The operating principle of the Li-S battery is based on the reduction of S at the cathode during discharge to form various polysulfides, which combine with Li to ultimately produce Li2Sand vice versa during charge. Li-S technology has many attractive features, including the natural abundance and low cost of S, and a high theoretical energy density (2,500 Wh/kg). However, despite decades of development, the Li-S battery has yet to reach mass commercialization. Several problems inherent in Li-S chemistry remain. These include: (i) limited reaction kinetics due to the insulating nature of sulfur and the solid reduction products (Li2Sand Li2S2); (ii) rapid capacity loss due to the production of various soluble Li2Sn(3 ≤ n ≤ 6) polysulfide intermediates, giving rise to redox shuttling; and (iii) a poorly controlled Li/electrolyte interface.
Several strategies have been explored to develop cathodes with porosity and structure capable of efficient electron transport to S and capture of polysulfides formed during discharge. The most significant advance was made by Nazar et al., who demonstrated that cathodes built on nanostructured mesoporous carbon-sulfur composites deliver higher and more sustained reversible capacities. The non-polar nature of carbon, however, limits effective sulfur trapping, hence its replacement by more polar oxides that are initially insulating and then conductive (Ti4O7), the choice of oxides for effective polysulfide trapping being rationalized by their redox potential. These aspects will be addressed, as will the improvement of the Li/electrolyte interphase in the presence of LiNO3 additives to combat redox shuttling (polysulfides) or other means of preventing polysulfide migration to the negative, such as functionalizing separators (e.g. fluorinated graphene) to trap them.
