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In order to ensure the healthy development of new energy vehicles, the safety of power lithium battery is very important. On June 5, Professor Jiang Jiuchun of Beijing Jiaotong University visited the 6th micro class of the Alliance, bringing a wonderful course on the health diagnosis technology of power lithium batteries.

1、 Research background

Since 2019, electric vehicle spontaneous combustion accidents still occur frequently. According to incomplete statistics, as of January July 2019, there were more than 40 electric vehicle safety accidents related to power lithium battery reported by domestic and foreign media; From April to May 2020, there were also many electric vehicle spontaneous combustion accidents. Jiang Jiuchun said that from the perspective of battery material system, a considerable number of accident vehicles are equipped with ternary lithium batteries. From the shape of the battery, more accidents come from the square cell. It was also found that the proportion of electric vehicles in the three states of parking, driving and charging was basically the same when the accident occurred. The high specific energy density battery not only improves the vehicle range, but also intensifies the safety accidents related to electric vehicle batteries.

It can be said that new energy vehicles are one of the seven strategic emerging industries of the country. The country has issued detailed policies on the development scale of new energy vehicles from the 15th, 11th, 12th and 13th Five Year Plan. Especially since the 13th Five Year Plan, the country has stepped up to promote the development of new energy vehicles, and also issued relevant policies and regulations on the detection and recovery of power lithium batteries. These have played an important role in regulating the orderly development of the power lithium battery industry. However, Jiang Jiuchun stressed that, on the whole, these policies and regulations focus more on industry access and battery recycling, preferring to the head and tail of the pipe, but there are fewer regulations and standards on the use process of power lithium batteries.

He pointed out that from the analysis of the causes of power lithium battery safety accidents, the main reason for battery thermal runaway is that a large amount of heat released when internal and external short circuits occur makes the battery temperature rise sharply, which leads to runaway chain reaction. The gradual internal short circuit can be identified at the beginning and middle of the evolution according to the electrical thermal characteristics, so as to prevent the internal short circuit from developing to the end. In a word, the health status diagnosis of power lithium battery is very important. Due to the uneven distribution of temperature field, different battery locations and other factors, the health status and safety of series connected batteries in the same battery pack are different. Therefore, the health status of each battery should be diagnosed to ensure the safety of battery use.

2、 Lithium iron phosphate battery&ternary battery health status diagnosis and lithium battery safety warning strategy

Firstly, the analysis method of aging mechanism of lithium iron phosphate battery is emphasized, pointing out that the common aging mechanisms of lithium battery are: lithium ion loss, active material loss, positive material loss, negative material loss. In addition, each aging mechanism has a specific change in the voltage curve (V-Q); The basic principle of capacity increment analysis method is to differentiate the voltage curve commonly used in lithium batteries to obtain the relationship between capacity increment and terminal voltage. One of the most important advantages of capacity increment analysis method is to transform the long and flat voltage platform on the V-Q curve of batteries into an easily recognizable capacity increment peak. Therefore, small changes of V-Q curve can be observed on the capacity increment curve, which is helpful to identify the aging mechanism of the battery.

Secondly, the aging mechanism analysis of ternary battery is emphasized. Among them, it points out the attenuation mechanism of lithium battery under different SOC interval cycles: when the battery is cycled between partitions at 20% depth, LLI is the dominant factor leading to the decline of battery capacity; LAMPE and LLI accounted for a high proportion in the aging process of the whole inter zone circulating battery; Compared with the five inter partition cycles, LAMPE accounts for the largest proportion in the 0-20% SOC range of battery aging.

With regard to the safety early warning strategy of lithium batteries, he analyzed and pointed out that a number of characteristic parameters reflecting the internal aging mechanism were extracted from the constant current charging voltage curve of each single battery in the series battery pack, and the battery cell outlier detection method based on battery pack consistency could quickly screen the problem batteries. In addition, he focused on the battery capacity diving recognition based on the inflection point of the characteristic parameter evolution track.

3、 From the perspective of cycle life comparison, lithium iron phosphate battery has a long cycle life, and there is no inflection point of accelerated capacity decline; The cycle life of ternary battery is relatively short, and there is an inflection point of accelerated capacity decline. Important battery failures caused by battery aging include: inconsistent battery aging and individual battery safety risks; The battery has an inflection point of accelerated capacity decline. The corresponding battery safety early warning strategies include the safety early warning method based on outlier detection (horizontal comparison) and the battery capacity diving recognition based on the inflection point of the characteristic parameter evolution track (vertical comparison).

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