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Quick activation of nanoporous anatase TiO2 as highrate and durable anode materials for sodium ion batteries Liming Ling, Ying Bai, Yu Li, Qiao Ni, Zhaohua Wang, Feng Wu, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13927 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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ACS Applied Materials & Interfaces

Quick Activation of Nanoporous Anatase TiO2 as High-Rate and Durable Anode Materials for Sodium Ion Batteries Liming Ling,† Ying Bai,*,†,‡ Yu Li,† Qiao Ni,† Zhaohua Wang,† Feng Wu†,‡ and Chuan Wu*,†,‡ †

Beijing Key Laboratory of Environmental Science and Engineering, School of

Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China

ABSTRACT: To understand the slow capacity activation behavior of anatase TiO2 as sodium-ion batteries anode during cycling, a nanoporous configuration was designed and prepared. Based on the comprehension of Na-ions storage mechanism, the behavior is demonstrated to be related with the gradual formation of amorphous phase resulting from the phase transition during discharge. And the level of phase transition is determined by the discharge rates and cycle numbers, which strongly affects the electrochemical performance of anatase TiO2. Via a quick formation process of amorphous phase in the initial cycles, the capacity activation is accelerated, and high initial capacity are achieved with no fading after 500 cycles. Particularly, anatase TiO2 displays surprisingly unique properties in the fast charge (even at 20 C, 6.7 A g-1) mode, delivering a 179 mAh g-1 charge capacity. This study is significant for the comprehensive understanding of the controversial sodium storage mechanisms and unclear special behaviors occurring in anatase TiO2, thus greatly contributing to better guidance on the computational studies and experiment technologies for further performance promotion. KEYWORDS: anatase TiO2, nanoporous, capacity activation behavior, phase transition, fast, durable, sodium ion batteries 1

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1. INTRODUCTION During the past decades, lithium-ion batteries (LIBs) have been successfully developed as dominant power sources for transportation systems and various portable devices.1,2 Considering the continuously growing demands for emerging applications, cost will become one of the major concerns for LIBs due to the very limited and unevenly distributed lithium resources.3 Therefore, rechargeable sodium ion batteries (SIBs) have been considered as a top alternative to LIBs for large-scale energy storage because of sodium’s lower cost and wider availability.4 However, it is a challenge to search for suitable SIBs electrode materials with high battery capacity and long cycle life at high rates because Na+ ions have larger ionic radius than Li+ ions.5 Recently, a variety of alternative cathode materials has been widely researched,6–15 such as layered metal oxide, polyanionic compounds, metal hexacyanometalates, and organic compounds. The potential anode materials for SIBs involve carbon-based, metal oxides, alloy-type, and metal disulfide.16–22 Among all proposed anode materials, for the reasons of exceptional stability, nontoxicity, natural abundance, and low cost, titanium based compounds have been studied successively; representatively, spinel lithium titanate (Li4Ti5O12),23 sodium titanate (Na2Ti3O7),24 and titanium dioxides (TiO2).25–29 Of these, as for anatase TiO2, unremitting efforts have been devoted to address the issue of intrinsically low electrical conductivity, such as elements doping,30,31 surface coating,32 composites constructing,33 nanomaterials and 2


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hierarchical structure design,34–37 leading to encouraging advances. Moreover, relevant researches were targeted to exploring the sodium storage mechanism of anatase TiO2. Despite some great efforts acquired so far, it is still scattered and controversial owing to different experiment evidences. Most results displayed that Na+ could be inserted and extracted reversibly into the host structure of anatase TiO2 coupled with Ti4+/Ti3+ redox reaction during cycling in SIBs.31,37–39 Several computational studies were also directly based on this intercalation mechanism.38,40,41 While Wu et al.42 reported the irreversible reduction of anatase TiO2 caused by the large Na+ ions insertion. Some studies support the viewpoint although the details of experiment data and analysis were not totally the same,43–45 such as the X-ray diffraction (XRD) peaks changes of the TiO2 anode during cycling and the chemical states of Ti in the reduction phase. Interestingly, some researchers claimed that the structural change of the TiO2 lattice is reversible during charge/discharge and the anatase phase could be partially recovered after desodiation.34,46,47 Apart from the controversial sodium storage mechanism, anatase TiO2 anode displayed some intrinsic features including the capacity activation behavior upon cycling32,39,48,49 and the rising trend of peak currents in the CV test.38,50,51 Most researchers simply ascribed these special phenomena to the gradual improvement of electrode kinetics, the increase of active area or the electrochemical activation of materials inner part, etc. There are no systematic studies and reasonable analyses on these special phenomena. Therefore, for anatase TiO2, the sodium storage mechanisms are still under debate and these special phenomena are unclear. Due to the lack of comprehensive understanding on 3



