Insights into the Transport Properties of Electrolyte Solutions in a Hierarchical Carbon Electrode by Molecular Dynamics Simulations

We report here the transport properties obtained through molecular dynamics simulations of a 1.5 M dimethyl sulfoxide (DMSO) lithium hexafluorophosphate (LiPF₆) electrolyte solution confined in a novel cathodic structure of a lithium-air battery. The cathode is composed of a hierarchical carbon structure, which was created from functionalized graphene plates using molecular simulations, mimicking the experimental nanocasting technique. Density profiles of the salt ions and the solvent were obtained under various electrode potentials and Li⁺ concentrations to investigate the effect of the hierarchical structure on the electrolyte and Li⁺ movement along with the diffusivities of the salt ions and the solvent. Results show that the hierarchical carbon structure boosts the diffusivity of Li⁺ ions by several orders of magnitude compared to typical porous carbon structures. In addition, the density profiles suggest that the hierarchical carbon cathode allows the Li⁺ ions to coexist in the small micropores along with the larger PF₆ ^- ions and the DMSO molecules under various conditions. This indicates the ability of the hierarchical carbon to promote the discharge reactions to form a passive Li₂O₂ discharge product within the micropores, while the existence of the high donor number solvent leads to the growth of Li₂O₂ in solution, preventing electrode passivation. In addition, the free space created in the mesopore will allow for more oxygen diffusion, supporting the hypothesis that materials with engineered porosity can help boost lithium-air battery performance.