Effect of ion doping on the electrochemical performances of LiFePO₄–Li₃V₂(PO₄)₃ composite cathode materials

Abstract

Due to its low intrinsic electronic and ionic conductivities, olivine-structured LiFePO₄ has been the focus of research into ionic modifications of LiFePO₄/C cathode materials. Various ionic doping processes have been developed for enhancing the electrochemical performances of lithium-ion batteries, including cation and anion doping of LiFePO₄. In particular, significant progress has recently been made in understanding and controlling the syntheses, nanostructures and electrochemical performances of LiFePO₄–Li₃V₂(PO₄)₃ composite materials. However, there are still many challenges in achieving good charge transfer kinetics and charge–discharge performances of LiFePO₄–Li₃V₂(PO₄)₃ composite cathodes. In this review, we summarize some of the recent progress in several typical cation modification methods.

---

Chao Jin

Chao Jin was born in 1989 in Shandong province, China. He graduated from Qilu University of Technology in 2012. Then, in the same year, he moved to the Institute of Materials Science and Engineering, Qilu University of Technology as a postgraduate student, and majored in materials chemistry. His scientific interests focus on nanomaterials for lithium batteries.

Xudong Zhang

Xudong Zhang is currently a full professor in the Department of Materials Science and Engineering at Qilu University of Technology. He received his PhD degree from Shandong University in 2004, in the State Key Lab of Crystal Materials. His research interests focus on materials chemistry, technology for the synthesis of nanometer materials, lithium batteries and electrode materials.

Introduction

Due to the rapid depletion of non-renewable resources and the effects of global warming, people's attention has been attracted to electric vehicles (EVs), hybrid electric vehicles (HEVs) and energy storage devices. The cathode material is the most important component for obtaining a lithium-ion battery with high energy density, high rate capability and long cycle life. Choosing cathode materials for large-scale lithium-ion batteries should be a major issue. Among the cathode materials, olivine-structured LiFePO₄ (LFP) and monoclinic Li₃V₂(PO₄)₃ (LVP) attract the most attention, due to their intrinsic structural and chemical stabilities that lead to safe, low cost batteries with long cycle life. LFP and LVP have been considered as the most competitive cathode candidates for next-generation large-scale lithium-ion batteries for use in HEVs or EVs. However, one of the main obstacles to the practical application of LFP and LVP is their poor rate capability, which can be attributed to slow lithium-ion diffusion kinetics and poor electronic conductivity.^1–3

Pure LFP is an electronically conductive material consisting mainly of an N-type semiconductor. It has a theoretical capacity of 170 mA h g⁻¹ at a voltage of 3.45 Volts versus lithium, a lithium-ion diffusion coefficient of 10⁻¹⁴ to 10⁻¹⁶ cm² s⁻¹ and an electron conductivity of 10⁻⁹ to 10⁻¹⁰ S cm⁻¹.^4,5 Prosini et al. measured the lithium ion migration parameter value as approximately −3.8 (when the lithium ion migration is 0, the interaction parameter g is −4).⁶ Pure LVP has a low electronic conductivity of about 2.3 × 10⁻⁸ S cm⁻¹ at 27 °C,⁷ which presents a major drawback to practical implementation of the material.

In order to improve the properties of existing LiFePO₄-based electrode materials, electrochemical researchers have made extensive efforts, which include particle size control and manipulation, surface modification of particles by coating with electronically conductive agents, and atomic-level doping with supervalent ions.^8–10 One early report that spawned much activity suggested that the poor electronic conductivity could be increased by 8 orders of magnitude by supervalent-cation doping, which was proposed to stabilize the minority of Fe³⁺ hole carriers in the lattice.¹¹ The dramatic increase in conductivity was later implied to be a result of carbon and metallic iron phosphides/carbophosphides on the LiFePO₄ surface arising from solid-state reactivity at the elevated temperatures used in processing.¹² In 2002, Chung et al.¹¹ investigated different cation dopants to determine the effects of aliovalent doping on the electronic conductivity of LiFePO₄. In 2012, Park et al.¹³ demonstrated that the undercoordinated Fe²⁺/Fe³⁺ redox couple at the surface causes a high barrier for charge transfer, but it can be stabilized by surface modification with nitrogen or sulfur anions. Surface modification greatly improves the charge transfer kinetics and the charge–discharge performance of an LiFePO₄ cathode. Reports indicated, via structural and electrochemical analyses, that ion doping could decrease the lithium miscibility gap, favor phase transformation kinetics in cycling, expand diffusion channels, and introduce controlled atomic disorder into the ordered olivine structure.¹⁴ Since the pioneering work of Jiajun Wang and coworkers,³ much work has been carried out to investigate the synthesis, structure and electrochemical properties of LiFePO₄. The present work focuses on the impact of cation modification on the electrochemical performances of LiFePO₄–Li₃V₂(PO₄)₃ composite cathodes. The effects of single and dual dopants on the cathode materials are compared. An insight into future research and development of LiFePO₄–Li₃V₂(PO₄)₃ composites is also discussed.

