Alkali polysulfide phosphorus complexation batteries : a quantum chemistry electrochemical study
Keywords: Engineering.
Mechanical Engineering.

The development of battery technologies has historically been largely based on trial and error, with theory and computation playing an auxiliary role. This work focuses on the use of quantum chemistry to understand the operation and inform the design of battery systems from a molecular perspective. The complexation and reaction mechanisms of lithium-sulfur and lithium-thiophosphate catholyte batteries are analyzed using density functional theory calculations, which are directly compared with experiments. Li-thiophosphate batteries exhibit high capacity and cyclability, thanks to the ability of phosphorus structures to accommodate the discharge product (Li2S). The accommodation mechanism informs the stoichiometry that leads to optimal electrochemical performance and improved performance, which achieves theoretical capacity, is confirmed experimentally. Electrochemistry calculations are also performed for Na and K batteries, allowing the thermodynamic and kinetic comparison of Li, Na and K for S-based catholyte batteries. Na is found to operate at lower voltages compared to Li, whereas K operates in a wide voltage window. Charge transfer energy barriers are evaluated using Marcus theory and reaction kinetics favor Na over Li and K over both Li and Na. The calculated activation potentials accurately predict the onset of charging in cyclic voltammetry experiments. The theoretical studies of alkali-sulfur batteries lead to important considerations that must be accounted for in comprehensive quntum-chemistry-based electrochemistry calculations. Specifically, levels of theory require comparison with electrochemical and structural experiments; the nature of sites on the battery anode nanostructure impacts electrochemical potentials significantly; and temperature-dependent terms, entropy especially, contribute substantially to theoretical electrochemical potentials and cannot be neglected, even at room temperature. Furthermore, the intercalation ability of Li, Na and K in graphite, the most common battery anode material, is assessed by binding energy calculations of alkali metal atoms and cations with aromatic molecules. It is observed that Na binding with aromatics is significantly weaker than Li or K binding. The non-monotonic binding behavior with atom size or mass arises from a competition between metal ionization energy and equilibrium distance between the metal and the aromatic, which leads to a different Coulomb attraction energy. In addition, alkali metal incorporation drastically reduces the band gap of aromatic molecules, but the effect is lost when the complex is charged, allowing the design of tunable band gap materials. Such an effect is also observed with transition metals, which bind strongly with aromatics. Between the choice of aromatic, metal element, charge state and spin state, a wide range of band gap values can be achieved. The optical behavior of metal-aromatic complexes is also found to be consistent with flame-formed carbon nanoparticle optical properties. Finally, a 2D quantum dot theory is developed to describe the band gap decrease of aromatic molecules with size, as they approach the zero band gap graphene limit. It is also observed that the band gap of molecular clusters is determined by the smallest band gap constituent molecule, which sheds light on the size vs. composition debate for the band gap of carbon nanoparticles.