It is generally believed that the vehicle battery system should have high energy density and high power density, and also need to meet the long calendar life and cycle life, and the system products have certain electrical, thermal and mechanical safety. In order to meet these requirements, the battery system health needs to be managed and detected, usually mainly through BMS, BMSThrough the analysis of the relevant data transmitted by some electrical sensors and thermal sensors, the health status of the battery system is accurately analyzed and instructions are made to ensure the safe operation of the system. On the other hand, lithium-ion batteries may fail and decline, and in the process of lithium-ion battery failure, it is often accompanied by changes in gas generation and shape and size, and chemical and mechanical sensors can be installed in the system to better detect the health status of the product. A more advanced topic relates to in-situ chemical sensors mounted directly in the battery, which monitor the safety of electrode or electrolyte materials in real time by detecting specific chemical markers to indicate degradation processes of electrode or electrolyte materials. This will significantly improve the safety of automotive battery systems, eliminating the risk of sudden and unpredictable thermal runaway events.
1. Mechanical sensors
In the process of charging and discharging lithium-ion batteries, with the phenomenon of expansion and contraction of lithium intercalation and deintercalation of positive and negative electrode materials, in addition to the volume change of electrode material, the occurrence of some side reactions in the battery often produces some gases and may also affect the change of battery pack size. Therefore, monitoring the size changes of the battery during its use is important to determine the state of health and prevent safety issues. According to relevant research, there are various technical means that can measure this change in situ, mainly including:
The first is to use optical technology for measurement, mainly digital image method (DIC), laser beam position detector (LBPD) and multi-beam optical stress sensor (MOSS). Among them, DIC is mainly used to characterize the stress change of electrode in lithium-ion batteries, LBPD and MOSS are the use of optical technology to measure the change of curvature, which can be used to study the bending degree of the electrode surface of lithium-ion batteries. Of the three methods, DIC is faster and less expensive, while LBPD and MOSS are characterized by easy data processing and accurate measurement of the degree of variation in the curvature of the pole piece. However, these three types of techniques can only be used to characterize electrodes or individual cells and cannot be integrated into battery management systems (BMS) of commercial battery systems.
Second, resistive strain sensors: strain sensors are resistive sensors used to measure the strain of objects, and their cost is low. The structure is shown in the figure below, by bonding the sensor to the object under study, when the stress of the object leads to deformation, the impedance of the sensor changes, and the strain of the object is detected through this change relationship. Some researchers used resistive strain sensors in 18,650 batteries, the change in cylindrical cell diameter increases as the battery ages, and suggest the use of strain gages as a diagnostic tool to predict sudden battery failure and potential safety issues. Although current resistive strain sensors cost between 1 and 3 euros, integrating them into each cell of the battery pack still comes with a large cost increase.
Third, fiber optic sensors: Fiber optic (FO) sensors are very promising in monitoring battery packs in cars. The main advantages of fiber optic sensors include: (1) insulation makes them immune to electromagnetic noise; (2) Light weight; (3) Corrosion resistance, even resistance to the corrosion of hydrofluoric acid. Optical fibers can be coupled into various sensors that indicate battery health by measuring the relationship between strain and temperature, etc. The researchers looked at the increased cost of fiber optic sensors in automobiles, with a price of $10 per fiber sensor and $3 per meter of fiber, as well as hardware additions in BMS, and found that monitoring each battery pack was too costly for existing products to afford.
Second, chemical substance sensors
Changes in the size of batteries are not the only indicator of lithium-ion battery health. In fact, gaseous chemicals that form inside the battery and emit to the outside are another important phenomenon to measure battery health and predict battery thermal runaway. Current studies on battery thermal runaway show that several continuous processes are involved in thermal runaway of batteries: including SEI decomposition, anode collapse, reaction with electrolyte, separator melting, decomposition of positive active materials and electrolyte lithium salts, electrolyte solvent oxidation, binder reaction with other substances, etc. The study occurred for batteries with NMC at 100% SoC, the main products are CO2 and H2 when overheated, and CO0 at 25 to 2% SoC , while the content ofH2 and VOCs is less than 15%, and traces of CO are present; SOC is between 50~143%, the main product is CO, followed by CO2 and H2, in the process of overcharging cylindrical batteries based on LFP cathode material, the main products are CO2, followed by H2 and VOC. Most gas sensors are sensitive to these chemicals, especially carbon monoxide and VOCs. Commonly used include electrochemical sensors, semiconductor sensors, NDIR sensors, and chemical sensors.
III. Conclusion
Through analysis, commercial mechanical and chemical sensors can accurately measure battery health in situ and provide early warning of thermal runaway. These mechanical and chemical sensors complement current BMS measurements. Among them, chemical sensors can be used to reveal battery failure by measuring CO or by paying attention to the leakage of organic volatile compounds from the battery electrolyte to warn in advance, compared to mechanical sensors, the main advantage of chemical sensors is that only by using a smaller number of sensors in the system, can achieve the purpose, without causing a large change in the cost of the battery pack. In contrast, a large number of mechanical sensors are required to achieve the purpose of corresponding SOH detection, which increases the cost greatly.