How to optimize the utilization of renewable energy sources (e.g. wind, solar) for a sustainable human future has become a global focus of attention. One of the most effective solutions is to use cost-effective and environmentally friendly rechargeable batteries to store renewable energy.256,257For these applications, lithium ion batteries (LIBs) are not the ideal options because lithium resources in the Earth’s crust suffer from low abundance combined with rising prices. By comparison, sodium is abundant and low-cost, and it also has similar physical and chemical properties to Li. Therefore, sodium ion batteries (SIBs), as the most attractive choice, have been given more attention in recent years.48,247,258,289
Cathode materials play a decisive role in the final electrochemical performance of a full cell. Recently, a number of transition metal oxides,98,126,259 phosphates,319-322 and organic compounds205,311,323 were reported as cathode host materials for sodium-ion storage. In particular, Prussian blue (PB) and its analogues (KMFe(CN)6 or NaxMFe(CN)6, 0 ≤ x < 2, M = Fe, Mn, Ni, Zn, Cu, etc.) have attracted more attention due to their environmentally friendly nature, inexpensive constituents and superior electrochemical performance.169,171,173,174,181,324 Rhombohedral Na1.72MnFe(CN)6 (denoted as NMHFC) prepared by Goodenough’s group,174
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analogues, including a reversible capacity as high as 134 mAh g-1, with a higher average potential plateau of 3.55 V, contributing to high energy storage and making this compound a promising cathode material for rechargeable SIBs. Its short cycle life (just 30 cycles shown), however, and high potential polarization at high rates, due to the intrinsic poor electronic conductivity of NMHFC, need to be substantially improved for this material to be used commercially. Here, we utilize the intrinsic oxidation power of the NMHFC, without an external chemical oxidant, to form conducting polymers on the surfaces of NMHFC particles, resulting in improved electronic conductivity, which leads to increased rate capability. Simultaneously, perchlorate ions (ClO4-) and sodium ions (Na+) are doped into the polymer chains and NMHFC crystals, respectively, during the polymerization. Perchlorate ions can act as a dopant to enhance the electronic conductivity of the polymer and function as redox- active sites to increase the capacity of the doped polymer. Additional sodium ion doping can improve the specific capacity by removing zeolitic water from crystal structure. In this way, the total capacity decrease resulting from the introduction of the conducting coating which has lower specific capacity than that of NMHFC crystals can be minimized. Compared with other coating methods, this method is facile, one-step, and energy-effective, especially because it does not need external oxidation as the driving force of the polymerization, eliminating the risks of residual oxidant or oxidant by-products.
8.2 Experimental Section
All the chemicals were purchased from Sigma Aldrich and used without further purification. 8.2.1 Synthesis of NMHFC
3 m mol manganese acetate was dissolved into 100 m L deionized water, denoted solution A. 1.5 m mol Na4Fe(CN)6 ∙ 10H2O and 4 m mol sodium citrate were dissolved into 100 m L deionized water, denoted solution B. Then, solution A was added dropwise into the solution B under magnetic stirring. The suspension was aged for several hours before centrifuging. The white precipitate was washed with deionized water for several times and then dried at 120 °C for 10 h.
8.2.2 Synthesis of NMHFC @ PPy
In a typical process, the above prepared NMHFC powder (100 mg) was dispersed in 40 m L of DMC solution (Dimethyl carbonate), irradiated by ultrasound for 1h. 24 mg NaClO4 was added as dopant into NMHFC suspension. 45 μ L pyrrole which dissolved into 20 m L DMC solution, was slowly added to the previous DMC solution under magnetic stirring. The
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polymerization was carried out in ice bath (at 0 - 5 °C) for 6 h, and the nitrogen gas was blowed into the solution during the whole process. Then, NMHFC @ PPy powder was collected by centrifugation and washed with ethanol for several times. Finally, the NMHFC @ PPy powder was dried at 50 °C for 4 h under vacuum. For comparison, the pure PPy was synthesized in the similar method and the FeCl3 was used as the oxidant instead of NMHFC. 8.2.3 Structural characterization
The microstructure of the NMHFC @ PPy powders was characterized by Powder X-Ray diffraction (XRD; GBC MMA diffractometer) with Cu Kα radiation at a scan rate of 2° min-1. The morphology of the samples was investigated by field emission scanning electron microscopy (FESEM; JEOL JSM-7500FA) equiped with energy-dispersive X-ray spectroscopy (EDS). Raman spectra were collected using a JOBIN Yvon Horiba Raman spectrometer model HR800, with a 10 mW helium/neon laser at 632.8 nm excitation in the range of 150 to 2000 cm-1. X-ray photoelectron spectroscopy (XPS) was conducted using a SPECS PHOIBOS 100 Analyser installed in a high-vacuum chamber with the base pressure below 10–8 mbar, X-ray excitation was provided by Al Ka radiation with photon energy hν = 1486.6 eV at the high voltage of 12 kV and power of 120 W. The XPS binding energy spectra were recorded at the pass energy of 20 eV in the fixed analyser transmission mode. Analysis of the XPS data was carried out using the commercial CasaXPS 2.3.15 software package. All the spectrum were calibrated by C1s=284.6ev. FTIR spectra were performed by a FTIR Prestige-21 (Shimadzu). The amount of PPy in the composites was determined using a Mettler - Toledo thermogravimetric analysis / differential scanning calorimetry (TGA / DSC) Stare System from 50-600 °C at 10 °C min-1 in Ar.The magnetic measurements were carried out using a 14 T physical properties measurement system (PPMS), equipped with a vibrating sample magnetometer (VSM), over a wide temperature range from 5 K to 300 K in a 1000 Oe magnetic field. In situ synchrotron XRD data were collected on the powder diffraction beamline at the Australian Synchrotron with a wavelength (λ) of 0.6888 , determined using the NIST LaB6 660b standard reference material. Data were collected continuously in 30s acquisitions. The cell used for the data collection was charged at a current density of 50 mA g-1 between 2 V and 4.2 V.
8.2.4 Electrochemical characterization
NMHFC @ PPy electrodes were prepared by mixing 70% active materials, 20% carbon black and 10% carboxymethyl cellulose (CMC) used as binder by weight to form an electrode slurry, and then was coated on Aluminum foil, followed by being dried in a vacuum oven
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overnight at 80 °C, and then pressed with the pressure of 30 MPa. The active materials loading rate was ~7 mg cm-2.The sodium foil was cut by the doctor blade technique from the sodium bulk stored in the mineral oil, and then was employed as both reference and counter electrode. A porous polypropylene film was used as the separator. The home-made electrolyte was 1.0 mol/L Sodium perchlorate (NaClO4 ACS reagent 98.0%, Sigma-Aldrich) in a 1 : 1 (v/v) mixture of ethylene carbonate (EC, anhydrous 99%, Sigma-Aldrich) and diethyl carbonate (DEC, anhydrous 99+%, Sigma-Aldrich). The chemicals were used without further drying. The cells were assembled in an argon-filled glove box. The electrochemical performance were tested by a Land Test System in a voltage range of 2 ~ 4.2 V (vs. Na+/Na). All of the capacities and energy density are calculated based on the mass of active materials including PPy and NMHFC.