Carbon-Free TiO2 Microspheres as Anode Materials for Sodium Ion

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Letter

Carbon-Free TiO2 Microspheres as Anode Materials for Sodium Ion Batteries Jang-Yeon Hwang, Hoang-Long Du, Bin-Na Yun, Min-Gi Jeong, Ji-Su Kim, Hyoungchul Kim, Hun-Gi Jung, and Yang-Kook Sun ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02510 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Energy Letters

Carbon-Free

TiO2

Microspheres

as

Anode

Materials for Sodium Ion Batteries Jang-Yeon Hwang, †,# Hoang-Long Du, ‡, # Bin-Na Yun,‡ Min-Gi Jeong, †,‡ Ji-Su Kim,§ Hyoungchul Kim,§ Hun-Gi Jung,* ‡,∥and Yang-Kook Sun* † ,

,

†

Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea

‡

Center for Energy Storage Research, Green City Technology Institute, Korea Institute of

Science and Technology, Hwarang-ro 14 gil 5 Seongbuk-gu, Seoul 02792, Republic of Korea §

High-Temperature Energy Materials Research Center, Korea Institute of Science and

Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea ∥

Division of Energy & Environment Technology, Korea University of Science and

Technology,Gajeong-ro 217 Yuseong-gu, Daejeon 34113, Republic of Korea

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] (H.-G. Jung) and [email protected] (Y.-K. Sun)

1 ACS Paragon Plus Environment

ACS Energy Letters 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

Page 2 of 22

ABSTRACT

In this study, we synthesize carbon-free anatase/bronze TiO microsphere (TiO (A/B)-MS) 2

2

via the solvothermal method and demonstrate their potential for use as a high-performance anode material for sodium-ion batteries. The highly compact structure of the microsphere constructed from nano-primary particles not only enhances the stability and subsequently leads to a good cycling performance but also enables the transport pathways for Na ions and +

electrons to be shortened, ensuring a fast Na storage performance. In addition, an anatase/bronze interfacial structure of the material further improves fast Na -ion diffusion +

kinetics. Benefiting from these merits, the proposed TiO (A/B)-MS demonstrate a high specific 2

capacity of 221 mAh g at 0.1 C, fast charge-discharge capability up to 50 C, and long-term –1

cycling stability over 1000 cycles at 1 and 10 C without using a conductive carbon matrix. A combination of various experiments and theoretical studies is used to verify the outstanding Na-storage performance of TiO (A/B)-MS. 2

TOC GRAPHICS

TiO2(A/B) Microsphere

Na+

anatase/bronze interfacial structure


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ACS Energy Letters

For the past few decades, lithium-ion batteries (LIBs) have been accepted as the dominant power source for portable electronic devise, electric vehicles (EVs) and large-scale energy storage systems (ESSs). Increased demand for mid- to large-scale batteries may, however, lead 1

to serious problems such as the depletion of lithium resources in the near future. Therefore, 2

alternatives to LIBs should be developed to help satisfy the rapidly growing public demand for rechargeable batteries. Recently, sodium-ion batteries (SIBs) have begun development in tandem with lithium-based batteries. More importantly, focuses on the development of 3

sodium-based batteries, which have arisen because of the potentially low cost and natural abundance of sodium resources, will become a meaningful advantage for the future of the battery field. Furthermore, SIBs possess a similar “rocking-chair” sodium storage mechanism 4

to that of LIBs, which has accelerated the development of SIBs. Recent interest in SIBs has been stimulated by the development of nonmetallic anode materials because of safety concerns on the practical use of Na metal as the negative electrode. Reflecting on these situations, several anode materials for SIBs have been widely investigated from the scientific to industrial fields. Among them, carbonaceous materials, particularly hard 5

carbon, have attracted attention as anodes in SIBs. Hard carbon allows an efficient insertion of Na with a voltage profile that is the lowest (near zero V vs Na/Na ) versus Na/Na , and with a +

+

+

reasonable capacity (~250 mAh g ) and cycling performance. Although the merits of hard -1

6

carbon as an anode material are undeniable, at this moment, the long-term cycling stability and power capability are still far below those required for high-power applications owing to its intrinsic physical properties and random structure. Even worse, the voltage plateau related to most capacities is too close to the sodium plating voltage (~ 0 V vs Na/Na ), causing safety +

concerns.

