Ag Composite as Anode Materials for Lithium Ion Batteries

Ag Composite as Anode Materials for Lithium Ion Batteries

Accepted Manuscript Title: Preparation and Electrochemical Performance of Ti2 Nb10 O29 /Ag Composite as Anode Materials for Lithium Ion Batteries Auth...

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Accepted Manuscript Title: Preparation and Electrochemical Performance of Ti2 Nb10 O29 /Ag Composite as Anode Materials for Lithium Ion Batteries Authors: Wutao Mao, Kecheng Liu, Ge Guo, Guangyin Liu, Keyan Bao, Jiali Guo, Min Hu, Weibo Wang, Beibei Li, Kailong Zhang, Yitai Qian PII: DOI: Reference:

S0013-4686(17)31943-6 http://dx.doi.org/10.1016/j.electacta.2017.09.072 EA 30271

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

4-6-2017 13-9-2017 13-9-2017

Please cite this article as: Wutao Mao, Kecheng Liu, Ge Guo, Guangyin Liu, Keyan Bao, Jiali Guo, Min Hu, Weibo Wang, Beibei Li, Kailong Zhang, Yitai Qian, Preparation and Electrochemical Performance of Ti2Nb10O29/Ag Composite as Anode Materials for Lithium Ion Batteries, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.09.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and Electrochemical Performance of Ti2Nb10O29/Ag Composite as Anode Materials for Lithium Ion Batteries Wutao Mao a,b, Kecheng Liu a, Ge Guo a, Guangyin Liu a*, Keyan Baoa,b*, Jiali Guo a, Min Hu a, Weibo Wang a, Beibei Li a, Kailong Zhangb, Yitai Qian b a

College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University,

Nanyang 473061, China b

School of Chemistry and Environment, Resource environment & Clean energy

laboratory, Jiangsu University of Technology, Changzhou 213001, China * Corresponding author. Tel.: +86 377 63513540 Graphical Abstract

E-mail address: [email protected] and [email protected] Abstract: In lithium ion batteries anode material is very crucial. Exploring the highperformance anode materials remains a great challenge. Here, Ti2Nb10O29/Ag composite was obtained. Electrochemical measurements demonstrate that Ti2Nb10O29/Ag composite shows good electrochemical performance than pure Ti2Nb10O29, mostly due to enhanced electronic conductivity after Ag-coating. The

specific capacities of Ti2Nb10O29/Ag composite can be as high as 253 and 173 mAh g1

at 1 and 10 C, respectively, and the specific capacity is 132 mAh g-1 at 20 C.

Moreover, the Ti2Nb10O29/Ag composite exhibited excellent cyclic stability, even after 500 cycles, its specific capacity at 10 C still stabilized at a large value of 142 mAh g-1, which was in sharp contrast to the corresponding values of pure Ti2Nb10O29 (68 mAh g-1). The high electrochemical performance demonstrated that Ti2Nb10O29/Ag composite is highly promising anode materials for Li-ion batteries. Keywords: Lithium ion battery; Ti2Nb10O29; Anode material; Electrochemical property.

1. Introduction In Li-ion batteries (LIBs), anode material is very crucial [1-3]. Graphite usually as the anode material of commercial LIBs because of its low redox potential, stable cycling performance and low-cost. However, graphitic materials display poor rate performance because of their low Li-ion diffusion coefficient. Moreover, the low Li insertion potential (~ 0.2 V vs. Li+/Li) of graphite may lead to the formation of lithium dendrites, which raises safety issues [4-6]. To solve this problem, many efforts have been devoted on new types of anode materials. During the past decade, titanium-based oxides (such as TiO2 and Li4Ti5O12) have draw widespread attention with even greater Li insertion/extraction potential (between one and two V vs. Li+/Li), which can efficiently suppress the formation of Li plating and guarantee the safety of the LIBs [5-8]. In particular, spinel-phase Li4Ti5O12 is

being considerably studied as a safe anode material for LIBs for its superior rate capability and long cycle life [5, 6]. However, small theoretical capacity (175 mAh g1

