Ultra-small TiO2 Coated Reduced Graphene Oxide Composite as

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Ultra-small TiO2 Coated Reduced Graphene Oxide Composite as High-rate and Long-cycle-life Anode Material for Sodium-ion Battery Yao Liu, Jingyuan Liu, Duan Bin, Mengyan Hou, Andebet Gedamu Tamirat, Yong-Gang Wang, and Yongyao Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Applied Materials & Interfaces

Ultra-small TiO2 Coated Reduced Graphene Oxide Composite as High-rate and Long-cycle-life Anode Material for Sodium-ion Battery Yao Liu,a Jingyuan Liu,a Duan Bin, a Mengyan Hou,a Andebet Gedamu Tamirat,a Yonggang Wang,*a Yongyao Xia,*a a

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, People’s Republic of China

Abstract: Owing to the low cost and abundant nature of sodium element, sodium-ion battery is attracting extensive attention, and variety of sodium-ion battery cathode materials have been discovered. However, the lack of high-performance anode materials are the major challenge of sodiumion battery. Herein, we have synthesized ultra-small TiO2 nanoparticles coated reduced graphene oxide composites by using one-pot hydrolysis method, which is then investigated as anode material for sodium-ion battery. The morphology of TiO2/reduced graphene oxide has been characterized using transmission electron microscopy, indicating the TiO2 nanospheres uniformly grows on the surface of reduced graphene oxide nanosheet. As-prepared TiO2/reduced graphene oxide composites exhibited a promising electrochemical performance in terms of cycle stability and rate capability, especially, the initial cycle coulombic efficiency of 60.7% that is higher than previous reports. The kinetics of electrode reaction has been investigated by cyclic voltammetry. The results indicate the sodium ions intercalation/extraction behavior is not controlled by the semi-infinite diffusion process, which give rise to outstanding rate performance. In addition, electrochemical performance of TiO2/reduced

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graphene oxide composites in full cells have been investigated, coupling with carbon coated Na3V2(PO4)3 as positive material. The discharge specific capacity was up to 117.2 mAh g-1 and 84.6 mAh g-1 was remained after 500 cycles under current density of 2 A g-1, which shows excellent cycling stability. Key words: TiO2, Reduced graphene oxide, Anode material; Initial cycle coulombic efficiency, Sodium-ion battery

1. Introduction With emphasis on the environmental protection and reducing the fossil fuel consumption, renewable energy sources, such as tidal energy; wind energy; solar energy; etc. are becoming increasingly important in order to sustain the development of our society. However, most of the renewable energy sources are discontinuous and not evenly distributed. To make full use of those renewable energy sources, developing on the larger scale energy storage systems are quite important.1-3 Among different kinds of energy storage systems, chemical energy storage system for its special advantages shows broad application prospects. Lithium-based batteries, for their superior electrochemical performance, have been thought as best choice for energy systems.4-5 However, once the large scale applications in energy storage system, the limited lithium resource would hardly full fill the large scale demand. Compared to lithium element, sodium element has similar chemical properties and it is wide availability in the Earth crust.6-8 Therefore, developing the sodium based batteries (SIBs) is much economically favorable, especially for finding excellent electrode materials. Recently, many researches are focusing on the cathode materials for SIBs, such as O3-type and O2type layer compounds, polyanion compounds (NaFePO4, FePO4, Na3Fe2(PO4)3, Na3V2(PO4)3), 2


