Toward First Principles Prediction of Voltage Dependences of Electrolyte/Electrolyte Interfacial Processes in Lithium Ion Batteries

In lithium ion batteries, Li⁺ intercalation into electrodes is induced by applied voltages, which are in turn associated with free energy changes of Li⁺ transfer (Δ<i>G</i>_<i>t</i>) between the solid and liquid phases. Using <i>ab initio</i> molecular dynamics (AIMD) and thermodynamic integration techniques, we compute Δ<i>G</i>_<i>t</i> for the virtual transfer of a Li⁺ from a LiC₆ anode slab, with pristine basal planes exposed, to liquid ethylene carbonate confined in a nanogap. The onset of delithiation, at Δ<i>G</i>_<i>t</i> = 0, is found to occur on LiC₆ anodes with negatively charged basal surfaces. These negative surface charges are evidently needed to retain Li⁺ inside the electrode and should affect passivation (“SEI”) film formation processes. Fast electrolyte decomposition is observed at even larger electron surface densities. By assigning the experimentally known voltage (0.1 V vs Li⁺/Li metal) to the predicted delithiation onset, an absolute potential scale is obtained. This enables voltage calibrations in simulation cells used in AIMD studies and paves the way for future prediction of voltage dependences in interfacial processes in batteries

Molecular Simulation of Carbon Dioxide, Brine, and Clay Mineral Interactions and Determination of Contact Angles

Capture and subsequent geologic storage of CO₂ in deep brine reservoirs plays a significant role in plans to reduce atmospheric carbon emission and resulting global climate change. The interaction of CO₂ and brine species with mineral surfaces controls the ultimate fate of injected CO₂ at the nanoscale via geochemistry, at the pore-scale via capillary trapping, and at the field-scale via relative permeability. We used large-scale molecular dynamics simulations to study the behavior of supercritical CO₂ and aqueous fluids on both the hydrophilic and hydrophobic basal surfaces of kaolinite, a common clay mineral. In the presence of a bulk aqueous phase, supercritical CO₂ forms a nonwetting droplet above the hydrophilic surface of kaolinite. This CO₂ droplet is separated from the mineral surface by distinct layers of water, which prevent the CO₂ droplet from interacting directly with the mineral surface. Conversely, both CO₂ and H₂O molecules interact directly with the hydrophobic surface of kaolinite. In the presence of bulk supercritical CO₂, nonwetting aqueous droplets interact with the hydrophobic surface of kaolinite via a mixture of adsorbed CO₂ and H₂O molecules. Because nucleation and precipitation of minerals should depend strongly on the local distribution of CO₂, H₂O, and ion species, these nanoscale surface interactions are expected to influence long-term mineralization of injected carbon dioxide