Thermodynamics of lithium intercalation into graphites and disordered carbons

Abstract

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 ΔS and enthalpy of intercalation ΔH were determined over a broad range of lithium concentrations. For the disordered carbons, ΔS is small. For graphite, an initially large ΔS decreases with lithium concentration, becomes negative, and then shows two plateaus associated with the formation of intercalation compounds. For all carbons ΔH is negative, and decreases in magnitude with increased lithium concentration. The measurements of ΔH 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 ΔS require another contribution to entropy, perhaps vibrational in origin.