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Prussian blue, etc.9-15 For anode material, various carbon based materials have been intensively researched. Unlike lithium and potassium, graphite cannot be used as an intercalation anode for SIBs. Disordered carbon materials, like hard carbon has been intensively studied, which has shown a large degree of sodium reversible intercalation.16-17 But for the security consideration, hard carbon was not suitable for practical applications owing to its low interaction potential of sodium ions. Some researchers reported alloy-type or metal sulfides anode materials for SIBs.18-22 But these kinds of anode materials usually suffer from large volume change and poor cyclibility. Based on the above consideration, transition metal oxides,23-24 especially TiO2-based anode with suitable potential window and cycle stability, are regarded as potential anode material for SIBs. Very recently, various polymorphs of titania based materials, such as amorphous TiO2,25 rutile TiO2,26 anatase TiO2,27 and TiO2(B)28 have been researched as anode materials in SIBs and showed promising sodium ions storage capabilities. Due to the radius of sodium ion is larger than that of lithium ion and electronic structure is difference, the mechanism of sodium ions storage is quite different from that of lithium ions and electrochemical performances are much poorer than that of in lithium-ion battery.29-32 During sodium ions intercalation process, antase TiO2 partly transforms into titanium and sodium oxide.33 Therefore, volume expansion occurs during sodium ions insertion and thereby decreases the cycle’s stability of the material. In addition, the practical applications of sodium-ion battery are still facing big challenges of low electronic conductivity, low ions diffusion and low initial coulombic efficiency. According to the previous reports, different methods were developed to improve the electrochemical performance.34 However, the specific capacity, cycle stability and rate capability need to be further improved. Most importantly, as we summarize recent reports of TiO2 anodes in SIBs (Table S1), the initial coulombic efficiency (CE) of TiO2 was relative low (most were less than 50%). Therefore, it is full of meaningful 3



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to improve the initial CE for TiO2 anodes in SIBs, although it is full of challenges owing to the formation of the solid electrolyte interface and partial conversion of TiO2 to metallic titanium and sodium peroxide.33 Stefano Passerini et al. have reported that decreasing the particle size could improve the first cycle coulombic efficiency.27 But conventional synthesis methods could hardly obtain small particle sizes of TiO2 based on previous reports.35-42 Herein, one-pot method was applied to synthesize the ultra-small TiO2 nanoparticles (about 10 nm) coated reduced graphene oxide composites (TiO2@RGO), which is quite different from other conventional methods. The morphology was characterized by electron microscopy, which showed the nanosheet morphology. Subsequently, the TiO2@RGO was employed as anode in SIBs. The electrochemical performance of TiO2@RGO sample showed high specific capacity, excellent rate capability and ultra-long cycle stability. Especially, the first cycle CE reached up to 60.7%. Furthermore, for the first time we have demonstrated the full cell electrochemical performance of sodium-ion battery, by coupling with Na3V2(PO4)3 cathode material. The discharge specific capacities were 117.2 mAh g-1 and maintained at 84.6 mAh g-1 after 500 cycles under current density of 2 A g-1. The results indicated that the full cell shows excellent cycle stability (capacity retention ration of 72.2% after 500 cycles), rivaling most sodium full cell reported to date.

2. Material preparation 2.1. Preparation of material All chemicals were analytical grade and used without further purification. The schematic representation of the preparation of TiO2@RGO composite is shown in Figure 1. Firstly, graphene

4


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Figure 1. Schematic representation of the preparation of TiO2/RGO composite. oxide was synthesized through modified Hummers’ method.43 Briefly, flake graphite, (3.0 g,SigmaAldrich, cat. 332461) NaNO3 (1.5 g were added in to concentrated H2SO4 (69 mL) and KMnO4 (9.0 g) were mixed under ice bath condition. The KMnO4 was added slowly to maintain the reaction temperature below 20 °C. After stirring for 30 mins, added the additional deionized water (DI, 420 mL) and 30% H2O2 (3 mL). Subsequently, the reaction was continued for 1 h. The obtained solution was filtered (using polyster fiber supplied by Carpenter Co.). Then taking out of the filtrate and centrifuging (4000 rpm, four hours) process was used to get the solid material. The obtained solid material was washed using DI, hydrochloric acid (30% vol.) and ethanol, successively, for three times and then dialyzed for a week. (The transmission electron microscopy images as-prepared is supplied in supplementary, Figure S1). Then, the TiO2@RGO composite was synthesized through one-pot method. As-prepared GO (0.32g) was dissolved in the 100 mL ethanol and sonicated for an hour. Then the tetrabutyl titanate (TBOT, 6.8 g, Sigma-Aldrich Co.) was added into the 100 mL ethanol under magnetic stirring. The ammonia (28%, 10ml) with DI (10mL) mixed solution were added in the solution in a slow way of dropwise. The reaction was kept in water bath under 45 °C with magnetic 5



