Thermal Runaway Mechanism And Safety Protection Measures of Polymer Lithium Batteries

Thermal runaway mechanism and safety protection measures of polymer lithium batteries

Due to their high energy density and excellent electrochemical properties, polymer lithium batteries have become the core components of portable electronic devices and new energy storage systems. However, safety issues caused by thermal runaway have always restricted the further development of this technology. This article will deeply analyze the induction mechanism of thermal runaway and propose a systematic safety protection plan.

1. Three-stage evolution mechanism of thermal runaway

1. Initial induction stage (80-120℃)

SEI film decomposition: When the temperature reaches 80℃, the solid electrolyte interface film begins to decompose irreversibly

Electrolyte vaporization: carbonate solvents vaporize violently above 90℃, and the internal pressure of the battery increases

Measured data: The temperature rise rate can reach 1.5-2℃ per minute

2. Chain reaction stage (120-250℃)

Positive electrode material oxygen evolution: layered oxide positive electrode releases active oxygen at 150℃

Electrolyte oxidation decomposition: PF6⁻ salt decomposes at 180℃ to produce corrosive gases such as PF5

Heat generation rate: This stage can reach more than 500W/kg

3. Thermal runaway outbreak stage (＞250℃)

Diaphragm meltdown: polyethylene diaphragm shrinks at 135℃, polypropylene diaphragm melts at 165℃

Internal short circuit: direct contact of electrodes causes instantaneous high current discharge

Temperature peak: experimentally measured up to 800℃

2. Five major factors causing thermal runaway

Mechanical abuse

Puncture test shows: 1mm steel needle puncture can cause temperature to soar 600℃ within 3 seconds

Extrusion deformation exceeding 15% may cause internal short circuit

Electrical abuse

When overcharged to 4.5V, the amount of positive lithium precipitation increases by 300 %

Fast charging above 2C causes lithium dendrite growth to increase by 5 times

Thermal abuse

120℃ environment for 30 minutes, thermal runaway probability reaches 75%

When the temperature difference in the local overheating area is greater than 50℃, the risk increases dramatically

Manufacturing defects

Self-discharge rate caused by micron-level metal particle contamination is greater than 5%/day

The risk of puncture increases significantly when the pole burr is greater than 20μm

Aging factors

SEI film thickness increases by 200% after 300 cycles

Thermal stability decreases by 40% when the capacity decays to 80%

III. Multi-level safety protection system

1. Improvements at the material level

Ceramic coating diaphragm (temperature resistance increased to 300°C)

Flame-retardant electrolyte (adding 10% trimethyl phosphate can shorten the self-extinguishing time by 80%)

Cathode material coating (Al2O3 coating increases the oxygen release temperature by 50°C)

2. Battery design protection

Three-level fuse design (overcurrent, overvoltage, overtemperature)

Pressure release valve (opening pressure 0.8-1.2MPa)

Thermal isolation layer (thermal conductivity of aerogel material <0.02W/m·K)

3. System-level management strategy

Distributed temperature sensing (one NTC is arranged every 2cm²)

Multi-parameter coupling warning (voltage + temperature + internal resistance change rate)

Liquid cooling system response time <3 seconds

4. Usage Specifications

Charging temperature should be controlled within the range of 0-45℃

Avoid continuous fast charging of >1C

Perform internal resistance test regularly (replacement is required if >30% of the initial value)

IV. Outlook of new safety technologies

Solid electrolytes

Oxide electrolytes have achieved 10⁻⁴S/cm ion conductivity at room temperature

Polymer-inorganic composite electrolytes can withstand high temperatures of 400℃

Smart diaphragms

Temperature-sensitive switch materials (response time <0.1 seconds)

Self-healing functional coatings (automatic healing of microcracks)

Early warning systems

Gas sensors detect characteristic gases such as HF and CO

Acoustic emission technology captures changes in internal structures

Conclusion

The safety issues of polymer lithium batteries need to be solved in a coordinated manner from three dimensions: materials, cells, and systems. With the development of new electrolyte materials and intelligent protection technologies, it is expected that the thermal runaway accident rate will be reduced to 1/10 of the current rate by 2025. It is recommended that manufacturers establish a full life cycle safety management system from raw material screening to terminal applications, and users must strictly abide by the usage specifications to jointly promote the safe and sustainable development of lithium battery technology.