Lattice dynamics, thermodynamics and elastic properties of monoclinic Li ₂CO ₃ from density functional theory

Monoclinic Li ₂CO ₃ has been identified as a critical component of the solid electrolyte interphase (SEI), a passivating film that forms on Li-ion battery anode surfaces. Here, lattice dynamics, finite temperature thermodynamics and the elastic properties of monoclinic Li ₂CO ₃ are examined with density functional theory (DFT) and various exchange-correlation functionals. To account for LO-TO splittings in phonon dispersion relations of Li ₂CO ₃, which is a polar compound, a mixed-space phonon approach is employed. Bond strengths between atoms are quantitatively explored with phonon force constants. Temperature variations of the entropy, enthalpy, isobaric heat capacity and linear (average) thermal expansion are computed using the quasiharmonic approach. The single-crystal elasticity tensor components along with polycrystalline bulk, shear and Young's moduli are computed with a least-squares approach based upon the stress tensor computed from DFT. Computed thermodynamic properties as well as structural and elastic properties of the monoclinic Li ₂CO ₃ are in close accord with available theoretical and experimental data. In contrast to a recent DFT study, however, computed vibrational spectra suggest that neither the monoclinic Li ₂CO ₃ nor its high-temperature hexagonal phase exhibits either elastic or vibrational instabilities.