TiP2O7 and Expanded Graphite Nanocomposite as Anode Material for

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TiP2O7 and Expanded Graphite Nanocomposite as Anode Material for Aqueous Lithium-Ion Batteries Yunping Wen, Long Chen, Ying Pang, Zhaowei Guo, Duan Bin, Yong-Gang Wang, Congxiao Wang, and Yongyao Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14856 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 21, 2017

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ACS Applied Materials & Interfaces

TiP2O7 and Expanded Graphite Nanocomposite as Anode Material for Aqueous Lithium-Ion Batteries

Yunping Wen, Long Chen, Ying Pang, Zhaowei Guo, Duan Bin, Yong-gang Wang, Congxiao Wang, and Yongyao Xia ∗ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, iChEm (Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai 200433, People’s Republic of China.

KEYWORDS: Aqueous lithium-ion batteries; Anode materials; TiP2O7; Expanded graphite; H2 evolution reaction.

ACS Paragon Plus Environment


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ABSTRACT: This paper reports a facile sol-gel synthesis method to successfully prepare the TiP2O7/expanded graphite (EG) nanocomposite as an advanced anode material for aqueous lithium-ion batteries. The constructed TiP2O7 nanocomposite (50-100 nm) are in-situ encapsulated in the pore and layer structure of expanded graphite with good conductivity and high specific surface area. As a consequence, the resulting TiP2O7/EG electrode exhibits a reversible capacity of 66 mAh g-1 at 0.1 A g-1 with an appropriate potential of -0.6 V before hydrogen evolution in aqueous electrolytes, and also demonstrates greatly enhanced cycling stability with 75% capacity retention after 1000 cycles at the current density of 0.5 A g-1. A full cell consisting of TiP2O7/EG anode, LiMn2O4 cathode and 1 M Li2SO4 electrolyte delivers a specific energy of 60 Wh kg–1 calculated on the weight of both cathode and anode materials with an operational voltage of 1.4 V. It also exhibits superior rate capability and remarkable cycling performance with a capacity maintenance of 66 % over 500 cycles at 0.2 A g-1 and 61% at 1 A g-1 over 2000 cycles.


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1. INTRODUCTION Since its commercialization in 1990, rechargeable lithium-ion batteries (LIBs) with diversified advantages of high energy density and lighter weight, are widely developed for large-scale applications in electricity storage stations, electric vehicles (EV/HEV/PHEVs), and other portable electric devices.1-3 However, with the shared concerns for safety and high cost, the further promotion and utilization of lithium-ion batteries are restricted due to the employment of flammable and toxic organic electrolytes. Aqueous lithium-ion batteries, which employ aqueous electrolytes of inorganic salt, can fundamentally solve these problems and have attracted considerate attention with the distinguished advantages of low cost, intrinsic safety, environmental friendly, high ionic conductivity of electrolytes and high power density. It provides the most promising potential for the stationary power sources application in the smart-grid.4-7 Nevertheless, the chemical and electrochemical processes in aqueous electrolytes are more complex as compared with those in organic electrolytes because of many side reactions from the O2 and water involved, such as the reactions between electrode materials and water or O2, H2/O2 evolution reactions and proton co-intercalation parallel to lithium ions intercalation. Above all, the H2/O2 evolution reaction is the first to be affected factor that must be carefully taken into account in the selection of appropriate electrode materials for aqueous lithium-ion batteries. Reasons are as follows: firstly, due to the inherent operate potential of anode or cathode, the H2/O2 evolution



