Ouyang Minggao's team cracked the combustion mechanism of lithium-ion power batteries

Since the beginning of summer, a number of electric vehicle fire accidents have occurred one after another, which has aroused widespread concern and worry from all walks of life. Why do new energy vehicle fires occur frequently? What causes the fires? Can they be controlled? Can they be prevented?

On June 23, at the Qinghai Forum of the China Electric Vehicle 100, Ouyang Minggao, a professor at Tsinghua University and an academician of the Chinese Academy of Sciences, delivered a report titled "Research Progress in Power Battery Safety Prevention and Control", introducing his team's progress in lithium-ion power battery safety research. People were delighted to see that Ouyang Minggao's team had successfully cracked the mechanism of lithium-ion power battery combustion, bringing hope to solving the safety problems of electric vehicles.

According to Ouyang Minggao, the Battery Safety Laboratory of Tsinghua University has found through multi-level research that the main cause of lithium-ion power battery combustion is battery thermal runaway, and has discovered three new thermal runaway mechanisms of lithium-ion power batteries that are different from traditional thermal runaway mechanisms, including: active lithium precipitation at the negative electrode, short circuit in the battery, and active oxygen release at the positive electrode. Through statistical analysis of new energy vehicle fire accidents in 2018, it was found that the largest proportion of accidents occurred during charging of electric vehicles, reaching 29%, spontaneous combustion during driving and static parking each accounted for 14%, accidents caused by wading accounted for 10%, and another 33% were of unknown causes. These three thermal runaway mechanisms can explain the causes of more than 99% of the above-mentioned battery thermal runaway accidents.

The fatal hidden danger of new energy vehicles is safety

On June 17, the Ministry of Industry and Information Technology issued the "Notice on Carrying out Safety Hazard Inspections on New Energy Vehicles", requiring all new energy vehicle manufacturers to carry out safety hazard inspections on the new energy vehicles they produce. This is the first time that the Ministry of Industry and Information Technology has suspended the inclusion of ternary lithium battery buses in the catalog of new energy vehicle promotion and application models in early 2016, and the "Notice on Further Improving Safety Supervision on the Promotion and Application of New Energy Vehicles" in November 2016.

This is the fourth notice on the investigation of safety hazards of new energy vehicles issued by the Ministry of Industry and Information Technology since the "Notice on Carrying out Special Investigation of Safety Hazards of New Energy Passenger Vehicles" in August 2018 and the "Notice on Carrying out Special Investigation of Safety Hazards of New Energy Passenger Vehicles and Trucks" on September 25, 2018. The warning targets have been expanded from large buses equipped with ternary lithium batteries to small passenger cars, indicating that the safety technology of ternary lithium batteries has not been broken through, and the safety problems of new energy vehicles have become more serious, to the extent that the government is unwilling to tolerate and the society cannot ignore. Some of these "Notices" focus on improving the sealing and waterproof performance of vehicles, strengthening flame retardant materials, improving fire resistance, and extending the evacuation time of passengers after a fire. Some of them list in detail the appearance, software, and air tightness of the power batteries that companies are required to check. However, there is no explanation for why lithium-ion power batteries catch fire and what causes the fire. In fact, whether it is the government department in charge, or the new energy vehicle manufacturers and professional power battery manufacturers who are responsible for safety first, they have been looking for a scientific answer to this question.