Structure and properties

The structural framework of olivine-type LiFePO₄ consists of FeO₆ octahedra and PO₄ tetrahedra; its space group is Pnma, and Li ions are located in the 1D channels along the [010] direction with relatively low migration energies.^15–17 One FeO₆ octahedron has common edges with two LiO₆ octahedra. The PO₄ groups share one edge with an FeO₆ octahedron and two edges with LiO₆ octahedra. This will greatly limit the transmission speed of the lithium-ions. Monoclinic Li₃V₂(PO₄)₃ possesses a NASICON-type structure consisting of slightly distorted VO₆ octahedra and PO₄ tetrahedra.^18–20 Li₃V₂(PO₄)₃ contains three independent lithium sites. The large polyanions replace the smaller O²⁻ ions in this open framework so as to stabilize the structure and allow fast ion migration. As the VO₆ octahedra are separated by PO₄ tetrahedra, the VO₆ octahedra cannot directly connect to each other, which limits the electronic conductivity.^21–26 XRD profiles and the structures of olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ are shown in Fig. 1. The other basic properties are shown in Table 1.²⁸

Fig. 1 The crystal structure of olivine LiFePO₄ projected along [001]²⁷ and its XRD profile (a); the crystal structure of monoclinic Li₃V₂(PO₄)₃ (ref. 18) and its XRD profile (b).

Table 1 Basic properties of olivine LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ (ref. 28)^a

	Olivine LiFePO4 (LFP)	Monoclinic Li3V2(PO4)3 (LVP)
a Note: WP: working potential; C: capacity at low C rates (<0.1 C); Ed: energy density; Ec: electrical conductivity.
WP (V vs. Li/Li^+)	3.45	3.62, 3.68, 4.08 and 4.55
C (mA h g^−1)	170 (theoretical), 150 (practical)	197 (theoretical), 160–170 (practical)
Ed	590 (W h kg^−1) (theoretical), 520 (W h kg^−1) (practical)	2330 mW h cm^−3 after carbon coating
Ec (S cm^−1)	10^−9 to 10^−8	2.3 × 10^−8
Cost ($ kg^−1)	20–25; possibly low, but limited by patent issues and process cost	Higher than LFP

---

Enhancing the electrochemical performance of LiFePO₄–Li₃V₂(PO₄)₃

LFP is cheap and environmentally friendly, and can be cycled thousands of times over years without decay, while monoclinic LVP offers the optimal combination of high operating voltage, high lithium capacity, good ion mobility, excellent thermal stability and the highest theoretical capacity of all the phosphates. They are potential cathode materials. However, both LFP and LVP have low electronic conductivities, which present a major drawback for their practical implementation in EVs or HEVs.

Many efforts have been made to improve the electrochemical performances of LFP and LVP, including nanosizing (so as to shorten the Li ion diffusion length in the solid state as well as decrease the anti-site defects to increase the Li ion conductivity²⁹),^30–36 conductive coating,^37,38 and doping modification.^9,11,39–42 Another method is preparing LiFePO₄-based compounds with the fast ion conductor additive Li₃V₂(PO₄)₃.^43–47

Conductive coating

Coating of LiFePO₄ and Li₃V₂(PO₄)₃ particles with electron and Li⁺ ion conductive materials is common, and carbon coating is an effective way to improve the electronic conductivity. Fig. 2 shows a schematic representation of conductive coating of LiFePO₄. Organic matter pyrolizes into carbon at high temperature, and the carbon increases the surface conductivity. Nanometer sized particles can be produced, expanding the conductive area which contributes to diffusion of lithium ions. The introduction of carbon can prevent the formation of Fe³⁺, and provides electron tunneling in the phosphate material to prevent grain growth, increasing the surface area and improving the electrochemical performance of the product. Barker et al.⁴⁹ first introduced a carbothermal reduction method to synthesize Li₃V₂(PO₄)₃/C using carbon black as the carbon source, and the electronic conductivity of the materials was greatly improved by the residual carbon. Pioneering work was carried out by Ravet et al.,^50,51 who showed that LiFePO₄ with a carbon coating can almost achieve its theoretical capacity.

