Thermodynamics of Lithium Intercalation into Graphites and Disordered Carbons

The temperature dependence of the open-circuit potential of lithium half-cells was measured for electrodes of carbon materials having different amounts of structural disorder. The entropy of lithium intercalation, DeltaS, and enthalpy of intercalation, DeltaH, were determined over a broad range of lithium concentrations. For the disordered carbons, DeltaS is small. For graphite, an initially large DeltaS decreases with lithium concentration, becomes negative, and then shows two plateaus associated with the formation of intercalation compounds. For all carbons DeltaH is negative, and decreases in magnitude with increased lithium concentration. For lithium concentrations less than x = 0.5 in LixC6, for the disordered carbons the magnitude of DeltaH is significantly more negative than for graphite (i.e., intercalation is more exothermic). The measurements of DeltaH provide an energy spectrum of chemical environments for lithium. This spectrum can be used to understand some of the concentration dependence of configurational entropy, but the negative values of DeltaS require another contribution to entropy, perhaps vibrational in origin.