Use of Thermal Analysis to Elucidate Irreversibility and Degradation in Lithium Based Batteries

<br/>At its essence, energy is the sum of heat and work: △E = q + w. As such, the ultimate goal for any energy storage system is to maximize useful work (w) and minimize the generation of waste heat (q). Ideal batteries would provide energy that can be efficiently converted to useful electrical work over many reversible cycles, however, degradation inevitably occurs. Operando isothermal microcalorimetry (IMC) is a precise technique that can monitor real-time heat flow released during battery operation and can thus be utilized as a diagnostic tool for quantifying the heat flow. Our detailed analysis of the thermal profiles enabled determination of thermodynamic reversibility as well as the contributions of parasitic reactions to total heat flow. <br/>Specifically, IMC was used to probe electrolyte interactions on conversion materials used as negative electrodes as a function of state of charge and voltage. The heat flow analysis separated the contributions from polarization, entropic changes, and parasitic reactions where the onset and magnitude of the electrolyte reaction could be determined. Further, IMC was used to probe insertion positive electrode materials also as a function of state of charge and associated voltage. Study of the heat flow under conditions of increased delithiation achieved through the use of higher charge voltage revealed differences in thermodynamic reversibility. The examples provided will illustrate the important mechanistic insights that can be gained through in depth analysis of thermal data.