Investigation of thermal running causes in lithium-ion batteries and investigation of gas release

Lithium-ion batteries (LIB) are preferred due to their advantages and are used intensively in many areas including electric vehicles (EV). The main reason for the demand for these batteries with high energy density is that they are advantageous in terms of environment and fuel cost. Some safety problems can be experienced in lithium-ion batteries with high energy density, low weight, fast charging feature and low memory effect. Thermal runaway fires can occur as a result of external factors. In some extreme conditions, a large amount of chemical energy stored in a limited area can suddenly escape and turn into heat. It has been observed by researchers that fire accidents are usually triggered by various abuses (thermal, mechanical and electrical abuse) such as overheating, nail penetration, crushing, overcharging and discharging. The typical thermal runaway process occurs in the form of the breakdown of the solid electrolyte interface layer, the reaction of the electrode material with the electrolyte and the decomposition of the electrolyte. As a result of these exothermic reactions, the temperature of the battery can increase and a vicious cycle can occur that will lead to accidents such as fire etc. At the same time, flammable and toxic gas products released during the reaction process can cause sudden pressure increases, causing the battery to swell, create jet flames or even explode. Therefore, it is very important to detect thermal runaway formation in a timely manner. Since gas release occurs before smoke and fire are observed, detection of thermal runaway gases in LIBs can detect the fault at an early stage and prevent spreading. Overcharge and discharge times are an issue that must be taken into consideration due to the chemistry of these batteries. Because temperature ranges are important for LIBs. In cases where the battery temperature is high, the battery ages rapidly, while long discharge times shorten the cycle life. These situations are important for lithium-ion battery users and battery safety. Lithium-ion batteries come together to form batteries, and batteries come together to form cells. When the internal temperature of the cells reaches the range of 90 °C to 120 °C, decomposition may begin and the solid electrolyte interface layer initiates an exothermic reaction. The hydrocarbon electrolyte can decompose above 200 °C. Thermal runaway can start when the separator ruptures due to impact or puncture. Overcharging outside the specified charging specifications can cause dendrites to form on the coating or anode surface of the lithium-ion battery. These dendrites can eventually penetrate the separator and come into contact with the electrodes, causing a short circuit between the electrodes. Internal short circuits can start due to damage to the battery separator due to high charging current density, low temperature conditions, failure of the cathode and anode active materials, and precipitation of the electrolyte material, etc., while external short circuits can start due to high temperatures. Battery management systems are incorporated into batteries to ensure safe operation and management of batteries. The safety features of lithium-ion batteries depend on the physical shape of the cell (geometry, materials), its physical condition (state of charge, aging effects), how it is stressed (low or high temperature, overcharge or overdischarge, heating), and preventive measures against thermal runaway (internal safety devices, cooling, limitation). Thermal runaway can be caused by a fault or by human misuse in daily use. The effects that may occur as a result of improper use can be divided into three groups. These are mechanical effects, electrical effects and thermal effects. In addition, heat release rate (HRR) is an important parameter for assessing the fire risk of the battery and electric vehicle. Lithium salts in the electrolyte of lithium-ion batteries can produce HCl during thermal runaway. CO release may occur due to compounds that cannot be fully burned. If the ideal combustion process is achieved, CO2 release may begin. Organic solvents used in the battery content can turn into organic compounds. In line with this information, it is important to be able to monitor the gases that may be released during the thermal runaway process through sensors. Early detection of gases can increase the safety of the battery. Thesis writing stages were planned after the literature research. In the thesis study where lithium-ion batteries with a cell size of 18650 were used, thermal runaway, gas emissions and temperature changes that may occur after the impacts that the battery may receive from external factors were observed. Lithium-ion batteries with a capacity of 2400-2800-3600-4800 mAh at 0-60-70-80% charge rates were pierced with a nail and the results were observed. In this study, a thermal camera is used to observe the temperature changes of the LIB, which contains sensors that detect CO2 (carbon dioxide), O2 (oxygen), HCl (hydrogen chloride), CO (carbon monoxide), H2 (hydrogen) and C2H4 (ethylene) emissions. It is of great importance to be able to determine the factors that can prevent lithium-ion battery fires, reduce or stop the spread of thermal runaways, minimize the loss and damage that may occur and increase safety in electric vehicles. Lithium salts in the electrolyte of lithium-ion batteries can produce HCl during thermal runaway. CO emissions may occur due to compounds that cannot be fully burned. If the ideal combustion process is provided, CO2 emissions may begin. Organic solvents used in the battery content can turn into organic compounds. In line with this information, it is important to be able to monitor the gases that may be released during the thermal runaway process via sensors. Early detection of gases can increase battery safety. Therefore, safe management of batteries is very important to reduce the risk of fire and explosion. The charging rates of batteries are closely related to the thermal runaway process. When batteries are above an average of 60-70% SOC levels and the battery temperature starts to exceed 90 ᵒC, the risk factor is high. In the experiments conducted, lithium-ion batteries with 2400-2800-3600-4800 mAh power and 18650 cell sizes at 0-60-70-80% SOC levels were pierced with a nail and the results were observed. 