Entropy Coefficient of a Blended Electrode in a Lithium-Ion Cell

The entropy coefficient of a blended positive electrode consisting of different types of active material (constituents) was derived from open circuit potential (OCP). Upon the imposition of a temperature step from an equilibrium state at a certain state of charge, the constituents should exhibit different OCPs due to their different entropy coefficients. Such a difference in OCPs shall induce an internal balancing current among the constituents, resulting in a transient OCP of the blended electrode as a mixed potential depending on the amount and charge transfer kinetics of the constituents. The state of charge of the constituents are adjusted until their respective OCPs equal to one another to define the equilibrium OCP of the blended electrode. An analytical expression relating the entropy coefficient of the blended electrode to that of the constituents, as well as the capacity fractions, is deduced and Validated with the experimental results. This method shall allow us to understand the charge transfer, energy loss, and heat generation pathway in the transient state under different charge and discharge conditions in the blended electrode to estimate the sources of heat in the cell reaction. (C) 2015 The Electrochemical Society.