LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ as cathode material with improved electrochemical performance for lithium ion batteries

Abstract

In this article, the pristine Li-rich layered oxide Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ porous microspheres have been successfully synthesized using a urea combustion method and then coated with 1 wt% LaF₃ via a facile chemical precipitation route. The structures and morphologies of both pristine and LaF₃ coated Li_1.2Mn_0.54Ni_0.16Co_0.08O₂ were investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HR-TEM). The results reveal that the obtained particles possesses the morphology of porous microspheres and a LaF₃ layer with a thickness of 5–8 nm coated on the surface of the Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ particles. As lithium ion battery cathodes, the LaF₃ coated sample, compared with the pristine one, has shown a significantly improved electrochemical performance; the initial coulombic efficiency improves from 75.36% to 80.01% and the rate compatibility increased from 57.4 mA h g⁻¹ to an extremely high capacity of 153.5 mA h g⁻¹ at 5 C. Decreased electrochemical impedance spectroscopy (EIS) reveals that the enhanced electrochemical performance of the surface coating was attributed to the lower charge transfer resistance of the sample.

---

Introduction

To meet the requirements for application in electric vehicles (EVs) and hybrid electric vehicles (HEVs), searching for safe, low-cost, long lifetime, high energy density and power density cathode materials has been one of the most important subjects in LIBs.^1–3 Li-rich layered oxides xLi₂MnO₃·(1 − x)LiMO₂ (M = Ni, Co, Mn or combinations) have been regarded as one of the most promising candidates due to their extraordinarily high theoretical discharge capacity of more than 250 mA h g⁻¹ and high operating potential over 3.7 V (vs. Li⁺/Li).^4–6 However, several drawbacks impede the commercialization of Li-rich cathode materials. The first problem is the enormous irreversible capacity loss of 40–100 mA h g⁻¹ in the first cycle, which depends on the composition when charged up to 4.6 V. The irreversible capacity loss can be attributed to the extraction of Li₂O followed by an elimination of the oxide ion vacancies from the structure during the first charge, leading to fewer insertion–extraction sites for lithium ions in the subsequent discharge process.^7–10 In addition, the relatively low electron conductivity of the Mn-containing layered component and high cut-off operating voltage result in a poor rate capability during the electrochemical cycling process.^11,12

Previous reports demonstrate that surface coating or modification are a valid method to enhance the electrochemical performance of cathode materials for LIBs when they are cycled on many occasions or charged at a high cut-off voltage (e.g., 4.7 V).^13–18 Among all surface modification materials, metal fluorides, such as AlF₃, CaF₂, CeF₃, and LiF, have been extensively studied and have turned out to be an effective approach to improve the electrochemical performance of the lithium rich layered oxides. Generally, the metal fluoride is believed to suppress HF corrosion, which is responsible for better cycle stability.^19–22 Furthermore, Zheng et al. have reported an AlF₃ coated layer that can reduce the activity of extracted oxygen and suppress the electrolyte decomposition at voltages above 4.5 V, resulting in an improved coulombic efficiency and cycling stability.²³ Sun et al. also indicated that the AlF₃ coating layer can induce the transformation of the layer phase to a spinel phase. The formation of a spinel phase can play the role as a fast lithium ion conductor and help to improve the rate capability.²⁴

LaF₃ is another commonly used metal fluoride. Herein, the opposite ion La³⁺ has an ionic radius of ∼106.3 pm, which is larger than that of Mn⁴⁺, Co³⁺ and Ni²⁺. Thus, a slight doping of La³⁺ on the surface of Li-rich layered oxides may lead to the formation of some defects and/or vacancies, and these defects and/or vacancies should be helpful for the intercalation/deintercalation of lithium ions.²⁵ In the present study, a small amount of LaF₃ (i.e., 1 wt%) was coated on the surface of Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ microspheres. The experimental results show that the LaF₃ coated Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, compared with pristine Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, exhibits a huge improvement in the initial coulombic efficiency and rate capability. The reasons for the improvement in the electrochemical performance via the LaF₃ coating are discussed in detail.

Experimental

Preparation of the pristine and LaF₃-coated samples

All the raw materials were of analytical-grade and used as received. The Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ porous spheres were prepared via a facile urea combustion method described as follows: 0.5112 g LiNO₃ (5% Li excess), 0.2792 g Ni(NO₃)₂·6H₂O, 1.301 g of an aqueous solution of Mn(NO₃)₂ (50 wt%), 0.1397 g Co(NO₃)₂·6H₂O and 1.098 g CO(NH₂)₂ were dissolved together in 5 ml distilled water to form a uniform solution. Subsequently, the obtained mixed solution was heated in a muffle furnace at 450 °C for 40 min under air to remove the organic contents. Then, the precursors were ground and further annealed in a muffle furnace at 800 °C for 10 h under air and then allowed to cool naturally to room temperature.