) is its drawbacks, influence the material's wide use of LIBs. Therefore, major point

is develop new type replacement materials with the same merits of Li4Ti5O12 but with high Li storage capacity. Recently, titanium-niobium oxides (such as Ti2Nb2O9 [9], TiNb2O7 [10-17], Ti2Nb10O29 [18-23], and TiNb6O17 [24]) has been also considered as a promising anode materials due to they are with high lithium storage potential (one~two V vs. Li+/Li). Moreover, these oxides can realize one or two electron transfer occur in Ti4+/Ti3+ or Nb5+/Nb3+ couple, respectively [12-15, 19-24]. Among these oxides, Ti2Nb10O29 is of very importance for its high theoretical capacities of 396 mAh g-1, two times higher than Li4Ti5O12 [19-24]. Despite its many advantages, electronic conductivity low is quite shortcomings of Ti2Nb10O29, which cause decrease in capacitance and rate capability [20, 21, 23]. Up to now, the effect of altering the intrinsic properties of Ti2Nb10O29 is reported. Takashima et al. through TiO2 and Nb2O5 annealed in vacuum atmosphere introduction of oxygen vacancy in Ti2Nb10O29, enhances the electronic conductivity of Ti2Nb10O29 to a certain extent [23]. Wang et al. synthesized Ti2Nb10O29/rGO composite, which exhibited a much better cycling performance and rate capacity than pure Ti2Nb10O29 [20]. Previous effort show that metal coating is a useful way to increase electrical conductivity of electrode and improved its rate capability and cycling stability, while not decrease the volumetric power density [25-27]. For example, Li4Ti5O12/Ag

composite is obtained by Liu shows a large rate capacity and exhibits excellent cycle stability [25]. Qian and co-workers fabricated micro-/nanoscale Cu/Li4Ti5O12 composite with high discharge capacity and exhibit excellent long-term cyclability at a high rate [26]. Up to now, there is no literature about the electrochemical characteristics of Ti2Nb10O29/Ag as anode material for LIBs. In this paper, Ti2Nb10O29/Ag composite was synthesized by two steps: first, Ti2Nb10O29 was prepared by a solid state reaction; second, Ti2Nb10O29/Ag composite was obtained via a simple solvothermal process. Ti2Nb10O29/Ag composite shows good electrochemical performance and low charge transfer resistance mostly due to enhanced electronic conductivity after Ag-coating. 2. Experimental Section 2.1. Synthesis of Ti2Nb10O29 material The Ti2Nb10O29 was prepared by a solid state reaction from TiO2 and Nb2O5, with molar ratio of Ti : Nb= 1: 5. TiO2 and Nb2O5 were mixed with a suitable amount of ethanol in a ball mill for 6 h, then the mixture was calcined at 1100 °C in a furnace under air for 24 h. 2.2. Preparation of Ti2Nb10O29/Ag composite Ti2Nb10O29 (0.5 g) and PVP (1 g) were dispersed in N, N-dimethylformamide (DMF, 60 mL), the solution introducing ultrasonic for 30 mins, flowed by addition of AgNO3 (0.025 g) and the solution continue under ultrasonic for 1 h. The final solution was

sealed in a 100 mL autoclave and and hold temperature at 160 °C for 1 h. The product was collected and conserved. 2.3 Characterization of Ti2Nb10O29/Ag and Ti2Nb10O29 The XRD measurements using a Bruker D8-a Advance X-ray powder diffractometer. FESEM images were taken on Hitachi SU8010 microscopy. TEM and elemental composition were taken on JEOL JEM-2100F microscopy. XPS measurements were performed with a Thermo Fisher ESCALAB 250Xi spectrometer equipped with a monochromatic Al X-ray source. 2.4 Electrochemical characterization The working electrode was prepared by mixing Ti2Nb10O29/Ag, conducting carbon black and a binder (PVDF) at a weight ratio of 7/2/1. The CR2025-type half-coin cells were assembled in an Ar-filled glove box with Celgard polypropylene as a separator and lithium foil as both the counter electrode and the reference electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (1:1:1 volume ratio). For comparison, a pure Ti2Nb10O29 was also prepared by the same process. The chargedischarge performance of the electrodes was measured by a LAND CT2001A battery tester under various charge/discharge rates in the voltage range of 1.0 to 2.5 V. Cyclic voltammetry measurements were performed using a CHI650D electrochemical workstation at a scanning rate of 0.1 mV s-1. 3. Results and discussion

