Unraveling the Role of Nitrogen-Doped Carbon Nanowires Incorporated with MnO_(2)Nanosheets as High Performance Cathode for Zinc-Ion Batteries

Manganese-based cathode materials are considered as a promising candidate for rechargeable aqueous zinc-ion batteries(ZIBs).Suffering from poor conductive and limited structure tolerance,various carbon matrix,especially N-doped carbon,were employed to incorporate with MnO_(2)for greatly promoted electrochemical performances.However,the related underlying mechanism is still unknown,which is unfavorable to guide the design of high performance electrode.Herein,by incorporating layered MnO_(2)with N-doped carbon nanowires,a free-standing cathode with hierarchical core-shell structure(denoted as MnO_(2)@NC)is prepared.Benefiting from the N-doped carbon and rational architecture,the MnO_(2)@NC electrode shows an enhanced specific capacity(325 mAh g^(−1)at 0.1 A g^(−1))and rate performance(90 mAh g^(−1)at 2 A g^(−1)),as well as improved cycling stability.Furthermore,the performance improvement mechanism of MnO_(2)incorporated by N-doped carbon is investigated by X-ray photoelectron spectroscopy(XPS),Raman spectrums and density functional theory(DFT)calculation.The N atom elongates the Mn-O bond and reduces the valence of Mn^(4+)ion in MnO_(2)crystal by delocalizing its electron clouds.Thus,the electrostatic repulsion will be weakened when Zn^(2+)/H^(+)insert into the host MnO_(2)lattices,which is profitable to more cation insertion and faster ion transfer kinetics for higher capacity and rate capability.This work elucidates a fundamental understanding of the functions of N-doped carbon in composite materials and shed light on a practical pathway to optimize other electrode materials.

One-step synthesis of novel core-shell bimetallic hexacyanoferrate for high performance sodium-storage cathode

Recently, the design of core-shell hierarchical architecture plays an important role in improving the electrochemical performance of Prussian blue analogue cathodes(PBAs). Unfortunately, the inconvenient stepwise preparation and the strict lattice-matching requirement have restricted the development of coreshell PBAs. Herein, we demonstrate a one-step synthesis strategy to synthesize core-shell manganese hexacyanoferrate(MnFeHCF@MnFeHCF) for the first time. And the formation mechanism of the core-shell hierarchical architecture is investigated by first-principles calculations. It is found that the as-obtained Mn FeHCF@MnFeHCF act out the superior intrinsic natures, which not only can obtain a larger specific surface area and lower Fe(CN)_(6) vacancies but also can activate more Na-storage sites. Compared with the manganese hexacyanoferrate(MnHCF), the iron hexacyanoferrate(FeHCF), and even the traditional coreshell nickel hexacyanoferrate(FeHCF@NiHCF) prepared by a stepwise method, the Mn Fe HCF@MnFeHCF demonstrates a superior rate performance, which achieves a high capacity of 131 mAh g^(−1) at 50 mA g^(−1) and delivers a considerable discharge capacity of about 100 mAh g^(−1) even at 1600 mA g^(−1). Meantime, the capacity retention can reach up to nearly 80% after 500 cycles. The improved performances could be mainly originated from two aspects: on the one hand, Mn substitution is helpful to enhance the material conductivity;on the other hand, the core-shell structure with matched lattice parameters is more favorable to enhance the diffusion coefficient of sodium ions. Beside, the structural transformation of MnFeHCF@MnFeHCF upon the extraction/insertion of sodium ions is instrumental in releasing the interior stress and effectively maintaining the integrity of the crystal structure.