Synthetic, Structural, and Electrochemical Study of Monoclinic

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Synthetic, structural and electrochemical study of monoclinic Na4Ti5O12 as a sodium ion battery anode material Pierre J.P. Naeyaert, Maxim Avdeev, Neeraj Sharma, Hamdi Yahia, and Chris D. Ling Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5035358 • Publication Date (Web): 04 Dec 2014 Downloaded from http://pubs.acs.org on December 5, 2014

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Chemistry of Materials

Synthetic, structural and electrochemical study of monoclinic Na4Ti5O12 as a sodium ion battery anode material Pierre J.P. Naeyaert†, Maxim Avdeev‡, Neeraj Sharma§, Hamdi Ben Yahia#, Chris D. Ling†,* †

School of Chemistry, The University of Sydney, Sydney 2006, Australia ANSTO, Locked Bag 2001, Kirrawee DC 2232, Australia § School of Chemistry, The University of New South Wales, Kensington NSW 2052, Australia # Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan ‡

KEYWORDS: Sodium-ion battery, anode material, lithium-ion battery, neutron diffraction, synchrotron, in situ, sodium titanate, lithium titanate ABSTRACT: The monoclinic phase of Na4Ti5O12 (M-Na4Ti5O12) has been investigated as a potential sodium ion battery anode material. In contrast to the previously investigated trigonal phase (T-Na4Ti5O12), MNa4Ti5O12 has continuous 2D channels with partially occupied sodium sites, providing broader pathways and more space for the intercalation of excess sodium. Electrochemical measurements show that it exhibits a comparable or higher reversible capacity than T-Na4Ti5O12. Neutron powder diffraction reveals the preferred sites and occupancies of the excess sodium. In situ synchrotron X-ray diffraction under electrochemical cycling shows that the crystal lattice undergoes a strongly anisotropic volume changes during cycling.

Introduction Since lithium ion batteries were commercialized in 19911 they have dominated the portable electronics market. In 2014 over a quarter of the world’s lithium mined was devoted to lithium ion battery applications2. Increased future demand will be driven by the electric/hybrid vehicle market and load-leveling solutions for renewable energy sources3. The development of new, low cost alternatives to lithium-ion batteries with comparable electrochemical properties is therefore highly desirable. Sodium is the most obvious choice, as a cheap, abundant and easily extractable element. As well as the decreased cost in starting material, sodium ion batteries can use aluminum as the current collector for the anode because, unlike lithium, sodium does not alloy with aluminum metal. Aluminum offers both a cheaper and lighter alternative to the copper usually used in lithium-ion batteries4. However challenges still remain in developing sodium intercalation materials capable of achieving a high reversible capacity and energy density. In particular, due to sodium ions’ inability to intercalate with graphite5, anode materials need to be significantly improved. Previous research into anode materials has focused on carbonbased materials, sodium-based alloys and transition metal oxides. Despite successfully intercalating sodium ions into carbon based nanospheres, nanowires and nanosheets, affording relatively high capacities of ~200-250 mAh/g6, the columbic efficiency and cycle life of these anodes is insufficient for practical applications7. Similarly, sodium-based tin alloy anodes as reported by Komaba et al.8 achieved very high capacities (~850 mAh/g); however, this was also accompanied

by pronounced volume changes on intercalation, leading to battery instability and safety issues. Multiple transition metal oxides have previously been reported to intercalate sodium ions with varying degrees of success9,10,11,12. The most promising in terms of practical, large scale applications are titanium-based metal oxides4,13,14,15, led by Na2Ti3O7,16 which has a reversible capacity approaching 200 mAh/g6,15. This is largely due to the low voltage redox couple of Ti(III)/Ti(IV) vs. sodium and the high availability of titanium oxide17. One of the most exciting titanium oxide anode materials previously investigated is spinel-type Li4Ti5O12. Its structure (Figure 1c) consists of face-sharing TiO6 octahedra with continuous 2D sodium ion conduction pathways. Li4Ti5O12 has been shown to be a zero strain anode for lithium ion batteries18, whilst also affording one of the highest reversible capacities for oxide based sodium anode materials.14,19,20 Based on these promising results, sodium-based analogues of the same stoichiometry have also been investigated. Two crystal structures have been reported with the Na4Ti5O12 stoichiometry: trigonal T-Na4Ti5O1213,21 and monoclinic MNa4Ti5O1222. The two forms are structurally similar in that they contain face and edge sharing TiO6 octahedra with potential 2D sodium conduction pathways. However, M-Na4Ti5O12 (Figure 1a) has a quasi-2D layered structure with 4 crystallographically distinct partially occupied sodium sites compared to T-Na4Ti5O12 which has completely filled sodium sites. TNa4Ti5O12, as examined by Woo et al.13, has been shown to successfully intercalate sodium ions, affording a reversible capacity of 50 mAh/g. However, M-Na4Ti5O12 has not been

