Improving the cycling stability of NaSubscript3

Authors: Huajun Zhou Z Ryan Tian Simon S Ang

Publish Date: 2016/02/08

Volume: 5, Issue: 1, Pages: 3-

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Abstract

Here we report the electrochemical performances of a Na3V2PO43/C nanocomposite as a cathode material for aqueous sodiumion batteries SIBs Compared to a previously reported Na3V2PO43 microparticle this nanocomposite demonstrated much improved cycling stability While the improvement mainly attributed to the right pH and the carbon matrixmediated protection against the electrolyte the capacity fade was mainly due to the deterioration of crystallinity and structure of the nanocomposite caused by various interactions between the nanocomposite and electrolyte This work not only help to understand the degradation of Na3V2PO43 in aqueous SIBs but also shed light on the design and fabrication of electrode materials with high cycling stability for aqueous SIBsCurrently widely used lithiumion batteries LIBs employ flammable and toxic organic solvents and expensive lithiumcontaining compounds and thus present two major issues of safety and cost and find limits in largescale applications To address these issues and meet the vast need for energy storage recently aqueous sodiumion batteries SIBs with their capacities comparable to those of LIBs have received considerable interests 1 2 3 4 5 On one hand aqueous systems are safer as they could be neither flammable nor toxic on the other hand sodium is much less expensive than lithium due to its much more abundant natural reserve Furthermore the high ionic conductivity 2 of aqueous electrolytes can support fast charge–discharge processesAmong electrode materials candidates for aqueous SIBs sodium vanadium phosphates with sodium Na super ion conductor NASICON structures such as NaVPO4F 6 Na3V2OPO42F 7 and Na3VTiPO43 8 have been widely investigated as cathodes for the following reasons First this structure features a highly covalent threedimensional 3D framework in which Naions can facilely diffuse in and out 9 second the strong covalent bonds can provide structural stability and safety Though extensive studies have been investigated on the use of Na3V2PO43 as electrode materials in nonaqueous SIBs 10 11 12 13 14 15 the use of Na3V2PO43 as electrode materials in aqueous SIBs was first studied by Ji and Banks only very recently 16 In that work Na3V2PO43 microparticle was tested as the cathode material and its capacities were found to decay in a rather significant fashion ca 31  capacity retention for the 30th cycle The mechanism responsible for the capacity fade was not investigated eitherHere we report the use of Na3V2PO43/C nanocomposite composed of Na3V2PO43 nanoparticles and a carbon matrix as the cathode material for aqueous SIBs The Na3V2PO43 nanoparticle provides shorter Naion diffusion lengths than its microsized counterpart does while the carbon support could enhance the electrical conductivity of the electrode 10 and slow down its dissolution 1 Compared to the previously reported Na3V2PO43 microparticle this nanocomposite indeed demonstrates much improved cycling stability The mechanism responsible for the capacity fade was also investigated hereThe nanocomposite was prepared by modifying a published procedure 10 To a roundbottom flask containing 30 mL tetraethylene glycol TEG was added 0246 g/3 mmol sodium acetate NaCH3COO 0697 g/2 mmol vanadium III acetylacetonate VOC5H7O22 and 0345 g/3 mmol ammonium dihydrogen phosphate NH4H2PO4 The mixture was then stirred overnight at room temperature to afford a homogenous green solution and the resultant solution was then heated at 320 °C for 72 h The precipitate was collected by centrifugation washed by ethanol and acetone each for 3 times respectively and dried in a vacuum oven at 80 °C for 2 h Then the resultant lightbrown powders were annealed at 650 °C under Ar/H2 40/2 sccm for 6 h and then at 800 °C under Ar flow 40 sccm for 6 h to afford black powdersThe phase purity was characterized by a Rigaku MiniFlex II Desktop Xray diffractometer using monochromatized CuKα radiation λ = 15418 Å at 30 kV and 15 mA A continuous scan mode was used to collect the diffraction data from 10 to 60° at a speed of 02°/min The morphologies were investigated by an FEI Nova NanoSEM scanning electron microscope SEM equipped with a field emission gun operated at 10 kV and by an FEI Titan 80–300 transmission electron microscope TEM To minimize charging problems samples for SEM were coated with thin Au layers Raman spectra were recorded on a homemade μRaman spectroscope that was composed of a 6328 nm He–Ne excitation laser an iHR 550 HORIBA spectrometer and a Si CCD detector Prior to Raman the measurement the system was calibrated using a bulk Si Elemental analysis was done in Atlantic Microlab Inc to determine the weight percentage ~20wt  of carbon species in the Na3V2PO43/C nanocomposite ICPMS analysis revealed the mole ratio of NaVP to be 300200300 further suggesting the pure phase of Na3V2PO43/CA threeelectrode beakertype cell was used to test the electrochemical performances of Na3V2PO43/C nanocomposite in an aqueous system A large piece of pure platinum a Ag/AgCl/1 M KCl electrode 0235 V vs NHE and 1 M Na2SO4 aqueous solution served as the counter electrode reference electrode and electrolyte respectively The working electrode was prepared by mixing the nanocomposite super P carbon black and polytetrafluoroethylene PTFE in a weight ratio of 75205 onto a stainless steel foil of ~03 cm2 area The electrodes were dried in a vacuum oven at 80 °C overnight and then pressed at a pressure of 20 MPa using a PHI manual compression presses The loading of each electrode was controlled to be 1–2 mg cm−2 and the thickness was ~50 μm Before the cell assembly the electrolyte was bubbled with Ar for 60 min while the platinum foil was first rinsed in acetone and then repeatedly ultrasonicated in deionized water Cells were assembled in an Ar atmosphere Cyclic voltammetry CV was performed on a Solartron SI 1287 Electrochemical Interface at a scan rate of 05 mV s−1 from 0 to 08 V vs the reference electrode while galvanostatic tests were performed on a BT 2000 battery tester The electrochemical performances of this nanocomposite in a nonaqueous system were also tested as a reference The electrode fabrication was the same as above except that here Al foil was used and electrodes were ~8 mm in diameter The loading and thickness of each electrode was 1–2 mg cm−2 and ~50 µm respectively 2032type coin cells were assembled in an MBraun glove box O2  01 ppm H2O  2 ppm while a Celgard 3501 microporous membrane was used as the separator CV was performed on a Solartron SI 1287 electrochemical interface at a scan rate of 005 mV/s while galvanostatic tests were performed on a BT 2000 battery tester 1 M NaClO4 in EC/DMC w/w = 12 and Na foil were used as the electrolyte and counter electrodea Ball stick model of a unit cell structure of Na3V2PO43 viewed along the a axis orange lines represent cell edges b spacefilling model of the anionic framework V2PO433− in which Naions reside indicating the channels are too small for physical adsorption of water molecules the scale bar represents the diameter of a water molecule

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