On Battery Fire of Electric Vehicle
There are many reasons for electric vehicle fire, but what we really focus on is the unique battery system fire problem of electric vehicle. The most important thing in the battery system is the energy storage unit: the cell.
Knowledge of battery cell fire.
1. Thermal runaway of battery cell.
At present, there are two main types of batteries commonly used by domestic and foreign electric vehicle enterprises according to the electrochemical composition: lithium iron phosphate battery (LFP) and three-component lithium ion battery. Early electric vehicles mostly used lithium iron phosphate batteries, to the early Rongwei e550, BYD Qin and so on. Later, with the subsidies policy linked to the battery energy density, plus the development of EV projects, users'requirements for pure electric mileage and the development of ternary lithium-ion battery technology, most of the new energy vehicles on the market now carry ternary lithium-ion batteries. So how about the safety of ternary lithium ion batteries and lithium iron phosphate batteries, especially after thermal runaway?
Thermal Out-of-Control Test of NCM Ternary Lithium Ion Core
Thermal runaway test of lithium iron phosphate cores
From the two videos, it can be found that lithium iron phosphate batteries have higher thermal stability than lithium ternary batteries, and the reaction of thermal runaway is milder. It mainly produces a lot of smoke, and basically does not appear the phenomenon of fire spray. Ternary lithium-ion batteries, however, are more explosive in nature, with very fast thermal runaway reactions, and with very intense flame and impurity injection, the temperature rises to 7,800 degrees Celsius. If the battery runs out of control, the whole car is hardly immune. Compared with lithium iron phosphate batteries, ternary lithium ion batteries have higher energy density, higher activity and lower safety.
2. Main Causes and Preventive Measures of Core Thermal Runaway
Most of the vehicle fire accidents are caused by the thermal runaway spread of the core. One of the single core thermal runaway may lead to the thermal runaway of the surrounding core and eventually cause chain reaction.
Current batteries basically adopt integrated structure design, single core integrated module, module in integrated battery pack. Therefore, each battery pack contains a large number of cell monomers to realize parallel or series structure, such as Tesla Model S, which integrates more than 7000 three-element cylindrical cores in a whole package with great creativity and boldness, and ponders over the terrible series.
Tesla Model S Internal structure of battery pack
In the current situation that the core technology can not achieve without thermal runaway, how to effectively prevent and insulate becomes extremely important. A few reasons are briefly analyzed.
(1) Internal Short Circuit
Analysis of lithium: Low temperature environment, high rate cycle cause lithium precipitation, continuous growth of lithium dendrites will pierce the diaphragm and cause internal short circuit (which is why it is recommended that we use slow charging as much as possible);
Impurities: In the production or manufacturing process of the core, due to improper production environment or operation, metal impurities are mixed into the core, which leads to internal short circuit as the core is recycled.
Local stress concentration in the core may induce precipitation of other metals.
Preventive measures: The occurrence of internal short circuit of the core has certain development rules, such as abnormal voltage and temperature of the core, which can be prevented in advance according to the changes of different stages. Detailed content related to enterprise secrets will not be repeated one by one.
Many thermal accidents occur in the charging process, and the thermal runaway caused by overcharging usually occurs in the early stage of micro-overcharging.
The consistency of the cores is poor: during the charging process, some of the cores may be filled, and some of the cores may not be filled.
Parallel structure design of cores: The parallel voltage is the same, among which cores with different voltages are difficult to detect and stop at any point.
This involves the charging process software control strategy, charging model and algorithm.
(3) Core aging
The aging of batteries will lead to the further expansion of the inconsistency of batteries and the deterioration of the consistency of capacities. The accuracy of BMS battery management system in estimating the health status of batteries will be compromised, so the corresponding safety prevention threshold and the consistency of charge-discharge power curve settings need to be verified. And aging in low temperature environment will seriously affect the thermal stability of batteries, which may eventually lead to thermal runaway.
At the stage of product development, it is necessary to have more adequate battery aging test and verification, and to optimize the accuracy of BMS calculation. This requires not only a lot of research and development, but also a lot of product data to support optimization. Companies that haven't sold a few cars need to refuel.
3. High specific energy is still the trend of development and faces more severe security problems.
From the practical experience, there are often some contradictions in the development of battery system. To design and develop products from an enterprise's point of view can only maximize the product strength in a limited time and cost, launch the market, and gain profits at the same time with competitiveness. The development of product safety has a bottom line, but no upper limit. If we want to achieve higher security, we need to pay higher costs under the current technical conditions, such as more robust structural design, better reliability of components, more adequate testing and verification, etc. At the same time, we may sacrifice the competitiveness of products, such as the use of lithium iron phosphate cores will shorten the driving mileage. From the development trend of battery energy density worldwide and the technical roadmap of battery energy density issued by the largest domestic supplier, high specific energy is still the development trend.
From the safety point of view, from lithium iron phosphate - NCM532 - NCM622 - NCM811, the activity of the core is increasing, and the safety problems are becoming more and more stringent. But the development of the industry can not be abandoned because of choking, not because of security issues, the key is how to control the balance between energy and security. How to consider battery safety design more comprehensively is a great test for every electric vehicle safety practitioner.
4. Technological Development Trends and Concerns
Considering the cost and development trend of batteries, the short-term development is mainly based on liquid electrolyte, high nickel ternary positive electrode and silicon carbon negative electrode. The control strategy of battery management system is designed to prevent and control the thermal runaway of batteries. At the same time, some external auxiliary insulation measures are designed to restrain the thermal runaway spread of batteries, so as to obtain enough time for users to escape and fire rescue. Meanwhile, to minimize the negative effects of fire accidents. The long-term development direction is still to improve the technical ability of the core, to essentially solve the risk of thermal runaway of the core, and to meet the requirements of no thermal runaway, such as the development of solid-state battery technology.