Electrochemical Lithiation and Sodiation of Nb-Doped Rutile TiO2

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Electrochemical Lithiation and Sodiation of Nb-Doped Rutile TiO Hiroyuki Usui, Yasuhiro Domi, Sho Yoshioka, Kazuki Kojima, and Hiroki Sakaguchi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01595 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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ACS Sustainable Chemistry & Engineering

Electrochemical Lithiation and Sodiation of NbDoped Rutile TiO2 Hiroyuki Usui†,§, Yasuhiro Domi†,§, Sho Yoshioka†,§, Kazuki Kojima†,§, Hiroki Sakaguchi*,†,§ †

Department of Chemistry and Biotechnology, Graduate School of Engineering,

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

ACS Paragon Plus Environment

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ABSTRACT

As

anode

materials

of

Li-ion

battery and

Na-ion

battery,

the

electrochemical

insertion/extraction reactions of Li and Na were investigated for a rutile-type Nb-doped TiO2 synthesized by a sol–gel method. We changed particle size and crystallite size of the Nb-doped rutile TiO2 powders by an annealing at various temperatures of 100–1000 oC, and prepared thickfilm electrodes consisting of the powders. The anode performances were remarkably improved not only in Li-ion battery but also in Na-ion battery with reducing the annealing temperature from 1000 oC to 400 oC. We revealed that the Nb-doped TiO2 showing better high-rate performances exhibited a larger ratio of the crystallite size to the particle size. The sizedependent enhancement in the performance of rutile TiO2 was much more drastic than that of anatase TiO2. These results suggest that rutile’s potential diffusivity of Li and Na appeared more obviously by increasing in the ratio because the diffusion coefficient is anisotropic and significantly high.

KEYWORDS. Rutile-type titanium oxide; Niobium doping; Li-ion battery; Na-ion battery; Anode material, Thick-film electrode


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INTRODUCTION Energy storage is expected to play a very important role in the future of a sustainable electrical grid system. For realization of a low-carbon society and a green sustainable community, we should develop electric vehicles and stationary rechargeable batteries1. Li-ion battery (LIB) and Na-ion battery (NIB) are favorable energy storage devices for electric vehicles and stationary batteries, respectively. Electric vehicles have a great advantages in respect of reducing greenhouse (CO2) emission. Stationary batteries can effectively utilize renewable energies such as sunlight and wind powers. To meet the demands in these applications, further improvements are required for the batteries in terms of cost reduction, higher safety, high-rate performances, and long-term cycle stability. Graphite has a theoretical capacity of 372 mA h g−1 and a good cycle stability. A natural graphite is widely used as the commercial LIB anodes due to its low and flat potentials (below 0.2 V versus Li/Li+). However, with an increased demand of performance for the next-generation LIB, its safety issue and poor rate capability become more serious2. The safety issue comes from the low charge−discharge potentials, leading to Li dendrite growth and decomposition of organic electrolyte forming solid electrolyte interface (SEI) films. A Li4Ti5O12 anode3 is one of the next-generation LIB anodes practically used for electric vehicles and stationary batteries. The Li4Ti5O12 is an excellent anode material because of its essential safety. The electrochemical lithiation and delithiation show very flat potential plateaus at about 1.55 V vs. Li/Li+ by a two-phase reaction between a spinel Li4Ti5O12 and an ordered rock-salt Li7Ti5O12. At the potential, there is no dangerous reaction such as Li dendrite growth (0 V vs. Li+/Li) and a cathodic decomposition of electrolyte (0.7−1.1 V vs. Li+/Li). The


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disadvantages of Li4Ti5O12, however, are an low electronic conductivity of 10−13 S cm−1 and a relatively high cost of about 4,400 $ kg−1 (see Table S1 in the Supporting Information, SI).

