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Simultaneous texturing and conductivity tailoring of mesoporous NaTi 2 (PO 4 ) 3 nanocrystals by gadolinium doping for enhanced Na storage G.B. Xu a , Z. Chen a , X. Liu a , Y. Zhang a , X.L. Wei a , L.W. Yang a, b, * , Paul K. Chu b, ** a School of Physics and Optoelectronics, Xiangtan University, Hunan, 411105, China b Department of Physicsand Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China article info Article history: Received 12 February 2019 Received in revised form 15 March 2019 Accepted 10 April 2019 Available online 14 April 2019 Keywords: Sodium-based energy storage devices Mesoporous nanocrystals Doping Gadolinium ion NASICON-type NaTi 2 (PO 4 ) 3 abstract Doping is an effective method to produce hybrid materials with desirable properties and functions. Herein, gadolinium ions (Gd 3þ ) are introduced to substitute Ti in mesoporous NaTi 2 (PO 4 ) 3 (MNTP) nanocrystals to tailor the texture and conductivity simultaneously to improve Na storage properties. Microstructure characterization and kinetics analysis reveal that the superior electrochemical charac- teristics arise from the expanded cell, optimized porous structure, enhanced intrinsic electron conduc- tivity, and better Na ion mobility as a result of Gd 3þ introduction into the MNTP nanocrystals. In the half cells for sodium-ion battery, the MNTP nanocrystals with 5% Gd 3þ exhibit a large ICE beyond 97% at 1 C, superior rate capability of 47.5 mAhg 1 at 50 C, and long cycling life time (>75% capacity retention at 5 C after 2000 cycles). Furthermore, using the Gd 3þ doped MNTP nanocrystals as the anode and activated carbon as the cathode, the assembled sodium-ion hybrid capacitor shows a high energy density of 63 Whkg 1 at a power density of 46 Wkg -1 . Even at a large current rate of 5 Ag 1 , a stable capacity is maintained after 30,000 cycles without obvious degradation. Gadolinium ion doping provides a new technique to improve the electrochemical properties of NASICON-type anodes in sodium-based energy storage devices. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction Increasing attention is being paid to sodium-based electro- chemical energy storage devices such as sodium-ion batteries (SIBs) [1e3], sodium dual-ion batteries [4], and sodium-ion hybrid capacitors (NHCs) [5,6] as alternatives to the lithium-related counterparts due to lower cost and more abundant resource of sodium. Particularly, NHCs combining the merits of SIBs and elec- trochemical double layer capacitors (EDLCs) are promising in of- fering both high energy and power densities that can meet the demands of large-scale applications such as electric vehicles and hybrid electric vehicles. However, most anode materials for SIBs have poor electrochemical properties and structural integrity due to the larger ion radius and slower diffusion coefcient of sodium ions compared to lithium ions, thereby stymieing wider application of sodium-based electrochemical energy storage devices [6,7]. Consequently, anode materials with large ion diffusion coefcients, fast electron transfer, as well as small volume change during sodium-ion insertion/extraction are highly desirable. New anode materials including Ti-related compounds [8], transition metal chalcogenides [9], phosphorus [10], 2D metal carbides [11], and sodium super ion conductor (NASICON)-type compounds [12] have been studied. Among them, NASICON-type NaTi 2 (PO 4 ) 3 (NTP) with large open 3D channels is attractive anode materials for SIBs because of the high theoretical capacity of 133 mAhg 1 , fast Na þ diffusion, small volume change during Na insertion/extraction, as well as intrinsically good safety related to high voltage plateau of about 2.1 V vs Na/Na þ [13]. Nevertheless, the poor intrinsic electrical conductivity of NTP leads to inferior high-rate capability and long-term cycle life. To overcome these hurdles, efforts have been made to improve the electron/ion transport kinetics by constructing the composite of nano- structured NTP with the modication of electro-conducting car- bon [14, 15]. The modication schemes such as application of thin * Corresponding author. School of Physics and Optoelectronics, Xiangtan Uni- versity, Hunan, 411105, China. . ** Corresponding author. E-mail addresses: [email protected] (L.W. Yang), [email protected] (P.K. Chu). Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2019.04.059 0013-4686/© 2019 Elsevier Ltd. All rights reserved. Electrochimica Acta 309 (2019) 177e186

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Electrochimica Acta 309 (2019) 177e186

Contents lists avai

Electrochimica Acta

journal homepage: www.elsevier .com/locate/electacta

Simultaneous texturing and conductivity tailoring of mesoporousNaTi2(PO4)3 nanocrystals by gadolinium doping for enhanced Nastorage

G.B. Xu a, Z. Chen a, X. Liu a, Y. Zhang a, X.L. Wei a, L.W. Yang a, b, *, Paul K. Chu b, **

a School of Physics and Optoelectronics, Xiangtan University, Hunan, 411105, Chinab Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

a r t i c l e i n f o

Article history:Received 12 February 2019Received in revised form15 March 2019Accepted 10 April 2019Available online 14 April 2019

Keywords:Sodium-based energy storage devicesMesoporous nanocrystalsDopingGadolinium ionNASICON-type NaTi2(PO4)3

* Corresponding author. School of Physics and Opversity, Hunan, 411105, China. .** Corresponding author.

