Solid-state synthesis and electrochemical performance of Ce-doped Li4Ti5O12 anode materials for lithium-ion batteries

Solid-state synthesis and electrochemical performance of Ce-doped Li4Ti5O12 anode materials for lithium-ion batteries

Electrochimica Acta 174 (2015) 369–375 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

3MB Sizes 0 Downloads 77 Views

Electrochimica Acta 174 (2015) 369–375

Contents lists available at ScienceDirect

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

Solid-state synthesis and electrochemical performance of Ce-doped Li4Ti5O12 anode materials for lithium-ion batteries T.P. Zhoua , X.Y. Fenga,c,d , X. Guoa , W.W. Wua , S. Chengb , H.F. Xianga,* a

School of Materials Science and Engineering, Hefei University of Technology, Anhui Hefei 230009, PR China Instrumental Analysis Center, Hefei University of Technology, Hefei, Anhui 230009, PR China Institute of Urban Environment, Chinese Academy of Sciences, PR China d Ningbo Urban Environment Observation and Research Station-NUEORS, Chinese Academy of Sciences, PR China b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 January 2015 Received in revised form 14 May 2015 Accepted 5 June 2015 Available online 6 June 2015

Ce-doped Li4Ti5O12 anode materials are prepared via a solid state reaction. The Ce is designed to replace of Ti (Li4Ti5 xCexO12) or take the place of octahedral interstice (Li4Ti5CexO12). The structure and morphology of these materials are characterized and their electrochemical performance is investigated. The structure analysis results show that it is difficult for Ce to dope into the lattice of Li4Ti5O12, only few Ce can dope in the lattice of Li4Ti5O12 and CeO2 impurity is inevitable in Ce-doped Li4Ti5O12 materials. As we designed, Li4Ti5Ce0.1O12 with Ce interstitial doping has the bigger lattice volume than Li4Ti4.9Ce0.1O12 with Ce substitution doping and pristine Li4Ti5O12. As for the electrochemical performance, Li4Ti5Ce0.1O12 delivers the highest capacity at 0.2 C (over 170 mAh g 1) and best rate performance among the pristine and Ce-doped Li4Ti5O12 materials. Ac impedance results show that the impedance of Li4Ti5Ce0.1O12 is much less than the other samples, which is the main reason for the best electrochemical performance. ã2015 Published by Elsevier Ltd.

Keywords: Lithium titanate Ce-doping solid-state reaction lithium-ion batteries

1. Introduction Lithium-ion batteries (LIBs) have been regarded as promising power sources for electric vehicles (EVs) and energy storage systems [1–5], owing to their advantages on energy density, power density and lifetime. This kind of battery draws much attention in the past decades with increasing energy demand and environmental problems caused by fossil fuels consuming. The development of LIBs in the past is based on the development of electrode materials, such as spinel LiMn2O4 and their derivation materials [6–8], olivine phosphate [9–11], silicon based materials [12–14], Li4Ti5O12 [15–22], besides the commonly used LiCoO2 and graphite. Among these electrode materials, Li4Ti5O12 has been demonstrated as one of the most promising anode materials for LIBs since it exhibits ultra-long lifetime with zero structural change during the lithium insertion/extraction process and a relatively higher operating voltage (1.55 V vs. Li/Li+) to ensure better safety of LIBs by avoiding the formation of lithium dendrites than the graphite electrode. Despite many advantages, the rate capability of Li4Ti5O12 is relatively low due to the poor electronic conductivity (<10 13 S cm 1) [15] and lithium ion diffusion. To overcome the significant

* Corresponding author. Tel.: +86-551-62901457; fax: +86-551-62901362 E-mail address: [email protected] (H.F. Xiang). http://dx.doi.org/10.1016/j.electacta.2015.06.021 0013-4686/ ã 2015 Published by Elsevier Ltd.

