ORIGINAL PAPER

Influence of carbon polymorphism towards improved sodiumstorage properties of Na3V2O2x(PO4)2F3-2x

P. Ramesh Kumar1 & Young Hwa Jung1 & Do Kyung Kim1

Received: 29 December 2015 /Revised: 11 July 2016 /Accepted: 11 August 2016 /Published online: 23 August 2016# Springer-Verlag Berlin Heidelberg 2016

Abstract The effects of 10 wt.% super P carbon, multiwalledcarbon nanotubes (MWCNTs), and reduced graphene oxide(rGO) on particles size and electrochemical properties ofNa3V2O2x(PO4)2F3-2x cathode material were studied. Amongthese three carbon polymorphs, using MWCNT in compositeis effective to obtain low particle size and enhanced electro-chemical properties of Na3V2O2x(PO4)2F3-2x. While usingrGO to make the composite, the particle size becomes quitebig, leading a smaller surface contact area with the electrolytesand thereby resulting in poor cyclability. Na3V2O2x(PO4)2F3-2x-MWCNT composite shows a high capacity of 98 mAh g−1at0.1 C rate for 100 cycles, and further, it exhibits a stable capacityof 89mAhg−1 at 0.2 C rate vs. NaTi2(PO4)3-MWCNTin full cellconfiguration.

Keywords Sodium ion . Sodium battery . Vanadium .

Oxy-fluorophosphate . Phosphate-carbon composite .

Polymorphism reduced . Graphene oxide-electrochemicalproperties

Introduction

During the last 5 years, sodium-ion battery (SIB) researchis focused which is highly possible to make a low-cost

energy storage device, which is also alternative for con-ventional lithium-ion batteries. Considerable research ef-forts have been made to investigate high-voltage,ecofriendly, and high-capacity electrode materials forSIBs [1–5]. Especially in cathode side, phosphate-basedmaterials have overhand due to their high-voltage andopen frame structure [6]. The reported sodium transitionmetalphosphates are FePO4, Na3V2(PO4)3, Na2MPO4F (M = Fe, Mn,and Co), NaVPO4F, Na3V2O2(PO4)2F, Na3V2(PO4)2F3,Na3V2O2x(PO4)2F3-2x, and Li1.1Na0.4VPO4.8F0.7 [7–21].Among these phosphates, Na3V2O2x(PO4)2F3-2x is particularlyattractive, because it has a theoretical specific capacity of128 mAh g−1 with two Na+ per formula unit cycled reversiblyand within the very low volume expansion (<2 %) during thecycling. Thus, Na3V2(PO4)2F3 and Na3V2O2(PO4)2F com-pounds have V+3 and V+4 oxidation states, respectively,whereas Na3V2O2x(PO4)2F3-2x intermediate compoundsare having V+3 and V+4 mixed-valence phases. InNa3V2(PO4)2F3, the fluorine is replaced by the oxygen,which will make this as a high stable Na3V2O2x

(PO4)2F3-2x cathode material. Moreover, Na3V2O2x

(PO4)2F3-2x shows high operating voltages at 3.7 and4.1 V vs. sodium [17]. Nevertheless, there is one com-mon problem that needs to overcome in the preparationof sodium-vanadium oxy-fluorophosphate, which is lowelectronic conductivity. The material poor electronicconductivity (10−7 S cm−1) nature requires compositewith carbon-based materials to enhance the electrontransport properties. Hence, researchers have made dif-ferent variety of composite electrode materials for SIBapplications [22–25]. For high-conductivity compositeelectrodes, amorphous carbon is the most widely useddue to their large surface area, low cost, and easy pro-cess. However, it shows relatively lower reversible ca-pacity and inferior rate capability. Other carbon

Electronic supplementary material The online version of this article(doi:10.1007/s10008-016-3365-6) contains supplementary material,which is available to authorized users.

* Do Kyung Kimdkkim@kaist.ac.kr

1 Department of Materials Science and Engineering, Korea AdvancedInstitute of Science and Technology, Daejeon 34141, Republic ofKorea

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polymorphs, thus, such as graphene and carbon nano-tubes (CNTs) can be exploited as potential candidatesfor making high-conductivity composites because oftheir fascinating electrical and mechanical properties aswell as their unique 2D and 1D structural features [26,27].

