Effect of oxygen-substituted lattice carbon in Ti3C2 MXene as anchoring material for Li-S battery: DFT calculations and experiments

Abstract

Electrochemical processes involving the adsorption/desorption of polysulfides are usually related to the composition of adsorbed-sulfur electrode material. Here, we investigate the moderate influence of non-metallic O and C elements on anchoring property towards lithium polysulphides (LiPSs), whereby the adsorption behavior of 2D-Ti3C2 MXene with continuous O-substitution into C site is analyzed via density functional theory (DFT) calculations in concerning of reported inevitable oxygen in the lattice. The introduction of O into 2D-Ti3C2 lattice reduces the density of states of Ti elements near the Fermi level, thus weakening the Ti-S binding interaction. Ti3C2 MXene with C/O ratio of 3:1 (Ti3C2-C12O4 model) exhibits moderate adsorption superior to other compositions, where the calculated binding energies of S8 and Li2Sn (n = 8, 6, 4) are −2.88, −2.57, −2.31 and −2.19 eV atom−1, respectively. The substitution of O atom theoretically hinders the transfer of Li+ and reduces the energy barrier of S-migration, thus promoting the anchoring and transformation of LiPSs. Notably, consistent result is obtained from the comparative study on electrochemical performance of titanium oxycarbide with precisely different C/O ratios as alternative, considering the unpractical controlling of O-content in 2D-Ti3C2 and the homologous crystal structure. Titanium oxycarbide with C/O ratio of 3:1 (Ti(C,O)-C12O4) as anchoring material in Li-S battery exhibits a superior initial specific capacity of 841 mAh/g at 1C and fasten ion diffusion coefficients (1.92 cm2 s−1 and 2.94 cm2 s−1). This study suggest attention on regulatory effect of non-metallic elements in lattice instead of surface towards performance optimization of titanium-carbide-based material as host for sulfur cathode utilized in Li-S battery.