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stirring for 24 h. The obtained samples were centrifuged and washed by ethanol for three times. The final samples were dried at 80 °C for 12 h. The obtained final samples were sintered in a tube furnace at 450 °C for 2 h with circulation of Ar (20 mL min-1). After heat treatment, the amorphous TiO2 was transformed into antase TiO2 and graphene oxide (GO) was reduced. For full cell electrochemical performance evaluation, carbon coated Na3V2(PO4)3 (Na3V2(PO4)3@C) cathode material was synthesized based on a previous report.44-45 2.2. Material Characterizations The structure of TiO2/RGO anode and Na3V2(PO4)3@C cathode were characterized using Bruker D8 X-ray diffractometer (XRD, Germany). The constant wavelength was 1.5406 Å, scanned between 10° and 90° at 0.5s/step with a step width of 0.02°. The specific surface area measurements and the analysis of the porosity of the as-prepared samples were performed through nitrogen adsorption-desorption isotherms at 77 K, using an Autosorb-iQ system (Quantachrome Instruments U. S. Co. America). Thermogravimetric (TG) measurements were used the SDT Q600 (America) thermal analyser with the temperature increasing from room temperature to 800 °C at a rate of 10 °C min-1, using air atmosphere (20 mL min-1). The pan in the TG test used the ceramic crucible. The morphology of samples was characterized by using scanning electron microscopy (SEM, Nova NanoSem 450, FEI CO., UAS). Transmission electron microscopy (TEM, Tecnai G2 F20 S-Twin, FEI CO., USA.) equipped with an energy dispersive X-ray spectrometer (EDX-Mapping) was used to study the morphological properties of the TiO2@RGO composite samples. 2.3. Electrochemical tests The anode electrodes were prepared by mixing active material (TiO2@RGO composite,), conductive material (carbon black, 10%), and sodium carboxymethylcellulose polymeric binder (CMC-Na, 10%, 6


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Sigma-Aldrich Co.) in deionized water. The resulting slurry was cast uniformly onto Cu foil to obtain the electrode film, with thickness of 75 μm. Subsequently, the films were dried at 80 °C for 2 h to remove most of DI before pressing. Then, the films were further dried in a vacuum oven at 80 °C for 12 h. The cathode electrodes were prepared by mixing active material (NVP@C composite,), conductive material (carbon black, 10%), and polyvinyl difluoride binder (PVDF, Sigma-Aldrich Co.), in 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich Co.). Then the treatment process was similar to that of anode electrodes. Before the coin cell fabrication, the films were pouched into in to disc with diameter of 12 mm, weighted and transformed them into glove box rapidly. Sodium metal (AR, SigmaAldrich Co.) was used as counter and reference electrode, which was punched into disc with diameter of 12 mm and 1mm thickness. The electrolyte was 1 M NaClO4 (AR, Sigma-Aldrich Co.) dissolved in ethylene carbonate (EC, Shanghai Xiaoyuan Energy Co. Ltd) and propylene carbonate (PC, Shanghai Xiaoyuan Energy Co. Ltd) in a volume ratio of 1:1 with addition of 2% (vol) fluorinated ethylene carbonate (FEC, Shanghai Xiaoyuan Energy Co. Ltd). Whatman glass microfibre filter (Grade GF/C) was used as separator (diameter of 16mm, Whatman Co.). All coin cells (2016-type) were assembled in glove box (H2O and O2 < 1ppm). The galvanostatic cycling was tested by using Land CT2001A battery test system (Wuhan Land Co. Ltd, China). Cyclic voltammetry (CV) measurements were performed on a Princeton chemical work station (PARSTAT MC-500, USA.). The electrochemical performance of TiO2@RGO//NVP@C full cell was evaluated by using 2016-type coin cell. Prior to full cells fabrication, the pre-sodiation and de-sodiation of TiO2@RGO anode material were performed to carry out the initial cycle activation. The mass loading of TiO2@RGO anode is 3.02 mg and 1.53 mg for NVP@C cathode (the weight ratio of Negative/Positive was calculated based on the specific capacity). The voltage window was 1.5-3.4 V. All electrochemical measurements were tested at room 7