E-mail addresses: [email protected] (L.W. Y(P.K. Chu).

https://doi.org/10.1016/j.electacta.2019.04.0590013-4686/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

Doping is an effective method to produce hybrid materials with desirable properties and functions.Herein, gadolinium ions (Gd3þ) are introduced to substitute Ti in mesoporous NaTi2(PO4)3 (MNTP)nanocrystals to tailor the texture and conductivity simultaneously to improve Na storage properties.Microstructure characterization and kinetics analysis reveal that the superior electrochemical charac-teristics arise from the expanded cell, optimized porous structure, enhanced intrinsic electron conduc-tivity, and better Na ion mobility as a result of Gd3þ introduction into the MNTP nanocrystals. In the halfcells for sodium-ion battery, the MNTP nanocrystals with 5% Gd3þ exhibit a large ICE beyond 97% at 1 C,superior rate capability of 47.5mAhg�1 at 50 C, and long cycling life time (>75% capacity retention at 5 Cafter 2000 cycles). Furthermore, using the Gd3þ doped MNTP nanocrystals as the anode and activatedcarbon as the cathode, the assembled sodium-ion hybrid capacitor shows a high energy density of 63Whkg�1 at a power density of 46 Wkg-1. Even at a large current rate of 5 Ag�1, a stable capacity ismaintained after 30,000 cycles without obvious degradation. Gadolinium ion doping provides a newtechnique to improve the electrochemical properties of NASICON-type anodes in sodium-based energystorage devices.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Increasing attention is being paid to sodium-based electro-chemical energy storage devices such as sodium-ion batteries(SIBs) [1e3], sodium dual-ion batteries [4], and sodium-ion hybridcapacitors (NHCs) [5,6] as alternatives to the lithium-relatedcounterparts due to lower cost and more abundant resource ofsodium. Particularly, NHCs combining the merits of SIBs and elec-trochemical double layer capacitors (EDLCs) are promising in of-fering both high energy and power densities that can meet thedemands of large-scale applications such as electric vehicles andhybrid electric vehicles. However, most anode materials for SIBshave poor electrochemical properties and structural integrity dueto the larger ion radius and slower diffusion coefficient of sodium

toelectronics, Xiangtan Uni-

ang), [email protected]

ions compared to lithium ions, thereby stymieing wider applicationof sodium-based electrochemical energy storage devices [6,7].Consequently, anode materials with large ion diffusion coefficients,fast electron transfer, as well as small volume change duringsodium-ion insertion/extraction are highly desirable.

New anode materials including Ti-related compounds [8],transition metal chalcogenides [9], phosphorus [10], 2D metalcarbides [11], and sodium super ion conductor (NASICON)-typecompounds [12] have been studied. Among them, NASICON-typeNaTi2(PO4)3 (NTP) with large open 3D channels is attractiveanode materials for SIBs because of the high theoretical capacity of133mAhg�1, fast Naþ diffusion, small volume change during Nainsertion/extraction, as well as intrinsically good safety related tohigh voltage plateau of about 2.1 V vs Na/Naþ [13]. Nevertheless,the poor intrinsic electrical conductivity of NTP leads to inferiorhigh-rate capability and long-term cycle life. To overcome thesehurdles, efforts have been made to improve the electron/iontransport kinetics by constructing the composite of nano-structured NTP with the modification of electro-conducting car-bon [14,15]. The modification schemes such as application of thin

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186178

carbon coatings [16,17], loading onto carbon nanotubes [18,19], andtwo-dimensional or three-dimensional graphene wrapping [20,21]have been demonstrated to improve the electrochemical perfor-mance in terms of capacity, capacity retention, and high-rate per-formance. Almost all the reported NTP anodes for SIBs have highICEs over 90% suggesting they are practical in NHCs applications.For instance, the NTP-based NHC anode reported by Thangavel et al.(composite of NTP grown on graphene nanosheets) delivers a highenergy density of~80 Whkg�1 at a specific power density of 8Wkg�1 [22]. However, it is difficult to improve the intrinsic (bulkphase) electron conductivity and sodium ion mobility by carbonmodification. In this respect, the intrinsic characteristics can beimproved alternatively by metal doping. Although metals such asMn and Mo have been introduced to replace V3þ in the NASICON-type compound of Na3V2(PO4)3 to improve the electrochemicalperformance [23,24], to the best of our knowledge, few studies havebeen performed on metal-doped nanostructured NTPs.

Mesoporous nanocrystals with a diameter of about 100 nm arepromising in high-performance sodium-based electrochemicalenergy storage devices on account of large surface area, abundantelectro-active sites, as well as optimal morphology for the fabri-cation of compact electrode layers [25,26]. Mesoporous nano-crystals with outstanding Na storage should also possess goodintrinsic conductivity for charge transfer and a suitable porousstructure for mass transport [27,28]. In spite of previous studiesabout mesoporous NTP (MNTP) nanocrystals with improved rate-capability [20,29e31], they are still insufficient in practice. Lower-valence lanthanide ions have many desirable characteristics suchas the large radius, high electric charge, strong self-polarizationability, as well as the yield of vacancies in NTP to enhance the ionconductivity [32,33]. In addition, lanthanide doping has beendemonstrated to be effective in tuning the texturing (shape,morphology and size) of nanocrystals such as NaYF4 [34], alkaline-earth fluoride [35], Li4Ti5O12 [36], and so on. Hence, substitution ofTi with lanthanide ions is an effective and practical strategy toimprove the electrochemical properties by tailoring of the textureand conductivity in MNTP nanocrystals at the same time.