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Chemistry of Materials electrochemically examined. Its partially occupied sodium sites however suggest a strong likelihood of improved electrochemical properties compared to T-Na4Ti5O12 (Figure 1b). Note that both these structures are distinct from that of Li4Ti5O12, which is spinel-type (Figure 1c). Herein we describe the synthesis of M-Na4Ti5O12. We report the electrochemical properties and in situ structural evolution of M-Na4Ti5O12 as an anode material in sodium ion batteries, and determine the preferred sites at which intercalated excess Na resides.

Figure 1. Crystal structures of (a) M-Na4Ti5O12, (b) TNa4Ti5O12 and (c) Li4Ti5O12. TiO6 octahedra are blue, O atoms are small red spheres, Na atoms are large black spheres with fractional occupancy indicated, Li atoms are large green spheres. Na sites in (a) are labeled in accord with Table 1. Experimental Synthesis of M-Na4Ti5O12 began with a sodium titanate precursor, Na16Ti10O28,23 which was itself synthesized by solidstate reaction of stoichiometric amounts of TiO2 (99.995%, Aithaca) and Na2CO3 (99.9%, Merck). The mixture was ground and calcined at 700 °C for 13 h before being re-ground and sintered at 850 °C for 13 h. As reported by Nalbandyan et al.22 M-Na4Ti5O12 only forms in the presence of excess Na to give an initial product of stoichiometry Na4.6Ti5O12. Na4.6Ti5O12 was synthesized by solid-state reaction from TiO2 (Anatase, 99.7%, Aldrich) and Na16Ti10O28 in a mole ratio giving 10% excess of sodium. The mixture was ground and pressed into a pellet before being reacted in a tube furnace at 900 °C for 1 h under flowing dry H2 gas (99.9%). Note that the reaction was only successful when the H2 gas was carefully dried over CaCO3 and glacial H2SO4. Note that this assynthesized Na4.6Ti5O12 phase is not stable in the atmosphere, losing Na until the stoichiometry becomes Na4Ti5O12. Reaction progress was monitored using X-ray powder diffraction (XRD) data collected on a PANalytical X'pert PRO in Bragg-Brentano geometry with a sealed-tube Cu-Kα source. Neutron powder diffraction (NPD) data were obtained at the OPAL research reactor, Australian Nuclear Science and Technology Organisation (ANSTO), using the high-resolution Echidna diffractometer. Air-sensitive powdered samples were sealed in 9 mm diameter vanadium cans and sealed using indium metal. Room-temperature data were collected at a neutron wavelength λ = 1.6215 Å. For electrochemical measurements, negative electrodes were prepared by mixing M-Na4Ti5O12 with carbon black and polyvinylidene fluoride (PVdF) in an 80:10:10 ratio. N-methylpyrrolidone (NMP) was added to the mixture as a solvent and

the solution mixed overnight. The electrode slurry was deposited onto aluminum current collector foil using a notch bar with a thickness of 200 µm. The electrodes were then dried under vacuum for 24 hours at 100 °C before being pressed to 100 kN and re-dried at 100 °C under vacuum. Research-scale coin cells (CR 2032) were assembled in a glove box under an inert argon atmosphere. The cells were assembled using sodium metal (1 mm thickness) and a glass fibre separator with 1 M NaPF6 dissolved in dimethyl carbonate and ethylene carbonate (1:1 wt%) electrolyte solution. The cells were cycled at 12.5 mA/g between 2.5 and 0.1 V using an MTI Corporation battery cycler. In situ synchrotron XRD (S-XRD) data were collected at the Australian Synchrotron on the powder diffraction (PD) beamline at a wavelength λ = 0.68954(1) Å, calibrated against a NIST LaB6 660b standard. Coin cells were prepared as described above except for 3 mm diameter holes in the coin cell casings and 5 mm holes in the stainless steel spacers. Rietveld-refinements were carried out using the GSAS24 software suite with the EXPGUI25 frontend. Due to the presence of both sodium and aluminum metal in the X-ray beam the refinement of in situ coin cell patterns proved quite difficult. Perhaps the most challenging aspect was the highly undulating background present in the patterns. It was ultimately necessary to use fixed background points together with a shifted 30-parameter Chebyschev function. Sodium and aluminum peaks were excluded from the refinement (large gaps in data evident in Figure 2). The refinement showed evidence for preferred orientation of sample crystallites, unsurprisingly given the large amount of pressure applied during the cell preparation.

