Spinel MnCo₂O₄ nanospheres as an effective cathode electrocatalyst for rechargeable lithium–oxygen batteries

Abstract

The air cathode is vital for lithium–oxygen batteries (LOBs) to achieve high performance, which will facilitate the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during the discharge and charge processes. In this paper, MnCo₂O₄ nanospheres prepared through the hydroxide co-precipitation method are investigated as an effective electrocatalyst for the ORR and OER in non-aqueous LOBs. The results show that a non-aqueous LOB with the electrocatalyst exhibits an excellent discharge capacity of 8518 mA h g_cathode⁻¹ with a narrow voltage gap of 0.85 V between the discharge and charge plateaus at a current density 100 mA g_cathode⁻¹, and can deliver cycling stability over 20 cycles. This study shows that MnCo₂O₄ nanospheres can be a promising cathode electrocatalyst in rechargeable lithium–oxygen batteries due to its enhanced catalytic activity towards the ORR and OER.

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1. Introduction

Rechargeable non-aqueous lithium–oxygen batteries (LOBs) have received worldwide attention since Abraham et al. presented the first true lithium–oxygen system with non-aqueous electrolyte in 1996.¹ However, there are still numerous challenges that need to be overcome before they can be used widely, such as lower practical energy density, larger polarization resistance, lower cycling rates, and the instability of the organic electrolyte.^2–8 Among the factors determining the performance of LOBs, the high catalytic activity air cathode is considered as a vital one for high performance LOBs.

Up to now, many researchers have explored various kinds of cathode catalysts to reduce the discharge–charge overpotentials, enhance the specific energy density and improve the cycle life, including noble metals and alloys as Pt, Au, Ag and Pd;^9–16 porous carbon materials as carbon black, nano-structured carbon, functionalized carbon and graphene;^17–21 transition metal oxides as manganese-based oxides, perovskite oxides and spinel oxides;^22–28 and non-oxide materials.^29–32 Among these cathode materials, the noble metals are the most effective electrocatalysts for their high conductivity, excellent ORR and OER activities. However, the use of noble metals will make the LOBs economically impractical. Recently, the transition metal oxides have attracted extensive attention due to their bi-functional activity and other attractive features as low price, relatively high electrical conductivity and stability.^33–35 Even more, the excellent catalytic activity of the transition metal oxides can lead to lower overpotentials and longer cycle lives of LOBs than noble metals. Among the transition metal oxides, spinel oxides are favored by many researchers due to their sufficient conductivity, electro-catalytic activity and easy preparation.^36–39

MnCo₂O₄ is a well-known cobalt–manganese spinel oxide, with Mn and Co occupying the tetrahedral and octahedral sites of the cubic lattice. Recently, nano-structured MnCo₂O₄ has gained much research interests owing to its electrochemical application in lithium ion batteries, fuel cells and metal air batteries.^39–42 In the field of LOBs using MnCo₂O₄ spinel oxide as catalyst, Wang et al.⁴⁰ prepared the cathode by direct nucleation and growth of MnCo₂O₄ nanoparticles on reduced graphene oxide. However, a relative lower discharge capacity of 3784 mA h g_cathode⁻¹ was obtained based on the total mass of the MnCo₂O₄/graphene hybrid. Ma et al.⁴¹ prepared multi-porous MnCo₂O₄ microspheres through the solvothermal method and achieved excellent charge capacity (∼4200 mA h g_cathode⁻¹). However, the charge voltage of LOB with MnCo₂O₄ catalyst was about 3.90 V, which is 0.94 V higher than the theoretical value.

In this paper, nano-sized MnCo₂O₄ spheres are prepared via co-precipitation method. The nano-spheric structure and sufficient electronic conductivity of MnCo₂O₄ can facilitate the diffusion of reactants and electrons, thus enhancing the ORR and OER during discharge and charge process of LOBs. The batteries display excellent capacity as high as 8518 mA h g_cathode⁻¹, and cycling performance over 20 cycles with much lower charge overpotential.