7

Recently, titanium dioxide (TiO ) has been considered one of the best alternative anode 2

materials for SIBs because of its unique advantages such as nontoxicity, potentially high 3 ACS Paragon Plus Environment


Page 4 of 22

capacity, safe working voltage (0.7 V vs. Na/Na ), and the allowance of a long lifespan for +

batteries originating from its low volumetric variation during the (de)intercalation process.

8,9

Amidst TiO polymorphs, anatase TiO (A),

10–12

2

2

rutile-TiO , and bronze TiO (B) have the 13

14

2

2

potential of performing similarly to insertion anode materials regarding the number of studies on SIBs. On the other hand, the advantages of biphasic interfaces consisting of anatase and bronze phases on the facile kinetics of Na ions have been also reported . However, in most +

15

cases, neither the anatase TiO -A or TiO -B or TiO (A/B) anode material has thus far achieved 2

2

2

a long-term cycling stability and high power capability in SIBs without the support of carbonaceous materials because of their low electronic conductivity. More seriously, they 16

have still been plagued by the issue of low powder tap densities that are closely related to the lower volumetric energy density of electrodes. Therefore, considering practical applications, more elaboration on the development of TiO materials should be required to satisfy not only 2

the long-term cycling stability and power capability but also the high powder tap density. Inspired by previous works, we aimed to develop a more advanced TiO anode material for 2

SIBs in this study. Herein, carbon-free and high-tap-density TiO (A/B) microsphere 2

(TiO (A/B)-MS) were synthesized and their potential for use as high-performance anodes for 2

SIBs was demonstrated. By taking the beneficial features of anatase/bronze interfacial structures and the compact microsphere constructed from nano-primary particles, the proposed material can produce a high capacity and power density without the supporting conductive carbon. Surprisingly, the TiO (A/B)-MS anode delivered a high capacity of 221 mAh g at 0.1 –1

2

C and exhibited a long-term cycling stability over 1000 cycles even at 10 C. Moreover, the Naion diffusion into/out of the TiO (A/B)-MS structure was extremely fast, resulting in a great 2

power capability of up to 50 C rate.


Page 5 of 22

Figure 1. Schematic of synthesis process for the TiO (A/B)-MS materials. 2

As illustrated in the schematic in Figure 1, by the facile solvothermal method, the hierarchical intertexture architecture of TiO (A/B)-MS was synthesized without the presence of 2

carbonaceous materials. Through the following calcination step, we obtained a highly pure material comprising microsphere with average diameters of 1–2 μm (see the scanning electron microscope (SEM) images in Figure 2a, b). As depicted in Figure 2c, transmission electron microscope (TEM) image clearly shows that the TiO (A/B)-MS have a spherical morphology 2

with average diameters of 1–2 μm and no contrast change from the particle center to surface, indicating a dense particle interior. b

a

500 nm

4 µm

1.5

100

d 1.0

80 (2.2 %)

60

0.5

(1.2 %)

40

0.0

20

-0.5

0

0

200

400

600

800

DTG (% /min)

c

Weight / %

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 Energy Letters

-1.0

o

Temperature / C

Figure 2. (a, b) SEM images, (c) bright field TEM image, and (d) TGA analysis data of asprepared TiO (A/B)-MS. 2


ACS Energy Letters

All the reflection peaks in the XRD patterns could be assigned to the bronze and anatase TiO

2

phases corresponding to JCPDS card numbers 00-046-1237 and 00-002-0406,

14,17

respectively.