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anatase TiO2, it would be difficult to further optimize the electrochemical performance, thus greatly restricting its practical application in SIBs. In this work, nanoporous anatase TiO2 is designed and used as SIBs anode on account of the beneficial effect of nanostructure (the unique combination of tiny nanocrystals and uniform nanopores). Thus, the research would be more accurate resulting from the exclusion of other factors such as the electrochemical activation of materials inner part. The slow capacity activation behavior during cycling was studied deeply and resolved basically based on the comprehension of Na-ions storage mechanism. Our results suggest that during the discharge process sodium ions tend to partially reduce mesoporous anatase TiO2 to form amorphous phase, which is electrochemical active for subsequent cycles. The incomplete phase transition is the main factor for the capacity activation behavior during cycling, which would remain before the finish of phase transition. At low discharge rate, the irreversible phase transition is fast. Thus, the capacity activation is accelerated and the remarkable steady capacity is achieved in the initial cycles.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Here, nanoporous anatase TiO2 is prepared on account of the beneficial effect of nanostructure on the research into the capacity activation behavior.52 The preparation process is shown in Scheme 1a. Typically, tetrabutyl titanate (TBOT, 0.5 ml) was added slowly to acetic acid (HAc, 25 ml) under constant magnetic stirring at room temperature. The obtained white suspension was then 4


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transferred to a 50-ml sealed Teflon reactor and heated to 210°C for 24 hours. The white products were collected by centrifugation, washed with deionized (DI) water and ethanol for several times, dried at 60°C for 24 hours, and finally calcined at 400°C for 30 min in a muffle furnace. 2.2. Physical Characterization. The crystal structure of electrode materials was analyzed in the 2θ range from 10° to 80° by Rigaku2400 powder X-ray diffraction (XRD) with Cu Kα radiation source. The surface morphologies were observed by using a FEI Quanta 250 field-emission scanning electron microscope (FE-SEM) and a Tecnai G2 F20 high-resolution transmission electron microscope (HR-TEM). Nitrogen adsorption and desorption isotherms were obtained by ASAP-2020 HD88 at 77 K. The element distribution information was verified by energy dispersive X-ray spectroscopy (EDX). X-ray photoelectron spectroscopy (XPS, PHI Quantera II SXM) was carried out to investigate the chemical state of samples. 2.3. Electrode Preparation and Electrochemical Testing. The working electrode was fabricated by blending 70 wt% TiO2, 20 wt% Super-P, and 10 wt% carboxyl methyl cellulose (CMC) binder in DI water into a uniform slurry and then coating on Cu foil. The coated electrodes were dried under vacuum at 120°C for 12 hours, and pressed at the pressure of 2 MPa. The average loading amount of active material is 1 mg cm-2. The quantity of the electrolyte per half-cell was 0.29 g, that is, 0.22 ml. Two-electrode coin cells (CR2025) employing sodium metal as counter electrodes and glass fiber as separators were assembled in an Ar-filled glovebox. The electrolyte consisted of 1 M solution of NaClO4 dissolved in ethylene carbonate (EC) and 5



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propylene carbonate (PC) (1:1 v/v). Galvanostatic measurements were tested on a LAND-CT2001A instrument, with 1 C = 335 mAh g-1. The voltage range was 0.02-2.5 V vs. Na/Na+. The cyclic voltammetry (CV) experiments were conducted with a CHI660e electrochemical workstation. CV curves were recorded between 0.02 V to 2.5 V (vs. Na/Na+) at a constant scan rate of 0.1 mV s-1.

3. RESULTS AND DISCUSSION 3.1. Structure and morphology The XRD patterns of pristine and calcined samples are shown in Figure 1a. The peaks of all samples agree well with the standard spectrum of anatase TiO2 (JCPDS PDF#04-0477) and no impurities are founded. The increase of peak intensity after thermal treatment indicates the improvement of crystallinity. As shown in Figures 1b and 1d, the particles are generally spindle-shaped. The results of N2 adsorption-desorption measurement are shown in Figure 1c. It displays the mesoporous characteristic with an average diameter of 8.8 nm,53 which is further confirmed by the microscopic voids and tiny nanocrystals of transmission electron microscope (TEM) image (Figure 1d).