In the current study, we focus on the electrochemical per-formance of carbon, graphene, and multiwalled carbon nano-tube (MWCNT)-based Na3V2O2x(PO4)2F3-2x composite ma-terials for use as SIB-positive electrodes. Herein, we present asynthesis of Na3V2O2x(PO4)2F3-2x submicron squares alongwith three different carbon polymorphs (conductive carbon,MWCNT, and reduced graphene oxide (rGO)) to understandthe effect the carbon forms on synthesis and electrochemicalproperties of Na3V2O2x(PO4)2F3-2x. The obtained submi-cron square Na3V2O2x(PO4)2F3-2x/MWCNT compositeprovides high capacity, good rate capability, and cyclingstability when compared to using other polymorphs, i.e.,conducting carbon and rGO. The Na3V2O2x(PO4)2F3-2x/MWCNT composite provides a stable and unbreakableelectronic conducting network to boost the electrochem-ical performance of the highly insulating electrodematerials.

Experiment

Synthesis and characterization of Na3V2O2x(PO4)

2F3-2x/carbon composites

Na3V2O2x(PO4)2F3-2x/carbon composite samples were preparedunder mild hydrothermal conditions by reacting NaF and[V(PO3)3]n/carbon in a 3.3:1 M ratio. The reaction mixture wassealed in a polytetrafluoroethylene (PTFE)-lined steel pressurevessel, which was maintained at 170 °C for 72 h. The excesscarbon source prevents the complete oxidation from V3+ to V4+

in an aqueous medium. [V(PO3)3]n/carbon is the important pre-cursor for above synthesis and prepared by a solid state reactionwith stoichiometric amounts of V2O5, NH4H2PO4, and GO/super P carbon (Timcal)/MWCNT (CM 95, Iljin Nanotech Co.,Ltd.), which act as a reducing agent (Fig. 1). All of the chemicalswere procured from Sigma-Aldrich and used without any furtherpurification. The graphene oxide was prepared by modifiedHummers’method [28]. This mixture was annealed twice underN2 atmosphere at 300 and 850 °C for 6 h. Other two carbonsources (super P carbon and MWCNT) were used without anyfurther treatment. The whole preparation can be explained ac-cording to the following reactions:

0:5 V2O5 þ 3 NH4H2PO4 þ C source→ VPO3ð Þ3� �

n= C sourceþ 3 NH3 þ CO þ 4:5 H2O

4 VPO3ð Þ3� �

n =C sourceþ 6 NaFþ 22x O2 þ 44x H2O→2Na3V2O2x PO4ð Þ2 F3−2x=Cþ 4xHFþ 8H3PO4

During the synthesis of [V(PO3)3]n, carbon source will actin two ways; first, it is a strong reducing agent for vanadium(V5+ to V3+), which is essential for this carbothermal reaction.Second, it controls the particle size during the reaction. Inhydrothermal synthesis of Na3V2O2x(PO4)2F3-2x/carbon com-posites, the excess carbon source will prevent the completeoxidation fromV3+ to V4+ in an aqueous medium. The startingmolar ratio of Na:V is 3:2, and the 10 weight percentage ofcarbon was used for making these three compositions basedon the final product. At last, the remaining carbon in the finalproduct will enhance the electrochemical properties ofNa3V2O2x(PO4)2F3-2x by creating unbreakable electronconducting network in the composites [29, 30].

Preparation of the NaTi2(PO4)3-MWCNT compositeanode material

Stoichiometric amounts of NaH2PO4:H2O, TiO2, andNH4H2PO4 were mixed with 10 wt.% of MWNTs. After thereagents were mixed in an agate mortar about 1 h, the mixturewas placed in an alumina crucible covered with graphite foil to

avoid contamination. Then, it was fired at 900 °C for 10 hunder an argon atmosphere.

