Single-Atom Catalysts as Promising Cathode Materials for Lithium–Sulfur Batteries

Abstract

A major challenge toward the practical application of lithium–sulfur (Li–S) batteries is the lithium polysulphide (LiPS) shuttling, caused by the rapid LiPS migration in the electrolyte and slow reaction kinetics. Single-atom catalyst (SAC) materials hold the promise of strong LiPS binding to the cathode and improved reaction kinetics in Li–S batteries. In this study, we examine the electrocatalytic properties of four SAC materials with TM–N4–C (TM = Co, Fe, V, and W) formation, from simulations based on the density functional theory. We study for the first time a W-based SAC as the Li–S cathode host and calculate the adsorption energy of five LiPS intermediates (Li2S n , n = 1, 2, 4, 6, and 8). We explore the mechanism for the conversion of Li2S2 to Li2S and present the energy profiles based on the Gibbs free energy for the LiPS reaction pathway from S8 to Li2S. V- and W-based SACs provide high LiPS binding, of up to 6.0 times of pristine graphene, and largely improved reaction kinetics by lowering the S dissociation barrier of the Li2S2 decomposition by ∼2.5 times compared to pristine graphene. Our work provides a detailed investigation into the adsorption and conversion of LiPSs on SAC cathode hosts, highlighting their potential on improving the electrochemical performance of Li–S batteries and mitigating the LiPS “shuttle” effect.

Accelerating the theoretical study of Li-polysulphide adsorption on single-atom catalysts via machine learning approaches

“…We note that when we include higher energy Li 2 S 2 CH vertexes at the relaxation stage, we identify a configuration with binding energy of -2.04eV, in line with what reported in the literature. In this regards, the binding energy change trend among the various LiPSs is also in good agreement with our previous study [13], which adopts the exact same DFT calculation set up.…”

“…Recent studies indeed suggest that SAC with atomically dispersed transition metals (TMs) on graphene lattices are particularly promising candidates. In particu-lar, calculations on the effect of SACs with TM-N 4 -C formation (TM = Co, Fe, Mn, Ru, V, W, Zn, and C as a graphene lattice) on the LiPS adsorption, the Gibbs free energy change and the reaction activation barrier [5][6][7][8][9][10][11][12][13], predict a remarkable improvement in the cathode properties which could potentially reduce the LiPS "shuttle" effect.…”

“…For example, in highorder LiPS intermediates (Li 2 S 4 , Li 2 S 6 and Li 2 S 8 ) the final binding energies varies over 0.5 eV [14] depending on the initial LiPS structure that is used as input in the relaxation. By the same token, reported values of Li 2 S 6 binding energy on Fe-based SAC, one of the most examined SAC material for Li-S batteries, vary by about 0.6 eV, from −0.9 eV to −1.5 eV [5,7,9,13]. The large discrepancy in the results, raises concerns regarding the quality of the protocols adopted to identify the minimum energy structure, and adds uncertainty when comparing SAC results from different studies.…”

Sulfur infiltration and allotrope formation in porous cathode hosts for lithium‐sulfur batteries

“…As the sulfur powder is initially on the top surface of the ACF, sulfur fluid infiltration proceeds in the y axis (transverse to the fabric) according to Equation (6). As the sulfur powder is intimately mixed with the AC powder, a thin layer of sulfur is assumed initially to wrap each particle (same layer thickness for all AC particles of size 2 R part , assuming homogeneous sulfur distribution after ideal milling of powder blend), and sulfur infiltration proceeds radially from the particle surface inwards according to Equation (7).…”

“…2 Additionally, effective cathode hosts offer means to trap the sulfur and polysulfides to limit polysulfide shuttling between cathode and anode, which would cause reduction in capacity and coulombic efficiency. 3,4 Suitable microstructural architectures, including small pores, pores with necks and hollow particles, 5,6 and hosts functionalized with chemical groups with high adsorption energy to sulfur and sulfides 7 can provide such good traps. Additional specifications for host porosity and pore size distribution (PSD) target a sulfur mass greater than 5 mg cm À2 , making up at least 70% of the cathode weight, to realize the high theoretical energy density of Li-S batteries.…”