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reactions inevitably happen throughout the whole charge/discharge process, failing to make full use of the capacity of the electrode materials before the electrolyte decomposition. Secondly, the H2/O2 evolution reactions are often accompanied by the change of the pH value, which greatly impacts the stability of the electrode materials.8, 9 Currently, LiMn2O4 has been fully proved to be a prominent cathode material owing to its high voltage potential and excellent cycling stability.10-13 In contrast, the electrochemical performance of anode materials for aqueous lithium-ion batteries are limited. As the most common anode material for aqueous lithium-ion batteries, LiTi2(PO4)3 has been synthesized in diverse ways and intensively studied.7, 14-17 Unfortunately, owing to its relatively low operating potential, the H2 evolution reaction in aqueous electrolyte plays a dominant role in impacting its stability. Hence developing high-performance anode materials with appropriate intercalation potential becomes the main challenge of the development of aqueous lithium-ion batteries. As a polyanionic compound, TiP2O7 has a three-dimensional (3D) framework with TiO6 octahedra and P2O7 double-tetrahedra sharing corners, which is similar to the NaCl-type cubic structure.18, 19 Such an open polyanionic network enables rapid Li-ion mobility and prominent electrochemical/thermal stability.20-23 Combined with its redox potential (2.6 V vs Li/Li+, -0.6 V versus saturated calomel electrode (SCE)) in aqueous electrolytes and a flat voltage plateau, TiP2O7 has become a promising anode material for aqueous batteries


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and intrigues keen interests since firstly proposed in 2007.14 And more notably, the appropriate operating potential is beneficial to minimise the negative effects of the hydrogen evolution. However, TiP2O7 typically suffers from inherently low electronic conductivity, leading to its poor cycling stability and rate performance.14,

24-28

Then designing feasible strategies to improving the

electrochemical performance of TiP2O7 are at the top of the agenda, including carbon coating25-27, nanotechnology23, 28 and metal ion doping24. In the present work, we introduce expanded graphite and report a facile sol-gel synthesis method to synthesize TiP2O7/EG nanocomposite as anode for aqueous lithium-ion batteries at one step. The phase composition and morphology of TiP2O7/EG are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, the electrochemical performance of TiP2O7/EG is evaluated both in organic and aqueous electrolytes. An aqueous lithium-ion battery consisting of TiP2O7/EG anode, LiMn2O4 cathode and 1 M Li2SO4 electrolyte is also investigated, including its cycling stability and rate capability.

2. EXPERIMENTAL SECTION 2.1 Material preparation. Commercial expanded graphite (EG) was preheated at 1000 oC for 15 min for further expansion. Analytical grade chemicals, tetrabutyl titanate (TBT) and orthophosphoric acid (H3PO4, 85%) were purchased from commercial sources and used as received. TiP2O7/EG was prepared by a sol-gel



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method. A certain amount of EG was dispersed in 200 mL ethanol solution, TBT dissolved in ethanol was added to the above solution with magnetic stirring. Then stoichiometric amounts of H3PO4 dispersed in ethanol was added dropwise to the prepared solution. The mixture was transferred to the water bath at 80 oC to evaporate the solvent. The resulting black precursors was ground to a fine powder and annealed at 750 oC for 5 h under Ar atmosphere with heating rate of 5 oC min-1. The resulting TiP2O7/EG composites with different EG content of 15%, 30% and 40% (based on the theoretical yield of TiP2O7) were named as TiP2O7/EG-15, TiP2O7/EG-30 and TiP2O7/EG-40, respectively. For comparison, pristine TiP2O7 was synthesized under the similar procedure without the addition of EG and finally sintered at 750 oC for 5 h in air. 2.2 Materials characterization. The morphology and particle sizes were characterized using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Tecnai G2 F20 S-Twin). The carbon content of the sample was determined by a thermogravimetric analysis (TG, Perkin-Elmer TGA 7) from 30 to 900 oC under O2 atmosphere (60 mL min-1). The phase identification was analyzed by X-ray diffraction (XRD, Bruker D8 X-ray diffractometer) using Cu Ka radiation (γ = 1.5406 Å) , the scan ranged from 2θ = 10o to 80o with step size 0.02. 2.3 Electrochemical measurements. Galvanostatic charge/discharge tests in organic electrolyte were performed using CR2016-type coin cells. The working electrode was prepared by mixing 80 wt% TiP2O7 composite material, 10 wt%


