Transition metal oxides have attracted tremendous attention as anode materials for lithium ion batteries (LIBs) recently. However, their electrochemical processes and fundamental mechanisms remain unclear. Here we report the direct observation of the dynamic behaviors and the conversion mechanism of Fe2O3/graphene in LIBs by in situ transmission electron microscopy (TEM). Upon lithiation, the Fe2O3 nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li2O layer. Single-crystalline Fe2O3 nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li2O matrix. Surprisingly, the delithiated product was not Fe2O3 but FeO, accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. The charge-discharge processes of Fe2O3 in LIBs are different from previously recognized mechanism, and are found to be a fully reversible electrochemical phase conversion between Fe and FeO nanograins accompanying the formation and disappearance of the Li2O layer. The macroscopic electrochemical performance of Fe2O3/graphene was further correlated with the microcosmic in situ TEM results.

Smart hybridization of active materials into tailored electrode structure is highly important for developing advanced electrochemical energy storage devices. With the help of sandwiched design, herein a powerful strategy is developed to fabricate three-layer sandwiched composite core/shell arrays via combined hydrothermal and polymerization approaches. In such a unique architecture, wrinkled MoSe 2 nanosheets are sandwiched by vertical graphene (VG) core and N-doped carbon (N-C) shell forming sandwiched core/shell arrays. Interesting advantages including high electrical conductivity, strong mechanical stability, and large porosity are combined in the self-supported VG/MoSe 2 /N-C sandwiched arrays. As a preliminary test, the sodium ion storage properties of VG/MoSe 2 /N-C sandwiched arrays are characterized and demonstrated with high capacity (540 mA h g −1 ), enhanced high rate capability, and long-term cycling stability (298 mA h g −1 at 2.0 A g −1 after 1000 cycles). The sandwiched core/shell structure plays positive roles in the enhancement of electrochemical performances due to dual conductive carbon networks, good volume accommodation, and highly porous structure with fast ion diffusion. The directional electrode design protocol provides a general method for synthesis of high-performance ternary core/shell electrodes.

Ferromagnetism and the Kondo effect are crucial for 3d electrons to become spin-separated and heavy at the same time.

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renewable energy is imperative and of great significance for human beings. As the ideal alternatives, green energy sources including wind, solar energy, water, and tide power have been widely applied in modern industry successfully, but their intermittent and variability feature cannot support all-weather utilization. [3][4][5] Hence, developing high-efficiency energy storage and conversion technology is a powerful measure to solve the above problems. Currently, there has been great interest in developing/refining high-performance electrochemical energy storage (EES) devices such as batteries, supercapacitors, and fuel cells. Among various EES technologies, secondary rechargeable batteries (e.g., lead-acid batteries, Ni-Cd batteries, nickel-metal hydride batteries, and lithium/sodium ion batteries) have been extensively studied and play an important role in modern electronics and transportation. [6] Typically, since the successful commercialization of lithium ion batteries (LIBs) by Sony in 1991, we have entered into an era of LIBs due to their high working voltage, long cycles, low self-discharge, large energy density, and low maintenance. [7] After rapid development over the past decades, the fabrication techniques and performance of LIBs have made great progress and matured significantly to be used as main power source for sophisticated electronics, [8] hybrid electric vehicles, and pure electric vehicles. [1,9] However, the high Scrupulous design and smart hybridization of bespoke electrode materials are of great importance for the advancement of sodium ion batteries (SIBs). Graphene-based nanocomposites are regarded as one of the most promising electrode materials for SIBs due to the outstanding physicochemical properties of graphene and positive synergetic effects between graphene and the introduced active phase. In this review, the recent progress in graphene-based electrode materials for SIBs with an emphasis on the electrode design principle, different preparation methods, and mechanism, characterization, synergistic effects, and their detailed electrochemical performance is summarized. General design rules for fabrication of advanced SIB materials are also proposed. Additionally, the merits and drawbacks of different fabrication methods for graphene-based materials are briefly discussed and summarized. Furthermore, multiscale forms of graphene are evaluated to optimize electrochemical performance of SIBs, ranging from 0D graphene quantum dots, 2D vertical graphene and reduced graphene oxide sheets, to 3D graphene aerogel and graphene foam networks. To conclude, the challenges and future perspectives on the development of graphene-based materials for SIBs are also presented.