drawbacks of Li4Ti5O12, several approaches have been developed to improve the conductivity or electron transfer of Li4Ti5O12. One route is to synthesis nanostructure Li4Ti5O12 materials including nanoparticles [16–18], nanorods [19], nanoplates [20] and so on. The nanostructures can reduce diffusion paths of both electron and lithium ions. Also, nanostructures can improve the intercalation kinetics by providing a larger electrode/electrolyte contact area. Another way to improve rate performance of Li4Ti5O12 materials is to coat conductive species including Ag [21], carbon [22–25], metal oxides [26,27] and fluorides [28] on the surface of Li4Ti5O12 particles. Those conductive materials can enhance the electron transfer on the surface and improve the rate performance of Li4Ti5O12. Besides the routes above, another common way to enhance conductivity and rate performance is ion doping [29–32]. The ion doping in the crystal lattice of the spinel can effectively increase the intrinsic electronic conductivity or lithium ion diffusion. For example, when Mg2+ with high valence is doped in Li4Ti5O12 to replace part of lithium ions, i.e. Li4 xMgxTi5O12, some of the Ti4+ ions transform to Ti3+ to maintain charge balance and the electronic conductivity increases to 10 2 S cm 2 when x increases to 1 (Li3MgTi5O12) [29]. When the doping ions has the same valence with replaced ions, i.e. Li[Li(1 x)/3CrxTi(5 2x)/3]O4, some of Cr3+ replace part of Li+ and Ti4+ at the same time to maintain charge balance, the change of lattice parameters makes the lithium ion diffusion coefficient increase from 10 10 cm2 s 1

370

T.P. Zhou et al. / Electrochimica Acta 174 (2015) 369–375

(x=0) to 10 9 cm2 s 1 (x=1) [30]. The increasing of electronic conductivity and lithium ion diffusion leads to better rate performance of Li4Ti5O12 materials. In previous literatures, the substitution doping in Li4Ti5O12 lattice has been widely reported, but to our best knowledge, effects of the interstitial doping in the Li4Ti5O12 lattice was seldom studied. In the present work, we design and synthesize Ce-doped Li4Ti5O12 materials using a facile solid-state reaction. Ce is not only doped in the site of Ti (Li4Ti5 xCexO12), but also in the O6 octahedral interstice (Li4Ti5CexO12). The Ce in different lattice sites leads to significant change of Li4Ti5O12 on the structure and electrochemical performance. While Ce is partially doped in the O6 octahedral interstice, the electrochemical performance of Li4Ti5O12 is improved, compared with the original Li4Ti5O12 and the Ti site doped Li4Ti5O12 (Li4Ti5 xCexO12).

(Li4CexTi5O12, x=0, 0.05, 0.1, 0.15, 0.2) with different Ce concentrations were marked with LTO (without doping), Ce0.05, Ce0.1 (same as Ce0.1Ti5.0), Ce0.15, Ce0.2, respectively. 2.2. Structure characterization The crystal structure of these Ce-doped Li4Ti5O12 samples was determined by X-ray diffraction (XRD) using a diffractometer (D/max 2500 V, Cu Ka radiation, l=0.15406 nm). The diffraction patterns were recorded at room temperature in the 2u range from 10 to 70 , and Rietveld refinement was carried out by using GSAS software. Morphology and particle sizes of these Li4Ti5O12 samples were characterized by scanning electron microscopy (JSM-6390LA, JEOL) and transmission electron microscopy (TEM, JEM-2100F). 2.3. Electrochemical measurement

2. Experimental 2.1. Synthesis of Ce-doped Li4Ti5O12 Ce-doped Li4Ti5O12 anode materials were prepared via a solidstate reaction. CeO2, TiO2 and Li2CO3 with different ratios were mixed together by ball milling using ethanol as dispersant agent for 5 h. After dried at 80 C, the mixture was calcined at 800 C for 12 h and then grinded into fine particles by hand. The different mole ratios of Ce and Ti were designed to prepare Li4Ti4.9Ce0.1O12 (denoted as Ce0.1Ti4.9) by Ce substitution doping on Ti sites and Li4Ti5Ce0.1O12 (denoted as Ce0.1Ti5.0) by Ce interstitial doping in O6 octahedral interstice. The interstitial doping products

The electrochemical properties of the Ce-doped Li4Ti5O12 samples were measured in the CR2032 coin-type cells. To make an electrode laminate, Li4Ti5O12 samples, acetylene black, binder (12 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2pyrrolidinone (NMP)) and proper amounts of solvent (NMP) were mixed together to form a uniform slurry. Then the slurry was casted on a copper foil (current collector). After being dried at 70 C, the laminate was punched into discs with diameter of 14 mm in order to fabricate the coin-type cells. The electrode consisted of 80 wt.% Li4Ti5O12 samples, 10 wt.% acetylene black and 10 wt.% PVDF binder, and the mass loading on each disc was about 5 mg cm 2. For comparison, CeO2 electrodes were prepared in a

Fig. 1. XRD pattern (a,b) of original and Ce-doped Li4Ti5O12 (Li4Ti4.9Ce0.1O12 and Li4Ti5Ce0.1O12), and Rietveld refinement patterns of Li4Ti4.9Ce0.1O12 (c) and Li4Ti5Ce0.1O12 (d).

