Transition Metal Sulfides Based on Graphene for Electrochemical Energy Storage

Abstract

factors: electrode materials, electrolytes, and separators. [18] Finding suitable electrode materials to promote and realize their commercialization is still considered one of the major challenges. [19][20][21] Up to now, electrode materials have commonly included carbon materials (CMs), [5,13,22] conducting polymer materials (CPMs), [23] transition metal oxides (TMOs), [24] and others. [25][26][27] However, they all have their respective shortcomings; for instance, CMs tend to restack and then decrease the specific surface area, which is an important index for CM performance. [28][29][30] CPMs show the same phenomenon that the original structure will collapse under long-term charge/discharge process. [23] Although the above two types of electrode materials have good electron conductivity, their changing specific surface area and structures during use make them unlikely to be promising electrode materials. TMOs always have a good reversible faradic reaction due to their valence flexibility. However, in comparison to CMs, their poor electron conductivity may lower their specific capacity. [31][32][33] Transition metal sulfides (TMSs) have attracted tremendous attention due to their high specific capacity. For example, nickel and cobalt sulfides (e.g., NiS x , CoS x ) have specific capacities that are double of their oxide counterparts (e.g., NiO x , CoO x ). [22,[34][35][36] This is because the replacement of oxygen with sulfur, an element with a lower electronegativity, increases the performance compared to TMOs. [37,38] However, their unyielding volume change during the cycling process has hindered their further development and application in lithium and sodium rechargeable batteries. [39] Though TMSs possess high specific capacities and excellent rate capabilities when used for SCs, it is difficult to achieve all these objectives simultaneously from a single material. Therefore, the hybridization of different materials with different properties is becoming an interesting research area. Recent papers have testified that the adulteration of graphene or graphene derivatives can solve these issues and increase the electrochemical performance of energy storage devices. [40][41][42] It can be attributed to the properties of graphene: 2D conductive networks, a large specific surface area, and good physicochemical stability. In addition to these properties, their porous structure can effectively promote the diffusion of electrolyte ions. Therefore, 2D graphene is one of the ideal support framework materials to prepare TMS@graphene composites for electrode materials. [43][44][45][46] Transition metal sulfides, as an important class of inorganics, can be used as excellent electrode materials for various types of electrochemical energy storage, such as lithium-ion batteries, sodium-ion batteries, supercapacitors, and others. Recent works have identified that mixing graphene or graphene derivatives with transition metal sulfides can result in novel composites with better electrochemical performance. This review summarizes ...

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