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Nanostructured Na2Ti9O19 for Hybrid SodiumIon Capacitors with Excellent Rate Capability Swetha S M Bhat, Binson Babu, Mikhail Feygenson, Joerg C. Neuefeind, and Manikoth M Shaijumon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13300 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Materials & Interfaces

Nanostructured Na2Ti9O19 for Hybrid Sodium-Ion Capacitors with Excellent Rate Capability Swetha S M Bhat,a Binson Babu,a Mikhail Feygenson,b Joerg C. Neuefeind c and M. M. Shaijumona*

a. School of Physics, Indian Institute of Science Education and Research Thiruvananthapuram, Maruthamala PO, Thiruvananthapuram, Kerala, 695551, India b. Jülich Centre of Neutron Science, Forschungszentrum Jülich, 52428, Jülich, Germany c. Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, United States

KEYWORDS: Sodium ion battery, titanates, nanostructure, average/local structure, hybrid capacitor

ABSTRACT Herein we report a new Na-insertion electrode material, Na2Ti9O19, as a potential candidate for Na-ion hybrid capacitors. We study the structural properties of nanostructured Na2Ti9O19, synthesized by hydrothermal technique, upon electrochemical cycling vs. Na. Average and local structure of Na2Ti9O19 are elucidated from neutron Rietveld refinement and pair distribution function (PDF), respectively, to investigate the initial discharge and charge events. Rietveld refinement reveals electrochemical cycling of Na2Ti9O19 is driven by single phase solid solution reaction during (de)sodiation without any major structural deterioration, keeping the average structure intact. Unit cell volume and lattice evolution on discharge 1

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process is inherently related to TiO6 distortion and Na ion perturbations, while the PDF reveals the deviation in the local structure after sodiation. Raman spectroscopy and X-ray photoelectron spectroscopy studies further corroborate the average and local structural behavior derived from neutron diffraction measurements. Also Na2Ti9O19 shows excellent Na-ion kinetics with a capacitve nature of 86% at 1.0 mV s-1 indicating that the material is a good anode candidate for sodium ion hybrid capacitor. A full cell hybrid Na-ion capacitor is fabricated by using Na2Ti9O19 as anode and activated porous carbon as cathode, which exhibits excellent electrochemical properties, with a maximum energy density of 54 Wh kg-1 and a maximum power density of 5 kW kg-1. Both structural insights and electrochemical investigation suggests that Na2Ti9O19 is a promising negative electrode for sodium ion batteries and hybrid capacitors.

1. INTRODUCTION Rechargeable Li-/Na-ion batteries that deliver higher energy and power densities are highly essential to meet the growing energy demand from portable electronics and automotive sectors. In this regard, trade-off between specific energy and power densities in these systems needs to be addressed with newer storage mechanisms. Recently, there is great interest in the development of hybrid Li-/Na-ion capacitors that brings together both faradaic and double layer processes resulting in improved electrochemical performance in terms of both energy and power densities. Studies on sodium ion batteries (SIBs) have attracted great interest in recent times as one of the most promising alternatives to lithium ion batteries (LIBs) due to high abundance of sodium in earth crust, low cost and similar intercalation chemistry to lithium.1–3 Though there have been several reports on the use of activated porous carbons for the cathode part, developing efficient sodium storage materials for the fabrication of high performance Na-ion capacitor devices still remains a challenge. 2


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Carbonaceous materials such as hard carbon and graphene, Sn- and Sb-based alloys and transition metal oxides have been investigated as anode materials for SIBs.

4–6

Among

transition metal oxides, layered titanate materials have attracted great interest due to their low redox potential, non-toxicity and good electrochemical performance.7–11 The recent reports on sodium titanate, Na2Ti3O7, show great potential as efficient anodes for sodium-ion batteries.12–18 Na2Ti3O7 reversibly intercalates two sodium ions in the layered structure, with a very low intercalation potential. The promising capacity of Na2Ti3O7 paved a way to investigate analogous of sodium titanates, Na2O.nTiO2.12,19 For instance, Rudola et al., studied Na2Ti6O13, which exhibits impressive cycling stability, as possible anode material for sodium ion batteries. Na2Ti7O15 nanotubes were synthesized on titanium net substrate which shows 130 mAh g-1 capacity over 200 cycles at 1.0 A g-1.20,21 The (de)insertion of sodium is generally due to the open structural framework of sodium titanates which can easily accommodate bigger alkali ion in the host matrix without any structural degradation. The lower titanates such as Na2Ti3O7 usually form layered structure whereas higher titanates, for example, Na2Ti6O13 exhibit a tunnel like structure.