In this work, gadolinium (Gd3þ) ions with a half-full 4f electronshell are incorporated into MNTP nanocrystals for partial substi-tution of Ti4þ to enhance the Na storage performance. The effects ofGd3þ doping on the texture, electrochemical performance, and ki-netics of the MNTP nanocrystals are investigated in details. TheNHC full cell assembled with the Gd3þ dopedMNTP nanocrystals asthe anode and activated carbon (AC) as the cathode showsimpressive energy/power densities and long durability, which arecomparable to most reported batteries and capacitors.

2. Experimental details

2.1. Sample synthesis

Synthesis of theMNTP nanocrystals and a series of samples withdifferent Gd3þ concentrations is schematically illustrated in Fig. 1.The process involves a modified hydrothermal reaction and ther-mal treatment at 600 �C. In the typical process, 2mmol CH3COONawas dissolved in 6mL of phosphoric acid (85wt%) to form solutionA. 2mmol titanium (IV) butoxide and a suitable amount ofGdCl3$6H2O were dissolved in 40mL of ethanol to form solution B.The two solutions were mixed under magnetic stirring and trans-ferred to a Teflon vessel to conduct the hydrothermal reaction in anoven at 160 �C for 3 h. Afterwards, the white powder on the bottomof the Teflon vessel was collected, washedwith ethanol 3 times, anddried at 60 �C for 5 h. Finally, the white hydrothermal product wasannealed at 600 �C for 2 h in argon to yield the final samples. Thesamples were designated as MNTP, MNTP-Gd-1, MNTP-Gd-2, and

MNTP-Gd-3 according to the nominal Gd3þ concentrations of 0,2.5mol %, 5mol % and 7.5mol%, respectively.

2.2. Materials characterization

The structure of the samples was determined by by X-raydiffraction (XRD) using Cu Ka radiation (l¼ 0.154 nm), scanningelectron microscopy (SEM), and transmission electron microscopy(TEM, JEOL 2100) equipped with selected-area electron diffraction(SAED) and Oxford energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) was performed using an Al Ka

source (Kratos Analytical Ltd., UK) and the 284.8 eV C 1s was usedfor calibration. The nitrogen adsorptionedesorption isothermmeasurements were conducted at 77 K (Quantachrome NOVA4000e) and Raman scattering spectra were acquired on a RenishawinVia system with a 532 nm laser as the excitation source.

2.3. Electrochemical characterization

The electrochemical tests were carried out using two-electrodeCR2025 type coin-cells. The working electrodes in the half cellswere prepared by pasting slurry containing the active materials,carbon black, and polyvinylidene fluoride (PVDF) with a weightratio of 7:2:1 onto a Cu foil. A metallic sodium foil and glass fiber(Whatman GF/D) served as the counter electrode and separators inthe sodium cells, respectively and the electrolyte was 1M NaClO4mixed with ethylene carbonate/propylene carbonate in a 1:1 ratio(volume). The coin cells were assembled in a glove box filled withhigh-purity argon. Galvanostatic charging/discharging measure-ments were carried out between 1.5 and 3 V (vs. Na/Naþ) using themulti-channel battery system (NEWARE BTS-610) and cyclic vol-tammetry (CV) was conducted between 1.5 and 3 V (vs. Na/Naþ) onan electrochemical workstation (CHI660E). The NHC full cells wereconstructed using commercial AC as the cathode and MNTP-Gd-2as the anode. The mass ratio of the anode to cathode was 1:1.8according to the delivered capacities measured from the half cells.All the tests were conducted at room temperature.

3. Results and discussion

The XRD patterns of theMNTP nanocrystals containing differentconcentrations of Gd3þ in Fig. 2a show that all the diffraction peakscan be indexed to the NASICON-type structure NTP (JCPDS no. 01-085-2265) without impurities, indicating that Gd3þ ions areincorporated successfully into the NTP host and the original crystalstructure of host is not effected. However, as shown in Fig. 2b, thecharacteristic peaks of the Gd3þ-doped samples shift towards asmaller angle compared to those of pure MNTP, indicating that thecell sizes of the doped samples are enlarged. It is because the largerradius (0.0938 nm) of Gd3þ than Ti4þ (0.0605 nm) expands thelattice further confirming Gd3þ replacement of Ti4þ. Fig. 2c showsthe Raman scattering spectra of the samples and unique peaks at155, 272, 306, 341, 438, 990 and 1085 cm�1 are observed from theNASICON-type structure.

The Raman peaks at 990 and 1085 cm�1 are attributed to char-acteristic symmetrical and asymmetrical stretching in the NTPlattice, the peaks at 155 and 272 cm�1 originate from translationalvibration of Ti4þ ions and those at 306, 341, and 438 cm�1 areassociated with PO4

3� [37]. No impurity phases are detected indi-cating that the small concentration of Gd3þ does not damage theNASICON structure. Figure SI and Table SI present the EDS results ofthe doped samples, indicating that Gd3þ has been doped success-fully in the NTP crystal structure. The detectable contents ofGd3þare below the nominal contents in sample preparation due tolarger ionic diameter of Gd3þ compared to Ti4þ. XPS is also

Fig. 1. Schematic illustration of the synthesis of pure MNTP nanocrystals and a series of samples with different Gd3þ doping contents. The corresponding crystal structures are alsodemonstrated.