Fig. 2 Schematic representation of LiFePO₄ nanoparticles fully coated using (a) an ionic conductivity layer and (b) a carbon layer.⁴⁸

Single doping

Regarding lithium iron phosphate and lithium vanadium phosphate modification, doping is necessary to enhance their electrochemical properties, and previous work has demonstrated that this improves the electrochemical performance. It is necessary to take into account the structure of the materials before doping. Experiments have shown that the conductivity of Li_1−xM_z+xFePO₄ is 10⁸ times that of the undoped sample. The XRD results show that the doping in the structure allows charge compensation by means of point defects.^14,52 Research on LiFePO₄ defect chemistry has provided some acceptable defect compensation mechanisms, as shown in Table 2.⁵³ Ions tend to be substituted by isovalent ions. A variety of dopants with charges of +1 to +6, including V⁵⁺,^54,55 Ti⁴⁺,^56–58 Cr³⁺,⁵⁹ Al³⁺,^60–62 Nb⁵⁺,^63,64 Zn²⁺,⁹ Mg²⁺,^65–67 Mo⁶⁺,⁶⁸ and La³⁺,⁶⁹ have been employed for both Li⁺ and M²⁺ substitution in the nano-sized LiFePO₄ or Li₃V₂(PO₄)₃ systems. Low favorable energies were only obtained for Na⁺ substitution in the Li⁺ sites and isovalent dopants (e.g., Mg²⁺) in the M²⁺ sites. In contrast, aliovalent doping appears to be unfavorable in both the Li⁺ and M²⁺ sites in all four phases.⁷⁰ The SEM images of LiFe_0.95V_0.05PO₄ with their elemental mappings (Fig. 3a1–a4) and the XRD patterns of M-doped LiFePO₄ samples (M = Ni, V, Mg, Al) (Fig. 3b) verify that the doping element is incorporated in the lithium iron phosphate crystals.⁷¹

Table 2 Defect compensation mechanisms for LiFePO₄ (ref. 53)

Ideal crystal composition	Defect compensation mechanism	Defect compensation Kroger–Vink notation
Li1−nyMy^n+FePO4	Li-substitution & Li-vacancy compensation
Li1−(n−2)yMy^n+Fe1−yPO4	Fe-substitution & Li-vacancy compensation
Li1−yMy^n+Fe1−(n−1)y/2PO4	Li-substitution & Fe-vacancy compensation
LiMy^n+Fe1−ny/2PO4	Fe-substitution & Fe-vacancy compensation
LiFePO4 + MxOy	Stoichiometric & impurity	Undetermined

---

Fig. 3 Elemental mapping and SEM image for an LiFe_0.95V_0.05PO₄ sample. (a1) SEM image of an LiFe_0.95V_0.05PO₄ sample, (a2) elemental mapping for Fe, (a3) elemental mapping for P, (a4) elemental mapping for V; (b) XRD patterns of M-doped LiFePO₄ samples (M = Ni, V, Mg, Al).⁷¹