To prepare the LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂, the as-prepared black powder was immersed into an aqueous solution of La(NO₃)₃·nH₂O with continuous stirring and then a solution of NH₄F was added dropwise to the suspension solution. The pH value of the obtained solution was adjusted to 7.0 by adding an ammonia solution. The molar ratio of La to F was regulated to 1 : 3 and the coating amount of LaF₃ was set to 1 wt% of the parent cathode material. The obtained solution was constantly stirred at 80 °C until the solvent was completely evaporated. Subsequently, the wet powder was dried at 80 °C in a vacuum drying oven until the solvent was completely removed. Finally, the dry powder was further annealed in an N₂ atmosphere at 450 °C for 4 h to obtain the LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂.

Structural and morphology characterization

The crystalline phases of the obtained samples were characterized by X-ray diffraction (DX-2007 LiaoNing DanDong) at a scanning rate of 0.03° s⁻¹ within 2θ degree of 10–80°. SEM images were performed using a Quanta FEG 250 field emission scanning electron microscope (FEI, Electron optics, B.V.). EDX spectroscopy was performed using an EDAX system. The surface microstructure and selected area electron diffraction (SAED) of the coated sample was observed by a transmission electron microscope (TEM, JEM-2100F).

Electrochemical characterization

Electrode slurry was fabricated by mixing 80 wt% of the active material, 10 wt% of carbon black and 10 wt% of polyvinylidene difluoride (PVDF) binder with a certain amount of N-methyl-2-pyrrolidine (NMP) solvent. Then, the slurry mixture was pasted onto an aluminum foil and dried at 80 °C for 8 h in a vacuum drying oven. Finally, the dried aluminum foil were cut into round discs with a diameter of 12 mm and the mean mass loading of active material was about 2.1 mg cm⁻². A coin cell (CR2016) was assembled in an argon-filled glove box using pure lithium foils as the reference and counter electrode. A commercial LBC 301 LiPF₆ solution (ShenZhen XinZhouBang) was used as the electrolyte and thin polymer acted as a separator. The charge–discharge test of the assembled cells was performed using NEWARE battery test systems in a voltage range of 2.0–4.7 V at room temperature (about 30 °C). Electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical workstation (CHI660D, ShangHai ChenHua) in a frequency range of 0.01–0.1 MHz and open circuit voltage of 3.2 V.

Results and discussion

Structure and morphologies of pristine and LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂

Fig. 1 shows the XRD patterns of pristine and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂. All the sharp diffraction peaks can be indexed to a hexagonal α-NaFeO₂ type structure with a space group R3̄m. Adjacent peaks of (006)/(012) and (108)/(110) were divided clearly,²⁶ indicating that each sample has a good crystal structure. Weak XRD peaks observed within the 2θ degree range of 20–25° (marked by *) suggest the periodic occupation of Li⁺ ions in the transition metal layers of crystalline LiMO₂ and the resultant LiMn₆-typed cation arrangements indicate the co-existence of both crystalline Li₂MnO₃ (also referred to as layered Li(Li_1/3Mn_2/3)O₂) and LiNi_0.4Co_0.2Mn_0.4O₂.^21,22 No diffraction peaks for LaF₃ and other impurity can be observed in the XRD pattern, indicating that a little amount of LaF₃ was only coated on the surface of the Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ and the bulk structure of the Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ remains unchanged after the surface modification process.

Fig. 1 XRD patterns of (a) pristine and (b) 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂.

The morphology and particle size of the pristine and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ were investigated using SEM and HR-TEM, as shown in Fig. 2. It can be clearly seen that the pristine Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ was composed of porous microspheres, and these microspheres were constructed of numerous nanoparticles that have a particle size of 3–8 μm (Fig. 2a and b). As is well known, the porous microsphere structure should be suitable as a lithium ion battery cathode. First, the sphere-like structure can have a good stability during cycling. Second, the primary nanoparticles within the microspheres can provide a short pathway for the intercalation/deintercalation of lithium ions.^27–29 Thus, the electrochemical performance should be improved as expected after surface modification with functional materials such as LaF₃.

