A calorimetric approach to fast entropy-variations extraction for lithium-ion batteries using optimized galvanostatic intermittent titration technique

A fast entropy-variations (ΔS) extraction method has been proposed based on calorimetry, which determines the heat associated with ΔS by analyzing the electro-thermal response of a battery to a sequence of constant current pulses, i.e., the galvanostatic intermittent titration technique (GITT). The rest times in GITT are reduced by only considering limited relaxation of the ionic concentration gradients inside the battery after the current interruptions while completely ignoring the thermal equilibrium conditions inside the calorimeter. The resulting thermal signal of the battery is analyzed using an algorithm that adopts exponential regression to characterize the generated heat energy corresponding to each current pulse. Additionally, the polarization heat inside the battery is investigated by taking into account the initial presence of the concentration gradients when a current pulse is applied. Thus, the optimized rest times between the successive current pulses can reduce the measurement time manyfold compared to the previously reported methods, which require the battery to reach both electrochemical and thermal equilibriums. This work shows that the ΔS profiles of a 1 Ah NMC811/graphite pouch cell with 2.5% state of charge (SOC) resolution can be extracted at least three times faster than the method with unoptimized rest times, in a highly repeatable manner.