Graphene modified sodium vanadium fluorophosphate as a high voltage cathode material for sodium ion batteries

Accepted Manuscript Title: Graphene modified sodium vanadium fluorophosphate as a high voltage cathode material for sodium ion batteries Author: Yan-Li Ruan Kun Wang Shi-Dong Song Xu Han Bo-Wen Cheng PII: DOI: Reference:

S0013-4686(15)00232-7 http://dx.doi.org/doi:10.1016/j.electacta.2015.01.186 EA 24238

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

10-10-2014 26-1-2015 29-1-2015

Please cite this article as: Yan-Li Ruan, Kun Wang, Shi-Dong Song, Xu Han, Bo-Wen Cheng, Graphene modified sodium vanadium fluorophosphate as a high voltage cathode material for sodium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.01.186 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphene modified sodium vanadium fluorophosphate as a high voltage cathode material for sodium ion batteries Yan-Li Ruan1,2*, Kun Wang2, Shi-Dong Song2, Xu Han2, Bo-Wen Cheng1,2 1

State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic

University, Tianjin, P.R.China 2

School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin,

P.R.China *

Corresponding Author. Tel: +86-22-83955672; E-mail address: [email protected]

Abstract Substantial interests have been paid to Na-based electrode materials due to the abundance and low cost of sodium resources on earth. In this study, we improved the electrochemical performance of NaVPO4F cathode by coating graphene sheets. The structure, composition and morphology of NaVPO4F/graphene composite were investigated by XRD, FT-IR, FE-SEM and TEM. FE-SEM and TEM analysis indicate that the graphene sheets were successfully coated on the surface of NaVPO4F. The NaVPO4F/graphene composite could be operated at a high working potential (3.95 V for average voltage), and displayed the highest capacity of 120.9 mAh g-1. After 50 charge/discharge cycles at 0.05 C, the capacity retention of the cathode could reach up to 97.7%. Although the discharge capacity of NaVPO4F/graphene decreased to 70.1 mAh g-1 under a high rate of 0.5 C, it returned back to 113.2 mAh g-1 once the rate was reduced to 0.05 C. EIS measurement further reveals that the NaVPO4F/graphene electrode exhibited a faster sodium-ion diffusivity and less resistance compared with 1

the pure NaVPO4F. Our results demonstrate that NaVPO4F/graphene is a promising cathode material for sodium ion batteries. Keywords: Sodium ion batteries; Sodium vanadium fluorophosphate; Cathode material; Electrochemical performance 1. Introduction Wide-scale implementation of renewable energy such as solar and wind power requires the large scale production of inexpensive and efficient energy storage systems (ESSs). At present, lithium-ion battery technology is quite mature and has been applied in ESSs. However, as the use of large format lithium-ion batteries (LIBs) becomes widespread in electric vehicles, increasing demands for Li commodity chemicals combined with geographically-constrained Li mineral reserves will drive up prices and limit their applications in ESSs [1]. Therefore, the development of cheaper and more sustainable energy storage technology as alternatives to LIBs is important [2-4]. Sodium-ion batteries (SIBs) have been considered as an attractive alternative to LIBs due to the abundant resources, low material cost and easy accessibility of new reserves. In addition, the well-established understanding of the lithium based electrochemical system is beneficial to the development of Na counterpart. Therefore, SIBs can compete with LIBs in the markets where weight and size are not important such as stationary land-based power applications and electrical grid stabilization [5]. The battery performances such as specific capacity and operation voltage are mainly determined by the electrochemical properties of electrode materials [1]. The 2

