Research Progress in Graphene as Sulfur Hosts in Lithium-Sulfur Batteries

Abstract

Lithium-sulfur batteries are considered to be one of the most promising new-generation energy storage devices, owing to their ultra-high theoretical energy density and the merits of sulfur cathodes, which include natural abundance, low cost, and no toxicity. However, the commercial application of lithium-sulfur batteries is still subject to various intractable challenges. First, the insulation of sulfur and its solid discharge products (Li2S2/Li2S) leads to low utilization of the active materials. Second, the cathode suffers from an 80% volume expansion after the discharge process, which adversely affects its structural stability. Finally, intermediary lithium polysulfides can easily dissolve into the electrolyte, which can trigger the “shuttle effect.” This results in the loss of active materials, fast capacity fading, and low Coulombic efficiency. Graphene has garnered significant interest as a host material to accommodate sulfur for high-performance lithium-sulfur battery. A graphene host featuring a high specific surface area, excellent conductivity, and excellent mechanical stability can ensure a good electrical contact between the sulfur species and the current collector and withstand the volumetric strain of the electrode during cycling. Unfortunately, lithium polysulfides are still prone to escape from cathodes owing to the open two-dimensional (2D) plane structure of graphene sheets. To address this issue, various graphene-based materials with unique structures and chemical compositions have been trialed as sulfur hosts. In this review, we summarize research progress regarding three-dimensional (3D) graphene, graphene with modified surface chemistry, graphene-based composites, and graphene-based flexible materials as sulfur hosts for lithium-sulfur batteries. Furthermore, we analyze the challenges of applying graphene host materials in high-performance lithium-sulfur batteries. This review is mainly divided into four parts: (1) 3D graphene materials as sulfur hosts: the interconnected 3D porous network structure assembled from 2D graphene sheets provides a half-enclosed cavity to accommodate sulfur and its discharge products, which can inhibit the diffusion of lithium polysulfides to a certain extent. (2) Graphene materials with modified surface chemistry as sulfur hosts: hydrophilic surface functional groups and doped non-metal or metal heteroatoms on graphene can chemically adsorb polar lithium polysulfides. (3) Graphene-based composites as sulfur hosts: in various graphene-based composites, graphene usually functions as a conductive and flexible substrate. Other components, such as other types of carbon or metal compounds, can play an important role in restricting lithium polysulfides and propelling their reaction kinetics. (4) Flexible graphene-sulfur electrodes: the excellent flexibility and conductivity of graphene endowed it and its composites with a broad range of prospective applications regarding flexible lithium-sulfur batteries.