Measurement of the entropy of reaction as a function of state of charge in doped and undoped lithium manganese oxide

Potentiometric measurements of the entropy and enthalpy of reaction at 298 K as a function of state of charge for LiMn2O4 and LiAl0.2Mn1.8O4-deltaF0.2 are presented. Lithium resides on nearest neighbor or next-nearest neighbor sublattices in lithium manganese oxide. The entropy of reaction is positive when a sublattice is nearly empty and negative when a sublattice is nearly full, and ranges from 0.25 to -0.35 mV/K in the undoped spinel and from 0.1 to -0.4 mV/K in the doped spinel. The potentiometric method includes a correction for self-discharge. Potentiometric measurements of entropy are compared to calorimetric measurements. The potentiometric method is most accurate for thermally thin cells, while calorimetric methods require large cells to improve the signal-to-noise ratio. (C) 2001 The Electrochemical Society