| dc.description.abstract | Li-ion batteries represent a pinnacle of compact energy storage. This size reduction makes them very energy dense systems, thus substantially increasing the chances of the mishap. Future applications demand a very good rate capability (i.e., fast charging), which invariably leads to higher heat generation. This heat if not dissipated properly, rapidly increases the cell temperature and eventually leads to thermal runaway. Thus, the knowledge of heat generation as a function of current is of utmost importance for the design of cooling systems. Heat generation rates are most commonly quantified using accelerating rate calorimeter.
In this study, a calorimeter-free method based on inverse heat transfer analysis is proposed. 18650 cells are electrochemically cycled at different currents (C-rate) with consecutive charge-rest-discharge-rest cycles in a constant temperature ambient. During the experiments cell temperature, ambient temperature, current and voltage data is recorded. An energy balance is carried out to model the thermal response of the 18650 cell during electrochemical cycling. The model involves volumetric heat generation rate and convective heat transfer coefficient as unknowns which are characterized by inverse heat transfer analysis. Convective heat transfer coefficient is computed from data during rest periods. It is then used to quantify heat generation rate as a function of charge/discharge capacity and C-rate. At low current operation, the contribution of reversible heat is of the similar order to irreversible heat and would lead to qualitatively different heat generation profiles during charging and discharging. On the other hand, at higher currents, irreversible heat dominates and the heat generation rates during charge and discharge are quite similar, both qualitatively and quantitatively.
The contribution of various sources towards total heat generation has been quantified. Effect of capacity fade on internal resistances and heat generation rates, while cycling at various C-rates, has been investigated. At higher C-rates, the contribution of reversible heat towards the total heat generation is found to be negligible while that of irreversible ohmic heat is found to be major and is closely related to the internal resistance. Internal resistance is found to be independent of C-rate of operation, and increasing with capacity fade in a cell. | en |
