Lithium iron phosphate batteries and ternary lithium batteries in batteries have the advantages of high energy density, wide operating temperature range, long cycle life, safety and reliability, and are widely used in power batteries for new energy vehicles. However, lithium batteries generate reversible reaction heat, ohmic heat, polarization heat and side reaction heat during the charge and discharge process. The heat generated by the battery is mainly affected by its internal resistance and charging current.
Power batteries are very “delicate”. Temperature has a very significant impact on the overall performance of power batteries, which is mainly reflected in three aspects: performance, lifespan and safety. The application of power batteries in electric vehicles generally requires comprehensive consideration of the impact of temperature on battery performance, life and safety to determine the optimal operating range of the battery, and to obtain the best balance of performance and life within this temperature range. It is generally believed that the optimal operating temperature range of the battery is 20℃~30℃. In actual projects, the optimal operating temperature of the battery needs to be determined based on the battery-related thermal test results.
The capacity of lithium battery will change as the temperature increases. Through tests, it was found that the capacity increases by 0.8% for every 1°C increase in temperature. However, the increase in temperature will also damage the battery, and the battery cycle life and capacity will gradually decrease. According to tests, in an environment with a normal temperature of 25°C, if the temperature rises by 6~10°C, the battery life will be reduced by half because the high temperature will increase the float charge current of the battery. Due to the accumulation of overcharge, the cycle life of the battery is shortened.
The capacity of lithium batteries increases with temperature. If the battery temperature increases and the total discharge remains unchanged, the depth of discharge will decrease. When the temperature of the battery rises to 45°C, the service life can be extended. If the battery is charged in an environment with a temperature higher than 50°C, the acid will accelerate corrosion on the battery plates, and the increase in temperature will accelerate the aging of the battery casing.
Changes in temperature cause the available capacity of lithium batteries to attenuate to varying degrees. The specific reference level is: the available capacity at -10°C is 70%, the available capacity at 0°C is 85%, and the available capacity at 25°C is 100%. Therefore, it is normal for battery performance to decrease when the weather gets cold. When the temperature decreases, the battery discharge voltage also decreases significantly. In this way, the battery will reach the discharge cut-off voltage faster when discharging at low temperature, resulting in the low-temperature discharge capacity being significantly lower than the normal temperature capacity. .
Effect of low temperature on battery performance
When a lithium-ion battery is at a low temperature, its available capacity is reduced and its charge and discharge power is limited. If the power is not limited, it will cause the precipitation of lithium ions inside the battery, causing irreversible attenuation of battery capacity and posing safety risks to the use of the battery. The lower the ambient temperature, the lower the activity of the active materials in the battery, the higher the internal resistance and viscosity of the electrolyte, and the more difficult it is for ion diffusion. Moreover, the diffusion rate of lithium ions in the electrode is slow at low temperatures, making it difficult to embed and easy to escape, thus making Capacity drops rapidly, so use at low temperatures will have a big impact on battery life.
I believe everyone has similar feelings. The use time of lithium batteries in winter is shorter than in summer. It can be seen that the performance of lithium battery is affected by the ambient temperature. Among all environmental factors, temperature has the greatest impact on the charge and discharge performance of lithium batteries. People in the lithium battery industry generally know that whether the charge and discharge state of lithium batteries is stable or not, temperature changes play a big influencing factor. When lithium batteries are charged and discharged in high and low temperature environments, the capacity retention rate of lithium batteries will decrease. .
What needs to be explained to everyone is that the capacity of lithium-ion batteries at low temperatures does not disappear, but it just cannot be fully discharged within the normal voltage range (≥3.0V). If the discharge cut-off voltage can be continued to be extended downward, the remaining capacity can be capacity is released.
The electrochemical reaction at the electrode/electrolyte interface is related to the ambient temperature, and the electrode/electrolyte interface is regarded as the heart of the battery. If the temperature decreases, the reaction rate of the electrode also decreases. Assuming that the battery voltage remains constant and the discharge current decreases, the battery’s power output will also decrease.
Figure 1 is a schematic diagram of the discharge capacity curve of lithium-ion batteries at different low temperatures (here used to represent the general trend). Compared with the room temperature of 20°C, the capacity attenuation at low temperature of -20°C is already more obvious. At -30°C, the capacity loss is even greater, and at -40°C, the capacity is less than half.
