Rational Design of High-Loading Cathodes for Lithium Sulphur Batteries
Authors: Wang Hui , Adams Brian , Pan Huilin , Estevez Luis , Zheng Jianming , Lu Dongping , Chen Shuru , Shao Yuyan , Xu Wu , Zhang Ji-Guang 
Published: February 2020

The lithium-sulphur (Li/S) battery has been the subject of intense research and development in recent years owing to its low cost and high-capacity. Fundamental challenges of Li-S batteries originate from the insulating properties of elemental sulphur and lithium sulfides, the dissolution of lithium polysulfides (LiPS) in the electrolyte, the cathode volume changes, and corrosion of lithium anodes.1-2 It is now widely realized that high sulphur loading and low electrolyte to sulphur (E/S) ratios are critical for Li-S technology in the marketplace. 3 These problems are exacerbated due to the difficulties in creating a continuous electron/ Li+ pathway over thick cathodes, 2 severely inhomogeneous distribution of LiPS within thick cathodes and resultant shuttle effect on both electrodes 4-5  and presence of internal inherent mesoscale porosities 2. To address these two imperative challenges, the concept of sparingly solvating electrolytes was proposed to control the dissolution of LiPS.6

Herein, we propose a new strategy on rational design of high-loading cathodes under lean operation for Li-S batteries by spatial homogeneity control. An activated carbon fiber cloth (ACFC) has been selected as both a sulphur-hosting material and the current collector. ACFC possesses a high micropore volume (ca. 0.8 cc/g) with a narrow pore distribution (< 2 nm), high surface area (ca. 1800 m2/g) but with the absence of mesopores which is common in conventional carbon matrices. The microporous nature of the ACFC allows sulfur to be confined, yet the macro-surface area (< 10 m2/g; excluding micropores) is low, which potentially allows high utilization electrolytes (i.e. complete wetting at low E/S ratios). An adsorptive impregnation method was developed successfully and achieved the theoretical sulphur loading (i.e. 100 % occupation of micropores) based on the empirical relationship. Preliminary cycling experiments indicate there is an optimum sulphur pore-loading range (5-7 mg cm-2) for this method if penetration of electrolytes, volumetric expansion upon lithiation, and escape of resultant LiPS from micropores into bulk electrolytes are taken into account. After sulphur loading, the effective sulphur to carbon (excluding the average weight contribution from Al current collector) weight ratio is approximate 58:42. The cell with sulphur loading of 5.4 mg cm-2 exhibits the best cycling retention, compared to cells with lower sulphur loading and the cell with comparable sulphur loading using melt-infusion method. After 16 cycles, the cell can deliver a favorable areal capacity of 5.2 mAh cm-2 with the E/S ratio of 7.3 along with decreasing nucleation barrier for the first liquid-solid discharge reaction. These results suggest encapsulation of most liquid LiPS in the micropores can be achieved under lean electrolyte condition using an optimum sulphur loading and a hybrid cycling protocol, which also shed light on approaches of delivering high areal capacities (6 mAh cm-2) in a lean electrolyte (E/S < 3). Others factors that affect the cell cycling performance will be discussed in terms of E/S ratios, different kinds of ACFC, temperature and cycling parameters.

[1] J-G. Zhang, W. Xu, and W.A. Henderson. Springer Series in Materials Science,vol. 249, Springer International Publishing, Cham, Switzerland, DOI: 10.1007/978-3-319-44054-5 (2017)

[2] Q. Pang, X. Liang, C.Y. Kwok and L.F. Nazar, Nat. Energy, 16132 (2016)

[3] D. Eroglu, K. R. Zavadil and K. G. Gallagpher, J. Electrochem. Soc.,162 (6) A982-A990 (2015)

[4] Y. V. Mikhaylik and J. R. AkridgeJ. Electrochem. Soc., 151 (11) A1969-A1976 (2004)

[5] H. J. Peng, J.Q. Huang, X.Y. Liu, X.B. Cheng, W.T. Xu, C.Z. Zhao, F. Wei and Q. Zhang, J. Am. Chem. Soc., DOI:10.1021/jacs.6b12358 (2017)

[6] L. Cheng, L.A. Curtiss, K.R. Zavadil, A.A. Gewirth, Y.Y. Shao and K.G. Gallagher, ACS Energy Lett., 1503-509 (2016)