The Function Mechanism of a Novel Quasi-Solid-State Li-S Battery
Authors: Cao Yi , Zuo Pengjian , Lou Shuaifeng , Huo Hua , Ma Yulin , Du Chunyu , Gao Yunzhi , Yin Geping 
Published: February 2020

We propose a [Li(triglyme)]+[TFSI]- solvent-salt complex with excellent stability towards Li10GeP2S12 electrolyte and demonstrate its application in the high-performance quasi-solid-state lithium-sulfur battery. A spot of LiG3 addition into cathode dramatically facilitates the Li+ ionic transfer and the transition of a solid-solid redox reaction for sulfur species to solid-liquid dual-phase redox reaction. And LiG3 addition accelerates the kinetic process at the beginning of discharge, and LGPS powders scattered in cathode could greatly strengthen the solid-phase redox reaction process, promoting the excellent electrochemical performance of the solid-state Li-S battery.

Experimental methods. The composite cathode powders were prepared by a two-step mechanical milling method. Firstly, 0.2 g LGPS electrolyte, 0.05 g VGCFs and 0.2 g sulfur were mixed with a rotate speed of 250 rpm and 10 h in a 60 mL zirconia jar with 5 mL mineral spirit solution, ratio of grinding media to material was controlled at 20:1, solid content was controlled at 40%. Subsequently, 0.05 g VGCFs was added into the mixture and remixed with a rotate speed of 100 rpm and 10 h. Then, the mixture was dried at 60℃ for 24 h, and the obtained pristine composite cathode powders were denoted as S-0 powder. Finally, 0.25 g LiG3 complex and 5.0 g the obtained S-0 were mixed in an agate mortar and formed the optimized composite cathode powders, denoted as S-5, the weight ratio of sulfur, LGPS electrolyte, VGCFs and LiG3 complex in the composite is 4: 4: 2: 0.5.

Results. Fig.1(a) presents the typical charge-discharge curves of the Li-S cells used in this work. The S-5 cells exhibit two discharge voltage plateaus, while there is only one reductive peaks at 1.5 V for the S-0 system. Fig.1(b) shows the surface morphology changes of S-0 and S-5 cells changes after cycling, and it suggests that the reaction mechanisms between S-0 and S-5 cells are different. Moreover, the XPS results (fig.1(c)) directly prove that S-0 cells perform a totally different reaction route with conventional Li-S battery, which is no polysulfide lithium intermediates generation. Nevertheless, with the LiG3 addition, the S-5 cells generate lithium polysulfide (Li2Sn) and follow an apparent solid-liquid dual-phase redox reaction. In addition, we attempt to replace LGPS within the S-5 cathode with the Li+ insulated SiO2 nano-filler, to deeply elucidate the role of LGPS electrolyte in the cathode, and cells present inferior solid-phase redox reaction with poor capacity and cycling stability. Therefore, it can be believed that LiG3 addition accelerates the kinetic process at the beginning of discharge, and LGPS powders scattered in cathode could greatly strengthen the solid-phase redox reaction process, promoting the excellent electrochemical performance of the solid-state Li-S battery.
Figure 1 (a) Charge-discharge curves of S-0 and S-5 cells at 50 μA/cm2, RT.; (b) SEM morphology of S-0 and S-5 cells before and after cycling tests and cycled S-5 after soaking by LiG3 for 5d; (c) XPS spectrum of the surface of discharged composite cathode S-0 and S-5.

Conclusions. The addition of LiG3 facilitates the Li+ ionic transport kinetics at the cathode/electrolyte interfaces, and changes the reaction mechanism from solid-state Li-S battery to solid-liquid dual-phase redox reaction without sacrificing the ability to inhibit shuttle effect. And it can be believed that LiG3 addition accelerates the kinetic process at the beginning of discharge, and LGPS powders scattered in cathode could greatly strengthen the solid-phase redox reaction process, promoting the excellent electrochemical performance of the solid-state Li-S battery.
References.
[1] Y. Cao, P. Zuo, et al, Journal of Materials Chemistry A, 2019, DOI: 10.1039/C9TA00146H

Figure 1