Fig. 2. (a) XRD patterns of the MNTP nanocrystals with different Gd3þ doping contents. (b) Partial enlarged view of the XRD patterns shown in (a). (c) Raman spectra of the MNTPnanocrystals with different Gd3þ doping contents. (d) High-resolution XPS spectra of Ti 2p acquired from the MNTP-Gd-2 and referenced pure MNTP.

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186 179

performed to determine the chemical composition as well asvalence states of Ti and Gd in the doped samples. The survey andhigh-resolution Na 1s, Ti 2p, P 2p, O, and Gd 3d XPS spectra(Figure S2) acquired fromMNTP-Gd-2 reveal the presence of Na, Ti,P, O, and Gd. As shown in Figure S1f, the broad peak at 1227.5 eV isrelated to the Gd 3d core level, suggesting that Gd3þ ions maintainthe valence state. Fig. 2d depicts the high-resolution XPS spectra of

Ti 2p revealing two peaks at 460.7 and 466.7eV assigned to Ti 2p1/2and 2p3/2, respectively. Compared to those from pure MNTP, thebinding energies of Ti 2p in MNTP-Gd-2 shift to lower energies,suggesting a slight change in the average crystal field in the NTPhost due to Gd3þ replacement of Ti4þ. As Gd3þ has a lower valence,nonequivalent substitution of Ti4þ by Gd3þ may induce Na or Tivacancies for charge balance [32,33,38], which is responsible for the

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186180

volume change of the unit cell and crystal field change surroundingresidual Ti. The expanded unit cell in the doped MNTP can providelarger Naþ transport channels and more space for Naþ (de)inter-calation boding well for the electrochemical performance.

Fig. 3a shows the TEM image of pure MNTP revealing that it iscomposed of quasi-spherical mesoporous nanocrystals with abroad size distribution between 50 and 180 nm. The formation ofthe meso-porous nanostructure originates from the transformationof micropores during crystallization of amorphous componentsduring annealing is similar to that described previously [20]. Asshown in Fig. 3b the morphology of MNTP-Gd-2 is similar to that ofpure MNTP, although the former has a narrower size distributionranging from 100 nm to 150 nm and the quasi-spherical meso-porous nanocrystals become more uniform. The results indicatethat Gd3þ doping affects the crystallization process of the NTPparticles during annealing. Fig. 3c shows the high-magnificationTEM image of MNTP-Gd-2 revealing hierarchical mesopores.Fig. 3d displays the high-resolution TEM (HR-TEM) image of a singleMNTP-Gd-2 nanocrystal and the lattice spacing of 0.61 nm can beindexed to the (012) lattice plane of the NASICON-type phaseindicating high crystallinity. Fig. 4 shows the HAADF-STEM imageand corresponding EDS elemental maps. Na, Ti, P, O, and Gd arehomogeneously distributed in the single MNTP-Gd-2 nanocrystalfurther corroborating doping of Gd3þ.

The influence of Gd3þ doping on the pore structure as well asspecific surface area of the MNTP nanocrystals is also investigatedby nitrogen gas porosimetry. As shown in Fig. 5, all the samplesexhibit isothermal adsorption-desorption curves with the typicaltype-IV feature. With regard to the pure MNTP nanocrystals(Fig. 5a), the hysteresis loop is not obvious and the Barrett-Joyner-Halenda (BJH) pore-size distribution curve (inset in Fig. 5a) showsthat most of pores are in the range of 1e5 nm together with a fewscattered ones beyond 10 nm. The specific surface area and porevolume of the pure MNTP nanocrystals are 12.6 m2g-1 and 0.11cm3g-1, respectively. Hence, the pure MNTP nanocrystals are less

Fig. 3. (a) Typical TEM image of pure MNTP nanocrystals; (b)e

porous and the transport properties from mesoporous channels inthe sample are not superior. After doping, the hysteresis loopsbecome distinct (Fig. 5bed) and the BJH pore-size distributioncurves (insets in Fig. 5bed) exhibit hierarchical porous featureswith increased mesopores in the range between 5 and 40 nm. Thecorresponding specific surface areas and pore volumes of the dopedsamples increase notably. Particularly, the MNTP-Gd-2 with amoderate amount of Gd3þ exhibits optimal hierarchical mesoporesin the range from 5 to 20 nm, which are mainly located at about9 nm, and it has the largest specific surface area of 93.4 m2g-1 andpore volume of 0.47 cm3g-1. The results indicate that the Na or Tivacancies induced by nonequivalent substitution of Ti4þ by Gd3þ

for charge balance have an effect on mesopore formation in theMNTP nanocrystals during annealing [39e41]. The doped MNTPnanocrystals with a large specific surface area and hierarchicalmesopore feature providemore abundant active sites, superior ionstransportation channels, as well as more efficient contact betweenelectrolytes and active materials to enhance the Na storageperformance.