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temperature (25°C).The energy density and powder density of full cell was calculated by following Eq.1 and 2. ∗

= =

E t

1 2

Herein, E represents the energy density (Wh kg-1). Q (mAh) is the specific discharge capacity of the cell. V is the average discharge voltage of the cell. MNVP and MNaxTiO2 are the mass loading of the Na3V2(PO4)3 in cathode and the pre-sodiation TiO2@RGO in the anode, respectively. P represents the power density (W kg-1), and t is the time for full discharge.

3. Results and discussions The XRD pattern of TiO2@RGO is displayed in Figure 2a. Each peaks located at 25.3, 38.1,48.0, 54.2, 54.9, 62.7, 69.3, 70.1, 75.4 and 82.7° can be assigned to the (101), (004), (200), (105), (211), (204), (203), (106), (105), (211), (215) and (224) facets of TiO2, respectively, which indicated the formation of pure phase antase TiO2 with space group of I41/amd (JCPDS No.21–1272). The average crystallite size is roughly calculated according to the Scherrer formula based on the XRD pattern. The result indicates that the average crystallite size is ca. 11 nm. The N2 adsorption-desorption isotherms are displayed in Figure 2b. As-prepared TiO2@RGO exhibited a type IV isotherm with an H1 hysteresis loop according to the IUPAC classification,46 demonstrating the characteristics of mesoporous feature. The Brunauer−Emmett−Teller (BET) analysis displays that the TiO2@RGO sample has a specific surface area of 135.2 m2 g-1. In addition, the pore size distribution curve calculated using the BarrettJoyner-Halenda (BJH) method shows that the size of the majority of the mesopores fell within a 8


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Figure 2. (a) XRD pattern; (b,c) N2 adsorption-desorption isotherms and corresponding BJH pore size distribution; (d) TG curve of ultra-small TiO2 nanoparticles wrapped reduced graphene oxide composite. relatively narrow range of 15-25 nm (shown in Figure 2c). The mesopores formation is attributed to the growth of anatase nanocrystallines during calcination process at 450 °C in Ar for two hours.32 The result of TG measurement indicated that the carbon content of the composite was about 8 wt% (Figure 2d). The SEM images (Figure S2) and TEM images (Figure 3) of the TiO2@RGO composite clearly show the uniform growth of antase TiO2 nanospheres on the RGO nanosheets. HR-TEM images of TiO2@RGO composite are shown in Figure 3a-3c. The size distribution curve displays a mean diameter centered at ∼10 nm (Figure 3a, inset). The RGO nanosheets are encapsulated tidily by antase TiO2 nanoparticles. To further investigated the crystal structure of TiO2@RGO composite, the selected9



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Figure 3. (a, b, c) HR-TEM images of mesoporous TiO2@RGO composite, (d,e) SAED patterns 1 and EDX-Mapping. area electron diffraction (SAED) patterns of TiO2@RGO composite is shown in Figure 3d. The SAED patterns show diffuse concentric rings, which correspond to the [101], [004], [200] and [211] facets of antase TiO2. The SAED results testified that the as-prepared TiO2 has antase structure and is consistent with aforementioned XRD pattern. The EDX-mapping of composite is shown in Figure 3e, which indicates the uniform distribution of elements throughout the entire composite matrix. The TiO2@RGO composite, which has unique structure with ultra-small TiO2 nanoparticles coated RGO, was evaluated as anode material SIBs. Figure 4a shows the initial charge/discharge curves of 10