Fig. 2 SEM images of the pristine Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ at (a) low magnification and (b) high magnification, (c) 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ and (d) HR-TEM and SAED (selected area electron diffraction) image of the LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂.

The SEM image of the LaF₃-coated porous microspheres is shown in Fig. 2c; compared with the pristine sample (Fig. 2b), a bright coating layer can be observed on the surface of the coated sample and the gap between nanoparticles is less distinguished due to the existence of the coating layer. To further study the structure of the surface coating layer, high-resolution transmission electron microscopy (HR-TEM) was carried out and the result is revealed in Fig. 2d. The lithium rich layered oxides are a nanosized mixture of Li₂MnO₃ and LiMO₂ components. Herein, Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ can also be denoted as 0.5Li₂MnO₃-0.5LiNi_0.4Co_0.2Mn_0.4O₂. In Fig. 2d, the Li₂MnO₃ domain has been successfully captured by HR-TEM and the lattice fringes with a spacing of 0.43 nm can be assigned to the (020) crystal face of the Li₂MnO₃ component. Moreover, in the SAED pattern displayed in Fig. S1,† the diffraction dots of the (003), (101), (104) and (107) crystal planes can be clearly presented, which are consistent with the XRD pattern shown in Fig. 1. It should be noted that a little amount of LaF₃ is difficult to detect by SAED. In particular, the measured thickness of the coating layer is about 5–7 nm. In comparison with the previous literature reports,^{19,20,30–32} the thickness of the coating layer is very suitable.

In order to further distinguish the difference in the surface structure after coating, XPS spectroscopy of the pristine and coated samples was carried out, as shown in Fig. S2.† The characteristic binding energies of La3d and F1s were found to be 834.89 eV and 684.64 eV, respectively, which are in accordance with those found for pure LaF₃.³³ Compared with the peaks of bare Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂, the Ni2p, Co2p and Mn2p peaks (in Fig. S2†) of the LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ have no obvious chemical shift, indicating that the Ni, Co and Mn ion environments in the structure have not been changed. However, the intensity of each peak decreases significantly after coating, which was attributed to the formation of the LaF₃ layer on the surface of Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂.¹⁴

Furthermore, the selected area EDS image and corresponding elemental analysis of the 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ is shown in Fig. 3. The calculated elemental ratio of Ni : Co : Mn, according to Fig. 4, is 0.560 : 0.159 : 0.082, which is close to the chemical formula of Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ (i.e., 0.560 : 0.160 : 0.08). The calculated atomic ratio of La : Mn is 1.0 : 50.2 from the EDS analysis, and the theoretical data based on a 1 wt% LaF₃ coating should be 1.0 : 56.0. These results indicate that the actual elemental compositions of the as-prepared pristine and 1 wt% LaF₃-coated sample are consistent with the experimental design. It should be emphasized that the content of O and F is difficult to be detected accurately using EDS. However, the EDS image also shows the co-existence of the elements O and F.

Fig. 3 Element dispersive spectrum (EDS) of 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂. The insets are the selected area image and detailed elemental analysis.

Fig. 4 The initial charge–discharge voltage profiles and corresponding differential capacity (dQ/dV) curves for the pristine Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ electrodes at a current density of 0.1 C between 2.0 and 4.7 V.

Fig. 4 comparatively reveals the initial charge–discharge and corresponding dQ/dV curves for the pristine and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ electrodes within 2.0–4.7 V at 0.1 C (1 C = 200 mA g⁻¹). The initial charge–discharge capacity of the pristine and 1 wt% LaF₃-coated sample are 330/249 and 339/272 mA h g⁻¹ (Fig. 4a), giving a coulombic efficiency of 75.4% and 80.4%, respectively. It is clear that the irreversible capacity loss has been decreased from 81 to 67 mA h g⁻¹. As is well known, one of the primary disadvantages of LLOs (lithium rich layered oxides) cathodes is the enormous irreversible capacity loss of 40–100 mA h g⁻¹ in the first cycle, and the irreversible capacity loss can be attributed to the extraction of Li₂O followed by an elimination of the oxide ion vacancies from the structure. The surface modification with metal fluoride can reduce the activity of extracted oxygen. Thus, the initial coulombic efficiency of the LLOs can be effectively improved. In other words, the improved coulombic efficiency should be attributed to the fact of that the LaF₃ coated sample, compared with the pristine one, presents a higher discharge capacity under close charge capacity.

As reported in many literature report s,^20,24,34 surface modification can effectively improve the initial coulombic efficiency. Zheng et al.²³ reported that a coating layer can act as a “buffer” layer to enhance the formation of inactive oxygen and restrain the secondary reaction of electrolyte oxidation caused by active oxygen species. Thus, an improved coulombic efficiency can be expected.

Fig. 4b is the dQ/dV curves for the pristine and coated samples. During the first charge process, an anodic peak near 4.00 V can be ascribed to the de-intercalation of Li⁺ ions from the LiNi_0.4Co_0.2Mn_0.4O₂ phase and the oxidation of Ni²⁺ ions, and the sharp anodic peak within 4.50–4.52 V corresponds to the transformation of Li₂MnO₃ to layered MnO₂ and the oxidation of Co³⁺ ions. In the initial discharge process, two cathodic peaks around 3.75 and 3.30 V indicate the intercalation of Li⁺ ions into the layered LiNi_0.4Co_0.4Mn_0.4O₂ and previously formed MnO₂, respectively. Compared with the pristine Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂, the coated sample has a high discharge voltage (Fig. 4b, red curve), which demonstrates that the coating layer can reduce the polarization to a certain degree.

The discharge capacity and cycling performance of the electrode materials at an elevated current density are significant parameters for a cathode. The discharge capacity and cycling stability of pristine and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ were studied, as shown in Fig. 5. Similar to the discharge capacity at the low current density (Fig. 4a), the LaF₃-coated samples have a higher discharge capacity (182.1 mA h g⁻¹) and initial coulombic efficiency of 75.36% compared to the pristine samples (158.0 mA h g⁻¹, 70.01%) at 1 C (Fig. 5a). The cycling stability of these samples is shown in Fig. 5b. A residual discharge capacity of 161.7 and 193.0 mA h g⁻¹ can be retained after 50 cycles. In comparison with initial value (158.0 and 182.1 mA h g⁻¹), the cycling stability of both samples is excellent, especially the LaF₃-coated sample.

Fig. 5 (a) The initial charge–discharge curves and (b) cycling performance of the Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ electrodes within 2.0–4.7 V at 1 C.

Although the Li-rich layered xLi₂MnO₃·(1 − x)LiMO₂ has an extraordinarily high theoretical discharge capacity of more than 250 mA h g⁻¹ and possesses a high operating potential of 4.6–4.8 V (vs. Li⁺/Li), the poor rate capacity was one of the most important drawbacks that impedes the commercialization of this cathode material.^35–37 In this paper, the rate capability of the pristine and LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ were analyzed as shown in Fig. 6a. Apparently, the discharge capacity decreased in each sample with an elevated current density. The LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ have a much better rate compatibility than the pristine sample. The LaF₃-coated electrode exhibits a stable discharge capacity of 273.2, 229.3, 202.7 or 178.2 mA h g⁻¹ at a current density of 0.1 C, 0.5 C, 1 C or 2 C, respectively. Even when the current density is increased to a high value of 5 C, the electrode still delivers a high discharge capacity of 153.5 mA h g⁻¹, while the pristine sample only shows a discharge capacity of 121 or 58.2 mA h g⁻¹ at a current density of 2 C or 5 C, respectively. The discharge profiles at various C rates of pristine and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ are shown in Fig. 6b and c. The capacity and voltage decay with elevated current density has been denoted by an arrow, and it can be seen that the pristine sample has a more serious decay. According to the abovementioned results, the rate capability of Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ has been greatly improved by a surface coating of 1 wt%. LaF₃ may act as a fast Li ion conductor and/or induce the formation of spinel Li–Ni–Mn–O oxide,^38–40 which facilitates the insertion/extraction of Li ions within the interface of electrode material and electrolyte.

Fig. 6 (a) Rate capability of the pristine Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ and 1 wt% LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ electrodes. The initial discharge profiles for (b) Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ and (c) LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ at a series of current densities. The arrow denotes a voltage decrease with increased current density.