T.P. Zhou et al. / Electrochimica Acta 174 (2015) 369–375

371

CHI 660D Electrochemical Workstation (Shanghai Chenhua Instruments Co., Ltd.), with the frequency range of 0.01 Hz to 105 Hz. 3. Results and discussion 3.1. Structure and morphology of Ce-doped Li4Ti5O12 samples

Fig. 2. XRD pattern of different Ce-doped Li4Ti5O12.

similar way. High-purity lithium metal was used as the counter and reference electrode. All the cells were assembled in an argonfilled dry-box (MBraun) with a polypropylene microporous membrane (Celgard 2400) as the separator and 1 M LiPF6/EC: DEC (1:1, w/w) as the electrolyte. They were cycled on a multichannel battery test system (NEWARE BTS-610) in the voltage range from 1.0 to 2.5 V with different current densities. Moreover, ac impedance of the cells was measured at room temperature on a

The X-ray diffraction patterns of original Li4Ti5O12 and Ce-doped samples (Li4Ti4.9Ce0.1O12 and Li4Ti5Ce0.1O12) are shown in Fig. 1. According to our design, Ce may take the place of Ti to form Li4Ti4.9Ce0.1O12 or interstitial site of O6 octahedral interstice to form Li4Ti5Ce0.1O12. In the XRD pattern (Fig. 1) we can see that all the peaks of the original Li4Ti5O12 can be attributed to a cubic spinel structure of Li4Ti5O12 (JCPDS card No.49-0207) with high crystallinity (sharp diffraction peaks). After Ce doping, some impurity of CeO2 comes out in both Li4Ti4.9Ce0.1O12 and Li4Ti5Ce0.1O12. Considering the amount of CeO2 we added is small, that means only few Ce or even no Ce is doped in the Li4Ti5O12 lattice. To confirm the effect of Ce doping, the strongest peak of Li4Ti5O12 at around 18 is picked out and zoomed in (Fig. 1b). It can be seen in Fig. 1b that the diffraction angle decreases after Ce doping  (18.38 of Li4Ti5O12, 18.36 of Li4Ti4.9Ce0.1O12 and 18.32 of Li4Ti5Ce0.1O12), which means that the lattice parameters and lattice volume increase. According to the pauling radius, Ce4+ (87.0 pm) is bigger than Ti4+ (60.5 pm), so the increase of lattice parameters can be attributed to Ce doping in Li4Ti5O12 lattice. To further identify the structures of Li4Ti4.9Ce0.1O12 and Li4Ti5Ce0.1O12, their Rietveld refinement patterns are shown in Fig. 1c and 1d. The lattice parameter and lattice volume of Li4Ti4.9Ce0.1O12 are 8.35433(7) Å and 583.09050(5) Å3, which are smaller than those of Li4Ti5Ce0.1O12 (a=8.35859(7) Å, V=583.98294(3) Å3). In Li4Ti4.9Ce0.1O12

Fig. 3. SEM image of original Li4Ti5O12 (a, c) and Ce-doped Li4Ti5Ce0.1O12 (b, d).

