1. National standards for power lithium batteries and the impact of temperature on lithium-ion power batteries:
(1) National standard requirements:
GB/T31486-2015, the national standard for electrical performance requirements and test requirements for power batteries for electric vehicles, is the latest national standard for individual cells and modules. The performance requirements for battery modules at high and low temperatures are as follows: 1C discharge capacity at -202℃ should not be less than 70% of the initial capacity; 1C discharge capacity at 552℃ should not be less than 90% of the initial capacity. After 7 days of storage at 100% SOC at 552℃, the charge retention rate should not be less than 85% of the initial capacity, and the capacity recovery should not be less than 90% of the initial capacity.
(2) The effect of temperature on the charge retention capability of lithium-ion batteries:
Self-discharge of lithium-ion batteries refers to the automatic discharge that occurs when a lithium-ion battery is left unused in an open-circuit environment. Self-discharge directly reduces the amount of electricity the battery can output, thus lowering its capacity. The occurrence of self-discharge is primarily due to the thermodynamic instability of the electrodes in the electrolyte, resulting in redox reactions occurring at each of the two electrodes.
Of the two electrodes, the self-discharge of the negative electrode is crucial. Self-discharge consumes the active material, converting it into unusable heat energy. The magnitude of self-discharge can be expressed as the self-discharge rate, which is the percentage decrease in capacity of a lithium-ion battery over a specified time.
In the formula: Y% is the self-discharge rate; C1 is the capacity of the valve-regulated sealed lead-acid lithium battery before it is stored; C2 is the capacity of the lithium battery after it is stored; T is the storage time of the lithium battery, which is generally expressed in days, weeks, months or years.
The self-discharge rate of lithium-ion batteries is determined by kinetic factors, which mainly depend on the nature of the electrode material, surface condition, composition and concentration of the electrolyte, impurity content, and also on the environmental conditions of storage, such as temperature and humidity.
Lithium-ion batteries experience the greatest capacity loss when stored at high temperatures and the least capacity loss when stored at low temperatures. They also possess strong charge retention capabilities and a wide operating temperature range. Self-discharge is significantly affected by manufacturing processes, materials, and storage conditions, making it a crucial parameter for evaluating lithium-ion battery performance. Lower storage temperatures generally result in lower self-discharge rates, but excessively low or high temperatures can damage the battery and render it unusable.
(3) The effect of temperature on the lifespan of lithium-ion batteries:
Increased temperature affects both the calendar life and cycle life of lithium-ion batteries. The effect of temperature on lithium-ion battery life follows the Arrhenius equation: as the temperature of a lithium-ion battery increases, the reaction rate accelerates. Related studies show that the degradation rate of a lithium-ion battery doubles for every 10°C increase in temperature. For the same battery cell, with 90% remaining capacity, the output capacity is 300 kWh at 25°C, but only 163 kWh at 35°C.
A 10°C increase in temperature reduces the cycle life of a battery cell by nearly 50%. This demonstrates the significant impact of temperature on the cycle life of lithium-ion batteries. Therefore, during the use of lithium-ion batteries, prolonged exposure to high temperatures should be avoided, especially high-rate charge-discharge cycles at high temperatures. This is a crucial reason why temperature control is essential for automotive lithium-ion batteries.
2. Low and high temperature characteristics of lithium-ion batteries for power applications:
(1) Low-temperature characteristics of lithium-ion batteries for power applications:
Figure 1 shows the discharge capacity curves of lithium-ion batteries at different low temperatures. Compared with room temperature of 20℃, the capacity decay is already quite obvious at -20℃, and the capacity loss is even greater at -30℃. At -40℃, the capacity is less than half.
From an electrochemical perspective, the solution resistance and SEI film resistance do not change much over the entire temperature range and have little impact on the low-temperature performance of lithium-ion batteries; however, the charge transfer resistance increases significantly with decreasing temperature, and its change with temperature over the entire temperature range is significantly greater than that of the solution resistance and SEI film resistance.
This is because as the temperature decreases, the ionic conductivity of the electrolyte decreases, while the resistance of the SEI film and the electrochemical reaction resistance increase. This leads to an increase in ohmic polarization, concentration polarization, and electrochemical polarization at low temperatures, which is reflected in the discharge curve of lithium-ion batteries as a decrease in both average voltage and discharge capacity as the temperature decreases.