For further investigating the mechanism of the enhanced electrochemical performance of LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂, the electrochemical impedance spectroscopy (EIS) of the pristine and LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ was carried out and collected after 3 charge–discharge cycles at 0.1 C, as shown in Fig. 7. It should be noted that the spectra were fitted using the impedance matching software ZSimDemo 3.30 original spectrum in Fig. S3.† The high-frequency semicircle and low-frequency slope line are given by the Nyquist plots, in which the high-frequency semicircle was related to the charge transfer resistance (R_ct) in the electrode/electrolyte and the low-frequency slope line on behalf of the impedance of lithium ion diffusion in the bulk electrode materials. Obviously, compared to the pristine samples, the LaF₃-coated samples have a much smaller R_ct value. The R_ct value of the pristine samples was 116.2 Ω, while the LaF₃-coated samples exhibited an R_ct value of 89.66 Ω. As mentioned above, it demonstrates that LaF₃ acted as a very stable conductor layer to improve the electrochemical conductivity of the as-prepared Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂.^24,35,41 This is the reason why the LaF₃ coating layer results in the excellent rate compatibilities and higher coulombic efficiency of the electrode.

Fig. 7 Electrochemical impedance spectroscopy (EIS) and the equivalent circuit used (inset) for the pristine and LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ after 3 cycles before charging.

Conclusions

As is well known, the initial coulombic efficiency and the poor rate performance are the main drawbacks that impede the possible commercialization of lithium rich layered oxides such as Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂. In this paper, pristine Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ porous microspheres have been successfully synthesized using a urea combustion method and then uniformly coated with LaF₃ via a simple chemical precipitation method. The coating layer has a reasonable thickness of 5–7 nm. As a lithium ion battery cathode, the LaF₃-coated Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ exhibits greatly enhanced electrochemical performance as compared to the pristine sample. The initial coulombic efficiency has been improved from 75.36% to 80.01%. In particular, the discharge capacity increases from 57.4 to 153.5 mA h g⁻¹ at 5 C. The coating layer of LaF₃ might have acted as both a buffer and a conductor to enhance the initial coulombic efficiency and rate capability.

Acknowledgements

This work was financially supported by the Science and Technology Program of LongYan (2014LY36) and the School Research Program of LongYan University (LC2013008).