A thermogravimetric analysis (TGA) plot of the TiO (A/B)-MS shows almost no weight loss 2

(only ~3.4%) up to 800 °C, indicating the successful synthesis of the carbon-free material (Figure 2d). Note that the former weight loss of 2.2% up to 300 °C was due to the evaporation of absorbed water or dehydration of hemihydrate. By taking advantage of the carbon-free and hierarchical micro-intertexture properties, the TiO (A/B)-MS demonstrated a high tap density 2

of about ~1.0 g cm ; this value highly compatible with previous reports (Table S1). In addition, –3

such a hierarchical architecture with a spherical morphology usually possesses a low surface energy, which results in less self-aggregation during the electrochemical process, leading to a high rate capability and long-term cycling stability.

18,19

a

10

b

20

30

40 50 2 q / Deg.

60

Obs. Fit. TiO2-B TiO2-A TiO2-B TiO2-A

Relative Intensity

Observed Calculated Difference TiO2-A TiO2-B

Intensity / a. u.

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

Page 6 of 22

70

80

220

200

180

160

140

120

100

-1

Raman Shift / cm

2 q/degree

Figure 3. (a) Rietveld refined XRD patterns and (b) Raman spectra of the TiO (A/B)-MS 2

sample. To better understand the crystal structure of TiO (A/B), a combination of Rietveld refinement, 2

Raman spectroscopy, and high-resolution transmission electron microscopy (HR-TEM) analysis was further performed. As shown in Figure 3a, Rietveld refined diffraction peaks for the TiO (A/B)-MS can be assigned to TiO -A and TiO -B, and all the diffraction peaks are 6 20

2

2

21

2

ACS Paragon Plus Environment

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ACS Energy Letters

almost identical to the simulated pattern. Based on the refinement analysis (see Figure 3a and Table 1), the weight fraction of the anatase phase and bronze phase in the TiO (A/B)-MS was 2

calculated to be ca. 59.06% for the anatase phase and ca. 40.94% for the bronze phase. The Raman active modes present an intensified peak at 143–146 cm , suggesting the presence of a –1

mixed phase of TiO -A for 144 cm and TiO -B for 144.96 cm . –1

–1 22–24

2

2

The remaining peaks,

especially the two major peaks at 123 and 195.96 cm , signify the Raman active modes of TiO –1

2

B and TiO -A, respectively. Based on the peak areas, the ratio of TiO -A and TiO -B quantifies 2

2

2

a mass percentage of ca. 58.33% for the anatase phase and ca. 41.67% for the bronze phase in the TiO (A/B)-MS, which are in good agreement with the Rietveld refined data. From the 2

refined XRD pattern and Raman analysis results, we can confirm that the anatase and bronze phase coexists in the TiO (A/B)-MS at a ratio of 2 to 3. 2

Table 1. Structural Properties of TiO (A/B)-MS Sample. 2

TiO (A/B)-MS 2

Lattice parameter

TiO -A

TiO -B

2

2

(Space group: I4 /amd) 1

(Space group: C12/m1)

a-axis [Å]

3.7871(2)

12.1974(3)

b-axis [Å]

-

3.7552(5)

c-axis [Å]

9.511(4)

6.523(2)

β angle [º]

-

107.1(2)

136.42(8)

285.61(5)

R [%]

3.74

3.74

Wt. frac. [%]

59.06

40.94

Unit Volume [Å ] 3

wp



Page 8 of 22

In general, calcination temperatures greatly affect the determination of TiO polymorphs. 2

When the TiO precursor obtained from the hydrothermal reaction was calcined at 300 °C, the 2

XRD patterns of the resultant powder (hereafter denoted as TiO -300) show that it is relatively 2

amorphous. The Rietveld refined diffraction peaks for the TiO -300 material can also be 2

assigned to TiO -A and TiO -B, but the weight fraction of the anatase phase and bronze phase 2