Intensity (counts ×102)

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3 2 1 0 5

10

15

20

25

2θ (°) Figure 2. Rietveld fit for M-Na4Ti5O12 to S-XRD data from an un-cycled coin cell. Data are shown as crosses, the calculated model as a line through the data, and the difference between the data and the model as the line below the data. Vertical markers are for M-Na4Ti5O12. Sodium and aluminum peaks were excluded from the refinement. Results and Discussion Rietveld-refinement against ex situ S-XRD data (obtained after the sample had extensive exposure to the atmosphere) confirmed that M-Na4Ti5O12 adopts the expected monoclinic structure (Figure 1a) with lattice parameters a = 26.4611(3), b = 2.95449(3), c = 6.35742(7) Å, β = 95.7489(6) °, V = 494.515(8) Å3. These values match those previously reported26 within 0.5% relative error. Atomic fractional coordinates, occupancies (for Na sites) and atomic displacement parameters


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(ADPs) were refined freely. ADPs were constrained to be identical for each element type, in response to an unreasonable variation among them when left unconstrained. Figure 3 shows the Rietveld fit to S-XRD data with crystallographic details displayed in Table 1. From the data it was clear that two impurities were present in our M-Na4Ti5O12 sample: the precursor Na16Ti10O28; and a sodium-deficient phase, Na2.08Ti4O9.27 Despite numerous attempts to purify MNa4Ti5O12 by varying reaction times and temperatures we were unable to completely eliminate these impurities. The need for excess Na to drive the formation of the monoclinic phase, accompanied by the need for high temperatures which lead to Na loss on a similar timescale and creates a dynamic situation: decreasing temperature or reaction time results in more unreacted precursor, while increasing temperature or reaction time leads to more sodium-deficient phase. Refined impurity phase fractions for the sample used in the S-XRD experiments are 13.7(2) wt% Na2.08Ti4O9 and 4.635(14) wt% Na16Ti10O28.

Figure 3. Final Rietveld-refined fit of Na4Ti5O12 to ex situ SXRD data. Data are shown as crosses, the calculated model as a line through the data, and the difference between the data and the model as the line below the data. The inset shows an enlarged 5 < 2θ < 15 ° region. Vertical markers indicate (from top to bottom) Na16Ti10O28 (green), Na2.08Ti4O9 (black) and Na4Ti5O12 (red). Nalbandyan et al.22 reported that as-made non-stoichiometric form Na4+xTi5O12, x = 0.6, is unstable to sodium loss at room temperature in air. This was also observed in our investigation, with the product changing from black to grey almost immediately upon grinding in air. This color change is consistent with the oxidation of Ti(III) to Ti(IV) as sodium is lost. In order to identify and quantify the preferred sites for excess sodium in Na4+xTi5O12, the compound was synthesized as previously described, before being immediately transferred to a vanadium can and sealed. This allowed for NPD data to be collected on the material before exposure to air. The sample was then removed, ground in air until it turned white, and another NPD data set collected. In order to obtain the most bias-free model for sodium site preference, the structure was first refined against NPD data for stoichiometric M-Na4Ti5O12 using fixed sodium site occupancies from S-XRD (from Table 1), which gave a total sodium content of 3.95(6) per formula unit (Figure 4a). The same structural and instrumental parameters were then refined against NPD data for as-made M-Na4+xTi5O12,

with the addition of sodium site occupancy (Figure 4b). Key refined parameters are compared in Table 2. Table 1. Refined structural parameters for M-Na4Ti5O12 based on S-XRD data. Space group C2/m, a = 26.4587(2), b = 2.95425(2), c = 6.35713(7) Å, β =95.7489(6) °, V= 494.413(9) Å3 Sit e

x (a)

y (b )

z (c)

Ti 1

Occ.

Wyko ff Pos.

