Increasing the safety of lithium-ion cells through pressure-controlled gas removal

Lithium-ion cells can release significant amounts of energy in the case of mishandling or production defects. This leads to the development of high pressure and very high temperature inside the cell followed with release of toxic, explosive and flammable gases, usually known as Thermal Runaway (TR). The impact of a TR on the immediate environment, especially if it affects other cells of the battery module, can be significant. The development of a TR includes several chain reactions involving mainly three components of the cell, namely the anode, cathode and electrolyte. A well-timed reduction in the concentration of one of these three reactants can significantly reduce the amounts of energy released during a TR and thus reduce the impact of the event on the surrounding environment.

The proposed pressure-controlled opening of the cell at early stages of fault development can remove the gaseous electrolyte from the system thereby the concentration of this reactant will be reduced. In addition, further evaporation of the electrolyte removes more energy from the cell in form of evaporation enthalpy.

The poster presents a proof-of-concept prototype consisting of a commercial prismatic cell equipped with a measurement unit. This measuring unit contains a temperature sensor, a pressure sensor and an electromagnetic valve that can be opened at different pressures. During preparation, the cell was opened in the inert atmosphere at the position of the seal pin and the measuring unit was mounted on this position.

The cell upgraded with this measuring unit and a non-manipulated cell were forced to TR by heating with a constant temperature ramp of 0.5 K/min using heating plates. The valve of the manipulated cell was opened for controlled gas venting at a temperature of 130 °C, whereas the burst vent of the non-manipulated cell opened at a temperature of 152 °C.

The experiments showed that the following can be achieved through the well-controlled gas release of the cell:

• Less severe exothermic reactions and TR process due to reduced concentration of one of the three reactants
• Cooling of the system during fault development due to the evaporation enthalpy of the electrolyte
• Reduction of heat power caused by ISC due to reduction of ionic conductivity of the electrolyte
• Reducing of the flammability of the gases emitted during TRs

Through this the TR could be delayed in time and accordingly to higher temperatures. In other words, a significantly higher energy input is needed to force the cell to TR. In addition, the maximum temperature of the cell during the TR was also reduced.

This concept can help increase the thermal stability of lithium-ion cells and reduce the impact of TRs on the environment.