After modification, the high rate performances or cycling performances of the two materials were improved. Na-doped Li_3−xNa_xV₂(PO₄)₃/C (x = 0.00, 0.01, 0.03, and 0.05) compounds were synthesized using a sol–gel method. Li_2.97Na_0.03V₂(PO₄)₃/C showed the highest initial capacity of 118.9 mA h g⁻¹, and the capacity retention rate was 88% after 80 cycles at 2.0C (Fig. 4a1 and a2).⁷² Li₃V_2−xSn_x(PO₄)₃/C cathode materials were synthesized quickly by a microwave solid-state synthesis method. At a discharge rate of 0.5C in the potential range of 2.5–4.5 V at room temperature, the initial discharge capacity of Li₃V_1.95Sn_0.05(PO₄)₃/C was 136 mA h g⁻¹ (Fig. 4b1 and b2).⁷³ Addition of Cu and Ag to an LiFePO₄ electrode did not affect the structure of the LiFePO₄ electrode material, but it contributed to the growth of small particles and reduced the resistance between the particles, improving the kinetics in terms of capacity delivery and cycle life.³² LFP particles were synthesized by a solid state reaction method and ball milled with polyethylene glycol (PEG) based ZnO nanopowders to form ZnO/C co-coated LFP particles. An LFP/PEG/ZnO (2 wt%) composite electrode showed maximum discharge capacities of 158.9 mA h g⁻¹ at 0.1C, 145.7 mA h g⁻¹ at 5C and 109.3 mA h g⁻¹ at 10C. The discharge capacity of the LFP/PEG/ZnO (2 wt%) composite electrode remained stable for 50 cycles at a 0.1C rate (Fig. 5a1 and a2).⁷⁴ Ti⁴⁺-doped LiFePO₄ was prepared by an ambient-reduction and post-sintering method. The electrochemical properties of lithium iron phosphate were significantly improved with Ti⁴⁺-doping. The Ti⁴⁺-doped sample sintered at 600 °C delivered initial discharge capacities of 150, 130 and 125 mA h g⁻¹ at 0.1C, 1C and 2C rates, respectively, and there was a negligible drop in the capacity after 40 cycles (Fig. 5b1 and b2).⁷⁵ Mg-doped LiFePO₄ and pure LiFePO₄ were prepared by a low-temperature sol–gel method using succinic acid as a chelating agent. LiMg_0.05Fe_0.95PO₄ showed initial charge and discharge capacities at a 0.2C rate of 159 and 141 mA h g⁻¹ respectively, compared to 121 and 107 mA h g⁻¹ obtained for pure LiFePO₄. After 60 cycles, the capacity retention rate of LiMg_0.05Fe_0.95PO₄ was still more than 89%.⁶⁷ An olivine-type lanthanum and magnesium-doped Li_0.99La_0.01Fe_0.9Mg_0.1PO₄/carbon aerogel composite was synthesized via a simple solution impregnation process using carbon aerogel (CA) as a template. At room temperature (20 °C), the capacity reached 160.2 mA h g⁻¹, 154.3 mA h g⁻¹, 142.7 mA h g⁻¹, 135.7 mA h g⁻¹, 124.3 mA h g⁻¹ and 101.8 mA h g⁻¹ at discharge rates of 0.2C, 1C, 5C, 10C, 20C and 50C, respectively (Fig. 5c1 and c2).⁷⁶ Dy-doping and carbon coating were used to synthesize an LiFePO₄ cathode material in a simple solution. The LiDy_0.02Fe_0.98PO₄/C composite cathode showed an initial discharge capacity of 153 mA h g⁻¹ at 0.1C. The electronic conductivity of Dy-doped LiFePO₄/C was enhanced to 1.9 × 10⁻² S cm⁻¹.⁷⁷

Fig. 4 Charge–discharge and cycling performances of samples of doped Li₃V₂(PO₄)₃ compounds (initial charge–discharge curves of Li_3−xNa_xV₂(PO₄)₃/C (x = 0.00, 0.01, 0.03, and 0.05) (a1), and rate capabilities at 0.2C, 0.5C, 1.0C and 2.0C (a2); initial charge–discharge curves of Li₃V_2−xSn_x(PO₄)₃/C (x = 0, 0.01, 0.02, 0.05, and 0.10) at a 0.5C rate in a voltage range between 2.5 and 4.5 V (b1), and cycling performance comparisons (b2)).^72,73

Fig. 5 Charge–discharge curves and cycling performances for doped LiFePO₄ (discharge capacity of LFP/PEG/ZnO (2 wt%) at different C-rates (0.1–10C) (a1), cycling performance comparison of LFP, LFP/PEG and LFP/PEG/ZnO discharge capacities at 0.1C (a2); initial discharge curves (b1) and cycling performances (b2) of LiFePO₄ and Ti⁴⁺-doped LiFePO₄ synthesized at 600 °C; discharge curves (c1) and cycling stability (c2) of doped LFP/CA at room temperature).^74–76

Single dopants play a big role in improving the electrochemical properties of both LiFePO₄ and Li₃V₂(PO₄)₃. The single doping element is uniformly distributed over all the particles; homogeneous doping can effectively inhibit the growth of crystalline grains and reduce the crystal particle diameter. If the dopant has a smaller radius than Fe, V, or Li, the composition powder has smaller lattice constants and unit cell volume than LFP or LVP. On the contrary, if the radius of the dopant is larger, the lattice constants and unit cell volume of the composition powder are larger. In addition, single dopants do not alter the structure of the material and result in electron holes in the structure, which are beneficial to the electrochemical performances of cathode materials.

Dual (mutual) doping and composite material modification

Because the structures and preparation methods of LiFePO₄ and Li₃V₂(PO₄)₃ are similar, Fe²⁺ and V³⁺ mutual doping may be unavoidable during the calcining process for preparing LiFePO₄ and Li₃V₂(PO₄)₃ composite materials. That is, Fe²⁺ (and/or Li⁺) in LiFePO₄ may be substituted by V,^55,78 and V³⁺ (and/or Li⁺) in Li₃V₂(PO₄)₃ may be substituted by Fe.²¹ The XRD pattern (Fig. 6a) and the EDS spectra of regions i (Fig. 6c) and ii (Fig. 6d) in the TEM image of 9LiFePO₄·Li₃V₂(PO₄)₃/C (Fig. 6b) correspond to V-doped LiFePO₄ and Fe-doped Li₃V₂(PO₄)₃, indicating that the synthesized compound is a mixture of LiFePO₄ and Li₃V₂(PO₄)₃.^79,80