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acetylene black (AB) as conducting agent and 10 wt% polytetrafluoroethylene (PTFE) as binder in isopropanol to form slurry thin sheet by roll-pressing. The electrode was punched in the form of disks typically with a diameter of 12 mm, and then pressed onto aluminum mesh at 10 MPa using a manual hydraulic press (Carver, Inc.). The typical mass loading of active material was 4-5 mg cm-2. The electrode was vacuum-dried at 80 oC for 12 h before assembled in an argon-filled glove box. The electrolyte solution was 1 M LiPF6/ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1 : 1 : 1 by volume). For the electrochemical measurements of TiP2O7 in aqueous electrolyte, the working electrodes, including the pristine TiP2O7, TiP2O7/EG composite and LiMn2O4 were prepared as the same as in organic electrolyte. The current collector used stainless steel grid in aqueous electrolyte. The electrochemical behaviors of the individual TiP2O7 composite electrode were characterized by galvanostatic charge/discharge measurements using a three-electrode cell under nitrogen atmosphere in which an activated carbon was used as counter electrode, a saturated calomel electrode (SCE, 0.242 V vs. NHE) as reference electrode and 1 M Li2SO4 solution as electrolyte. The charge/discharge tests were characterized on the HOKUTO DENKO Battery Charge/Discharge System HJ Series controlled by a computer. A full cell consisting of TiP2O7 anode, LiMn2O4 cathode and 1 M Li2SO4 electrolyte was assembled and characterized on LAND CT2001A Battery Cycler. The mass ratio of anode to cathode was 1.5:1.

3. RESULTS AND DISCUSSION



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Figure 1a shows the XRD patterns of TiP2O7, TiP2O7/EG-30 and expanded graphite (EG). The patterns of both TiP2O7/EG-30 and TiP2O7 are phase pure and can be indexed to the reference pattern (JCPDS NO. 38-1468) with a Pa3 space group and cell parameter a=23.6342 Å. These results are in good agreement with the previous reports on crystalline TiP2O7.19, 29-31 In addition, the TiP2O7/EG-30 composite also presents the additional diffraction peaks for expanded graphite at 26.5° and 54.6°. To further detect the mass ratio of carbon, the thermogravimetric (TG) curve was performed in oxygen for TiP2O7/EG-30. As shown in Figure 1b, there is only one significant weight loss on the curve, from which can calculate the content of carbon coated. The significant descent from 450 oC to 600 oC, whose mass loss is approximately 30 wt.%, could account for the combustion of the expanded graphite, which is basically consistent with the raw input ratio in preparation. The morphology of TiP2O7 samples and expanded graphite was characterized by SEM and shown in Figure 2. From the SEM picture of expanded graphite (Figure 2a-b), it displays the morphology of petal-liked flakes with the size of 20-30 µm. As seen, expanded graphite has a porous structure due to the exfoliation of graphite to ultrathin sheets after the thermal treatment. As shown in Figure 2c-d, there are only a small part of TiP2O7/EG-15 particles piling up on the surface of expanded graphite, most of them are highly aggregated and develop to serried micro-sized secondary particles. Also, the composite with much higher contents of expanded graphite, for example, TiP2O7/EG-30 and TiP2O7/EG-40 samples were synthesized. From


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Figure 2e-f, TiP2O7/EG-30 primary particles distribute evenly in the layer and pore structure of expanded graphite without tightly packed agglomeration or stacked layers. Identical with those reported in literature, the introduction of expanded graphite with a good conductive network is beneficial to suppress the aggregation and reduce the particle size of TiP2O7 to a certain extent. The nano-sized TiP2O7 particles implies the deep penetration of electrolyte, short diffusion paths for Li ions and a consequent improvement of the anode performance.32-34 Figure 2g-h shows SEM images of TiP2O7/EG-40. It can be seen that the superabundant and massive expanded graphite nanosheets stack together, and TiP2O7 particles scatter in the pores or on the surface of expanded graphite. Figure 3a displays the transmission electron microscopy (TEM) images of TiP2O7/EG-30. It can been seen that the nano-sized (50-100 nm) TiP2O7/EG-30 particles uniformly distribute in expanded graphite sheets. The shrunk particle size might result from the feasible sol-gel synthesize method and the introduction of expanded graphite which could effectively restrict the growth and the agglomeration effects of primary TiP2O7 particles so as to efficaciously confine their size at the nanoscale. The lattice fringe of TiP2O7 particles can be clearly seen in the HRTEM image (Figure 3b). The average d-spacing of 0.398 nm agrees well to the Miller indices (600) of TiP2O7. Combined with these characterization images, we could conclude that the TiP2O7/EG hybrid material with hierarchical structure has been successfully fabricated.



