Ab initio simulations of Li/Pyrite- MS₂ (M=Fe, Ni) battery cells

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.