Semi-Flooded Sulfur Cathode with Ultralean Absorbed Electrolyte in Li-S Battery

Lean electrolyte (small E/S ratio) is urgently needed to achieve high practical energy densities in Li-S batteries, but there is a distinction between the cathode's absorbed electrolyte (AE) which is cathode-intrinsic and total added electrolyte (E) which depends on cell geometry. While total pore volume in sulfur cathodes affects AE/S and performance, it is shown here that pore morphology, size, connectivity, and fill factor all matter. Compared to conventional thermally dried sulfur cathodes that usually render "open lakes" and closed pores, a freeze-dried and compressed (FDS-C) sulfur cathode is developed with a canal-capillary pore structure, which exhibits high mean performance and greatly reduces cell-to-cell variation, even at high sulfur loading (14.2 mg cm^-2) and ultralean electrolyte condition (AE/S = 1.2 µL mg^-1). Interestingly, as AE/S is swept from 2 to 1.2 µL mg^-1, the electrode pores go from fully flooded to semi-flooded, and the coin cell still maintains function until (AE/S)_min ≈ 1.2 µL mg^-1 is reached. When scaled up to Ah-level pouch cells, the full-cell energy density can reach 481 Wh kg^-1 as its E/S ≈ AE/S ratio can be reduced to 1.2 µL mg^-1, proving high-performance pouch cells can actually be working in the ultralean, semi-flooded regime.