Performance of Lithium Sulfur Batteries Consisting of Li2s/Carbon Composite Cathode
Authors: Asano Hikari , Seita Taisho , Sasagawa Shohei , Tatara Ryoichi , Liu Jiali , Ueno Kazuhide , Dokko Kaoru , Watanabe Masayoshi 
Published: December 2020

Lithium sulfur (Li-S) battery is a rechargeable battery typically consisting of sulfur as a cathode, Li metal as an anode, and a Li-ion conductive electrolyte. The sulfur has many advantages, such as high gravimetric theoretical capacity, high terrestrial abundance, and low price. Despite this potential, Li-S batteries are facing some challenges. Firstly, sulfur is an electrical insulator, which requires a large amount of conductive additive such as carbon for the formation of the electron conduction path. As a result, the loading amount of sulfur on the cathode is hard to increase. Secondly, lithium polysulfide intermediates dissolve in conventional organic electrolytes, leading to a redox shuttle. Thirdly, the volume expansion/shrinkage of sulfur during lithiation/delithiation is large, resulting in the mechanical degradation of the electron-conduction path in the electrode, which deteriorates the capacity. Finally, Li dendrites grow on Li metal during cycles, which causes a short circuit.
Our group has proposed the combination of Li2S/carbon composite cathode, a graphite/Si anode, and a sparingly solvating electrolyte for solving these problems.1-4 Li2S is a reduction product of sulfur and has a high theoretical capacity. Replacing sulfur with Li2S enables us to use a non-Li anode such as graphite/Si,1,2 where the lithium dendrites are not growing. Furthermore, Li2S can improve the mechanical stability of the composite cathode because it starts with shrinkage in the initial charge process. The instability and low processability of Li2S was the problem; however, we recently reported the preparation process by the simple reduction of Li2SO4/carbon mixture, where carbon acts as a reductant, at a high temperature.3 The sparingly solvating electrolyte makes the solubility of polysulfide intermediates significantly low, which can suppress the redox shuttle, and the usage of graphite anode possible.2

Up to now, we used three-dimensional (3D) porous Al as the current collector for Li2S/carbon electrode to increase the loading amount of Li2S.4 However, to enhance the energy density and to simplify fabrication process of the cells, the use of simple Al foil (2D) is desirable. In this study, we prepared Li2S/carbon composite cathodes using the 3D and 2D current collectors and compared the electrochemical performances.
Li2S/carbon composite was synthesized by the reduction of Li2SO4/carbon mixture. A slurry was prepared by mixing poly(N-vinyl pyrrolidone) (PVP), N-methyl pyrrolidone (NMP) and Li2S/carbon composite, and was inserted and coated to the 3D and 2D current collector, respectively, followed by drying. The 1:2 mixture of lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and sulfolane (SL), [Li(SL)2][TFSA], diluted by 1,1,2,2,-tetrafluoroethyl-2,2,3,3,-tetrafluoropropyl ether (HFE) was employed as the sparingly solvating electrolyte. Charge/discharge tests were performed at C/12.
It was rather difficult to make a uniform slurry suitable for the 2D electrodes due the aggregation of carbon-rich Li2S cathode materials. The usage of two different molecular weight PVPs (10,000 and 360,000 Da) as dispersant and binder polymers, respectively, was effective to disperse the carbon-rich materials in NMP. Fig. 1 shows a comparison of the capacity retention with cycling between Li2S cathodes using 3D and 2D current collectors, where the Li2S loading amount was 3.2 mg cm-2 and 1.9 mg cm-2, respectively. For the both cells, the Coulombic efficiency was ca. 98% due to the low solubility of polysulfide intermediates, and cycle characteristic was relatively stable. The discharge capacity of the 2D electrode was a little lower than that of the 3D electrode. The lower apparent electronic conductivity of the electrode could be a reason. Further, the loading amount of Li2S on the 2D electrodes cannot be as high as that on the 3D electrodes. In the presentation, merits and demerits to use 2D and 3D electrode will be discussed.
Acknowledgements
This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). The authors are indebted to Lithium Ion Battery Technology and Evaluation Center (LIBTEC) for the preparation of the 2D cathode.
References

Zhe Li et al., ACS Appl. Mater. Interfaces, 8, 16053-16062 (2016).

Zhe Li et al., Electrochemistry, 84, 887-890 (2016).

Zhe Li et al., Nanoscale, 7, 14385-14392 (2015).

Taisho Seita et al., ACS Energy Lett, 5, 1-7 (2020)

Figure 1