Carbon nanotubes (CNTs) are of great interest for many potential applications because of their extraordinary electronic, mechanical and structural properties. However, issues of chaotic staking, high cost and high energy dissipation in the synthesis of CNTs remain to be resolved. Here we develop a facile, general and high-yield strategy for the oriented formation of CNTs from metal-organic frameworks (MOFs) through a low-temperature (as low as 430 °C) pyrolysis process. The selected MOF crystals act as a single precursor for both nanocatalysts and carbon sources. The key to the formation of CNTs is obtaining small nanocatalysts with high activity during the pyrolysis process. This method is successfully extended to obtain various oriented CNT-assembled architectures by modulating the corresponding MOFs, which further homogeneously incorporate heteroatoms into the CNTs. Specifically, nitrogen-doped CNT-assembled hollow structures exhibit excellent performances in both energy conversion and storage. On the basis of experimental analyses and density functional theory simulations, these superior performances are attributed to synergistic effects between ideal components and multilevel structures. Additionally, the appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function, hence efficiently enhancing its oxygen reduction activity. The viable synthetic strategy and proposed mechanism will stimulate the rapid development of CNTs in frontier fields.

K-ion battery (KIB) is a new-type energy storage device that possesses potential advantages of low-cost and abundant resource of K precursor materials. However, the main challenge lies on the lack of stable materials to accommodate the intercalation of large-size K-ions. Here we designed and constructed a novel earth abundant Fe/Mn-based layered oxide interconnected nanowires as a cathode in KIBs for the first time, which exhibits both high capacity and good cycling stability. On the basis of advanced in situ X-ray diffraction analysis and electrochemical characterization, we confirm that interconnected KFeMnO nanowires can provide stable framework structure, fast K-ion diffusion channels, and three-dimensional electron transport network during the depotassiation/potassiation processes. As a result, a considerable initial discharge capacity of 178 mAh g is achieved when measured for KIBs. Besides, K-ion full batteries based on interconnected KFeMnO nanowires/soft carbon are assembled, manifesting over 250 cycles with a capacity retention of ∼76%. This work may open up the investigation of high-performance K-ion intercalated earth abundant layered cathodes and will push the development of energy storage systems.

Nanowires and nanotubes have been the focus of considerable efforts in energy storage and solar energy conversion because of their unique properties. However, owing to the limitations of synthetic methods, most inorganic nanotubes, especially for multi-element oxides and binary-metal oxides, have been rarely fabricated. Here we design a gradient electrospinning and controlled pyrolysis method to synthesize various controllable 1D nanostructures, including mesoporous nanotubes, pea-like nanotubes and continuous nanowires. The key point of this method is the gradient distribution of low-/middle-/high-molecular-weight poly(vinyl alcohol) during the electrospinning process. This simple technique is extended to various inorganic multi-element oxides, binary-metal oxides and single-metal oxides. Among them, Li3V2(PO4)3, Na0.7Fe0.7Mn0.3O2 and Co3O4 mesoporous nanotubes exhibit ultrastable electrochemical performance when used in lithium-ion batteries, sodium-ion batteries and supercapacitors, respectively. We believe that a wide range of new materials available from our composition gradient electrospinning and pyrolysis methodology may lead to further developments in research on 1D systems.

The emerging electrochemical energy storage systems beyond Li‐ion batteries, including Na/K/Mg/Ca/Zn/Al‐ion batteries, attract extensive interest as the development of Li‐ion batteries is seriously hindered by the scarce lithium resources. During the past years, large amounts of studies have focused on the investigation of various electrode materials toward emerging metal‐ion batteries to realize high energy density, high power density, and a long cycle life. In particular, vanadium‐based nanomaterials have received great attention. Vanadium‐based compounds have a big family with different structures, chemical compositions, and electrochemical properties, which provide huge possibilities for the development of emerging electrochemical energy storage. In this review, a comprehensive overview of the recent progresses of promising vanadium‐based nanomaterials for emerging metal‐ion batteries is presented. The vanadium‐based materials are classified into four groups: vanadium oxides, vanadates, vanadium phosphates, and oxygen‐free vanadium‐based compounds. The structures, electrochemical properties, and modification strategies are discussed. The structure–performance relationships and charge storage mechanisms are focused on. Finally, the perspectives about future directions of vanadium‐based nanomaterials for emerging energy storage devices are proposed. This review will provide comprehensive knowledge of vanadium‐based nanomaterials and shed light on their potential applications in emerging energy storage.

Intercalation of ions in electrode materials has been explored to improve the rate capability in lithium batteries and supercapacitors, due to the enhanced diffusion of Li(+) or electrolyte cations. Here, we describe a synergistic effect between crystal structure and intercalated ion by experimental characterization and ab initio calculations, based on more than 20 nanomaterials: five typical cathode materials together with their alkali metal ion intercalation compounds A-M-O (A = Li, Na, K, Rb; M = V, Mo, Co, Mn, Fe-P). Our focus on nanowires is motivated by general enhancements afforded by nanoscale structures that better sustain lattice distortions associated with charge/discharge cycles. We show that preintercalation of alkali metal ions in V-O and Mo-O yields substantial improvement in the Li ion charge/discharge cycling and rate, compared to A-Co-O, A-Mn-O, and A-Fe-P-O. Diffraction and modeling studies reveal that preintercalation with K and Rb ions yields a more stable interlayer expansion, which prevents destructive collapse of layers and allow Li ions to diffuse more freely. This study demonstrates that appropriate alkali metal ion intercalation in admissible structure can overcome the limitation of cyclability as well as rate capability of cathode materials, besides, the preintercalation strategy provides an effective method to enlarge diffusion channel at the technical level, and more generally, it suggests that the optimized design of stable intercalation compounds could lead to substantial improvements for applications in energy storage.

