Due to the rapid development of new energy vehicles, the power lithium-ion battery industry has also attracted much attention. Power lithium-ion battery is the power source that supplies the power source for tools, and mostly refers to the storage battery that supplies power for electric cars, electric trains, electric bicycles, and golf carts.
The first National Li-ion Battery Safety Technology Seminar hosted by Tsinghua University was held in Beijing. This meeting focused on the thermal runaway problem of power lithium-ion batteries (mainly high temperature thermal runaway), discussed various factors affecting battery safety, and how to further improve the technical means and technical measures of lithium-ion battery safety. Professor Ouyang Minggao of Tsinghua University broke down the three causes of thermal runaway of power lithium-ion batteries at the meeting and put forward some solutions and suggestions.
Generally speaking, the so-called thermal inducement is the external high temperature environment, including external fire, poor battery heat dissipation, etc. At high external temperatures, due to the structural characteristics of the lithium-ion battery, the SEI membrane, electrolyte, etc. will undergo analytical reactions. The analytes in the electrolyte will also react with the positive and negative electrodes, and the cell diaphragm will melt for analysis. Various reactions lead to a large number of The appearance of heat. The melting of the diaphragm causes an internal short circuit, and the release of electrical energy increases the heat processing. This cumulative and mutually reinforcing destructive use results in the breakdown of the explosion-proof membrane of the battery cell, the ejection of the electrolyte, and the occurrence of combustion and fire.
Experimental data shows that when the battery cell temperature reaches 135°C, the diaphragm begins to melt and the voltage drops; the battery voltage at 150°C drops rapidly; when the temperature reaches 245°C, the diaphragm completely collapses, and the battery will burst into flames.
In this regard, manufacturers can deal with two aspects of battery design and bMS battery management system. From the perspective of battery design, materials can be developed to guard against thermal runaway and block the response of thermal runaway; from the perspective of battery management, different temperature ranges can be predicted to mean different safety levels, so as to perform hierarchical alarms.
At present, the power lithium-ion batteries of electric vehicles on the market all contain a thermal management system, which uses air-cooling or water-cooling methods to dissipate heat from the battery. Relevant users should start to eliminate heat inducements from their usage habits, such as guarding against direct sunlight on vehicles and not placing flammable materials in vehicles, and keeping on-board fire extinguishers at the same time to eliminate spontaneous combustion factors. In addition, always pay attention to the battery temperature information on the dashboard or central control screen. Generally speaking, the operating temperature of a battery cell is between 40°C and 50°C. Anything higher or lower than this temperature range is not conducive to battery use.
Battery manufacturing impurities, metal particles, expansion and contraction of charge and discharge, lithium evolution, etc. may all cause internal short circuits. This kind of internal short circuit occurs slowly, for a very long time, and it is not known when it will be thermally out of control. If an experiment is carried out, the verification cannot be repeated. At present, experts all over the world have not found a process that can repeat the internal short circuit caused by impurities, and they are all under study.
To deal with 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 make a safety prediction for the internal short circuit. Before thermal runaway occurs, it is necessary to find the monomer with an internal short circuit. This means that if you have to find the characteristic parameters of the monomer, you can start with consistency. The battery is inconsistent, and the internal resistance is also inconsistent. As long as you find the monomer with variation in the middle, you can distinguish it. In detail, the equivalent circuit of a normal battery and the equivalent circuit of a micro-short circuit are actually the same in the form of the equation, except that the parameters of the normal cell and the micro-short-circuit cell have changed. You can study these parameters and see some of their characteristics in internal short-circuit changes.
A large number of lithium ions are embedded in the negative electrode of a fully charged battery. After overcharge, lithium precipitation occurs on the negative electrode sheet, and needle-like lithium metal crystals appear, which pierce the diaphragm and cause a short circuit. In the bMS battery management system, there will be an overcharge protection strategy. When the system detects that the battery voltage reaches the threshold, the charging circuit will be shut down to protect the battery. Although the manufacturer will conduct a series of electrical performance detections for bMS before leaving the factory, in order to prevent any accident, it is not recommended that users charge electric vehicles for a long time and choose regular charging equipment to eliminate the hidden danger of overcharging.
Mechanical and electrical incentives
Collision is a typical mechanical way to trigger thermal runaway, that is, battery damage caused by car collision. When the battery is damaged, an internal short circuit will also cause thermal runaway, but this short circuit is different from the short circuit caused by electrochemical inducement. Mechanical damage generally occurs instantaneously, corresponding to unexpected accidents in real life, such as strong collisions and overturns. Both squeezing and squeezing can cause the battery to vigorously and mechanically damage in a short period of time.
The way to deal with the impact (mechanical) triggering thermal runaway is to do a good job in 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 framework;
2. Reliability design: use battery pack vibration isolation connector to reduce vibration and wear; elastic floating plate to ensure connection reliability; Ip67 method dust-proof design;
3. Anti-collision lightweight design: optimization of anti-collision CAE structure; lightweight battery modules that meet the strength requirements, and the mass efficiency of the square shell system is 90%;
4. Battery pack positioning and locking technology: use limit self-locking and single locking mechanism to accurately position and lock the battery pack.
Experts at the meeting believed that electric vehicle batteries should meet performance and safety-related requirements, and safety detection verification should meet thermal detection (high temperature hazard, thermal stability, no thermal management cycle, thermal shock cycle, passive propagation resistance), electrical detection (short circuit, Safety requirements for overcharge and overdischarge) and mechanical detection (impact, drop, puncture, rollover, immersion, crushing). However, this does not mean that power lithium-ion battery companies can sit back and relax. Safety is endless. Improving the safety of electric vehicles requires the joint efforts of the state, scientific research institutions, and the entire industrial chain of power lithium-ion batteries.
In the more than one hundred years of history of the development of fuel vehicles, accidents have also occurred continuously, and setbacks are the law of the development of everything. Therefore, with regard to various accidents, the various industrial chains of electric vehicles should not stand still, but should examine and improve their own various problems and deficiencies. At the same time, it should be realized that consumers' requirements for safety are endless, and safety must be the first condition for satisfying all functions.