Titanium dioxides (TiO2) are regarded as potential anode materials because of their natural abundance, ready availability, and low cost. Among various polymorphs of TiO2, chemists have investigated antase4-9, bronze10,11, brookite12, and rutile13-17. Anatase TiO2 has been the most extensively studied because it has been believed that anatase phase is more active for Liinsertion/extraction reactions. On the other hand, rutile phase is less active in a bulk form. For a long time, rutile TiO2 has been considered as a poor Li-ion insertion material owing to its limited Li-activity17. By contrast, when the TiO2’s size is reduced to several ten nanometers, Maier13 and Tarascon14 have reported that rutile TiO2 exhibited an enhanced reversible capacity and an improved cycle stability. The authors have focused the remarkable size effect on the anode performance because rutile TiO2 has the most unique characteristics of Li-ion diffusion in TiO2 polymorphs: its diffusion coefficient along the c-axis direction is approximately 10–6 cm2 s–1, while it is only 10–14 cm2 s–1 in the ab-plane direction18 (Table S1 in the SI). We considered that three-dimensional Li-ion diffusion is kinetically restricted in micrometer-sized rutile TiO2, and that its potential ability of Li-ion diffusion can be more effectively extracted by the size effect. By reducing the rutile TiO2 size to about 30 nm, significantly improved anode performances were attained for TiO2-coated Si electrodes19 and TiO2 electrodes20. A niobium (Nb)-doping into rutile TiO2 raised its electronic conductivity to 1000 times and enhanced the high-rate performance. In addition, the authors have discovered for the first time that not only Li ions but also Na ions can be reversibly inserted into rutile and Nb-doped rutile through its diffusion paths along c-axis20. Rutile TiO2 has a better availability and a much lower cost compared with Li4Ti5O12 in practical use for LIB anode. Less expensive electric vehicles and stationary


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rechargeable batteries would rapidly spread if rutile TiO2 anode is applied to the next-generation LIB and NIB, which will make a contribution to realization of the low-carbon society and the green sustainable community. For further improvement in Nb-doped rutile TiO2 performance, its mechanism should be clarified. As we demonstrated, the anode performance depends on its particle size. It is easily speculated that its crystallite size also would affect the performance because Li ions and Na ions move along diffusion paths in the crystal structure. In this study, we prepared Nb-doped rutile TiO2 powders with different particle sizes and crystallite sizes by changing an annealing temperature for the powders, and investigated the relationship between these sizes and LIB/NIB anode performances obtained for electrodes consisted of the powders.

EXPERIMENTAL SECTION Synthesis of active materials.

Nb-doped rutile TiO2 (Ti0.94Nb0.06O2) particles were

synthesized by a typical sol–gel method using hydrochloric acid, titanium(IV) tetraisopropoxide, and niobium(V) ethoxide20. We firstly diluted 4 mL of hydrochloric acid (HCl, Wako Pure Chemical Industries, 35–37% assay) with 56 mL of deionized water in a flask. After the dilution, 2 mL of titanium(IV) tetraisopropoxide [Ti(OCH(CH3)2)4, Wako Pure Chemical Industries, 95%] was dripped into the system. Niobium(V) ethoxide [Nb(OC2H5)5, Wako Pure Chemical Industries, 99.9%] was also added to the system with stirring. The stirring process was maintained for four hours at 55 oC and 1000 rpm. The resulting colloidal suspensions were centrifuged and washed with deionized water three times. The washed precipitates were dried


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under vacuum at 85 oC for 24 hours, then heated at 100−1000 oC in air for four hours. Whitecolored powders of the Nb-doped rutile TiO2 particles were obtained after the heat treatment. Alternatively, for the preparation of Nb-doped anatase TiO2 (Ti0.94Nb0.06O2), we diluted 28 mL of ethanol with 28 mL of deionized water in a flask. After the dilution, we added sodium dodecyl sulfate (C12H25SO4Na, Wako Pure Chemical Industries, 95%) so that its concentration becomes 0.01 M, and then Ti(OCH(CH3)2)4 and Nb(OC2H5)5 were added to the system with stirring. The stirring was carried out for four hours at 55 oC. The obtained suspensions were centrifuged and washed. The resulting precipitates were dried under vacuum at 85 oC for one day, and then heated at 400 oC in air for four hours to obtain Nb-doped anatase TiO2 powder. For comparison, we synthesized undoped TiO2 powders with rutile structure or anatase structure by the sol–gel method. In addition, we studied for the anode properties for commercially available ones [rutile TiO2 (Wako Pure Chemical Industries, 99%) and anatase TiO2 (Wako Pure Chemical Industries, 98.5%)] as undoped TiO2 with larger particle sizes. Electrode preparation.

