Tantalum-Doped Titanium Oxide with Rutile ... - ACS Publications

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Tantalum-Doped Titanium Oxide with Rutile Structure as a Novel Anode Material for Sodium-Ion Battery Hiroyuki Usui, Yasuhiro Domi, Kunihiko Takama, Yuri Tanaka, and Hiroki Sakaguchi ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Energy Materials

Tantalum-Doped

Titanium

Oxide

with

Rutile

Structure as a Novel Anode Material for Sodium-Ion Battery Hiroyuki Usui†,§, Yasuhiro Domi†,§, Kunihiko Takama†,§, Yuri Tanaka‡,§, and Hiroki Sakaguchi†,§,*

† Department

of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Minami, Koyama-cho, Tottori 680-8552, Japan

‡ Course

of Chemistry and Biotechnology, Department of Engineering, Graduate School of Sustainability Science, Tottori University, 4-101 Minami, Koyama-cho, Tottori 680-8552, Japan

§ Center

for Research on Green Sustainable Chemistry, Tottori University, 4-101 Minami, Koyama-cho, Tottori 680-8552, Japan

Corresponding Author Keywords:

* Tel./Fax: +81-857-31-5265, E-mail: [email protected]

Rutile TiO2; Ta doping; Na-ion battery; Anode material; Electron charge density

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ABSTRACT

We hydrothermally synthesized various-element-doped rutile TiO2 for Na-ion battery anodes. Sn- and In-doped TiO2 electrodes showed poor performances. In contrast, Ta- and Nb-doped TiO2 electrodes exhibited larger discharge capacities, which is attributed to expanded diffusion path and improved electronic conductivity. Among them, the Ta-doped one delivered excellent cycling performance and better rate capability. A first principle calculation revealed that Ta doping reduced electron charge density in rutile’s Na+-diffusion path because Ta5+ has larger effective nuclear charge to attract strongly outermost electron. Therefore, Na+ was not bound by electron to easily diffuse in TiO2 particle, which leading to the enhanced capacity.


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1. INTRODUCTION Na-ion batteries (NIBs) have been recently considered as an attractive alternative to Li-ion batteries (LIBs) for large-sized rechargeable batteries because Na resource has a lower cost and a wider availability than Li resource. Na+ has a 2.4 times larger ionic volume than Li+, resulting in slower kinetics of its host materials of negative electrode (anodes) and positive electrodes (cathodes).1 A fundamental development of NIB requires a new material research for active material because electrochemical insertion-extraction properties of Na+ appear similar, but are in fact quite different from those of Li+. Titanium oxide-based materials are one of the promising candidates of anode materials emphasizing the long-term durability and the high safety. Since 2014, reversible Na-insertion/extraction reactions have been reported for anatase TiO2,2 TiO2(B),3 Na2Ti3O7,4 and Na2+xTi6O13.5,6 Among them, many researchers have intensively studied anatase-type TiO2 because it has a low cost, high availability, and relatively high electrochemical activity with Na+. By contrast, nobody has reported on rutile TiO2 until 2015 owing to its less activity originated from highly anisotropic Na+-diffusion along c-axis direction, which had been well known in research as LIB anode. Under such situation, the authors have revealed for the first time Na+-insertion/extraction of rutile TiO2 as shown in Figure 1(a).7 The authors have controlled particle size and crystallite size of rutile TiO2 by using a

G' synthesis and a subsequent annealing to clarify a significant size

effect. By increasing a ratio of the crystallite size to the particle size, one-dimensional Na+diffusion could enable Na storage in the inner part of TiO2 particle because it could reduce the density of grain boundary preventing Na+ diffusion.7,8 In addition to this, the authors have discovered the remarkable effect of Nb-doping for the first time. By Nb-doping into rutile TiO2, its anode performances were drastically improved.7-9 Two effects of the doping have been confirmed (Figure 1(b)). The first effect is significant enhancement of electronic conductivity: we have confirmed that the Nb-doped rutile TiO2 showed a three orders of magnitude higher conductivity compared with undoped one.7 When Nb is doped into anatase TiO2, 4d orbitals of Nb are strongly hybridized with 3d ones of Ti to form a d-nature conduction band.10,11 Thus, Nb-doped anatase was well studied as a transparent conductor material.10,11 It has been predicted that a doping into rutile TiO2 also improves an electronic conductivity by the generation of defect level in its



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2. EXPERIMENTAL Impurity elements (Ta, Nb, Sn, and In)-doped TiO2 powders were synthesized by a hydrothermal method. The synthesis conditions were summarized in Figure S1. These TiO2 have a crystal structure of tetragonal rutile-type TiO2 (Inorganic Crystal Structure Database, ICSD No. 00-021-1276), which was confirmed by analysis using an X-ray diffraction (XRD, Ultima IV, Rigaku) as shown in Figure 2G :5 A single phase of rutile TiO2 could be formed for the doping amounts of 6.7 at.% Ta, 5.7 at.% Nb, 2.6 at.% Sn, and 0.5 at.% In. The doping amount was evaluated by the analysis using an energy-dispersive X-ray fluorescent (XRF) analyzer. The morphologies were observed by using a field emission scanning electron microscope (FE-SEM, JSM-6701F, JEOL Ltd.). Ta-doped TiO2 particles showed rectangular shape with size of about 170 nm, which is very similar to that of Nb-doped TiO2 (Figure S8). Electrodes were prepared by using of TiO2/acetylene black/carboxymethyl cellulose/styrene-butadiene rubber with the weight ratio of 70/15/10/5 wt.%. We prepared coin cells consisting of the TiO2 electrodes, Na metal sheets, and an electrolyte of 1.0 mol dmG; sodium bis(fluorosulfonyl)amide (NaFSA)-dissolved/PC. The ' G

' tests were performed in the potential range between 0.005 and 3.0 V vs. Na+/Na

at 303 K at 50 mA gG, (0.15 C). Other conditions were described in the previous paper.6,7,24 To discuss electron charge density and Na+ behavior in the diffusion path of doped rutile TiO2, a firstprinciple calculation was carried.

