The five modes of heat generation in a Li-ion cell under discharge

Abstract

A lithium-ion cell under discharge generates thermal energy (Q) through five different internal parameters or modes: the electrolyte resistance (Rs), anode resistance (Ra), cathode resistance (Rc), and entropy changes in the cathode (ΔSc), and the anode (ΔSa). This work demonstrates a set of tools to measure/quantify the heat generated by each parameter separately during discharge. These five sources are not dependent upon each other; they are dependent on the state of charge and the environmental temperature (Tenv). The Q generated by each mode varies with degree of discharge and Tenv. Rs generates most of the Q in the −10 °C to 40 °C range; Rc becomes significant at Tenv <20 °C. Constant current discharge does not cause a monotonic increase in anode and cathode temperatures (Ta and Tc), due to the direction of change in ΔSc and ΔSa. Negative change in ΔSa for the carbon anode cools it, causing the Ta to level off and even decrease with increased discharge. ΔSc for lithium manganese oxide cathode is positive at some SoC and negative at others, preventing a monotonic increase in Tc. Measuring the five Qs separately opens the opportunity to study thermal-runaway from the perspective of the anode, cathode and electrolyte.