372

T.P. Zhou et al. / Electrochimica Acta 174 (2015) 369–375

(Fig. 1c), the most possible composition is Li4Ti4.9804Ce0.0196O12, which suggests that about 20% Ce ions are doped on the Ti sites in the lattice and 80% CeO2 is residue. But for Li4Ti5Ce0.1O12 (Fig. 1d), the best refinement result is obtained from the composition of Li4Ti5Ce0.0075O12. Since 0.1 mol CeO2 were added in 1 mol Li4Ti5O12 in the preparation procedure, 7.5% Ce ions take the place of O6 octahedral interstitial site of Li4Ti5O12 lattice and 92.5% CeO2 is residue. It is reasonable for less Ce doping in Li4Ti5Ce0.1O12 than in Li4Ti4.9Ce0.1O12 because of a bigger steric hindrance for Ce ions to enter the interstitial sites than to substitute Ti ions in 16d sites. Even though less Ce ions are introduced into the Li4Ti5O12 lattice for Li4Ti5Ce0.1O12, its lattice parameter and lattice volume are bigger than those of Li4Ti4.9Ce0.1O12. Since Ce can be doped into the O6 octahedral interstitial site of Li4Ti5O12 lattice, we further investigate the effect of Ce doping content. From Fig. 2, we can see that the more amount of CeO2 was detected in the final product when more CeO2 was introduced into the reactants. Only 5% CeO2 is still excessive for the interstitial doping in the Li4Ti5O12 lattice, indicating that only few Ce can dope in the lattice of Li4Ti5O12. Even though CeO2 impurity is seemingly inevitable in Ce-doped Li4Ti5O12 materials, the distinct shift of (111) peak in all Ce-containing products indicates that Ce can be introduced into the O6 octahedral interstitial site of the Li4Ti5O12 lattice to prepare the Ce-doped Li4Ti5O12 materials. The diffraction  angle of the (111) peak decreases after Ce doping (18.38 of  Li4Ti5O12, 18.36 of Li4Ti5Ce0.05O12 and 18.32 of Li4Ti5Ce0.1O12). The lattice parameter and the lattice volume of Li4Ti5Ce0.05O12 are 8.35457(6) Å and 583.13932(4) Å3, which are smaller than those of Li4Ti5Ce0.1O12. However, when the CeO2 amount is over 0.15, the  diffraction angle increases (18.41 of Li4Ti5Ce0.15O12 and 18.40 of Li4Ti5Ce0.2O12). It is a strange change but we repeated this

experiment and just got the same results. Also, this change of lattice is related to the electrochemical performance, which can be seen later. Fig. 3 shows the SEM images of various Li4Ti5O12 samples. The particle size of pristine Li4Ti5O12 is less than 1 mm and quite uniform. Most of pristine Li4Ti5O12 particles are quasi-circular and the rest has spindle shape, like two particles interconnecting together. After Ce doping, the particle size does not change much, but more spindle particles can be found. It seems that Li4Ti5Ce0.1O12 particles become easier to interconnect together. The possible reason is that the presence of some CeO2 could promote the formation of more spindle particles, and this spindle morphology could be advantageous over single quasi-circular shape for charge transfer. The TEM images and EDX images of Ce-doped Li4Ti5Ce0.1O12 are shown in Fig. 4. It is distinct that Ce-doped Li4Ti5Ce0.1O12 particles join together through sintering neck (red circle in Fig. 4a). The EDX results also support the Ce doping conclusion from the XRD results. Firstly, the Ti element and Ce element are distributed homogeneously in the product, proving that Ce has been doped in Li4Ti5O12, as we can see in Fig. 1b, peaks of L4Ti5O12 move to the low angle after Ce doping. Also, we can see Ce element enrichment in some small particles (seen in Fig. 4d), which should be CeO2 impurity. 3.2. Electrochemical performance of Li4Ti5O12 and Ce-doped samples. The initial charge-discharge curves of original Li4Ti5O12 and Cedoped Li4Ti5O12 samples are shown in Fig. 5a. Both the discharge capacities of pristine Li4Ti5O12 and Li4Ti4.9Ce0.1O12 at the first cycle are very close to 160 mAh g 1, which indicates that there is no

Fig. 4. TEM image (a) and EDX of Ce-doped Li4Ti5Ce0.1O12 samples (b, c, d).

T.P. Zhou et al. / Electrochimica Acta 174 (2015) 369–375

Fig. 5. Initial voltage profiles at 0.2 C (a, 1st cycle) of different Li4Ti5O12 samples and commercial CeO2 (b, 1st to 5th cycle).

Fig. 6. Rate performance of different Li4Ti5O12 samples (from 0.2C to 2C).

improvement after Ce doping in the Ti site. When Ce was doped in the interstitial site, the discharge capacity of Li4Ti5Ce0.1O12 increases to over 180 mAh g 1, 20 mAh g 1 higher than the other two samples and slightly higher than the theoretical capacity

373

Fig. 7. Initial voltage profiles (a) at 0.2 C and rate performance (b) from 0.2 C to 2 C of different Ce-doped Li4Ti5O12 samples.

