Electrochemical properties of crystallized dilithium squarate: insight from dispersion-corrected density functional theory

The stacking parameters, lattice constants, and bond lengths of solvent-free dilithium squarate (Li2C4O4) crystals were investigated using density functional theory with and without dispersion corrections. The shortcoming of the GGA (PBE) calculation with respect to the dispersive forces appears in the form of an overestimation of the unit cell volume up to 5.8%. The original Grimme method for dispersion corrections has been tested together with modified versions of this scheme by changing the damping function. One of the modified dispersion-corrected DFT schemes, related to a rescaling of van der Waals radii, provides significant improvements for the description of intermolecular interactions in Li2C4O4 crystals: the predicted unit cell volume lies then within 0.9% from experimental data. We applied this optimised approach to the screening of hypothetical framework structures for the delithiated (LiC4O4) and lithiated (Li3C4O4) phases, i.e. oxidized and reduced squarate forms. Their relative energies have been analysed in terms of dispersion and electrostatic contributions. The most stable phases among the hypothetical models for a given lithiation rate were selected in order to calculate the corresponding average voltages (either upon lithiation or delithiation of Li2C4O4). A first step towards energy partitioning in view of interpretating crystal phases relative stability in link with (de)-intercalation processes has been performed through the explicit evaluation of electrostatic components of lattice energy from atomic charges gained with the Atoms in Molecules (AIM) method.