13,20,22,23

Unfortunately, the practical

applications of these materials are limited because of the fast capacity fading upon cycling. For example, Na2Ti3O7 failed to deliver sustained cycling, while Na2Ti6O13 exhibits a very low capacity of 49 mAh g-1 at 1C rate, and the monoclinic phase of Na4Ti5O12 exhibits low electrochemical performance with capacity not at par with the practical anode materials.17,20,24 Nanoscale approaches for the fabrication of electrode materials result in increased surface area and shorter diffusion length, leading to enhanced electrochemical properties.21 Various morphologies of sodium titanates have been fabricated and reducing their size down to nanometer scale improved the Na+ insertion kinetics, while maintaining the stability of the nanostructure.8,16,23,30–32 For example, nanorods of Na2Ti6O13 show impressive long cycle life due to its high surface area exposed to electrolyte.20 For Na-ion capacitor applications, it 3



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would be ideal to design insertion electrode materials that exhibit pseudocapacitive behavior. When designing new and efficient electrode materials, it is also important to focus on their fundamental understanding in terms of extraction and insertion of sodium ions. There have been few attempts to get structural insights into the capacity fading mechanism of sodium titanate upon prolonged cycles.12,18,23 The high quality neutron diffraction data would be useful to obtain the fine details of the average structure of the material, as position of light atoms such as oxygen can be accurately determined. Neutron PDF is a complementary method to probe the local structural behavior of the anode material, which reveals a potential short range order within a material. The PDF analysis has proven to be an excellent tool for investigating the local environment of anode/cathode materials. The information provided by PDF analysis helped to rationalize the electrochemical mechanism and cycling performances.25–29 Motivated by the electrochemical performance and the unique structure of sodium titanate systems, herein, we explore the structural and electrochemical properties of Na2Ti9O19 as a new electrode material for sodium ion capacitor, with a focus on its structural behavior upon (de)sodiation. Nanostructured Na2Ti9O19, with a flower-like morphology is synthesized for the first time, by facile hydrothermal technique and we show that the material exhibits remarkable pseudocapacitive behavior as a new electrode material for sodium ion hybrid capacitor. The crystal structure of Na2Ti9O19 is studied by the Rietveld and PDF analysis of neutron powder diffraction data. The obtained results are further supported by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) studies. We investigate the electrochemical properties and the kinetic studies reveal very high capacitive nature of 86% at 1.0 mV s-1, which shows its potential as anode material for hybrid Na-ion capacitor. Further a full cell Na-ion hybrid capacitor is fabricated with Na2Ti9O19 as anode and commercially available porous carbon (PC) as cathode, which exhibits excellent electrochemical properties with a good cycling 4


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stability. 2. EXPERIMENTAL SECTION 2.1 Materials synthesis and characterization Nanostructured Na2Ti9O19 has been synthesized by hydrothermal technique. In a typical procedure, TiO2 (1.42 g) was added to 15 M NaOH solution while stirring. After continuous stirring for 3 h, solution was transferred to 40 ml Teflon vessel and kept at 130 ˚C for 72 h. Excess NaOH was removed from the product by thorough water wash. The obtained white precipitate was calcined at 600 ˚C for 2 h under air to achieve pure phase of Na2Ti9O19. The phase purity of the sample was characterized by using PANalytical Empyrean powder X-ray diffractometer with Cu Kα radiation. Nano Nova field emission scanning electron microscope (FE-SEM) and Technai F20/F40 200 KeV Transmission electron microscope (TEM) were employed to ascertain the morphology and size of Na2Ti9O19 ‘flowers’.

Thermal stability of

Na2Ti9O19 was analyzed

by using

SDT Q600

thermogravimetric analyzer. The changes in the oxidation states of titanium was analyzed by using X-ray photoelectron spectroscopy (XPS, AXIS ultra DLD). Raman spectra were recorded with LaBRAM HR Raman spectrometer (Horiba Jobin Yvon) by using 633 nm He– Ne laser source. Time-of-flight (TOF) neutron powder diffraction experiments were performed at Nanoscale Ordered Materials Diffractometer (NOMAD) at Spallation Neutron Source in Oak Ridge National Laboratory.26 The samples were loaded in quartz capillaries with 2 mm diameter and data was collected for 2 h at room temperature. The PDF was calculated by using the Fourier transform at Qmax = 22 Å-1 of the structure factor which was obtained from normalizing scattering intensity to the scattering from solid vanadium rod and subtracting the background from the identical quartz capillary. The Rietveld refinements were carried out by using GSAS/EXPGUI33 suite. The PDF analysis was performed by using PDFGui.34 The Brunauer-Emmett-Teller (BET) specific surface area of Na2Ti9O19 and porous 5