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Figure 4. Electrochemical performance of TiO2@RGO composite in SIBs:(a) charge/discharge curves under 0.05 A g-1, (b) charge/discharge curves at 0.05, 0.1, 0.5, 1.0 and 2.0 A g-1 (10th, 20th, 30th, 40th, 50th), (c) rate performance; (d) cycle performance under 1.0 A g-1. All measurements using the voltage range from 0.01 to 3.0 V. TiO2@RGO composite at current density of 0.05 mA g-1. It were found to be 460 and 279.5 mAh g-1 respectively for initial discharge and charge specific capacities, corresponding to the initial CE of 60.7%, which is much higher than that of commercial rutile TiO2 anode (28%)26 and much better than other reports (Table S1 summarized some TiO2 anode based on previous reports). The relatively high initial coulombic efficiency is attributed to the particles size with diameter of 10 nm and the reduced graphene oxide adding to decrease the polarization. After 200 cycles, the discharge specific capacity of 206.9 mAh g-1 was maintained (Figure 4a and Figure S4). The rate capability of TiO2@RGO are displayed in Figure 4b and 4c. The discharge specific capacities were 248.5, 204.2, 167.3, 151.3 mAh g-1 and 118.8 mAh g-1 at current density of 0.05, 0.1, 0.5, 1.0 and 2.0 A g-1, respectively. The cycle 11



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Figure 5. (a) CV curves measured at different scan rates, (b) analysis of b-value for anodic and cathodic peak current, (c, d) the separations for capacitive-controlled behavior under measured at scan rates of 0.08 and 0.5 mV s-1, (e) the ratio of capacitive-controlled and diffusioncontrolled capacities at different sweep rates (0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 1.0 mV s-1). stability of TiO2@RGO was evaluated under current density of 1.0 A g-1. A discharge specific of 150.3 mAh g-1 and 127.7 mAh g-1 can still be achieved after 1000 cycles (Figure 4d.), corresponding to a capacity retention ratio of 84.9%, which was superior to some recent TiO2-based anode materials in SIBs (Table S2). The results indicated the TiO2@RGO composite shows high initial CE, outstanding 12


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rate capacity and superior cycle stability. The cycle performance under current density of 2 A g-1 is given as Figure S5. It can be observed that the discharge specific capacity is 120.2 mAh g-1, with a capacity retention of 117.1 mAh g-1 after 550 cycles. To further clarify the high rate property of as-prepared TiO2@RGO samples, we have investigated the Na-storage kinetic properties of the corresponding electrodes, using the CV method quantificationally to calculate the contributions of diffusion-controlled and capacitive-controlled. The results are shown in Figure 5. Figure 5a displays the CV curves at different scan rates (0.08, 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 1.0 mV s-1). The results of CV curves display similar shapes with board cathodic and anodic peaks (The CV at the scan rate of 0.5 mV s-1 for 1 to 20 cycles is shown in Figure S3). The peaks current density (i) was increasing with increasing of scan rate (ν). Utilizing the relationship between i and ν, sodium ions intercalation/extraction kinetics can be analyzed, by using the following Eq.3 and 4.47 i = av

3

logi = loga + b ∗ logv

4

The b-value is the parameter for evaluating the electrode process. Specifically, b-value of 0.5 corresponds to a diffusion-controlled process caused by ‘guest ions ’ intercalated into ‘host electrode material’, while the b-value equals to 1.0 indicates the capacitive behavior for surface faradaic redox reaction.48 The a-value is coefficient. To reveal the mechanism of TiO2@RGO composite for sodium ion storage, log(i) versus log(v) was plotted, which is shown in the Figure 5b. The line fitting results show that the b-values of 0.904 and 0.871 correspond to cathodic and anodic current peaks, respectively. This indicates that the surface faradaic redox reaction mechanism of TiO2@RGO composite for sodium ion storage is capacitive behavior. The quantitative capacitive contribution from 13



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the total current response is determined using Eq.5 and 6.47 i(V) = k v + k v i(V) ∗ v

