RESUMEN
A previous combined experimental and theoretical study found that the position of anchoring groups on a phenanthrene (PHE) backbone played a large role in determining the single-molecule conductance of the PHE derivative. However, a consistent 0.1 G0 feature was found across all PHE derivatives. To understand this, the previously investigated PHE derivatives were placed flat on a simulated Au substrate with a scanning tunneling microscope (STM) tip over PHE and conductance was calculated using the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT). The location of the tip was varied to find the most conductive and most energetically favorable arrangements, which did not coincide. Furthermore, the variation in conductance found in erect junctions was not present when PHE derivatives were lying flat, with all derivatives calculated to have conductance values around 0.1 G0.
RESUMEN
Calcium-ion batteries offer many advantages to the current lithium-ion technology in terms of cost, sourcing materials, and potential for higher energy density. However, calcium-ion batteries suffer from lack of a stable electrolyte due to reduction from the anode. Building off of our recent work investigating the stability of two representative electrolyte solvents, tetrahydrofuran (THF) and ethylene carbonate (EC), we now use ab initio molecular dynamics (AIMD) and the non-equilibrium Green's function technique in conjunction with density functional theory (NEGF-DFT) to investigate charge transport as the solvent molecules dynamically interact with the anode surface. THF maintained a relatively consistent conductance throughout the trajectory, although some jumps in the conductance were attributed to THF molecular rearrangement. EC exhibited a large amount of molecular decomposition, and a corresponding decrease in conductance of several orders of magnitude was noted. Through this analysis, we show that molecular decomposition and early-stage solid-electrolyte interphase (SEI) formation plays a major role in the robustness of charge transport as the system evolves in time and with temperature.
RESUMEN
Current electrolytes in calcium-ion batteries suffer from a lack of stability and degradation caused by reduction from the anode. The solid-electrolyte interphase (SEI) that forms on the anodes during operation stems the flow of electrons from the anode to the electrolyte. CaF2 is a common inorganic compound found in the SEI, and is derived from electrolyte salts such as Ca(PF6)2. CaF2 can exist in crystalline, polycrystalline, and amorphous phases in the SEI, and as recent work has shown, different phases of the same compound can have vastly different electronic conductivities. Using the non-equilibrium Green's function technique with density functional theory (NEGF-DFT), we find that amorphous phase systems enhance electron tunneling in thin CaF2 films by 1-2 orders of magnitude when compared to crystalline and polycrystalline CaF2 systems. Transport through several amorphous structures was considered showing that, despite their random structures, their conductance properties are similar. Through analysis of the decay constant ß and the low-bias conductance of each system, we show that crystalline and polycrystalline CaF2 offer greater protection of the electrolyte than amorphous CaF2.