search for new intercalation host materials with high energy density batteries is essential to push forward the implementation of new SIB technologies [1, 6-7]. Recently, more attention has been paid to sodium vanadium phosphates Na3V2(PO4)3 [8-10] and sodium vanadium fluorophosphates materials, such as NaVPO4F [11-12], Na3V2(PO4)2F3 [13-14], Na3(VO)2(PO4)2F [15] and Na3V2O2x(PO4)2F3-2x [16], because of their good electrochemical performances. Among these cathode materials, sodium vanadium fluorophosphate (NaVPO4F) shows good electrochemical properties, especially a high energy density. The inductive effects of both PO43- and Fanions lead to a high working potential of this material (plateaux at 3.7 and 4.2 V vs. Na/Na+) [17]. Meanwhile, a high specific capacity and a good cycling ability can also be expected due to the structural rigidity of the fluorophosphate material [11]. Barker et al. [11] combined NaVPO4 F and hard carbon to form sodium-ion batteries, and found that the reversible specific capacities for cathode and anode were 82 and 202 mAh g-1, respectively. However, the discharge capacity of this battery decayed significantly after the cell cycled more than 30 times. In order to improve the electrochemical performance of NaVPO4 F, Zhuo and co-workers [18] doped Cr3+ into NaVPO4F. Although the reversible specific capacities were improved insignificantly, the Cr-doped material showed a better capacity retention than the un-doped material, and the reversible capacity retention of the Cr-doped material could maintain 91.4% after 20 cycles. Carbon coating is an effective way to improve the electrode performance [19-21]. Recently, Lu et al. [19] improved the electronic conductivity and electrochemical 3

performance of the NaVPO4F via carbon coating, and found that the highest capacity of this material could reach 97.8 mAh g−1 and the capacity retention was 89% after 20 cycles. Graphene (G), the basic building unit of all carbon materials (fullerenes, carbon nanotubes, graphite, etc.) has remarkably high electric conductivity and has been widely investigated in LIBs [22]. To the best of our knowledge, few studies have been

reported

on

the

electrochemical

performances

of

sodium-vanadium

fluorophosphates materials modified with graphene (NaVPO4F/G). In this study, we report the synthesis and electrode performance of NaVPO4F/G in a sodium cell. Our results showed that graphene was an effective carbon source to obtain the micrometer-sized NaVPO4F by a simple solid state method, and the electrode performance was drastically improved. The highest capacity achieved for NaVPO4F/G was 120.9 mAh g-1 and the capacity retention was 97.7% after 50 cycles at 0.05 C. When it was tested at 0.5 C, the initial discharge capacity was 70.1 mAh g-1 and it could be maintained after 10 cycles. From these results, we demonstrated the possibility of graphene coated NaVPO4F as the positive electrode for SIBs. 2. Experimental 2.1. Sample preparation NaVPO4F/graphene (NaVPO4F/G) composite was synthesized via two step solid-state reaction. In the first step, VPO4 powders were synthesized via a carbothermal reduction (CTR) method. Stoichiometric proportions of NH4H2PO4, V2O5 and 5 wt% excessive glucose powder which could ensure the complete reduction of vanadium were put in the agate jar and ball-milled at 500 rpm for 12 h. 4

The mixture was then pressed into a pellet and heated at 750 oC for 5 h in N2 atmosphere. The reaction was shown in Eq. (1). 3V2O5+6NH4H2PO4+C6H12O6→6VPO4+6NH3+15H2O+6CO

(1)

In the second step, stoichiometric proportions of NaF, VPO4 and a 10% mass of graphene were ball-milled for 6 h at 500 rpm. After that, they were transferred to a temperature-controlled tube furnace and heated to 750 oC in N2 atmosphere for a dwell period of 1 h. The reaction was shown in Eq. (2). NaF+VPO4+Graphene→NaVPO4F/G

(2)

Graphene could be obtained by chemical reduction of graphene oxide (GO) by hydrazine hydrate. GO was prepared using a modified Hummers method [23]. GO was then washed and exfoliated before centrifugation. After that, the centrifuged GO was dried under vacuum conditions for 24 h. NaVPO4F in the absence of graphene was also synthesized and used as the control. 2.2. Material characterization In order to understand the crystalline structure of the material, X-ray diffraction (XRD) analysis was conducted using X-ray diffractometer (D8 DISCOVER) with Cu Kα radiation in the 2θ range of 10 ~ 60o with a scan rate of 5o min-1. The morphology of the material was observed using field emission-scanning electron microscopy (FE-SEM, Hitachi, S4800) and transmission electron microscopy (TEM, JEOL, 2100F). The infrared (IR) spectrum was obtained using a FT-IR spectrometer (FTIR, Thermo scientific, Nicolet 6700) under a transmission mode in the range of 500 2000 cm-1. 5

