Facile Synthesis for LiFePO4 Nanospheres in Tridimensional Porous

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Facile Synthesis for LiFePO4 Nanospheres in Tridimensional Porous Carbon Framework for Lithium Ion Batteries Jianqing Zhao, Jianping He,* Jianhua Zhou, Yunxia Guo, Tao Wang, Shicao Wu, Xiaochun Ding, Ruiming Huang, and Hairong Xue College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing, Jiangsu 210016, P. R. China

bS Supporting Information ABSTRACT: The excellent electronic conductivity and high surface area are two crucial factors for electrode materials to achieve high energy and power capabilities in lithium ion batteries. This article presents a feasible method to obtain nanospherical electrode materials in the versatile carbon framework. A facile synthesis has been developed to prepare LiFePO4 nanospheres with an average diameter of ∼300 nm lodged in the tridimensional (3D) porous carbon structure. This LiFePO4/C composite possesses the considerably enhanced electronic conductivity of ∼10-2 Scm-1 and amazing high surface area of 200.5 m2g-1, and the lithium ion diffusion coefficient of ∼10-15-10-14 cm2s-1 is calculated. The LiFePO4/C cathode material delivers discharge capacities of 155.0 mAhg-1 at 0.1 C and 69.5 mAhg-1 at 20 C. Furthermore, the pristine LiFePO4/C entity has exhibited discharge capacity of 127.8 mAhg-1 at 0.1 C without conductive carbon additives.

’ INTRODUCTION Lithium iron phosphate has shown various advantages in terms of high theoretical capacity, outstanding high-rate capability, prominent long-term cycle performance, safety, environmental benignity, and low-cost raw material, and is ongoing to occupy the rapid demand in the market for power portable electronic devices and plug-in hybrid electric vehicles.1-8 However, the state-of-the-art development of LiFePO4 cannot perfectly match the required high power density in lithium ion batteries for high-end consumer need. The most challenging issue of LiFePO4 is its extremely poor electronic conductivity in nature.9-12 Numerous effective approaches have been investigated to circumvent this main drawback by ameliorating the intrinsic character of bulk LiFePO4 or with extrinsic modifications, including metallic cation doping in crystal,9,10 typical carbon coating or fabricating a carbonaceous matrix,13-19 mixing with noble metal particles,20 and introducing conductive inorganic compounds21,22 or organic polymers.23 It is also suggested that emphases should be placed on lithium ion diffusion kinetics, the direct aspect related to superior high-power electrochemical performance and properties. The ability of lithium ions to travel across the interface between LiFePO4 and electrolyte phases is crucial for ultrafast diffusion,11 and aliovalent metallic cation doping has been demonstrated to accelerate the mobility of lithium ions.24 However, perfect surface coating and intradoping in crystal are difficult to achieve. Creating large surface area in nanoscale electrode materials can contribute to the high-rate capability by shortening the diffusion path length of lithium ions r 2011 American Chemical Society

and maximizing the interfacial contact between active particles and the surrounding electrolyte.25,26 In general, the electrochemical performance of electrode materials can be affected by different properties,27-29 involving purity, crystallinity, morphology, structure, particle size, surface area, and carbon content, and so on. From the previous investigations,14,16-19,30 the two-phase LiFePO4/C composite has been considered as an alternative optimum structure design for cathode material by combining host LiFePO4 particles with guest carbon framework into one incorporated entity for lithium ion storage.19 The solid carbon fabric can significantly restrict the growth of LiFePO4 particles on the nanometer scale in synthesis to shorten the diffusion path of lithium ions and effectively prevent unfavorable aggregations of active material in usage. Furthermore, the excellent electronic conductivity and high surface area provided by the constructed 3D porous carbon framework are favorable to achieve high energy and power capacities in lithium ion batteries. The 3D carbon network can bridge embedded LiFePO4 nanoparticles to overcome the key limitation of extremely low charge transfer, and the sufficient porosity in high-surface-area carbon structure facilitates the immersion of electrolyte and its subsequent accommodation to ensure the facile diffusion of lithium ions for high-rate capabilities. The rigid carbon skeleton is also a scaffold to release the strain on/from Received: September 2, 2010 Revised: December 26, 2010 Published: January 25, 2011 2888

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The Journal of Physical Chemistry C LiFePO4/FePO4 two-phase transformation during the lithium ion insertion/extraction.18 To create this versatile carbon framework, sort of high carbon content cannot be inevitable, and more efforts are being taken to solve this dilemma17,18. Tuning the morphology and structure to obtain novel LiFePO4/C composite is a very exciting route to pursue energy and power versatility in electrochemical performance. Both hard and soft templates, involving carbon monolith16, colloidal crystal17, triblock copolymer,16,18 and citric acid,31 have been employed to prepare the porous carbon frameworks for lodging LiFePO4 nanoparticles. Template approaches always suffer from disadvantages related to high cost and complicated synthetic procedures that are difficult to expand to large-scale commercial applications26,32. Therefore, the simple methodology for preparing LiFePO4/C composite with LiFePO4 nanoparticles in appropriate carbon structure should be developed. Moreover, spherical LiFePO4 nanoparticles have been demonstrated to improve its high volumetric density in working electrodes and increase reaction active sites for electrode materials, which benefit the enhancement for electrochemical performance33-35. Herein, the onestep preparation of LiFePO4 nanospheres within a 3D porous carbon framework has been significantly optimized using glucose as carbon source. LiFePO4 nanospheres with an average diameter of ∼300 nm are homogeneously lodged in the 3D carbon framework full of macro/mesoporous pores. The LiFePO4/C composite possesses the considerably enhanced electronic conductivity, remarkable high surface area, and an improved lithium ion diffusion coefficient.