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Figure 1 (a) XRD patterns of pristine and calcined TiO2. The black vertical line is the standard spectrum of anatase TiO2 (JCPDS, card no: 04-0477). (b) SEM image, (c) N2 adsorption-desorption isotherms and the corresponding pore size distribution (Inset) and (d)

transmission electron microscope (TEM) image of calcined TiO2.

3.2. Phase transition and capacity activation behavior Figure 2a shows the cycle performances of the TiO2 sample for sodium ion storage at a current density of 0.1 C. The charge capacities are 226, 233 and 200 mAh g-1 in the 1st, 10th and 110th cycle, respectively, indicating good stability and high capacities of the nanoporous anatase TiO2 for the unique combination between mesoporous structure and tiny nanocrystals.54 The initial coulombic efficiency is ~60%, which is high enough compared with previous studies.30,32,33,37,42 As shown in Figure 2b, the initial discharge profile is different from subsequent cycles. In the first discharge profile, the 7



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voltage drops to 1.0 V rapidly with a capacity less than 10 mAh g-1, while the voltage decreases slowly in the range from 1.0 to 0.3 V. Most of the sodium ions uptake into the anode happened in the third section between 0.3 V to 0.02 V. There is a voltage plateau around 0.1 V. All subsequent charge-discharge curves could no longer be divided into three parts clearly as revealed above and show similar features with slope-shape plateaus. A slight capacity increase with improving coulombic efficiency upon cycling is found in Figure 2a and further revealed by the corresponding discharge/charge profiles for the 2nd, 5th and 10th cycles (Figure 2b). When cycled at higher rates, TiO2 sample still displays super durability over 3000 cycles with charge capacities of ~145 mAh g-1 at 1 C and ~75 mAh g-1 at 5 C. However, the capacity activation behaviors are even more pronounced compared with the rate of 0.1 C, and it lasts for near 1000 cycles.

Figure 2 (a) Cycling stability and (b) the corresponding discharge/charge curves of TiO2 cycled at 8


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0.1 C. (c) Cycling stability of TiO2 cycled at 1 C and 5 C.

To understand the capacity activation behavior, we firstly researched the Na-ions storage mechanism of TiO2 in the initial discharge/charge cycle at 0.1 C. As shown in Figure 3a, seven states of discharge/charge are selected. The decreases of (101) reflection intensity (Figure 3b) indicate that the crystal structure is gradually transformed from anatase to the amorphous phase during the discharge process. This transition is prominent between 0.3 to 0.02 V, which is consistent with the large capacity contribution of this voltage range in the whole discharge process (Figure 3a). At the end of the discharge step (0.02 V), most anatase reflections vanish and (101) reflection still partially remains. Upon the subsequent charge to 2.5 V, there are almost no ex-situ XRD patterns changes such as the total disappearance of (101) reflection and the reappearance of anatase phase. The X-ray photoelectron spectroscopy (ex-situ XPS) (Figure 3c) tests demonstrate that the peak of metallic Ti0 (Ti 2p3/2 at 453.6 eV) appears at 0.02 V and remains at 2.5 V. These results suggest that during discharge the sodium ions appear to partially reduce anatase TiO2 to form the amorphous phase and metallic Ti0. The phase transition is irreversible and only happens during discharge (especially within the voltage range from 0.3 to 0.02 V). Then the amorphous phase would be electrochemical active for subsequent cycles. It is reflected by the special change of corresponding discharge/charge curves (Figure 2b).

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Figure 3 (a) The initial discharge/charge profiles for TiO2-based electrodes cycled at 0.1 C between 0.02 and 2.5 V. Point 1, 2, 3 and 4 represent discharge states of not cycled, 1, 0.3 and 0.02 V, respectively. Point 5, 6 and 7 represent charge states of 0.5, 1.25 and 2.5 V, respectively. (b) The corresponding ex-situ XRD patterns and (c) ex-situ XPS analysis of TiO2-based electrodes.

Moreover, the ex-situ SEM analysis on the electrode surface was performed 10


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(Figures S1a-g). To observe the real changes in the normal galvanostatic cycling, the discharge/charge rate is still at 0.1 C. In the whole initial discharge/charge process, the morphologies generally maintain well and are similar to the pristine electrode (Figure S1a). As the energy dispersive X-ray (EDX) mapping shows in Figure 4, the element distribution of electrode discharged to 0.02 V is uniform. The storage of sodium ions is identified. The origination of element Cl is the conductive sodium salt (NaClO4) contained in the electrolyte. And the atomic ratio of Cl/Ti is only 0.025, indicating the very little existence of Cl in the electrode. Figure S1h shows the Na/Ti atomic ratios of electrode at different discharge/charge states acquired by EDX measurements. It displays the same change trend with the ratios via capacity calculation of Figure 2a and reveals the Na ions storage in TiO2 during the initial cycle.

