Entropy of Li intercalation in LixCoO2

Abstract

The entropy of lithiation of LixCoO2 for 0.5 < x less than or equal to 1.0 was determined from measurements of the temperature dependence of equilibrated voltages of electrochemical cells. Measured changes in the entropy of the lithiation reaction were as large as 9.0 k(B)/atom, and as large as 4.2 k(B)/atom within the "O3" layered hexagonal structure of LixCoO2. Three contributions to the entropy of lithiation for the O3 phase were assessed by experiment and calculation. The phonon entropy of lithiation was determined from measurements of inelastic neutron scattering. Phonon entropy can account for much of the negative entropy of lithiation, but its changes with lithium concentration were found to be small. Electronic structure calculations in the local density approximation gave a small electronic entropy of lithiation of the O3 phase. The configurational entropy from lithium-vacancy disorder was large enough to account for most of the compositional trend in the entropy of lithiation of the O3 phase if ordered structures exist at lithium concentrations of x=1/2 and x=5/6. The electrochemical measurements showed the existence of a two-phase region in the composition range between x=5/6 and 0.95. Electronic structure calculations gave evidence that these phases were metallic and insulating, respectively. Changes of the electronic and configurational entropy were found to be of comparable importance for this metal-insulator transition.

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