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carbon are measured from the N2 adsorption-desorption studies conducted at liquid nitrogen temperature (77 K) by using Micromeritics 3-Flex surface characterization analyser. 2.2 Electrochemical characterization Galvanostatic charge discharge, cyclic voltammogram (CV), and electrochemical impedance spectroscopy (EIS) measurements were carried out by using VMP3 BioLogic electrochemical workstation. Na2Ti9O19 electrodes were fabricated by making a slurry of 75% of the active material, 15% of conductive carbon black (super P) and 10% of sodium carboxy cellulose (CMC) binder with the help of buffer solution (citric acid pH 3). The prepared slurry was casted on to the copper foil. Electrodes were dried in air at 65 ˚C and then in vacuum at 120 ˚C for 12 h. Typical electrode mass loading of the active material was ~1.2 mg/cm2. Electrochemical performance of the material was evaluated in CR2032 twoelectrode coin cell in which metallic sodium was used as a counter and reference electrode for the half cell configuration. The solution of 1M NaPF6 in ethylene carbonate (EC)dimethylene carbonate (DMC) in 1:1 ratio was used as electrolyte and the electrochemical measurements were performed in a voltage window of 0.0-2.5 V. The porous carbon (PC) electrodes were prepared by making a thick slurry with 80 wt% active material, 10 wt% conductive carbon black (super P) and 10 wt% polytetrafluoroethylene (PTFE, 60 wt% in H2O, Sigma) in ethanol, which is rolled into a uniform thin film and is pressed onto stainless steel mesh (Alfa Aesar) with a mass loading of ~2.7 mg/cm2 and dried in 1200C for 12 hr in vacuum. The Na-ion full cell hybrid capacitor was assembled with Na2Ti9O19 as anode and porous carbon (PC) as cathode in an optimum mass ratio of 1:3.25 in the same organic electrolyte and the electrochemical studies are carried out in a suitable potential range of 1 – 4.5 V.

3. RESULTS AND DISCUSSION 6


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Powder X-ray diffraction (XRD) analysis confirms the phase purity of the material (Figure S1a) and matches well with the ICSD card no 62766. Na2Ti9O19 belongs to a monoclinic crystal system with C2/m space group. Thermogravimetric analysis depicts the thermal stability of the material (Figure S1b) and Na2Ti9O19 shows no thermal decomposition up to 800 ˚C, except the release of physisorbed water below 200 ˚C. The scanning electron microscopy (SEM) studies of Na2Ti9O19, synthesized by hydrothermal technique, revealed the flower-like morphology that has evolved from a single base of quasi sphere (Figure 1a). Transmission microscopy (TEM) image show the nanopetals, which are protruded from the core of the quasispheres (Figure 1b). The lattice fringes observed in Figure 1c are approximately 0.268 nm, which corresponds to (-404) plane of C2/m space group in monoclinic crystal system. Selected area electron diffraction (SAED) pattern further shows the crystalline nature of the material (inset of Figure 1c). The nanopetals, 10 – 15 nm wide, emerge from the quasi sphere forming flower-like morphology. Each such quasisphere is connected to each other forming the clusters of flowers as seen in Figure 1a. The evolution of the flower-like morphology and hierarchical assembling of the nanopetals to form individual flowers usually follow the Ostwald ripening mechanism.35 Hence, we believe that the possible mechanism for the formation of 3D-flowerlike architecture of Na2Ti9O19 involves the following pathways; (i) formation of nanopetals (ii) the growth of the nanopetals via Ostwald’s ripening process (iii) the formation of hierarchical quasi-flower like morphology from the nanopetals through dissolution and recrystallization process.36 Most of the sodium titanates reported in the literature usually exhibit nanoribbons, platelets or nanorods like morphology, however, Na2Ti9O19 exhibits unique morphology comprising flower-like particles with nanopetals evolving from the quasisphere.16,37,38 Detailed porosity measurements were carried out and BET surface area of ~14 m2 g-1 was obtained for Na2Ti9O19, and corresponding N2 adsorption-desorption isotherm and pore 7



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volume distribution curve (inset), measured at 77 K are given in Figure S2a. The Na2Ti9O19 was studied as an electrode in Na-half cells, which were galvanostatically cycled within the voltage window of 0.0-2.5 V vs. Na/Na+. The charge-discharge curves of Na2Ti9O19 electrode cycled at a rate of 20 mA g-1 vs. Na exhibit a sloppy voltage plateau (Figure 2a), which could be an indicative of a single phase sodiation behavior.17 It can be noticed that the electrode delivers high capacity of 220 mAh g-1 for the first discharge process with a Coulombic efficiency of 85%, while it relatively decreases to a reversible capacity of 120 mAh g-1 after 100 cycles. The decreased initial coulombic efficiency is due to the inevitable formation of solid electrolyte interface (SEI). This behavior is consistent with initial cyclic voltammogram (CV) curves (Figure 2b), where it exhibits a small shoulder at 0.5 V in the cathodic scan, which is absent in second cycle onward. No significant change is observed in the reversible capacity over 100 cycles. The electrode delivers remarkably steady capacity of 117 mAh g-1 with coulombic efficiency close to 100% (Figure 2c).

Figure 1. Morphological characterization of Na2Ti9O19 grown by hydrothermal technique. (a) SEM image of Na2Ti9O19 depicting the flower like morphology, inset shows high resolution SEM image of Na2Ti9O19 nanopetals. (b) TEM and (c) HRTEM images of Na2Ti9O19, SAED pattern revealing the crystallinity is shown in the inset of (c).