Characterization

The final phase of the Na3V2O2x(PO4)2F3-2x/carbon compos-ite was confirmed by X-ray diffractometer (Rigaku D/Max-2500) with a Cu Kα radiation (λ = 1.5418 Å) at room tem-perature. The particle size and morphology were characterizedusing a field emission scanning electron microscope (FE-SEM; Hitachi S-4800, Japan). The Raman spectra of the pow-ders were recorded at room temperature on a HR 800 Ramanspectrophotometer (Jobin Yvon-Horiba, France) using amonochromatic He-Ne laser (514 nm) operating at 20 mW.Elemental analysis was carried out using a Thermo ScientificFlash 2000 Series element analyzer. 23Na NMR spectra wererecorded on Bruker Advance 400-MHz spectrometer at roomtemperature. Inductively coupled plasma–optical emissionspectrometry (Agilent ICP-OES 720, USA) was used for thedetermination of analysis with transitions at 589.592,213.618, and 292.401 nm, i.e., Na, P, and V, respectively.

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Electrochemical testing

The electrochemical studies of the as-synthesizedNa3V2O2x(PO4)2F3-2x/carbon composites were conductedin CR2032 coin cells. The composite electrode was pre-pared by mixing 80 wt.% of active materials with10 wt .% super P carbon b lack and 10 wt .%polyvinylidene fluoride (PVDF) binder in N-Methyl-2-pyrrolidone (NMP) solvent. The obtained slurry wascoated on Al foil and cut into circular electrodes withdiameter of 12 mm. Sodium metal was used as an an-ode, and 1 M NaClO4 in propylene carbonate (PC) with2 vol.% of fluoroethylene carbonate (FEC) was used asthe electrolytes. The coin cells were assembled in anargon-filled dry glove box using a borosilicate glass-fiber separator (Whatman GF/D). For the full-cell tests,the NaTi2(PO4)3-MWCNT composite on the 14-mm Alfoil was used as the anode for testing Na-ion non-aque-ous full cells. The cathode loading and area for theNa3V2O2x(PO4)2F3-2x/carbon composite electrode were~1.3 mg and 1.13 cm−2, respectively, and the anodeloading and area for the NaTi2(PO4)3-MWCNT compos-ite electrode were ~2.2 mg and 1.54 cm−2, respectively.The cyclic voltammetry and galvanostatic cycling of theNa-ion full cells were measured using a potentiostatVMP3 (Biologic, France). The Na coin half cells weregalvanostatically cycled between 2.5 and 4.5 V using anautomatic battery cycler WBCS3000 (Wonatech, Korea).The all full cells were cycled from 1.0 to 2.5 V at a rateof 0.2 C based on the cathode active material weight.

Results and discussion

Structural characteristics

Figure 2 shows the XRD patterns for the all Na3V2O2x(PO4)2F3-2x/carbon polymorphs composites along with their JCPDS data.The prepared all three Na3V2O2x(PO4)2F3-2x/carbon polymorphscomposites were matched with tetragonal symmetryNa3V2O2(PO4)2F compound with I4/mmm space group bySauvage et al. [31]. Hence, it can be proposed that these com-posites belong to a family with the general formula ofNa3V2O2x(PO4)2F3-2x. From Fig. 2, no impurities or residues

Fig. 2 XRD patterns for all three compositions alongwith JCPDS data, aNa3V2O2x(PO4)2F3-2x/C, b Na3V2O2x(PO4)2F3-2x/MWCNT, and cNa3V2O2x(PO4)2F3-2x/rGO

Fig. 1 Illustration ofNa3V2O2x(PO4)2F3-2x synthesiswith different carbon sources

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have been detected, indicating a high purity of the prepared ma-terials. The Raman spectroscopy results for the Na3V2O2x

(PO4)2F3-2x/ carbon composites are shown in Fig. 3. The peaksat 2700 and 2910 cm−1 correspond to the G′ and D + G bands.Another high-intensity two bands at 1350 and 1580 cm−1 wereappeared, corresponding to the disordered (D) and graphitebands (G) of carbon-based materials. Furthermore, theNa3V2O2x(PO4)2F3-2x/MWCNThas spitted graphite band,which

is directly represented to the diameter of carbon nanotubes. Thethree samples display almost similar ratio of D band and G bandintensities for carbon (ID/IG = 1.56),MWCNT (ID/IG = 1.32), andrGO (ID/IG = 1.29), respectively, which indicates a relativelysmall size of the ordered domains [32]. In addition to thesepeaks, two peaks were observed in the Raman spectra ofthe all three Na3V2O2x(PO4)2F3-2x/carbon form compos-ites at 930 and 1040 cm−1 due to the symmetric P–Ostretching vibration and anti-symmetric stretching bandsof the PO4, respectively [33].