To investigate the effectiveness of Gd3þ doping on Na storagecapability of the MNTP nanocrystals, electrochemical assessment iscarried out on half cells with sodiummetal as the both counter andreference electrodes. Fig. 6a shows the CV curves of the dopedMNTP nanocrystals and reference sample acquired at a scanningrate of 0.5mVs�1 in the potential range between 3.0 and 1.5 V vs.Naþ/Na. Obvious redox peaks at about 2.2 V (cathodic peak) and2.0 V (anodic peak) can be observed from all the samples indicatingNaþ insertion/extraction in the NTP lattice as follows [20,31]:NaTi2ðPO4Þ3 þ 2Naþ þ 2e�44Na3Ti2ðPO4Þ3. Compared with theundoped sample, the doped samples exhibit larger peak currentsrevealing that Gd3þ doping results in faster Na diffusion and higherNa storage capacity. In general, the potential difference betweenthe anodic and cathodic peaks in the CV curves reflects the degreeof electrochemical polarization and as shown in Fig. 6a, the po-tential difference of pure MNTP nanocrystals is larger than those of

(d) Typical TEM and HR-TEM images of the MNTP-Gd-2.

Fig. 5. Nitrogen-adsorption isotherms/corresponding pore-size distribution of the MNTP nanocrystals with different Gd3þ doping contents: (a)MNTP, (b)MNTP-Gd-1, (c)MNTP-Gd-2and (d) MNTP-Gd-3.

Fig. 4. Representative HAADF-STEM image and corresponding TEM elemental maps of the MNTP-Gd-2. The results show that Na, Ti, P, O and Gd elements are homogeneouslydistributed in single MNTP-Gd-2 nanocrystal.

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186 181

the doped samples. Especially, MNTP-Gd-2 with a moderateamount of Gd3þ shows the least electrochemical polarizationwith apotential difference of about 0.11 V, suggesting that the reactionkinetics is improved due to higher electron and ion conductivity ofthe Gd3þ doped MNTP nanocrystals.

Fig. 6b shows the galvanostatic charging/discharging profilesacquired from the MNTP-Gd-2 electrode at a current rate of 1 C(1 C¼ 133mAhg�1). There is a well-defined and symmetrical flatvoltage plateau at 2.1 V corresponding to the redox reaction of Ti4þ/Ti3þ. The electrode shows small electrochemical polarization of

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in the potential range between 3.0 and 1.5 V vs. Naþ/Na. (b)Representative galvanostatic charge/discharge profiles acquired from the MNTP-Gd-2 electrode at a current rate of 1C(1C¼ 133mAhg�1). (c) Rate performance from 0.5C to 50C. (d) Long stable cycling performance of the MNTP-Gd-2 at a rate of 5C.

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186182

100mV which is almost invariant with cycling. The low and stableelectrochemical polarization of the MNTP-Gd-2 electrode implieshigh ion/electron conductivity. The initial discharging and chargingcapacities of theMNTP-Gd-2 electrode are 105.2 and 102.0mAhg�1,respectively, corresponding to an initial Coulombic efficiency (ICE)of 97%. The ICE value is larger than those of carbon [42,43], phos-phorus [44], and metal alloy based anode materials [45e47]. Itimproves to close to 99% during cycling, indicating outstandingelectrochemical reversibility. Fig. 6c presents the rate performanceof the dopedMNTP nanocrystals and reference sample from 0.5C to50 C and Fig. S3 compares the corresponding galvanostaticcharging/discharging curves at different rates. All the Gd3þ dopedMNTP nanocrystals have better rate capability than pure MNTPnanocrystals as a result of faster Naþ diffusion and higher reversibleelectrochemical reactions in the dopedMNTP nanocrystals. At ratesof 0.5, 1, 2, 5, 10, 20, and 30 C, the MNTP-Gd-2 electrode showsdischarge capacities of 106.6, 96.2, 90.5, 80.0, 74.0, 65.3, and56.9mAhg�1, respectively. Even at 50 C, a reversible capacity of47.5mAhg�1 is maintained implying rapid charging/discharging inhigh energy/power density application within a few minutes. Incontrast, the referenced MNTP anode delivers poor rate perfor-mance showing a capacity of only 10.8mAhg�1 at 50 C. The dopedMNTP nanocrystals also deliver long and stable cycling perfor-mance at a high rate. As shown in Fig. 6d, the MNTP-Gd-2 electrodeat a rate of 5 C has a reversible capacity of 84.5mAhg�1 in the firstcycle and after 2000 cycles, a high capacity of 63.4mAhg�1 with75.0% retention and high Coulombic efficiency beyond 99% ismaintained. The large ICE, superior rate capability and long cyclinglife achieved from the doped MNTP nanocrystals are crucial tosodium-based electrochemical energy storage devices. The

electrochemical properties of the doped MNTP nanocrystals arefurther investigated by EIS after the rate capability test. As shown inthe high-medium frequency region of the Nyquist plots (Figure S4),the diameters of the semicircle representing the charge transferresistance (Rct) between the electrolyte and electrode of the dopedMNTP nanocrystals are smaller than that of the pure MNTP sample.Based on the inset equivalent circuit model in Figure S3, the Rctvalues for the MNTP-Gd-1, MNTP-Gd-2, and MNTP-Gd-3 are 154,145, and 158U, respectively (Table SII), which are smaller than thatof the pure MNTP sample. According to EIS spectra, the Naþ

chemical diffusion coefficient (DNa) is calculated and as shown inTable SI, the DNa values of the doped MNTP nanocrystals are largerthan that of the referenced MNTP sample. EIS reveals that thecharge transport kinetics in the doped MNTP nanocrystals are su-perior to that of the referencedMNTP sample and improved sodiumstorage is demonstrated.