8


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CV curves for Na2Ti9O19 at a scan rate of 0.1 mV s-1 in 0.0 - 2.5 V voltage window for the initial five cycles are represented in Figure 2b. Broad oxidation and reduction peaks are observed. It is also interesting to note a large area under the curve, indicative of pseudocapacitive contribution to the total charge storage. The rate capability measurements were carried out at different current densities, which showed an excellent rate performance (Figure 2d). The performance of Na2Ti9O19 electrode was further investigated by electrochemical impedance spectroscopy (EIS) measurements. Typical Nyquist plots obtained at different depth of discharge and charge are displayed in Figure S3. EIS spectrum was fitted to an equivalent circuit which is shown in Figure S4 (a) and (b) and Table S1. Interestingly, the shape of the Nyquist plots changes during the discharge event. Nyquist plots obtained for the electrodes discharged up to 1.0 V comprise of a depressed semicircle at high frequency and medium frequency region and a linear Warburg line at low frequency region. However, medium frequency region semicircle becomes larger and the inclined line moves towards imaginary axis as the sample was discharged to 0 V. The Nyquist profile of the charged sample at different voltages shows reversed phenomenon to the discharging event. The semicircle can be attributed to the charge transfer resistance at the electrode/electrolyte interface and the Warburg line corresponds to diffusion of sodium ions towards the bulk of the electrode. Nyquist plot illustrates that with the depth of sodium ion insertion, the charge transfer resistance increases. As it leaves the host structure, the charge transfer reaction was reduced which is evident from the Nyquist curve of charged sample. Diffusion coefficient was calculated for Na2Ti9O19 and found to be 1.624 x 10-18 cm2 s-1 (Supporting information). Rietveld refinement of neutron time of flight diffraction data was carried out to obtain the crystallographic information about Na2Ti9O19. Na2Ti9O19 adopts a monoclinic crystal system 9



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with centrosymmetric space group C2/m.39–41 Rietveld refinement fit is shown in Figure 3a

+

Figure 2. Electrochemical performance studies of Na2Ti9O19 as anode vs. Na/Na . (a) -1

Galvanostatic discharge/charge profiles, cycled at a rate of 20 mA g . (b) Cyclic -1

voltammograms for initial five cycles at a scan rate of 0.1 mV s (c) Capacity retention -1

of the electrode at a rate of 20 mA g . (d) Rate capability plots cycled at different rates -1

-1

from 0.02 A g – 5 A g .

and crystallographic information obtained from the fit are listed in Table 1 and S2. We found that there is a slight increase in the unit cell of Na2Ti9O19 compared with that of the previously reported Na2Ti9O19 (ICSD #62766). The expansion of the lattice could be due to the reduction of the particle size of Na2Ti9O19. As reported in the literature for several metal oxides, unsaturated dangling bonds could be responsible for the elongation of the anion cation bond lengths which in turn increases the unit cell.42,43 Figure 4a depicts the structure of Na2Ti9O19 along (010) direction. The crystal structure of Na2Ti9O19 is similar to a typical

10


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Figure 3. Observed, Calculated and Difference plot obtained from Rietveld refinement of TOF neutron diffraction data of (a) Na2Ti9O19 (b) sodiated Na2Ti9O19 and (c) desodiated Na2Ti9O19.

sodium titanate consisting of tunnel-like structure formed by the association of corner and edge shared octahedral framework of five crystallographically different titaniums. The infinite chains of titanium octahedra are formed along a axis. The sodium atoms are distributed along b axis within the tunnel structure. In order to investigate the structural evolution during sodium insertion/extraction, electrode specimen were collected after first discharge and charge processes. Figure 3b, c shows the Rietveld refinement of such specimen after first discharge and charge, respectively. Atomic coordinates are listed in Table S3 and S4. Although, discharge profile shows sloppy curve implying topotactical sodium insertion into the host matrix, neutron diffraction data revealed the change in the lattice parameters. The unit cell parameters for pristine, sodiated and desodiated samples are listed in Table 1. Electrochemical discharge leads to increase in the unit cell volume. Even at full sodiation, peaks can be indexed to monoclinic crystal system with C2/m space group. This behavior is in stark contrast with Na2Ti3O7, which is known to follow a biphasic electrochemical reaction while (de)sodiating, whereas, the higher sodium titanate such as Na2Ti6O13 undergoes single phase reaction during electrochemical cycling.17 Understanding the change in the lattice

11



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parameter is important as it influences the cycling performances. As reported earlier in the literature for sodium titanates, sodium occupation induces biphasic reaction.11 Usually, the two phase reaction brings capacity loss as there is lattice differences. Hence, change in the lattice parameter is discussed in detail. Figure S5 illustrates the change in the lattice parameters upon sodium insertion and extraction from the pristine sample.