Here we look at the factors that affect low temperature performance. By comparing the relationship between capacity and electrolyte conductivity (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 manifested as an increase in the resistance of the internal reaction of the battery (this resistance is represented by impedance in electrochemistry), resulting in a decrease in discharge capacity, that is, a decrease in capacity. Furthermore, by measuring the impedance of each part of the battery (positive electrode, negative electrode, electrolyte), we can see the impact of each part on the battery impedance (Figure 3). When the temperature is <-10℃, the interface impedance of the positive and negative electrodes (graphite is used as an example in the figure) increases rapidly, and the impedance of the electrolyte rises rapidly after about -20℃. The combined results of these impedances are manifested as a rapid increase in battery impedance at <-10℃ (represented by Li-ion cell in the figure).
The relationship between battery capacity and electrolyte conductivity at different temperatures
The impedance of each part of the battery at different temperatures
Compared with low-temperature discharge, the performance of low-temperature charging of lithium-ion batteries is even more unsatisfactory. Charging below 0 at low temperatures will increase the internal pressure of the battery and may cause the safety valve to open. First, charging at low temperatures will quickly reach the constant voltage stage, and will reduce the charging capacity to a certain extent, while increasing the charging time. Not only that, when lithium-ion batteries are charged at low temperatures, lithium ions may not have time to embed into the graphite negative electrode, thereby precipitating on the surface of the negative electrode to form metal lithium dendrites. This reaction will consume the lithium ions in the battery that can be repeatedly charged and discharged, and greatly reduce the battery capacity. The precipitated metal lithium dendrites may also pierce the diaphragm, thereby affecting safety performance.
The low-temperature discharge capacity of lithium-ion batteries will decrease, but it can be restored after normal temperature charging and discharging, which is a reversible capacity loss; however, low-temperature charging will cause lithium precipitation, which is a permanent capacity loss. Since the harm of lithium precipitation during low-temperature charging is greater, the low-temperature charging of lithium-ion batteries is more strictly controlled than low-temperature discharge.
When charging in winter, the outdoor temperature is low, and the environment is below 0℃, the battery charging speed will decrease, or even may not be able to charge. This is normal. Please charge the battery in an appropriate ambient temperature to ensure the charging effect.
Effect of high temperature on battery safety performance
Lithium battery temperature is too high, exceeding 45℃. Lithium-ion batteries are increasingly widely used in people’s production and life, which makes its temperature environment a key point of concern. Relatively speaking, lithium batteries are more likely to have safety problems in high temperature environments. Therefore, it is necessary to test the high temperature performance of lithium batteries and compare them with their normal temperature test data. When lithium-ion batteries are abused or misused, such as use at high temperatures or charger control failure, they may cause violent chemical reactions inside the battery and generate a large amount of heat. If the heat cannot be dissipated in time and quickly accumulates inside the battery, the battery may leak, release gas, smoke, etc. In severe cases, the battery will burn violently and explode.
The chemical reactions that occur in batteries at high temperatures mainly include:
(1) Decomposition of SEI film: The protective film is metastable and will decompose and release heat at 90-120℃.
(2) Reaction of embedded lithium and electrolyte: Above 120℃, the film cannot isolate the contact between the negative electrode and the electrolyte, and the lithium embedded in the negative electrode reacts with the electrolyte to release heat.
(3) Electrolyte decomposition: Decomposition occurs at temperatures above 200°C and releases heat.
(4) Positive electrode active material decomposition: In the oxidized state, the positive electrode material decomposes exothermically and releases oxygen, which in turn reacts exothermically with the electrolyte, or the positive electrode material reacts directly with the electrolyte.
(5) Exothermic reaction of embedded lithium and fluoride binder.
Saft, a famous French battery company, once studied the effect of high temperature on battery performance through 2Ah cylindrical batteries (positive electrode material NCM, using PVdF binder, negative electrode material carbon, using CMC/SBR binder), and compared the conditions of two batteries at different high temperatures:
B2 battery – first cycled twice at 60℃, then cycled at 85℃
B3 battery – first cycled twice at 60℃, then cycled at 120℃
As can be seen from Figure 4, after 26 cycles at 85℃, the capacity of the B2 battery lost about 7.5%, and the battery impedance increased by 100%; after 25 cycles at 120℃, the capacity of the B3 battery lost about 22%, and the battery impedance increased by as much as 1115%.
The model in Figure 5 is used to illustrate the changes in the positive electrode of the battery at a high temperature of 120°C. At 120°C, part of the positive electrode binder PVdF migrates from the Part 1 area to the positive electrode surface, which causes the binder content in the Part 1 area to decrease. The active material NMC material has a reduced ability to react electrochemically due to the lack of binder. In the Part 2 area, which is the main body of the positive electrode, the binder content is normal, and the high temperature has little effect, and the active material can react normally.