The XRD patterns of Ti2Nb10O29 and Ti2Nb10O29/Ag indicated in Fig.1. The high intensities of the diffraction peaks reveal the good crystallinity of the samples. It shows that the typical peaks of Ti2Nb10O29 (Fig.1a), corresponding to the (200), (10-4), (300), (011), (400), (21-1), (11-3), (21-5), (41-1), (206), (51-5), (30-10), (700), (020) and (902) faces of the monoclinic structure (JCPDS No. 40-0039), appear in both patterns at 12.4o, 17.3o, 18.6 o, 23.8o, 24.9o, 26.0o, 26.7o, 32.2o, 33.3o, 35.3o, 38.9o, 44.1o, 44.4o, 47.6o and 54.7o, respectively. Fig. 1b is local view of Fig.1a in the 2 theta range of 3646o. In comparison, the diffraction peak of Ag appear at 38.2o in the XRD pattern of Ti2Nb10O29/Ag spectrum, corresponding to the (111) face of cubic Ag. The second peak of silver, (200) plane at 44.1o, overlap with the (020) plane of Ti2Nb10O29. Those results indicate that Ag is successfully introduced in the composite. Peaks are shifted to higher degree in composite sample, which due to the reduce of particle size of the composite. From the XRD pattern ( Fig. 1b ) also can seen that the diffraction peaks of Ti2Nb10O29/Ag composite is wider than that of pure Ti2Nb10O29. FESEM images of Ti2Nb10O29 and Ti2Nb10O29/Ag composite are show in Fig. 2. The panoramic FESEM image of Ti2Nb10O29 indicating that the product consists of a uniform columnar structure with high quantity and their surfaces are glossy and smooth (Fig.2a). The average sizes of these columnar are approximately 1.0 µm. Fig. 2b is a panoramic FESEM image of the as-prepared Ti2Nb10O29/Ag composite. From Fig.2b can be seen that the morphology and size of Ti2Nb10O29/Ag composite is similar with pure Ti2Nb10O29. It is worth noting that some Ag nanoparticles are evenly distributed on the surface of Ti2Nb10O29. ICP-MS analysis shows that the amount of Ag in the final

composite is estimated to be 2.35%, which is close to the initial Ag content in the precursor (3.1%). The structures of Ti2Nb10O29 and Ti2Nb10O29/Ag composite were also investigated by TEM. Fig. 3a and 3b are TEM images of pure Ti2Nb10O29 and Ti2Nb10O29/Ag composite, respectively. By comparison, can be seen that the TEM image of the Ti2Nb10O29/Ag composite shows that some Ag nanoparticles are evenly distributed on the surface of Ti2Nb10O29, which is in keeping with FESEM result (Fig. 2b). The high-resolution TEM (HRTEM) images of Ti2Nb10O29/Ag composite are show in Fig. 3c and 3d. The calculated d-spacing from the HRTEM image of Fig. 3c is 0.374 nm, corresponding to the crystal spacing with (011) plane of Ti2Nb10O29. The spacing between contiguous planes in Fig. 3d is measured to be 0.205 nm, which matches well with the (200) plane of Ag. The corresponding EDX maps are provided in Figs. 2g-i, dense elemental distribution of Ti and Nb confirm the presence of the underlying Ti2Nb10O29 substrate. In addition, uniform Ag loading can be seen on the whole Ti2Nb10O29 substrate. The results of XRD analysis and EDX elemental mappings reveal that Ag nanoparticles has really loaded on the surface of Ti2Nb10O29. In order to analyze the surface chemical states and to confirm their corresponding valence state, the XPS spectra of Ti2Nb10O29/Ag composite are shown in Fig. 4. Fig. 4a, the peak at 458.5 eV, named Ti 2p3/2, corresponds to Ti4+ oxidation state [28]. In Fig. 4b, the peak at 207.05 eV is correspond to the 3d5/2 and the peak at 209.80 eV is attributed to the 3d3/2 of Nb 3d. The Nb 3d3/2 peak reflects the existence of Nb5+ oxidation state [28, 29]. The values of 530.1eV (O1s) in Fig. 4c is reported for

