Lithium-ion batteries are generally safe; however, reports of safety incidents occasionally surface. Notable examples include the recent fires of Boeing 737 and B787 aircraft batteries, BYD electric vehicle fires, and Tesla Model S fires. These lithium-ion battery safety incidents first came to public attention four or five years ago. Even now, safety remains a key factor limiting the application of lithium-ion batteries in high-energy/high-power applications. Thermal runaway is not only the root cause of safety problems but also one of the shortcomings limiting the performance of lithium-ion batteries.
Starting with the preparation of electrode materials, a series of steps are required, such as stirring, slurry pulling, cutting, powder scraping, powder brushing, roller forming, electrode lug riveting, welding and connecting, applying adhesive tape, testing, and formation.
Even if all steps in this process are completed, there is still a possibility that improper execution could lead to increased internal battery resistance or short circuits, creating potential safety hazards. For example:
During the welding process, issues such as incomplete welds (between positive/negative electrode plates and tabs, between positive electrode plates and caps, between negative electrode plates and shells, and high internal resistance of rivets and contacts) may occur, along with material dust, diaphragm paper that is too small or not properly padded, holes in the diaphragm, and burrs that have not been cleaned properly.
An incorrect capacity ratio between the positive and negative electrodes may also lead to the deposition of a large amount of metallic lithium on the surface of the negative electrode. Insufficient uniformity of the slurry may also lead to uneven distribution of active particles, resulting in large volume changes of the negative electrode during charging and discharging and lithium deposition, thereby affecting its safety performance.
Furthermore, the quality of the SEI film formation during the formation process directly determines the cycle performance and safety performance of the battery, and affects its lithium intercalation stability and thermal stability.
Factors affecting the SEI film include the types of negative electrode carbon materials, electrolytes, and solvents, as well as the setting of parameters such as current density, temperature, and pressure during formation. By appropriately selecting materials and adjusting the parameters of the formation process, the quality of the generated SEI film can be improved, thereby enhancing the safety performance of the battery cell.
