Protection and optimization of power battery thermal runaway

Thermal runaway process of lithium-ion batteries

Thermal runaway of the battery is caused by the fact that the heat generation rate of the battery is much higher than the heat dissipation rate, and a large amount of heat accumulates but is not dissipated in time. In essence, “thermal runaway” is a positive energy feedback loop process: rising temperatures cause the system to heat up, and when the system heats up, the temperature rises, which in turn makes the system hotter. The process of battery thermal runaway can be divided into three stages:

Stage 1: Battery internal thermal runaway

Due to internal short circuit, external heating, or the battery itself generating heat during high-current charging and discharging, the internal temperature of the battery rises to about 90℃ to 100℃, and the lithium salt LiPF6 begins to decompose.

The chemical activity of the carbon cathode in the charged state is very high, which is close to metallic lithium, so the SEI film on the surface decomposes at high temperatures, and the lithium ions embedded in the graphite react with the battery electrolyte and binder, further pushing the battery temperature to 150℃.

At this temperature, a new violent exothermic reaction occurs, for example, the electrolyte decomposes in large quantities to generate PF5, which further catalyzes the decomposition reaction of organic solvents, etc.

Stage 2: Battery bulging

When the battery temperature reaches above 200℃, the anode material decomposes, releasing a large amount of heat and gas, and continues to heat up. At 250-350℃, the lithium-embedded cathode begins to react with the electrolyte.

Stage 3: Battery thermal runaway and even explosion failure

During the reaction, the charged cathode material begins to undergo a violent decomposition reaction, the electrolyte undergoes a violent oxidation reaction, releases a large amount of heat, generates high temperature and a large amount of gas, and the battery burns and explodes.

Thermal runaway prevention measures

With the goal of blocking, delaying, and preventing thermal runaway of power batteries, many researchers have conducted research based on battery thermal management and high-strength battery pack structural design.

Single battery safety design

① Research on the safety of diaphragm design

The core of improving the safety of the separator is to increase the temperature at which the separator shrinks, melts and decomposes, and enhances the isolation capability under high-temperature conditions. The high-temperature isolation capability of the separator ensures that the micropores of the separator are sealed in high-temperature environments, blocking the outflow of lithium ions. The widely used diaphragm materials are generally covered with ceramic coatings or other materials with closed cell effect.

② Research on the safety of cathode materials

The most common lithium-ion cathode active materials used in power battery market applications are generally LiCoO2, LiFePO4, LiMn2O4, LiNixCoyMnzO2 (NCM), LMFP, etc. The positive electrode is covered with materials to block and alleviate thermal runaway side reactions and improve battery cycleability and thermal stability, such as ZrO2 and AlF3.

③ Research on the safety of anode materials

The improvement of anode in lithium ion battery is mainly through material coating or adding additives to the electrolyte to improve the thermal stability of the SEI film. The liquid alloy GaSnIn is added to the electrolyte to improve the thermal stability of the battery. Experiments show that the prepared gradient SEI layer greatly reduces voltage polarization and improves Coulomb efficiency to 99.06%. An ultrathin aramid nanofiber (ANF) film was prepared to inhibit lithium dendrite growth.

In experimental tests, in a high current density environment of 50 mA/cm2, the capacity of the ANF-Li | LiFePO4 battery decreased to 80.2% after 1,200 cycles. And its research discovered fibrous lithium deposition for the first time. The nanoscale gaps in the prepared ANF film promoted electrolyte diffusion, accelerated the efficiency of lithium transport, and eliminated the disadvantages of micron-scale lithium dendrites penetrating the separator.

④ Research on electrolyte safety

Electrolyte is involved in most thermal runaway accidents, so it is critical to improve electrolyte safety and prevent thermal runaway. Overcharge prevention additives such as flame retardants, solid polymeric substances or ionic liquids are often added to the electrolyte.

Fluorinated ethylene carbonate (FEC) is the most common electrolyte additive. Its advantage is to improve the coulombic efficiency of reversible delithiation of the anode by changing the composition of the SEI membrane. Using lithium difluoroborate (LiDFOB) as the main salt, an SEI membrane with double-layer crystallization and polymer solid electrolyte interphase was designed in a phosphate ester mixed electrolyte.

Flame retardant experiments show that the self-extinguishing time of the flame-retardant electrolyte is 6.1 s, the reversible efficiency of Li is 98.2%, and 89.7% of the battery capacity is still maintained after 150 charge and discharge cycles.

Power battery system safety protection and optimised design

① Optimized design of lithium ion battery structure

Battery pack structural design and vehicle installation location optimization are crucial to improving safety. A classification experiment was conducted on the impact of thermal runaway range based on the 18650 battery arrangement. Experiments show that the space with a larger heating area takes a shorter time to ignite, and the spread speed and scope are greater. However, its experiment only considered the overall heating of the power battery module and did not consider the local overheating caused by internal short circuit.

When the battery pack design was optimized based on the three-dimensional heat dissipation model of the power battery pack, heat dissipation simulation was performed. Experiments show that the peak temperature of the optimized lithium-ion battery dropped from 46℃ to 34℃, and the temperature difference between single cells was controlled within 5℃.

② Battery thermal management system design

Lithium-ion batteries are highly thermally sensitive, and improving low-temperature discharge efficiency and high-temperature safety is the core of the battery thermal management system. Battery pack cooling methods include liquid cooling and air cooling.

The electric vehicles produced by Tesla all use liquid cooling technology, and electric buses generally use air cooling. In recent research, aerogels, phase change materials and hybrid materials have been used in battery thermal management systems due to their excellent heat absorption performance.

③ Cooling, fire extinguishing, blocking and gas guidance design for battery thermal runaway

When battery thermal runaway is unavoidable, it is particularly important to promptly block thermal spread, cool down and guide high-temperature gases so as not to affect batteries that are installed in similar locations. The figure below shows a three-dimensional model of thermal runaway diffusion.

Ways to block the spread of thermal runaway mainly include: filling with flame-retardant media, using insulation materials to isolate the thermal runaway battery, or guiding flames and high-temperature gases through paths to discharge the battery pack.

A researcher developed a high-temperature gas heat pipe with a rectangular cross-section arranged along the battery as shown in the figure below. Although it cannot prevent the occurrence of thermal runaway of a single battery, it can effectively prevent the spread of local thermal runaway of the battery pack.

Summary

Research on power battery thermal runaway has made great progress, but safety accidents still occur from time to time. The role of battery system grouping technology in this requires in-depth research and practical verification.

Based on a large number of thermal runaway safety tests and battery system group design practices, in order to solve the battery thermal runaway safety problem, main loop fuse protection, collection loop overcurrent protection, safe creepage distance, electrical clearance design, and distribution were adopted in product design.

There are many structural, electrical, and active safety measures such as external short-circuit protection design of battery cells, application of high flame-retardant materials, multi-dimensional module thermal expansion structural protection, breathable explosion-proof devices, active fire extinguishing devices, etc.