Fig. 6 XRD pattern of LiFePO₄–Li₃V₂(PO₄)₃ synthesized at 700 °C for 12 h (a); TEM image of 9LiFePO₄·Li₃V₂(PO₄)₃/C (b); EDS spectra of regions i (c) and ii (d) (regions i and ii are shown in (b)).^79,80

LiFe_1−xV_xPO₄/C samples were synthesized using a two-step solid-state reaction route. Experiments showed that V incorporation significantly enhances the electrochemical performance of LiFePO₄. Particularly, the LiFePO₄/C sample with 5 wt% vanadium-doping showed the best performance with a specific discharge capacity of 129 mA h g⁻¹ at 5C after 50 cycles; the capacity retention rate was over 97.5% at 0.1C, 1C, 2C and 5C (Fig. 7a1 and a2).⁴⁵ Mg²⁺, Ni²⁺, Al³⁺, or V³⁺ ions, which have atomic radii similar to or smaller than that of the Fe²⁺ ion, were doped into the Fe sites to synthesize LiFe_0.95M_0.05PO₄ samples using a solution method. All samples contained a carbon content of about 3 wt% and had similar Brunauer–Emmett–Teller surface areas. The LiFe_0.95V_0.05PO₄ powder had the largest unit cell volume (longest Li–O bond lengths) and exhibited the highest discharge capacities of 152 and 136 mA h g⁻¹ at 0.1C and 1C rates, respectively (Fig. 7b1 and b2).⁷¹ Fe-doped Li₃V₂(PO₄)₃ cathode materials for Li-ion batteries were synthesized by a conventional solid-state reaction. The initial discharge capacity of Li₃Fe_0.02V_1.98(PO₄)₃ was 177 mA h g⁻¹, and after the 80th cycle it was 126 mA h g⁻¹. The retention rate of the discharge capacity was about 71%, much higher than that of the undoped system (58%) (Fig. 7c1 and c2).²¹ 9LiFePO₄·Li₃V₂(PO₄)₃/C was synthesized via a carbon thermal reaction using petroleum coke as a reducing agent and carbon source. Fig. 8a1–a3 show the electrochemical properties. The first discharge capacity of 9LFP·LVP/C in 18650 type cells was 168 mA h g⁻¹ at 1C, and the material showed a high reversible discharge capacity of 125 mA h g⁻¹ at 10C, even after 150 cycles. At −20 °C, the reversible capacity of 9LFP·LVP/C was maintained at 75% of that at room temperature.⁴⁶ The hybrid materials xLiFePO₄·(1−x)Li₃V₂(PO₄)₃ were synthesized through a sol–gel method. The sample 0.7LiFePO₄·0.3Li₃V₂(PO₄)₃ showed the advantages of both LiFePO₄ and Li₃V₂(PO₄)₃, exhibiting initial discharge capacities of 166 mA h g⁻¹ at a 0.1C rate and 109 mA h g⁻¹ at a 20C rate, with a capacity retention rate of 73.3% and an excellent cycling stability. xLiFePO₄·(1−x)Li₃V₂(PO₄)₃ with x = 0, 0.3, 0.5, 0.7, and 1 correspond to samples A, B, C, D, and E in Fig. 8b2, respectively. The electrochemical performances of xLiFePO₄·(1−x)Li₃V₂(PO₄)₃ are shown in Fig. 8b1 and b2.⁸¹

Fig. 7 Charge–discharge and cycling performances of Fe²⁺ and V³⁺ mutually doped samples (charge–discharge profiles of samples at 0.1C (a1), and cycling performances of samples at a 5.0C rate (a2); the discharge curves of LiFe_0.95M_0.05PO₄ samples at 0.1C during the 10th cycle (b1), and the discharge curves of LiFe_0.95M_0.05PO₄ samples at a 1C rate during the 10th cycle (b2); typical charge–discharge curves of Fe-doped (a) and undoped (b) Li₃V₂(PO₄)₃ (c1), and discharge capacity as a function of cycle number at 0.2C and 25 °C for various Li₃Fe_xV_2−x(PO₄)₃ samples: (a) x = 0.00; (b) x = 0.01, (c) x = 0.02, (d) x = 0.04 and (e) x = 0.06 (c2)).^21,45,71

Fig. 8 Charge–discharge and cycling performances of xLiFePO₄·yLi₃V₂(PO₄)₃ composites (the first charge–discharge curves of the xLiFePO₄·yLi₃V₂(PO₄)₃/C composites (a1) and cycling performances of 9LiFePO₄·Li₃V₂(PO₄)₃/C at different discharge rates (a2) and at different working temperatures (a3); charge–discharge curves of sample D (b1) and cycling performances of the samples at different discharge rates (b2)).^46,81

