Lithium-ion battery safety assessment and testing methods. In recent years, there have been frequent safety incidents involving lithium-ion batteries in electric vehicle power batteries. How to assess and test the safety of lithium-ion batteries and better prevent such incidents has been a major concern in the industry.
The temperature display of a lithium battery reveals its thermal state, which is essentially a result of heat generation and transfer within the battery. Studying the thermal characteristics of lithium batteries, and their heat generation and transfer properties under different states, allows us to understand the main pathways of exothermic chemical reactions occurring inside the battery.
Unsafe behavior
Unsafe behaviors of lithium batteries, including overcharging, over-discharging, rapid charging and discharging, short circuits, mechanical abuse, and high-temperature thermal shock, can easily trigger dangerous side reactions inside the battery, generating heat and directly damaging the passivation film on the surfaces of the negative and positive electrodes.
When the cell temperature rises to 130°C, the SEI film on the surface of the negative electrode decomposes, causing the highly active lithium-carbon negative electrode to be exposed to the electrolyte and undergo a violent redox reaction. The heat generated puts the battery into a high-risk state.
When the local temperature inside the battery rises above 200°C, the passivation film on the positive electrode surface decomposes, causing oxygen evolution. This process continues with a violent reaction with the electrolyte, generating a large amount of heat and creating high internal pressure. When the battery temperature reaches above 240°C, a violent exothermic reaction also occurs between the lithium-carbon negative electrode and the binder.
Lithium battery safety assessment instrument
In lithium battery safety research, calorimeters are the most important instruments used. The most commonly used calorimeter is the accelerating calorimeter (ARC). ARC is a new type of thermal analysis instrument recommended by the United Nations for the assessment of hazardous materials, and it can provide time-temperature-pressure data of chemical reactions under adiabatic conditions.
Designed based on the adiabatic principle, ARC can use a large sample volume, has high sensitivity, and can accurately measure the initial temperature of sample thermal decomposition, as well as the temperature and pressure changes over time during adiabatic decomposition. In particular, it can provide information on the slow pressure change process of substances during thermal decomposition that cannot be obtained by differential scanning calorimetry and differential thermal analysis.
ARC security assessment method
ARC provides a near-insulated environment by precisely tracking temperature and avoiding heat exchange between the sample and the environment. It is mainly used to test and analyze the exothermic behavior of the sample.
In addition to testing thermal runaway, the ARC can provide an adiabatic environment. By connecting the ARC with a DC constant current source and charging/discharging equipment, the specific heat capacity of the battery and the adiabatic temperature rise during the charging and discharging process can be tested.
Temperature has a significant impact on the safety of lithium batteries. The environment in which they are used naturally has a temperature, and lithium batteries also generate heat during operation. Importantly, temperature greatly affects the internal chemical reactions of lithium batteries; excessively high temperatures can even shorten the battery's lifespan and, in severe cases, lead to safety issues.