2.3. Electrochemical properties The cathode mixture was composed of 80 wt% the active material (NaVPO4F/G or NaVPO4F), 10 wt% conductive carbon (acetylene black) and 10 wt% polyvinylidenefluoride

(PVDF)

binder,

which

was

then

dissolved

in

1-methyl-2-pyrrolidone (NMP). The mixed slurry was plastered onto the aluminum foil as the current collector, which was then dried at 80 oC under vacuum for 24 h. The circular electrodes in a diameter of 10 mm were cut from the foil, dried at 120 oC under vacuum overnight and finally transferred in an argon filled glove box (MBRAUN, LABstar). The mass loading of the active material in the electrode was approximately 3.0 mg. 2032 coin-type testing cells were assembled with the as-prepared cathode, an anode (metallic sodium) and a glass microfiber separator (Whatman, GF/A). The electrolyte was 1 M NaClO4 dissolved in propylene carbonate (PC). Galvanostatic charge/discharge reactions were carried out in the voltage of 1.5 4.3 V using the CT2001A LAND battery tester. The charging rates in the measurements were based on the following relationship: 1C=143 mA g-1. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured using an electrochemical workstation (ChenHua, CHI660e), respectively. The CV characterization was performed at a scan rate from 0.1 to 1 mV s-1, and the EIS was characterized the amplitude of 5 mV in the frequency range of 0.1-100 mHz. All electrochemical tests were carried out at room temperature. 3. Results and discussion 3.1. Material Characterization 6

The XRD patterns of the as-synthesized NaVPO4F and NaVPO4F/G powders are shown in Fig. 1. The position and intensity of the diffraction peaks agree well with those reported by Barker et al. [11]. As it is shown in Fig. 1, the peak positions, intensity ratios and peak sharpness of these two samples are similar, indicating the good crystallinity of NaVPO4 F and NaVPO4F/G (Fig.1). Structural refinement of the data using the hkl indices for a tetragonal symmetry structure with space group I4/mmm generated the following crystallographic parameters: a = b = 0.641 nm, c = 1.068 nm, which are in good accordance with the structure analysis of NaVPO4 F phase (a = 0.639 nm, c = 1.073 nm [11]) and Na3Al2(PO4)3F2 (a = 0.621 nm, c = 1.042 nm [24]). This structure demonstrates the facile diffusion of sodium ion through an extended three-dimensional framework stacked by VO4F2 octahedra and PO4 tetrahedra. FT-IR spectra of NaVPO4F and NaVPO4F/G in Fig. 2 show that the vibration from V3+-O2− bonds in VO4F2 octahedra is detected at 630 and 892 cm−1 [10]. However the bands at 760 and 950 cm-1 which are characteristic peaks of V5+ in VO4F2 octahedra are not observed. This indicates that V5+ in V2O5 is completely reduced to V3+ in both NaVPO4F and NaVPO4F/G composites. The presence of P-O bonds of PO4 tetrahedra is detected at 578 and 1056 cm−1 [10]. The infrared bands in the range of 1150-1250 cm−1 can be attributed to the stretching vibration of PO4 units [25]. It is well known that FT-IR spectrum of phosphates is mainly consisted of the inner-vibrated mode peaks of PO43- group [18]. From the FT-IR spectra of the samples, it can therefore be confirmed that this material is in the form of phosphate salts. 7