Figure 4 Energy dispersive X-ray (EDX) mapping of discharged electrode (0.02 V). (a) SEM image of the investigated electrode area. (b-f) The elemental mappings for Na (green), Ti (yellow), C (blue), O (red), and Cl (purple), respectively. 11



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As revealed by the Na ions storage mechanism of nanoporous anatase TiO2, the phase transition happens during discharge. Here, different discharging/charging conditions are used to further understand the phase transition during cycling. The anatase characteristic of pristine TiO2 electrode slice is shown in Figure 5a I. When cycled at 0.1 C, the transition is incomplete associated with the existence of weak peaks (2θ = 25.3 and 48°) at the end of the initial discharge step (0.02 V) (Figure 5a II). Upon discharge for 110 cycles, all anatase reflections vanish and there are no new peaks (Figure 5a III). Similarly, anatase TiO2 is nearly amorphous via three times extra treatments (Resting for 30 minutes and discharging to 0.02 V again) after initially discharging to 0.02 V (Figure 5a IV). Then the anatase phase can’t reappear when charged to 2.5 V for the irreversible characteristic of phase transition (Figure 5a V). Ex-situ transmission electron microscopy (ex-situ TEM) was conducted to identify the phase transition as shown in Figure 5b. The anatase characteristic of pristine TiO2 is revealed by high-resolution transmission electron microscope (HR-TEM) (Figure 5b I), in which lattice fringes correspond to (101) planes with an interplanar spacing of d = 0.35 nm. After directly discharge to 0.02 V, the sample is not well crystalline compared to the as-synthesized (Figure 5b II). In particular, with further three times treatments (Resting for 30 minutes and discharging to 0.02 V again), the anatase structure disappears, and there is no new crystalline phase (Figure 5b IV). These results are confirmed in the corresponding selected-area diffraction (SEAD) patterns and agree with the ex-XRD patterns. When cycled at a high rate of 1 C, at the end of the initial discharge step (0.02 V), the (101) reflection is apparent with 12


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high intensity (Figure 5c VI). Upon discharge for 3000 cycles, the phase transition is complete with no anatase reflections (Figure 5c VII). However, when at a higher rate of 5 C, there are still little maintain of (101) reflection even after 3000 cycles (Figure 5c VIII).

Figure 5 (a) Ex-situ XRD patterns of TiO2-based electrodes under different cycle numbers at 0.1 C. (b) The corresponding ex-HRTEM images and related SEAD patterns of TiO2. (c) Ex-situ XRD patterns of TiO2-based electrodes under different cycle numbers at 1 C and 5 C.

As shown in Figure 5, these experiments strongly demonstrate that the level of phase transition could be controlled by the discharge rates and cycle numbers. When discharged for the same cycle number, the lower the discharge rate, the more complete the phase transition. When discharged at the same rate, the longer the discharge number, the more complete the phase transition. That is, the discharge rates and numbers display the similar effect on the level of phase transition. The reasons are 13



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speculated as follows. Firstly, high discharge rates cause less discharge time resulting from the high current density and aggravated electrode polarization. While long discharge cycle numbers mean more discharge time. Thus, the discharge rates and cycle numbers can affect the total discharge time. Secondly, the discharge time can affect the diffusion time of sodium-ions, the reaction time of phase transition and the quantity of sodium-ions to induce the phase transition. As revealed by the Na ions storage mechanism of nanoporous anatase TiO2, the phase transition only happens during discharge. According to the relevant theory of reaction dynamics, the total discharge time would have great effect on the crystal structure transition process. Therefore, the total discharge time, which is associated with the discharge rates and numbers, can affect the phase transition. The lower the discharge rate or the longer the discharge number, the longer the total discharge time; thus the more complete the phase transition. The phase transition process is shown in Scheme 1b. At present, the possible sodium storage mechanisms of anatase TiO2 include Na+ insertion reaction into the host anatase structure31,37–39 and Na+ insertion in the amorphous sodium titanate converted from anatase TiO2.42–44 The research conflicts might be attributed to the different level of phase transition, which forms the anatase/amorphous mixture phase or the complete amorphization, thus resulting in the potential cognitive imperfection of the crystal structure during cycling. Moreover, some researchers claimed that the structural change of the TiO2 lattice is reversible during charge/discharge and the anatase phase could be partially recovered after desodiation.34,46,47 We speculate it is 14