By analyzing the surface of the negative electrode, we can see the effect of high temperature on the negative electrode (Figure 6). Figure 6a is the initial state of the negative electrode. After cycling at 85°C, a common solid electrolyte phase appears on the surface of the negative electrode (Figure 6b shows that the surface of the negative electrode is covered by newly generated substances, causing the surface morphology to be different from the initial morphology, with some small spherical substances. SEI: Solid Electrolyte Interface). When the temperature rises to 120°C, more SEI is generated (Figure 6c, the surface of the negative electrode is covered by more particles), consuming more active lithium ions and causing a decrease in capacity.
Effect of high temperature on battery life
Excessive operating temperature: On the one hand, the anode reduction electrolyte at a low potential for a long time causes the loss of active lithium ions, resulting in a decrease in electrochemical performance; on the other hand, high temperature increases the side reactions of the anode reduction electrolyte, and the inorganic products of the reaction are deposited on the anode surface, hindering the deintercalation of lithium ions and accelerating the aging of the battery. Battery side reactions increase at high temperatures, such as the decomposition, rupture or dissolution of the SEI film on the surface of the negative electrode, which leads to continuous consumption of lithium ions during the cycle at high temperatures and a rapid decrease in capacity.
Ahmad A. Pesaran’s research shows that when the battery operating temperature exceeds 40°C, the battery cycle life will be halved for every 10°C increase. The battery pack is closely arranged in the battery compartment of new energy vehicles, and the heat accumulation generated by the single cells causes a temperature difference inside the battery pack, resulting in different attenuation rates of the single cells, 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 charging and discharging with a small current, the highest temperature position of the battery pack is in the middle of the battery where it is not easy to exchange heat with the outside world. When charging and discharging with a large current or the ear structure design is unreasonable, the highest temperature of the battery pack is at the ear.
Therefore, according to the characteristics of the power battery and the working environment, the battery cooling system can be reasonably designed to improve not only the endurance performance of the vehicle, but also the safety and reliability of the vehicle.
The impact of temperature difference on battery performance
The battery temperature difference is mainly divided into two types: the internal temperature difference of the battery, which is manifested as the uniformity of the battery temperature; the temperature difference between battery cells, which is manifested as the consistency of the battery temperature.
Causes of internal temperature difference: Generally, in low-temperature heating conditions or high-temperature heat dissipation conditions of water cooling systems, when the battery module is in single-sided heating or single-sided cooling, a large internal temperature difference will occur due to the large thermal resistance of the battery cell itself. This temperature difference is related to the internal structure and material components of the battery, and it is difficult to avoid from the perspective of thermal management system design.
Causes of temperature difference between cells: The temperature difference between battery cells is mainly determined by the battery module layout and the battery thermal management structure, and the temperature difference can be reduced by optimizing the thermal management design.
Effect of temperature difference inside cells on batteries
Excessive temperature difference inside the battery will cause uneven impedance, uneven current distribution, and uneven heat generation inside the battery, which will affect the battery performance and accelerate the battery capacity decay. However, the difference between cells is generally small, and the impact on consistency is small.
Effect of temperature difference between cells on batteries
Excessive temperature difference between battery cells will cause inconsistent performance and capacity decay rates of each battery cell in the assembly. Since the battery cells in the battery pack are connected in series, the performance decline and capacity decay of any battery cell will affect the overall performance of the assembly. Therefore, it is very important to control the consistency of battery temperature.
In addition, the temperature difference between cells will have a continuous cumulative impact on the battery. Cells with high temperatures age faster, generate more heat, and are more likely to generate high temperatures.
Conclusion
Different types of lithium batteries have different operating temperature ranges. Too high or too low temperature will affect the performance of lithium batteries, and in severe cases may even shorten the service life of the battery. In order to charge effectively, the ambient temperature range of lithium batteries should be between 20-30℃.
In general, the factors that affect the high and low temperatures of batteries can be summarized as: electrolyte conductivity, interface impedance, SEI film, etc. These factors work together to affect the performance of the battery. Generally speaking, improving the conductivity or electrical conductivity of each component of the battery (including selecting active materials with better conductivity, optimizing electrolyte composition, improving the negative electrode SEI film composition, inhibiting the dissolution of positive electrode surface substances, etc.), thereby reducing the overall impedance of the battery, is helpful for improving high and low temperature performance. The adaptability of lithium-ion batteries to temperature is the same as that of the human body. Too high or too low temperatures are not conducive to their maximum function. Only by selecting suitable materials, optimizing structural design, and customizing appropriate use conditions can their performance be fully exerted.
Post time: Jul-04-2024