The SEM images were presented in Fig. 4 in two differentmagnifications. In Fig. 4a–e, it is clearly observed that uni-form distribution of carbon forms in the Na3V2O2x(PO4)2F3-2xsubmicron squares, which will help to increase the electronicconductivity of electrode materials. The average diagonallength of smooth edge Na3V2O2x(PO4)2F3-2x squares with su-per P carbon, MWCNT, and rGO are 2, 2.1, and 3.8 μm,respectively. Furthermore, the carbon percentage in thesethree composite materials was found by the element analyzer(Thermo Scientific Flash 2000 Series), and the values were ca.9.6, 7.9, and 8.0 wt.%, respectively.

In electronic supplementary information (ESI†), Fig. S1shows the three 23Na solid-state NMR spectra of theNa3V2O2x(PO4)2F3-2x/carbon form composites. The NMR

Fig . 3 Raman spec t ra for a Na3V2O2 x (PO4)2F3 - 2 x /C , bNa3V2O2x(PO4)2F3-2x/MWCNT, and c Na3V2O2x(PO4)2F3-2x/rGO

Fig. 4 SEM images at different magnifications a, b Na3V2O2x(PO4)2F3-2x-C, c, d Na3V2O2x(PO4)2F3-2x-MWCNT, and e, f Na3V2O2x(PO4)2F3-2x-rGO

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spectra for the three samples were fitted to three main signalsat 95, 126, and 172 ppmwith relative intensities of 44, 31, and25 %, respectively. The observed chemical shifts indicate thepresence of hyperfine interactions between the 23Na and theparamagnetic V ions [20]. The three signals observed in thespectrum were assigned to the V4+-V4+ (95 ppm), V3+-V4+

(126 ppm), and V3+-V3+ (172 ppm) pairs. Using the fitted dataof the 23NMR spectra, the obtained average oxidation state of thevanadium in the Na3V2O2x(PO4)2F3-2x/C, MWCNT, and rGOcomposites are 3.56, 3.46, and 3.55, respectively. Finally, basedon the NMR results, we have proposed Na3V2O1.2(PO4)2F1.8,Na3V2O0.92(PO4)2F2.08, and Na3V2O1.1(PO4)2F1.9 as formulasfor prepared three composite materials [19]. Also, the ICP-OESanalysis of final product shows that the ratio of Na/V is approx-imately 1.35 and NA:P is 1.3. Hence, the XRD, SEM, TEM,Raman, and 23Na NMR results confirm the phase, purity, anduniform distribution of carbon forms over the Na3V2O2x

(PO4)2F3-2x microsquares.

Electrochemical characteristics

Figure 5 shows the first 20 cyclic voltammetry cycles of threeNa3V2O2x(PO4)2F3-2x/carbon composites at the scan rate of0.2 mV s−1 vs. Na+/Na. In case of Na3V2O2(PO4)2F (x = 1),only couple of oxidation peaks (3.6 and 4.1 V) are responsiblefor the extraction of two Na ions from the structure associatedwith V4+/V5+ reaction, resulting from the ion migration fromtwo Na sites with 0.5 occupation and one Na site with oneoccupation for this NASICON-type structure. Secondly,Na3V2(PO4)2F3 (x = 0) exhibits a little higher oxidation potentialof 3.7 and 4.2 Vwith the V3+/V4+ redox couple. From Fig. 5, allcomposites are showing three sets of oxidation and reductionpeaks; because all three composites are in mixed phase of abovetwo compounds, the splitting of second redox peak might berelated to these mixed phase characteristics. Figure 5a, b showsthat the composites with carbon and MWCNT electrodes arevery stable, when compared to the rGO composite electrode.From Fig. 5c, a decrease in the current peak height for theNa3V2O2x(PO4)2F3-2x/rGO CV plot is observed as well as asignificant decrease in the electrochemical reversibility of theredox couple, which will be explained later in this section.