Nb-doped TiO2 thick-film electrodes were prepared by a gas-

deposition (GD) method21,22. Unlike conventional slurry-based electrodes, any binder or conductive material are not required for the GD method. In this method, active material particles are accelerated to high speeds of about 150–500 m s−1 by a carrier gas ejected from a gas nozzle. A high impact energy of collision between the particles and substrates can result in a strong adhesion of the particles22. An active material layer consists only of active material particles, which is a remarkable advantage to evaluate the fundamental electrode reactions. In this study, the GD was performed under conditions with a nozzle diameter of 0.5 mm, a Ti current collector thickness of 20 µm, a He carrier gas differential pressure of 5.0×105 Pa, and a nozzle–substrate distance of 10 mm. Even when the particles are very small, the authors have confirmed that a


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thick film can be successfully obtained under these optimized conditions, and that particle size and crystalline size are basically not changed by the deposition: representative particle sizes before/after deposition in this study were 29.6 (±8.9) nm/30.7 (±6.8) nm and crystallite sizes before/after deposition were 12.6 (±2.0) nm/16.7 (±4.3) nm, respectively. The weights of the active material layers were within the range of 110–130 µg. The deposition areas of the active materials were approximately 0.79 cm2. Thus, the loading amount is 150 µg cm–2. The active layer thickness of the Nb-doped TiO2 was estimated to be 14 µm by observation using a confocal scanning laser microscope (CSLM, VK-9700, Keyence). Compositions of the active material powders and the thick-film electrodes were analyzed by an energy dispersive X-ray fluorescence (XRF) spectrometer (EDX-720 Shimadzu Co. Ltd.). Characterizations. The crystal information was acquired by X-ray diffraction (XRD) using an X-ray diffractometer (Ultima IV, Rigaku) with CuKα radiation (λ = 0.154178 nm). The crystallite sizes of the TiO2 powders were estimated by using the Scherrer equation and full width at half maximums of diffraction peaks. The surface morphologies of the powders and the electrodes were observed by a field emission scanning electron microscope (FE-SEM, JSM6701F, JEOL Ltd.) with an acceleration voltage of 3 kV. The particle sizes of the TiO2 powders were obtained from the SEM images. In our previous study20, Nb-doping into TiO2 has been well confirmed by XRD analysis and an electronic conductivity measurement. A thermogravimetric (TG) analysis was conducted for as-prepared Nb-doped TiO2 powders to check residual water molecules on the surface by using a TG analyzer (Thermo plus EVO II, Rigaku Co., Ltd.) with a heating rate of 10 oC min−1 from room temperature to 1000 oC under argon atmosphere. Charge−discharge tests.

For evaluation as the anode of the LIB, we assembled 2032-type

coin cells consisting of the Nb-doped TiO2 electrode as the working electrode, Li foil counter


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electrode, electrolyte, and glass fiber separator. The electrolytes used in this study were 1 M lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)-dissolved in propylene carbonate (PC; C4H6O3, Kishida Chemical Co., Ltd.). The cell assembly were carried out in a purge-type glove box (Miwa MFG, DBO-2.5LNKPTS) filled with an Ar atmosphere in which O2 concentration was below 1 ppm and dew point was below –100 °C (corresponding to H2O concentration of 0.01 ppm). Constant current charge–discharge tests were performed using an electrochemical measurement system (HJ-1001 SM8A, Hokuto Denko Co., Ltd.) in the potential range between 1.0 and 3.0 V vs. Li+/Li at 303 K at the constant current density of 33.5 mA g–1 (0.1C) or 335 mA g–1 (1.0C). High-rate performances were evaluated under the current densities from 33.5 mA g–1 (0.1C) to 16.75 A g–1 (50C). As NIB anodes, we prepared coin cells consisting of the Nb-doped TiO2 electrode, Na foil, and an electrolyte of 1 M NaClO4-dissolved/PC. The charge–discharge tests were performed in the potential range between 0.005 and 3.000 V vs. Na+/Na at 303 K at 50 mA g–1 (0.15C). Nb amount in Nb-doped TiO2 has been already optimized in our previous report20. The authors have investigated NIB anode performances of Nb-doped TiO2 (Ti1−xNbxO2) electrodes with various Nb amounts x from 0 to 0.18. As a result, Ti0.94Nb0.06O2 (x=0.06) electrode exhibited the best performance. When x was larger than 0.06, Nb exceeds the solid solubility limit of Nb to Ti in rutile TiO2 to form an impurity phase such as TiNb2O720. Therefore, the authors judged that x=0.06 is the most favorable doping amount. In the case of LIB anode also, the authors confirmed that the Nb-doped TiO2 electrode with this composition (x=0.06) showed the best performance.


