Graphene Mesosponges for the Conductive Additive of Cathode Composites in Lithium−Sulfur All-Solid-State Batteries

Abstract

All-solid-state lithium−sulfur batteries are a class of promising energy storage technologies because of the low cost of sulfur as the active material, high safety due to no use of volatile organic solvents, high theoretical energy density (2600 Wh kg−1), and being operative at high temperature. However, they suffer from the limited utilization of sulfur when the rate of charge/discharge processes is fast, partly because the active material is highly insulating with s of 10−30 S cm−1. Moreover, it is highly desired to use as much sulfur in the cathode composite as possible while maintaining the electrochemical performance for the practical application, but the literatures reported thus far utilized lower surface loading of sulfur ranging from 9 to 50wt%. The choice of conductive additive such as activated carbons in the cathode composites will be crucial in improving and controlling the electrochemical processes. In this regard, graphene mesosponges (GMS) are a class of unprecedentedly functional carbon materials having the high electrochemical stability, electric conductivity, structural flexibility, and mesostructured nanopores (Nishihara, H. et al. Adv. Funct. Mater. 2016, 26, 6418–6427; Energy Environ. Sci. 2019 DOI: 10.1039/C8EE03184C), and therefore, are promising as the conductive supports in cathode composites of lithium−sulfur all-solid-state batteries. The utilization of GMS as the conductive additive indeed showed a good electrochemical performance with a gravimetric capacity of 690 mA h g−1 at 80 ºC at a rate of 0.2C even with 50wt% of sulfur loaded in the cathode composites. The elastic nature with the bulk modulus of ca. 0.8 GPa and large pore volume (~5 mL g−1) of the 3D graphene materials will be suitable for the charging/discharging processes that require a large volumetric change of the cathode composites (ca. 80%) as going from sulfur (~2 g cm−3) to lithium sulfide (1.66 g cm−3) with the following equation 2Li++S+2e− ⇄ Li2S. Moreover, the phase segregation in micrometer scale occurred with high loading of the active material (50−60wt%) was suppressed by the confinement of insulating sulfur into the mesopore of the elastic 3D graphene, achieving the durable three-phase contact between a solid-electrolyte, insulating sulfur, and the electric conductive carbon. As a result, both the rate characteristic and durability of the all-solid-state batteries were higher than those with a typical activated carbon, and the electrochemical operation was feasible under the harsh operation conditions up to 80 ºC with the specific charge/discharge capacities of 3.4 mA h cm−2 (2 C) even with the unprecedentedly high sulfur-loading (60wt%).