References

1. A. Ito, D. Li, Y. Ohsawa and Y. Sato, J. Power Sources, 2008, 183, 344–346.
2. T. A. Arunkumar, E. Alvarez and A. Manthiram, J. Mater. Chem., 2008, 18, 190–198.
3. J. Bareno, C. H. Lei, J. G. Wen, S. H. Kang, I. Petrov and D. P. Abraham, Adv. Mater., 2010, 22, 1122–1127.
4. N. Yabuuchi, K. Yoshii, S. T. Myung, I. Nakai and S. Komaba, J. Am. Chem. Soc., 2011, 133, 4404–4419.
5. H. Kobayashi, Y. Takenaka, Y. Arachi, H. Nitani, T. Okumura, M. Shikano, H. Kageyama and K. Tatsumi, Solid State Ionics, 2012, 225, 580–584.
6. L. Simonin, J.-F. Colin, V. Ranieri, E. Canévet, J.-F. Martin, C. Bourbon, C. Baehtz, P. Strobel, L. Daniel and S. Patoux, J. Mater. Chem., 2012, 22, 11316.
7. S. Rajarathinam, S. Mitra and R. K. Petla, Electrochim. Acta, 2013, 108, 135–144.
8. X. Zhang, C. Yu, X. Huang, J. Zheng, X. Guan, D. Luo and L. Li, Electrochim. Acta, 2012, 81, 233–238.
9. J. Wang, G. Yuan, M. Zhang, B. Qiu, Y. Xia and Z. Liu, Electrochim. Acta, 2012, 66, 61–66.
10. J.-L. Liu, J. Wang and Y.-Y. Xia, Electrochim. Acta, 2011, 56, 7392–7396.
11. D. Mohanty, A. S. Sefat, S. Kalnaus, J. Li, R. A. Meisner, E. A. Payzant, D. P. Abraham, D. L. Wood and C. Daniel, J. Mater. Chem. A, 2013, 1, 6249.
12. S. Hy, W.-N. Su, J.-M. Chen and B.-J. Hwang, J. Phys. Chem. C, 2012, 116, 25242–25247.
13. Y. Chen, G. Xu, J. Li, Y. Zhang, Z. Chen and F. Kang, Electrochim. Acta, 2013, 87, 686–692.
14. X.-H. Liu, L.-Q. Kou, T. Shi, K. Liu and L. Chen, J. Power Sources, 2014, 267, 874–880.
15. J. Liu, Q. Wang, B. Reeja-Jayan and A. Manthiram, Electrochem. Commun., 2010, 12, 750–753.
16. Y. J. Kang, J. H. Kim, S. W. Lee and Y. K. Sun, Electrochim. Acta, 2005, 50, 4784–4791.
17. W. C. West, J. Soler, M. C. Smart, B. V. Ratnakumar, S. Firdosy, V. Ravi, M. S. Anderson, J. Hrbacek, E. S. Lee and A. Manthiram, J. Electrochem. Soc., 2011, 158, A883.
18. Q. Y. Wang, J. Liu, A. V. Murugan and A. Manthiram, J. Mater. Chem., 2009, 19, 4965.
19. X. Liu, J. Liu, T. Huang and A. Yu, Electrochim. Acta, 2013, 109, 52–58.
20. C. Lu, H. Wu, Y. Zhang, H. Liu, B. Chen, N. Wu and S. Wang, J. Power Sources, 2014, 267, 682–691.
21. P. Mohan and G. Paruthimal Kalaignan, Ceram. Int., 2014, 40, 1415–1421.
22. T. Shi, Y. Dong, C.-M. Wang, F. Tao and L. Chen, J. Power Sources, 2015, 273, 959–965.
23. Z. Lu and J. R. Dahn, J. Electrochem. Soc., 2002, 149, A815–A822.
24. Y. K. Sun, M. J. Lee, C. S. Yoon, J. Hassoun, K. Amine and B. Scrosati, Adv. Mater., 2012, 24, 1192–1196.
25. T. Ohnishi, K. Mitsuishi, K. Nishio and K. Takada, Chem. Mater., 2015, 27, 1233–1241.
26. O. Toprakci, H. A. K. Toprakci, Y. Li, L. Ji, L. Xue, H. Lee, S. Zhang and X. Zhang, J. Power Sources, 2013, 241, 522–528.
27. W.-M. Chen, L. Qie, Y. Shen, Y.-M. Sun, L.-X. Yuan, X.-L. Hu, W.-X. Zhang and Y.-H. Huang, Nano Energy, 2013, 2, 412–418.
28. J. Liu, L. Chen, M. Hou, F. Wang, R. Che and Y. Xia, J. Mater. Chem., 2012, 22, 25380.
29. X. Wei, S. Zhang, Z. Du, P. Yang, J. Wang and Y. Ren, Electrochim. Acta, 2013, 107, 549–554.
30. S. Zhao, Y. Bai, L. Ding, B. Wang and W. Zhang, Solid State Ionics, 2013, 247–248, 22–29.
31. S.-H. Kang and M. M. Thackeray, Electrochem. Commun., 2009, 11, 748–751.
32. S. Francis Amalraj, B. Markovsky, D. Sharon, M. Talianker, E. Zinigrad, R. Persky, O. Haik, J. Grinblat, J. Lampert, M. Schulz-Dobrick, A. Garsuch, L. Burlaka and D. Aurbach, Electrochim. Acta, 2012, 78, 32–39.
33. H. Yu, Y. Shen, Y. Cui, H. Qi, J. Shao and Z. Fan, Appl. Surf. Sci., 2008, 254, 1783–1788.
34. B. Xiao, J. Liu, Q. Sun, B. Wang, M. N. Banis, D. Zhao, Z. Wang, R. Li, X. Cui, T.-K. Sham and X. Sun, Adv. Sci., 2015, 2, 1500022.
35. J. W. Min, J. Gim, J. Song, W.-H. Ryu, J.-W. Lee, Y.-I. Kim, J. Kim and W. B. Im, Electrochim. Acta, 2013, 100, 10–17.
36. S. K. Martha, J. Nanda, Y. Kim, R. R. Unocic, S. Pannala and N. J. Dudney, J. Mater. Chem. A, 2013, 1, 5587.
37. B. Liu, Q. Zhang, S. He, Y. Sato, J. Zheng and D. Li, Electrochim. Acta, 2011, 56, 6748–6751.
38. K. Kubota, T. Kaneko, M. Hirayama, M. Yonemura, Y. Imanari, K. Nakane and R. Kanno, J. Power Sources, 2012, 216, 249–255.
39. B. Song, Z. Liu, M. O. Lai and L. Lu, Phys. Chem. Chem. Phys., 2012, 14, 12875–12883.
40. D. Mohanty, S. Kalnaus, R. A. Meisner, K. J. Rhodes, J. Li, E. A. Payzant, D. L. Wood and C. Daniel, J. Power Sources, 2013, 229, 239–248.
41. Q. Q. Qiao, H. Z. Zhang, G. R. Li, S. H. Ye, C. W. Wang and X. P. Gao, J. Mater. Chem. A, 2013, 1, 5262.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06243h

---