metallic oxides, which is in agreement with O1s electron binding energy for Ti2Nb10O29 [30, 31]. C 1s spectrum has been updated to confirm the precise peak position of every element. The high-resolution C1s spectrum shows the peak centered at 284.8 eV (Fig. S1), which agrees with the reported C-C bonding [32]. Thus, the binding energy value of Ag3d might not be calibrated using the C1s peak. Two obvious peaks located at 367.6 eV and 373.6 eV are found, which are associated with the Ag 3d5/2 and Ag 3d3/2 of metallic Ag, respectively ( Fig. 4d ) [33, 34]. We also provided wide-scan and narrow-scan data for Ti2Nb10O29 powder for comparison with Ti2Nb10O29/Ag composite, the peak positions have a little change (Fig. S2). To investigate the redox kinetic properties of Ti2Nb10O29 and Ti2Nb10O29/Ag composite, cyclic voltammetry (CV) was tested in the cut-off range of 2.5-1.0 V at a scan rate of 0.1 mV s-1. As shown in Fig.5a, a main oxidation peak at 1.72 V, with two shoulder oxidation peaks at 1.19 V and 1.92 V corresponding to the Nb5+/Nb4+, Nb4+/Nb3+ and Ti4+/Ti3+ redox couples, respectively [18, 20-22]. Fig.5b demonstrates the CV curve of the Ti2Nb10O29/Ag electrode, the result is similar to the Ti2Nb10O29 electrode, however the redox peak currents increase obviously. Importantly, the Ti2Nb10O29/Ag electrode exhibits much higher redox kinetic property than that of the pure Ti2Nb10O29 electrode, confirming that the Ag-loading can improve the electronic conduction of the Ti2Nb10O29 electrode. To determine the electrochemical properties of Ti2Nb10O29 and Ti2Nb10O29/Ag as anode for rechargeable LIBs, a series of electrochemical tests were performed. The Li-insertion properties of Ti2Nb10O29 and Ti2Nb10O29/Ag composite electrodes were

investigated. Fig. 6a shows a typical voltage-capacity profile for the obtained Ti2Nb10O29/Ag composite after 1, 2 and 3 cycles at 0.2 C. It can be seen that the Ti2Nb10O29/Ag electrode shows narrower voltage plateau around 1.9 V and wider plateau around 1.65 V, which is consistent with the results as previously reported [19, 22]. The initial discharge capacity is 297 mAh g-1 and the corresponding charge capacity values is 282 mAh g-1. The initial coulombic efficiency of the Ti2Nb10O29/Ag electrode is as high as 94.9% which is higher than that of commercial graphite anode (around 90%). Subsequently, the coulombic efficiency of second and third cycle maintain high values are 97.9% and 99.6%, implying highly reversible property of Ti2Nb10O29/Ag electrode in charge/discharge process. Fig. 6b and Fig. 6c show the initial charge-discharge curves of the Ti2Nb10O29/Ag composite and Ti2Nb10O29 at various current rates, respectively. When tested at 1, 5, 10 and 20 C, the Ti2Nb10O29/Ag composite sample respectively delivers initial charge capacities of 253, 200, 169 and 130 mAh g-1, respectively, which are significantly larger than those of the Ti2Nb10O29 sample (234, 164, 133 and 88 mAh g-1, respectively). The corresponding discharge capacities values of the Ti2Nb10O29/Ag composite are 253, 209, 173 and 132 mAh g-1 at 1, 5, 10 and 20 C, respectively, while those values are 235, 173, 135 and 88 mAh g-1 for Ti2Nb10O29, respectively. In comparison, the specific capacity of Ti2Nb10O29/Ag composite can be as high as 253 mAh g-1 at 1 C, and the specific capacity is 132 mAh g-1 at 20 C. With increasing current rate, the specific capacity of the Ti2Nb10O29 electrode rapidly fades with values of 88 mAh g-1 at 20 C. With increasing the C-rate, all the charge curves rise,