2. Experimental

2.1. Synthesis of MnCo₂O₄ nanospheres

0.01 mol of Mn(NO₃)₂ and 0.02 mol of Co(NO₃)₂·6H₂O were dissolved into 50 ml of deionized water. After magnetic stirred for 1 h, a pink solution was obtained. Then 40 ml of NaOH solution (1 mol L⁻¹) was added dropwise to the above solution under vigorously stirring and the pH value was controlled at ca. 11 to facilitate the complete precipitation of the precursor. The precipitate was then aged in the solution for 12 h at 70 °C under continuous stirring, followed by rinsing to remove Na⁺ and NO₃⁻ species with deionized water. Finally the precipitate was collected by centrifugation and then dried at 100 °C for 24 h. The final product MnCo₂O₄ powders can be obtained after calcining the precipitate at 700 °C for 3 h in air.

2.2. Assembly of lithium–oxygen battery

The lithium–oxygen batteries were assembled with Li metal disc (ϕ 15.6 mm × 0.4 mm) as anode, glass fiber (Whatman) as separator, 1 mol L⁻¹ LiTFSI (lithium bis(tri-fluoromethanesulfonyl)imide) in DMSO (dimethyl sulfoxide) as electrolyte, and as-synthesized air electrode as cathode. The used electrolyte was dried with activated 4 Å molecular sieves in advance, ensuring that water concentration was less than 10 ppm (measured by Karl Fischer titration). For the preparation of the air electrode, carbon paper (Toray carbon, 0.27 mm thickness) was first cut into circular discs with the diameter of 15.6 mm as current collector, and then the cathode slurry composing of 26 wt% MnCo₂O₄ catalyst, 62 wt% KB (Ketjen black EC-600JD, Fuhua Industry (Shanghai) Ltd.) and 12 wt% PVdF (polyvinylidene fluoride, Sigma Aldrich, MW 534 000, 99.9%) was coated onto the discs, followed by drying at 80 °C overnight in the vacuum oven. The average loading of the slurry is about 1 mg cm⁻². For comparison, cathode with 88 wt% KB and 12 wt% PVdF was also prepared. The batteries were assembled layer by layer in the argon-filled glovebox with both water and oxygen concentrations less than 0.1 ppm.

2.3. Material characterizations

The powder X-ray diffraction (XRD) patterns were recorded on X'Pert PRO X-ray Diffractometer with Cu-Kα radiation over 2θ range of 10–80°. The morphologies and composition were investigated using a field emission scanning electron microscope (FE-SEM, FEI, Sirion 200) and X-ray photoelectron spectroscopy (XPS, ESCA-LAB 250 photoelectron spectrometer). Nitrogen adsorption–desorption measurements were performed at 120 °C using a Micromeritics ASAP 2020 adsorption analyzer.

2.4. Electrochemical measurements

Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) data were obtained by rotating disk system on a Zahner Zennium IM6 station. Li metal discs were used as the reference electrode and the counter electrode, a glass carbon disc with the diameter of 3 mm was used as the working electrode, and 1 M LiTFSI in DMSO saturated with O₂ was used as electrolyte. For the preparation of the working electrode, 4 mg of KB or 1.18 mg of MnCo₂O₄ mixed with 2.82 mg of KB were dispersed in 10 μl of Nafion solution (5 wt%) that mixed with 0.6 ml DI water, 0.3 ml ethanol ahead, and then sonicated for 30 min to prepare a homogeneous ink. Then 10 μl inks were then added on the glass carbon and dried at 80 °C overnight before used. For the test of CV, the experiment was conducted at the sweep rate of 10 mV s⁻¹ between the voltage window of 2.0 V and 4.5 V at the rotating speed of 900 rpm. And for LSV, the voltage window was set between 2.8 V and 4.5 V with the sweep rate of 1 mV s⁻¹ at the rotating speed of 900 rpm.

The galvanostatic discharge–charge tests were conducted on a Hantest cycler (Wuhan Hantest Technology Co. Ltd.) with the voltage range of 2.2–4.5 V (vs. Li/Li⁺). The capacity was calculated based on the mass of the cathode for capacity comparison. The batteries were rested for 4 h to reach equilibrium of the oxygen concentrations of the electrolyte before test.

3. Results and discussion

XRD pattern of MnCo₂O₄ is demonstrated in Fig. 1. It can be seen that the sample shows sharp peaks at corresponding diffraction 2θ angles of 31°, 36°, 44°, 58° and 64°, which can be indexed to well-crystallized MnCo₂O₄ phase (ICDD: 23-1237) with no impurity phase.