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possibly ascribed to the different discharge condition settings when discharged to 0.02 V (Figure 5a IV) and charged to 2.5 V (Figure 3b). Importantly, the deep understanding of the crystal structure transition process contributes to more accurate computational studies, which were generally based on the intercalation mechanism into the host structure of anatase TiO2.38,40,41

Scheme 1 (a) Schematic diagram of the synthetic route for anatase TiO2 anode. (b) Schematic illustration of the phase transition process for anatase crystal structure during discharge.

3.3. Quick activation and electrochemical analysis The electrochemical performances are also further investigated by regulating the discharge/charge conditions (mode). As revealed in Figures 1c-d, TiO2 microparticles display the mesoporous microstructure, which possibly causes the delayed surface wetting of electrolyte. Thus, the capacity activation step can be partially attributed to the delayed wetting of electrolyte into the 3D nanoporous structure of the composite electrode.55 15



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To eliminate the potential influence of surface wetting, the coin cell aged before electrochemical tests. The aging time is forty days, which is longer than the actual activation time when cycled at 1 C as shown in Figure 2c. Here, four cycle modes were used and the results are shown in the Figure 6a. (i) The capacity increases with the increasing cycle numbers at 1 C. Therefore the mesoporous structure hardly hampers the surface wetting between the material and the electrolyte. (ii) Similarly, when discharged at 1 C and charged at 0.1 C, the capacity still increases slowly. It demonstrates that the low charge rate can’t achieve a high initial capacity. (iii) In contrast, when discharged at 0.1 C and charged at 1 C, high capacity is acquired in the initial cycles. Low discharge rate indeed contributes to the performance improvement and the effective elimination of capacity activation behavior. (iv) Moreover, the capacity activation (compared with Figure 2c) is also accelerated at 1 C after cycled at 0.02 C for three cycles. As discussed above, the lower the discharge rates or the longer the discharge cycle numbers, the more complete of the phase transition. Therefore, the incomplete phase transition would be the main factor for the capacity activation behavior, which would remain before the finish of phase transition. The apparent differences of charge capacities between partial and total phase transition imply the amorphous rather than anatase phase is mainly electrochemical active for the whole cycling. As inspired by the speculation, the coin cells are tested in new discharge/charge conditions. The rate performances of the sample were tested at different charge rates, from 0.1 to 20 C; while all cells were subjected to discharge at a constant rate of 0.1 C 16


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(Figure 6b). The reversible charge capacities of 230, 218, 217, 207, 204, 193, 187, and 179 mAh g-1 were obtained at charge rates of 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 C (6.7 A g-1), respectively. It demonstrates the unique properties and surprisingly high capacities of TiO2 in the slow discharge and fast charge mode (especially at high charge rates), which offers significant advantages and has rarely been explored before. Particularly, when discharged at 0.1 C and charged at 30 C (10 A g-1, namely fully charged in 120 s), it delivers remarkable performance with a 154 mAh g-1 initial charge capacity and 140 mAh g-1 in the 250th cycle (Figure 6c). The rapid energy supply and long cycle life could be applicable in many fields, such as public transport and power demand management. The detailed performance at 1 C after cycled at 0.02 C for three cycles is shown in Figure 6d. High initial charge capacity of 163 mAh g-1 is obtained and there is no capacity fading after 500 cycles. The capacity activation is accelerated in both tests, ascribing to the quick phase transition of initial cycles. At low discharge rate, the irreversible phase transition is fast. Thus, the capacity activation is accelerated and the remarkable steady capacity is achieved in the initial cycles. As known above, the amorphous phase is mainly electrochemical active for cycling. To further promote the rate performance of anatase TiO2, the level of phase transition is the very key factor to consider.

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Figure 6 (a) Galvanostatic cycling of TiO2-based electrodes under different discharge/charge mode. (b) Rate performance at the different charge rates (at a constant discharge rate of 0.1 C). (c) Cycling stability of TiO2 when discharged at 0.1 C and charged at 30 C. (d) Cycling stability of TiO2 at 1 C after cycled at 0.02 C for three cycles.

