Lithium-ion batteries are typically smaller, lighter, have a lower self discharge rate, a higher voltage and hold a charge much longer than other types of rechargeable batteries. Amongst other uses, lithium-ion batteries have been one of the most promising energy sources to power the next generation of vehicles. However, the energy density requirements for the desired 40-mile all-electric range are three- to five-times more than what is achievable by the current lithium ion technology. Therefore, lithium-ion battery chemistries with significantly higher energy densities need to be developed in order to provide plug-in hybrid electric vehicles (PHEVs) with a sufficient charge-depleting range.
One such battery includes the lithium-sulfur battery, which can afford a significantly higher energy density than the current lithium-ion system (i.e., lithium transition metal oxide cathode with a graphite anode). Sulfur, which is abundant, non-toxic, and inexpensive, is an ideal compound in that it has a high theoretical capacity of 1,675 mA/g. In addition, a sulfur cathode can have a high specific energy density (e.g., 2,500 watt-hour/kg (Wh/kg)) and the highly ordered lithium polysulfides can provide intrinsic overcharge protection from a redox shuttle mechanism.
However, despite these benefits, a number of issues remain for the practical application of lithium-sulfur batteries. Some of these issues include i) the capacity fading during cycling and self-discharge rates, ii) lower energy efficiency due to the internal “shuttle mechanism” especially at low charge/discharge rates, and iii) cycle life and safety risk due to the lithium metal anode.
The electrochemical reaction in the lithium-sulfur battery system proceeds according to 16Li+S8→Li2S via a series of lithium polysulfide intermediates (LiSx, 1<x<8). The higher order ordered lithium polysulfides (4<x<8) are soluble in the electrolyte, thus they can diffuse to the anode and react directly with the lithium metal anode. This side reaction forms lower order polysulfides, including the less soluble Li2S2 and Li2S, which may then precipitate on the lithium anode and become non-electroactive. This loss of ‘cyclable’ active mass results in severe capacity fading upon cycling. On the other hand, the reversible conversion of lower ordered polysulfide to higher ordered polysulfide, and vice versa on the electrode surface, causes a redox shuttle phenomenon which can then significantly lower the columbic efficiency.
Much research has been devoted to stabilizing the sulfur cathode and improving the life and cycling performance of the lithium-sulfur battery system. Most has been aimed at maintaining the mechanical and electrical integrity thereof by constraining sulfur or lithium polysulfides within a designated framework using a host material, such as various mesoporous carbon, carbon nanotubes, carbon spheres, graphene, and graphene oxide. Such studies have shown that porous carbon materials with controlled pore diameters may have the ability to restrain the solubility and mobility of the polysulfide anions. However, such porous carbon materials have been difficult to both synthesize and to scale up as the synthetic process either requires the use of a template, with removal thereof at harsh conditions (e.g., strong base, HF, etc.), or requires calcination of small organic molecules at high temperature. See, US 2011/0052998, US 2009/0311604, and Zhang et al. Energy Environ. Sci. 2011, vol. 4, pp. 5053.