To exploit the advantages of the doped MNTP nanocrystals withsuperior electrochemical characteristics, an NHC using the MNTP-Gd-2 as the anode and commercial AC as the cathode is con-structed. To maximize the energy density of the system, an organicelectrolyte of 1M NaClO4 in ethylene carbonate/propylene car-bonate is employed and the mass ratio of the anode to cathode is1:1.8. Before the battery performance is evaluated between 0 and2.5 V, the electrochemical performance of the AC in a SIB half-cellwas determined in a potential window of 2.5e4.5 V. As shown inFigure S5, the CV curves of the AC cathode showa quasi-rectangularshape suggesting that the capacitive behavior can be attributed toreversible surface desorption/adsorption of ClO4� anions.Figure S5b shows the charging/discharging profiles of the ACcathode at 5mVs�1 with a perfect triangular-shape profile due to

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186 183

surface desorption/adsorption mechanism of ClO4�. The AC cathode

also has high discharge capacity of 65mAhg�1 at 0.2 Ag�1, superiorrate performance of 31mAhg�1 at 10 Ag�1 (Figure S5c), and long-cycling ability with 45mAhg�1 at 5 Ag�1 after 600 cycles(Figure S5d). Fig. 7a displays the schematic illustration of themechanism of the NHC. When the NHC is charged, Naþ cations inthe electrolyte are intercalated into MNTP-Gd-2 via a redox reac-tion accompanying charge imbalance and at the same time, thesystem stabilizes with balance charges and ClO4� anions adsorbquickly onto the surface of AC to form a double layer. During dis-charging, both Naþ ions and ClO4� anions return to the electrolyteas the former are deintercalated from MNTP-Gd-2 and simulta-neously, the latter desorb from the surface of AC. As shown inFig. 7b, the CV curves of the NHC acquired at a scanning rate of5mVs�1 are irregular and different from that observed from anEDLC. Obvious humps can be observed between 0.8 and 2.1 Vindicating that the CV curves comprise a combination of a rectan-gular region and redox peaks. The redox peaks can be attributed torapid intercalation/deintercalation of Naþ ions into/from MNTP-Gd-2, whereas the rectangular region is due to the adsorption/desorption of ClO4� on the AC. The CV results confirm the coexis-tence of battery and capacitor behavior in the system therebymaking it a hybrid case. The CV curves overlap during successivescans demonstrating outstanding cyclability. Fig. 7c shows the CVcurves of the NHC at a sweep rate of 10 mVs�1 to a high rate of 100mVs�1. The peak currents appear in direct response to theincreasing sweeping rates and the well-defined sharp redox peaks

Fig. 7. (a) Schematic illustration of the operating mechanism of the assembled NHC using thethe assembled NHC for the initial five cycles at a sweep rate of 5mVs�1. (c) Typical CV curvchargeedischarge curves of the assembled NHC at a current density of 0.2 Ag�1.

are maintained implying excellent rate performance. Fig. 7d dis-plays the chargingedischarging curves of the NHC at a currentdensity of 0.2 Ag�1 and they are neither linear nor triangular inshape. The chargingedischarging curve is a combination of theplateau related to Naþ ion intercalation/deintercalation into theNTP host by the Ti redox reaction and ClO4� adsorption/desorptionon the AC. The results suggest a dual charge storage mechanism forthe hybrid capacitor. In addition, the charging�discharging profilesare stable (inset in Fig. 7d) during subsequent cycling, suggestingexcellent cycling ability.

To determine the electrochemical properties of the NHC, therate capability is monitored by chargingedischarging at differentcurrent densities. Fig. 8a shows the galvanostatic charging/dis-charging profiles at different current densities between 0.1 and10 Ag-1 (current densities according to the mass of the anode ma-terials). When the current density increases, the GCD profile of theNHC retains the unique shape indicative of effective sodiumintercalation/deintercalation in MNTP-Gd-2 as well as rapidadsorption and desorption on AC. The average discharge capacitiesare 51, 45, 42, 40, 35, and 30mAhg�1 for current densities of 0.1, 0.2,0.5, 1, 2, and 5 A g�1, respectively (Fig. 8b). Even at a large currentdensity of 10 Ag�1, the NHC delivers a high discharge capacity of25mAhg�1 reflecting superior rate capability. The long-termcycling durability is evaluated at a large current density of 5 Ag�1

and as shown in Fig. 8c, the NHC is stable after the initial charging/discharging cycles showing little degradation. After 30,000chargingedischarging cycles, the retention rate is still about 90%

MNTP-Gd-2 as anode material and commercial AC as cathode material. (b)CV curves ofes of the assembled NHC tested at high sweep rates from 10 to 100mVs�1. (d) Typical

Fig. 8. Electrochemical performance of the assembled NHC using the MNTP-Gd-2 as anode material and commercial AC as cathode material.(a)Chargeedischarge curves at differentcurrent densities ranging from 0.1 to 10Ag-1. (b) Rate capabilities of the NHC. (c) Cycling performance at 5Ag-1. The inset shows the photograph of a logo consisting of 17 LEDspowered by one NHC. (d)Nyquist plots collected from pristine and cycled NHC.