To further understand the speculation over the capacity activation behavior, the cyclic voltammetry (CV) of the mesoporous TiO2 electrode was tested at a scan rate of 0.1 mV s-1. Figure 7a shows the CV curves of the initial 11 cycles between 0.02 V and 2.5 V (vs Na/Na+). The corresponding changes of peak currents for 11 cycles are shown in Figure 7b. During the first cathodic scan, the peak around 0.03 V is mainly ascribed to the structure transition from anatase to amorphous phase. Its intensity decreases in the following cathodic sweeps, indicating the gradual finish of amrophization. Meanwhile, a new redox couple at 0.85 V (anodic) and 0.65 V (cathodic) become pronounced and prominent during subsequent cycles. It is attributed to the reversible sodium ions storage in the new amorphous phase. The 18


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rising trend is in reasonable agreement with the galvanostatic discharge/charge. Moreover, the peak around 0.03 V is partially connected with the electrolyte decomposition and the capacity contribution of conductive carbon black for its corresponding anodic peak of 0.08 V.56 The capacity of carbon black always exists in subsequent cycles. While it does not affect the overall evaluation of battery capacity due to the little contribution with the charge capacity value of ~15 mAh g-1. In contrast, after cycled at 0.02 C for three cycles, the result of CV tests shows that the curves of the initial 11 cycles are almost overlapped (Figure 7c), showing an excellent reversibility of TiO2 anode and demonstrating the acceleration effect of low discharge rate on the phase transition process.

Figure 7 (a) CV measurement of TiO2-based electrode over the potential window of 0.02–2.5V versus Na/Na+ for 11 cycles. (b) The corresponding peak currents for 11 cycles. (c) CV measurement of TiO2-based electrodes over the potential window of 0.02–2.5V versus Na/Na+ for 19



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11 cycles after cycled at 0.02 C for 3 cycles. (d) CV measurement of TiO2-based electrodes over the potential window of 0.02–2.5V versus Li/Li+ for 11 cycles.

The prepared TiO2 was also tested as LIBs anode. It is known that anatase TiO2 exhibits the typical lithium-ions insertion/extraction mechanism into the host anatase structure.57 Therefore, there are no peaks for phase transition and the electrochemical reaction of other phase in the CV curves (Figure 7d). The cathodic/anodic peaks located at 1.7 and 2.0 V (versus Li/Li+) directly decrease with cycles increasing. There is no capacity activation behavior when cycled at 5 C (Fig. S2). The comparison further reveals the particularity of sodium storage in anatase TiO2, contributing further research on anatase TiO2 and other electrode materials for sodium-ions batteries.

4. CONCLUSION Here, nanoporous configuration of anatase TiO2 is designed and used as SIBs anode, and it displays the slow capacity activation behavior during cycling at high rates despite its super durability over 3000 cycles. Hence, we try to shed light on the capacity activation behavior based on the deep comprehension of sodium storage mechanism. Various material characterization techniques were conducted on it, including the ex-situ EDX, ex-situ HRTEM and CV tests of long cycles (after tested at a low rate), which were seldom used in the previous relevant researches. Thus, we identify that during discharge sodium ions tend to partially reduce mesoporous 20


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anatase TiO2 to form amorphous phase. The phase transition is irreversible and only happens during the discharge process. Interestingly, the level of phase transition is determined by the discharge rates and cycle numbers, which strongly affects the electrochemical performance of anatase TiO2. By regulating the discharge/charge mode, the capacity activation is accelerated and excellent properties are achieved ascribing to the quick phase transition in the initial cycles. The rapid energy supply and long cycle life of the new formed amorphous phase could be applicable in many fields, such as public transport and power demand management. Moreover, the formation of the anatase/amorphous mixture phase or the complete amorphization could cause the potential cognitive imperfection of the crystal structure during cycling, thus leading to the general conflicts on the sodium storage mechanism occurring in anatase TiO2. Generally, to address the issue of low electrical conductivity, we would adopt strategies including elements doping, surface coating, composites constructing, nanomaterials and hierarchical structure design, etc. We also use some computational studies to analyze the reasons for performance improvement. As for anatase TiO2 used in SIBs anode, the new formed amorphous rather than anatase phase is mainly electrochemical active for the whole cycling and thus should be the key factor to consider. Therefore, we should combine the general strategies for performance improvement with the special phase transition process occurring in anatase TiO2, thus contributing to an optimization of the electrochemical properties.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.******. Cycling stability of TiO2 cycled at 5 C for lithium-ions battery, and ex situ SEM images for TiO2-based electrodes cycled at 0.1 C (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Bai); [email protected] (C. Wu). NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS The present work is supported by the National Basic Research Program of China (Grant No. 2015CB251100).

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