Fig. 1 XRD patterns of the prepared MnCo₂O₄ sample calcined at 700 °C.

The morphology of the as-prepared MnCo₂O₄ sample is investigated by SEM, as shown in Fig. 2a. Obviously, those particles exhibit rather regular spheric structure. And the particles are well dispersive with an average diameter of 70 nm after calcined at 700 °C. The N₂ adsorption–desorption isotherms and the pore-size distribution of MnCo₂O₄ calcined at 700 °C are listed in Fig. 3. The isotherms of as-synthesized MnCo₂O₄ nanospheres exhibit the characteristics of type IV with H1 hysteresis loop appearing at high P/P₀ range of 0.8–1.0. From the inset figure of the pore size distribution, it is indicated that the average pore size of the MnCo₂O₄ sample is less than 50 nm, confirming that the mesoporous structure of MnCo₂O₄. For MnCo₂O₄, the BET specific surface area is 14.43 m² g⁻¹ and a total pore volume is 0.05 cm³ g⁻¹.

Fig. 2 SEM of MnCo₂O₄ nanospheres (a) and pore size distribution of MnCo₂O₄ nanospheres (b).

Fig. 3 Nitrogen adsorption–desorption isotherms of MnCo₂O₄ nanospheres and the pore size distribution (inset).

To investigate the surface chemical composition of Mn and Co elements of MnCo₂O₄ nanospheres, XPS analysis is conducted and the results are shown in Fig. 4. As seen in Fig. 4a, the Co 2p spectrum is well fitted, with two shakeup satellite (indicated as “Sat”) surrounding around the two spin–orbit doublets characteristic of Co²⁺ and Co³⁺. The binding energies of Co³⁺ are located at 780.2 eV and 795.1 eV and the peaks at the binding energies of 782.4 eV and 797.0 eV are ascribed to Co²⁺. For Mn 2p in Fig. 4b, the two main peaks of Mn 2p_3/2 and Mn 2p_1/2 can be resolved into four peaks: two peaks at 641.50 eV and 652.85 eV are attributed to the binding energy of Mn³⁺, while the other two peaks at 644.60 eV and 654.83 eV are indexed to the existence of Mn⁴⁺. Thus, the XPS results confirm that MnCo₂O₄ has mixed valence states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺, ensuring the possible redox catalytic activity during LOBs process.

Fig. 4 XPS spectra of MnCo₂O₄ nanospheres: Co 2p (a) and Mn 2p (b).

Cyclic voltammetry test is performed using the rotating disk electrode (RDE) to investigate the electro-catalytic activity of MnCo₂O₄ nanospheres toward ORR and OER. Fig. 5a presents the CV curves of MnCo₂O₄/KB and pure KB in O₂-saturated non-aqueous electrolyte at the sweep rate of 10 mV s⁻¹. The peak potential of MnCo₂O₄/KB for ORR is 75 mV higher than that of the pure KB, and so is the peak current density, showing a faster kinetic process for ORR. To further investigate the OER catalytic activity, LSV test is also performed as shown in Fig. 5b. The onset potential of MnCo₂O₄/KB for the OER is about 3.2 V, and the peak potential is 3.4 V, showing a better OER catalytic activity than that of the pure KB, which may be accounted for the lower OER overpotential for the batteries with MnCo₂O₄ catalyst.

Fig. 5 CV curves of MnCo₂O₄/KB and pure KB in O₂-saturated LiTFSI/DMSO electrolyte at the sweep rate of 10 mV s⁻¹ with the rotating speed of 900 rpm (a) and linear scanning voltammograms on RDE at the sweep rate of 1 mV s⁻¹ with the rotating speed of 900 rpm (b).

The discharge–charge properties of the batteries are studied at various current densities to measure the capacity of the batteries, as shown in Fig. 6. Fig. 6a shows the discharge–charge curves of LOB with MnCo₂O₄ nanospheres, which reveals that high discharge capacity of 8518 mA h g_cathode⁻¹ and charge capacity of 7036 mA h g_cathode⁻¹ are obtained at 100 mA g_cathode⁻¹. However, for LOB with pure KB (Fig. 6b), the corresponding discharge capacity is only 6403 mA h g_cathode⁻¹ at 100 mA g_cathode⁻¹. It can be found that the capacity of LOB with MnCo₂O₄ nanospheres is always higher than that with pure KB at all discharge current densities. The results confirm that MnCo₂O₄ nanospheres can increase the capacity of LOBs, which may be attributed to the outstanding catalytic performance of MnCo₂O₄ nanospheres during the discharge process.

Fig. 6 The full discharge–charge capacity of LOB with MnCo₂O₄/KB (a) and pure KB (b) catalysts at current densities of 100, 200 and 300 mA g⁻¹.

The full discharge–charge cycle performance of the LOBs with MnCo₂O₄/KB and pure KB are also studied at 100 mA g_cathode⁻¹ within voltage range of 2.2–4.5 V, as shown in Fig. 7. With MnCo₂O₄ as catalyst, the battery has a relative high capacity retention about 63.4%, which is 21.3% higher than that of the pure KB on the second discharge process, showing a relative better catalytic activity. But the batteries degrade seriously as the discharge–charge process goes on, which may be caused by the accumulation of insulated product Li₂O₂ during the full discharge and charge process. So the capacity is limited to 1000 mA h g_cathode⁻¹ to further confirm the cycle stability as reported elsewhere.^43,44

Fig. 7 The full discharge–charge cycle of LOBs with MnCo₂O₄/KB (a) and pure KB (b) as catalysts at the current density of 100 mA g⁻¹.

The overpotential of discharge–charge process is an important factor to evaluate the performance of LOBs. Fig. 8 shows the discharge–charge curves of LOBs with MnCo₂O₄/KB and pure KB cathode at current density of 100 mA g_cathode⁻¹ and the capacity restriction of 1000 mA h g_cathode⁻¹. It can be found that LOB with MnCo₂O₄/KB cathode provides a much higher discharge potential (2.86 V) than that of LOB with pure KB (2.75 V). Thus MnCo₂O₄/KB cathode shows enhanced ORR electro-catalytic activity to decrease the overpotentials. The catalytic role of MnCo₂O₄ nanospheres is not terminated on discharge, it is also active for OER during the charge process. The potential at capacity of 1000 mA h g_cathode⁻¹ for the MnCo₂O₄/KB cathode is still substantially lower than that for the pure KB cathode. It can be revealed that the discharge–charge voltage plateaus are much more stable and the voltage gap is sharply reduced by the introduction of MnCo₂O₄ nanospheres catalyst.

Fig. 8 Discharge–charge curves of LOBs with MnCo₂O₄/KB and pure KB catalysts at current density of 100 mA g⁻¹ and the capacity restriction of 1000 mA h g⁻¹.

The capacity cycle performance of LOBs with MnCo₂O₄ nanospheres is also test. Fig. 9a shows a typical galvanostatic discharge–charge cycles up to 20 times at 100 mA g_cathode⁻¹ under a capacity restriction of 1000 mA h g_cathode⁻¹. The voltage curves are stable and exhibit much higher discharge voltages (about 2.83 V) and lower charge voltages (about 3.8 V). The charge voltage during first cycle contains two stages: the first stage with a slopping profile is attributed to the delithiation of the outer part of Li₂O₂, which could then chemically disproportionate to evolve O₂; the second stage is attributed to the oxidation of bulk Li₂O₂.^45,46 With cycling, it is concluded by FTIR spectroscopy that the rising U_chg is caused by insoluble decomposition products, including Li₂CO₃ and Li formate, as shown in Fig. 9c. The peak at 1000 cm⁻¹ corresponding to the bond S=O, is an indication of DMSO. The peaks assigned to a mixture of Li₂CO₃ and HCO₂Li (1484 cm⁻¹, 1412 cm⁻¹, 863 cm⁻¹)^47,48 appear except the pristine and first discharged cathode. The data indicate not only the accumulation of Li₂CO₃ and Li formate but also explain the rising U_chg on cycling. The byproduct accumulation plays a critical role in this phenomenon. Besides that, no byproduct is observed after first discharge process and the data collected by Raman spectrum prove the presence of Li₂O₂ after 1st discharge, with no other species being detected. And the result is in accordance with the explanation of charge profile during first cycle. Fig. 9d shows the Raman spectra of composite cathode (MnCo₂O₄/KB) after different cycles. The characteristic peaks of Li₂O₂ (790 cm⁻¹) appear on cycling, confirming the existence of Li₂O₂ in the discharged product. However, after 20 cycles, the characteristic peaks Li₂CO₃ at 1088 cm⁻¹ are much stronger than those of Li₂O₂. The analysis could illustrate the reason of the capacity fade severely after 20 cycles.^49–52

Fig. 9 Cycle performance of LOB with MnCo₂O₄/KB catalyst up to 20 cycles at current density of 100 mA g⁻¹ under capacity restriction of 1000 mA h g⁻¹. (a) Discharge–charge curves. (b) Discharge voltage tendency. (c) FTIR spectra of the pristine cathode and after the first discharge, 5 cycles, 10 cycles and 20 cycles. (d) Raman spectra of cathode after the first discharge, 10 cycles and 20 cycles.

To further explore the influence of the MnCo₂O₄ on ORR and OER process, the composition and morphology of the discharge products accumulated on the cathode should be confirmed. Thus, XRD and SEM have been conducted and the results are shown in Fig. 10. Since MnCo₂O₄ is coated on carbon paper, the diffraction peaks corresponding to MnCo₂O₄ at these three stages are not very strong in Fig. 10d. After discharge to 1000 mA h g_cathode⁻¹, peaks corresponding to the discharge product Li₂O₂ appear, confirming that the film coated on the surface of MnCo₂O₄ consists of Li₂O₂, as shown in Fig. 10b. After the LOB is recharged to 1000 mA h g_cathode⁻¹, the discharge product Li₂O₂ disappears completely and the cathode is almost the same as the pristine one, which verifies a relative good rechargeable performance of LOB with MnCo₂O₄.

Fig. 10 SEM of cathodes of LOBs with MnCo₂O₄/KB before discharge (a), at first discharge stage (b) and recharge stage (c), XRD patterns for the products of pristine, discharge and recharge stage (d).

For all the phenomena observed above, the synergy effect of MnCo₂O₄ and KB is deduced. Fig. 11 shows the schematic illustration for the formation of film-like product Li₂O₂ during discharge and recharge process for LOB with MnCo₂O₄/KB cathode. For the discharge process, the high surface area (1350 m² g⁻¹) of KB can improve the adsorption of oxygen molecules, greatly increasing the three-phase interface. The abundant oxygen vacancy and high electronic conductivity of MnCo₂O₄ nanospheres can promote the electronic/ionic diffusion within the electrode, enhancing the kinetics of film-like Li₂O₂ formation on its surface.^53,54 During the recharge process, the better OER electrochemical activity of MnCo₂O₄ connected with the good contact for film-like Li₂O₂ may account for the lower overpotential for OER, realizing the bi-functional catalytic activity for discharge and charge process.

Fig. 11 Schematic illustration for the formation of film-like product Li₂O₂ during discharge and recharge process.

4. Conclusions

In summary, MnCo₂O₄ nanospheres are successfully synthesized by co-precipitation method, showing excellent electro-catalytic activity toward ORR and OER in non-aqueous solution. LOBs with MnCo₂O₄ cathode exhibits excellent discharge capacity as high as 8518 mA h g_cathode⁻¹ and low overpotential of only 0.85 V during the charge–discharge process, and for 20 cycles, the battery shows little performance degradation. The observed enhanced battery performance is considered to result from the outstanding catalytic activity of MnCo₂O₄ for ORR and OER process. The results confirm that MnCo₂O₄ nanospheres are potential effective catalysts for high performance lithium–oxygen batteries.

Acknowledgements

The authors would like to thank Materials Characterization Center of Huazhong University of Science and Technology for samples characterization assistance. This research was financially supported by Guangdong Province (2013B090500051) & Shenzhen City (20140419131733975).

References

1. K. M. Abraham and Z. Jiang, J. Electrochem. Soc., 1996, 143, 1–5.
2. M.-K. Song, S. Park, F. M. Alamgir, J. Cho and M. Liu, Mater. Sci. Eng., R, 2011, 72, 203–252.
3. Y. Wang, D. Zheng, X. Q. Yang and D. Qu, Energy Environ. Sci., 2011, 4, 3697–3702.
4. X. Ren, S. S. Zhang, D. T. Tran and J. Read, J. Mater. Chem., 2011, 21, 10118–10125.
5. R. Padbury and X. Zhang, J. Power Sources, 2011, 196, 4436–4444.
6. T. Ogasawara, A. Débart, M. Holzapfel, P. Novák and P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 1390–1393.
7. A. Kraytsberg and Y. Ein-Eli, J. Power Sources, 2011, 196, 886–893.
8. H. Zheng, M. Zhang, B. Chi, J. Pu and J. Li, Progr. Chem., 2013, 25, 260–269.
9. Y. C. Lu, Z. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli and Y. Shao-Horn, J. Am. Chem. Soc., 2010, 132, 12170–12171.
10. H.-J. Qiu, X. Shen, J. Q. Wang, A. Hirata, T. Fujita, Y. Wang and M. W. Chen, ACS Catal., 2015, 5, 3779–3785.
11. D. A. Stevens and J. R. Dahn, Carbon, 2005, 43, 179–188.
12. F. Li, K. Y. Chan and H. Yung, J. Mater. Chem., 2011, 21, 12139–12144.
13. D. Zhu, L. Zhang, M. Song, X. Wang and Y. Chen, Chem. Commun., 2013, 49, 9573–9575.
14. D. Su, H. S. Kim, W. S. Kim and G. Wang, J. Power Sources, 2013, 244, 488–493.
15. A. K. Thapa, Y. Hidaka, H. Hagiwara, S. Ida and T. Ishihara, J. Electrochem. Soc., 2011, 158, A1483–A1489.
16. A. K. Thapa, G. Park, H. Nakamura, T. Ishihara, N. Moriyama, T. Kawamura, H. Wang and M. Yoshio, Electrochim. Acta, 2010, 55, 7305–7309.
17. X. Lin, X. Lu, T. Huang, Z. Liu and A. Yu, J. Power Sources, 2013, 242, 855–859.
18. E. Yoo and H. Zhou, J. Am. Chem. Soc., 2011, 5, 3020–3026.
19. J. Xiao, D. Mei, X. Li, W. Xu, D. Wang, G. L. Graff, W. D. Bennett, Z. Nie, L. V. Saraf, I. A. Aksay, J. Liu and J. G. Zhang, Nano Lett., 2011, 11, 5071–5078.
20. G. Wu, N. H. Mack, W. Gao, S. Ma, R. Zhong, J. Han, J. K. Baldwin and P. Zelenay, ACS Nano, 2012, 6, 9764–9776.
21. B. Zheng, J. Wang, F. B. Wang and X. H. Xia, Electrochem. Commun., 2013, 28, 24–26.
22. L. Zhang, X. Zhang, Z. Wang, J. Xu, D. Xu and L. Wang, Chem. Commun., 2012, 48, 7598–7600.
23. H. Zheng, M. Zhang, B. Chi, J. Pu and J. Li, Catal. Commun., 2015, 61, 44–47.
24. H. Zheng, M. Zhang, B. Chi, J. Pu and J. Li, J. Alloys Compd., 2015, 626, 173–179.
25. Q. Liu, Y. Jiang, J. Xu, D. Xu, Z. Chang, Y. Yin, W. Liu and X. Zhang, Nano Res., 2015, 8, 576–583.
26. J.-J. Xu, Z.-L. Wang, D. Xu, F.-Z. Meng and X.-B. Zhang, Energy Environ. Sci., 2014, 7, 2213–2219.
27. J.-J. Xu, D. Xu, Z.-L. Wang, H.-G. Wang, L.-L. Zhang and X.-B. Zhang, Angew. Chem., Int. Ed., 2013, 52, 3887–3890.
28. Y. Li, L. Zou, J. Li, K. Guo, X. Dong, X. Li, X. Xue, H. Zhang and H. Yang, Electrochim. Acta, 2014, 129, 14–20.
29. K. Zhang, L. Zhang, X. Chen, X. He, X. Wang, S. Dong, P. Han, C. Zhang, S. Wang, L. Gu and G. Cui, J. Phys. Chem. C, 2013, 117, 858–865.