The electrochemical profiles of the three Na3V2O2x

(PO4)2F3-2x/carbon form cathodes in a sodium-ion recharge-able battery for 100 cycles shown in Fig. 6. All three compos-ites deliver a stable capacity of 89, 99, and 91 mAh/g after100 cycles at 0.1 C rate (13 mA/g). Figure 6b–d presents thecharge/discharge profile discharge curves of Na3V2O2x

(PO4)2F3-2x/carbon form composites. The charge/dischargeprofiles show that there are two plateaus with average voltagesof about 3.7 and 4.1 V. The average voltage of Na3V2O2x

(PO4)2F3-2x family is 3.9 V, one of the highest among cathodematerials with the same redox couple of V4+/5+. From Fig. 6d,it is clearly observed that rGO composite has high-capacity

fading compared to the other two composites. But, it is gen-erally accepted that large number of graphene-based compos-ites exhibit enhanced battery performance in terms of the ca-pacity and cyclic ability due to the enhanced interlayer spac-ing, high surface area improvements in the adsorption, andimmersion of the electrolyte, and also, it produces a pathwayfor electron transport. Here, we have prepared reducedgraphene oxide composite from reduction of graphene oxideby thermal reduction process [34]. Therefore, there are stillsome oxygen functional groups in or on the reduced grapheneoxide surface. Different reducing agents will lead to variouscarbon to oxygen ratio and chemical compositions in reducedgraphene oxide. Reproducibility is also a huge problem ingraphene, and graphene synthesis method influences

Fig. 5 CV plots for the Na3V2O2x(PO4)2F3-2x with a carbon, b MWCNT(Reproduced from Ref. 29 with permission from the Royal Society ofChemistry), and c rGO at a scan rate of 0.2 mV s−1

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performance. One advantage of CNTs is that they are availableoff the shelf and cost lot less than graphene. Hence, it has beenclaimed that graphene may not be beneficial as an electrode

material, due to its lower edge surface area, leading to slowheterogeneous electron transfer. The orientation of grapheneon the electrode may also affect its electrochemical

Fig. 6 a Cyclability ofNa3V2O2x(PO4)2F3-2x along withthree different carbonpolymorphs. Charge-dischargecurves for theNa3V2O2x(PO4)2F3-2x with bcarbon, c MWCNT (Reproducedfrom Ref. 29 with permissionfrom the Royal Society ofChemistry), and d rGO

Fig. 7 a Rate capability ofNa3V2O2x(PO4)2F3-2x along withthree different carbonpolymorphs. Charge-dischargecurves at different current rates forthe Na3V2O2x(PO4)2F3-2x with bcarbon, c MWCNT (Reproducedfrom Ref. 29 with permissionfrom the Royal Society ofChemistry), and d rGO

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performance. Furthermore, the size of microsquares is big inNa3V2O2x(PO4)2F3-2x/rGO composite, which will have asmaller surface contact area with the electrolytes and poorcyclability [35].

The slow heterogeneous electron transfer effect on electro-chemical performance of rGO composites can be inferredfrom the rate capability tests in Fig. 7. The discharge capacityof all three composites at current rates of 130 mA g−1 (1 C),260 mA g−1 (2 C), 650 mA g−1 (5 C), 1300 mA g−1 (10 C),and 2600mA g−1 (20 C) for 10 cycles is shown. From Fig. 7a,the initial discharge capacity of Na3V2O2x(PO4)2F3-2x/rGOcomposite at 1 °C rate is higher than that of other two com-posites, but the capacity values are lower with increasing thecurrent density compared to others. The charge-dischargevoltage profiles at various C rates of the Na3V2O2x(PO4)2F3-2 x / C , N a 3 V 2 O 2 x ( P O 4 ) 2 F 3 - 2 x / MWCN T, a n dNa3V2O2x(PO4)2F3-2x/rGO composites were shown inFig. 7b–d, respectively. From Fig. 7d, Na3V2O2x(PO4)2F3-2x/rGO composite delivers very low discharge capacity value(15 mAh g−1) at high current rate (20 C) compared to othertwo composites, supporting the broken electron transferpathway.

