Room-Temperature Lithium Phases from Density Functional Theory

Metallic lithium is a promising electrode material for developing next generation rechargeable batteries, but it suffers from dendrite formation upon recharging. This compromises both efficiency and safety of these systems. The surface phenomena responsible for dendrite formation are difficult to characterize experimentally, making it important to use Quantum Mechanics (QM) to provide the understanding needed to design improved systems. The most accurate practical level of QM for such studies is the PBE form of density functional theory (DFT.) We report here an assessment of the accuracy for PBE to predict the most stable four phases of bulk lithium. PBE predicts three phases, bcc, fcc, and hcp, to be nearly equal in stability (within 5 meV). Including the zero-point energy and enthalpy corrections at standard conditions (298 K and 1 atm), we predict bcc most stable, fcc at 0.0029 eV, hcp at 0.0036 eV, and cI16 at 0.0277 eV. Indeed their experimental free energies under ambient conditions differ only by a few meV, with bcc considered most stable. Experimentally, the fourth phase becomes stable at high pressure (>30-40 GPa) and low temperature (<200 K). To use QM calculations to predict growth mechanisms, it is essential to bias the calculations by imposing the desired crystal structure in the underlying layers. We consider that this will allow QM studies of dendritic growth far from the crystalline phase due to the lower surface energy of bcc. (Figure Presented).