Ouyang Minggao: Solving the problem of thermal battery runaway requires three axes

From May 6th to 7th, the National Lithium Ion Battery Safety Technology Seminar hosted by Tsinghua University was held in Beijing. This conference focused on the thermal runaway problem of power batteries (mainly due to high temperature thermal runaway), discussed various factors affecting battery safety, and how to further improve the safety and safety of lithium ion batteries. At the meeting, Professor Ouyang Minggao of Tsinghua University analyzed three incentives for thermal battery thermal runaway and proposed some solutions and suggestions.

Heat incentive

Generally speaking, the so-called heat incentive is the external high temperature environment, including external fire, battery heat dissipation and so on. At the external high temperature, due to the characteristics of the structure of the lithium ion battery, the SEI film, the electrolyte, etc. will undergo a decomposition reaction, and the decomposition product of the electrolyte will react with the positive electrode and the negative electrode, and the cell diaphragm will melt and decompose, and a plurality of reactions cause a large amount of reaction. The generation of heat. The melting of the diaphragm causes an internal short circuit, which in turn increases the production of heat. This cumulative mutual reinforcement damage has the effect of causing the explosion-proof membrane of the battery core to rupture, the electrolyte to be ejected, and the combustion to ignite.

The test data shows that when the temperature of the battery cell reaches 135 °C, the diaphragm begins to melt and the voltage drops; the battery voltage drops rapidly at 150 °C; when the temperature reaches 245 °C, the diaphragm collapses completely and the battery will explode.

In this regard, manufacturers can solve from two aspects of battery design and BMS battery management system. From the perspective of battery design, it can be developed to prevent thermal runaway materials and block thermal runaway reactions. From the perspective of battery management, different temperature ranges can be predicted to define different safety levels for grading alarms.

The power batteries of electric vehicles on the market now include a thermal management system that uses air-cooled or water-cooled solutions to dissipate heat from the battery. For users, it is necessary to eliminate the heat incentives from the habit of using, such as avoiding direct sunlight in the vehicle, not placing flammable materials in the car, and keeping a car fire extinguisher at the same time to eliminate the spontaneous combustion factor. In addition, always pay attention to the battery temperature information on the instrument panel or the central control panel. Generally, the operating temperature of the battery cell is between 40 ° C and 50 ° C. Above or below this temperature range is not conducive to battery use.

Electrochemical incentive

Impurities in battery manufacturing, metal particles, shrinkage of charge and discharge expansion, and lithium deposition may cause internal short circuits. This internal short circuit occurs slowly, for a very long time, and it is not known when it will be out of control. If the test is performed, the verification cannot be repeated. At present, experts around the world have not found a process capable of repeating internal short circuits caused by impurities, which are under study.

To solve this problem, first improve the manufacturing process to reduce impurities in battery manufacturing. It is necessary to select a battery manufacturer with good product quality, and secondly, to make a safety prediction for the internal short circuit, and to find a monomer with an internal short circuit before the thermal runaway occurs. This means that you must find the characteristic parameters of the monomer, you can start with consistency. The batteries are inconsistent, and the internal resistance is also inconsistent. As long as you find a monomer with a variation in the middle, you can identify it. Specifically, the equivalent circuit of a normal battery and the equivalent circuit in which a micro short circuit occurs, the form of the equation is actually the same, except that the parameters of the normal single cell and the micro short circuit are changed. It is possible to study these parameters to see some of the characteristics of the internal short circuit changes.

A large amount of lithium ions are embedded in the negative electrode of the battery in a fully charged state. After overcharging, lithium deposition occurs on the negative electrode sheet, and needle-shaped lithium metal crystals appear, and a short circuit occurs in the piercing diaphragm. In the BMS battery management system, there is an overcharge protection strategy. When the system detects that the battery voltage reaches the threshold, it will turn off the charging circuit and protect the battery. Although the manufacturer will perform some electrical performance tests for the BMS before leaving the factory, in order to prevent it, it is not recommended that the users charge the electric vehicle for a long time, and choose a regular charging device to eliminate the hidden danger.

Mechanical and electrical incentives

Collision is a typical way of mechanically triggering thermal runaway, which is a battery crash that causes damage to the battery. When the battery is damaged, an internal short circuit will also occur to cause thermal runaway. However, this short circuit is different from the short circuit caused by electrochemical inducement. Mechanical damage generally occurs instantaneously, corresponding to sudden accidents in real life, strong impact and rollover. Extrusion can cause mechanical damage to the battery in a short period of time.

The solution to the collision (mechanical) triggering thermal runaway is to make the structural safety protection design of the battery. To this end, Professor Ouyang Minggao gave four design routes:

1 Assembly structure design: plastic frame support + steel belt pre-tightened assembly structure and high-strength skeleton;

2 Reliability design: use battery pack vibration isolation connector to reduce vibration and wear; elastic floating board to ensure connection reliability; IP67 mode dustproof design;

3 anti-collision lightweight design: anti-collision CAE structure optimization; battery module that meets the strength requirements for lightweight, square shell system quality group efficiency > 90%;

4 Battery pack positioning and locking technology: The battery pack is accurately positioned and locked by the limit self-locking and single locking mechanism.

Experts attending the meeting believe that electric vehicle batteries should meet performance and safety requirements. Safety test verification should meet thermal tests (high temperature hazard, thermal stability, no thermal management cycle, thermal shock cycle, passive propagation resistance), electrical test (short circuit, Safety requirements for overcharge and overdischarge) and mechanical testing (impact, drop, puncture, tumbling, immersion, crushing). However, this does not mean that power battery companies can rest easy. There is no end to safety, and improving the safety of electric vehicles requires the joint efforts of the country, scientific research institutions, and the entire industrial chain of power batteries.

In the more than one hundred years of the development of fuel vehicles, there have been accidents, and the setbacks are the law of any development. Therefore, for all kinds of accidents, the various industrial chains of electric vehicles should not stop, but should review and improve their various problems and deficiencies. At the same time, it should be realized that consumers' requirements for security are endless, and that safety must be the primary condition for meeting all functions.