Moreover, in the FT-IR spectrum of NaVPO4F/G, it is found that the absorbance of peak increases compared with NaVPO4F, and the band peak moves to higher wave numbers. It is explained that the strength of V-O band increases in NaVPO4F/G. It therefore can be expected that the stability of materials is enhanced and the cycle performance is improved by coating graphene on NaVPO4F. SEM and TEM analysis were used to characterize the structure and morphology of the samples. Significant differences of morphology and particle sizes between NaVPO4F and NaVPO4 F/G can be found in Fig. 3a and b. The particle size of NaVPO4F is approximately 1 μm, and some ellipsoid shape particles are found in the sample (Fig. 3a). Compared with the un-modified sample, the particle size of NaVPO4F/G is smaller, and exfoliated graphene sheets can be clearly detected on the particles, which alleviate the agglomeration of the particles (Fig. 3b). The introduced graphene in NaVPO4 F improves the contact between the electrode material and the graphene, which is beneficial for the improvement of electrochemical performance. TEM image (Fig. 3c) shows that graphene was successfully coated on the surfaces of NaVPO4F particles. Clear and continuous lattice-fringe observed in Fig. 3d indicates the high crystallinity of NaVPO4F/G, and the distance of 0.287 nm between neighboring fringers is corresponded to the (222) plane of NaVPO4 F/G. 3.2. Electrochemical evaluation The electrochemical performance of NaVPO4 F and the NaVPO4F/G composite was investigated by cyclic voltammetry and galvanostatic cycling. Cyclic voltammetry was carried out between 3.0 and 4.3 V vs. Na/Na+ at scanning rate from 8

0.1 to 1 mV s-1. The comparison of the two samples between the voltammograms at a low scan rate (0.1 mV s-1) is shown in Fig. 4a. Two pairs of peaks in the anodic and cathodic sweeps are found in CV curves, which agree well with the two voltage plateaus of the charge/discharge curves for the cathode materials in Fig. 5. It suggests that the intercalation and deintercalation of sodium ions are carried out in two steps. In the low voltage range, both pristine NaVPO4F and NaVPO4F/G show a pair of sharp and reversible peaks at around 3.65 V, which related to the V3+/V4+ redox couple, indicating the good electrode kinetic for both electrodes. However, the peak intensity of the NaVPO4F is weaker than that of NaVPO4F/G, suggesting the NaVPO4F/G cathode has a higher activity in the electrochemical reaction [26]. In the high voltage range, the broadness of the redox peaks of the two samples implies the complicated electrochemical mechanism induced by F atom, which increases the equilibrium voltage of V3+/V4+ redox couple [27]. The potential differences between anodic and cathodic peaks for NaVPO4F and NaVPO4F/G are calculated and are listed in Table 1. The φpla and φpha are the potential of anodic peaks, φplb and φphb are the potential of cathodic peaks, Δpl and Δph are the difference between anodic and cathodic peak potentials. Obviously, NaVPO4F/G composite shows a lower potential difference (Δpl = 0.167 V, Δph = 0.164 V) than the pure NaVPO4F (Δpl = 0.281 V, Δph = 0.187 V), indicating NaVPO4F/G has a lower polarization and a higher reversibility of the sodium ion intercalation. The cyclic voltammograms of the NaVPO4F/G composite at different scan rates are shown in Fig. 4b. The anodic (extraction) potential shifts to higher voltages and 9

the cathodic (insertion) potential shifts to lower voltages with the increase of the scanning rate from 0.1 to 1 mV s-1. This is due to the increased polarization at higher sweep rates because of kinetic limitations are associated with the sodium diffusion through the active material [16]. As the scanning rate increased, the peak current values increased, which agrees with the Randlese-Sevcik equation [28]: i p  (2.69  105 )n3/ 2 AD1/ 2 Cv1/2

(3)