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The electrochemical performance of TiP2O7/EG-30 in organic electrolyte was evaluated by a coin-type half cell. Figure 4a shows the typical galvanostatic charge/discharge curve of TiP2O7/EG-30 composite at 0.1 A g-1 between 1.5 and 3.5 V. The electrode delivers a voltage plateau of 2.6 V and a discharge capacity of 75 mAh g-1 calculated on the weight of TiP2O7/EG-30 electrode, and 108 mAh g-1 based on the weight of TiP2O7, approximating the theoretical capacity of TiP2O7 (121 mAh g-1)14, 20, . Figure 4b depicts the cycling performance of TiP2O7/EG-30 at 0.1 A g-1. After 500

35

cycles, it demonstrates excellent cycling stability with capacity retention of 90%, which effectively proves that the present synthetic strategies make the TiP2O7/EG nanocomposite an eligible candidate of high performance lithium-ion batteries. It is particularly noted that the water decomposition has become the great challenge limiting the performance of aqueous lithium-ion batteries. Therefore, the key factor to construct aqueous batteries is to choose suitable anode materials with an appropriate operating potential before hydrogen evolution.8, 9 Figure 5 compares the linear scan voltammetry of TiP2O7, LiTi2(PO4)3 and activated carbon (AC) in 1 M Li2SO4 at scan rate 0.2 mV s-1 within the potential of 0 V and -1.2 V versus SCE. As seen, there is no peak observed on the curve of AC, which implies its inclined charge/discharge curve and the limited capacity. When it comes to LiTi2(PO4)3, the most commonly used anode material for aqueous LIBs, its Li-ion inserting potential is -0.78 V, much lower than the hydrogen evolution potential, resulting in the evolution of H2 and the decrease of cycling stability during the charge process of the full aqueous lithium-ion batteries. On the contrary, the peak potential of TiP2O7 is slightly


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higher than the hydrogen evolution potential, and 0.15 V higher than that of LiTi2(PO4)3. This promising intercalation potential could effectively mitigate the inevitable side reaction of H2 evolution. With this clarification, TiP2O7 may become a suitable candidate as the anode material for aqueous lithium-ion batteries on account of its comparably low working potential and enhanced cycling stability. It must be considered that in aqueous electrolytes, the hydrogen evolution may occur on the negative electrode after complete Li insertion. Therefore, the charge/discharge potential of the TiP2O7 anode in 1 M Li2SO4 electrolyte was controlled between 0 and -0.8 V (vs. SCE). Figure 6 presents the galvanostatic charge/discharge curves of TiP2O7, TiP2O7/EG-15, TiP2O7/EG-30 and TiP2O7/EG-40 in 1 M Li2SO4 under nitrogen atmosphere at the current of 0.2 A g-1. Apparently, pristine TiP2O7 (Figure 6a) delivers much lower discharge capacity of 6 mAh g-1 and a less flat curve from -0.7 V to -0.8 V, lower than the theoretical potential of TiP2O7 (about -0.6 V), which might be ascribe to the increase of polarization resulting from poor electronic conductivity as well as the aggregation of TiP2O7 primary particles.14, 25

Therefore, the expanded graphite is introduced to prepare TiP2O7/EG composite

nanomaterials. The charge/discharge curves of TiP2O7/EG-15 is presented in Figure 6b. Similar to TiP2O7, TiP2O7/EG-15 exhibits the unsatisfied capacity, sloped voltage curve and the obvious polarization. In association with the previous SEM data, we speculate that due to the insufficient expanded graphite, most TiP2O7 particles still tend to aggregate together and wrap around the expanded graphite sheets, obstructing the electron transfer capability. On this basis, we increased the content of expanded