(175 mAh g 1) of Li4Ti5O12. The extra capacity could come from the side reactions between 1.0 V to 1.5 V, which can be seen in many other Li4Ti5O12 samples [20], such as lithium insertion into CeO2. Since there is some CeO2 impurity in the Li4Ti5Ce0.1O12 and Li4Ti4.9Ce0.1O12 samples, the side reaction for lithium insertion into CeO2 leads to higher irreversible capacity and lower Coulombic efficiency (90%, charge capacity divided by discharge capacity) of the Ce-doped products than that of pristine Li4Ti5O12. To prove this speculation, commercial CeO2 was used as an anode material and its electrochemical performance in the CeO2/Li cells was investigated (Fig. 5b). The discharge capacity of CeO2 is about 15 mAh g 1 and most of this capacity is irreversible. The discharge plateau at 1.47 V is possibly corresponding to irreversible lithium insertion reaction into CeO2. The difference of specific capacity between Li4Ti4.9Ce0.1O12 and Li4Ti5Ce0.1O12 should be related to the difference on lithium ion diffusion or conductivity of these two samples. Since the lattice parameter increased, the lithium ions may transfer faster and faster. Also, according to our previous analysis, the Ce atom may be doped in the Ti site of Li4Ti4.9Ce0.1O12 and in the octahedral interstice of Li4Ti5Ce0.1O12. When in the octahedral interstice, redundant Ce should lead to charge imbalance and part of Ti may transform from Ti4+ into Ti3+, the existing of Ti3+ would improve the electronic conductivity of Li4Ti5O12. Since the amount of Ce doping in Li4Ti5O12 is small, it is difficult to detect the trace amount of Ti3+.

374

T.P. Zhou et al. / Electrochimica Acta 174 (2015) 369–375

Fig. 6 shows the rate capability of original Li4Ti5O12 and Cedoped Li4Ti5O12 samples. The reversible specific capacities of Li4Ti5Ce0.1O12 are 172, 165, 160 and 140 mAh g 1 at 0.2C, 0.5C, 1C and 2C, respectively. Herein, the capacity of 140 mAh g 1 at 2 C is quite good for the Li4Ti5O12 materials prepared via a solid-state reaction. Although the specific charge capacity of Li4Ti5Ce0.1O12 decreases with the current density increasing, it is still much higher than that of the other two samples at the same current density. The specific capacity and rate performance indicate that Ce doping in the O6 octahedral interstice is more helpful for lithium insertion/extraction kinetics in the Ce-doped Li4Ti5O12. On the other hand, the existence of CeO2 impurity leads to lower capacity at different currents, that is why the capacity of Li4Ti4.9Ce0.1O12 is a little lower than that of pristine Li4Ti5O12. According to the results above, we find that Li4Ti5Ce0.1O12 with Ce interstitial doping shows better electrochemical performance than the pristine and Ce substitution doping Li4Ti5O12. Then we optimize the doping amount of Ce in order to obtain better electrochemical performance in the Li4Ti5CexO12 (x=0, 0.05, 0.1, 0.15, 0.2). As shown in Fig. 7a, Li4Ti5Ce0.1O12 shows the highest capacity among these samples, and less or excess Ce doping leads to lower capacity than Li4Ti5Ce0.1O12. From the XRD results (Fig. 2), we can see that the lattice parameter of Li4Ti5Ce0.1O12 is the biggest, which may lead to the fastest lithium ion diffusion. Also, as we discussed before, the presence of more Ti3+ in Li4Ti5Ce0.1O12 would enhance electronic conductivity and thus improve the electrochemical performance. For Li4Ti5Ce0.15O12 and Li4Ti5Ce0.2O12, more inactive CeO2 also reduced the specific capacity of Ce-doped Li4Ti5O12. The rate performance of Ce-doped Li4Ti5CexO12 with different Ce contents is shown in Fig. 7b. Li4Ti5Ce0.1O12 shows the best rate performance among these Ce-doped samples. With less Ce doping, the rate capability of Li4Ti5Ce0.05O12 is close to that of Li4Ti5Ce0.1O12, both of which are higher than pristine Li4Ti5O12. The better electrochemical performance of Li4Ti5Ce0.05O12 proves the effectiveness of Ce-doping in the octahedral interstice further. With more Ce introduction, more CeO2 impurity in the final product (XRD pattern, Fig. 2) results in worse electrochemical performance. Although the specific capacity of Li4Ti5Ce0.15O12 at 0.2C is higher than that of pristine Li4Ti5O12, it becomes less than pristine Li4Ti5O12 at 2C. When the amount of Ce reaches 0.2 in Li4Ti5Ce0.2O12, its electrochemical performance gets even worse. The electrochemical performance is tightly related with impedance. Herein these Ce-doped Li4Ti5O12 samples were cycled

