Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Badri Narayanan Applicant's institution: ANL Applicant's division: NST Project Name: LiSx Project title: Solvation dynamics of lithium polysulfide species in sparingly solvating electrolytes Associated funding: JCESR Other Systems: Carbon (CNM): 500,000 core hours ALCF: 50 million core hours Science: Lithium-sulfur (Li-S) batteries hold tremendous potential for next-generation energy storage devices owing to their high theoretical capacity (1675 mAhg-1), nearly 10 times that of a conventional Li-ion battery (140 mAhg-1), while using inexpensive and abundantly available materials. The Li-S battery system, however, is very challenging, mainly due to the complex redox reactions taking place at the S cathode; these cause reduction of S8 into various soluble polysulfide species that diffuse away from the cathode, and consequently, decreases Coulombic efficiency and capacity retention of the cell, as well as result in undesirable side reactions at the Li anode, termed polysulfide shuttle. Recently, the use of sparingly solvating electrolytes (e.g., acetonitrile with lithium bis(trifluoromethane sulfonyl) imide (LiTFSI)) have shown great promise in impairing solubility of Li-polysulfide species. These studies utilize high concentrations of salt with com mon ions (Li+), which shift the equilibrium between solid and solvated Li+ towards solid and decrease the solubility. This, however, comes at the price of increased viscosity and poor diffusion kinetics. To mitigate this effect, secondary solvents called diluents (e.g., 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE)) thought to be inert to the solvate structure are added to reduce the viscosity, and enhance the diffusion kinetics. Although, experimental works by our JCESR collaborators have shown marked success in the use of sparingly solvating primary solvents in combination with diluents for electrolytes, a fundamental understanding of the solvation dynamics is still lacking. A molecular level knowledge of the solvation structure, and dynamics around LiSx species is crucial for designing new electrolytes with exceptional electrochemical performance. Such an understanding is invaluable particularly under conditions necessary for high energy density such as a moderately high S loading (4 mg/cm2), low E/S ratio (< 3 mL/g) and room temperature. In this project, we will employ ab-initio molecular dynamics simulations, solvation free-energy density functional theory calculations, and electronic structure calculations to investigate the solvation structure around various LiSx species in two ACN-LiTFSI (primary solvent), and TTE/HFE diluents. These studies will identify the effect of diluent on electrochemic al performance, electrolyte stability, and physical properties (e.g., viscosity) of the electrolyte system. Furthermore, these will unravel the molecular/electronic origin of the improved performance of electrolyte; these, in turn, will enable computationally-aided discovery of new and better diluent structures. Project description: The first set of simulations will employ ab-initio molecular dynamics to identify the solvation structure around various LiSx species in numerous electrolyte systems. Three LiSx species: 1) S8, 2) Li2S8, and 3) Li2S6 will be investigated in electrolyte systems comprising of (ACN)2-LiTFSI (primary), and two diluents (TTE, HFE). The AIMD simulations will be performed using the plane wave density functional theory package VASP; the k-point will be sampled only at the Γ-point, and modest energy cut-off (300 eV) will be employed. The Perdew-Burke-Ernzerhof pseudopotentials will be employed for exchange correlations within the generalized gradient approximation. Three different ratios of primary/diluent will be studied for each LiSx species and diluent. For each simulation (system size ~200 atoms), AIMD runs for 10 ps will be performed at 300 K and 400 K (operation temperatures); each of these runs are expected to take 100 hours on 96 cores. So we need (3 LiSx species) x (3 ratios) x (2 diluents) x (2 temperatures) x (96 cores ) x (100 hours) = 345600 core hours.

From the AIMD simulations, we will choose 12 solvation structures on which we will perform more accurate electronic structure, and charge transfer calculations. Each of these will take 144 hours on 96 cores. So we need (12 structures) x (96 cores) x (144 hours) = 165888 core hours.

In total, require 500,000 core hours. Industry partnership: Project URL: Requested allocation: 499999 Q1: 125000 Q2: 125000 Q3: 125000 Q4: 124999 Justification: Storage requirements: The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System