Roles of N doping and titanium suboxide in pyrolytic waste plastics-derived carbon for immobilizing polysulfides in the cathode of Li-S batteries

Abstract

Lithium-sulfur (Li-S) batteries exhibit high theoretical energy densities, but suffer from shuttling phenomena, which is fatal to long-term cycle stability in the way that lithium polysulfide gradually diffuses at the anode interface. To solve this problem, the pyrolysis residue of waste plastic was used as a carbon source, and this was applied as a sulfur host for a cathode material of a Li-S battery to suppress the shuttle effect. Hierarchical porosity was secured by giving mesopores of 30% of the total pore volume in as-synthesized carbon through potassium hydroxide and subsequent ammonia heat treatment. Nitrogen defects in the carbon were mostly introduced into the carbon framework at the edge, which played a positive role in chemical interaction with the Li moiety of polysulfides. In addition, it was proved through electron spin resonance (ESR) and static adsorption experiments that titanium suboxide ($TiO_{2-x}$) was formed as an impurity after heat treatment to suppress the diffusion of the intermediates. Based on the overlapping effect of N-defects and $TiO_{2-x}$, the pyrolysis residue-derived carbon from waste plastics exhibited a reversible capacity of more than 500 mAh and a small decay level of 0.089% up to 500 cycles at 1C rate. This study is expected to provide a precedent for the design of a practical and eco-friendly carbon electrode by applying the pyrolysis residues of waste plastics as a carbon source.