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2017-12-05

Graphene microsheet/sulfur cathodes for Li-S batteries deliver long-term cyclability and high coulombic efficiency




Electrochemical performances of Li-S batteries with GMs/S cathode and lithium anode. (a) Cyclic voltammograms of GMs/S cathode (50 wt% S) at a scan rate of 0.1 mVs-1, (b) EIS plots of GMs/S cathode, (c) Charge-discharge voltage profiles of GMs/S cathode). The first cycle is at 0.05C corresponds to an activation cycle. (d) Rate performance of GMs/ S cathodes with sulfur content of 62% and 38% at the rates from 0.1C to 2 C. (e) Rate performance of the GMs/S (50% S) and microcrystalline graphite minerals/S (50% S) at the rates from 0.1C to 2 C. (f) Cycling performance and coulombic efficiency of graphene microsheets/S (50%) and microcrystalline graphite minerals/S (50%) cathodes at 0.2C. (g) Cycling performance and coulombic efficiency of the GMs/S cathode at 0.5C (62%), 1C (50%) and 2C (38%).

A team in China has used graphene microsheets (GMs)—prepared from microcrystalline graphite minerals by an electrochemical & mechanical approach—as a special conductive support for sulfur for the cathode of a lithium-sulfur battery. The graphene microsheets feature excellent conductivity and low-defect, small sheet sizes of <1 µm2 and = 6 atomic layers.
In a paper in the Journal of Power Sources, the researchers report an average coulombic efficiency of 99.7% for Li-S batteries using the GMs/S cathodes, with long-term cyclability of 2000 cycles at 1C. The work suggests that graphene microsheets from micro-crystalline graphite minerals can be developed into high-stable and long-term lithium-sulfur batteries.
The lithium-sulfur batteries have attracted considerable academic and industrial interest as the next-generation rechargeable battery systems owing to its competitive cost, environmental-friendly and most importantly, ultrahigh theoretical energy density (2600 Wh kg-1), which is almost five times higher than the commercial lithium-ion batteries. However, many challenges including potential safety risk of Li-dendrite formation, poor cycle stability due to the dissolution of the lithium polysulfide still hinder its application. These are three problems to be solved: (i) the poor conductivity of sulfur and the emergence of discharging products (Li2S and Li2S2), (ii) the shuttle effect of polysulfide and (iii) the volume changes from sulfur to Li2S during discharging and charging processes.
Recently, graphene has emerged as an excellent material to solve these challenges due to its high electrical conductivity, fast electron/ charge transfer, high surface area and open space as well as excellent electrochemical performance, which has been proved to be the effective carbon matrix for sulfur cathode. However, achieving long-cycle reversible capability of Li-S battery at high discharge current density is still on the batteries often by-side reactions and obstruct the penetration of electrolyte, which usually results in degrading the cycle stability and rate performance.
To create the graphene microsheets, the researchers first used natural microcrystalline graphite mineral (MGM) as a positive electrode immersed in 25 wt% H2SO4 solution. After charging under +5V for 3 days, the powder was filtered and exfoliated by a mill ball milling for 2 hours. After a water wash and drying at 80 °C, the graphene was collected.
To prepare the GMs/S material, the team sealed a mixture of graphene microsheets and sulfur powder in a glass vessel in an argon atmosphere, and heated it to 155 °C for 12 hours.
The team found that an impurity residue of natural silicate covering the graphene proved useful in absorbing sulfur and polysulfide.