Table 1. Crystallographic parameters obtained from Rietveld refinement of neutron data. Crystallographic

Na2Ti9O19

Parameters

Na2Ti9O19

Na2Ti9O19

First Discharge

First Charge

Crystal System

Monoclinic

Monoclinic

Monoclinic

Space group

C 2/m

C 2/m

C 2/m

a (Å)

11.913(16)

12.025(12)

12.460(17)

b (Å)

3.942(3)

4.635(2)

3.770(04)

c (Å)

15.336(14)

14.827(21)

16.689(21)

β (˚)

104.96(14)

105.89(4)

104.64((13)

Unit Cell Volume (Å3)

695.69 (30)

794.87(64)

758.48 (62)

Rwp (%)

7.1

8.2

7.9

12


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As the sodium content increases in the host matrix, lattice parameters a, b and β increases, whereas c axis decreases. Consequently, the expansion of unit cell volume is observed. Once the sodium is de-intercalated from the parent structure, b and β parameters decrease. However, we note that a and c parameters increase after the sodium extraction. Change in unit cell volume upon sodiation was ~14% and decrease is only ~6%. This result demonstrates that material undergoes single-phase reaction keeping the integrity of the structure. To understand the structural evolution during full discharge and charge, careful investigation of Ti-O and Na framework was elucidated from the Rietveld refinement results. Figure 4a depicts the modification of the pristine structure during Na ion insertion and extraction. The TiO6 was distorted after the sodium insertion. Interestingly, only Ti(3)-O and Ti(4)-O average distances in Na2Ti9O19 were increased considerably (Ti(3)-O

2.03 to 2.23

Å ; Ti(4)-O 1.87 to 2.08 Å) while Ti(1)-O and Ti(2)-O mean distances decreased (Ti(1)-O 2.18 to 2.03 Å and Ti(2)-O 2.46 to 2.03 Å) (Table S5). The increased bond length predicts the transformation of Ti(IV) to Ti(III), as the reduced phase Ti(III) has larger ionic radius than that of Ti(IV). This variation in the structural framework of Ti-O influenced the change in the tunnel structure. Similar to modification in Ti-O, the distribution of Na ions also disturbed from the incoming sodium atoms. There is only one crystallographically distinct sodium atom at 4i Wyckoff position, which is fully occupied. The incoming sodium ion needs to occupy the different Wyckoff position. It is noticed that pristine sodium ions are displaced from their initial position once the new sodium ions are entered to the pristine structure. The Na-Na distance was elongated as the sodium is migrated from its initial position. Na-Na distance varies from 2.34 to 2.62 Å. However, their coordination number is retained after the desodiation though it induced the structural rearrangement of the Na-O coordination during sodium insertion. The Na-O distances in pristine structure ranges from 2.35 to 3.28 Å, 13



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whereas in fully discharged state Na coordination ranges from 2.37 to 3.19 Å. As the Na-O distance decreases, the distance between two original sodium increased. The change in Na-Na distance is shown in Figure 4b along with the rearrangement of Ti-O framework. It is noteworthy that the increase in the bond lengths of Ti-O and Na-Na resulted in expansion of a and b axis of the pristine structure to accommodate the inserted sodium cations.

Figure 4. The crystal structure of (a) Na2Ti9O19 upon sodiation and desodiation. (b) TiO6 octahedra and Na-Na in (i) pristine (ii) desodiated and (iii) sodiated Na2Ti9O19.

Though there is a structural reorganization of Na and Ti-O framework in average structure, probing the local structure gives an insight of the (de)sodiation mechanism. PDF offers an 14


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opportunity to study the short range order of the material to investigate the change in local environment of the material.44 The PDF method analyzes both Bragg and diffuse scattering. The PDF is obtained by converting a conventional diffraction data into real space using the Fourier transformation. It provides information about the bond length distribution in short, intermediate and long range order while the Rietveld refinement gives only long range average structure. Therefore, PDF reveals the structural transformations during sodium (de)insertion in the host structure on the local scale which would affect the electrochemical performance. The PDF method found numerous applications in alloys for tracing the structural deterioration and capacity fading mechanism, however it was rarely used for the materials undergoing insertion mechanism in anodes.

6,45

Neutron PDF was employed to analyze the short range structure of Na2Ti9O19.

