High-Performance Lithium-Sulfur Batteries with Metal-Deposited Graphite Cathode and Lithium Polysulfide Catholyte
Authors: Kazahaya Natsuho , Uesugi Nanami , Katayama Yu , Tsutsumi Hiromori 
Published: December 2020

Lithium-sulfur (Li-S) batteries have attracted remarkable attention as next-generation batteries due to their low cost and high theoretical specific energy density.[1] However, it has encountered several difficulties, including low specific capacity due to the insulating nature of sulfur as well as poor cycle performance, which is caused by the dissolution of reaction intermediates (lithium polysulfide, LiPS) into the electrolyte. [2] Most strategies to overcome the above disadvantages are focused on designing carbon-based host materials, such as highly conductive and/or mesoporous carbon. [3] [4] However, it is difficult to completely suppress the dissolution of LiPS by the physical adsorption. Recent research thus focused on the utilization of chemical interactions between LiPS and electrode additive, mainly metal particles, to suppress the dissolution of LiPS. [5] Although it has been reported that the addition of metal improves the capacity retention and suppresses the self-discharge, the mechanistic understanding is still lacking.
In this study, we selected Au and Ag as a metal additive, which has similar electrical conductivity but different affinity towards sulfur, to clarify the effect of metal-sulfur interaction on battery performances. We selected catholyte, in which LiPS (Li2S8) was previously dissolved, was selected as an electrolyte to achieve a continuous supply of LiPS to the positive electrode. Galvanostatic charge-discharge measurements revealed that the charge/discharge capacity affected by the metal addition; 75.8 mAh cm-3 (G) < 77.4 mAh cm-3 (Ag/G) < 99.5 mAh cm-3 (Au/G), with excellent initial discharge capacity for Au-decorated electrode. Furthermore, we confirmed the effect of the amount of metal additive on the charge/discharge capacity. The result thus indicates (1) addition of metal with a strong affinity towards sulfur and (2) optimization of its amount is crucial to improve Li-S battery performances. The improvement in capacity retention rate was confirmed for the metal-decorated electrode; capacity retention at the 20th cycle was G = 68.1 % < Ag = 89.3 % < Au = 94.9 %. The interaction between the metal additives and LiPS may contribute to anchor LiPS at the vicinity of the electrode, leading the improvement in the cyclability.
The reason for the improvement in the cyclability will be discussed based on the surface product analysis of the electrode after initial charge and discharge, as deduced from on-line gas chromatography-mass spectrometry (GC-MS) measurements. We will also discuss the effect of metal additives on the reaction mechanism of sulfur redox reaction by in situ probing of the reaction intermediates using infrared spectroscopy. Elucidation of the reaction mechanism opens up the new avenue to optimize both electrode and electrolyte materials, which can further improve the performance of the Li-S battery technology.
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