100Uis (Å2)

0

0

0

1

2a

1.40(6 )

Ti 2

0.70586( 15)

½

0.0989(6 )

1

8j

1.40(6 )

Ti 3

0.59456( 11)

0

0.8381(5 )

1

4i

1.40(6 )

Na 1

0.8094(2)

0

0.4015(8 )

0.961(9 )

4i

2.2(2)

Na 2

0.5989(4)

½

0.3250(1 3)

0.594(9 )

8j

2.2(2)

Na 3

0.9439(6)

0

0.597(2)

0.371(8 )

4i

2.2(2)

Na 4

½

0

½

0.100(1 1)

4i

2.2(2)

O1

0.7042(3)

0

0.2856(1 2)

1

4i

2.59(1 0)

O2

0.7798(2)

½

0.1070(1 3)

1

8j

2.59(1 0)

O3

0.5413(2)

0

0.1209(1 3)

1

4i

2.59(1 0)

O4

0.6349(3)

½

0.9831(1 2)

1

8j

2.59(1 0)

O5

0.9531(3)

0

0.2166(1 3)

1

4i

2.59(1 0)

O6

0.3805(3)

0

0.3923(1 3)

1

4i

2.59(1 0)

o

The refined total sodium content of the as-made sodium-rich (black) sample was 4.35(18) per formula unit, significantly higher than for the stoichiometric air-ground (white) sample but below the theoretical maximum of 5 (taking into account the fact that Na2 and Na3 are less than 1.3 Å apart and therefore should be considered as a split site, both parts of which cannot be simultaneously occupied). Figure 5 shows the structure viewed along the c-axis, with a sodium-containing layer exposed in order to highlight the refined positions and occupancies of sodium sites in the white (a) and black (b) samples. In Table 2 and Figure 5 it can be seen that the excess sodium goes almost exclusively to the Na4 site, with the occupancy for Na1, and the sum of occupancies for Na2 and Na3, staying close to 100% in both cases. Interestingly, there is also a shift in occupancy from Na3 to Na2 over the split site in the black (vs. white) sample. This is consistent with the increased occupancy of Na4: note that Na4 is surrounded most closely (2.3 Å) by Na3, with Na2 at a greater distance (3.3 Å).


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Figure 5. Structures of stoichiometric (air-ground) MNa4Ti5O12 and as-made M-Na4+xTi5O12, as refined against NPD data, viewed along the c-axis, with a sodium-containing layer exposed. TiO6 octahedra are blue and Na atoms are yellow spheres with partial occupancies indicated by wedges. Figure 4. Final Rietveld fits of M-Na4Ti5O12 (a) and MNa4.6Ti5O12 (b) to NPD data. Data are shown as crosses, the calculated model as a line through the data, and the difference between the data and the model as the line below the data. The Vertical markers indicate (from top to bottom) MNa4Ti5O12/Na4+xTi5O12 (red) and Na16Ti10O28 (black). Refined impurity phase fractions were 83.8(3) wt% M-Na4Ti5O12 and 16.2(6) wt% % Na16Ti10O28. Table 2. Sodium site occupancies and unit cell parameters for stoichiometric (air-ground) M-Na4Ti5O12 and as-made MNa4+xTi5O12, as refined against NPD data. Stoichiometric M-Na4Ti5O12

As-made M-Na4+xTi5O12

Site

Occupancy

Site

Occupancy

Na1

0.961(9)

Na1

0.96(2)

Na2

0.594(9)

Na2

0.67(3)

Na3

0.371(8)

Na3

0.32(2)

Na4

0.100(11)

Na4

0.45(4)

a (Å)

26.5016(7)

a (Å)

26.5471(8)

b (Å)

2.95274(7)

b (Å)

2.95241(7)

c (Å)

6.33577(18)

c (Å)

6.33496(17)

β (°)

95.689(2)

β (°)

95.764(2)

Cell Parameter

3

V (Å )

Cell Parameter

493.35(2)

3

V (Å )

494.01(2)

