The Rational Design of Nanocarbon and Metal Composite Materials for Lithium–Sulfur Battery Cathode: Li- or S-Binding?

Abstract

Given the high theoretical capacity 1672 mAh g−1 of sulfur cathode, lithium–sulfur (Li–S) batteries have been intensively concerned to fulfill the urgent demands of high capacity energy storage. However, quite a few obstacles are still hindering the practical applications of Li–S batteries. One of the major issues is the complex diffusion of lithium polysulfide intermediates, which in combination with the subsequent paradox reactions is known as the shuttle effect. Heteroatom-doped carbon and polar compound additives are two promising routes in ascendant for the adsorption and immobilization of polysulfide intermediates.In this work, a series of heteroatom-doped graphene nanoribbons (GNR) was modeled and their binding behaviors toward both polar lithium polysulfide and nonpolar elemental sulfur were evaluated. The first-principle calculation was conducted to systematically describe the complex interactions in terms of different doping atoms. By meticulously analyzing the configuration, binding energy, bond length, charge transfer, and deformation charge density, how the doping atoms including B, N, O, F, P, S, and Cl affect the adsorption of sulfur and Li2Sx species was explored. The local polarization of binding sites within the carbon lattice was correlated to the subsequent dipole–dipole interactions, both of which were considered to play critical role in the system. A set of principle rules and a volcano plot correlating the electronegativity of doping atoms to the adsorption energies were proposed to guide the future screening and design of scaffolding materials with chemical dopants.Besides doped carbon hosts, various metal composites have been considered as additives to enhance the binding effects and promoting the conversion of high-order polysulfides. The first-row transition-metal sulfides (TMSs) are selected as the model system to obtain a general principle for the rational design of a sulfur cathode. The strong S-binding that is induced by charge transfer between transition-metal atoms in TMS slabs and S atoms in Li2S is confirmed to be of great significance in TMS composite cathodes. An analogous periodic law of anchoring strength was find in 3D-metal sulfide system, which can be extended well to the 3D-metal oxide system well, and VS was predicted to be an excellent polar host candidate for sulfur cathodes. This finding provides a general and practical principle for rational design of advanced sulfur cathode materials, which is expected to be an effective toolbox to screen composite sulfur cathode for Li–S batteries with high energy density and long cycle life.