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RESULTS AND DISCUSSION Characterizations of Nb-doped TiO2 particles.

Figure 1(a) shows SEM images of Nb-

doped rutile TiO2 particles annealed at 400 oC and 1000 oC. No significant change in particle morphology was observed up to the annealing temperature of 400 oC. By contrast, a noticeable growth in the particle size was found for the particles annealed at 1000 oC. This morphology change is a reasonable result because the sintering of rutile TiO2 starts at 600–800 oC23. Results of the SEM observations (Figure S1 in the SI) and the XRD analyses (Figure S2 in the SI) estimated the particle sizes and the crystallite sizes as summarized in Table S2 in the SI. Figure 1(b) represents the dependence of these sizes on the annealing temperature. Both the particle size and the crystallite size were increased with raising the annealing temperature from 400 oC to 1000 oC. In particular, the increase in the particle size is more significant: the crystallite size and the particle size in case of 1000 oC are 5 times and 10 times larger than those in case of 400 oC, respectively.


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LIB anode performances of Nb-doped TiO2 electrodes.

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Figure 2 presents

charge−discharge curves of the Nb-doped rutile TiO2 electrodes as LIB anodes at the first cycles. The lithiation (charge) profiles display two potential plateaus at 1.4−1.5 V and 1.0−1.2 V vs. Li+/Li. These plateaus agree well with other nanosized rutile electrodes, and can be attributed to the Li insertion into rutile structure13-17. The electrode reactions can be basically recognized as follows. The potential slopes at 1.4−1.5 V vs. Li+/Li in the cathodic profiles are attributed to Liinsertion into rutile TiO2 up to the composition of Li0.5TiO2. The lower slopes at 1.0−1.2 V vs. Li+/Li are caused by deeper Li-insertion and irreversible phase transformation to rock-salt type LixTiO2 14 which passivates the TiO2/electrolyte interface after the first cycle24. In the anodic profiles, Li-extraction occurs from the lithiated rutile TiO2 in wide potential region of 1.5−2.5 V vs. Li+/Li 17. These potential slopes are very typical features observed for nanosized rutile TiO2 25. The typical features in cathodic and anodic profiles were confirmed by cyclic voltammetry also (Figure S3 in the SI). These inclined potential slopes are due to an inhomogeneous electrochemical potential required for Li-ion diffusion into the “nanosized” rutile TiO2, though the detailed mechanism is still unclear. In a smaller nanoparticle, the volumetric ratio of surface part to whole particle should become much larger. This size effect can vary the electrochemical potential for Li-ion diffusion. Therefore, nanosized active materials basically exhibited not a flat potential plateau but an inclined potential slope. The electrodes of Nb-doped TiO2 annealed at 300 oC and 400 oC exhibited the highest initial Coulombic efficiencies of 68%. The most highly crystallized Nb-doped TiO2, annealed at 1000 o

C, showed the lowest initial Coulombic efficiency of 34%. Although the origin of an

irreversible capacity for submicron-sized rutile is recognized as the phase transformation from rutile TiO2 to rocksalt LiTiO217, the irreversible capacity of nanosized rutile at the first cycle is


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still not clearly understood15. On the other hand, the capacity reversibility was drastically improved at the second cycle (Figure S4 in the SI). The improvement was especially pronounced for the Nb-doped TiO2 annealed at 400 oC: the Coulombic efficiency quickly reached 100% at the second cycle and maintained 100% in the subsequent cycles in spite of its highest charge and discharge capacities among them.