Electrochemical measurements showed that M-Na4Ti5O12 has a relatively high initial capacity of approximately 137 mAh/g. However, after this initial cycle the capacity dropped by approximately 50% to 64 mAh/g. The capacity continued to diminish slowly for 25 cycles before stabilizing at 56 mAh/g (Figure 6a), consistent with the theoretical capacity when 1 Na is transferred per unit formula (51.5 mAh/g). The charge/discharge curve in Figure 6b) also shows diminishing capacity. However, the shape of the curve is almost identical between cycles, suggesting a good reversibility of sodium insertion/extraction reactions. When the cells were cycled at 31.25 mA/g instead of 12.5 mA/g (the rate used for all other measurements), the reversible capacity dropped only slightly to ~48 mAh/g. Once the stable state had been reached (after 25 cycles), the number of sodium ions intercalated per formula unit of MNa4+xTi5O12 as calculated from electrochemical data was x = 1.08, consistent with a maximum stoichiometry of Na5.08Ti5O12 at full discharge and close to the theoretical maximum of x = 1. This suggests that the high initial capacity in the first cycle is an artifact, typically observed due to sodium incorporation into the 10%wt carbon black used in the preparation of the cell and/or an amorphous surface layer at the electrolyte interface. The small (~8%) apparent excess capacity over the stoichiometric maximum may be due to the intercalation of some Na into the impurity phases Na2.08Ti4O9 and Na16Ti10O28 (accounting for ~15 wt.% in total). These phases have not been electrochemically examined to the best of our knowledge, therefore their effects on the capacity achieved cannot be quantified; however, based on their crystal structures they would not be expected to intercalate Na as readily as M-Na4Ti5O12, and this prediction is consistent with our results.


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(a)

from 6.3675(8) to 6.3416(16) Å, for an overall volume decrease of ~0.64%. M-Na4Ti5O12 is therefore not a zero strain material like its lithium-based counterpart. However, the relative size of the volume change is quite small compared to the ~10% increase for graphite undergoing lithium intercalation28,29, the ~13% increase for Li4Ti5O12 undergoing sodium intercalation14 and the ~56% increase for sodium alloy (Na-Sn2) systems undergoing sodium intercalation30.

(b)

Figure 6. (a) Electrochemical capacity of M-Na4Ti5O12 over 50 cycles. (b) Voltage versus capacity for M-Na4Ti5O12. The cell was discharged to 0.1 V and charged to 2.5 V at 12.5 mA/g. Figure 7 shows variations in the unit cell parameters as sodium ions are intercalated/de-intercalated during the first cycle, from our in situ S-XRD experiment. The intercalation of sodium into the anode produces a strongly anisotropic volume change in M-Na4Ti5O12, with a exhibiting a 0.93% increase while c shows a 0.70% decrease when fully discharged. The direction of these unit cell variations on sodium intercalation/de-intercalation are consistent with the differences between unit cells for the “white” and “black” samples determined from ex situ NPD data (Table 2), but of a larger magnitude due to the greater variation in sodium content. The first 45 minutes shows a rapid and almost immediate decrease in c. This can be attributed to decreased anion-anion repulsion between oxygen anions in adjacent TiO6 octahedral layers following the insertion of additional Na+ cations between them. Conversely, perpendicular to c, cation-cation repulsion among Na+ increases as sodium is intercalated and the a and b parameters increase. These lattice changes are clearly evident in the evolution of individual peaks associated uniquely with the a, b and c lattice parameters (Figure 8). Note in Figure 7 that there is a slight irreversibility associated with these lattice parameter changes: After cycling, a has increased from 26.4156(15) to 26.492(2) Å and c has decreased

Figure 7. Effects of sodium intercalation/de-intercalation on the unit cell of M-Na4Ti5O12, from Rietveld-refinement against in situ S-XRD data. Error bars are smaller than symbols. The purple line superimposed on the volume data shows the discharge/charge curve for the 1st cycle.

Figure 8. Evolution of in situ S-XRD data at the (0 0 1) (left), (0 2 0) (center) and the (10 0 0) (right) reflections of MNa4Ti5O12. Conclusion Monoclinic M-Na4Ti5O12 exhibits comparable or slightly higher electrochemical capacity over the previously studied trigonal polymorph T-Na4Ti5O12, with a reversible capacity of 56 mAh/g. Neutron and synchrotron X-ray diffraction, brought to bear on M-Na4Ti5O12 for the first time, confirm the presence of continuous 2D channels with partially occupied sodium sites. A large capacity drop after the first cycle is likely to be


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related to an anisotropic volume change of 0.64% during that cycle, observed in an in situ synchrotron X-ray diffraction experiment. The observed capacity at this stage does not correspond to that of a practical anode material, as it would limit the overall capacity of a working cell when combined with the best available cathode materials. Nevertheless, noting that sodium insertion/extraction reactions in M-Na4Ti5O12 are highly reversibly after the initial volume change and drop in capacity, it would now be worthwhile trying synthetic methods aimed at decreasing particle size – known to improve kinetics and capacity – in pursuit a viable new anode material for sodium-ion batteries.