Fig. 9. Cycling performance of different Ce-doped Li4Ti5O12 samples at 2 C.

for 5 times and then discharged to 1.7 V for the ac impedance measurements. In Fig. 8, each impedance curve consists of a semicircle in high-to-medium frequency region and an inclined line in low frequency region. The semicircle is related to the charge transfer impedance and the inclined line is considered as Warburg impedance. Here it is clear that the improved electrochemical performance of some Ce-doped Li4Ti5O12 samples is attributed to the reduced charge transfer impedances, and especially Li4Ti5Ce0.1O12 with the best electrochemical performance has the lowest charge transfer impedance. Except for Li4Ti5Ce0.2O12 (excessive CeO2 impurity induces the higher impedance), Ce interstitial doping in a certain extent could reduce the charge transfer impedance in the following ways. Firstly, Ce doping results in the enlargement of the lattice volume of Li4Ti5O12, which can enhance the lithium insertion/extraction kinetics. Secondly, introduction of Ce4+ into the O6 octahedral interstitial site could significantly transfer Ti4+ into Ti3+, which can increase the electronic conductivity of the Li4Ti5O12 material. Last but not least, the presence of some CeO2 could promote the formation of more spindle shape particles, and this change on morphology could also be one of the reasons for the reduced charge transfer impedance, even though the exact mechanism is still pending at present. Additionally, Li4Ti5O12 is a zero-strain material, always with very long lifetime, even at high current density. When cycling at 2.0 C (Fig. 9), all these Li4Ti5O12 samples are stable with almost no capacity loss after 100 cycles. Therefore, the Ce interstitial doping doesn’t damage the structure stability of Li4Ti5O12 even for cycling at high current density. 4. Conclusions

Fig. 8. Ac impedance of different Ce-doped Li4Ti5O12 samples at 1.7 V (after 5 cycles).

In this work, Ce-doped Li4Ti5O12 samples in different sites and with different Ce contents were successfully synthesized by a facile solid-state reaction. The Ce is designed to replace of Ti (Li4Ti5 xCexO12) or take the place of octahedral interstice (Li4Ti5CexO12). When Ce was doped in the site of Ti to replace part of Ti, the electrochemical performance like capacity became lower, but when Ce was doped in the octahedral interstice, the specific capacity at different current densities from 0.2 C to 2 C became higher. Therefore, the doping site of Ce in the Li4Ti5O12 lattice results in different electrochemical performance. On the other hand, Ce content in the Li4Ti5O12 lattice affected the electrochemical performance of the products, x=0.1 in Li4Ti5Ce0.1O12 is the optimal content and introducing less or excessive Ce can not significantly improve or even damage the electrochemical

T.P. Zhou et al. / Electrochimica Acta 174 (2015) 369–375

performance of Li4Ti5O12. The main reason for the improved electrochemical performance of Li4Ti5Ce0.1O12 is its reduced electrochemical impedance. While Ce was doped in the octahedral interstice, part of Ti4+ could be turned into Ti3+ to reach charge balance, so the electronic conductivity of Li4Ti5O12 increased. But excessive Ce added to the material would introduce CeO2 impurity, which leads to high impedance because of its poor conductivity. Acknowledgements This study was supported by National Science Foundation of China (Grant Nos. 21006033, 51372060 and 5140021137), the Fundamental Research Funds for the Central Universities (2013HGCH0002) and National Undergraduate Innovation and Entrepreneurship Training Program (201310359014). X. Feng acknowledges the Ningbo Natural Science Foundation (grant No. 2013A610134). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

