The development of low-cost, high-energy cathodes from nontoxic, broadly available resources is a big challenge for the next-generation rechargeable lithium or lithium-ion batteries. As a promising alternative to traditional intercalation-type chemistries, conversion-type metal fluorides offer much higher theoretical capacity and energy density than conventional cathodes. Unfortunately, these still suffer from irreversible structural degradation and rapid capacity fading upon cycling. To address these challenges, here a versatile and effective strategy is harnessed for the development of metal fluoride-carbon (C) nanocomposite nanofibers as flexible, free-standing cathodes. By taking iron trifluoride (FeF 3 ) as a successful example, assembled FeF 3 -C/Li cells with a high reversible FeF 3 capacity of 550 mAh g −1 at 100 mA g −1 (three times that of traditional cathodes, such as lithium cobalt oxide, lithium nickel cobalt aluminum oxide, and lithium nickel cobalt manganese oxide) and excellent stability (400+ cycles with littleto-no degradation) are demonstrated. The promising characteristics can be attributed to the nanoconfinement of FeF 3 nanoparticles, which minimizes the segregation of Fe and LiF upon cycling, the robustness of the electrically conductive C network and the prevention of undesirable reactions between the active material and the liquid electrolyte using the composite design and electrolyte selection.

Highly flexible porous carbon nanofibers (P-CNFs) were fabricated by electrospining technique combining with metal ion-assistant acid corrosion process. The resultant fibers display high conductivity and outstanding mechanical flexibility, whereas little change in their resistance can be observed under repeatedly bending, even to 180°. Further results indicate that the improved flexibility of P-CNFs can be due to the high graphitization degree caused by Co ions. In view of electrode materials for high-performance supercapacitors, this type of porous nanostructure and high graphitization degree could synergistically facilitate the electrolyte ion diffusion and electron transportation. In the three electrodes testing system, the resultant P-CNFs electrodes can exhibit a specific capacitance of 104.5 F g(-1) (0.2 A g(-1)), high rate capability (remain 56.5% at 10 A g(-1)), and capacitance retention of ∼94% after 2000 cycles. Furthermore, the assembled symmetric supercapacitors showed a high flexibility and can deliver an energy density of 3.22 Wh kg(-1) at power density of 600 W kg(-1). This work might open a way to improve the mechanical properties of carbon fibers and suggests that this type of freestanding P-CNFs be used as effective electrode materials for flexible all-carbon supercapacitors.

NiCo2S4@CoSx core/shell nanotube arrays have been successfully synthesized and used to optimize the capacitive performance of electrochemical supercapacitors.

Hierarchical ZnCo2O4@NixCo2x(OH)6x core/shell nanowire arrays have been successfully constructed for hybrid supercapacitors with outstanding performance.

devices, backup power supplies, and a broad range of other important applications. [2] Recent developments of smart textiles [3] and flexible electronics [4] will benefit greatly from the development of robust, safe-to-use, low-cost, and flexible energy storage that could be integrated into fabrics. [5] Commercial SCs are electrical doublelayer capacitors (EDLCs) based on activated carbons that store energy by the accumulation of electrostatic charge on their internal surface area. However, the low density of activated carbon (typically below 0.7 g cm −3 ) and limited specific capacitance of carbon electrodes (typically 70-250 F g −1 , depending on the electrolyte used) result in rather moderate volumetric energy density characteristics of EDLCs. [6] Besides, the common use of organic electrolytes in EDLCs induces additional fire hazard, [7] which is a particularly sensitive topic if those are integrated into clothing. In contrast to EDLCs, asymmetric SCs (ASCs) that use a dense pseudocapacitive (or a battery-like) electrode together with an EDLC (or a different kind of pseudocapacitive) electrode may offer higher volumetric energy storage characteristics even with nonflammable aqueous electrolytes. [8][9][10] In the latter case, such a hybrid design relies on different potential windows for positive and negative electrodes, achieving high electrode capacity, a relatively high cell voltage, and a resulting high-energy density. [9,11] Transition metal oxides (TMOs) are attracting great interest for applications in ASCs, because they can benefit from fast reversible redox reactions and offer significantly higher volumetric capacitance than porous carbon materials. [12] Among TMOs, ruthenium oxide (RuO 2 ) is considered as "gold standard" pseudocapacitive materials with high conductivity, high specific capacitance, and excellent rate performance, but its extremely high cost seriously limits its large-scale applications. [9,13] Vanadium oxides were demonstrated to exhibit very large specific capacitance, [14] but are too toxic for wearable applications. In sharp contrast, both iron and manganese oxides (Fe 2 O 3 and MnO 2 ) offer good electrochemical activity, very low cost, environmental friendliness, and great abundance in Earth's crust. [15][16][17] Besides, the large potential windows of Fe 2 O 3 (−0.8-0 V vs Ag/AgCl) and MnO 2 (0−0.8 V vs Ag/AgCl) allow ASCs based on such electrodes have a moderately high operating Aqueous asymmetric supercapacitors (ASCs) may offer comparable or higher energy density than electric double-layer capacitors (EDLCs) based on organic electrolytes. As such, ASCs may be more suitable for integration into smart textiles, where the use of flammable organic solvents is not acceptable. However, reported ASC devices typically suffer from poor rate capability and low areal loadings. This study demonstrates the development of nitrogendoped carbon (N-C) nanowire/metal oxide (Fe 2 O 3 and MnO 2 ) nanocomposite electrodes directly produced on the internal surface of a conductive fabric ...

Lithium-metal fluoride batteries promise significantly higher energy density than the state-ofthe-art lithium-ion batteries and lithium-sulfur batteries. Unfortunately, commercialization of metal fluoride cathodes is prevented by their high resistance, irreversible structural change, and rapid degradation. In this study, we demonstrate substanial boost in metal fluoride (MF) cathode stability by designing nanostructure with two layers of protective shells -one deposited ex-situ and the other in-situ. Such methodology achieves over 90% capacity retention after 300 charge-discharge cycles, This article is protected by copyright. All rights reserved.3 producing the first report on FeF 3 as a cathode material, where a very high capacity utilization in combination with excellent stability is approaching to the level needed for practical applications of FeF 3 . The CEI containing lithium oxalate and B-F bond containing anions is found to effectively protect the cathode material from direct contact with electrolytes, thus greatly suppressing the dissolution of Fe. Quantum chemistry (QC) and molecular dynamics (MD) calculations provide unique insights into the mechanisms of CEI layer formation. As a result, our work not only demonstrates unprecedented performances, but also provides the readers with a better fundamental understanding of electrochemical behavior of MF cathodes and the positive impact observed with the application of a LiBoB salt in the electrolyte.

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