An electrochemical energy profile on the charge-discharge cycle model of Li/pyrite- MS2 (M=Fe, Ni) secondary battery cells was simulated using density functional theory. For a full-discharge reaction of MS2 under enough Li-ion concentration, the calculated results indicate that the final products are Li2 S and M metal via the intermediate compound of commonly suggested Li2 MS2 (unknown for M=Ni), which is the intermediate product that continues the self-decomposition as Li2 MS2 →MS+ Li2 S. For a full-charge reaction Li 2 S+M metal at the cathode, the reproduction of the initial pyrite FeS2 is a more favorable scheme than the production of other iron sulfides (FeS and Fe3 S4), indicating the capability of the reversible charge-discharge cycle of Li/ FeS2 cell. However, for Li/ NiS2, there is difficulty in the reproduction of the initial pyrite NiS2 due to a closer formation enthalpy between NiS 2 and the other nickel sulfides (NiS, Ni3 S2, and Ni3 S4). This is one of the reasons that the Li/ FeS2 cell shows an experimentally better charge-discharge cycle performance than Li/ NiS2. The temperature dependence of the open-circuit voltage (OCV) was also estimated using the Nernst's equation, namely, the change in the Gibbs free energy derived from the enthalpy and entropy obtained by phonon calculations for the crystal lattices by applying the density functional perturbation theory. There is little temperature dependence of the OCV in the temperature range 0-100°C, and the maximum OCVs at the thermodynamic standard state (1.67 V for Li/ FeS2 and 1.84 V for Li/ NiS2) are consistent with the experimental results.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry