Improved rate capability and cycling stability of novel terbium-doped lithium titanate for lithium-ion batteries

Electrochimica Acta 210 (2016) 935–941

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Improved rate capability and cycling stability of novel terbium-doped lithium titanate for lithium-ion batteries Ping Zhanga , Yudai Huanga,* , Wei Jiaa , Yanjun Caia , Xingchao Wanga , Yong Guoa , Dianzeng Jiaa,* , Zhipeng Suna , Zaiping Guob a Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, China b Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia

A R T I C L E I N F O

Article history: Received 11 February 2016 Received in revised form 1 June 2016 Accepted 3 June 2016 Available online 3 June 2016 Keywords: Lithium titanate Terbium-doping Rate capability Cycling stability Lithium ion battery

A B S T R A C T

Li4Ti4.94Tb0.06O12-d has been synthesized via a facile co-precipitation method. The structure and morphology of the as-prepared sample have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and particle size distribution analysis. The results show that Tb3+ is successfully doped into the Li4Ti5O12 structure and that the average particle size is about 111 nm. Li4Ti4.94Tb0.06O12-d exhibits a high initial discharge capacity of 166.5 mAh g
1 at 20 C, and the discharge capacity retention is nearly 93% after 500 cycles. In addition, the sample shows much improved rate capability at 20 C compared with pure Li4Ti5O12. Tb3+ doping not only enhances the electronic conductivity, but also improves the Li+ ion diffusivity in Li4Ti5O12, which indicates that Tb3+ doping is beneficial for promoting the electrochemical performance of Li4Ti5O12. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Spinel Li4Ti5O12 (LTO) is regarded as a promising anode material for high power lithium ion batteries (LIBs) because it has many virtues, such as a higher Li+-insertion platform voltage that can avoid reduction of electrolyte, which ensures reversibility after cycling and high thermodynamic stability [1]. There is almost no volume variation during the Li+ ion insertion-extraction process, which promises high cycling stability and long potential life [2]. Based on these merits, LTO can be considered as an appropriate candidate for high power LIBs. Nevertheless, its empty Ti 3d orbitals give LTO an insulating character, so the rate performance of LTO at high current density is limited by its low electronic conductivity and low Li+ ion diffusion coefficient [3]. It follows that the electrochemical properties of LTO cannot completely meet the requirements for practical application. To enhance the electrochemical performance of this material, several methods have been proposed, such as synthesis of nanoparticles. Nanoparticles of LTO can significantly shorten the pathways for electron conduction and the pathways for Li+ ion transportation within the particles, which can greatly improve the

* Corresponding author. Tel.: +86 991 8588209; fax: +86 991 8588209. E-mail addresses: [email protected] (Y. Huang), [email protected] (D. Jia). http://dx.doi.org/10.1016/j.electacta.2016.06.017 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

rate performance [4–6]. Coating carbon on LTO particle surfaces is another method to improve the surface electronic conductivity and the electrical contact with the electrolyte [7–9]. In spite of these efforts, the conductivity inside the LTO particle is still poor. Doping with metal ions, such as cations [10–13] or anions [14–16], to substitute onto Li or Ti sites can fundamentally solve this intrinsic defect. According to some recent investigations, rare earth doping into Li4Ti5O12 [17–19] could not only enhance the stability of this material, but also improve the cycling performance, reversible capacity, and high-rate performance. Ce-doped LTO@CeO2composite exhibits much improved rate capability and cycling stability. (*) The discharge capacity is 152 mAh g
1 at 10 C after 180 cycles [17]. Gadolinium doped and carbon-coated LTO nanosheet exhibits the initial discharge capacity of 213 mAh g
1, 182 mAh g
1, and 171 mAh g
1 at 1 C, 5 C, and 10 C, respectively [18]. Carbon-coated Li4Ti4.95Nd0.05O12 composite featured a first discharge capacity of 147 mAh g
1, 128 mAh g
1, and 110 mAh g
1, corresponding to the discharge rate of 2 C, 5 C, and 10 C, respectively [19]. As mentioned above, although highly crystalline rare earth doped LTO has been prepared, many researchers are still making unremitting efforts toward the goal of enhancing the electrochemical performance of LTO by rare earth doping. Doping with rare earth elements has a direct influence on the electronic conductivity of LTO, which is determined by the concentration of the electronic charge carriers. Moreover, doping

936

P. Zhang et al. / Electrochimica Acta 210 (2016) 935–941

at a low level would stabilize the structure, reduce polarization, and improve the electronic conductivity of LTO. To the best of our knowledge, full details of the effect of Tb3+ doping in spinel LTO have not been reported. Herein, we chose Tb3+ as a dopant ion, mainly due to the belief that Tb3+ with sufficient electrons in the 4f orbital could then transfer them into the empty 3d orbital of Ti [20–22]. The ability of LTO to conduct electrons will then be enhanced, which will lead to an obvious dramatic increase in the rate capability of LTO, as well as in its electronic conductivity. 2. Experimental 2.1. Synthesis Li4Ti4.94Tb0.06O12-d was synthesized by a co-precipitation procedure. Typically, 12 mL of tetrabutyl titanate was thoroughly mixed in ethanol to form a faint yellow homogeneous solution. 1.208 g of LiOHH2O and 0.1903 g of Tb(NO3)36H2O were fully dissolved in deionized water to obtain a solution, and the solution was slowly dropped into the previous yellow solution under vigorous stirring (mole ratio of Li:Ti:Tb = 4.03:4.94:0.06). The yellow transparent solution gradually turned into a white suspension. The suspension was stirred for 24 h, dried at 100  C to fully remove the solvent, and heated at 600  C for 5 h in air to obtain the final product. Excess Li was used to compensate for the volatilization of Li at high temperature. The product was denoted as LTO-t4. For comparison, pure Li4Ti5O12 material was prepared by the same procedure without the addition of Tb(NO3)36H2O, and this sample was denoted as LTO-t1. 2.2. Material characterization The crystal structures of both samples were examined by X-ray diffraction (XRD: D8 Advance equipped with Cu Ka radiation, Bruker, Germany). The surface chemical compositions of the powders were monitored by X-ray photoelectron spectroscopy (XPS: ESCALAB 250Xi, Thermo Fisher Scientific, American). The morphologies of the two samples were characterized by scanning electron microscopy (SEM: H-600, Hitachi, Japan). The particle size distributions of both samples were analyzed with Zetasizer Nano (ZN, Z90, America). 2.3. Electrochemical characterization Composite electrodes were constructed by mixing the active material, acetylene black, and poly(vinylidene fluoride) in a weight ratio of 80:10:10 in N-methyl pyrrolidinone solvent. The mixture was prepared and spread onto pure Cu foil using the doctor blade

technique, dried at 110  C in a vacuum oven for 12 h, and then cut into pellets 12 mm in diameter. Coin-type cells (2032) were assembled, using a solution of 1 M LiPF6 in a 1:1 (volume ratio) mixture of ethylene carbonate and dimethyl carbonate as the electrolyte, a Celgard 2300 microporous polypropylene/polyethylene/polypropylene membrane as the separator, and a lithium disk as the counter electrode. All the cells were assembled in an argonfilled glove box, where water and oxygen concentrations were kept at less than 1 ppm. Before electrochemical testing, the batteries were allowed to rest for 24 h to ensure sufficient soakage. Chargedischarge tests were carried out using a battery test system (CT2001A, Land, China) in the voltage range of 1.0-3.0 V (vs. Li+/Li). Cyclic voltammograms (CVs) were collected in the voltage range of 1.0-3.0 V using a CHI660D electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was conducted on a Zahner Elektrik electrochemical workstation in the frequency range of 10
1–105 Hz, with an applied DC potential that was equal to the open circuit voltage of the cell and an AC oscillation of 5 mV. All the electrochemical tests were conducted at room temperature. 3. Results and discussion 3.1. Powder characterization Fig. 1(a) shows the XRD patterns of the as-prepared LTO-t1 and LTO-t4. It can be seen that major diffraction peaks of LTO-t1 and LTO-t4 match very well with those of spinel LTO (JCPDS Card No.49-0207), and no new phase form is observed, which suggests that Tb3+ doping cannot obviously change the spinel structure of LTO [23]. For further observation, the (111) diffraction peaks were magnified for both samples and are shown in Fig. 1(b). It can be seen that the (111) diffraction peak of LTO-t4 is slightly shifted to a smaller angle after Tb3+ doping, which indicates that the lattice parameter increases after Tb3+ doping. The lattice parameter of LTO-t1 and LTO-t4 was calculated to be 8.362 Å and 8.367 Å, respectively. This may be attributed to the changed occupancy situation in the spinel structure of LTO, where some Ti4+ ions in 16d sites may be partially replaced by Tb3+, which can be confirmed by a structure model based on the neutron diffraction data [24]. Then, oxygen vacancies [25–27] and deformed polyhedra may also be present in the LTO-t4 structure, since the ionic size of Tb3+ (0.092 nm) is larger than that of Ti4+ (0.061 nm) [28]. It may be concluded that the dopant Tb3+ enters into the lattice structure of LTO-t4. As no Tb3+ located at the 8a sites, the intensity of the (220) peak in LTO-t4 is not strong [29]. Based on above analyses, the structure of Tb3+ doped LTO may be [Li3]8a[LiTi5-xTbx]16d[O12-d]32e. More details of the structure of Tb3+ doped LTO will need be further investigated.

Fig 1. XRD patterns of LTO-t1 and LTO-t4 (a), with an enlargement of the (111) diffraction peaks for both samples (b).

P. Zhang et al. / Electrochimica Acta 210 (2016) 935–941

937

Fig. 2. Ti 2p XPS spectra of LTO-t1 (a) and LTO-t4 (b).

In order to obtain more detailed information on the valence of elements, XPS characterizations were conducted on the corresponding Ti 2p spectra for both samples, as shown in Fig. 2(a) and (b). The two spectra show similar peak types. Two peaks at about 459.1 eV and 464.8 eV in Fig. 2(a) for LTO-t1 can be assigned to the Ti 2p3/2 and Ti 2p1/2 core level binding energies of Ti4+, respectively. The binding energies of Ti in LTO-t4 are 458.3 eV and 464.2 eV, however, which are approximately 0.8 eV and 0.6 eV lower than in LTO-t1. Additionally, the LTO-t4 spectrum can be fitted with small Ti3+ peaks at 456.3 eV and 460.8 eV, corresponding to Ti3+ 2p3/2 and Ti3+ 2p1/2. This implies that doping Tb3+ onto Ti4+ sites and oxygen vacancies will cause the transition of a small quantity of Ti ions from Ti4+ to Ti3+ to maintain charge compensation [30]. Therefore, the number of electrons is increased, and the electronic conductivity is also enhanced, which will improve the electrochemical performance of LTO-t4. Fig. 3 presents SEM images of LTO-t1 and LTO-t4, with the insets showing the corresponding particle size distributions of the two samples. The materials are well crystallized, and the average particle size of LTO-t1 and LTO-t4 is 156.7 nm and 111 nm, respectively. The particle size of LTO-t4 is smaller than that of LTO-t1, which could be ascribed to following reasons: (I) the dopant Tb3+ ions could enter into the lattice structure of the LTO, resulting in lattice distortion [31], which would hinder the particle growth during heat-treatment; (II) the addition of Tb3+ in the structure leads to a decrease in the particle size by reducing the formation of long-range order, which can improve the rate capability [32]. In addition, the smaller particle size is beneficial for increasing the contact area between the active materials and the electrolyte, hence making the Li+ ion insertion/extraction in the

LTO host structure more efficient, which will improve the electrochemical performance of LTO-t4 [33]. The electrochemical performances of LTO-t1 and LTO-t4 were evaluated within the potential window of 1.0-3.0 V. Fig. 4(a) and (b) show the rate performances of LTO-t1 and LTO-t4 at various current densities. Obviously, both samples show similar capacity decline. At the high current rates of 10 C and 20 C, LTO-t4 delivers an initial discharge capacity of 177.2 mAh g
1 and 166.2 mAh g
1, which are higher than for LTO-t1, which merely delivers 116.8 mAh g
1 and 104.2 mAh g
1, respectively. Compared with the reported results [28,33–37], which are shown in Table 1. LTO-t4 still shows better electrochemical performance. The improved rate capability of LTO-t4 can be ascribed to not only the electrons provided by Tb3+ ions, but also the appearance of Ti3+ after Tb3+ doping, which is consistent with the result of XPS (Fig. 2(b)). Fig. 4(c) and (d) present the discharge-charge curves of LTO-t1 and LTO-t4 at current rates from 0.5 C to 20 C. The discharge capacity gradually decreases with increasing discharge-charge rates. The results clearly show that the first discharge capacity of LTO-t4 is 204.2 mAh g
1, 199.7 mAh g
1, 188.9 mAh g
1, 177.2 mAh g
1, and 166.2 mAh g
1, corresponding to the discharge rate of 0.5 C, 1C, 5 C, 10 C, and 20 C, while the first discharge capacity of LTO-t1 is only 132 mAh g
1, 129.5 mAh g
1, 124 mAh g
1, 116.8 mAh g
1, and 104.2 mAh g
1 at the same current densities, respectively. The initial discharge capacity of the Tb3+ doped sample is 204.2 mAh g
1 at 0.5 C, which is higher than theoretical capacity of spinel Li4Ti5O12 (175 mAh g
1). The additional discharge capacity could be attributed to the increasing amount of Ti ions that are transformed from Ti4+ to Ti3+ by the doping with Tb3+ ions due to charge compensation [16,35]. It is well known that almost all the

Fig. 3. SEM images of LTO-t1 (a) and LTO-t4 (b), with the insets showing the corresponding particle size distributions of the samples.

938

P. Zhang et al. / Electrochimica Acta 210 (2016) 935–941

Fig. 4. Rate capability of LTO-t1 and LTO-t4 (a, b); the voltage plateaus at different rates (0.5 C, 1 C, 5 C, 10 C, and 20 C) of LTO-t1 (c) and LTO-t4 (d); comparison of the chargedischarge plateau potential difference for LTO-t1 and LTO-t4 (e); and the cycling performance of LTO-t1 and LTO-t4 at 10 C (f).

electrochemical energy comes from the reversible redox reaction between the trivalent titanium ion and the tetravalent titanium ion [38]. In addition, electrons in the 4f orbital of Tb3+ can transfer into the empty 3d orbital of Ti, which will lead to an obvious increase in electronic conductivity and a dramatic increase in the rate capability of Li4Ti5O12. Furthermore, Tb3+ ion doping into Li4Ti5O12 will lead to lattice distortion and introduce defects into Li4Ti5O12 [36]. The defects in Li4Ti5O12 could serve as Li+ storage sites for improved lithium capacity and rate performance [25–27]. In order to evaluate the polarization of the two samples, the potential differences between the discharge and charge plateaus at different rates were calculated. As shown in Fig. 4(e), for LTO-t4, the difference between the voltage plateaus at 0.5 C, 1 C, 5 C, 10 C, and 20 C is 82 mV, 146 mV, 274 mV, 375 mV, and 407 mV, while the potential difference in LTO-t1 is 94 mV, 157 mV, 344 mV, 435 mV, and 478 mV at the same current densities, respectively. It is obvious that the polarization of LTO-t4 is significantly lower than that of LTO-t1, which is attributed to the better reaction kinetics after Tb3+ doping. Fig. 4(f) shows the long-term cycling performances of LTO-t1 and LTO-t4 at 10 C. The initial discharge capacity of LTO-t1 is 140 mAh g
1, and it decreases to 123.6 mAh g
1 after 500 cycles with capacity retention of 88%. In the case of LTO-t4, the discharge capacity is 168.4 mAh g
1 in the first cycle and still remains at

150.5 mAh g
1 after 500 cycles, with capacity retention of 89%. The Li4-x/3Ti5-2x/3GdxO12 (x = 0.05) composite prepared by Zhang et al. [39] shows a discharge capacity of 110.8 mAh g
1 after 100 cycles at 10C. The Li4Ti5O11.7Br0.3 synthesized by Wang et al. [40] exhibits a reversible capacity of 138 mAh g
1 after 100 cycles at 10 C. LTO-t4 demonstrates remarkably improved cycling stability. Moreover, the coulombic efficiency of both samples is approximately 99% throughout the cycling, which means that Li+ ion insertionextraction are very reversible. Fig. 5 presents the discharge capacity vs. cycle number and the coulombic efficiency for LTO-t1 and LTO-t4 between 1.0 and 3.0 V at 20 C over 500 cycles. Due to the small volume change during the Li+ ion insertion-extraction process, both samples exhibit superior cycling properties. It is worth noting that the initial discharge capacity of LTO-t4 (166.5 mAh g
1) is much higher than that of LTO-t1 (121.8 mAh g
1), which may be attributed to the smaller particle size of LTO-t4. In addition, the coulombic efficiency of LTOt4 (99.75%) is higher than that of LTO-t1 (96.06%) at 20C after 500 cycles, indicating that Tb3+ doping could reduce the polarization of LTO-t4, which is consistent with the results from the chargedischarge profiles (Fig. 4e). The reversible discharge capacity of LTO-t1 decreases to 94.2 mAh g
1 after 500 cycles. The reversible discharge capacity is still 154.9 mAh g
1 for LTO-t4 after 500 cycles, however, which is better than that of LTO-t1. Here, the long-term

P. Zhang et al. / Electrochimica Acta 210 (2016) 935–941

939

Table 1 The rate performance of LTO-t4 compared with previous results.

Specific Current capacity -1 (mAh g ) Material

0.5 C

1C

5C

10 C

20 C

Potential Window

Ref

Sc3+ doped LTO

----

174

161

143

121

1-3 V

[28]

Li4Ti4.9W0.1O12

169.7

163

150

133

110.4

1-2.5 V

[33]

Li3.9Ca0.1Ti5O12

174.7

170

150

140

122.4

1-2.5 V

[34]

Li4Ti4.95Nb0.05O12

178.8

173

--

135

127

1-2.5 V

[35]

Y0.04LTO

---

165.8

161.3 146.8

115.5

1-2.5 V

[36]

Li3.94La0.06Ti5O12

---

153.4

146.9 142.2

133.2

1-2.5 V

[37]

Tb3+ doped LTO

204.2

199.7

188.9 177.2

166.2

1-3 V

This work

Fig. 5. Cycling performance and Coulombic efficiency of LTO-t1 (a) and LTO-t4 (b) between 1.0 V and 3.0 V at 20 C.

cycling performance of LTO-t4 is obviously enhanced by doping with Tb3+. To further investigate the electrochemical behavior of the two samples, cycle voltammograms (CVs) were collected within a potential range of 1.0-3.0 V (vs. Li+/Li) at a scan rate of 0.1 mV s
1 and are shown in Fig. 6(a). It can be seen clearly that the two curves each has a pair of redox peaks corresponding to insertion and extraction of Li+ ions. In addition, the potential difference between the anodic and cathodic peaks decreases from 200 mV for LTO-t1 to 180 mV for LTO-t4, as shown in Table 2. This decrease in potential difference can be ascribed to the change in the intrinsic properties of LTO after Tb3+ doping, which produces good electrical contact that facilitates charge transfer. For the purpose of comparing the reaction kinetics properties of both samples, the CVs of LTO-t1 and LTO-t4 at different scan rates were collected, and are displayed in Fig. 6(b) and (c). With the increase in scan rate, the potential difference becomes larger, indicating increased electrode polarization. Nevertheless, when the rate increases to 1.3 mV s
1, the redox peaks of LTO-t4 retain a more stable shape than that of LTO-t1, implying that LTO-t4 has weaker polarization. In addition, Fig. 6(d) displays the linear

relationship between the peak current density and the square root of the scan rate in the cathodic process for both LTO-t1 and LTO-t4, implying that the charge-discharge capability is determined by a diffusion controlled process [41]. The Li+ ion diffusion coefficient in the electrode can be calculated according to the slope of line by using the following Eq. (1) [42]: ip ¼ 2:75  105 n3=2 AD1=2 C Liþ n1=2

ð1Þ

where ip is the peak current value (A), n is the charge transfer number (for Li+, n = 1), A is the surface area of the electrode (1.13 cm2 in this case), D is the chemical diffusion coefficient (cm2 s
1), CLi+ is the concentration of Li+ ions (given as 4.37  10
3 mol cm
3 for LTO), and y is the scanning rate (V s
1). According to Eq. (1), the lithium diffusion coefficients of LTO-t1 and LTO-t4 are determined to be 7.95  10
10 cm2 s
1 and 7.86  10
9 cm2 s
1, respectively, as indicated in Fig. 6(d). A faster chemical diffusion coefficient is obtained in LTO-t4, implying great improvement of the diffusion dynamics. In order to investigate the effect of Tb3+ doping on the electronic conductivity, EIS spectra of the two samples were conducted, and the results are shown in Fig. 7. The EIS curves consist of two

940

P. Zhang et al. / Electrochimica Acta 210 (2016) 935–941

Fig. 6. CVs of LTO-t1 and LTO-t4 between 1.0 V and 3.0 V at a scan rate of 0.1 mV s
1 (a); CVs of LTO-t1 (b) and LTO-t4 (c) at various scan rates; Anodic peak current density plotted against the square root of the scan rate for LTO-t1 and LTO-t4 (d).

Table 2 Values of the anodic and cathodic peak potentials from CVs of LTO-t1 and LTO-t4 between 1.0 V and 3.0 V. Samples

’pa/V

’pc/V

4’p/mV

D/cm2 s
1

LTO-t1 LTO-t4

1.64 1.65

1.44 1.47

200 180

7.95  10
10 7.86  10
9

semicircles and one slope from high frequency to low frequency. The first semicircle represents the contact resistance, explained as the resistance of Li+ ions diffusing through the surface of the electrode [43,44]. The second semicircle accounts for the charge transfer resistance. The low frequency straight line indicates the Warburg impedance caused by Li+ ion diffusion within the

electrode material. As shown in Fig. 7, LTO-t4 shows lower charge-transfer resistance than LTO-t1. This result reveals that a suitable amount of Tb3+ doping can enhance the electronic conductivity of the material. Therefore, as we expected, it is much easier for charge transfer to take place at the electrode/electrolyte interface of the Tb3+ doped sample, and the internal resistance of LTO-t4 is reduced at the same time. According to above analyses, the excellent electrochemical performance of LTO-t4 may be due to the following reasons: Firstly, the special charge-transfer reaction caused by Tb3+ will enhance the rate capability of LTO. In addition, after Tb3+ doping, the appearance of Ti3+ and oxygen vacancies will sufficiently increase the electron concentration, and a larger proportion of Ti3+ ions can efficiently enhance bulk electronic conductivity. Secondly, the small particle size is beneficial for increasing the contact area between the active materials and the electrolyte, which will make the Li+ ion insertion-extraction in the LTO host structure more efficient and improve the electrochemical performance of LTO. Thirdly, the larger Li+ diffusion coefficient value of LTO-t4 demonstrates that the electrochemical reaction kinetics is improved after Tb3+ doping. 4. Conclusions

Fig. 7. EIS spectra of LTO-t1 and LTO-t4.

In this work, Li4Ti4.94Tb0.06O12-d has been synthesized via a simple co-precipitation method. Li4Ti4.94Tb0.06O12-d exhibits a remarkable initial discharge capacity of 166.2 mAh g
1 and a good discharge capacity of 154.9 mAh g
1 is still maintained after 500 cycles at 20 C. Furthermore, Li4Ti4.94Tb0.06O12-d has smaller charge transfer resistance and higher Li+ ion diffusivity than Li4Ti5O12. All the results demonstrate that Tb3+ doping is beneficial for the reversible intercalation and extraction of Li+ ions in LTO. Therefore,

P. Zhang et al. / Electrochimica Acta 210 (2016) 935–941

Tb3+ doped LTO could be used as an efficient anode material for high-rate and long-life large energy storage devices. Acknowledgments This work was supported by the Program for New Century Excellent Talents in University (NCET-12-1076), the National Natural Science Foundation of China (21466036), the High-Tech Project of Xinjiang Province (201515105), the Science and Technology Assistance Foundation of Xinjiang Province (201491128), and the Graduate Research Innovation Project of Xinjiang (XJGRI2015014 and XJGRI2015011). References [1] S.L. Chou, J.Z. Wang, H.K. Liu, S.X. Dou, J. Phys. Chem. C 115 (2011) 16220–16227. [2] R.J. Inada, K.T. Shibukawa, C.K. Masada, Y.T. Nakanishi, Y.J. Sakurai, J. Power Sources 253 (2014) 181–186. [3] H.S. Li, L.F. Shen, X.G. Zhang, P. Nie, L. Chen, K. Xu, J. Electrochem. Soc. 159 (2012) A426–A430. [4] K. Nakahara, R. Nakajima, T. Matsushima, H. Majima, J. Power Sources 117 (2003) 131–136. [5] Y.S. Lin, M.C. Tsai, J.G. Duh, J. Power Sources 214 (2012) 314–318. [6] E.N. Pohjalainen, T.N. Rauhala, M.K. Valkeapää, J.N. Kallioinen, T.J. Kallio, J. Phys. Chem. C 119 (2015) 2277–2283. [7] H.Y. Yu, X.F. Zhang, A.F. Jalbout, X.D. Yan, X.M. Pan, H.M. Xie, R.S. Wang, Electrochim. Acta 53 (2008) 4200–4204. [8] C.C. Yang, H.C. Hu, S.J. Lin, W.C. Chien, J. Power Sources 258 (2014) 424–433. [9] B. Zhang, Z.D. Huang, S.W. Oh, J.K. Kim, J. Power Sources 196 (2011) 10692–10697. [10] C.F. Lin, M.O. Lai, L. Lu, H.H. Zhou, J. Phys. Chem. C 118 (2014) 14246–14255. [11] C.F. Lin, M.O. Lai, L. Lu, H.H. Zhou, Y.L. Xin, J. Power Sources 244 (2013) 272–279. [12] J. Wolfenstine, J.L. Allen, J. Power Sources 180 (2008) 582–585. [13] G.B. Xu, L.W. Yang, X.L. Wei, J.W. Ding, J.X. Zhong, P.K. Chu, J. Power Sources 295 (2015) 305–313. [14] Y.D. Huang, Y.L. Qi, D.Z. Jia, X.C. Wang, Z.P. Guo, W.I. Cho, J. Solid State Electrochem. 16 (2012) 2011–2016. [15] Y.L. Qi, Y.D. Huang, D.Z. Jia, S.J. Bao, Z.P. Guo, Electrochim. Acta 54 (2009) 4772–4776. [16] M.D. Ji, Y.L. Xu, Z. Zhao, H. Zhang, D. Liu, C.J. Zhao, X.Z. Qian, C.H. Zhao, J. Power Sources 263 (2014) 296–303. [17] X.J. Yang, Y.D. Huang, X.C. Wang, D.Z. Jia, W.K. Pang, Z.P. Guo, X.C. Tang, J. Power Sources 257 (2014) 280–285.

