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Abstract Ammonium vanadate with bronze structure (NH 4 V 4 O 10 ) is a promising cathode material for zinc-ion batteries due to its high specific capacity and low cost. However, the extraction of $${\text{NH}}_{{4}}^{ + }$$ NH 4 + at a high voltage during charge/discharge processes leads to irreversible reaction and structure degradation. In this work, partial $${\text{NH}}_{{4}}^{ + }$$ NH 4 + ions were pre-removed from NH 4 V 4 O 10 through heat treatment; NH 4 V 4 O 10 nanosheets were directly grown on carbon cloth through hydrothermal method. Deficient NH 4 V 4 O 10 (denoted as NVO), with enlarged interlayer spacing, facilitated fast zinc ions transport and high storage capacity and ensured the highly reversible electrochemical reaction and the good stability of layered structure. The NVO nanosheets delivered a high specific capacity of 457 mAh g −1 at a current density of 100 mA g −1 and a capacity retention of 81% over 1000 cycles at 2 A g −1 . The initial Coulombic efficiency of NVO could reach up to 97% compared to 85% of NH 4 V 4 O 10 and maintain almost 100% during cycling, indicating the high reaction reversibility in NVO electrode. 

Iron hexacyanoferrate (FeHCF) particles were synthesized at room temperature with ethylenediaminetetraacetic acid (EDTA) at varying pH. The presence of EDTA produced faceted particles and increasing synthesis pH resulted in slower reaction kinetics and larger particles with lower water content and fewer anion vacancies determined by TGA and Mössbauer spectroscopy. Electrochemical testing of sodium metal half cells revealed higher capacity in FeHCF particles grown at lower pH with EDTA, obtaining a maximum discharge capacity of 151 mA h g −1 with 79% capacity retention after 100 cycles at 100 mA g −1 and a rate capability of 122 mA h g −1 at 3.2 A g −1 . In contrast, particles grown at higher pH had stunted low-spin Fe redox activity but with improved long-term cyclic stability. These findings demonstrate that small changes in synthesis pH can greatly affect the growth and electrochemical properties of FeHCF when using a pH sensitive chelating agent such as EDTA. 

Manganese dioxide (MnO 2 ) with a conversion mechanism is regarded as a promising anode material for lithium-ion batteries (LIBs) owing to its high theoretical capacity (∼1223 mA h g −1 ) and environmental benignity as well as low cost. However, it suffers from insufficient rate capability and poor cyclic stability. To circumvent this obstacle, semiconducting polypyrrole coated-δ-MnO 2 nanosheet arrays on nickel foam (denoted as MnO 2 @PPy/NF) are prepared via hydrothermal growth of MnO 2 followed by the electrodeposition of PPy on the anode in LIBs. The electrode with ∼50 nm thick PPy coating exhibits an outstanding overall electrochemical performance. Specifically, a high rate capability is obtained with ∼430 mA h g −1 of discharge capacity at a high current density of 2.67 A g −1 and more than 95% capacity is retained after over 120 cycles at a current rate of 0.86 A g −1 . These high electrochemical performances are attributed to the special structure which shortens the ion diffusion pathway, accelerates charge transfer, and alleviates volume change in the charging/discharging process, suggesting a promising route for designing a conversion-type anode material for LIBs. 

Sodium superionic conductor (NASICON)‐type materials are getting more and more attention due to their high capacity and good cycling ability compared with other cathode materials in aqueous zinc ion batteries (AZIB). The present paper was to study the synthesis and electrochemical properties of two NASICON compounds of Na₃V₂(PO₄)₃and Na₃V₂(PO₄)₂F₃and to understand the impacts of fluorine. Both Na₃V₂(PO₄)₃and Na₃V₂(PO₄)₂F₃are synthesized by hydrothermal growth followed with annealing at 800°C in inert gas. With 3 mol/L Zn(CF₃SO₃)₂in water as electrolyte, Na₃V₂(PO₄)₃offered a high storage capacity, while Na₃V₂(PO₄)₂F₃demonstrated a high discharge voltage though low storage capacity. It was also found that the storage capacity of Na₃V₂(PO₄)₂F₃increases with increased cycles; however, the compound undergoes a gradual phase transition. It is discussed possible approaches to attain both high discharge voltage and large capacity with good cycling stability.

 

Micro-supercapacitor is a member of the miniaturized energy storage device family, which offers great advantages on power density and life span. However, the limited device capacitance and narrow voltage window limit its energy density, hindering its application. In the present work, a novel micro-pseudocapacitor (MPC) constructed via the facile extrusion-based 3D printing technique has been demonstrated to deliver efficient charge storage with high device capacitance and moderate voltage window. Such an asymmetric MPC is constructed with 3D-printing-enabled asymmetric interdigitated cellular microelectrodes; in which, one is Ni–Co–O nanosheets grown on macroporous 3D reduced GO (3DG) microelectrode and the other is MnO 2 nanosheets grown on 3DG. Such an MPC offers facilitated fast electron transport, ionic diffusion, large number of active sites and desired porosity for electrolyte penetration. The asymmetric MPC shows a high specific capacity of 500 mC cm −2 , an energy density of 90 μW h cm −2 and a voltage window of 1.3 V. A device cycling stability with 10 000 charge and discharge cycles is also achieved for the as-fabricated asymmetric MPCs. These encouraging results may open a new avenue to design and fabricate state-of-the-art miniaturized electrochemical energy storage devices with customized geometries.