2D framework C2N as a potential cathode for lithium–sulfur batteries: an ab initio density functional study

Abstract

One of the biggest challenges of Li–S batteries (LSBs) is the dissolution of polysulfides at the cathode side. An ideal LSB cathode should be able to prevent polysulfide (Li₂S_n, 1 ≤ n ≤ 8) dissolution, and should have a strong binding energy, high capacity and a light weight, as well as being electrically conductive. Herein, we present our quantum chemical calculations combined with a genetic algorithm for a global structure search using a C₂N nanosheet as a LSB cathode. The Gibbs free energies of isolated Li₂S_n and Li_mS_n_C₂N (1 ≤ m, 1 ≤ n ≤ 8) are calculated to set forth the studies of both the voltages and stabilities of lithiation reactions in every step of the discharging process. Our results indicate that the polysulfides can adhere to the C₂N host substrate in a thermodynamically stable cluster state without dissolution, and the system has an energy capacity of up to 1122.21 W h kg⁻¹. The calculated band-structure shows that Li_mS_n_C₂N is metallic though the original C₂N nanosheet has an optical gap of 1.96 eV. This shows that C₂N could be a promising cathode material for LSBs.