ASSOCIATED CONTENT Supporting Information available: Crystallographic information files (CIFs) for NPD refinements of M-Na4Ti5O12 and NNa4+xTi5O12, and ex situ S-XRD refinement of M-Na4Ti5O12. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Funding Sources NS was supported by the Australian Institute of Nuclear Science and Engineering (AINSE) Research Fellowship Scheme.

ABBREVIATIONS

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(15) Rudola, A.; Saravanan, K.; Mason, C. W.; Balaya, P. Journal of Materials Chemistry A 2013, 1, 2653. (16) Andersson, S.; Wadsley, A. D. Acta Crystallographica 1961, 14, 1245. (17) Senguttuvan, P.; Rousse, G.; de Dompablo, M.; Vezin, H.; Tarascon, J. M.; Palacin, M. R. Journal of the American Chemical Society 2013, 135, 3897. (18) Ohzuku, T.; Ueda, A.; Yamamoto, N. Journal of the Electrochemical Society 1995, 142, 1431. (19) Jaiswal, A.; Horne, C. R.; Chang, O.; Zhang, W.; Kong, W.; Wang, E.; Chern, T.; Doeff, M. M. Journal of the Electrochemical Society 2009, 156, A1041. (20) Venkateswarlu, M.; Chen, C. H.; Do, J. S.; Lin, C. W.; Chou, T. C.; Hwang, B. J. Journal of Power Sources 2005, 146, 204. (21) Avdeev, M.; Kholkin, A. Acta Crystallogr. Sect. C-Cryst. Struct. Commun. 2000, 56, E539. (22) Nalbandyan, V. B.; Avdeev, M. Y.; Lukov, V. V. Zhurnal Neorg. Khimii 1998, 43, 198. (23) Mayer, M.; Perez, G. Revue De Chimie Minerale 1976, 13, 237. (24) Larson, A. C. V. D., R. B. Los Alamos National Laboratory Report LAUR, 1994. (25) Toby, B. H. J. Appl. Crystallogr. 2001, 34, 210. (26) Werthmann, R.; Hoppe, R. Z. Anorg. Allg. Chem. 1984, 519, 117. (27) Akimoto, J.; Takei, H. Journal of Solid State Chemistry 1989, 83, 132. (28) Qi, Y.; Guo, H. B.; Hector, L. G.; Timmons, A. Journal of the Electrochemical Society 2010, 157, A558. (29) Reynier, Y.; Yazami, R.; Fultz, B. J. Power Sources 2007, 165, 616. (30) Hong, S. Y.; Kim, Y.; Park, Y.; Choi, A.; Choi, N. S.; Lee, K. T. Energy Environ. Sci. 2013, 6, 2067.

XRD, X-ray diffraction; S-XRD, synchrotron X-ray diffraction; NPD, neutron powder diffraction.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

7

a)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

b)


Page 8 of 16

a

Na(1) Na(3)

cc

Na(2) b b Na(4)

c) a a ACS Paragon Plus Environment

b

c

b

c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Intensity (counts ×102)

Page 9 of 16


3

2 1 0 5

10

15

2θ (°)


20

25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Intensity (counts x105)


Page 10 of 16

Na2.08Ti4O9

Na16Ti10O24 1.0

0.5 8

7

10

9

0.0

10

20

30

2θ (º)


40

50

60

Page 11 of 16

6

Intensity (counts x103)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

a)


χ2 = 2.633 R2 = 0.0444

4 2 0

10

b)

χ2 = 3.101 R2 = 0.0439

5

0

20

60 Plus80Environment 100 40 Paragon ACS

2θ (º)

120

140


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 12 of 16

Na1 Na2 Na3 Na4 Na3 Na2 Na1 a

b Stoichiometric “White” Na4Ti5O12

As-made “Black” Na4+xTi5O12


Page 13 of 16


(a) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

2nd charge

(b)

10th charge 20th charge

2nd discharge 10th discharge 20th discharge


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Charge

Page 14 of 16

2.5

1.2

Potential

Discharge Chemistry of Materials

0.1


Page 15 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13



Chemistry of Materials Page 16 of 16

1 2 3 4


Synthetic, Structural, and Electrochemical Study of Monoclinic

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