Compared with single dopants, large numbers of defects can also be produced by dual dopants (especially by Fe and V); this would greatly improve the electrochemical properties of the materials. Li₃V₂(PO₄)₃ has an inherently high ionic conductivity, with the large polyanions helping to stabilize the structure as an open 3D framework and allowing fast ion migration. Binding LiFePO₄ and Li₃V₂(PO₄)₃ together would create more Li⁺ transport channels, so V-doped LFP and Fe-doped LVP play an important part in the electron transfer activities and the lithium ion diffusivities of the composite materials.

Future directions

Various aspects of the performances of lithium vanadium phosphate and lithium iron phosphate have gradually been explored, however there are plenty of problems that are still unresolved. These problems are classified as follows.

The relationship between the power performance and the electrode–electrolyte interface is still a mystery. The stability of nano-sized LiFePO₄ under atmospheric conditions is bad. Poor low temperature performance and low tap density seriously limit the volumetric power density of lithium-ion batteries for use in portable devices.

Further work is needed to ensure that lithium iron phosphate and lithium vanadium phosphate batteries can withstand operation in EVs, HEVs and other equipment.

Conclusion

Various modification strategies have improved the electrochemical properties of LiFePO₄ and Li₃V₂(PO₄)₃. Modification by doping and composite modification have been mainly explored in this article, and some obstacles to the use of LiFePO₄ and Li₃V₂(PO₄)₃ remain.

Single and dual dopants (which modify the composites in a different way) are compared in this article. The influence of single dopants on anode materials such as lithium iron phosphate and lithium vanadium phosphate has been explored. Composite materials are formed by dual dopants and exhibit the advantages of both materials. According to our comparison, single dopants paved the way for dual dopant modification.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (grant no. 51272144, 51172132 and 51042003), and we also thank the Taishan scholarship program in the field of glass and ceramics for the technological support.

