Theoretical Insights into the Charge and Discharge Processes in Aluminum-Sulfur Batteries

Abstract

With the limited number of studies available (mostly experimental), the field of aluminum-sulfur (Al-S) batteries is in urgent need of understanding their complex electrochemical reactions. Herein, the ab initio molecular dynamics (AIMD) simulations are used to obtain a detailed understanding of the involved charging and discharging processes in Al-S batteries by analysis of interfacial systems S8(001)/[EMIM]AlCl4 and Al2S3(001)/[EMIM]AlCl4-electrolyte, respectively. We observe that during the discharging process, the reduction of S8 follows a layer-by-layer mechanism and involves the formation of various cationic and anionic intermediate species, which drives the formation of Al polysulfides during the course of the discharging process. The evolution of the discharge voltage profile studied over a limited timescale shows two voltage domains: the first voltage domain (1.87-2.10 V) corresponds to the interface effects of surface-electrolyte and second the voltage domain of 1.38-1.50 V starts coinciding with the experimental value of 1.30 V and involves the reduction of S8 to higher-order polysulfides. We also observe the diffusion of these higher-order Al polysulfides to the electrolyte, which is in accordance with the experimentally observed solvation of higher-order Al poylsulfides into the electrolyte. The evolution of the atomistic structure and reaction voltage during charging shows that the top atomic sublayer structural distortions are mainly limited to the layer where Al atoms are removed and not the other inner atomic layers, which could be the reason for the poor electrochemical reversibility and an increased overall charging voltage (1.75 V) in experimentally studied Al-S batteries. We believe that the new atomistic insights obtained about the formation of various intermediate species, competition of different reaction mechanisms, and importance of local Al concentration could help improve the understanding of the complex electrochemical processes observed in Al-S batteries. © 2020 American Chemical Society.