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Amorphous MnO2 has been prepared from the reduction of KMnO4 in ethanol media by a facile one-step wet chemical route at room temperature. The electrochemical properties of amorphous MnO2 as cathode material in sodium-ion batteries ( SIBs) are studied by galvanostatic charge/discharge testing. And the structure and morphologies of amorphous MnO2 are investigated by X-ray diffraction ( XRD), scanning electron microscopy ( SEM), transmission electron microscopy ( TEM) and Raman spectra. The results reveal that as-synthesized amorphous MnO2 electrode material exhibits a spherical morphology with a diameter between 20 and 60 nm. The first specific discharge capacity of the amorphous MnO2 electrode is 123.2 mAh•g−1 and remains 136.8 mAh•g−1 after 100 cycles at the current rate of 0.1 C. The specific discharge capacity of amorphous MnO2 is maintained at 139.2, 120.4, 89, 68 and 47 mAh•g−1 at the current rate of 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, respectively. The results indicate that amorphous MnO2 has great potential as a promising cathode material for SIBs. [ABSTRACT FROM AUTHOR]

[Display omitted] • NFMPP could deliver a higher average working potential of 3.27 V with a theoretical capacity of 120 mAh g−1. • In-situ XRD and ex-situ XPS revealed NFMPP presented good electrochemical reversibility. • Carbon coating NFMPP exhibits excellent electrochemical performance. • The NFMPP-HC full cell shows a maximum power density of 260 W kg−1 with a maximum specifc energy of 130 Wh kg−1. Iron-based phosphates as a low cost and high structural stability cathode materials for sodium ion batteries (SIBs) have been widely studied. However, the working potential basing on Fe3+/Fe2+ redox is very low (less than 3.05 V vs. Na/Na+), which has obviously affect on the energy/power density. In this work, we choose the non-precious metal manganese (Mn) to improve the working potential of Na 3 Fe 2 (PO 4)(P 2 O 7) (NFFPP). A series of mixed-phosphate Na 3 Fe 2-x Mn x (PO 4)(P 2 O 7) (x = 0.0, 0.5, 1.0, 1.5 and 2.0) compounds are firstly proposed as cathode materials for SIBs. After optimization, the as-prepared Na 3 FeMn(PO 4)(P 2 O 7) (NFMPP) could deliver a higher average working potential of 3.27 V (vs. Na/Na+) with a theoretical capacity of 120 mAh g−1. Furthermore, the carbon coating NFMPP cathode shows a reversible initial specific capacity of 105 mAh g−1 at 0.1C with a corresponding coulombic efficiency of 99%. The in-situ XRD and ex-situ XPS results indicate that this cathode material goes through the reversible redox Fe3+/Fe2+ and Mn3+/Mn2+ with a reversible two Na-ion insertion/deinsertion process. [ABSTRACT FROM AUTHOR]

• Na x+2 Fe x+1 (PO 4) x (P 2 O 7) with different molar ratio of PO 4 3- to P 2 O 7 4- were studied. • Na x+2 Fe x+1 (PO 4) x (P 2 O 7) (x = 3, 4) are mixed-phases combining of maricite NFP and Na4. • Voltage plateaus of the single-phases are influenced by proportion of PO 4 3- to P 2 O 7 4-. • Some NFP in mixed-phases is converted to amorphous NFP, mixed-phases form three-phase mixtures. Relationship between structure and performance during PO 4 3- substitution was explored. Na x+2 Fe x+1 (PO 4) x (P 2 O 7) with different proportion of PO 4 3- to P 2 O 7 4- have different phases, structure and electrochemical performance. [Display omitted] Iron-based phosphates are low cost and high structural stability cathode materials for sodium ion batteries, which are considered to be the most promising power source for large-scale energy storage systems. Here, we report the electrochemical properties of Na x+2 Fe x+1 (PO 4) x (P 2 O 7) (x = 1, 2, 3, 4) by adjusting the molar ratio of PO 4 3- to P 2 O 7 4-. When PO 4 3-/P 2 O 7 4- is one or two, there are single-phases with higher capacity and similar structure. When PO 4 3-/P 2 O 7 4- is three or four, there are mixed-phases with lower capacity containing Na 4 Fe 3 (PO 4) 2 (P 2 O 7) and maricite NaFePO 4. Both single-phases and mixed-phases have good stability and reversibility. This study provides a new strategy to regulate PO 4 3- and P 2 O 7 4- in the mixed-polyanion system for enriching materials design strategies for new sodium ion battery cathode materials. [ABSTRACT FROM AUTHOR]

An iron-base phosphate Na 4 Fe 3 (PO 4) 2 (P 2 O 7) nano-sphere growing on the multi-walled carbon nanotubes (MCNTs) is scalable synthesized via a convenient spray drying way. The Na 4 Fe 3 (PO 4) 2 (P 2 O 7)@MCNTs composite as cathode material for sodium-ion battery (SIBs) exhibits a high discharge capacity of 115.7 mA h g−1 at 0.1 C and an outstanding cycle stability with a with a capacity retention of 95% after 1200th at 2 C rate. It also shows an excellent rate capability with a capacity of 62.8 mA h g−1, even at high rate of 20 C. The outstanding electrochemical performance of Na 4 Fe 3 (PO 4) 2 (P 2 O 7)@MCNTs cathode are attributed to its' particular P 2 O 7 dimers for reducing the rotation/distortion and higher electronic conductivity improves by MCNTs. A sodium-ion battery using the Na 4 Fe 3 (PO 4) 2 (P 2 O 7)@MCNTs cathode and the hard carbon anode shows a working voltage of 3.04 V with a specific energy of 210 Wh Kg−1 bases on the total weight of both electrode materials. Image 1 • A Na 4 Fe 3 (PO 4) 2 P 2 O 7 nano-sphere on carbon nanotubes composite is synthesized. • The composite cathode exhibits a reversible capacity of 115.7 mA h g−1. • A sodium-ion full cell shows a specific energy of 210 Wh.Kg−1. [ABSTRACT FROM AUTHOR]