Researchers at the Argonne National Laboratory and Oregon State University in the U.S have designed a novel cathode architecture for lithium-sulphide batteries that consists of crystalline di-lithium sulphide nanoparticles encapsulated in few-layer graphene. The design is said to allow the maximum amount of active sulphur species to be incorporated into the electrode and so greatly improves its electrical conductivity. It also overcomes many of the major challenges associated with existing sulphur electrodes and di-lithium composites.
The Li2S-graphene nanocapsules architecture can boast superior electrochemical properties. The electrodes have a high reversible capacity of 1160 mAh/g and area capacity of 8.1 mAh/cm2. The team synthesized the Li2S@graphene nanocomposites in a one-step reaction in which they reacted lithium metal foils with CS2 vapour carried by argon gas at 650°C. Li2S nanocrystals and the tight wrapper of few-layer graphene are spontaneously generated, thus forming the nanocapsules. The Li2 nanoparticles are between 50 and 80 nm in size and are uniformly and seamlessly encapsulated in about 10–20 graphene layers. This significantly reduces the charge-transfer resistance between the two materials and greatly improves the electric conductivity of Li2.
In their experiments, the researchers found that the Li2 is redox-active and oxidizes to sulphur during electrode charging and reduces back during discharging. During this electrochemical conversion, the graphene capsules effectively retain the active sulphur species so the electrode does not expand in volume.
“Other advantages of the design are that Li2 nanoparticles only form in the presence of co-formed graphene layers, so the graphene framework functions as an overall electrical conduit,” explains the team. “This means that nearly all of the Li2 nanoparticles in the electrode are electrochemically active. The compact graphene shells also have good physical properties and preserve the structural integrity of the composites”. The graphene shells also reduce the amount of polysulphides diffusing into the electrolyte during cycling, he adds.