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graphite and further synthesized TiP2O7/EG-30 samples (shown in Figure 6c). TiP2O7/EG-30

demonstrates

a

significant

enhancement

of

electrochemical

performance, exhibits a distinct and characteristic Li-ion insertion/extraction voltage plateaus of TiP2O7 at -0.6 V, and a significant higher discharge capacity of 65 mAh g-1 based on the total weight of composite material, and 93 mAh g-1 on the weight of pure TiP2O7. With the further addition of EG, TiP2O7/EG-40 (Figure. 6d) shows the similar charge/discharge plateau, while it delivers a reduced capacity of 55 mAh g-1 based on the weight of composite material (92 mAh g-1 based on pure TiP2O7). Considering both the capacity and electrode polarization, the optimal ratio of TiP2O7/EG was chosen at 30. Its electrochemical performance was systemically investigated in half and full cells, including cycling stability and rate capability. The rate capability of TiP2O7/EG-30 was investigated at various current densities from 0.1 A g-1 to 10 A g-1 between 0 and -0.8 V in 1 M Li2SO4 under nitrogen atmosphere. As shown in Figure 7a, TiP2O7/EG-30 presents superior rate performance in terms of high capacity retention and reversibility. It delivers a discharge capacity of 66 mAh g-1 at 0.1 A g-1 and 32 mAh g-1 at 10 A g-1. Notably, it takes only 36 seconds to complete one circle of charge and discharge at the current density of 10 A g-1. Few researches have conducted a charge/discharge test at such high rates. Such outstanding electrochemical performance could be elucidated as follows: the expanded graphite can not only furnish good electrical conductivity, but also prevent the tendency of self aggregation of TiP2O7 nanoparticles. Ulteriorly, it may work as buffer layers to


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accommodate the volume change of TiP2O7 nanoparticles during the Li-ion intercalation/deintercalation process.28,

36-38

The cycling performance of

TiP2O7/EG-30 was also evaluated at a current density of 0.5 A g-1 (see in Figure 7b). After 1000 cycles with no rest, it presents remarkable cycling stability with about 25% capacity loss, that is to say, an average capacity decay of 0.025 % per cycle. In addition, the coulumbic efficiency near 100% is because we take the measure of deoxygenation during the electrochemical tests to avoid the hydrogen evolution and the oxidization of discharged TiP2O7. The rate capability and cycling performance of this composite material is moderate compared with previous reports about TiP2O7 composite anode for non-aqueous lithium-ion batteries.20, 39 Cyclic voltammetry (CV) was employed to determine the lithium-ion apparent diffusion coefficient. Figure 8a shows the CV profiles of TiP2O7/EG-30 in 1 M Li2SO4 solution at various scan rates 0.2, 0.5, 1, 2, and 5 mV s-1 between -0.2 to -0.95 V (vs. SCE). A pair of redox peaks is clearly observed within each curve, indicating only one well-defined step in the electrochemical process of reduction-oxidation for TiP2O7.14, 25, 27

The potential difference (∆Ep) and peak current (ip) change systematically with

the increase of scan rates. From Figure 8a, details of potential difference (∆Ep) and peak current (ip) for the TiP2O7/EG-30 sample at different scan rates could be easily obtained. And |Ep-E0| could be correspondingly calculated as half the value of ∆Ep, where E0 is the formal potential. The plots of |Ep-E0| vs log v and ip vs v1/2 are illustrated in Figure 8b and 8c, respectively. From Figure 8b, the values of |Ep-E0| at



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testing scan rates are obviously larger than 28 mV and increase linearly as the scan rates increase. Meanwhile, it could be seen from Figure 8c that ip-v1/2 shows a good linear relationship. As we know, owing to solid-state reaction and solution resistance, the process for Li-ion insertion/extraction in solid-state materials is extremely complicated and rarely ideal. Therefore, the process of TiP2O7 + Li + e = LiTiP2O7 could be considered as an irreversible electrochemical system and analyzed with the following equations40: ip = 2.99 ×105(αn)1/2A∆CD1/2 Liv1/2

(25 ºC)

|Ep-E0| = (RT/αnF)[0.780 + ln(D1/2 Li/k0) + ln(αnαFv/RT)1/2]