Electrochemical impedance spectroscopy (EIS) measure-ments were carried out during first cycle at different potential,as shown in Figs. 8, 9, and 10. In Fig. 8, scatters represent the

Fig. 8 Nyquist plots for the Na3V2O2x(PO4)2F3-2x with super P carbonvs. Na/Na+ during a charging and b discharging states

Fig. 9 Nyquist plots for the Na3V2O2x(PO4)2F3-2xwithMWCNT vs. Na/Na+ during a charging and b discharging states

Fig. 10 Nyquist plots for the Na3V2O2x(PO4)2F3-2xwith rGO vs. Na/Na+

during a charging and b discharging states

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experimental data points and fitted data are represented ascontinuous lines in the Na3V2O2x(PO4)2F3-2x/C composite.The obtained data were analyzed by fitting with an electricalequivalent circuit consisting of resistors and constant phaseelements (CPEs) shown in Fig. S2 (ESI†). The electricalequivalent circuit consists of electrolyte resistance (Re), theseparable surface film (RSF), and charge transfer (RCT) resis-tances, also constant phase elements CPESF and CPECT andthe finiteWarburg impedance (Wd). The depressed semicircles

in the spectra were represented by parallel combination ofCPEs and resistance. The experimental data was fitted byusing Z-fit software with fitting except at low frequencies.

In all three composites, result shows that the electrolyteresistance (Re) remained almost constant at 5.4 Ω. FromFigs. 8a, 9a, and 10a, Nyquist plot of the cell at open-circuitvoltage (OCV) consists of a depressed semicircle followed byWarburg element at low frequencies. The depressed semicirclewas observed in high-frequency range, which is related to the

Table 1 Re, RCT, RSF, and Wd values of the Na3V2O2x(PO4)2F3-2x with different carbon polymorphs vs. Na during the first charge-discharge cycle atvarious voltages

V Na3V2O2x(PO4)2F3-2x/C Na3V2O2x(PO4)2F3-2x/MWCNT Na3V2O2x(PO4)2F3-2x/rGO

Re(Ω) RSF(Ω) RCT(Ω) Wd Re(Ω) RSF(Ω) RCT(Ω) Wd Re(Ω) RSF(Ω) RCT(Ω) Wd

OCV(Cha.) 5.6 350 274 0.8 5.3 183 40 0.8 6.0 200 235 1

3 5.6 324 355 0.8 5.7 315 4.6 0.8 6.1 234 250 1

3.5 5.7 720 2460 0.6 5.6 353 2015 0.8 6 491 1572 0.6

3.7 5.5 340 155 0.7 5.0 384 36 0.7 5.8 482 383 0.5

4.2 5.8 257 114 0.7 6 98 345 0.7 5.9 256 263 0.6

4.6 5.8 248 7520 0.7 5.4 3000 314 0.7 5.9 6690 534 0.6

4(Dis.) 5.8 341 254 0.7 5.4 426 118 0.7 5.9 822 108 0.6

3.7 5.7 712 1140 0.7 4.9 484 472 0.8 5.8 967 704 0.6

3.3 5.7 892 8520 0.7 4.7 473 706 0.8 5.7 1173 2415 0.6

2.5 5.6 843 952 0.8 5.4 441 287 0.7 5.9 107 359 0.6

Fig. 11 a Cyclability and bcharge-discharge curves for theNa3V2O2x(PO4)2F3-2x with threedifferent carbon polymorphs/NaTi2(PO4)3-MWCNT full cell

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formation of passivation film on the surface (SF) and thecharge transfer process at interface (CT). At OCV, only sur-face film contribution will occur and it is depending on con-centration of electrolyte. In addition, solid electrolyte inter-phase (SEI) formation/partial dissolution/reformation alsotakes place on cycling. This is reflected in the changes in thevalue of surface film impedance (RSF) up on cycling. Also, at3 and 3.5 V, the impedance plots consist like OCV. At 3.7 and4.2 V, the Nyquist plots contain two semicircles in the high-and intermediate-frequency range and a sloping line in thelow-frequency region. Further charge to 4.6 V, the size ofhigh-frequency semicircle is reduced and the second semicir-cle disappears [36, 37].

