High-Performance All-Solid-State Lithium–Sulfur Batteries Enabled by Amorphous Sulfur-Coated Reduced Graphene Oxide Cathodes

DOI: 10.1002/aenm.201602923 suffer from safety problems arising from lithium anode and fast capacity fading due to the insulating nature of sulfur, the dissolution-induced polysulfide shuttle reaction, and large volume changes.[4–6] To address these issues, carbonaceous material[7,8] and conducting polymers[9] have been used to trap the high-order polysulfides in the cathodes; protective layers and electrolyte additives are employed for protection of metallic-lithium anodes from reactions with polysulfide.[10,11] However, the shuttle reaction still exists, and the safety issue induced by lithium dendrite is still a great challenge. All-solid-state Li–S batteries can completely inhibit the dissolution of polysulfide, eliminate the polysulfide shuttle, and avoid lithium dendrite formation.[12–19] However, the use of rigid solid electrolytes in all-solid-state Li–S batteries also increases the stress/strain and interface resistance and reduce the reaction kinetics.[20–22] The key challenge is to minimize stress/strain and to construct a robust electronic and ionic pathway in the sulfur cathode, due to the electronic/ionic insulting nature of sulfur. For enhancing the electronic conductivity and reducing the electronic contact resistance, Kobayashi et al. synthesized a sulfur and acetylene black (AB) nanocomposite cathode using a gas-phase mixing method, and reported a reversible capacity of 900 mA h g−1 at a current density of 0.013 mA cm−2 in all-solid-state batteries.[23] The sulfur and carbon-nanofibers composite cathode also shows a high capacity in the all-solid-state Li–S batteries.[24] To ensure high ionic conduction in the sulfur cathode, Lin et al. synthesized core–shell structured lithium–sulfide nanoparticles with an Li3PS4 electrolyte as shell, showing six orders of magnitude higher in ionic conductivity than that of bulk lithium– sulfide. Excellent cyclic performance was demonstrated for allsolid-state Li–S batteries at 60 °C.[13] By incorporation of five sulfur atoms in the Li3PS4 electrolyte, the Li3PS4+5 cathode with loading density of 0.25–0.6 mg cm−2 exhibits excellent cycling stability for all-solid-state Li–S batteries.[14] These studies demonstrate that a close contact of the nanosulfur, either to carbon or to electrolytes, and uniformly distributing these composites into an ionic/electronic conducting matrix, can significantly improve the electrochemical performances of solid-state Li–S cell because the nano-sulfur contacts both the highly ionic and Safety and the polysulfide shuttle reaction are two major challenges for liquid electrolyte lithium–sulfur (Li–S) batteries. Although use of solid-state electrolytes can overcome these two challenges, it also brings new challenges by increasing the interface resistance and stress/strain. In this work, the interface resistance and stress/strain of sulfur cathodes are significantly reduced by conformal coating ≈2 nm sulfur (S) onto reduced graphene oxide (rGO). An Li–S full cell consisting of an rGO@S-Li10GeP2S12-acetylene black (AB) composite cathode is evaluated. At 60 °C, the all-solid-state Li–S cell demonstrates a similar electrochemical performance as in liquid organic electrolyte, with high rate capacities of 1525.6, 1384.5, 1336.3, 903.2, 502.6, and 204.7 mA h g−1 at 0.05, 0.1, 0.5, 1.0, 2.0, and 5.0 C, respectively. It can maintain a high and reversible capacity of 830 mA h g−1 at 1.0 C for 750 cycles. The uniform distribution of the rGO@S nanocomposite in the Li10GeP2S12-AB matrix generates uniform volume changes during lithiation/delithiation, significantly reducing the stress/strain, thus extending the cycle life. Minimization of the stress/ strain of solid cells is the key for a long cycle life of all-solid-state Li–S batteries.