Chitosan-Based N-, S-Doped Nanostructured Carbon/Graphene Oxide Composite as a Sulphur Host in the Lithium-Sulphur Batteries

Among the all next-generation batteries the lithium-sulphur battery, which uses extremely cheap sulphur as the positive electrode and the ultra-high-capacity lithium metal as the negative electrode, is at the forefront of emerging battery technologies by offering higher specific energy, at a lower price and lower CO₂ footprint compared to today's lithium-ion (Li-ion) batteries. Because of the complex, multi-electron chemical reaction the theoretical gravimetric energy density of the Li-S batteries is 2 567 Wh/ kg, which is 4.5-fold higher than the conventional lithium-ion technologies have. However, the instability of the sulphur cathode and lithium metal anode, which results in the poor cycling stability of the battery are the main factors that have limited the successful commercialization of this technology. In Li-S batteries, the liquid electrolyte and the sulphur cathode act like a couple to form the highly soluble lithium polysulfides (LiPS), which causes the well-known “shuttle effect” and lead to the loss of active material, Li metal anode corrosion, and low coulombic efficiency during interaction of the LiPS with Li metal anode. Moreover, the sulphur by itself has a high-volume change over the cycling and low conductivity that also have a negative effect on the electrochemical performances of Li-S batteries. Currently, the most perspective solution to address the above drawbacks is incorporating sulphur with a porous carbon host, which will tolerate sulphur volume expansion, improve the conductivity of the electrode, prevent the shuttle effect and allow reversible lithium-ion migration during charging and discharging. The high conductive carbon materials such as graphene and graphene oxide were suggested as one of the best options as hosts for sulphur in the Li-S battery cathode. These materials not only have a high electrical conductivity but may be doped with different functional groups, which improve the electrochemical properties of the Li-S battery cathodes. However, the currently available synthesis methods for these materials are complex, expensive, and usually require using of the highly toxic chemical. Moreover, the sulphur loading in such a host is limited by the material porosity and restacking possibility of the graphene or graphene oxide sheets.<br/>In our current work, we investigated the possibility to decrease the amount of graphene oxide during the creation of the sulphur-graphene oxide composite material. We proposed a one-step carbonization method to prepare 3D nanostructured porous nitrogen and sulphur dopped carbon materials where graphene oxide is used as the conductive support for the carbon. Such 3D nanostructured carbon materials were created by using chitosan hydrogel as the main carbon source. Chitosan is mainly produced by the deacetylation of chitin which is the second most abundant and renewable biopolymer after cellulose. The chitosan has large amounts of amino groups, which leads to the creation of the self-nitrogen-doped carbon after the pyrolysis process. In addition, thiourea and potassium carbonate are introduced into the chitosan hydrogel and play key roles in optimizing the structure of the carbon material. Thiourea not only generates gas through pyrolysis to form pores, but its own rich nitrogen and sulphur elements can be partially retained in the finally obtained carbon material. The structural, compositional, and morphological properties of the created material were investigated using SEM, EDS, Raman spectroscopy, FTIR and TGA techniques. The produced N,S-doped nanostructured materials have a high surface area of up to 1500 m²/g and because of the heteroatom doping allow to improve the electrochemical performance of the Li-S battery cathode, when the developed material is used as the sulphur host.