where ip is the peak current, n is the electron stoichiometry, A is the electrode area, D is the diffusion coefficient, C is the concentration, and v the scan rate. The initial charge/discharge curve of NaVPO4F and NaVPO4 F/G composites is shown in Fig. 5. The cells were galvanostatically charged and discharged at different current density from 0.05 to 0.5 C between 1.5 and 4.3 V. In Fig. 5a, there are two charge plateaus at 4.2 V and 3.8 V as well as two discharge plateaus at 4.1 V and 3.6 V as for the NaVPO4F/G composite, which agree with the CV results (Fig. 4). The charge/discharge profile of NaVPO4F is comparable to that of NaVPO4F/G composite (Fig. 5b). The average voltage of NaVPO4 F (ca. 3.95 V) is one of the highest cathode materials with the same redox couple V3+/V4+ (for example, 3.4 V for the Na3V2(PO4)3 [29]). Moreover, it is higher than that of Na2V2O5 with V4+/V5+ redox couple (ca. 3.1 V [30]). The high potential of V3+/V4+ in this material is believed to originate from the stronger inductive effects of fluorine ion as well as phosphorus ion than that of oxygen, which substantially increases the equilibrium potential of the V3+/V4+ redox couple [11]. The high voltage results in a higher energy density of the electrode, which is beneficial for practical applications. The NaVPO4F/G composite 10

exhibits the initial discharge capacity of 120.9, 99.1, 89.3 and 69.2 mAh g-1 at 0.05, 0.1, 0.2 and 0.5 C rates, respectively (Fig. 5a). While NaVPO4F delivers a relatively lower initial discharge capacity of 103.2, 82.9, 71.3 and 49.6 mAh g-1 compared with NaVPO4F/G (Fig. 5b). It is worthwhile noting that with the increase of the current, the electrodes showed a shorter charge/discharge curve, a smaller reversible capacity, a higher charge voltage and a lower discharge plateau, which is due to the increase of electrode polarization. Compared with NaVPO4 F, the polarization of the NaVPO4 F/G electrode was greatly alleviated. The significant improvement of the electrochemical properties can be attributed to the enhanced electronic/ionic conductivity of the NaVPO4F/G composite. Rate capabilities of NaVPO4F/G and NaVPO4F with different discharge rate from 0.05 to 0.5 C are shown in Fig. 6a. Both NaVPO4F and NaVPO4F/G cathodes exhibited good cycling stabilities under different charge/discharge currents. This indicates that NaVPO4F structure maintained stable even under a high current, which is a promising property required for high power application. The specific capacity of the NaVPO4F/G composite maintained 119.2 mAh g-1 at a rate of 0.05 C after 10 cycles. Although the discharge capacity of NaVPO4F/G decreased to 70.1 mAh g-1 under high rate (0.5 C), it returned back to 113.2 mAh g-1 once the rate was reduced to 0.05 C again. Compared with the NaVPO4F/G composite, the discharge capacity of the pure NaVPO4F was 95.4 mAh g-1 when the rate changed from 0.5 to 0.05C. Fig. 6b shows the specific capacity and coulombic efficiency of the NaVPO4F/G composite during charge and discharge cycling at 0.05 C. The capacity retention was 11

97.7% of the initial specific capacity after 50 cycles. A low coulombic efficiency was observed in the first cycle, and it gradually increased to 97% after 6 cycles and maintained consistent after 50 cycles. Compared with the initial capacity of 97.8 mAh g−1 and the capacity retention of 89% after 20 cycles as reported previously [19], the NaVPO4F/G composite in this study is more competitive. The improved capacity and rate capability of NaVPO4F/G cathode are due to the well coating of graphene on NaVPO4F surface. The graphene network improves the electronic conductivities of NaVPO4F and decreases the electrochemical impedance. Furthermore, the presence of graphene inhibits the growth of NaVPO4F particles during the high temperature calcination process and results in small particles of NaVPO4F, which shortens the transport path of Na+ to migrate inside of the active material. On the other hand, graphene coating can also stabilize the structure of NaVPO4F and prevent electrode from directly contacting with electrolyte. To further demonstrate the role of graphene in NaVPO4F/G composite, the electrochemical impedance spectroscopy (EIS) was measured. Fig. 7a shows the Nyquist plots collected at a 5.0 mV AC voltage signal in the frequency range of 0.1 -100 mHz. Nyquist plots of the two NaVPO4F cathodes are composed of a semi-circle and a sloping line, representing the high frequency component and the low frequency component, respectively. The semi-circle in the high frequency region is due to the charge transfer resistance (Rct), while the sloping line corresponds to the diffusion of sodium ions in the electrode bulk, namely the Warburg impedance [31-32]. As shown in Fig. 7a, the charge transfer resistance of the NaVPO4F/G composite is much 12