During the low-temperature charging process, lithium-ion batteries will experience increased ohmic polarization, concentration polarization, and electrochemical polarization, leading to lithium metal deposition, electrolyte decomposition, and ultimately, thickening of the SEI film on the electrode surface and increased SEI film resistance. This manifests as a decrease in the discharge plateau and discharge capacity on the discharge curve.
Under low-temperature conditions, the chemical reactivity of lithium-ion batteries decreases, and the migration of lithium ions slows down. Before lithium ions on the negative electrode surface can be inserted into the negative electrode, they are reduced to metallic lithium and precipitate on the negative electrode surface to form lithium dendrites. This can easily puncture the separator, causing a short circuit inside the battery, which can damage the battery and lead to a safety accident.
(2) High-temperature characteristics of lithium-ion batteries for power applications:
At 120℃, some of the PVdF binder in the positive electrode of a lithium-ion battery migrates from the Part 1 region to the positive electrode surface, resulting in a decrease in the binder content in Part 1. This lack of binder in the active material leads to a decrease in electrochemical reactivity. In Part 2, which is the main body of the positive electrode, the binder content is normal, and the high temperature has little impact, allowing the active material to react normally.
When lithium-ion batteries are cycled at 85°C, a solid electrolyte forms on the surface of the negative electrode, covering it completely. When the temperature rises to 120°C, even more solid electrolyte is generated, further covering the negative electrode surface and consuming more active lithium ions, leading to a decrease in the battery's capacity.
3. Selection and placement of temperature sensors for lithium-ion batteries:
(1) Key points for selecting NTC thermistors:
When using NTC thermistors to collect the temperature inside a lithium-ion battery module, the following factors should be considered when selecting an NTC thermistor:
1) The NTC thermistor casing should be smooth, uniform in color, free from cracks, deformation, and severe scratches. The color of each batch of products (including leads) should be consistent and free from any corrosion. A permanent model number and serial number should be printed on the surface of the casing of each NTC thermistor.
2) Temperature range. Select NTC thermistors of different materials according to the operating temperature range of the application. NTC thermistors are generally composed of a temperature sensing head (metal shell or plastic shell), wires, terminals, connectors, epoxy resin or other filling materials, etc. When selecting, you should choose NTC thermistors of different materials according to different operating ambient temperatures.
3) Accuracy (overall measurement error within 2℃). NTC thermistors exhibit good linearity across the entire temperature detection range, and their characteristics conform to the entire parameter range. The influence of the NTC thermistor resistance accuracy on temperature detection accuracy; the influence of the NTC thermistor B constant accuracy on temperature detection accuracy; and the influence of the NTC thermistor thermal diffusivity constant C on temperature detection accuracy must also be considered.
Accuracy is a crucial performance indicator for NTC thermistors, as it significantly impacts the overall measurement accuracy of a measurement system. Higher accuracy NTC thermistors are generally more expensive; therefore, the accuracy of an NTC thermistor only needs to meet the overall accuracy requirements of the measurement system. Factors determining the accuracy of an NTC thermistor include:
① The inherent error of the NTC thermistor. The smaller the resistance error and B-value error of the NTC thermistor, the higher the measurement accuracy.
② The contact method between the temperature sensing head and the object being measured by the NTC thermistor. Direct contact offers higher measurement accuracy than indirect contact. However, because the RT curve of an NTC thermistor is non-linear, it cannot guarantee consistent accuracy over a wide operating temperature range. Therefore, to obtain higher measurement accuracy, the center operating temperature point of the working environment should be selected (generally, the center operating temperature point has the highest accuracy; due to the dispersion of the RT curve, the accuracy error gradually increases with temperature points farther from the center operating temperature point).
4) During measurement, the NTC thermistor must have a fast response speed, and the time to reach the closest temperature should be as short as possible, not exceeding 10 seconds; otherwise, it will not meet the efficiency requirements for practical application. Different applications require different response speeds for the NTC thermistor, and different materials have different thermal conductivity. Factors affecting the response speed of the NTC thermistor include:
① Thermal time constant of NTC thermistor chip. A smaller thermal time constant results in a faster response speed.
