Lithium-ion battery safety assessment and testing methods. In recent years, there have been frequent safety incidents involving lithium-ion batteries used in electric vehicles. 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-ion battery reveals its thermal state, which is essentially a result of heat generation and transfer within the battery. Studying the thermal characteristics of lithium-ion batteries, and their heat generation and transfer properties under different states, allows us to understand the important pathways through which exothermic chemical reactions occur inside the lithium-ion battery.
Unsafe behavior
Unsafe behaviors of lithium-ion batteries include overcharging, over-discharging, rapid charging and discharging, short circuits, mechanical abuse, and high-temperature thermal shock. These conditions 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 resulting heat 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 significant 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-ion battery safety assessment instrument
In lithium-ion 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 for 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 the temperature to prevent heat exchange between the sample and the environment, which is crucial for testing and analyzing the exothermic behavior of the sample.
In addition to testing thermal runaway, the ability of the ARC to provide an adiabatic environment can be utilized to connect the ARC with a DC constant current source and charge/discharge equipment to test the specific heat capacity of the battery and the adiabatic temperature rise during the charge/discharge process.
Temperature has a significant impact on the safety of lithium-ion batteries. The environment in which they are used naturally has a temperature, and lithium-ion batteries also generate heat during use. Importantly, temperature greatly affects the internal chemical reactions of lithium-ion batteries; excessively high temperatures can even shorten the battery's lifespan and, in severe cases, lead to safety issues.
