In grid energy storage applications, lithium batteries are rapidly replacing traditional energy storage systems. By the end of 2022, lithium batteries will account for 94.5 percent of global energy storage systems. However, when thermal runaway occurs, an exothermic reaction occurs inside the battery, and heat and reaction gas continue to accumulate inside the battery, resulting in an increase in the internal pressure of the battery. For the energy storage system, the thermal runaway of the monomer will cause heat propagation, which will cause the entire battery pack to catch fire and explode. In addition, thermal runaway produces a large amount of toxic gas, causing serious harm to the safety of users and firefighters and the environment.
Therefore, the early monitoring of battery thermal runaway is very necessary, and the research of gas production detection technology has become a hot spot in the field of energy storage. Ankejie will introduce the gas production process and gas production detection method of the battery, it provides reference for the development and standardization of gas production analysis and detection technology.
under the conditions of thermal abuse, electrical abuse and mechanical abuse, exothermic chemical reactions will occur inside the battery, releasing a large amount of heat and gas, such: solid electrolyte phase interface film decomposition reaction (90~120 ℃), negative electrode and electrolyte reaction (100~350 ℃), electrolyte decomposition reaction (110~300 ℃), diaphragm shrinkage and melting reaction (>130 ℃), positive electrode and electrolyte reaction (200~300 ℃) and binder decomposition reaction (200~300 ℃), etc. (fig. 1). From the internal reaction of the thermal runaway battery, it can be seen that the gas produced by the battery mainly contains CO. 2, CO, H 2, C x hy, C x H y O z, C x H y f, POF 3 and HF, etc.
2.1. Gas production and detection method
gas production is an important parameter of thermal runaway, which is not only related to the pressure relief design of the battery, but also related to the concentration of each component. By measuring the gas temperature before and after the reaction, the amount of gas generated by thermal runaway was calculated. In the actual thermal runaway process, the gas will have a large temperature gradient, which leads to the deviation of the calculated gas production. In addition, the gas production is also related to the battery material system, the SOC of the battery and other factors. Under the same capacity or quality, the gas production of the ternary system is higher than that of the iron lithium system; the higher the SOC, the higher the gas production.
2.2. Gas composition and detection method
as described in the gas production mechanism section, thermal runaway produces a variety of gases, among which, CO 2, CO, H 2 and C x H y of it is relatively large. At full charge, the thermally runaway gas H 2, CO 2 and the mass fraction of CO is higher than 80%, and the rest is C. x H y. The thermal runaway gas composition of the battery will be affected by the SOC. When the SOC increases from 50% to 150, the CO 2 the volume fraction decreased and the CO volume fraction increased. The determination of gas components can be carried out by the following characterization means:
2.2.1. Gas chromatography-mass spectrometry (GC-MS)
after the mixed gas enters the chromatographic column, due to the different adsorption rates of the adsorbent for different components, the different components are separated into the mass spectrum, and the gas-producing components are identified by the charge-mass ratio m/Z. The method of GC-MS detection of gas production components is mature and simple, and a small amount of gas production components can be injected into the GC-MS for measurement. After the standard gas is obtained, the quantitative analysis of the gas components can also be realized. In addition to the mass detector, FID,TCD the detector is also commonly used in gas chromatography to analyze the composition of battery gas. The advantage of TCD is that it can respond to both organic and inorganic substances. The disadvantage is that it is very sensitive to parameters such as temperature and gas flow, and it is more demanding on environmental conditions.
2.2.2. Fourier transform infrared absorption spectroscopy (FTIR)
the principle of FTIR analysis is to analyze the composition of mixed gas through the characteristic absorption spectra of different gases to infrared rays. Unlike GC-MS method, infrared gas analysis can make online measurement. ISO 19702:2015 "Guidelines for sampling and analysis of toxic gases and vapors in flame exhaust gas by Fourier transform infrared (FTIR) spectroscopy" details the sampling and analysis methods for detecting the composition of mixed gas by infrared method.
2.2.3. Differential Electrochemical Mass Spectrometer (DEMS)
mass spectrometry through the ionization of each component gas, according to the different charge ratio of separation statistics, the formation of mass spectrometry, through the different component gas characteristic mass spectrometry for component identification. By coupling with electrochemical reaction devices and mass spectrometers, instruments that can be used to detect volatile gas products in electrochemical reactions on-site have gradually been developed. Differential electrochemical mass spectrometry-electrochemical infrared spectroscopy is a combination of mass spectrometry and infrared spectrometer (DEMS-DEIRS) with electrochemical system, which can more accurately measure the changes of common battery gas production components on-line.
2.3. Gas production distribution and detection method
the research on the distribution of gas production is very important for evaluating the stability of the internal structure of the battery and monitoring the internal pressure and stability of the battery. The following advanced methods are mainly adopted for characterization:
2.3.1. Synchrotron X-ray Tomography
synchrotron X-ray tomography is the detection of high-energy X-rays through the sample under test, and based on the detection of the attenuation or phase shift of the rays, the reconstruction of a cross-section through the object. By stacking these cross sections together, three-dimensional stereoscopic visualization structure reproduction can be achieved. As shown in Figure 5, synchrotron X-ray tomography can achieve three-dimensional visualization of gas production and visually display the generation and flow of gas production at the electrode.
2.3.2. Ultrasonic nondestructive testing
ultrasonic testing is a non-destructive technology that uses ultrasonic waves to detect the internal structure of materials. Ultrasonic in the gas phase will occur significant attenuation, and in the solid, liquid phase attenuation is small, and in the gas-liquid, gas-solid interface reflection, using this phenomenon can detect the distribution of gas production inside the battery. 3 μL of gas was injected into the cell and observed under ultrasonic scanning in comparison with a cell without gas injection, as shown in FIG. 6, although the amount of gas was small, the state of gas diffusion in the cell could be clearly observed under ultrasound.
Thermal runaway is a hot spot in the safety research of lithium-ion battery. The existing research on the internal chemical reaction of thermal runaway battery is more thorough, and the main gas composition of thermal runaway is relatively clear, but the gas production measurement is lack of accurate and operable method research, the research on gas toxicity and explosion risk is not high, and the research depth needs to be improved.
The main direction of future energy storage battery gas production research will focus on large-size battery test methods and standardization, leading battery manufacturers to improve their technical level, ensure battery safety, and maintain consumers' lives and health.
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