3. RESULTS AND DISCUSSION Figure 2 shows

' G

' curves of various impurity-element-doped rutile TiO2

electrodes in the initial three cycles. The all electrodes showed initial irreversible capacities: the Coulombic efficiencies at the first cycles were +1KG/-K5 The trigger of the irreversible capacities is possibly an electrolyte decomposition or a side reaction of hydroxyl groups on TiO2 surface. These efficiencies improved to 00KGH-K at the third cycle. The undoped TiO2 electrode exhibited gentle potential shoulders in charge (sodiation) and discharge (desodiation) reactions. Na-insertion and Na-extraction mainly occurred at ,51G- V and at -52G25- V vs. Na+/Na (Figure S9). These 5 ACS Paragon Plus Environment



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Ta-doped TiO2 is expected to have larger discharge capacities at high current rates because Na+-diffusion path was expanded and its charge density was lowered. Figure 5 compares rate capabilities of Ta-doped TiO2 electrode and other-element-doped ones. To evaluate the potential performances, the rate capability tests were subsequently carried out after the capacity decay/rising in the initial ten cycles at their cycling tests. As we predicted, Ta-doped TiO2 electrode exhibited superior rate performances: the discharge capacities at the all C-rates from 0.15 C to 20 C were higher than those of the other electrodes. The capacity of 130 mA h gG, was attained even at a high current density of 10 C (3.35 mA gG,), which is much higher than that of undoped anatase TiO2 (40 mA h gG, at 10 C)2, Mo-doped anatase (100 mA h gG, at 5 C)26, Co-doped anatase (85 mA h gG, at 6 C)27, and (N, S)-doped anatase (90 mA h gG, at 6 C).28 The outstanding rate capability is attributed to the three effects of Ta doping: (i) expanded diffusion path, (ii) improved electronic conductivity, and (iii) reduced electron charge density in the path. This doping strategy gives a valuable information for new material design of TiO2-based anode materials. As the concept of Nb doping7 has been intensively applied to various rutile TiO28,9,19,20 and anatase TiO221-28, the strategy of Ta doping also will trigger to develop new TiO2-based anode materials.

0.15 C

1

Discharge capacity / mA h g

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


250

0.3 C

1.5 C

3.0 C

5.0 C

10 C

20 C

0.15 C

Ta-doped TiO2 200

In-doped TiO2

150 100

Nb-doped TiO2 Undoped TiO2

50

Sn-doped TiO2 (1 C = 335 mA g 1 )

0

0

5

10

15

20

25

30

35

40

Cycle number Figure 5 Rate capabilities of Ta-doped TiO2 electrode and other-element-dopedTiO2 electrodes. To evaluate the potential performances, rate capability tests were



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4. Conclusions To further enhance Na-storage performance of rutile TiO2 with ion-diffusion path along its caxis direction, we hydrothermally synthesized various element-doped TiO2 using Ta5+, Nb5+, Sn4+, and In3+ with larger ionic sizes and higher effective nuclear charges than those of Ti4+. 6.7 at.% of Ta5+ and 5.7 at.% of Nb5+ were successfully doped into rutile, whereas the doping amounts of Sn4+ and In3+ reached only 2.6 at.% and 0.5 at.% because of their too larger ionic sizes than Ti4+. Owing to these low doping amounts, the Sn- and In-doped TiO2 electrodes showed inferior performances as NIB anode to that of undoped one. On the other hand, the Ta- and Nb-doped TiO2 electrodes exhibited larger discharge capacities, which is attributed to the two doping effects: an expanded diffusion path and an improved electronic conductivity. In particular, the Ta-doped one delivered the highest initial capacity of 290 mA h gG,. As the results of first principle calculation, we revealed that Ta doping could reduce the charge density in the diffusion path more effectively because Ta5+ has a larger effective nuclear charge (20.5) than Nb5+ (14.1) to attract more strongly outermost electron existing in the path. Therefore, Na+ was not bound by electron in the path to diffuse toward the inner part of TiO2 particle, which leads to the enhancement of the discharge capacity. As we expected, Ta-doped TiO2 electrode exhibited an excellent rate performance with the reversible capacity of 130 mA h gG, even at a high current density of 10 C (3.35 mA gG,). These results demonstrated the significance of the third effect of doping: the reduction of electron charge density in the Na+-diffusion path.

ACKNOWLEDGMENT This study was partially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Number 19H02817, 17H03128, 17K17888, 16K05954). A part of this work was supported by the Japan Association for Chemical Innovation (JACI). The authors appreciate Mr. S. Ohnishi and Ms. R. Takaishi for their assistances of hydrothermal synthesis of doped TiO2 samples and discussion of first principle calculation results.


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ORCID Hiroyuki Usui: 0000-0002-1156-0340 Yasuhiro Domi: 0000-0003-3983-2202 Hiroki Sakaguchi: 0000-0002-4125-7182

ASSOCIATED CONTENT Supporting Information Preparation of active materials, Conditions of first-principle calculation, XRD patterns, SEM images, Charge–discharge curves.

AUTHOR INFORMATION Corresponding Author: Hiroki Sakaguchi

*Email: [email protected]



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