30. J.-L. Shui, N. K. Karan, M. Balasubramanian, S.-Y. Li and D.-J. Liu, J. Am. Chem. Soc., 2012, 134, 16654–16661.
31. F. Li, R. Ohnishi, Y. Yamada, J. Kubota, K. Domen, A. Yamada and H. Zhou, Chem. Commun., 2013, 49, 1175–1177.
32. E. Yoo and H. Zhou, J. Power Sources, 2013, 244, 429–434.
33. J. P. Singh and R. N. Singh, J. New Mater. Electrochem. Syst., 2000, 3, 131–139.
34. M. Hamdani, R. N. Singh and P. Chartier, Int. J. Electrochem. Sci., 2010, 5, 556–577.
35. C. Xu, M. Lu, Y. Zhan and J. Y. Lee, RSC Adv., 2014, 4, 25089–25092.
36. T.-F. Hung, S. G. Mohamed, C.-C. Shen, Y.-Q. Tsai, W.-S. Chang and R.-S. Liu, Nanoscale, 2013, 5, 12115–12119.
37. J. Li, M. Zou, W. Wen, Y. Zhao, Y. Lin, L. Chen, H. Lai, L. Guan and Z. Huang, J. Mater. Chem. A, 2014, 2, 10257–10262.
38. X. Ge, Y. Liu, F. W. T. Goh, T. S. A. Hor, Y. Zong, P. Xiao, Z. Zhang, S. H. Lim, B. Li, X. Wang and Z. Liu, ACS Appl. Mater. Interfaces, 2014, 6, 12684–12691.
39. B. Hua, J. Pu, W. Gong, J. Zhang, F. Lu and L. Jian, J. Power Sources, 2008, 185, 419–422.
40. H. Wang, Y. Yang, Y. Liang, G. Zheng, Y. Li, Y. Cui and H. Dai, Energy Environ. Sci., 2012, 5, 7931–7935.
41. S. Ma, L. Sun, L. Cong, X. Gao, C. Yao, X. Guo, L. Tai, P. Mei, Y. Zeng, H. Xie and R. Wang, J. Phys. Chem. C, 2013, 117, 25890–25897.
42. C. Fu, G. Li, D. Luo, X. Huang, J. Zheng and L. Li, ACS Appl. Mater. Interfaces, 2014, 6, 2439–2449.
43. Y. Liu, L.-J. Cao, C.-W. Cao, M. Wang, K.-L. Leung, S.-S. Zeng, T. F. Hung, C. Y. Chung and Z.-G. Lu, Chem. Commun., 2014, 50, 14635–14638.
44. F. Lu, X. Cao, Y. Wang, C. Jin, M. Shen and R. Yang, RSC Adv., 2014, 4, 40373–40376.
45. Y.-C. Lu and Y. Shao-Horn, J. Phys. Chem. Lett., 2013, 4, 93–99.
46. B. M. Gallant, D. G. Kwabi, R. R. Mitchell, J. Zhou, C. V. Thompson and Y. Shao-Horn, Energy Environ. Sci., 2013, 6, 2518–2528.
47. S. A. Freunberger, Y. Chen, Z. Peng, J. M. Griffin, L. J. Hardwick, F. Barde, P. Novak and P. G. Bruce, J. Am. Chem. Soc., 2011, 133, 8040–8047.
48. Z. Peng, S. A. Freunberger, Y. Chen and P. G. Bruce, Science, 2012, 337, 563–566.
49. N. P. S. Selladurai, Ionics, 2014, 20, 479–487.
50. D. Zhai, H. H. Wang, K. C. Lau, J. Gao, P. C. Redfern, F. Kang, B. Li, E. Indacochea, U. Das, H. H. Sun, H. J. Sun, K. Amine and L. A. Curtiss, J. Phys. Chem. Lett., 2014, 5, 2705–2710.
51. J. Xie, X. Yao, I. P. Madden, D. E. Jiang, L. Y. Chou, C. K. Tsung and D. Wang, J. Am. Chem. Soc., 2014, 136, 8903–8906.
52. P. Pasierb, S. Komornicki, M. Rokita and M. Rekas, J. Mol. Struct., 2001, 596, 151–156.
53. X. Cao, C. Jin, F. Lu, Z. Yang, M. Shen and R. Yang, J. Electrochem. Soc., 2014, 5, 296–300.
54. T. Y. Ma, Y. Zheng, S. Dai, M. Jaroniec and S. Z. Qiao, J. Mater. Chem. A, 2014, 2, 8676–8682.

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