Neutron Rietveld refinement which revealed the average structure of Na2Ti9O19 was adopted as the starting model for the PDF refinement. Figure 5a shows the PDF fit for Na2Ti9O19 refined in the short range of 1.4 – 4.8 Å for Qmax = 22 Å-1. The cell parameters and atomic coordinates obtained from the PDF refinement on the local scale are given in Table S6 and S7. We note that there is slight variation of average structure with the local structure. This could be due to the different instrument resolution function and asymmetric shape of the diffraction peak.46 On the basis of the structural studies obtained from the least square PDF refinement for the pristine sample, key features are outlined here; as the titanium has negative neutron scattering length, the correlation arising from Ti will appear as a negative peak in neutron PDF. Multiple peaks of negative correlations emerged at 1.59, 2.01 and 2.60 Å correspond to Ti-O bonds. The correlation peak at 2.5 Å can be ascribed to Na-O bond lengths and Na-Na peaks at 3.35 Å. The bond distances obtained from short range correlations are consistent with the average structure from neutron Rietveld refinement. The PDF data was collected for the sodiated 15



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sample along with the pristine sample (Figure 5b). Interestingly, pronounced change in the local structure of the sodiated sample was observed from the host matrix. The decrease in the intensity of the fully sodiated sample could be due to the presence of conductive carbon added while fabricating the electrode. The contribution of the carbon in the PDF data was negligible as it has short C-C bond length below 1.5 Å.

Figure 5. PDF and Raman spectroscopy analysis of Na2Ti9O19. (a) PDF analysis of pristine Na2Ti9O19 (b) comparison of PDF plots of pristine, sodiated and desodiated Na2Ti9O19. (c) Raman spectra of pristine, sodiated and desodiated Na2Ti9O19.

It was difficult to fit the data as the structure of the sodiated phase has not been previously reported. The drastic change in the local structure of the sodiated electrode confirms the insertion of sodium into the host structure and caused local structural change. Na-Na bond distances shifted to higher bond lengths compared to the pristine, once the sodium enters to the host structure. Presumably, the shift in the Na-Na correlation has occurred to minimize 16


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the coulombic repulsion force between the already existed sodium atoms and new sodium atoms in the parent structure, and hence the Na-Na distance has been increased. This observation is consistent with the average structure of the fully discharged sample. Na-O correlations arise in 3.10 Å range which is slightly shifted to shorter bond lengths. This difference can be attributed to the distortion of the Na-O polyhedra where Na-O bond length has been decreased. In order to independently confirm results of the PDF analysis, local structure of sodium titanate was further investigated from ex situ Raman scattering experiments. Raman spectra were collected for pristine, fully discharged and charged samples and compared for the structural modifications upon (de)sodiation, so that the experimental conditions for PDF and Raman measurements are the same. The Raman modes displayed in Figure 5c, clearly demonstrate the significant changes in the structure of Na2Ti9O19 on sodiation and desodiation. The Raman bands appeared in between 600-800 cm-1 correspond to Ti-O stretching vibrations, while the ones below 400 cm1

can be ascribed to Na-O stretching vibrations.37,47 Higher frequency bands at 800-950 cm-1

can be attributed to Ti-O short bond lengths emerging from the TiO6 octahedra, whose bond distance is less than 1.90 Å. The minor peak at 950 cm-1 in the pristine sample, appeared due to the presence of short bond length which is below 1.70 Å. The peak disappears after the sodiation, because the process alters the TiO6 octahedra and corresponding Ti-O distances. Hence, sodiated sample exhibits a red shifted peak at 880 cm-1, which could be due to the elongation of Ti-O distance from 1.70 to 1.90 Å, in excellent agreement with results of the PDF analysis. The major peak at 634 cm-1 in the pristine sample is due to Ti-O-Ti stretching frequency from the edge shared TiO6 octahedra, which are diffused in the (de)sodiated sample (indicating the modification of the Ti-O-Ti framework). The intensity of the peak at 270 cm-1 in pristine sample increases when sample is fully discharged, indicating the increase of the number of Na-O atomic pairs. However, fully charged sample also shows similar behavior, 17



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implying the residual sodium still remains in the sample. The bands below 150 cm-1 are due to Ti-O-Ti vibrations from the tunnel structure as seen in Na2Ti6O13 system. The change in the position and intensity of the peak confirms the modification in the tunnel structure due to incoming sodium. The red shift in the Raman bands from 400 – 600 cm-1 for the (de)sodiated sample reveals the increased bond length of Ti-O present in edge and corner shared TiO6 consistent with the local and average structures. XPS technique was adopted to follow the oxidation state of the Ti atoms during electrochemical cycling (Figure 6). Figure 6a depicts the XPS wide spectrum showing the characteristics corresponding to Ti 2p state with (IV) oxidation state, which is in agreement with previous results.48–51 However, spectra obtained for fully discharged sample reveals the presence of Na, Ti, O and C elements. The oxidation state of titanium was traced for fully discharged and charged samples. The pristine sample exhibits two peaks appearing at 458 eV and 464 eV, corresponding to Ti (III) and Ti (IV), respectively (Figure 6b).