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186184

and Coulombic efficiency is nearly 100%. To compare the perfor-mance of the fabricated NHC with other batteries and capacitorsreported previously, the energy and power densities based on themass of active materials are calculated and presented in the Ragoneplot (Figure S6). Our NHC has a high energy density of 63Whkg�1 ata power density of 46 Wkg-1 and even at a high power density of4125 Wkg-1, the energy density is 20 Whkg�1. Hence, the NHC issuperior to other batteries and capacitors such as NaMnO2//NaTi2(PO4)3 [48], LiCoPO4F//C [49]. NVP//AHD [50], as well asNaTi2(PO4)3//Na2NiFe(CN)6 [51]. On account of the excellent energyretention at high power and durability, the NHC has large potentialin electric and hybrid electric vehicles. As a demonstration, theinset in Fig. 8c shows that a logo consisting of 17 LEDs is powered byone NHC.

To gain more insights into the superior energy storage mecha-nism of the assembled NHC, EIS spectra are obtained in the pristineand different cycling states at 0.5 Ag. As shown in Fig. 8d, theNyquist plots are composed of an inclined line in the low frequencyregion and one semicircle in the high to medium frequency region.In general, the semicircle is associated with the charge transferresistance (Rct) between the electrolyte and electrode, whereas theinclined line is related to ion diffusion in the host materials. Ac-cording to the inset equivalent circuit model, the fitting Rct result ofthe pristine full cell is merely 3.7U which is comparable to that ofmany reported carbon-based aqueous electric double-layer capac-itor systems [52e54] and Rct of subsequent cycles increase slowly(see Table SIII). The results shows that diffusion of Naþ in MNTP-Gd-2 is fast enough and the surface of AC is highly active foradsorption/desorption of ClO4�. EIS also indicates that the kineticbalance between the MNTP-Gd-2 anode and AC cathode is main-tained and superior sodium-storage properties are observed from

the assembled NHC. The morphology and structural stability ofMNTP-Gd-2 after long-term cycling are investigated by TEM andRaman scattering. As shown in Fig. 9a and b, the structure of theMNTP-Gd-2 nanocrystals is well preserved and the distinct latticefringes in the HR-TEM images (Fig. 9c) disclose excellent crystal-linity after the long-term tests. The Raman results acquired fromMNTP-Gd-2 after cycling are shown in Fig. 9d. Besides two strongcarbon peaks assigned to D band and G band from conductive ad-ditive of Super P in the range of 1200e1700 cm�1, characteristicpeaks associated with the NASICON-type NTP can be observedfurther corroborating the excellent structural stability in repeatedNaþ ion intercalation/deintercalation.

4. Conclusion

In summary, we have demonstrated a strategy to improve Nastorage performance of the MNTP nanocrystals via synchronoustailoring of texturing and conductivity using Gd3þdoping. Micro-structure characterization and kinetics analysis reveal that thelarger cell size, optimal porous structure, larger intrinsic electronconductivity, and higher Na ion mobility result from the intro-duction of a moderate amount of Gd3þ into the crystal structure ofthe MNTP nanocrystals. As a result, superior electrochemicalcharacteristics are observed from the doped samples. In the halfcells of sodium-ion battery, the MNTP nanocrystals with 5% Gd3þ

exhibit a large ICE beyond 97% at 1 C, superior rate capability of47.5mAhg�1 at 50 C, and long cycling life (>75.0% capacity reten-tion at 5 C after 2000 cycles). The sodium-ion hybrid capacitorcomposed of the Gd3þ doped MNTP nanocrystals as the anode andactivated carbon as the cathode delivers a high energy density of 63Whkg�1 at a power density of 46Wkg�1. Even at a high current rate

Fig. 9. (a)e(c) Typical TEM and HRTEM images acquired from the MNTP-Gd-2 electrode after prolonged 30000 cycles; (d) Raman spectra of the cycled MNTP-Gd-2 electrode andinitial referenced one.

G.B. Xu et al. / Electrochimica Acta 309 (2019) 177e186 185

of 5 Ag�1, it shows stable capacity after 30,000 cycles withoutobvious degradation. Our results provide insights into the designand fabrication of efficient NASICON-type anodes for advancedsodium-based energy storage devices.

Acknowledgements

This work was financially supported by National Natural ScienceFoundation of China (Grant Nos. 11474242 and 11774298), ScientificResearch Fund of Hunan Provincial Education Department (No.18A064) and City University of Hong Kong Strategic Research Grant(SRG) No. 7005105.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.electacta.2019.04.059.

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1

Electronic Supplementary Information

Simultaneous Texturing and Conductivity Tailoring of Mesoporous

NaTi2(PO4)3 Nanocrystals by Gadolinium Doping for Enhanced Na

Storage

G. B. Xu a, Z. Chen a, X. Liu a, Y. Zhang a, X. L. Wei a, L. W. Yang a, b, Paul K. Chu b*

a School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, China

b Department of Physics and Department of Materials Science and Engineering, City

University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Corresponding authors: [email protected] (L.W. Yang); [email protected]

(P.K. Chu)

2

Figure S1. (a-c) SEM images of MNTP-Gd-1, MNTP-Gd-2, and MNTP-Gd-3; (d-f)

EDS spectra of MNTP-Gd-1, MNTP-Gd-2, and MNTP-Gd-3

(a) (b) (c)

(d)

(f)

(e)

3

Figure S2. (a) Survey XPS spectrum; (b-f) High-resolution Na 1s, Ti 2p, P 2p, O 1s,

and Gd 3d XPS spectra of MNTP-Gd-2.

