One-Dimensional Structure Comprising Oriented Na3V2(PO4)3 Nanoparticles in a Carbon Sheath for a Na Cathode Material

Introduction

Development of Na-ion batteries is desired as alternative energy storage devices to Li-ion batteries. NASICON frameworks (Na₃M₂(XO₄)₃; X = Si⁴⁺, P⁵⁺, S⁶⁺, Mo⁶⁺, As⁵⁺) are suitable to Na-ion intercalation electrode materials due to pass for fast ion transport. In particular, Na₃V₂(PO₄)₃ is a promising candidate for cathode materials in Na-ion batteries, because Na₃V₂(PO₄)₃ exhibits relatively high redox potential of V³⁺/V⁴⁺ at 3.37 V (vs. Na/Na⁺) and moderate theoretical capacity of 117.6 mAh/g. However, low electronic conductivity and lattice volume change during sodiation/desodiation of Na₃V₂(PO₄)₃give rise to low performance about micrometer-scale particles that are often yielded through traditional solid-state reaction.

Structural and morphological control can improve electrochemical properties of electrode materials. In the present study, we have designed mesostructure that consists of oriented Na₃V₂(PO₄)₃ nanoparticles in a carbon sheath, so as to meet the requirements as high-performance Na-ion electrode materials. The assembly comprising oriented nanoparticles possesses a short Na-ion transport pass and effectively relieves the elastic strain during sodiation/desodiation. In addition, the carbon sheath surrounding nanoparticles supplies a sufficient electron conductive pass. These features enable the long cycle performance and high rate capability of the electrode.

Experimental Section

The typical procedure for preparation of Na₃V₂(PO₄)₃ nanowire with the mesostructure from electrospun precursor fiber is as follows: 0.15 mol·dm⁻³ NH₄VO₃ and stoichiometric amount of NH₄H₂PO₄ and NaOH were added to 10 ml of citric acid aqueous solution (0.12 mol·dm⁻³). To dissolve the reagents, the aqueous solution was aged for 1 h at 90 ºC. After cooling at room temperature, 60 g·dm⁻³poly(acrylic acid) was dissolved into the solution. Precursor fiber obtained by electrospinning was heated at 800 ºC for 10 h in Ar flow condition.

The resultant materials were observed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For crystal structural analysis, X-ray diffraction was measured with Cu Ka radiation in steps of 0.01° over the range from 10 to 80º. Thermogravimetric analysis was conducted up to 1000 °C in air flow condition. Raman spectroscopy was recorded to confirm existence of amorphous carbon.

Electrochemical properties were measured in three electrode beaker cells. The resultant nanowire including amorphous carbon (85 wt%) were ground with further acetylene black (10 wt%) and polytetrafloroethylene (5 wt%) into a paste for electrochemical measurements. Sodium metal pressed on a SUS-304 mesh was used for counter and reference electrodes. 1M NaClO₄propylene carbonate solution was used as electrolyte. The cutoff voltages were 3.8 V for charging (Na-ion extraction) and 2.5 V for discharging (Na-ion insertion). Specific current rate and capacity were calculated for only active materials.

Results and Discussion

Obtained compounds were confirmed with powder X-ray diffraction (XRD) measurement and the Rietveld refinement. The XRD pattern was refined by Na₃V₂(PO₄)₃. Morphology of the product was observed with scanning electron microscopy (SEM). Precursor fiber exhibited fibrous morphology, estimated to be 500 nm in diameter with SEM. Even after the heating treatment, nowoven fabric consisting of nanowires was observed, though the diameter is reduced to 200 nm.

To confirm the formation of the mesostructure, transmission electron microscopy (TEM) obeservation was carried out. Figure 1a shows core-sheath structure in nanowires, where amorphous carbon sheath that is around 50 nm in thickness enfolds with aggregated nanoparticles 20-50 nm in size. Indeed, the Raman spectrum of the nanowires showed the broaden G-band and D-band of amorphous carbon around 1580 cm^-1 and 1350 cm^-1. The thermogravimetric curve of nanowire measured in air flow condition also suggested the existence of the carbon sheath, the amount of which in the product was estimated to be around 12 wt%. Selected-area electron diffraction (SAED) analysis of the nanowire shows an arched pattern, suggesting that nanoparticles in the carbon sheath are crystallographically oriented into the carbon sheath (Figure 1b).

The electrochemical properties of the nanowire were evaluated by the charge-discharge experiments at various current rate (0.1, 0.2, 0.5, 1 and 2C rate). The charge-discharge curves show the flat plateau potential of the redox V⁴⁺/V³⁺ couple at 3.37 V vs. Na/Na⁺, which is in good agreement with previous studies. The core-sheath nanowire shows an initial discharge capacity of 109 mAh/g at 0.1 C rate (theoretically 117.6 mAh/g) (Figure 2) and the initial discharge capacity at 1C rate is 94 mAh/g (80% of the theoretical capacity). Comparing with electrochemical properties of bulk Na₃V₂(PO₄)₃ that were obtained by solid-state reaction, The core-sheath nanowire exhibited lager capacity. The designed structure enabled efficient charge-discharge reaction at moderate current rate.