Coordination compounds such as Prussian blue and its analogues are acknowledged as promising candidates for electrochemical sodium storage owing to their tailorable frameworks. However, a key challenge for these electrode materials is the trade-off between energy and power. Here, we demonstrate that Prussian white (Na 3.1 Fe 4 [Fe(CN) 6 ] 3 ) hierarchical nanotubes with fully open framework configurations render extrinsic Na + intercalation pseudocapacitance. The cathode exhibits a capacity up to 83 mAh g -1 at an ultra-high rate of 50 C and an unprecedented cycle life over 10000 times for sodium storage. In situ Raman spectroscopy together with In situ X-ray diffraction analysis reveal that intercalation pseudocapacitance enables full reaction of N-Fe III /Fe II sites in Prussian white with a negligible volume expansion (< 2.1%). The discovery of surface-controlled charge storage occurring inside the entire bulk of intercalation cathodes paves a new way for developing high power, high energy, and long life-span sodium-ion batteries.

The calcium-ion battery is an emerging energy storage system that has attracted considerable attention recently. However, the absence of high-performance cathode materials is one of the main challenges for the development of calcium-ion batteries. Herein, a bilayered Mg0.25V2O5·H2O as a stable cathode for rechargeable calcium-ion batteries is identified. Remarkably, an unexpected stable structure of the material for Ca2+ storage is demonstrated. It is found that the interlayer spacing shows only a tiny variation of ∼0.09 Å during Ca2+ insertion/extraction, which results in its outstanding cycling stability (capacity retention of 86.9% after 500 cycles) for Ca2+ storage. On the basis of in situ/ex situ experimental characterizations and ab initio simulation, the origin of such superior structural stability is revealed. This ultrastable cathode together with the understanding lays a strong foundation for developing high-performance calcium-ion batteries.

is located below lithium in the periodic table and shares similar physical/chemical properties with lithium in many aspects. Thus, sodium is a promising candidate for replacing lithium in energy storage systems. [ 11 ] Recently, sodium ion batteries have been widely reconsidered for largescale applications. Undoubtedly, the exploration and development of sodium-ion batteries is a new and important direction in the fi eld of energy storage.Recently, potassium containing compounds have been investigated in sodiumion batteries. Liu et al. found that the potassium ion intercalated manganese oxide (K 0.27 MnO 2 ) with large ion diffusion channels shows superior cycling stability and rate capability for sodium storage. [ 12 ] This inspiring work indicates that the potassium-containing compounds have great potentials in energy storage. On the other hand, phosphates have been widely studied because of their high redox potential, good safety, and low cost. Compared with metal oxides, phosphates possess higher electrochemical voltage and thus higher energy density due to the inductive effect of PO 4 3− . [ 4,5 ] In addition, the phosphates also provide higher thermal stability for elevated temperature operation. However, the phosphates face the defects of regular impure phase and poor electronic conductivity. [ 4,5 ] The formation of impurity phases can be suppressed by thoroughly mixing the reactants before sintering, and the conductivity can be improved by compositing with carbon. [13][14][15][16][17][18][19] Among the phosphate compounds, Li 3 V 2 (PO 4 ) 3 [ 20,21 ] and Na 3 V 2 (PO 4 ) 3 [22][23][24][25] have been widely studied as lithium or sodium-ion battery cathodes. For example, Jian et al. reported the synthesis of Na 3 V 2 (PO 4 ) 3 /C composite by a one-step solid state reaction; when used as the cathod for sodium-ion battery, it delivers an initial discharge capacity of 93 mAh g −1 .[ 23 ] Saravanan et al. reported the preparation of porous Na 3 V 2 (PO 4 ) 3 /C with excellent cycling stability and superior rate capability in sodium-ion battery. [ 25 ] Despite the numerous reports on Li 3 V 2 (PO 4 ) 3 and Na 3 V 2 (PO 4 ) 3 , the crystal structure and electrochemical performance of K 3 V 2 (PO 4 ) 3 has never been reported. Herein, a novel potassium containing phosphate material, K 3 V 2 (PO 4 ) 3 , is designed and explored in energy storage. The K 3 V 2 (PO 4 ) 3 /C bundled nanowires were synthesized by a facile organic acid-assisted method. With a highly stable framework for sodium storage, porous nanostructure for fast Sodium-ion battery has captured much attention due to the abundant sodium resources and potentially low cost. However, it suffers from poor cycling stability and low diffusion coeffi cient, which seriously limit its widespread application. Here, K 3 V 2 (PO 4 ) 3 /C bundled nanowires are fabricated usinga facile organic acid-assisted method. With a highly stable framework, nanoporous structure, and conductive carbon coating, the K 3 V 2 (PO 4 ) 3 /C bundled nanowires manifest excellent e...