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 ...

Lithium‐ion batteries (LIBs) have been widely used in the field of portable electric devices because of their high energy density and long cycling life. To further improve the performance of LIBs, it is of great importance to develop new electrode materials. Various transition metal oxides (TMOs) have been extensively investigated as electrode materials for LIBs. According to the reaction mechanism, there are mainly two kinds of TMOs, one is based on conversion reaction and the other is based on intercalation/deintercalation reaction. Recently, hierarchically nanostructured TMOs have become a hot research area in the field of LIBs. Hierarchical architecture can provide numerous accessible electroactive sites for redox reactions, shorten the diffusion distance of Li‐ion during the reaction, and accommodate volume expansion during cycling. With rapid research progress in this field, a timely account of this advanced technology is highly necessary. Here, the research progress on the synthesis methods, morphological characteristics, and electrochemical performances of hierarchically nanostructured TMOs for LIBs is summarized and discussed. Some relevant prospects are also proposed.

C-H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C-H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C-H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe(II) to Fe(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C-C coupling and finalizing the C-H functionalization. Regeneration of the Fe(II) catalyst for the next round of C-H activation involves SET oxidation of the Fe(I) species generated after the C-C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C-H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C-H activation. It is predicted that stabilization of low-spin Mn(II) using an appropriate ligand sphere should produce another promising candidate for efficient C-H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C-H activation reactions.