results of these impedances are expressed as batteries. The impedance rises rapidly around <-10 °C.

Saft, a famous French battery company, studied the effect of high temperature on battery performance through 2Ah cylindrical batteries(positive material NCM, PVdF binder, negative material carbon, CMC/SBR binder) and compared the two batteries at different temperatures. The situation:

· 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

After the B2 battery circulates 26 times at 85 °C, the capacity loss is about 7.5 %, and the battery impedance increases by 100 %; After the B3 battery is recycled 25 times at 120 °C, the capacity loss is about 22 %, and the battery impedance increases by up to 1115 %.

Change of the positive electrode of the battery at a high temperature of 120 °C. At 120 °C, some of the positive polar binders PVdF migrated from the Part1 region to the positive surface, which caused the binder content in the Part1 region to decrease, and the active material NMC material caused the electrochemical reaction due to the lack of binders. The ability to decrease. In the Pat2 region, this part is the main body of the positive pole. The content of the binder is normal, the high temperature has little effect, and the active material can react normally.

The effect of high temperature on the negative electrode, the initial state of the negative electrode, after the cycle at 85 °C, the common solid electrolyte phase appears on the negative electrode surface(Figure 6B negative electrode surface is covered by newly generated material, resulting in the surface morphology and initial morphology. Different, Some small spherical matter. SEI: Solid Electronic Interface). When the temperature rises at 120 °C, more SEI(Figure 6C, the negative surface is covered by more particles) is generated, and more active lithium ions are consumed, resulting in a decrease in capacity.

In general, the factors that affect the high and low temperature of the battery can be summarized as follows: the conductivity of the electrolyte, the interface impedance, and the SEI membrane. These factors combine to affect the performance of the battery. In general, increasing the conductivity or conductivity of each component of the battery(including selecting a more conductive active material, optimizing the electrolyte composition, improving the negative SEI membrane composition, inhibiting the dissolution of positive surface substances, etc.), thus reducing the overall battery impedance, It is helpful to improve the high temperature and low temperature performance. The adaptability of lithium-ion batteries to temperature is the same as that of the human body. Too high and too low temperatures are not conducive to its maximum function. Choosing suitable materials, optimizing structural design, and customizing suitable conditions for use can give full play to its performance.

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