Analysis and Prevention of Lithium Battery Thermal Runaway

1. Lithium Battery Thermal Runaway Principle

How thermal runaway occurs in lithium batteries：

Phase 1

125°C, the beginning of thermal runaway.

The SEI film reacts and decomposes, exposing the negative electrode to the electrolyte, causing a reaction between the electrolyte and the lithium in the negative electrode, leading to the generation of gas.

① 2Li + C₃H₄O₃(EC) → Li₂CO₃+ C₂H₄

② 2Li + C₄H₆O₃(PC) → Li₂CO₃+ C₃H₆

③ 2Li + C₃H₄O₃(DMC) → Li₂CO₃+ C₂H₆

Phase 2

125~180°C, gas release and accelerated heating inside the battery.

Gas generation rate increases during this phase, and the positive electrode materials decompose, such as LiCoO2 decomposing to produce O2.

Lithium salts also decompose, for example, LiPF6 decomposes to generate LiF and the Lewis acid PF5.

The Lewis acid reacts with the electrolyte at high temperatures, producing a large amount of gas.

Phase 3

Above 180°C, thermal runaway occurs.

During this phase, the exothermic reactions between the positive / negative electrode materials and the electrolyte, as well as the decomposition of the electrolyte, increase significantly in rate.

The internal temperature of the battery also rises sharply, leading to the opening of pressure relief valves or spontaneous ignition.

Some researchers further subdivide thermal runaway into the following ranges:

No.	Temperature/ ℃	Chemical reactions	Heat/(J·g-1)	Explanation
1	90-120	SEI membrane decomposition	–	–
2	110-150	Reaction between Li4C6 and electrolyte	350	Passivation film rupture
3	130-180	PE membrane melting	-190	Endothermic
4	160-190	PP membrane melting	-90	Endothermic
5	180-500	Decomposition of Li0.3NiO2 with electrolyte	600	Oxygen release temperature peak ~200°C
6	220-500	Decomposition of Li0.45NiO2 with electrolyte	450	Oxygen release temperature peak ~230°C
7	150-300	Decomposition of Li0.2Mn2O4 with electrolyte	450	Oxygen release temperature peak ~300°C
8	130-220	Solvent reaction with LiPF4	250	Low energy
9	240-350	Reaction between Li2C4 and PVDF	1500	Intense chain growth
10	660	Melting of aluminum current collector	-395	Endothermic

2. Causes of Lithium Battery Thermal Runaway

(1) Mechanical Abuse

Such as squeezing, impact, puncture, etc., under the action of external force, the lithium battery (cell) undergoes deformation, the separator is damaged, and a short circuit between the positive and negative electrodes occurs, leading to thermal runaway.

(2) Thermal Abuse

The main sources of heat during the long-term operation of lithium batteries in high-temperature environments include external high-temperature environments, polarization heat, reaction heat, and decomposition heat generated during use.

(3) Electrical Abuse

Overcharging of lithium batteries leads to the destruction of the active material structure, electrolyte decomposition and gas generation, resulting in an increase in internal battery pressure.

This also includes over-discharging and charging at high rates (exceeding specifications).

3. Lithium Battery Thermal Runaway Mechanism at Different Environmental Temperatures

(1) Low Temperature

The main risk factors come from lithium deposition on the negative side and the generation of lithium dendrites.

(2) Room Temperature

The main risk factors come from heat generation due to polarization (Ohmic polarization, electrochemical polarization, etc.) or heat generation during high-rate charging/discharging.

(3)High Temperature

The main risk factors come from material failure, including SEI decomposition, separator shrinkage, and more.

4. Preventive Measures for Lithium Battery Thermal Runaway

(1) Installation of safety valves, with strict control over the pressure range.

(2) Installation of thermal resistors to prevent overcharging or short-circuiting of the battery.

(3) Precise thermal management by the Battery Management System (BMS), using methods like water cooling or air cooling to lower the battery temperature during usage.

(4) The use of additives in the electrolyte to reduce its flammability.

(5) Improvement of SEI film quality, such as adding LiCF3SO3 to the electrolyte to increase the inorganic components in SEI.

(6) Prevention of reactions between the positive electrode material and the electrolyte, such as using additives in the electrolyte or coating the positive electrode material.

(7) Increasing the melting point of the separator, such as applying ceramic layers on both sides of the separator.

(8) Standardized use of lithium batteries to minimize or eliminate human factors such as overcharging and over-discharging.