2

in the material was calculated to be ca. 36.36% for the anatase phase and ca. 64.64% for the bronze phase (Figure S1 and Table S2). With the increase in temperature to 500 °C, the bronze phase in TiO tends to convert its monoclinic phase into a tetragonal phase of anatase TiO 2

2

(hereafter denoted as TiO -500), leading to the formation of a pure anatase phase. Although 2

6.03% of the bronze phase exists in the TiO -500 structure, the profile-matched XRD patterns 2

of TiO -500 using Rietveld refinement are almost identical to the simulated pattern of the 2

anatase phase TiO structure (Figure S2 and Table S3). 2

a

b

3.55 Å

TiO2-A (101) 3.48 Å

TiO2-B (110) 3.55 Å

3.48 Å

c

d TiO2-B (110) 3.55 Å

TiO2-A (101) 3.48 Å

5 nm

Figure 4. (a) HR-TEM image (orange line: bronze phase; blue line: anatase phase), (b) FFT pattern of HR-TEM image, and (c) inversed FFT image of the TiO (A/B)-MS sample. (d) HR2

TEM image of TiO (A/B)-MS interface with co-existed anatase and bronze phase. 2


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ACS Energy Letters

The microstructural interfacial features of the TiO (A/B)-MS were investigated by HR-TEM 2

analysis (Figure 4a). HR-TEM image of the selected area from the edge of one TiO (A/B)-MS 2

particle and the corresponding fast Fourier transformation (FFT) image further confirm the anatase/bronze interfacial structure. As seen in Figure 4b and c, the lattice fringes with a dspacing of 3.48 Å for the (101) plane of TiO (A) and 3.55 Å for the (110) plane of TiO (B) are 2

2

in accordance with the literature. The inverse fast Fourier transform image in Figure 4d shows 15

the region of the anatase phase for the (101) plane (blue color) and bronze phase for the (110) plane (orange color) in the TiO (A/B)-MS particle, which matches well with HR-TEM image 2

in Figure 4b. The magnified HR-TEM image of TiO (A/B)-MS interface (Figure 4d) clearly 2

shows the co-existence of anatase/bronze that has the lattice fringes with d-spacing of 3.48 Å for the (101) plane and 2.4 Å for the (004) plane of TiO -A, and 5.8 Å for the (200) plane of 2

TiO -B. In addition, we calculated the lattice mismatch for the interface relationships using the 2

following equation by considering the angular mismatch (θ);

𝑚=

|𝑑% 𝑠𝑖𝑛𝜃 − 𝑑+ | 0.5 × (𝑑% 𝑠𝑖𝑛𝜃 + 𝑑+ )

As seen in HR-TEM image (Figure 4d), d and d correspond to the interplaner spacing of anatase 1

1

(101) and bronze (200) and θ was measured as 70º in Figure. m is calculated to be 0.579 for the interface between anatase (101) and bronze (200), which indicates the possibility of interface match.

25-26

Moreover, an obvious lattice distortion at the interface of TiO2(A/B)-MS

could be beneficial for providing more active sites for Na storage and transformation. From a combination of XRD and TEM data, we have confirmed the co-existence of the anatase and bronze phase in the TiO (A/B)-MS material on the bulk structure as well as the surface 2

interfacial layer. 9 ACS Paragon Plus Environment


Page 10 of 22

The Na-storage performance of TiO (A/B)-MS was evaluated using Na | 0.5M sodium 2

hexametaphosphate (NaPF ) dissolved in propylene carbonate (PC) with a 2% fluoroethylene 6

carbonate (FEC) | TiO (A/B)-MS cell in the voltage range of 0.01–2 V (vs. Na/Na ). In order to +