smaller than that of the normal electrode, suggesting that the graphene can reduce the electrochemical reaction impedance. The EIS results were also analyzed with the software of Zsimpwin using the equivalent circuit model shown in the inset of Fig. 7a, and the fitted values are listed in Table 2. In this equivalent circuit model, Rs represents the solution resistance, Rct is the charge transfer resistance, Zw is the Warburg impedance and CPE is the constant phase element. From the fitted data (Table 2), the Rs value of the NaVPO4F/G electrode is slightly lower than that of the pure NaVPO4F electrode. However, Rct of NaVPO4F/G is much smaller than that of the pure NaVPO4F. The decrease is ascribed to the improved electronic conductivity and less contact between the active material and electrolyte due to the graphene coating layer. The EIS is also used to calculate the sodium ion diffusion coefficient (D) using Eq. (4) [32]:

D

0.5R 2T 2 S 2 n 4 F 4C 2  2

(4)

Where R is the gas constant, T is the absolute temperature, S is the surface area of the cathode (S = 0.79 cm2), n is the number of electrons transferred in the half-reaction for the redox couple of V3+/V4+ (n = 1), F is the Faraday constant, C is the concentration of sodium ion in solid calculated based on the crystallographic cell parameter of NaVPO4F (C = 7.29×10-4 mol cm-3), and σ is the Warburg factor, which has a relationship with Z' as follows [33]: Z '  RS  RCT  1/ 2

(5)

Fig. 7b displays the linear fitting of Z' vs. ω-1/2. The values of σ are obtained through linear fitting by Eq. (4), and are listed in Table 2. The ratio (K) of the sodium 13

ion diffusion coefficient between NaVPO4F and NaVPO4F/G is calculated according to Eq. (6):

K

DNaVPO4 F /G DNaVPO4 F



 2NaVPO4 F

(6)

 2NaVPO4 F / G

As a result, the chemical diffusion coefficients (D) are 1.854×10-12 and 5.902× 10-12 cm2 s-1 for the pure NaVPO4F and the NaVPO4F/G composite, respectively. The magnitude of D value in this study is comparable with that of Na3V2(PO4)2F3 [31], and is significantly higher than that of LiFePO4 measured by Prosini et al. (10-16-10-14 cm2 s-1) [34] and Franger et al. (10-14-10-13 cm2 s-1) [35]. The ratio of the sodium ion diffusion coefficient K is 3.18. EIS results further confirm that kinetics of sodium ion diffusion and electron conduction were enhanced by introducing graphene into the cathode, and thus the electrochemical performance of NaVPO4F/G was significantly improved. NaVPO4F/G is therefore the promising cathode material for SIBs because of the higher discharge capacities as well as its high potential. 4. Conclusions Graphene modified NaVPO4 F as cathode material for sodium-ion batteries has been successfully synthesized through a simple solid-state reaction. Compared with the pure NaVPO4F, the NaVPO4F/G composite exhibited better electrochemical performance. The highest capacity achieved for NaVPO4F/G was 120.9 mAh g-1 and the capacity retention was 97.7% after 50 cycles at 0.05 C. When it was tested at 0.5 C, the initial discharge capacity was 70.1 mAh g-1 and it could be maintained after 10 cycles. EIS results further demonstrated that graphene network improved the electronic conductivities, decreased the cathode impedance and contributed to 14