18


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Figure 6. (a) XPS wide scan of pristine, discharged and charged Na2Ti9O19. XPS of Ti 2p for (b) pristine Na2Ti9O19, (c) sodiated Na2Ti9O19 and (d) desodiated Na2Ti9O19. When the sample is fully discharged, some of the Ti (IV) is converted to Ti (III) (Figure 6c). It is in agreement with Rietveld refinement, which suggested that sodiation increases the bond length of some of the TiO6 significantly, reflecting a reduction of Ti(IV) to Ti(III). The subsequent desodiation converts Ti (III) to Ti (IV), however XPS shows that the traces of Ti (III) still remain in the sample (Figure 6d). The combination of average and local structural analysis of neutron data, Raman studies, XPS measurements and electrochemical investigation suggests the following scenario; according to average structure, there was no phase change except for variation in the unit cell parameters, hence sodium insertion and extraction in Na2Ti9O19 proceeds through single phase solid solution mechanism. The insertion of Na can thus be represented as:

Na2Ti9O19 Na+ →

Na2+xTi9O19 19



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The sodium insertion in the host matrix causes TiO6 distortion and subtle migration of sodium, however, the distortion was reduced once sodium is extracted from the host. It was confirmed by the oxidation state change from Ti(IV) to Ti(III) during discharge, despite some fraction of Ti(III) remained after charging. The PDF analysis confirms the change in local structure of the material, which is permanent even after the desodiation and these results are further supported by Raman measurements. The local structural changes could in principle contribute to the reversible capacity and cycling stability of the material. Though, the average structure of the sodium titanates has been studied for a few systems, local structure analysis has not carried out and detailed mechanism of sodium storage remained obscured. Na2Ti3O7 and Na2Ti6O13 usually exhibit capacity fading over prolonged cycles, which cannot be explained by the Rietveld analysis alone, thus the local structure studies of these system might give insights in to how to improve their cycling stability. Our work presents both

20


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Figure 7. (a) Plot showing logarithm of specific peak current vs. sweep rates, ranging from 0.1-1000 mV s-1 (b) CV of Na2Ti9O19 at 1.0 mV s-1, representing the contribution of nonfaradaic capacitance (shaded region) and faradaic region, (c) bar diagram showing total stored charge of Na2Ti9O19 at different scan rates, clearly indicating the respective contribution from sodium intercalation and non-Faradaic capacitance to the total capacity.

average and local structural analysis of Na2Ti9O19, which would be beneficial to design efficient anode material for sodium ion batteries by overcoming the capacity fading. The 21



Page 22 of 36

efforts are in progress to unveil the position of the incoming sodium site in Na2Ti9O19 utilizing the density functional theory. It is noteworthy that the CV curves of Na2Ti9O19 suggests the material exhibits combined faradaic and capacitive behavior and thus could be an ideal electrode for Na-ion hybrid capacitor. The capacitive contribution associated with the sodium ion insertion on Na2Ti9O19 has been quantitatively studied by analyzing the CVs.

The kinetics study of the charge

storage in Na2Ti9O19 is analysed by recording CVs at different scan rates from 0.1 - 1000 mV s-1 (Figure S6) obtaining a b-value of ~0.9 for both cathodic and anodic peak, indicative of surface-controlled fast capacitive processes and a b -value of ~0.55 obtained after 20 mV s-1, clearly shows that the Na-ion kinetics are diffusion limited (Figure 7a).52 Figure S7b shows the b-value at each potential during charging and discharging. Further quantitating the capacitive contribution associated with sodium ion insertion on Na2Ti9O19 shows that the capacitive nature increases with scan rates and found to be 86% at 1.0 mV s-1 (Figure 7b and c).

53,54

The total charge stored by the material and the charge

stored during fast kinetics is further investigated using Trasatti’s procedure which reveals that about 67% of the qtotal is attributed to pure surface phenomenon and hence is more capacitive in nature (Figure S8).55 Therefore, Na2Ti9O19 is suitable as Na-ion capacitor anode material due to its high capacitive nature, which provides power that is compatible with the cathode part. Thus Na-ion hybrid capacitor is fabricated with Na2Ti9O19 as anode and the commercially available porous carbon (PC) with a BET surface area of 1677 m2 g-1 (Figure S2b) as cathode in 1M NaPF6 in ethylene carbonate (EC)-dimethylene carbonate (DMC) electrolyte. To achieve maximum electrochemical performance, the mass of the cathode and anode must be balanced by using the relation,

= ACS Paragon Plus Environment

22

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Figure 8. (a) Galvanostatic charge-discharge profiles of Na2Ti9O19/PC hybrid Na-ion capacitor (full cell) tested between 1 and 4.5 V at different current densities (b) cyclic voltammograms of Na2Ti9O19/PC hybrid Na-ion capacitor (full cell) capacitor at various scan rates (c) the rate capability study of hybrid capacitor at different current densities (d) Comparison of Ragone plot obtained for Na2Ti9O19/PC hybrid Na-ion capacitor with that of reported Li-ion and Na-ion hybrid devices.

where m is the mass of the active material and Q is the specific capacity calculated from the individual electrochemical measurements of each electrode vs. Na-metal at the same current density of 100 mA g-1 (Figure S9). The pseudocapacitive and double layer capacitive nature of Na2Ti9O19 and porous carbon (PC) are clearly seen from the voltage profile in Figure S9.

