Meaning
Critical point in an electrochemical cell where the rate of internal heat generation begins to exceed the rate of heat dissipation defines this safety event. Once thermal response inversion occurs, the temperature of the battery will continue to rise even if the electrical load is removed. This phenomenon is a precursor to a full thermal runaway event.
Exothermic Trigger
Breakdown of the solid electrolyte interphase layer provides the initial burst of energy for the temperature rise. When thermal response inversion starts, the heat released by these chemical reactions accelerates further decomposition.
Reaction Mechanism
Speed of the chemical decay increases exponentially with every degree of temperature rise according to the Arrhenius relationship. Since thermal response inversion involves multiple simultaneous reactions, the overall heat output can become very high in a short time. Most of the energy comes from the reaction between the lithiated anode and the electrolyte.
Safety Threshold
Identification of the specific temperature where this process begins is a requirement for the design of battery enclosures. Designing for thermal response inversion involves ensuring that the cooling system can handle the peak heat load or that the cells are sufficiently isolated. Systems that can detect the early signs of this inversion provide more time for safety protocols to be enacted.
The test for this limit usually involves heating a cell in an insulated environment until the self-heating rate exceeds a predefined value.