[1] [2]

where ip is the peak current (A), α is the transfer coefficient, n is the number of electrons transferred per molecule during intercalation, A is the surface area of the electrode, and v is the scan rate (mV s-1). According to the plot of ip vs v1/2 (Eq. 1), the apparent diffusion coefficient D1/2 Li can be calculated from the slope value in case of known the values of α and the variation of Li-ion concentration (A∆C). Based on Eq. 2, the transfer coefficient (α) can be obtained from the slope of corresponding plot and the result was 0.53. The electrode area (A) was calculated as 0.261 cm2. Therefore, the value of A∆C was 3.286×10−3 mol cm-1. Consequently, the values of D1/2 Li for TiP2O7/EG-30 sample calculated using Eq. 1 and 2 was 8.46×10-13 mol cm-1, which agrees well with the literature value,27 but is large than 6.2×10-14 mol cm-1 for pure TiP2O7.39


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Considering the excellent electrochemical properties of TiP2O7/EG-30 as anode material in aqueous solution, we constructed a full cell composed of TiP2O7/EG-30 anode and LiMn2O4 cathode in 1 M Li2SO4 electrolyte. Figure 9a displays the charge/discharge curves of TiP2O7/EG-30, LiMn2O4 and the full cell. The mass-load ratio of anode to cathode is 1.5:1. As shown in Figure 9a, the cell exhibits a long voltage plateau around 1.4 V with remarkable reversibility, delivers a capacity of 42 mAh g–1 and a specific energy of 60 Wh kg–1 calculated on the weight of both cathode and anode materials, which is comparable to the current commercial aqueous lithium-ion batteries. Moreover, the flat voltage plateau and the consistency of charging and discharging voltage, are believe to greatly improve the energy-utilization efficiency. Figure

9b

shows

the

galvanostatic

charge/discharge

curves

of

TiP2O7/EG-30//LiMn2O4 cell between 0.5 V and 1.7 V at different rates from 0.1 to 10 A g-1. The current density is calculated based on the mass of TiP2O7/EG-30 electrode. As illustrated in Figure 9b, the cell delivers a discharge capacity of 42, 40, 38, 36, 33, 31, 28 and 24 mAh g-1 with capacity retention of 100, 95, 90, 86, 79, 74, 67 and 57 % as the cycling rate gradually increasing from 0.1, 0.2, 0.5, 1, 2, 5 to 10 A g-1. To the best of our knowledge, the poor cycling stability of aqueous Li-ion batteries at low current densities is still

a

critical

challenge.

The

long-term

cycling

performance

of

TiP2O7/EG-30//LiMn2O4 at various rates was extensively evaluated and shown in Figure 9(c−d). At a low rate of 0.2 A g-1, it delivers a discharge capacity of



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40 mAh g−1 and the capacity retention of 66 % after 500 cycles (Figure 9c). At 1.0 A g-1, a superior capacity retention of 61.0% is also obtained even after 2000 cycles (Figure 9d). This outstanding cycling stability is superior to that of aqueous lithium-ion batteries previously reported, 30% after 100 cycles at 2 C for carbon-coated TiP2O7//LiMn2O435, 85% after 10 cycles at C/10 rate for TiP2O7//LiMn2O4, 75% after 10 cycles for LiTi2(PO4)3/LiMn2O4,14 36 % after 100 cycles and 65% after 40 cycles at 1 C for LiV3O8//LiCoO2.41 Such excellent cycling stability and rate capability can be put down to the synergistic effects of nanocrystallization synthesis and high conductivity of expanded graphite.

4. CONCLUSIONS In this paper, we have introduced expanded graphite (EG) as support medium to reduce the particle size of TiP2O7 and improve its electronic conductivity by a facile sol-gel method. The TiP2O7/EG composite is evaluated as anode materials both in organic and aqueous electrolytes. Also the effect of the ratio of TiP2O7 to EG on its electrochemical performance was studied. Considering both the capacity and rate capability, the optimal ratio of TiP2O7 to EG was chosen at 30% w.t. of EG. This anode material exhibits an appropriate potential of -0.6 V (vs. SCE) before hydrogen evolution, an excellent rate performance in 1 M Li2SO4 solution by maintaining 50% of the capacity at a rate as high as 10 A g-1 (vs. 0.1 A g-1), as well as excellent cycling stability with capacity retention above 75% after 500 charge/discharge cycles at the