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Figure 3 compares high-rate performances of these Nb-doped TiO2 electrodes cycled under various C-rates from 0.1C to 50C. The rate capability was not improved by the annealing at 100 o

C and 300 oC, whereas an obvious improvement was recognized at 400 oC. A result of TG

analysis demonstrated that water molecules and hydroxyl groups still remained on the TiO2 surface below 400 oC (Figure S5 in the SI). These cause unfavorable side reactions with Li ions to form an inactive Li2O layer on the TiO2 surface26, which probably degrades the anode performance of these electrodes. At about 400 oC, the desorption of the water molecules and the hydroxyl groups was completed. It was confirmed that the best high-rate performance was attained at the annealing temperature of 400 oC: the discharge capacity of 120 mA h g−1 was maintained even at the high current density of 16.75 A g–1 (50C)20. In contrast, above 400 oC, the rate capability was reduced with increasing the annealing temperature. We considered that the variation in the rate capability can be explained by the relationship between the crystallite size and the particle size. Thus, a ratio of the crystallite size to the particle size, Rs, was calculated for the Nb-doped TiO2 particles. The calculation of Rs excluded the particles annealed at the


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temperature less than 400

o

C because these electrodes could not show their potential

performances owing to the formation of Li2O surface layer. As the authors expected, Nb-doped TiO2 with better high-rate performances exhibited larger Rs values. The authors are here suggesting the mechanism of improvement in the performance as illustrated in Fig. 4. When Li ions reach the surface of particles during the charge, they can quickly diffuse along c-axis directions of rutile structure because of its extremely high diffusion coefficient (10−6 cm s−1) of Li-ion. In the case of higher annealing temperatures, Li ions should move toward ab in-plane directions also owing to the smaller Rs values for the Li insertion into all parts of one particle. The lithiation was, however, kinetically restricted by its low diffusion coefficient of 10−14 cm s−1 along the ab in-plane direction, which is a possible reason for the poorer rate capabilities. On the other hand, in the case of lower annealing temperature, Li ions can easily reach the inner part of particle only by the diffusion along c-axis because of larger Rs values. For this reason, the excellent high-rate performance was observed for the electrode of Nb-doped TiO2 with the largest Rs value.


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Figure 5 displays long-term cycling performances of Nb-doped TiO2 electrodes. For evaluation of the practicality as LIB anode, the electrodes were cycled under a relatively-high current density of 1.0C rate, enough for practical use. The cycle performance was not improved by the annealing at 300 oC. In contrast, the performance was clearly upgraded by the annealing at 400 C: the reversible capacity of 176 mA h g−1 and the capacity retention of 88% were achieved

o

even after 1000 cycles. In addition to this, the Nb-doped TiO2 electrode did not show the change in surface morphology (Figure S6 in the SI). This is very contrast with electrodes consisted of other anode materials such as Si showing large volumetric changes during charge−discharge reactions27. These results well evidence that the advantages of rutile TiO2 are not only low cost but also high safety, excellent high-rate performance, and good long-term cycling stability. Above 400 oC, the performance significantly dropped with increasing the annealing temperature.

NIB anode performances of Nb-doped TiO2 electrodes. Rutile TiO2 can play an NIB anode material also as the authors have revealed20. Thus, an investigation of correlation between LIB


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and NIB anode properties would give valuable knowledge. Figure 6 shows potential profiles of galvanostatic charge (sodiation) and discharge (desodiation) for the Nb-doped rutile TiO2 electrodes at the 20th cycles. Potential slopes appeared at 0−0.7 V vs. Na+/Na in the charge profile and 0.7−1.5 V vs. Na+/Na in the discharge profile. These slopes are attributed to the insertion/extraction reactions of Na ions into/from rutile structure. Other research groups also have reported that Na-insertion and Na-extraction take place in wide potential regions of 0−0.7 V vs. Na+/Na and 0.5−0.9 V vs. Na+/Na in association with redox reactions of Ti4+/Ti3+

28,29

. This

agreement is basically supported by our previous result revealing reversible Nainsertion/extraction into/from rutile TiO2 by ex-situ XRD analysis20. Although low Coulombic efficiencies of 23%−45% were obtained at the first cycle (Figure S7 in the SI), the efficiencies were improved to 68%−81% at the 10th cycle (Figure S8 in the SI) and 83%−87% at the 20th cycle. The main reason of the low efficiencies is a continuous cathodic decomposition of carbonate-based electrolyte30. For this problem, the authors have reported that a fluoroethylene carbonate (FEC) as an electrolyte additive can effectively suppress the electrolyte decomposition and can improve Coulombic efficiency20. In this study, however, we did not use such electrolyte additive for the charge−discharge tests to simply compare LIB results obtained without FEC and NIB results.