23



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The Na2Ti9O19/PC hybrid capacitor is assembled with an optimum mass ratio of 1:2.25 and the electrochemical studies are carried out in a suitable potential range of 1 – 4.5 V. The galvanostatic charge-discharge measurements are carried out at different current densities (based on the total active mass loading, m ~ 3.03 mg) and are shown in Figure 8a. During charging, the Na-ions from the electrolyte intercalate into the Na2Ti9O19 anode, while the PF6¯ anions get simultaneously adsorbed on the surface of porous carbon electrode and the reverse process happens during discharge. Figure 8b represents the cyclic voltammograms obtained at different scan rates, which show a combination of both pseudo and double layer nature. Further, the rate capability studies are performed, which shows good capacity retention even after cycling at higher current density of 5 A g-1 (Figure 8c). The specific power density ( ) and specific energy density ( ) of the Na2Ti9O19 /PC can be calculated from the following relations,

=

×

and

= ×

where, =

!

, "# and "$ are the potentials at the beginning and end of

galvanostatic discharge curve, is the applied current, is the total active mass of both electrodes and is the time taken for discharging.56 A higher energy density of ~ 54 Wh kg-1 is obtained at a power density of ~ 687 W kg-1 and even at a higher power density of 5 kW kg-1, the cell shows an energy density of ~20 Wh kg-1. Figure 8d shows a comparison of Ragone plot of the fabricated Na-ion capacitor with several Na-ion hybrid capacitors reported in the literature and it is found that our device exhibit superior performance.10,57–66 Figure S10 24


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shows the capacity retention of Na2Ti9O19/PC hybrid capacitor upon cycling at a current density of 2 Ag-1. Na2Ti9O19/PC hybrid capacitor retains 75% of the initial capacity even after 2000 cycles. The pseudo capacitance behavior of Na2Ti9O19 could be due to the tunnel structure of the material along with its unique hierarchical flower-like morphology enhancing the charge transfer at electrochemically active site. The nanopetals present in Na2Ti9O19 enable enhanced diffusion of ions from electrolyte to electrode, thus shortening the diffusion length.

4. CONCLUSION The sodium titanate, Na2Ti9O19, with flower-like morphology embedded by nanopetals has been synthesized by controlled hydrothermal technique and was studied as a new electrode material for hybrid sodium ion capacitors. The structural studies concerning the average structure of Na2Ti9O19 and (de)sodiated Na2Ti9O19, obtained from neutron diffraction, reveals that the electrochemical cycling is accompanied by a single phase solid solution mechanism, which is advantageous compared to other sodium titanates and TiO2. The unit cell volume expansion was observed, while the structure of Na2Ti9O19 was conserved during discharging. The pronounced change in Na-Na distance and structural changes of the TiO6 octahedra were also responsible for the unit cell elongation. XPS analysis independently confirmed the average structural modifications and provided the evidence of oxidation state change for Ti(IV) in discharge and charge cycles. However, PDF study reveals that there is a deviation in the local structure after sodium is inserted into host matrix, which is further supported by the Raman spectroscopy studies. This study provides detailed insights into the structural changes during electrochemical cycling. Na2Ti9O19 exhibited excellent Na-ion insertion and extraction kinetics with a huge capacitive nature of ~ 86% at 1.0 mV s-1 indicating that it is a potential anode for hybrid Na-ion hybrid capacitor. Further, 25



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Na2Ti9O19/PC full cell hybrid Na-ion capacitor was fabricated which showed excellent electrochemical properties with a high energy density of ~ 54 Wh kg-1 at a power density of ~ 687 W kg-1 and an energy density of ~20 Wh kg-1 even at maximum power density of 5 kW kg-1.

ACKNOWLEDGEMENTS Financial support from Technology Mission Division (TMD), Department of Science & Technology, Govt. of India [DST/TMD/MES/2k16/114], is gratefully acknowledged. Neutron-scattering experiments were conducted at the SNS, which is operated with the support from the Division of Scientific User Facilities, Office of Basic Energy Sciences, US Department of Energy.

ASSOCIATED CONTENT Supporting information Supporting information is available. XRD pattern, TGA spectra, BET surface area measurements,

Electrochemical impedance spectra, lattice parameters, atomic coordinates,

CV, plot showing the linear relationship of log v vs.log i for anodic (charge) and cathodic (discharge) sweeps of cyclic voltammogram, plot showing the voltammetric charge density ‘q’ vs. % &/! , the voltage profile of Na2Ti9O19 and porous carbon (PC) vs. Na-metal at constant current density of 100 mA g-1, capacity retention vs. cycle number showing the cycling performance of Na2Ti9O19/PC hybrid Na-ion capacitor measured at constant current rate of 2 A g-1.

AUTHOR INFOMRATION Corresponding author: *E-mail: [email protected]

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