②The thermal conductivity of the NTC thermistor temperature sensor housing material; materials with high thermal conductivity have excellent thermal conductivity.
③ The size of the NTC thermistor temperature sensor: a smaller temperature sensor will have a shorter heat conduction time and a faster response speed.
④ The temperature sensor head of NTC thermistor is filled with thermally conductive adhesive. The temperature sensor head filled with thermally conductive silicone grease with high thermal conductivity will have a faster response speed than the one without thermally conductive silicone grease or filled with thermally conductive silicone grease with low thermal conductivity.
5) For self-heating within a certain range, the resistance value should be selected considering its own heat generation to avoid overheating. Otherwise, the heat generated by the NTC thermistor itself will affect the temperature measurement, and it should have high reliability (excellent thermal shock resistance) and a small thermal time constant (fast response speed).
6) Stability: The ability of an NTC thermistor to maintain its performance unchanged after a period of use is called stability. Factors affecting the long-term stability of an NTC thermistor include the stability and reliability of the NTC thermistor chip, the sensor itself, and its structure, as well as the operating environment. For an NTC thermistor to have good stability, it must have strong environmental adaptability. Factors to consider when selecting an NTC thermistor for stability include:
① Select a highly reliable NTC thermistor.
② Select NTC thermistors with reasonable structure and strong mechanical strength.
③ Appropriate selection of different filling materials for different usage environments.
7) Lifespan: Not less than 6 years, including a 2-year storage period.
8) The NTC temperature sensor should show no mechanical damage or loosening after being subjected to three impacts in an environment of -55℃ to 70℃.
9) Insulation resistance: greater than 10MΩ/500V.
(2) NTC thermistor arrangement:
NTC thermistor
A power lithium battery module consists of multiple cells. Under normal operation, the temperature of the cells in a power lithium battery module is uniform. However, when a power lithium battery module malfunctions, there will be a large temperature difference between the cells in different power lithium battery modules.
Typically, 3 to 4 data collection points are selected to monitor the temperature of the entire power lithium battery module. After the collected temperature data is input into the power lithium battery module management unit, the power lithium battery module management unit calculates the temperature of the entire power lithium battery module management unit.
When designing temperature acquisition points for power lithium battery modules, the following temperature acquisition methods are used:
1) Directly collecting cell temperature usually involves placing an NTC thermistor on the surface of the lithium-ion battery module cell. When the characteristics of the lithium-ion battery module cells are relatively uniform, the NTC thermistor can be placed on the surface of the lithium-ion battery module cell by adhesive bonding.
2) Indirectly sensing the temperature of the battery cells: A typical method is to embed NTC thermistors on the two end plates of the power lithium battery module. This allows for accurate sensing of the temperature of the first and last power lithium battery cells, and the temperature of the entire power lithium battery module can be calculated based on the temperature of the first and last power lithium battery cells.
3) Collect the temperature at the top of the interconnect board of the power lithium battery cell, that is, embed the NTC thermistor into the internal interconnect board of the power lithium battery cell to accurately sense the highest temperature of the power lithium battery cell.
4) Collect the temperature of the power lithium battery module bus. A groove is provided on the power lithium battery module bus. The temperature sensor is fixed in the groove. The groove is provided with fixing adhesive for fixing the temperature sensor.
5) Collect the temperature of the surface of the power lithium battery module cover plate and directly attach the NTC thermistor to the cover plate. When connecting the NTC thermistor to the power lithium battery busbar, cell interconnection board, or bonding it to the surface of the power lithium battery module cells or the cover plate, the influence of the operating process on the NTC thermistor should be considered. Improper operation during the fixing process may cause the NTC thermistor to break, short-circuit, or have its lead coating broken.
Because the ceramic substrate inside an NTC thermistor is a fragile material, excessive pressure or impact must not be applied during connection or bonding. Otherwise, the connection between the lead and the component may break, or the component may crack. Furthermore, the coefficient of thermal expansion of the NTC thermistor across its various materials must be considered during connection or bonding; otherwise, internal stress may damage the NTC thermistor itself.
