2D Carbon Phosphide for Trapping Sulfur in Rechargeable Li-S Batteries: Structure Design and Interfacial Chemistry

Abstract

Rechargeable lithium-sulfur batteries (LiSBs) assembled with earth-abundant and safe Li anodes are less prone to form dendrites on the surface, and sulfur-containing cathodes offer considerable potential for achieving high energy densities. Nevertheless, suitable sulfur host materials and their interaction with electrolytes are at present key factors that retard the commercial introduction of these batteries. Here we propose a two-dimensional metallic carbon phosphorus framework, namely, 2D CP₃, as a promising sulfur host material for inhibiting the shuttle effect and improving electronic conductivity in high-performance Li-S batteries. The good electrical conductivity of CP₃ eliminates the insulating nature of most sulfur-based electrodes. The dissolution of lithium polysulfides (LiPSs) into the electrolyte is largely prevented by the strong interaction between CP₃ and LiPSs. In addition, the deposition of Li₂S on CP₃ facilitates the kinetics of the LiPS redox reaction. Therefore, the use of CP₃ for Li-S battery cathodes is expected to suppress the LiPS shuttle effect and to improve the overall performance, which is ideal for the practical application of Li-S batteries.

Keywords: 2D CP3; DFT; Electrochemical properties; Lithium polysulfides; MD; Organic electrolyte; Shuttle effect.

Figure 1

Schematic illustration of (a) shuttle effect phenomena in Li–S batteries and its suppression through (b) anchoring process and (c) catalytic effects of the 2D carbon phosphide host material.

Figure 2

(a, b) Top and side views of free-standing (a) blue phosphorus and (b) CP₃ monolayers. (c, d) Projected band structures of the (c) blue phosphorus and (d) CP₃ monolayers computed using GGA-PBE with the corresponding projected densities of states.

Figure 3

(a) The top and side views of the fully optimized structures of S₈/Li₂S_n molecules on CP₃ monolayer. (b) Computed binding energies of Li₂S_n molecules on blue phosphorene, graphene, and CP₃ monolayers. (c) Computed binding strengths of S₈/Li₂S_n molecules on CP₃ monolayer through DFT and DFT-D3 correction and their corresponding ratio (R_vdW). (d) Computed binding energy with different Li-concentrations on the CP₃ monolayer.

Figure 4

Distributed charge density for S₈/Li₂S_n molecules adsorbed on the CP₃ monolayer. Yellow and blue colors indicate the electron accumulation and depletion, respectively. The value of the isosurface is set to be 0.001 e Å^–3.

Figure 5

Free energy landscape of the sulfur reduction reaction on graphene and CP₃ surfaces, with insets highlighting the optimized geometries of reaction intermediates.

Figure 6

Comparison of Radial Distribution Function (g(r)) and Coordination Number (N(r)) average profiles for S₆^2– and S₈^2– anions (S^δ−, where δ denotes the partial charge on each S atom) surrounding a single Li⁺ ion in simulated electrolytes near the CP₃ surface, with varying solvent amounts. (a, b) depict Li₂S₆/DME and Li₂S₆/DOL electrolytes, as well as Li₂S₈/DME and Li₂S₈/DOL electrolytes, respectively. (c, d) illustrate the average profiles of solvent oxygen atoms around a single Li⁺ ion in the same electrolytes.

Figure 7

Relevant dynamics properties for the simulated electrolyte near the CP₃ monolayer: (a) Li⁺ transference number (t_Li⁺) and the Li⁺ self-diffusion coefficient, calculated for Li₂S₆/DME, Li₂S₆/DOL, Li₂S₈/DME and Li₂S₈/DOL simulated systems. (b) Ionic conductivity values (σ) and Nernst–Einstein conductivity (σ_ne) and its comparison. The dotted lines serve as visual guides. (c) and (d) Log–Log plot of the Li⁺ MSD variation as a function of time. (c) Li₂S₆/DME and Li₂S₆/DOL, (d) Li₂S₈/DME and Li₂S₈/DOL. The MSD plots confirm that a normal diffusion mode is achieved, which confirms the quality of the calculated dynamic properties.