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current of 0.2 A g-1. Such good electrochemical performance of the nanocomposite could be explained as the cooperation of the size reducing of TiP2O7 particles in cooperation with the good electronic conductivity of expanded graphite. An aqueous lithium-ion battery consisting of a TiP2O7 anode, a LiMn2O4 cathode shows a voltage plateau around 1.4 V with a specific energy of 60 Wh kg–1 based on the total weight of active electrode materials. Moreover, it shows good rate performance with the capacity maintenance of 60% at 10 A g-1 (vs. 0.1 A g-1), as well as remarkable cycling stability by maintaining 66% of capacity after 500 cycles at 0.2 A g-1 and 61% after 2000 cycles at 1.0 A g-1. Although the energy density is not comparable enough to that of organic lithium battery, the aqueous lithium battery with versatile and tunable properties, such as high safety, environmentally friendly, low cost, high power and long cycling life would have a great potential for extensive applications as stationary power sources for smart-grid.

AUTHOR INFORMATION Corresponding Author Tel & Fax: 0086-21-51630318. E-mail address: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partially supported by the National Natural Science Foundation of China (No. 21333002), the National Key Research and Development Plan (2



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016YFB0901503), and Shanghai Science & Technology Committee (13JC14079 00).

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Figure 1. (a) X-ray diffraction (XRD) patterns of the pristine TiP2O7, TiP2O7/EG-30, and expanded graphite. (b) Thermal gravimetric (TG) curves of TiP2O7/EG-30 measured from 30 to 900 ºC under the O2 flow of 60 mL min-1 with a heating rate of 10 ºC min-1.


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Figure 2. SEM images of (a, b) expanded graphite, (c, d) TiP2O7/EG-15, (e, f) TiP2O7/EG-30 and (g, h) TiP2O7/EG-40.



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Figure 3. (a) TEM and (b) HRTEM images of TiP2O7/EG-30.


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Figure 4. (a) Typical charge/discharge curve of TiP2O7/EG-30 composite at 0.1 A g-1 between 1.5 and 3.5 V in organic electrolyte. (b) Cycling performance of TiP2O7/EG-30 electrode material at 0.1 A g-1 in an organic electrolyte.



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Figure 5. The linear scan voltammetry of TiP2O7, LiTi2(PO4)3, activated carbon (AC) in 1 M Li2SO4 between 0 V and -1.2 V at scan rate 0.2 mV s-1.


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Figure 6. The galvanostatic charge/discharge profiles of (a) pristine TiP2O7, (b) TiP2O7/EG-15, (c) TiP2O7/EG-30, (d) TiP2O7/EG-40 in 1 M Li2SO4 under nitrogen atmosphere within the voltage of -0.8 V and 0 V at the current of 0.2 A g-1.



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Figure 7. (a) Charge-discharge curves of of TiP2O7/EG-30 in 1 M Li2SO4 under nitrogen atmosphere within the voltage of -0.8 V and 0 V at various current rates. (b) Cycle performance of TiP2O7/EG-30 in 1 M Li2SO4 solution at the current rate of 0.5 A g-1 between -0.8 V and 0 V.


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Figure 8 (a) Cyclic voltammetry curves of TiP2O7/EG-30 composite in 1 M Li2SO4 at scan rates range from 0.2 mV s-1 to 5 mV s-1 in potential ranges of -0.2 V and -0.95 V versus SCE; Plots of (b) |Ep-E0| vs log v and (c) ip vs v1/2 for TiP2O7/EG-30 composite.



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Figure 9. (a) The typical charge/discharge curves of the individual electrode TiP2O7/EG-30 and LiMn2O4 (vs. SCE) along with the voltage profile of the aqueous lithium-ion battery in range of 0.5-1.7 V at a current rate of 0.2 A g-1; (b) Charge/discharge curves of TiP2O7/EG-30//LiMn2O4 cell at different rates; Cycling stability of of TiP2O7/EG-30//LiMn2O4 cell at (c) 0.2 A g-1 for 500 cycles and (d) 1 A g-1 for 2000 cycles.


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TiP2O7 and Expanded Graphite Nanocomposite as Anode Material for

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