30 different batteries were pierced under the same environmental conditions. The onset of thermal runaway and gas emissions that may occur in the batteries were examined. It is known that fire occurs when flammable gases contained in lithium-ion batteries are released due to the impact effect due to their chemistry. In this study, the release of 6 different gases was detected and examined with sensors. As a result of the information obtained from the puncture of many batteries, it was observed that when thermal runaway started, gas emission started before the batteries caught fire. The ppm values reached by HCl, CO2, O2, CO, H2, C2H4 and VOCs were recorded. Temperature measurements of the batteries that caught fire were made with a thermal camera. After the highest temperature reached was recorded, the batteries were observed for 20-30 minutes. Batteries whose temperatures started to drop were collected in a separate area and new battery drilling was started. 9 batteries with different characteristics, some with a power of 2800 mAh. were exposed to various external factors and observed at 0% SOC level. 9 batteries, each with a power of 4800 mAh and a level of 60% SOC, were punctured separately and their data was recorded. No flame was observed in any of these batteries. The highest temperature reached by the batteries was 177 ᵒC. Gas emission occurred in the range of 1-29 seconds. At 70% SOC level, 7 batteries with 2400 mAh. power were individually punctured and observed. Gas release occurred between 2-18 seconds. 5 types of gas release were recorded by the sensors. While 5 batteries caught fire, no flame was observed in 2 of them. The highest temperature reached by the batteries was recorded as 290 ᵒC. At 80% SOC level, 8 batteries with 3600 mAh. power were individually punctured and observed. Gas release occurred between 3-23 seconds. While 7 batteries caught fire, no flame was observed in 1 of them. The highest temperature reached was recorded as 265 ᵒC. It is envisaged that LIB fires can be prevented through early detection of the released gases and improvement studies to be carried out in the battery packs before the fires break out. A postgraduate thesis study was completed by comparing the experimental data with the literature. Impact angle, battery charge level, exposure of the internal structure of the battery to oxygen, puncture occurrence in one or two notes, etc. Many factors such as affect the thermal runaway process. The postgraduate thesis was completed by comparing the experimental data with the literature. 9 batteries, some of which were 2800 mAh. and different types, were observed by exposing them to various external factors at 0% and 80% SOC levels. TR formation varies according to the SOC level of the batteries. At 60% SOC level, 9 separate batteries with 4800 mAh. were pierced and their data was recorded. No flame was observed in any of these batteries. The maximum temperature reached by the batteries was recorded as 177 ᵒC. Gas release occurred in the range of 1-29 seconds. At 70% SOC level, 7 separate batteries with 2400 mAh. were pierced and observed. Gas release occurred in the range of 2-18 seconds. The release of gases was recorded by sensors. While 5 of the batteries caught fire, 2 did not. The maximum temperature reached by the batteries was recorded as 290 ᵒC. 8 separate batteries with a capacity of 3600 mAh. at 80% SOC level were observed by piercing. Gas release occurred in the range of 3-23 sec. While 7 batteries caught fire, 1 did not catch fire. The maximum temperature reached was recorded as 265 ᵒC. Being able to determine the factors that can prevent lithium-ion battery fires, reduce or stop the spread of thermal runaway, is of great importance in order to minimize the possible losses and damages and increase the safety in electric vehicles. Many factors such as the angle of impact on the batteries, the SOC level of the battery, the contact of the battery chemistry with the air, the location and number of impacts, etc. affect the thermal runaway process. In the experiments conducted, the lowest gas release period occurred in 1 second, while the highest battery temperature was recorded as 325 ᵒC. One of the batteries that started releasing gas at the 1st second was observed to have a temperature of 177 ᵒC and the other at 110 ᵒC, but no flame occurred in these batteries. The other lowest gas release started at the 3rd second. The temperature reached by this battery was recorded as 262 ᵒC, and the battery caught fire at the 24th second. The battery with the highest temperature recorded as 325 ᵒC has a power of 2800 mAh and an SOC level of 80%. While gas release started at the 7th second, one of the batteries showed an explosion reaction at the 22nd second, and the other at the 29th second. There was no flare-up in batteries at 0% level. There was no flare-up in batteries at 60% level. Only HCl release was observed in these batteries. The highest ppm value of H2 in batteries at 70% level was observed as 349. The highest ppm value of H2 in batteries at 80% level was recorded as 227 ppm.