References

1. M. Thackeray, Nat. Mater., 2002, 1, 81.
2. X. Y. Du, W. He, X. D. Zhang, Y. Z. Yue, H. Liu, X. G. Zhang, D. D. Min, X. X. Ge and Y. Du, J. Mater. Chem., 2012, 22, 5960.
3. J. J. Wang and X. L. Sun, Energy Environ. Sci., 2012, 5, 5163.
4. M. Takahashi, S. Tobishima, K. Takei and Y. Sakurai, J. Power Sources, 2001, 97–98, 508.
5. M. Takahashi, S. Tobishima, K. Takei and Y. Sakurai, Solid State Ionics, 2002, 148, 283.
6. P. P. Prosini, M. Lisi, D. Zane and M. Pasquali, Solid State Ionics, 2002, 148, 45.
7. S. C. Yin, P. S. Strobel, H. Grondey and L. F. Nazar, Chem. Mater., 2004, 16, 1456.
8. P. S. Herle, B. Ellis, N. Coombs and L. F. Nazar, Nat. Mater., 2004, 3, 147.
9. H. Liu, Q. Cao, L. J. Fu, C. Li, Y. P. Wu and H. Q. Wu, Electrochem. Commun., 2006, 8, 1553.
10. C. A. J. Fisher and M. S. Islam, J. Mater. Chem., 2008, 18, 1209.
11. S.-Y. Chung, J. T. Bloking and Y.-M. Chiang, Nat. Mater., 2002, 1, 123.
12. M. Wagemaker, B. L. Ellis, D. Lützenkirchen-Hecht, F. M. Mulder and L. F. Nazar, Chem. Mater., 2008, 20(20), 6313.
13. K.-S. Park, P. Xiao, S.-Y. Kim, A. Dylla, Y.-M. Choi, G. Henkelman, K. J. Stevenson and J. B. Goodenough, Chem. Mater., 2012, 24, 3212.
14. N. Meethong, Y.-H. Kao, S. A. Speakman and Y.-M. Chiang, Adv. Funct. Mater., 2009, 19, 1060.
15. M. S. Islam, D. J. Driscoll, C. A. J. Fisher and P. R. Slater, Chem. Mater., 2005, 17, 5085.
16. S.-I. Nishimura, Y. Kobayashi, K. Ohoyama, R. Kanno, M. Yashima and A. Yamada, Nat. Mater., 2008, 7, 707.
17. D. Morgan, A. Van der Van and G. Ceder, Electrochem. Solid-State Lett., 2004, 7, A30.
18. S. C. Yin, H. Grondey, P. Strobel, H. Huang and L. F. Nazar, J. Am. Chem. Soc., 2003, 125, 326.
19. M. Y. Saidi, J. Barker, H. Huang, J. L. Swoyer and G. Adamson, J. Power Sources, 2003, 266, 119.
20. X. H. Rui, N. Ding, J. Liu, C. Li and C. H. Chen, Electrochim. Acta, 2010, 55, 2384.
21. M. M. Ren, Z. Zhou, Y. Z. Li, X. P. Gao and J. Yan, J. Power Sources, 2006, 162, 1357.
22. M. Sato, H. Ohkawa, K. Yoshida, M. Saito, K. Uematsu and K. Toda, Solid State Ionics, 2000, 135, 137.
23. L. Zhang, X. L. Wang, J. Y. Xiang, Y. Zhou, S. J. Shi and J. P. Tu, J. Power Sources, 2010, 195, 5057.
24. L. S. Cahill, R. P. Chapman, J. F. Britten and G. R. Goward, J. Phys. Chem. B, 2006, 110, 7171.
25. M. Y. Saidi, J. Barker, H. Huang, J. L. Swoyer and G. Adamson, Electrochem. Solid-State Lett., 2002, 5, A149.
26. M. Y. Saidi, J. Barker, H. Huang, J. L. Swoyer and G. Adamson, J. Power Sources, 2003, 119–121, 266.
27. J. M. Tarascon and M. Armand, Nature, 2001, 414, 359.
28. O. K. Park, Y. Cho, S. Lee, H. C. Yoo, H. K. Song and J. Cho, Energy Environ. Sci., 2011, 4, 1621.
29. B. Xu, D. N. Qian, Z. Y. Wang and Y. S. Meng, Mater. Sci. Eng., R, 2012, 73, 51.
30. A. Yamada, S. C. Chung and K. Hinokuma, J. Electrochem. Soc., 2001, 148, A224.
31. C. Delacourt, P. Poizot, S. Levasseur and C. Masquelier, Electrochem. Solid-State Lett., 2006, 9, A352.
32. F. Croce, A. D. Epifanio, J. Hassoun, A. Deptul, T. Olczac and B. Scrosati, Electrochem. Solid-State Lett., 2002, 5, A47.
33. Y. Q. Hu, M. M. Doeff, R. Kostecki and R. Finones, J. Electrochem. Soc., 2004, 151, A1279.
34. K. S. Park, J. T. Son, H. T. Chung, S. J. Kim, C. H. Lee and H. G. Kim, Electrochem. Commun., 2003, 5, 839.
35. G. Meligrana, C. Gerbaldi, A. Tuel, S. Bodoardo and N. Penazzi, J. Power Sources, 2006, 160, 516.
36. S. F. Yang, P. Y. Zavalij and M. S. Whittingham, Electrochem. Commun., 2001, 3, 505.
37. P. P. Prosini, D. Zane and M. Pasquali, Electrochim. Acta, 2001, 46, 3517.
38. Z. Chen and J. R. Dahn, J. Electrochem. Soc., 2002, 149, A1184.
39. D. Y. Wang, H. Li, S. Q. Shi, X. J. Huang and L. Q. Chen, Electrochim. Acta, 2005, 50, 2955.
40. J. Barker, M. Y. Saidi and J. L. Swoyer, Electrochem. Solid-State Lett., 2003, 6, A53.