B.L. Ellis, K.T. Lee, L.F. Nazar, Chem. Mater. 22 (2010) 691–714. B. Dunn, H. Kamath, J.M. Tarascon, Science 334 (2011) 928–935. J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2010) 587–603. V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243–3262. X.H. Rui, N. Ding, J. Liu, C. Li, C.H. Chen, Electrochim. Acta 55 (2010) 2384–2390. H.W. Lee, P. Muralidharan, R. Ruffo, C.M. Mari, Y. Cui, D.K. Kim, Nano Lett. 10 (2010) 3852–3856. Q.T. Qu, L.J. Fu, X.Y. Zhan, D. Samuelis, J. Maier, L. Li, S. Tian, Z.H. Li, Y.P. Wu, Energy Environ. Sci. 4 (2011) 3985–3990. L. Zhou, D.Y. Zhao, X.W. Lou, Angew. Chem. Int. Ed. 124 (2012) 243–245. C.W. Sun, S. Rajasekhara, J.B. Goodenough, F. Zhou, J. Am. Chem. Soc. 133 (2011) 2132–2135. V.A. Alyoshin, E.A. Pleshakov, H. Ehrenberg, D. Mikhailova, J. Phys. Chem. C 118 (2014) 17426–17435.

375

[11] Q.D. Truong, M.K. Devaraju, Y. Ganbe, T. Tomai, I. Honma, Scientific Reports 4 (2014) 3975–3982. [12] W.P. Si, X.L. Sun, X.H. Liu, L.X. Xi, Y.D. Jia, C.L. Yan, O.G. Schmidt, J. Power Sources 267 (2014) 629–634. [13] D.B. Polat, O. Keles, K. Amine, J. Power Sources 270 (2014) 238–247. [14] M. Joe, Y.K. Han, K.R. Lee, H. Mizuseki, S. Kim, Carbon 77 (2014) 1140–1147. [15] J.J. Huang, Z.Y. Jiang, Electrochim. Acta 53 (2008) 7756–7759. [16] W.J. Liu, D. Sao, G.E. Luo, Q.Z. Gao, G.J. Yan, J.R. He, D.Y. Chen, X.Y. Yu, Y.P. Fang, Electrochim. Acta 133 (2014) 578–582. [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. [18] X.R. Li, H. Hu, S. Huang, G.G. Yu, L. Gao, H.W. Liu, Y. Yu, Electrochim. Acta 112 (2013) 356–363. [19] W. Wang, Y.Y. Guo, L.X. Liu, S.X. Wang, X.J. Yang, H. Guo, Journal of Power Sources 245 (2014) 624–629. [20] Y.J. Sha, B. Zhao, R. Ran, R. Cai, Z.P. Shao, J. Mater. Chem. A 1 (2013) 13233–13243. [21] S.H. Huang, Z.Y. Wen, J.C. Zhang, X.L. Yang, Electrochim. Acta 52 (2007) 3704–3708. [22] K. Gao, S.D. Li, J. Power Sources 270 (2014) 304–311. [23] D. Song, M.R. Jo, G.H. Lee, J. Song, N.S. Choi, Y.M. Kang, J. Alloy Compd. 615 (2014) 220–226. [24] X. Guo, H.F. Xiang, T.P. Zhou, W.H. Li, X.W. Wang, J.X. Zhou, Y. Yu, Electrochim. Acta 109 (2013) 33–38. [25] X. Guo, H.F. Xiang, T.P. Zhou, X.K. Ju, Y.C. Wu, Electrochim. Acta 130 (2014) 470–476. [26] P. Svens, R. Eriksson, J. Hansson, M. Behm, T. Gustafsson, G. Lindbergh, J. Power Sources 270 (2014) 131–141. [27] X.Y. Feng, N. Ding, Y.C. Dong, C.H. Chen, Z.L. Liu, J. Mater. Chem. A 1 (2013) 15310–15315. [28] W. Xu, X. Chen, W. Wang, D. Choi, F. Ding, J. Zheng, et al., J. Power Sources 236 (2013) 169–174. [29] C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, T. Goacher, M.M. Thackeray, J. Electrochem. Soc. 148 (2001) A102–A104. [30] P. Martin, L. Lopez, C. Pico, M.L. Veiga, Solid State Sci. 9 (2007) 521–526. [31] D. Wang, H.Y. Xu, M. Gu, C.H. Chen, Electrochem. Commun. 11 (2009) 50–53. [32] B. Tian, H. Xiang, L. Zhang, Z. Li, H. Wang, Electrochim. Acta 55 (2010) 5453–5458.