2

investigate the overall electrochemical response, cyclic voltammogram (CV) measurements were conducted at a scan rate of 0.1 mV s (Figure 5a). A broad reduction peak is observed in –1

the first cathodic process under 1.0 V and then disappears in the following cycles. This irreversible peak was usually caused by the solid electrolyte interphase (SEI) layer formation, irreversible sites for Na-ion insertion in the crystal lattice defects, electrolytes, FEC additives, and other organic material decompositions in the Na cell. This behavior is well reflected in the first galvanostatic voltage profiles of the TiO (A/B)-MS anode shown in Figure 5b. As the 2

cycling progresses, the TiO (A/B)-MS exhibited reversible oxidation/reduction peaks at 0.5 V/ 2

0.8 V (Na/Na ) in the CV profiles, suggesting an excellent electrochemical reversibility. In a +

similar manner to the reaction mechanism in the TiO -based anode materials in previous studies, 2

the Na insertion-extraction process into the TiO (A/B)-MS at first cycle is divided into five +

2

regions (marked in Figure 5b) and described as follows: (1) pseudo-capacitive, (2) cathodic decomposition plateau of the electrolyte at the surface, (3) Na insertion and structure +

rearrangement and disproportionation reaction (formation of Ti and O ), and (4) reversible Na 0

+

2

extraction process. To clarify the electrochemical reaction at Stage 3, we conducted ex situ 9

XPS measurement of the TiO (A/B)-MS electrode at pristine and discharged state at 0.1 V 2

(Figure S3). The spectrum of pristine electrode shows only two peaks at 458.3 eV and 463.8 eV, which correspond to the binding energies of Ti 2p and Ti 2p , respectively, i.e., Ti . 4+

3/2


1/2

Page 11 of 22

0.00

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

-0.05

-0.10

-0.15

0.1mV/s 0.0

0.5

1.0

1.5

2.0

d

1C 3C

150

5C

10C 20C 30C

100

2

Sodiation 3

0

100 200 300 400 500 600 700 800

Discharge capacity / mAh g-1

2.5 2.0

Voltage / V

200

1.5 1.3 1.0

0.1C 0.5C

4

1

50C

0.1 C 0.2 C 0.5 C 1C 3C 5C 10 C 20 C 30 C 50 C

1.5 1.0 0.5

50

1 C = 250 mA g-1 0

0

5

0.0 0

10 15 20 25 30 35 40 45 50 55

Cycle Number

50

100

150

200

250


300

300 100

250

80

200

60

150

40

100

1C cycling 10C cycling

50 0

0

100

200

20

300

400

500

600

700

800

900

0 1000

Coulombic Efficiency %

e

0.2C

2.0

-0.5

2.5

300 250 0.1C

Desodiation

2.5

0.5 0.3 0.01

Voltage / V


c

b

0.05

Voltage / V

Current / mA

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 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Cycle Number

Figure 5. Comprehensive electrochemical performances of TiO (A/B)-MS in Na cell: (a) 2

Cyclic voltammogram, (b) initial galvanostatic voltage profiles, (c) discharge capacities at various C rate from 0.1 C (25 mA g ) to 50 C (12. 5 A g ), (d) corresponding charge-discharge -1

-1

voltage profiles, and (e) discharge capacities for long-term cycling at 1 C and 10 C rate of the cell. All cells are tested in the voltage range of 0.01–2.0 V at 30 °C.



Page 12 of 22

When the electrode is discharge to 0.1 V, we clearly observed that Ti is reduced to Ti and Ti , 4+

3+

0

indicating that some sodium ions are somehow inserted into the crystal lattice while, at the same time, the TiO is reduced to metallic Ti and oxygen. On the other hand, to further quantify 2

O gas evolution, in situ analysis of evolving gaseous products formed upon the initial discharge 2

of TiO (A/B)-MS will be investigated and reported in our next work. Following the sodiation2

desodiation reaction above, the TiO (A/B)-MS deliver a high specific capacity of 221 mAh g

