Figure 6. a) Schematic illustration of the electron transport pathway in a SEI-wrapped 3D CNT NAs-sulfur cathode. Reproduced with permission. [99] Copyright 2017, Wiley-VCH. b) SEM images of CC@ZnO anode after cycling test without (b 1 ) and with (b 2 ) 3D CF NAs. Reproduced with permission. [103] Copyright 2016, Wiley-VCH. c) The local current density and electric field distribution of Zn anode on a planar current collector (c 1 ) and a 3D current collector (c 2 ). d) Regulated growth of SEI on 3D titanate NAs with ether electrolyte. e) Discharge/charge profiles of the Na 2 Ti 2 O 5 anode with ether electrolyte (inset: HRTEM image of the fully discharged Na 2 Ti 2 O 5 NSs). d,e) Reproduced with permission. [112] Copyright 2019, Wiley-VCH.

Lithium-ion batteries have achieved tremendous success in the fields of portable mobile devices, electric vehicles, and large-scale energy storage owing to their high working voltage, high energy density, and long-term lifespan. However, lithium-ion batteries are ultimately unable to satisfy increasing industrial demands due to the shortage and rising cost of lithium resources. Sodium is another alkali metal that has similar physical and chemical properties to those of lithium, but is more abundant. Therefore, sodium-ion batteries (SIBs) are promising candidates for next-generation energy storage devices. Nevertheless, SIBs generally exhibit inferior electrochemical reaction kinetics, cycling performance, and energy density to those of lithiumion batteries owing to the larger ion radius and higher standard potential of Na+ compared to those of Li+. To address these issues, significant effort has been made toward developing electrode materials with large sodiation/desodiation channels, robust structural stability, and high theoretical capacity. As electrode performance is closely related to its architecture, constructing an advanced electrode structure is crucial for achieving high-performance SIBs. Conventional electrodes are generally prepared by mixing a slurry of active materials, conductive carbon, and binders, followed by casting on a metal current collector. Electrodes prepared this way are subject to shape deformation, causing the active materials to easily peel off the current collector during charge/discharge processes. This leads to rapid capacity decay and short cycle life. Moreover, binders and other additives increase the weight and volume of the electrodes, which reduces the overall energy density of the batteries. Therefore, binder-free, three-dimensional (3D) array electrodes with satisfactory electronic conductivity and low ion-path tortuosity have been proposed. In addition to solving the aforementioned issues, this type of electrode significantly reduces contact resistance through the strong adhesion between the array and the substrate. Furthermore, electrolyte infiltration is greatly facilitated by the abundant interspacing between individual nanostructures, which promotes fast electron transport and shortens ion diffusion, thus enabling the electrode reaction. The array structure can also readily accommodate substantial volume variations that occur during repeated sodiation/desodiation processes and release the generated stress. Therefore, it is of great interest to explore binder-free array electrodes for sodium-ion storage applications. This review summarizes the recent advances in various 3D array anode materials for SIBs, including elemental anodes, transition metal oxides, sulfides, phosphides, and titanates. The preparation methods, structure/morphology characteristics, and electrochemical performance of various array anodes are discussed, and future opportunities and challenges from employing array electrodes in SIBs are proposed.

Solid-state polymer lithium-ion batteries with better safety and higher energy density are one of the most promising batteries, which are expected to power future electric vehicles and smart grids. However, the low ionic conductivity at room temperature of solid polymer electrolytes (SPEs) decelerates the entry of such batteries into the market. Creating polymer-in-salt solid electrolytes (PISSEs) where the lithium salt contents exceed 50[Formula: see text]wt.% is a viable technology to enhance ionic conductivity at room temperature of SPEs, which is also suitable for scalable production. In this review, we first clarify the structure and ionic conductivity mechanism of PISSEs by analyzing the interactions between lithium salt and polymer matrix. Then, the recent advances on polyacrylonitrile (PAN)-based PISSEs and polycarbonate derivative-based PISSEs will be reviewed. Finally, we propose possible directions and opportunities to accelerate the commercializing of PISSEs for solid polymer Li-ion batteries.

TiNb 2 O 7 anode materials (TNO) have unique potential for applications in Li-ion capacitors (LICs) due to their high specific capacity of ca. 280 mA h g −1 over a wide anodic Li-insertion potential window. However, their highrate capability is limited by their poor electronic and ionic conductivity. In particular, studies on TNO for LICs are lacking and that for flexible LICs have not yet been reported. Herein, a unique TNO porous electrode with cross-linked nanorods tailored by post-annealing and its application in flexible LICs are reported. This binder-free TNO anode exhibits superior rate performance (~66.3% capacity retention as the rate increases from 1 to 40 C), which is ascribed to the greatly shortened ion-diffusion length in TNO nanorods, facile electrolyte penetration and fast electron transport along the continuous single-crystalline nanorod network. Furthermore, the TNO anode shows an excellent cycling stability up to 2000 cycles and good flexibility (no capacity loss after continuous bending for 500 times). Model flexible LIC assembled with the TNO anode and activated carbon cathode exhibits increased gravimetric and volumetric energy/power densities (~100.6 W h kg −1 /4108.8 W kg −1 ; 10.7 mW h cm −3 / 419.3 mW cm −3 ), more superior to previously reported hybrid supercapacitors. The device also efficiently powers an LED light upon 180°bending. Figure 1 Schematic illustration of the merits of TNO electrode architecture for Li-ion storage.

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