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Figure 7 represents rate capabilities of the Nb-doped TiO2 electrodes as NIB anodes under the current densities from 0.06C to 3.0C. As the annealing temperature raises from 100 oC to 400 oC, the rate capability was gradually enhanced. More than 400 oC, the rate capability was drastically reduced with increasing the temperature. The discharge capacities of all the electrodes recovered when the C-rate was lowered from 3.0C to 0.15C, indicating that the capacity fading is induced not by the disintegration of the active material layer but by the kinetic restriction of Na-ion diffusion. The dependence of the rate capability on the annealing temperature basically agrees with that in LIB anode (Fig. 3). To elucidate the dependences, relationships between Rs values and the rate performances of the Nb-doped rutile TiO2 electrodes as LIB and NIB anodes are described in Fig. 8. Both rate performances of LIB and NIB anodes were evidently enhanced with increasing Rs values. In particular, the LIB rate performance showed a drastic improvement depending on Rs values. It is a noteworthy result the electrode in the case of 400 oC is superior to


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a high-rate performance obtained for highly-crystallized Li4Ti5O12 nanoparticles with 20−50 nm in size31.


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The GD electrode is favorable for evaluating original electrochemical properties of the active material because it does not include conductive additive and binder. However, the loading amount is quite low (less than 150 µg cm−2) compared with conventional slurry-based electrode using conductive additive and binder, which would be advantageous for better performance. Thus, the authors prepared conventional slurry-based electrodes of Nb-doped rutile TiO2 using a mixture of active material annealed at 400 oC, conductive additive of acetylene black (AB), and binder of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR). The weight ratio of active material/AB/SBR/CMC/conductive additive/binder was 70/15/10/5 wt.%. Their charge−discharge tests were performed in an organic electrolyte comprised of sodium bis(fluorosulfonyl)amide (NaFSA)-dissolved in PC and an ionic liquid electrolyte using Nmethyl-N-propylpyrrolidinium and bis(fluorosulfonyl)amide (Py13-FSA). The loading amount of the electrodes was 1.3 mg cm−2, which is ten times larger than that of the GD electrode. Figure 9


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shows cycling performances of slurry-based Nb-doped TiO2 electrodes. In spite of much larger loading amount, the slurry-based electrode exhibited a good performance in the organic electrolyte after an activation process for the initial 20 cycles, which is comparable to the GD electrode. This result reveals that Nb-doped TiO2 can show good cycling performance not only for GD electrode but also for slurry-based electrode.


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Comparison of LIB anode performances of rutile and anatase TiO2. From the viewpoint of a low-cost anode material based on titanium oxides, anatase TiO2 is another potential candidate. We thus synthesized anatase TiO2 particles, and evaluated the LIB anode performances. Figure 9 compares initial charge−discharge curves for LIB anodes consisted of rutile TiO2 and anatase TiO2 with different particle sizes. By reducing the particle size of rutile TiO2, the charge and discharge capacities were notably increased (Fig. 10(a)). The anatase TiO2 electrodes showed flat potential plateaus at 1.7 V vs. Li+/Li in lithiation profile and 1.9 V vs. Li+/Li in delithiation profile (Fig. 10(b)), resulting from Li-insertion/extraction via the diffusion channels of anatase TiO2. Gradually decayed tails were observed for the lithiation profile below 1.7 V vs. Li+/Li. The potential tails correspond to the Li storage into surface structure9. For anatase TiO2 also, Nb-doping is effective to enlarge the charge/discharge capacities, which is probably achieved by improvement of electronic conductivity. However, the enhancement of the charge/discharge capacities by the size decreasing was less significant than that of rutile TiO2.


