Electric vehicles frequently catch fire. Why can’t the safety of power batteries be guaranteed?

Recently, electric vehicles have caught fire frequently. Compared with previous years, this year's electric vehicle fire accidents are very serious. Industry experts said that domestic electric vehicles have shortcomings in battery technology. Battery safety is a systemic problem. Whether it is battery cells, modules or PACK, there are technical barriers.

The development of domestic battery technology seems to be a bit too fast.

The 18650 battery currently used is small and difficult to dissipate heat. It is also 811, and the decomposition temperature of the positive electrode material is over 200 degrees, which requires sophisticated thermal management. Once the thermal runaway is not well controlled, it is easy to catch fire.

my country's battery companies all started with the production of nickel-cobalt-manganese 111 type. The most mature domestic ternary technology is nickel-cobalt-manganese 523 type, followed by nickel-cobalt-manganese 622 type. The ratio of nickel-cobalt-manganese was 1:1:1 and later became 8:1:1. The larger the number in front, the higher the nickel content, and the greater the probability of subsequent problems, and this is to pursue high energy density.

The current combination of nickel, cobalt and manganese in the ternary material is designed to increase energy density. Another reason for the frequent safety problems is the rapid development of the industry. The industry is developing rapidly and the demand is large, but battery technology cannot keep up.

Charging, especially fast charging, can cause damage to the battery. Excessive temperature can affect the diaphragm inside the battery cell, and once the temperature gets out of control, it can easily cause a fire. Excessively high energy density also places higher demands on the module's vibration resistance, impact resistance, expansion resistance, insulation and heat insulation properties.

Although everyone in the industry knows that safety is the top priority, there is no balance between safety and performance. In terms of scientific research, the development and verification of batteries or materials requires more than two years or even longer to find a stable and optimal solution. Although our policies are good, they change every year, which is a bit impatient.

According to the plan, by 2020, the energy density of my country's power battery cells will reach more than 300wh/kg, and we will strive to achieve 350wh/kg. The system energy density will strive to reach 260wh/kg, and the cost will be reduced to less than 1 yuan/watt-hour. Ouyang Minggao, executive vice president of the Electric Vehicle 100 Forum and academician of the Chinese Academy of Engineering, believes that the current energy density target of 350 Wh/kg for single cells and 260 Wh/kg for systems by 2020 is inconsistent with safety demands and should not be forced forward.

More than half of the recent electric vehicle fires were caused by ternary lithium batteries, which have poorer thermal stability. The batteries of the vehicles that caught fire were mainly cylindrical lithium batteries. Since these batteries are wrapped in a solid steel shell, the internal pressure increases sharply when they are heated and decomposed, making them prone to explosions and quickly igniting the entire battery module. In addition, most fires occurred during the charging stage, especially when the battery was fully charged.

However, the vehicles involved in the fire accidents were mainly old electric vehicles, and the battery system specific energy was not high. Therefore, the main reason for the frequent electric vehicle accidents cannot be simply attributed to the power batteries with too high energy density. The main reason for the frequent electric vehicle fire accidents is the thermal runaway of the battery. Thermal runaway means that the temperature of the power battery exceeds a certain limit, triggering a chain reaction, and the battery temperature rises rapidly in a short period of time (up to 1,000 degrees per second), causing combustion.

As a winner of Toutiao's Qingyun Plan and Baijiahao's Bai+ Plan, the 2019 Baidu Digital Author of the Year, the Baijiahao's Most Popular Author in the Technology Field, the 2019 Sogou Technology and Culture Author, and the 2021 Baijiahao Quarterly Influential Creator, he has won many awards, including the 2013 Sohu Best Industry Media Person, the 2015 China New Media Entrepreneurship Competition Beijing Third Place, the 2015 Guangmang Experience Award, the 2015 China New Media Entrepreneurship Competition Finals Third Place, and the 2018 Baidu Dynamic Annual Powerful Celebrity.