Free-Standing and Flexible Carbon Membrane for Lithium-Sulphur Batteries

In the last two decades, rechargeable Li-ion batteries have dominated the market. However, the increasing demand for energy storage creates new challenges for further use of lithium-ion batteries mainly due to the problems associated with limited energy density and high cost. To improve the specific energy of rechargeable batteries, a lithium-sulphur (Li-S) battery attracts particular interest in recent years as next-generation energy storage device due to its highest energy density (2600 Wh/kg) and theoretical capacity (1672 mAh/g) among all known systems. Furthermore, sulphur is cheap, abundant and environmentally benign. Despite this, due to its insulating nature, large volume changes during cycling of cell, and solubility of the polysulfides formed during these processes, the practical application of lithium-sulphur rechargeable batteries has not been successful yet [1].

To overcome these problems, we developed and investigated a free-standing and flexible carbon membrane as an interlayer between the sulfur/PANI composite cathode and the separator to prevent the shuttling of the intermediate polysulfides in Li-S batteries.

Sulphur/PANI composites were prepared based on the as-prepared sulphur powder via a self-assembly process. 15 ml of aqueous suspension, containing 1 g sulphur was sonicated for 10min. 15 ml of aniline (An) monomer solution in water was dissolved into the suspension and stirred magnetically for 0.5 h at room temperature. pH of the suspension was adjusted to pH 2.0 with 1 M HCl solution then replaced into ice bath. A precooled FeCl₃ aqueous solution was added into the resulting mixture together with diluted HCl. The mixture was reacted for 5 h at 0−5 °C. A black precipitate was separated by filtration and then thouroghly washed several times with deionized water, ethanol, acetone and 0.01 M HCl, and dried overnight under reduced pressure at room temperature to get the final composite product. Different molar ratio of aniline to sulphur were used to optimize the formation condition for preparation of the sulphur/PANI composites.

Cathode electrodes were prepared by mixing 80 wt% sulphur/PANI, 10 wt% polyvinylidene fluoride (PVdF, Kynar, HSV900) as a binder and 10 wt% acetylene black as a conducting agent in 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich). The resultant slurry was coated on aluminium foil as a current collector, and vacuum dried at 60 °C for 3 h.

Free-standing and flexible carbon membranes have been prepared by mixing carbon source and a binder at different molar ratio, coated on aluminium foil and vacuum dried at 60 °C for 4 h. After drying the composite has been detached from aluminium foil, cuted and set between the sulphur/PANI cathode and separator (Celgard). Lihium metal used as anode.

The coin cell type cells were assembled in an argon filled glove box (MBRAUN LABMASTER). 1 M LiPF₆solution in EC:DMC:DEC (1:1:1) was used as electrolyte.

In summary, we have successfully prepared flexible and free-standing carbon membrane for Li–S batteries. Carbon membrane adsorbed the intermediate polysulfides and prevented their shuttling, and  a battery with the sulphur/PANI composite exhibited excellent lithium storage performance, enhanced cycle stability and high rate capacity.

This publication has been made within the Sub-project  #157-2013  which is funded under the Technology Commercialization Project, supported by the World Bank and the Government of the Republic of Kazakhstan

Keywords:  lithium-sulphur batteries, free-standing membrane, energy storage.

References:

1. Zhao Y, Zhang Y, Bakenova Z and Bakenov Z (2015). Carbon/sulfur composite cathodes for flexible lithium/sulfur batteries: status and prospects. Front. Energy Res. 3:2.