–1

2

at a 0.1 C rate in a voltage range of 0.01–2 V. The cycling performance of the TiO (A/B)-MS 2

was investigated at different current densities of 1 C and 10 C rate in the voltage range of 0.01– 2 V. For both current densities, the TiO (A/B)-MS anode exhibited an outstanding cycling 2

stability of 100% (vs. the initial capacity) over the 1000 cycles (in Figure 5e). It should be noted that although the FEC additive could stabilize the Na-metal anode by inducing the good passivation layer, instability of Na-metal anode at high current density is inevitable and thus 27

it could affect to fluctuation of long-term cycling of Na-metal anode. Moreover, the TiO (A/B)2

MS anode demonstrated an outstanding power capability (in Figure 5c and d) with a capacity retention of 30% at 50 C rate (vs. the initial capacity at 0.2 C) without the support of a conductive carbon matrix. When the rate was restored to 0.1 C, the specific capacity returned to 218 mA h g (similar to 98.6 % of initial capacity), indicating an excellent structural stability –1

of TiO (A/B)-MS. These excellent electrochemical performances of the TiO (A/B)-MS can be 2

2

attributed to the synergetic effect of the unique structural and morphological features. First, the anatase/bronze interfacial structure in the TiO (A/B)-MS provides fast Na -ion diffusion +

2

kinetics. In addition, the highly compact structure of microsphere constructed from nano15

primary particles not only enhances the stability and subsequently leads to excellent cycling performances but also enables the transport pathways for Na ions and electrons to be shortened, +

ensuring an excellent rate capability. To the best our knowledge, the presented electrochemical


Page 13 of 22

performance of carbon free TiO (A/B)-MS is similar to the nanoscale and/or carbon-coated 2

samples but much better than the carbon-free samples reported in the literature (Figure S4).

15

2.5

1.5

6.0x10-11

1.0

4.0x10-11

0.5

2.0x10-11

0

50

100

150

200

250

Specific Capacity

2.5

Desodiation

2.0

3.0x10-10 2.5x10-10 2.0x10-10

1.5

1.5x10-10

1.0

1.0x10-10

0.5 0.0

DNa+ / cm2 S-1

8.0x10-11

0.0

b

1.0x10-10

Sodiation

2.0

DNa+ / cm2 S-1

Voltage / V (vs Na/Na+)

a

Voltage / V (vs Na/Na+)

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 Energy Letters

5.0x10-11

0

50

100

150

200

250

Specific Capacity

Figure 6. Galvanostatic intermittent titration technique (GITT) curves and Na-ion diffusion coefficient (D ) of TiO (A/B)-MS during (a) sodiation and (b) desodiation process. Na

+

2

To better understand the dynamic behavior of the TiO (A/B)-MS anode in Na-ion cell, the 2

Na-ion diffusion coefficient of TiO (A/B)-MS was measured by the galvanostatic intermittent 2

titration technique (GITT) in Figure 6 and Figure S5. Chemical Na diffusion was measured for +

TiO (A/B)-MS by the GITT. Sodium diffusion (D ) was calculated from Eq. (1) derived by +

2

Huggins:

Na

28

𝐷

456

=

+ ? @A 7 9: + > (@B) = G ( ) 𝑓𝑜𝑟 𝑡 @A 8 ;< ( ) @CD/F


(

@ F ) FN

≪ O

PQ6

(1)


Page 14 of 22

where 𝑉S is the molar volume, 𝐹 is the Faraday constant, 𝐴 is the contact area between the electrolyte and sample, 𝐼 W is the applied constant electric current, 𝑑𝜀/𝑑𝛿 is the slope of the coulometric titration curve, and 𝑑𝜀/𝑑𝜀 is the slope of the short-time transient voltage charge. The equation is valid for times shorter than the diffusion, (𝑑/2𝜋𝜋)+ /𝐷456 , where d is the average diameter of the grains (~1 µm on average). The 𝐷456 values derived from Eq. 1 are based on the following assumption: V remains unchanged with the variation in Na content in m

the compounds. Note that the average Na ion concentration is considered to be 0.044 mol at +

each step. During the sodiation process (Figrue 5a), the diffusion coefficient of Na ions gradually decreases from 9.54 × 10 to 1.14 × 10 cm S because of Na insertion and the –11