/

/

=k v

5 /

+k

6

Where k1 and k2 are adjustable parameters. The total current response is ascribed to the diffusioncontrolled (k2ν1/2) and capacitive-controlled contributions (k1ν). According to the Eq.5, the values of k1 and k2 can be obtained from the line fitting of ν1/2 versus i(V)*ν-1/2. After that, the capacitive and diffusion limited contributions could be calculated at each specific potential. The calculated results are shown in Figure 5c and 5d measured at the scan rates of 0.08 and 0.5 mV s-1, respectively. The percentage of surface capacitance-controlled in the total was increased from 54.4 to 83.5%. Figure 5e shows the proportion of surface capacitance controlled capacity at different scan rates. When the scan rate was 1.0 mV s-1, the percentage for capacitance-controlled was up to 88.6%. The results indicate that the sodium ions storage kinetic is not controlled by the semi-infinite diffusion process, which gives rise to outstanding rate performance. On the basis of the above discussions, the sodium-ion storage of TiO2@RGO composite demonstrated pseudocapacitive which resulted in excellent rate capability. In addition, ultra-small size mesoporous TiO2 nanoparticles with conducting RGO support provides high electronic transport. To further investigate structural changes of TiO2@RGO composite during sodium-ion intercalation-extraction process, TEM combining with mapping testing were performed to understand the structural changes at initial full discharge and full charge states. The results are illustrated in Figure 6a and 6b illustrate the TEM images of TiO2@RGO composite with different resolution at 0.01 V cut off (discharge at low current density of 0.01 A g-1). The morphology was similar to that of initial state. The size of TiO2 nanoparticles was uniformly grown on the RGO nanosheets. The results indicate that the structure of TiO2@RGO composite was kept stable after 14


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Figure 6. (a. b, c) TEM images and EDX-Mapping image at full discharge state; (d, e, f) TEM images and EDX-Mapping image at initial full charge state. sodium ion intercalation. The high resolution images clearly depict the TiO2 nanospheres with radius of about 10 nm. The EDX-Mapping images show that each element was distributed uniformly (Figure 6c). After initial full charge state, the TEM images of TiO2@RGO composite with different resolution are shown in Figure 6d and 6e. We have found that the micro-structure morphology was unchanged. Similarly, the morphology after full charge state were also similar to that of discharge state and as expected the amount of sodium elements obviously decreased (Figure 6f). From all the above observations, the high structural stability of the TiO2@RGO composite observed during sodium-ion 15



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intercalation/extraction process is the main reason for high cycle stability and the relatively high initial coulombic efficiency. The electrochemical performance of TiO2@RGO composite was also investigated in full-cell configuration by coupling with NVP@C as positive material. The XRD pattern (Figure S6) of asprepared Na3V2(PO4)3@C suggest that the sample is single-phase without impurities. The TG test indicated the carbon content is about 10% wt. (Figure S7). TEM images shown in Figure S8 indicates the nano-size particle with ca. 5nm thickness carbon layer. Before full cell test, Na3V2(PO4)3@C was first investigated in half cell configuration, and the cycling curves and cyclibility test are shown in Figure S9. The full cell voltage (V) and each positive/negative electrode potential were recorded to characterize the charge/discharge profile. As illustrated in Figure S10, an additional reference electrode of Na metal (as reference electrode) was used to assemble three-electrode customized cell. The initial charge/discharge curves for fabricated three-electrode sodium-ion full cell, shown in Figure S10, displayed the initial discharge and charge specific capacity of 450 and 208 mAh g-1, respectively. Although the TiO2@RGO composite has the relatively high initial coulombic efficiency compared with other reports as mentioned previously, for practical application, the initial coulombic efficiency of TiO2@RGO anode material should further be improved, which is presently under development in our laboratory. The rate capability and cycle stability have been investigated by using CR2016 coin cells. Prior to full cells fabrication, the pre-sodiation and de-sodiation of TiO2@RGO anode material were performed to carry out the initial cycle activation.54 The cell balance was achieved by setting the electrode mass ratio of positive/negative to about 2:1. The voltage window was 1.5-3.4 V. The results are shown in Figure 7. The charge and discharge curves of full cells are displayed in Figure 7a (0.1 A g-1). The discharge specific capacities were 197.5 mAh g-1. It were found to be 188.4, 168.7, 148.1 and 16