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Figure 11(a) presents the high-rate performances of the electrodes of undoped TiO2 with rutile and anatase structures. At the C-rates less than 1C, the rutile TiO2 with the particle size of 210 nm showed a very poor rate capability than the anatase TiO2 with the size of 220 nm. The reason is suggested to be the dimensionality of Li-ion diffusion paths. The Li-ion diffusion paths in rutile and anatase are descried in Fig. 11(b). The crystal structures were created using VESTA package32. Anatase TiO2 has three-dimensional zigzag channels for Li-ion diffusion. In bulk form, the channels are more favorable than the one-dimensional channel in rutile TiO2. With respect to anatase TiO2, the rate performance was improved by a twelve-fold reduction of its particle size. By contrast, rutile TiO2 exhibited more drastic improvement in the rate performance even though the particle size was reduced to only one third. The rutile’s superiority to anatase can not be explained only by decreasing in the particle size. This result indicates that rutile TiO2 is more favorable in smaller particle size because its potential ability of Li-ion


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diffusion along c-axis can be shown more obviously. This presumably comes from the difference in the dimensionality of Li-ion diffusion paths in the crystals. The authors consequently suggest that the crystallite size of TiO2 impacts on the Li-ion diffusion, and that not only particle size but also crystallite size should be concerned as discussed by using Rs values in this paper. To reveal the impact of crystallite size on the performance, it is necessary to evaluate electrodes consisting of TiO2 with the same particle size and with different crystallite sizes. Although we are currently performing this evaluation by a mechanical milling treatment for annealed TiO2 particles, a detailed study on the difference in the crystallite sizes lies outside the scope of this brief paper. The authors will report the impact of crystallite size on anode properties in a future study. It was demonstrated in this study that we successfully improved LIB/NIB performances of the thickfilm anodes consisted of Nb-doped rutile TiO2 by controlling their particle sizes and crystallite sizes. This knowledge would be very valuable to develop oxide-based anode materials for LIB/NIB with high-rate performance and low cost. The authors strongly believe that these batteries will promote the realization of the low-carbon society and the green sustainable community.


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CONCLUSIONS We synthesized Nb-doped rutile TiO2 particles with different particle sizes and crystallite sizes by changing annealing temperature after the sol–gel synthesis, and investigated electrochemical properties of thick-film electrode consisted of the particles for LIB anode and NIB anode. XRD analyses and SEM observations revealed that the particle size and the crystallite size were increased with raising the annealing temperature from 400 oC to 1000 oC. As the LIB anode, the high-rate performance and the long-term cycling performance were drastically improved with reducing the annealing temperature from 1000 oC to 400 oC though no improvement was observed by the annealing at temperatures below 400 oC. It was clarified that Nb-doped TiO2 with the better performances as LIB and NIB exhibited the larger ratio of the crystallite size to


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the particle size. With respect to an electrode consisted of undoped anatase TiO2 particles, the rate capability was enhanced by its particle size reduction. However, a more significant enhancement was found for undoped rutile TiO2 particles, indicating that rutile TiO2 is more favorable in smaller particle size because its potential ability of Li-ion diffusion along c-axis can be shown more obviously.

ACKNOWLEDGMENT This work has been partially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Number 24350094, 15K21166, 16K05954), and the Matching Planner Program from the Japan Science and Technology Agency (JST) (No. MP28116808236). A part of this work was supported by the Japan Association for Chemical Innovation (JACI) and the Izumi Science and Technology Foundation. The authors thank Mr. K. Wasada, Mr. S. Ohnishi, and Dr. M. Shimizu for their helpful assistances in LIB and NIB experiments.

ASSOCIATED CONTENT Supporting Information TG analysis results, FE-SEM images, XRD patterns, Charge–discharge curves. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: Hiroki Sakaguchi


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*Email: [email protected]


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Table of Contents Graphic

Electrochemical Lithiation and Sodiation of Nb-Doped Rutile TiO2 Hiroyuki Usui, Yasuhiro Domi, Sho Yoshioka, Kazuki Kojima, Hiroki Sakaguchi


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Electrochemical Lithiation and Sodiation of Nb-Doped Rutile TiO2

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