941

[18] G.B. Xu, L.W. Yang, X.L. Wei, J.W. Ding, J.X. Zhong, P.K. Chu, J. Power Sources 295 (2015) 305–313. [19] C.Y. Xia, C.C. Nian, Z. Huang, Y. Lin, D. Wang, C.M. Zhang, J. Sol-Gel Sci. Technol. 75 (2015) 38–44. [20] Y.F. Pan, Y.Z. Zhan, Y. Wang, J. Alloys Compd. 637 (2015) 524–530. [21] Y.R. Jhan, J.G. Duh, Electrochim. Acta 63 (2012) 9–15. [22] E. Dy, R. Hui, J. Zhang, Z.S. Liu, Z. Shi, J. Phys. Chem. C 114 (2010) 13162–13167. [23] J. Mosa, J.F. Vélez, J.J. Reinosa, M. Aparicio, A. Yamaguchi, K. Tadanaga, M. Tatsumisago, J. Power Sources 244 (2013) 482–487. [24] L. Aldon, P. Kubiak, M. Womes, J.C. Jumas, J. Olivier-Fourcade, J.L. Tirado, J.I. Corredor, C.P. Vicente, Chem. Mater. 16 (2004) 5721–5725. [25] X. Chen, X. Guan, L. Li, G. Li, J. Power Sources 210 (2012) 297–302. [26] J.H. Ku, J.H. Ryu, S.H. Kim, O.H. Han, S.M. Oh, Adv. Funct. Mater. 22 (2012) 3658–3664. [27] K.E. Swider-Lyons, C.T. Love, D.R. Rolison, Solid State Ionics 152–153 (2002) 99–104. [28] Y.Y. Zhang, C.M. Zhang, Y. Lin, D.B. Xiong, D. Wang, X.Y. Wu, D.N. He, J. Power Sources 250 (2014) 50–57. [29] B. Zhang, H.D. Du, B.H. Li, F.Y. Kang, Electrochem. Solid-State Lett. 13 (2010) A36–A38. [30] C.C. Chen, Y.N. Huang, C.H. An, H. Zhang, Y.J. Wang, L.F. Jiao, H.T. Yuan, ChemSusChem 8 (2015) 114–122. [31] X. Li, S.H. Tang, M.Z. Qu, P.X. Huang, W. Li, Z.L. Yu, J. Alloys Compd. 588 (2014) 17–24. [32] H.N. Song, S.W. Yun, H.H. Chun, M.G. Kim, K.Y. Chung, H.S. Kim, B.W. Cho, Y.T. Kim, Energy Environ. Sci. 5 (2012) 9903–9913. [33] Q.Y. Zhang, C.L. Zhang, B. Li, D.D. Jiang, S.F. Kang, X. Li, Y.G. Wang, Electrochim. Acta 107 (2013) 139–146. [34] Q.Y. Zhang, C.L. Zhang, B. Li, S.F. Kang, X. Li, Y.G. Wang, Electrochim. Acta 98 (2013) 146–152. [35] B. Tian, H. Xiang, L. Zhang, Z. Li, H. Wang, Electrochim. Acta 55 (2010) 5453–5458. [36] Y.J. Bai, C. Gong, N. Lun, Y.X. Qi, J. Mater. Chem. A 1 (2013) 89–96. [37] Y.J. Bai, C. Gong, Y.X. Qi, N. Lun, J. Feng, J. Mater. Chem. 22 (2012) 19054–19060. [38] T.F. Yi, Y. Xie, J. Shu, Z. Wang, C.B. Yue, R.S. Zhu, H.B. Qiao, J. Electrochem. Soc. 158 (2011) A266–A274. [39] Q.Y. Zhang, M.G. Verde, J.K. Seo, X. Li, Y.S. Meng, J. Power Sources 280 (2015) 355–362. [40] J.Q. Wang, Z.Z. Yang, W.H. Li, X.W. Zhong, L. Gu, Y. Yu, J. Power Sources 266 (2014) 323–331. [41] C.C. Chen, Y.N. Huang, H. Zhang, X.F. Wang, G.Y. Li, Y.J. Wang, L.F. Jiao, H.T. Yuan, J. Power Sources 278 (2015) 693–702. [42] J.Y. Lin, C.C. Hsu, H.P. Ho, S.H. Wu, Electrochim. Acta 87 (2013) 126–132. [43] Y.Q. Ge, H. Jiang, K. Fu, C.H. Zhang, J.D. Zhu, C. Chen, Y. Lu, Y.P. Qiu, X.W. Zhang, J. Power Sources 272 (2014) 860–865. [44] C.F. Lin, X.Y. Fan, Y.L. Xin, F.Q. Cheng, M.O. Lai, H.H. Zhou, L. Lu, Nanoscale 6 (2014) 6651–6660.