–11

2

–1

+

subsequent structural rearrangement in the TiO (A/B)-MS. Following the desodiation process 2

(Figure. 5b), the diffusion coefficient of Na ions is rapid, showing a high average value of about 2.0 × 10 cm S . The mixed-phase interface including crystal boundaries and lattice distortion –10

–1

of TiO (A/B)-MS effectively increase the reversible capacity and promote the reaction kinetics, 2

providing fast diffusion channels and active sites for Na ions. In the initial charge-discharge +

process where the interface reaction has a great influence on the diffusion rate, hence, Na ions +

easily inserted/extracted into/out TiO (A/B)-MS, showing a high Na ion diffusivity values. +

2

However, as mentioned in the Na-storage mechanism, the irreversible formation of Ti and O 0

2

by disproportionation reaction was occurred after deep discharge process, which leads to the relatively large polarization of the material (see the distribution of the polarization increases in GITT curve) and thus inhibits the Na ion diffusion into the structure. At a high desodiation +

level, relatively large polarization was observed in the GITT curve, resulting in decrease of Na

+

diffusion coefficient; this phenomenon might be attributed to the structural strain of TiO (A/B)2

MS. Notably, the measured diffusion coefficient of Na ions in the TiO (A/B)-MS is highly 2

compatible with the previously reported values (5.72 × 10 cm s ) in the carbon-coated –11


2

–1

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ACS Energy Letters

TiO (A)/TiO (B)/C material. The unique morphological and structural features of TiO (A/B)2

15

2

2

MS provide a larger number of Na-ion transport paths in the material, facilitating Na-ion transport during the sodiation-desodiation processes. On the other hand, to clarify the effect of lattice size on Na diffusion kinetic, we further +

characterized TiO -500 sample and tested its rate capability with Na ion diffusivity in Na-cell. +

2

From profile-matched XRD patterns refined by Rietvled method (Figure S2 and Table S3), we confirmed that TiO -500 sample has a nearly anatase phase TiO structure. In addition, as seen 2

2

in HR-TEM and corresponding IFFT image (Figure S6), the TiO -500 sample was nearly 2

crystallized in anatase phase. The TiO -500 sample has relatively large lattice parameters 2

compared to TiO (A/B)-MS, but it exhibited poor rate capability (Figure. S7) and low average 2

Na ion diffusion coefficient during desodiation process (Table S4). Although the TiO (A/B)+

2

MS has relatively constricted lattice parameters compared to bulk anatase and bronze TiO

2

structure, the mixed-phase interface including crystal boundaries and lattice distortion effectively increase the reversible capacity and promote the reaction kinetics; this could provide fast diffusion channels and active sites for Na ions, resulting in an additional capacity +

and enhanced rate capability.

25,29,30

To further verify the excellent Na-ion transport properties of the micro-intertextured TiO (A/B)-MS, the migration barrier energy (E ) of Na ions was calculated using density2

m

functional theory (DFT) calculations. In addition to the co-existence of the anatase and bronze phase in the bulk structures proved by XRD data, we also confirmed that the TiO (A/B)-MS 2

have a anatase/bronze interfacial layer through TEM analysis. Nudged elastic band (NEB) calculations (see Figure 7a) revealed the structural characteristics of the intertexture TiO (A/B)2

MS, which have both a fixed layer (gray region), corresponding to the bulk characteristics, and interfacial layer (red region) characteristics, in which the two phases are mixed. Further details on the construction of the slab structure and NEB calculations are presented in Table S5. 15 ACS Paragon Plus Environment


