Post Li-ion batteries are of utmost importance for developing low-cost and sustainable energy storage systems. Al-S batteries (ASBs), built from cost-efficient materials, have emerged as one of the most attractive technologies, according to Heng Zhang, research manager of the polymer electrolyte group at the CIC EnergiGUNE, an energy research center in Spain. Sulfur is a cathode material that improves the energy density of a battery and lowers its cost. Aluminum, the most abundant metal in earth’s crust, is a non-flammable, cost-effective, and recyclable material that can be safely used directly as an anode, yielding energy densities close to or even higher than lithium-ion batteries (LIBs) and other battery systems. Until recently, however, sulfur cathodes in rechargeable ASBs and LIBs were not able to survive an adequate number of charge/discharge cycles. 

“Many materials used for the ASB cathodes are stable for the first couple of cycles of charging and discharging, but after that they come with a lot of shortages,” says Mohammadreza Shokouhimehr, a researcher at Seoul National University (SNU). Shokouhimehr and a team of researchers from SNU, Korea Institute of Science and Technology (KIST), and Palacky University in Olomou in the Czech Republic, have introduced a hybrid, two-dimensional (2D) material that solves one of the main problems with sulfur for energy storage technology, the polysulfide shuttle effect. As reported in a recent issue of Scientific Reports, the new ASB cathode is made of commercially available and inexpensive materials, 2D-layered boron nitride (BN), and with carbon- and sulfur-nanoparticles uniformly adsorbed on the surface of the BN layers. 

Until now, the problem in most pioneering studies on ASBs has been that despite the high initial discharge capacity, polysulfide shuttling effect causes rapid capacity decay, which severely restricts ASBs’ practical applications. When shuttling occurs, various intermediate polysulfide compounds, which form during the reduction of sulfur at the cathode, dissolve in the electrolyte and reach the anode where they participate in parasitic reductions. At the final stage, Li₂S is formed, which blocks further aluminum transport. Thus, shuttling not only depletes the electrodes of their active materials, but also reduces the capacity of a battery. 

“Sulfur [by] itself cannot be used without a supportive platform for electrochemical applications, because sulfur aggregates very quickly; it forms various intermediates and shuts down the circuit,” Shokouhimehr says. A polysulfide “fixer” can solve the problem; the fixer bonds with sulfur and sulfide compounds and immobilizes them. Two-dimensional materials, like BN, can do so by adsorbing elemental sulfur and polysulfides in their exposed active sites found on their surface. This way, they protect the capacity of ASBs from decaying during repeated charge/discharge processes and enhance energy densities. 

In the present study, the researchers designed and tested three types of 2D-layered materials as sulfur fixers: BN, molybdenum disulphide (MoS₂), and tungsten disulphide (WS₂); the latter two sulfide compounds are also known as transition-metal dichalcogenides. 

“We first thought that MoS₂ would work better because it has better semiconducting behavior in comparison to BN; but at the end, BN, which is a very rigid 2D material, gave us a battery with very high stability compared to the other [two] 2D materials,” Shokouhimehr says. 

Powders of the 2D materials were mixed together with carbon nanoparticles—which acts as the conductive material—and sulfur nanoparticles, in various combinations, including MoS₂/S/C, WS₂/S/C, and BN/S/C. These were mechanically ground by means of ball-milling using zirconia balls at 1500 rpm for 2 days. During the grinding process, the S and C nanoparticles were adsorbed on the layers of MoS₂, WS₂, and BN. A semi-liquid mixture was then prepared by dispersing the ground powders into a constantly stirred solution. The mixture was spread onto a Pt-coated organic polymer film current collector, and the as-prepared electrode was dried in a vacuum oven at 60°C overnight. 

The electrochemical properties were characterized in pouch cells assembled with the dried electrode as a cathode and an Al metal foil (0.5 mm thickness) as an anode. A glass-fiber paper soaked with 1-ethyl-3-methylimidazolium chloride (EMIm) was inserted between the two electrodes to isolate the anode and cathode. 

Although the initial electrochemical charge capacities are similar for the BN, MoS₂ and WS₂ systems (all around 500 mAh g⁻¹, at a current density of 100 mA g⁻¹), the BN/S/C sample is the only one that retains it, after 300 charge/discharge cycles. The MoS₂ sample showed a decay that reached values below 50 mAh g⁻¹ after the first 50 cycles, a result that suggests that the intermediate compounds are not captured by the layered MoS₂ via the ball-milling process. Similar results are observed with the WS₂ sample, where the initial discharge capacity decreases to ~50 mAh g⁻¹ after the first 25 repeated charge/discharge cycles. At the same time, cutting-edge research on Al-ion batteries (AIB) with cobalt sulfide electrodes currently reports a discharge capacity of ~300 mAh g⁻¹ at the same current density after 200 cycles. 

Furthermore, the Coulombic efficiency was found to be 94.3% at the 300th charge/discharge cycle, when charged/discharged at a current density of 100 mA g⁻¹ and a discharge voltage plateau was observed at ~1.15 V versus AlCl^{4 −}/Al. Pyrolytic graphite, which is one of the most successful examples of material used for a AIB cathode, has a higher charge and discharge stability (~7000 repeated charge/discharge cycles) and also higher discharge voltage (~2 V versus AlCl^{4 −}/Al), but nevertheless has a capacity as low as ~60 mAh g⁻¹ and cannot satisfy the demands of high energy density. Until now this is the highest capacity value for any graphite-based or other composite cathode materials used for AIBs, for such a long life span. 

Shokouhimehr is now working to develop a bottom-up approach that will allow him to engineer a BN/S/C system with even better stability and efficiency for market applications. 

“An efficient sulfur fixer based on boron nitride, such as the one developed by Shokouhimehr and his team, could significantly improve the performance of ASBs, thus facilitating their practical implementation in the market,” Zhang says. 

Read the article in Scientific Reports.