Battery thermal runaway is the culprit for new energy vehicle safety accidents

According to public opinion information obtained by the State Administration for Market Regulation, more than 80 electric vehicle fire accidents have occurred across the country in the past year and a half from 2018 to now, an average of one every seven days, which is really a bit shocking. After analyzing and studying these accidents, Ouyang Minggao's team found that the proportion of fire accidents in electric vehicles during charging is the highest, and improper fast charging triggers battery lithium deposition, which leads to rapid decay of battery life and deterioration of thermal stability. This is the main reason. Through battery lithium deposition testing, simulation and safe fast charging technology experiments, the team discovered the mechanism of thermal runaway after fast charging of lithium-ion power batteries-active lithium deposition at the negative electrode, and revealed the mechanism of lithium deposition and re-embedding on the negative electrode surface during battery charging. Based on this, a power battery lithium deposition simulation model with lithium deposition and re-embedding mechanism was designed, simulating the voltage platform of the battery during the static process after charging lithium deposition, and developing a non-destructive detection method for power battery charging lithium deposition, and derived the optimal charging current without lithium deposition, realizing the rapid calibration of the battery safe fast charging MAP without lithium deposition.

Internal short circuit is a common link in battery thermal runaway

Ouyang Minggao's research team found through experiments that mechanical abuse can cause extrusion and puncture of batteries, electrical abuse can cause overcharging and overdischarge of batteries, and thermal abuse can cause overheating of batteries, which can damage the battery diaphragm and cause internal short circuits in the battery, leading to thermal runaway, smoke, fire, and explosion. Through lithium analysis during charging, it was found that the internal short circuit caused by the growing lithium dendrites piercing the diaphragm is a major threat to the safety of battery use. In 2013, the Boeing 787 aircraft battery system series fire accident and the Samsung Galaxy Note7 series battery fire accident in 2016 were all caused by internal short circuits in the battery.

Ouyang Minggao's team implanted a special memory alloy internal short-circuit trigger element with a spike structure into the battery. The temperature rises to make the spike structure rise and pierce the diaphragm, successfully simulating the internal short-circuit process. The internal short-circuit simulation test method was selected into the China Electricity Council standard T/CEC 169-2018 - "Internal Short Circuit Test Method for Lithium-ion Batteries for Power Storage". Based on this method, the research on the self-induced internal short circuit problem has achieved the results of early warning and detection of thermal runaway caused by internal short circuit.

The positive electrode releases active oxygen, causing thermal runaway of the battery

Ouyang Minggao's team also conducted research on the mechanism of thermal runaway of lithium-ion power batteries - the release of reactive oxygen from the positive electrode. They found that batteries without internal short circuits can also experience thermal runaway. They established a thermal runaway modeling and simulation model for single-cell power batteries based on the reaction kinetics of component materials, and proposed a series of improvement methods for battery thermal runaway characteristics based on the statistics of high-temperature thermochemical mechanisms of various battery material systems.

In the research on the thermal spread test, simulation and suppression technology of battery system, Ouyang Minggao's team invented the thermal runaway spread suppression technology of power battery system based on the model - thermal insulation design and heat dissipation design. For the battery system with the worst thermal stability, they invented the "firewall" technology combining thermal insulation and rapid cooling. This achievement technically supports the Global Technical Regulation Working Group on Electric Vehicle Safety EVS-GTR-TF5: the model analysis results provide a feasible solution for the formulation of international thermal runaway expansion regulations, complete the feasibility verification of the solution, and contribute to the inclusion of the Chinese solution in the first phase of the regulations; the national "Safety Technical Conditions for Electric Buses" and "Lithium-ion Power Battery Packs and Systems for Electric Vehicles: Test Methods and Safety Requirements (Draft)" continue to use the solution in EVS-GTR.

Tsinghua University's research results on lithium-ion power battery safety control technology have been applied to mainstream new energy vehicle manufacturers and battery manufacturers at home and abroad, and the intellectual property rights have been licensed to Daimler-Benz, Samsung SDI and SKI in South Korea, and transferred to domestic companies. The research results support the formulation of the national "Lithium-ion Power Battery Packs and Systems for Electric Vehicles: Test Methods and Safety Requirements" and participate in the formulation of international standards.

At present, Tsinghua University's research on lithium-ion power battery safety control technology is still ongoing. Ouyang Minggao hopes to achieve a comprehensive breakthrough in this key technology through three to five years of continuous efforts.