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Figure Captions Fig.1 The XRD patterns of NaVPO4F/G composite (a) and pure NaVPO4F (b). Fig.2 The FT-IR spectrum of NaVPO4F/G composite and the pure NaVPO4F recorded in the range of 500~2000 wavenumbers. Fig.3 SEM images of (a) pure NaVPO4F and (b) the NaVPO4F/G composite. TEM images of (c) and (d) are the NaVPO4F/G composite. Fig.4 Cycle voltammograms of NaVPO4F/G composite and pure NaVPO4F at a scan rate of 0.1 mv s-1 (a), Cycle voltammograms of NaVPO4F/G composite at different scan rates (b). Fig.5 Initial charge-discharge profiles of NaVPO4F/G (a) and NaVPO4F (b) at different C-rates. Fig.6 (a) Cycling performances at various C-rates for NaVPO4F/G composite and pure NaVPO4F in a voltage range of 1.5-4.3 V, and (b) specific capacity and coulombic efficiency for NaVPO4F/G during cycling at rate of 0.05 C. Fig.7. (a) Nyquist plots of the pure NaVPO4F and the NaVPO4F/G composite, the insert shows equivalent circuit model. (b) Linear fitting of Z' vs. ω-1/2. 19

333 125 440 135 016

330

124 332

222

300 310 311

211 0 0 2 (G)

110 111 020

Intensity / a.u.

(a)

(b)

10

20

30

40

50

60

2 / degree

3+

2-

)

2000

PO4)

1500

1042 1056

20

1172

40

630

NaVPO4F/G

578

880

(V -O

892

60

NaVPO4F

1182

Transmittance (%)

80

619 569

Fig. 1

1000

Wavenumber / cm-1

Fig. 2

20

 P-O

500

Fig. 3

21

100

(a)

NaVPO4F/G

Current / A

NaVPO4F 50

0

-50

-100 2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.2

4.4

Potential vs. Na / V

750 600

(b)

0.1 mv/s 0.3 mv/s 0.5 mv/s 0.7 mv/s 1 mv/s

Current /A

450 300 150 0 -150 -300 -450 -600 2.8

3.0

3.2

3.4

3.6

3.8

4.0

Potential vs. Na / V Fig. 4

22

Potential vs. Na / V

4.5

(a)

Charge

4.0

0.05 C 0.1 C 0.2 C 0.5 C

3.5 3.0 2.5 2.0 1.5

Discharge

-20

0

20

40

60

80

100

120

140

-1

Specific capacity / mAh g

Potential vs. Na / V

4.5

(b)

Charge

4.0

0.05 C 0.1 C 0.2 C 0.5 C

3.5 3.0 2.5 2.0 1.5

Discharge 0

20

40

60

80

100 -1

Specific capacity / mAh g Fig. 5

23

120

NaVPO4F/G NaVPO4F

(a)

140

Specific capacity / mAh g

-1

0.05 C 120

0.05 C

0.1 C

100

0.2 C

80

0.5 C

60 40 20 0

0

10

20

30

40

50

Cycle number

(b)

Specific capacity / mAh g

140

120

120

100

100

80

80

60

60

40 20 0

40

Discharge capacity, 0.05 C Coulombic efficiency

20 0

0

10

20

30

Cycle number Fig. 6

24

40

50

Coulombic efficiency (%)

-1

140

800 (a)

NaVPO4F

700

NaVPO4F/G

-Z'' / ohm

600 500 400 300

CPE RS

200 100 0

Rct ZW

0

100

200

300

400

500

600

700

800

Z' / ohm

700

(b)

NaVPO4F/G NaVPO4F

Z' / ohm

600 500 400 300 200 0.0

0.2

0.4

0.6

0.8

1.0 -1/2



/s

Fig. 7

25

1.2 1/2

1.4

1.6

1.8

2.0

Table 1 Values of the CV peaks for the NaVPO4F and NaVPO4F/G composite. sample

φpla(V)

φplb(V)

Δpl(V)

φpha(V)

φphb(V)

Δph(V)

NaVPO4F

3.812

3.531

0.281

4.235

4.048

0.187

NaVPO4F/G

3.760

3.593

0.167

4.213

4.049

0.164

Table 2 ACI results for the NaVPO4F and NaVPO4F/G composite. sample

RS (Ω cm-2)

RCT (Ω cm-2)

σ (S s1/2 cm-2)

CPE (F cm-2)

NaVPO4F

7.64

214.50

241.31

4.30E-6

NaVPO4F/G

6.45

196.51

135.25

4.21E-6

26