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Figure 7. (a) Atomistic structure of anatase/bronze interface in intertexture TiO (A/B). The red 2

and grey areas correspond to the interfacial and fixed regions, respectively. (b) Pathway schematics of single and multiple Na-ion migrations along the TiO (A/B) interface. (c) 2

Migration energy variation in single and multiple Na-ion migrations. The black and red lines correspond to the single- and multiple-ion migrations, respectively. As reported in the literature,

16, 31

bulk structures of anatase and bronze usually exhibit relatively

high migration barrier energies of 0.56 and 2.2 eV, respectively, which may be one of the major drawbacks for the use of TiO materials as anodes for practical SIBs. By contrast, the fast 2

diffusion characteristics of Na ions in the intertextured TiO (A/B)-MS calculated based on the 2

GITT results are very surprising. As seen in the NEB calculation results in Figure 7b, the anatase/bronze interfacial structure consists of successive tetrahedral sites for effective Na-ion transport, and there is a 4-Å-long pathway through the most stable A-site. The E values of the m

single and multiple Na ions for the anatase/bronze interfacial structure are 0.49 and 0.30 eV, respectively, which are lower than those of the two bulk phases of TiO (i.e., anatase and 16 2

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ACS Energy Letters

bronze). Notably, the concerned ionic diffusion with multiple Na ions, such as high Na-ion concentrations or fast charge-discharge conditions, has a lower E than that of single ions by m

0.19 eV; this strengthens our experimental results that the TiO (A/B)-MS material can deliver 2

ultrafast Na -ion storage performance. +

In this study, carbon-free and high-tap-density TiO (A/B) microsphere was successfully 2

synthesized and demonstrated great potential as anodes for outstanding Na storage. By taking the beneficial features of anatase/bronze interfacial structures and the compact microsphere constructed from nano-primary particles, the TiO (A/B)-MS anode exhibited excellent Na

+

2

storage performances; specifically, the proposed material exhibited an unexpectedly high capacity of 221 mAh g at 0.1 C and delivered an exceptional capacity retention of 100% even –1

after 1000 cycles at 10 C. Moreover, the TiO (A/B)-MS anode exhibited an outstanding power 2

capability with a capacity retention of 30% at 50 C rate (vs. the initial capacity at 0.1 C) without the support of a conductive carbon matrix. Compared to a previously reported carbon-free TiO 2

based anode, the proposed TiO (A/B)-MS produced better electrochemical performances in 2

terms of tap density, capacity, rate capability, and long-term cycling stability in SIBs. However, further investigations are needed in the future to prove that TiO (A/B)-MS anode can overcome 2

their limitation of practical acceptability, due to the large irreversible capacity at initial cycle. To minimize irreversible interface reaction between electrode and electrolyte, we will further engineer and optimize microsphere particle morphology that has a much denser and lower surface area than reported one. Although the practical use of a SPAN electrode will require further work, we believe that the results presented here will be helpful for the development of TiO -based materials for efficient sodium storage anodes with a high energy density, high 2

power, and low cost.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Experimental Methods, Rietvled refined XRD patterns, diffusion coefficient and rate capability of TiO2-300 and TiO2-500 samples, TEM and HR-TEM images for TiO2-500 sample, comparison table and figure, ex situ XPS data for TiO2-MS crystal structures of TiO2 anatase and bronze phase used in DFT calculations.

AUTHOR INFORMATION Authors Contributions #

Equally contributed to this work

Corresponding Authors * Hun-Gi Jung E-mail: [email protected] * Yang-Kook Sun E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. 2017R1A2B2006275) and by the Korea Institute of Science and Technology (KIST) 18 ACS Paragon Plus Environment

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ACS Energy Letters

Institutional Program (2V05940). This work was mainly supported by the Global Frontier R&D Program (2013M3A6B1078875) of the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, Information & Communication Technology (ICT). REFERENCES (1)

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