To ensure the healthy development of new energy vehicles, the safety of power lithium batteries is of paramount importance. On June 5th, Professor Jiang Jiuchun from Beijing Jiaotong University was a guest speaker at the 6th session of the alliance's micro-lecture series, delivering an insightful course on power lithium battery health diagnosis technology.
I. Research Background
Since 2019, spontaneous combustion incidents involving electric vehicles have remained frequent. According to incomplete statistics, from January to July 2019, domestic and international media reported over 40 safety incidents related to power lithium batteries in electric vehicles; multiple spontaneous combustion incidents also occurred in April and May 2020. Jiang Jiuchun stated that, in terms of battery materials, a significant number of the vehicles involved in these incidents were equipped with ternary lithium batteries. In terms of battery shape, a large proportion of the incidents involved prismatic batteries. Furthermore, it was found that the proportion of electric vehicles in the three states—stationary, driving, and charging—was roughly equal at the time of the incidents. While high energy density batteries increase vehicle range, they also exacerbate battery-related safety incidents in electric vehicles.
As one of the seven strategic emerging industries in China, new energy vehicles have received detailed policies regarding their development scale in the 15th, 11th, 12th, and 13th Five-Year Plans. Especially since the beginning of the 13th Five-Year Plan, the government has intensified its efforts to promote the development of new energy vehicles and has also introduced relevant policies and regulations on the testing and recycling of power lithium batteries. These have played an important role in regulating the orderly development of the power lithium battery industry. However, Jiang Jiuchun emphasized that, overall, these policies and regulations focus more on industry access and battery recycling, tending to regulate the beginning and end of the process, but there are relatively few regulations and standards concerning the use of power lithium batteries.
He pointed out that, based on the analysis of the causes of power lithium battery safety accidents, the main reason for battery thermal runaway is the large amount of heat released when internal and external short circuits occur, causing a rapid increase in battery temperature and inducing a chain reaction of thermal runaway. Regarding the gradual development of internal short circuits, identification can be made in the early and middle stages of the evolution based on electro-thermal characteristics, thus preventing the internal short circuit from developing to its final stage. In summary, the health status diagnosis of power lithium batteries is crucial. Influenced by factors such as uneven temperature field distribution and different battery locations, the health status and safety of batteries connected in series within the same battery pack vary. Therefore, the health status of each battery must be diagnosed to ensure safe battery use.
II. Health status diagnosis of lithium iron phosphate batteries & ternary lithium batteries and lithium battery safety early warning strategies
The presentation first focuses on the analysis methods for the aging mechanisms of lithium iron phosphate batteries, pointing out that common aging mechanisms in lithium batteries include lithium-ion loss, active material loss, positive electrode material loss, and negative electrode material loss. Furthermore, each aging mechanism exhibits a specific change in the voltage curve (VQ). The basic principle of capacity increment analysis is to differentiate the commonly used voltage curve of lithium batteries to obtain the relationship between capacity increment and terminal voltage. A key advantage of capacity increment analysis is that it transforms the long and flat voltage plateau on the battery's VQ curve into easily identifiable capacity increment peaks. Therefore, subtle changes in the VQ curve can be observed on the capacity increment curve, aiding in the identification of battery aging mechanisms.
Secondly, the analysis of the aging mechanism of ternary lithium batteries was emphasized. It pointed out the degradation mechanism of lithium batteries under different SOC ranges: when the battery is cycled at a depth of 20%, LLI is the dominant factor causing battery capacity degradation; during the aging process of batteries in the full range of cycles, both LAMPE and LLI account for a high proportion; compared to the five cycle ranges, LAMPE has the largest proportion in battery aging in the 0-20% SOC range.
Regarding lithium battery safety early warning strategies, he pointed out that extracting multiple characteristic parameters reflecting the internal aging mechanism from the constant current charging voltage curves of each individual cell in a series battery pack, and employing a cell outlier detection method based on battery pack consistency, can quickly screen problematic batteries. He also emphasized the identification of battery capacity drops based on the inflection points of the characteristic parameter evolution trajectory.
III. Summary From the perspective of cycle life comparison, lithium iron phosphate batteries have a long cycle life and have not shown an inflection point of accelerated capacity degradation; ternary lithium batteries have a relatively short cycle life and have shown an inflection point of accelerated capacity degradation. Important battery failures caused by battery aging include: inconsistent battery aging, individual batteries posing safety risks; and the appearance of an inflection point of accelerated capacity degradation. Corresponding battery safety warning strategies include safety warning methods based on outlier detection (horizontal comparison) and battery capacity drop identification based on the inflection point of characteristic parameter evolution trajectory (vertical comparison).
