New Insights using Isothermal Calorimetry and High Precision Cycling

Summary:

In-operando isothermal calorimetry is a powerful tool for the study of active materials for energy storage. It has been used to study positive [1–3] and negative [4–7] electrode materials in half cells, as well as in symmetrical [4,5] and full cells.[8–15] Calorimetry can be used to quantify parasitic reactions enabling lifetime predictions but also identify structural and entropy changes in materials.

Parasitics and Lifetime Predictions in Commercial Cells
High precision calorimetry and coulometry was performed at various temperatures on commercial 18650s allowing a quantification of the temperature dependence of parasitics. Identical commercial cells underwent long term cycling. Parasitics and cycle life as a function of temperature and rate are compared. The results demonstrate the applicability of isothermal calorimetry as a technique for commercial cell development or evaluation.

Calorimetric Signature of Structural Changes in Silicon [16]
Unlike graphite, the structural evolution of Si during lithiation and delithiation is uniquely dependent on the cycling conditions and can show either reversible or path dependent behavior. However, the calorimetric signature of this path dependence had never been characterized. Metallurgical Si is cycled to exhibit both reversible and path dependent cycling while in-operando calorimetry is performed with a high precision isothermal calorimeter. For the first time, the large exothermic signature of the crystallization to Li15Si4 at full lithiation is measured allowing the quantification of the enthalpy of crystallization of Li15Si4. The results of this in-depth calorimetric study of metallurgical Si provides valuable insights into the heat production and energy efficiency of Si as a negative electrode material in Li-ion batteries.

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