Furthermore, the impedance plots are following same trendlike first discharge. In Figs. 8b, 9b, and 10b at 4 and 3.7 V, theNyquist plots contain high-frequency semicircle along withlow-frequency depressed semicircle, which is due to interca-lation of Na ions into the electrode reaction (another hoppingmechanism). Further discharging down to 3.3 and 2.5 V, thesurface film and charge transfer impedances are merge andform one depressed semicircle. Hence, we concluded thatthe electrochemical impedance studies are supporting the ob-served electrochemical results. The fitted values of all Re, RSF,RCT, and Wd are listed in Table 1.

Na-ion full cell with NaTi2(PO4)3-MWCNT

NaTi2(PO4)3-MWCNT/Na3V2O2x(PO4)2F3-2x-carbon formNa-ion full cells were assembled to demonstrate the feasibilityof Na3V2O2x(PO4)2F3-2xmicrosquare sample as a cathode ma-terial for real Na-ion battery system. Figure 11a shows the full-cell cycling performance of all three composites withNaTi2(PO4)3-MWCNT [18]. The full cell delivers an initialdischarge capacity of 98 mAh g−1 at a rate of 0.2 C(26 mA g−1). The discharge capacity is decreased as89mAh g−1 after 100 cycles; however, charge/discharge efficien-cy has gradually increased to 99 % from the initial efficiency of97 %. The cycling stability for a Na-ion full cell should be en-hanced; this result indicates the feasibility of Na3V2O2x(PO4)2F3-2x-MWCNT electrode as a cathode for Na-ion batteries.However, Na3V2O2x(PO4)2F3-2x-C and Na3V2O2x(PO4)2F3-2x-rGO composites are delivering low-capacity values of 51 and40 mAh g−1, respectively, at the same C rate. Although the prac-tical capacity and cyclability of a full cell could be enhanced byoptimizing other factors such as electrode formula and balance,electrolytes, and cell process conditions, this full cell result con-firms again that the MWCNT composite shows better electrodeperformance than other carbon forms in Na3V2O2x(PO4)2F3-2x.The charge-discharge voltage profiles for the full cell duringcycling are also presented in Fig. 11b–d. The NaTi2(PO4)3-MWCNT has shown a flat plateau at ~2.1 V vs. Na+/Na [38],while Na3V2O2x(PO4)2F3-2x composites exhibit two flat plateauvoltage profile as shown in Fig. 6. The XRD pattern,

morphology, CV, and galvanostatic cycling results of theNaTi2(PO4)3-MWCNT half cell in non-aqueous electrolytes areshown in Fig. S3 (ESI†).

Conclusions

Na3V2O2x(PO4)2F3-2x-carbon composites are prepared byusing three different carbon polymorphs (super P carbon,MWCNT, and rGO) through combination of solid state andhydrothermal reactions. We investigated the effect of the car-bon source on synthesis as well as electrochemical propertiesof Na3V2O2x(PO4)2F3-2x cathode material. Among these threecarbon sources, MWCNT is more effective to obtain moderateparticle size with enhanced electrochemical properties. As acontradiction result, Na3V2O2x(PO4)2F3-2x-rGO shows a lowcyclic stability and poor rate capability due to the larger par-ticle size and low-quality thermal reduction of rGO. The pre-pared Na3V2O2x(PO4)2F3-2x-MWCNT composite shows thestable capacity of 98 and 89 mAh g−1 in half cell and full cellwith NaTi2(PO4)3-MWCNT configurations, respectively.Other parameters for the positive electrodes that were notdiscussed in this study such as the nanosized particle, binder,and hydrothermal reaction temperature could also have signif-icant impact on the battery performance and will be examinedin the future study.

Acknowledgments This work was supported by the Program to SolveClimate Changes (NRF-2010-C1AAA001-2010-0029031) of Korea(NRF), funded by the Ministry of Science, ICT, and Future Planning. Itwas also supported by the Climate Change Research Hub of KAIST(Grant No. N01150034).

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