41. N. Hua, C. Y. Wang, X. Y. Kang, T. Wumair and Y. Han, J. Alloys Compd., 2010, 503, 204.
42. X. Q. Ou, G. C. Liang, J. S. Liang, S. Z. Xu and X. Zhao, Chin. Chem. Lett., 2008, 19, 345.
43. J. C. Zheng, X. H. Li, Z. X. Wang, J. H. Li, L. J. Li, L. Wu and H. J. Guo, Ionics, 2009, 15, 753.
44. M. R. Yang, W. H. Ke and S. H. Wu, J. Power Sources, 2007, 165, 646.
45. L. L. Zhang and G. Liang, J. Phys. Chem. C, 2011, 115, 13520–13527.
46. J. Y. Xiang, J. P. Tu, L. Zhang, X. L. Wang, Y. Zhou, Y. Q. Qiao and Y. Lu, J. Power Sources, 2010, 195, 8331.
47. L. Wang, Z. Li, H. Xu and K. Zhang, J. Phys. Chem. C, 2008, 112, 308.
48. Y. G. Wang, P. He and H. S. Zhou, Energy Environ. Sci., 2011, 4, 805.
49. J. Barker, M. Y. Saidi and J. L. Swoyer, J. Electrochem. Soc., 2003, 150, A684.
50. N. Ravet, J. B. Goodenough, S. Besner, M. Simoneau, P. Hovington and M. Armand, J. Electrochem. Soc., 1999, 99(2), 172.
51. N. Ravet, Y. Chouinard, J. F. Magnan, S. Besner, M. Gauthier and M. Armand, J. Power Sources, 2001, 503, 97.
52. A. Goni, L. Lezama and A. Pujana, Int. J. Inorg. Mater., 2001, 3, 937.
53. A. Goni, L. Lezama and M. I. Arriortua, J. Mater. Chem., 2000, 10, 423.
54. C. S. Sun, Z. Zhou, Z. G. Xu, D. G. Wang, J. P. Wei, X. K. Bian and J. Yan, J. Power Sources, 2009, 193, 841.
55. J. Hong, C. S. Wang, X. Chen, S. Upreti and M. S. Whittingham, Electrochem. Solid-State Lett., 2009, 12, A33.
56. G. Wang, Y. Cheng, M. Yan and Z. Jiang, J. Solid State Electrochem., 2007, 11, 457.
57. S. Wu, M. Chen, C. Chien and Y. Fu, J. Power Sources, 2009, 189, 440.
58. L. Li, X. Li, Z. Wang, L. Wu, J. Zheng and H. Guo, J. Phys. Chem. Solids, 2009, 70, 238.
59. H. C. Shin, S. B. Park, H. Jang, K. Y. Chung, W. I. Cho, C. S. Kim and B. W. Cho, Electrochim. Acta, 2008, 53, 7946.
60. K. Hsu, S. Tsay and B. Hwang, J. Power Sources, 2005, 146, 529.
61. H. Xie and Z. Zhou, Electrochim. Acta, 2006, 51, 2063.
62. R. Amin, C. Lin and J. Maier, Phys. Chem. Chem. Phys., 2008, 10, 3519.
63. Z. D. Gao, Z. X. Bing, X. Jian, T. Jian, Z. T. Jun and C. G. Shao, Acta Phys.-Chim. Sin., 2006, 22, 840.
64. Z. Li, Z. M. Shou, W. D. Dan, S. Ou, D. Rui-Ping and M. Jian, Chin. J. Inorg. Chem., 2009, 25, 1724.
65. X. Ou, G. Liang, L. Wang, S. Xu and X. Zhao, J. Power Sources, 2008, 184, 543.
66. S. Yang, Y. Liu, Y. Yin, H. Wang and C. Cui, J. Inorg. Mater., 2007, 22, 627.
67. D. Arumugam, G. P. Kalaignan and P. Manisankar, J. Solid State Electrochem., 2009, 13, 301.
68. C. Yu, W. Z. Li, Y. C. Yang, X. D. Guo and W. Z. Yu, Acta Phys.-Chim. Sin., 2008, 24, 1498.
69. Y. Cho, G. T. Fey and H. Kao, J. Solid State Electrochem., 2008, 12, 815.
70. C. A. J. Fisher, V. M. H. Prieto and M. S. Islam, Chem. Mater., 2008, 20, 5907.
71. M. R. Yang and W. H. Ke, J. Electrochem. Soc., 2008, 155, A729.
72. Q. Kuang, Y. M. Zhao and Z. Y. Liang, J. Power Sources, 2011, 196, 10169.
73. H. Liu, S. F. Bi, G. W. Wen, X. G. Teng, P. Gao, Z. J. Ni, Y. M. Zhu and F. Zhang, J. Alloys Compd., 2012, 543, 99.
74. J. Lee, P. Kumar, J. Lee, B. M. Moudgil and R. K. Singh, J. Alloys Compd., 2013, 550, 536.
75. L. Wu, Z. X. Wang, X. H. Li, L. J. Li, H. J. Guo, J. C. Zheng and X. J. Wang, Trans. Nonferrous Met. Soc. China, 2010, 20, 814.
76. H. Zhang, Y. L. Xu, C. J. Zhao, X. Yang and Q. Jiang, Electrochim. Acta, 2012, 83, 341.
77. H. Göktepe, J. Chin. Chem. Soc., 2013, 60, 218–222.
78. J. Ma, B. Li, H. Du, C. Xu and F. Kang, J. Electrochem. Soc., 2011, 158, A26.
79. J. C. Zheng, X. H. Li, Z. X. Wang, S. S. Niu, D. R. Liu, L. Wu, L. J. Li, J. H. Li and H. J. Guo, J. Power Sources, 2010, 195, 2935.
80. S. K. Zhong, L. Wu and J. Q. Liu, Electrochim. Acta, 2012, 74, 8.
81. H. Tang, X. D. Guo and B. H. Zhong, J. Solid State Electrochem., 2012, 16, 1537.

---