while all the discharge curves drops, indicating the polarization in both the samples increases and thus resulting in their decreased capacities. However, at all the C-rates, the Ti2Nb10O29/Ag sample always exhibits smaller polarization and larger capacities than the Ti2Nb10O29 sample. The rate capabilities of Ti2Nb10O29/Ag and Ti2Nb10O29 from 1 C to 30 C were tested. Each rate stage was cycled 10 times. As shown in Fig. 6d, the capacities of the Ti2Nb10O29/Ag overtop that of Ti2Nb10O29 at all charge-discharge rates. For example, the specific capacities of Ti2Nb10O29/Ag are as high as 253 mAh g-1 at 1 C, 201 mAh g-1 at 5 C, 172 mAh g-1 at 10 C and 130 mAh g-1 at 20 C. Even at a high current rate of 30 C, Ti2Nb10O29/Ag can still exhibit a specific capacity of 93 mAh g-1. It also can be seen that there is more capacity loss of Ti2Nb10O29, which exhibits about 28 mAh g-1 at 30 C. Moreover, after cycling at 10 C, the specific capacity of Ti2Nb10O29/Ag is increased to 247 mAh g-1 as the rate is returned back to 1 C. Thus, Ti2Nb10O29/Ag composite exhibit good rate capacity performance. Fig. 7a shows the cycling performance of Ti2Nb10O29 at a current rate of 10 C. The initial discharge capacity is 147 mAh g-1, and the reversible discharge capacity stabilizes at 68 mAh g-1 after 500 cycles. Fortunately, the Ti2Nb10O29/Ag sample exhibits excellent cyclic stability. As shown in Figs. 7b and 7c, even after 500 cycles, its capacity at 10 C still stabilizes at a large value of 142 mAh g-1, which are in sharp contrast to the corresponding values of the pure Ti2Nb10O29. The initial discharge and charge capacities of Ti2Nb10O29/Ag are 175 and 172 mAh g-1, respectively, and the Coulombic efficiency is 98%; after 500 cycles, the discharge and charge capacities

are 142 and 142 mAh g-1, respectively, and the Coulombic efficiency is almost 100%. The electrochemical testing results demonstrated that Ti2Nb10O29/Ag composite is a promising alternative anode material for Li-ion batteries. To better understanding the electrode kinetics, the electrochemical impedance spectroscopic (EIS) test for the Ti2Nb10O29 and Ti2Nb10O29/Ag composite were carried out, the data is shown in Fig.8. Fig. 8a shows the Nyquist plots of the Ti2Nb10O29 and Ti2Nb10O29/Ag after 500 cycles. As can be seen, both the Nyquist plots are characteristic of one semicircle in the high frequency range, one semicircle in medium frequency range and a sloping straight line in the low-frequency range [13, 30, 35]. The semicircle may be consistent with the absence of a SEI layer on the electrode surface. The semicircle in medium frequency range represents an electrochemistry-controlled process, relating to the charge transfer through the electrode/electrolyte interface. The inclined straight line in low-frequency range represents a diffusion-controlled process, namely, reflects the solid-state diffusion of Li-ions in the electrode materials. The diffusion coefficient of lithium ion can be calculated from the plots in the low-frequency region according to the following equations: D = R2T2/2A2n4F4C2σ2 and Z' = Rct + R1 +σω-1/2 where R is the gas constant, T is the absolute temperature, n is the number of electron(s) per molecule oxidized, A is the surface area, F is Faraday’s constant, C is

the concentration, D is the diffusion coefficient, σis the Warburg factor, and ω is frequency. Based on the experimental results, the relationship between Z' and the reciprocal square root of ω in the low-frequency region can be obtained, as shown in Fig. 8b. Theσvalues of Ti2Nb10O29 electrode and Ti2Nb10O29/Ag electrode can further be drawn as 1.26 and 2.97. Therefore, the ratio of diffusion coefficient of lithium ion in Ti2Nb10O29 to Ti2Nb10O29/Ag is calculated to be around 0.42. It is clear that the diffusion coefficient of lithium ion is greatly increased in Ti2Nb10O29/Ag, suggesting that coating Ag contributes to the enhancement of conductivity, which will be favorable for its cycle ability.

