(Invited) Advances in Pyrolyzed Polymeric Sulfur Cathodes for Low-Cost Lithium Metal Batteries

Sulfur is a highly desirable cathode material for lithium batteries but its complex reaction mechanisms involving soluble species have limited its cycle life. When sulfur exists as a very short chain or substituted function groups on aromatic rings, the dissolution process appears to be mitigated, leading to a long cycle life. A good example is Sulfurized-poly(acrylonitrile) (SPAN), which is synthesized by pyrolyzing a mixture of PAN and elemental S at 300-500oC. SPAN is essentially a nitrogen doped carbon host for sulfur and has shown exceptional cycling stabilities in common carbonate electrolytes.We have recently launched an intensive effort to advance the Li/SPAN battery technology to an energy density of > 300 Wh/kg. Its exceptional cycle life and low cost will make the technology a promising choice for electric vehicle applications. In this talk, we will review several interrelated efforts to realize this goal: 1) searching for SPAN-like materials with higher S loading and higher capacities by tuning N:C:S ratios with different polymer and sulfur precursors; 2) improving the packing density of SPAN particles in the cathode by controlling particle shape and size using new synthesis approaches. A highlight is a um-sized, platelet-like morphology that greatly increase packing densities; 3) developing a solvent-free homogenous chemical lithiation method for the synthesis of lithium containing SPAN. The lithiated SPAN can greatly reduce the irreversible capacity loss during the first discharge cycle and minimize the volume change associated with the initial lithiation process; and 4) continuing the development of new electrolyte compositions that can further improve the lithium metal anode efficiency without sacrificing SPAN cycle life. These studies hold the promise to enable a practical, low-cost and long-life Li/S battery.