1065 1070 1075 1080

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Na 1s

(b)

450 455 460 465 470 475

Ti 2p 3/2

Inte

nsity (

a.u

.)

Binding Energy (eV)

Ti 2p 1/2

(c)

125 130 135 140 145

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

P2p

(d)

525 530 535 540

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

O1s

(e)

200 400 600 800 1000 1200

Inte

nsi

ty (

a.u

.)

Binding Energy (eV)

Ti2p

C1s

P2p

O1s

Na1sO a

(a)

1170 1180 1190 1200 1210 1220 1230 1240

Inte

nsi

ty (

a.u

)

Binding Energy (eV)

Gd (3d)

(f)

4

Figure S3. Charging-discharging profiles acquired from the MNTP-Gd-2 electrodes at

different currents.

0 20 40 60 80 100 1201.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

1C2C5C10C20C30C

Vo

ltag

e (V

)

Capacity (mAhg-1)

0.5C50C

5

Figure S4. (a) Nyquist plots and equivalent circuit of the cell (inset image) and (b) Z'

plots against ω-1 in the low frequency region of MNTP-Gd-2 and other reference

samples

0 200 400 600 800 1000

MNTP-Gd-1

MNTP-Gd-2

MNTP-Gd-3

MNTP

Z'ohm

0

100

200

300

400

500

600(a)

Fitting lines

Z''o

hm

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

500

1000

1500

2000

2500

3000

MNTP

MNTP-Gd-1

MNTP-Gd-2

MNTP-Gd-3

Linear fitting

Z¢o

hm

w-1/2

(b)

6

Figure S5. (a) CV curves, (b) Charging-discharging profiles, (c) Rate performance,

and (d) Cycle performance of the AC electrode in half cells.

2.5 3.0 3.5 4.0 4.5-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Cu

rre

nt

(mA

)

Voltage (V)

1st

2nd

3rd

4th

5th

(a)

0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100

0.2 Ag-1

10 Ag-1

5 Ag-1

2 Ag-1

1 Ag-1

0.5 Ag-1

Cap

acit

y (

mA

hg

-1)

Cycle number

0.2 Ag-1

(c)

0 100 200 300 400 500 6000

10

20

30

40

50

60

Cap

acit

y (

mA

hg

-1)

Cycle number

1 Ag-1

(d)

0 100 200 300 400 500 6002.5

3.0

3.5

4.0

4.5

Vola

tge

(V)

Capacity (mAhg-1)

(b)

7

Figure S6. Ragone plots of SIC in this work in comparison with other representative

energy storage systems in the literature.

1 10 100 1000 100001

10

100

[7]

[6]

[5]

[4]

[3]

[2]

Een

erg

y d

ensi

ty (

Wh

kg

-1)

Power density (W kg-1)

Na2CuFe(CN)

6//NaTi

2(PO

4)

3

NaMnO2//NaTi

2(PO

4)

3

NaTi2(PO

4)

3//Na

2NiFe(CN)

6

NTP@rGO//NVP/C

LiCoPO4F//C

NVP//AHD

NTP//AC

This work

[1]

8

Table SI. EDS results of MNTP, MNTP-Gd-1, MNTP-Gd-2, and MNTP-Gd-3

corresponding to Figure S2 (d-f).

MNTP-Gd-1 MNTP-Gd-2 MNTP-Gd-3

Eleme

nt

Weight

%

Atomic

%

Eleme

nt

Weight

%

Atomic

%

Eleme

nt

Weight

%

Atomic

%

O 48.63 54.40 O 48.80 61.93 O 39.82 45.16

Na 5.07 3.95 Na 6.13 5.41 Na 3.84 3.03

P 15.04 8.69 P 18.63 12.21 P 14.51 8.50

Ti 11.31 4.23 Ti 16.54 7.01 Ti 13.62 5.16

Gd 0.71 0.08 Gd 2.12 0.27 Gd 3.18 0.37

Table SII. Rs, Rct, σ and Di values of MNTP, MNTP-Gd-1, MNTP-Gd-2, and

MNTP-Gd-3 after the rate capacity test of the half-cells according to Figure S3.

Compound Rs(Ω) Rct(Ω) σ Di(cm2s-1)

MNTP 5.894 308.5 567.8 6.62×10-15

MNTP-Gd-1 5.719 183.3 46.4 9.91×10-13

MNTP-Gd-2 3.528 167.8 31.5 2.15×10-12

MNTP-Gd-3 4.068 173.3 49.8 1.30×10-13

Table SIII. Rs and Rct values of MNTP-Gd-2 before cycling and after 1, 200, 2000

cycles according to the EIS fitting results of Figure 8c.

Compound Rs(Ω) Rct(Ω)

MNTP-Gd-2 before cycling 4.4 3.7

MNTP-Gd-2 after 1 cycle 4.4 5.0

MNTP-Gd-2 after 200 cycles 5.1 5.4

MNTP-Gd-2 after 2000 cycles 6.1 5.5

9

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