4 Conclusion In summary, Ti2Nb10O29/Ag composite was synthesized by two steps: first, Ti2Nb10O29 was prepared by a solid state reaction; second, Ti2Nb10O29/Ag composite was obtained via a solvothermal process. The as-prepared Ti2Nb10O29/Ag composite showed remarkable electrochemical performance as anode material for LIBs. The specific capacities of the Ti2Nb10O29/Ag composite exceeded that of Ti2Nb10O29 at all charge-discharge rates from 1 to 30 C. Fortunately, the Ti2Nb10O29/Ag sample further exhibited excellent cyclic stability, even after 500 cycles, its capacity at 10 C still stabilized at a large value of 142 mAh g-1, which was in sharp contrast to the corresponding values of pure Ti2Nb10O29 (68 mAh g-1). Ti2Nb10O29/Ag composite showed good rate capability and favorable cycling stability mostly due to enhanced

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Figure Captions

Fig. 1. (a) XRD patterns of Ti2Nb10O29 (below) and Ti2Nb10O29/Ag composite (above); (b) local view of (a) in the 2 theta range of 36o-46o.

Fig. 2 FESEM images of (a) Ti2Nb10O29 and (b) Ti2Nb10O29/Ag composite.

Fig. 3 TEM images of (a) Ti2Nb10O29 and (b) Ti2Nb10O29/Ag composite, (c) and (d) HRTEM images of Ti2Nb10O29/Ag composite, (e) SEM image of Ti2Nb10O29/Ag composite, (f-h) EDX elemental mapping of Nb, Ti and Ag respectively.

Fig. 4 XPS spectra of Ti2Nb10O29/Ag composite (a) Ti 2p3/2 and Ti 2p1/2, (b) Nb2p5/2 and Nb2p3/2, (c) O1s, (d) Ag 3d5/2 and Ag 3d3/2.

Fig. 5. Cyclic voltammogram curves of (a) Ti2Nb10O29 and (b) Ti2Nb10O29/Ag composite in the voltage range of 2.5-1.0 V at a scan rate of 0.1 mV s-1.

Fig. 6 (a) Charge-discharge curves of Ti2Nb10O29/Ag composite at 0.2 C in the first three cycles. Initial charge-discharge curves of the electrodes at different rates: (b) Ti2Nb10O29/Ag composite and (c) Ti2Nb10O29. (d) Charge-discharge capacity versus cycle number plots of Ti2Nb10O29/Ag and Ti2Nb10O29 at different current rates.

Fig. 7 Cycling performance of (a) Ti2Nb10O29 and (b) Ti2Nb10O29/Ag composite at 10C. (c) Coulombic efficiency of Ti2Nb10O29/Ag composite.

Fig. 8 (a) Nyquist plots of the Ti2Nb10O29 and Ti2Nb10O29/Ag composite after 500 cycles; (b) Relationship between real impedance with low frequency of the Ti2Nb10O29 and Ti2Nb10O29/Ag composite.

Supplementary Information Preparation and Electrochemical Performance of Ti2Nb10O29/Ag Composite as Anode Materials for Lithium Ion Batteries Wutao Mao a,b, Kecheng Liu a, Ge Guo a, Guangyin Liu a*, Keyan Baoa,b*, Jiali Guo a, Min Hu a, Weibo Wang a, Beibei Li a, Kailong Zhangb, Yitai Qian b a

College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University,

Nanyang 473061, China b

School of Chemistry and Environment, Resource environment & Clean energy

laboratory, Jiangsu University of Technology, Changzhou 213001, China * Corresponding author. Tel.: +86 377 63513540 E-mail address: [email protected] and [email protected]

Fig. S1 XPS spectra of Ti2Nb10O29/Ag composite: (a) survey; (b) C 1s.

Fig. S2 XPS spectra of Ti2Nb10O29: (a) survey; (b) Ti 2p; (c) Nb 3d; (d) O 1s; (e) C 1s.