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Figure 7. Electrochemical performance of TiO2@RGO composite coupling with NVP@C as positive material in sodium-ion full cell: (a) charge/discharge curves at 0.1 A g-1; (b) rate performance; (c) cycle performance at current density of 2.0 A g-1; (d) Ragone plots of NVP@C//TiO2@RGO cell and other reported works.49-53 116.5 mAh g-1 at the current density of 0.2, 0.5, 1.0 and 2.0 A g-1, respectively (Figure 7b). The cycle stability and corresponding coulombic efficiency were tested under the current density of 2.0 A g-1. The result is shown in Figure 7c. A discharge specific of 117.2 mA g-1 in the first cycle can be achieved, and 72.2% (86.4 mA g-1) of the capacity can still be maintained even after 500 cycles. All capacities and current densities in this section were calculated based on negative electrode. The results indicated the electrochemical performances of NVP@C//TiO2@RGO cell were better than other reports (table S3). To further clarify the advantage of the full cells, power densities and energy densities of NVP@C//TiO2@RGO cells were calculated based on the total mass of active material. The results are 17



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given in the form of Ragone plot (Figure 7d). The cells exhibit the maximum energy densities of 167 Wh kg-1 and maximum powder densities of 1333 W kg-1. However, the disadvantages of TiO2-based anode materials for sodium-ion battery could not be ignored, especially the initial low initial coulombic efficiency might be the big challenge for its practical application and need to further investigate. In the lithium ion battery, several approaches have been proposed to address this capacity inefficiency in the first cycle: these include excess cathode material loading, high concentration lithium salt, lithium sacrificial salts, and lithium-rich cathode and stabilized lithium metal powder and so on. Therefore, I think all of those approaches are potential application in sodium ion battery to offset inefficiency in the first cycle.

4. Conclusion In summary, the TiO2@RGO composite was prepared using one-pot hydrolysis method. As-prepared TiO2@RGO composite disclosed ultra-small well dispersed TiO2 (10 nm) nanoparticles on the RGO sheet. BET analysis indicated a specific surface area of 135.2 m2 g-1 with the size of the majority of the mesopores fell within a relatively narrow range of 15-25 nm. The discharge specific capacity in SIBs was 150.3 mAh g-1 and 127.7 mAh g-1 was remained after 1000 cycles, corresponding to a capacity retention ratio of 84.9%. Especially, the first cycle coulombic efficiency of composites was 60.7%, which is higher than other previous reports. Furthermore, TiO2@RGO composite coupled with NVP@C sodium-ion full cells has been investigated. The discharge specific capacity of 117.2 mAh g1

was achieved and 84.6 mAh g-1 was remained after 500 cycles, corresponding to the capacity

retention ration of 72.2% and the full cells exhibit the maximum energy densities of 167 Wh kg-1 and maximum powder densities of 1333 W kg-1. The results indicated that the strategy obviously enhanced 18


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the electrochemical performance of TiO2 as anode material in sodium-ion batteries.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. Compared to previous reports about TiO2 as anode for sodium-ion battery; SEM, TEM images of graphene oxide; SEM images of ultra-small TiO2 nanoparticles coated reduced graphene oxide composite; CV curves; Cycle vs. specific capacity at low current densityy; Characterizations and electrochemical performance of Na3V2(PO4)3 cathode in half cell; Charge/discharge curves for fabricating three-electrode in sodium-ion full cells.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]; [email protected]; Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the State Key Basic Research Program of China (2016YFA0203302) and the National Natural Science Foundation of China (21333002).

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262x199mm (150 x 150 DPI)


Ultra-small TiO2 Coated Reduced Graphene Oxide Composite as

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