’ EXPERIMENTAL SECTION Synthesis of LiFePO4/C Composite. A facile ball-milling technique was adopted to prepare LiFePO4/C electrode material, followed by the typical carbothermal reaction. Stoichiometric amounts of precursors Fe(NO3)3 3 9H2O, CH3COOLi 3 2H2O, NH4H2PO4, and C6H12O6 (in molar ratio = 1:1:1:2) were homogeneously mixed in ethanol by ball milling in 4 h and subsequently aged for 12 h in tanks after operations. After being heated in a vacuum oven at 80 °C for 24 h, the dried mixtures were sintered at 700 °C in the tube furnace for 10 h with a heating rate of 5 °C/min under flowing nitrogen, and the products were obtained when cooling at room temperature. The pure carbon component in LiFePO4/C composite was separated by the removal of LiFePO4 particles in concentrated HCl solution, followed by centrifugations, washing-up with distilled water several times, and desiccations. To remove the carbon framework, LiFePO4/C powders were heated in air at 700 °C in 2 h. Characterizations. The carbon content in LiFePO4/C composite was determined by Thermogravimetric(TG)/Differential Thermal Analysis (DTA) on a PerkinElmer TGA 7 instrument in a temperature range from ambient to 900 °C under flowing oxygen. The crystallographic structure and purity of LiFePO4/C composite was examined by powder X-ray diffraction (XRD) on a Bruker D8 X-ray diffraction meter with monochromatic Cu KR radiation. The particle size and surface morphology were observed using a Hitachi S-4800 field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) images were captured on the JEM-2100 instrument microscopy at an acceleration voltage of 200 kV to investigate the diameter and distribution of LiFePO4 particles within the carbon framework as well as the characteristics of the carbon coating layer. The properties of the porous system in final products were estimated

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by the nitrogen adsorption and desorption isotherms at 77 K applying a Micromeritics ASAP 2010 instrument. The specific surface area was calculated using the Brunauer-Emmet-Teller (BET) method. The electronic conductivity of the LiFePO4/C composite was measured by the four-point dc method on diskshaped samples in diameter of 10 mm lapped to 900 μm thicknesses on the Wentworth Laboratories Aspect L1 detector. The element contents in the separated C framework were analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES) on a PerkinElmer Optima 5300DV analyzer. Electrochemical Measurements. The traditional cathode electrodes (abbreviated as T-LEP/C) were composed of 75% LiFePO4/C active material, 15% acetylene black (conductive carbon), and 10% poly vinylidenefluoride (PVDF) as a binder, dissolved in N-methyl-2-pyrrolidone (NMP) solvent. The resultant viscous slurry was coated onto the aluminum current collector and dried at 120 °C overnight under vacuum. 3 mg active material was loaded in the circular working electrode with a diameter of 16 mm. The pure carbon electrodes (abbreviated as P-C) were prepared with the separated carbon member as another cathode material mixed with conductive carbon and PVDF binder at a weight ratio of 75:15:10, and the bare LiFePO4/C working electrodes (abbreviated as B-LEP/C) without conductive carbon additives consisted of 90% LiFePO4/C powders and 10% PVDF binders. The electrochemical performance and properties of three different working electrodes were carried out in two-electrode system with the prepared electrode as the cathode electrode, metallic lithium foil as the counter and reference electrode, and Celgard-2400 membrane as the separator. The homemade testing cells were assembled in an argon-filled glovebox. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volumetric ratio of 1:1. Galvanostatic charge/discharge reactions were performed in the voltage range of 2.5-4.2 V on the CT2001A LAND Battery Tester. The electrochemical storage capacities of samples were calculated on the mass of active materials. The cyclic voltammetry (CV) of T-LEP/C were carried out at a set of scan rates of 0.05, 0.1, 0.5, 1, 5, 10, 25, and 50 mVs-1 respectively on the electrochemical workstation (CHI 660C). The lithium ion diffusion coefficient (DLiþ) was calculated from the relationship between anodic/cathodic peak currents (ip.a/ip.c) and the square root of scan rates (v1/2) in CV measurements.

’ RESULTS AND DISCUSSION A one-step ball milling has been applied to synthesize LiFePO4 nanospheres in the tridimensional porous carbon framework, followed by the carbothermal reduction reaction for the feasibility of mass production.7,13 It is worth noting that the enhanced electronic conductivity of ∼10-2 Scm-1 and amazing high surface area of 200.5 m2g-1 have been achieved from the 3D carbon structure with great porosity in the prepared LiFePO4/C composite, to the best of our knowledge, both of which are superior in LiFePO4/C analogues.14,16-18,30,31 The XRD pattern of LiFePO4/C powders in Figure 1 illustrates the good crystallinity and pure phase of LiFePO4 active material. All intense peaks are well indexed to the olivine orthorhombic structure (space group Pnmb), and its lattice parameters of crystal cell are generated: a = 6.013 Å, b = 10.325 Å, and c = 4.708 Å. By comparison with that of standard JCPDS Card No. 40-4199: a = 6.0189 Å, b = 10.347 Å, and 2889

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The Journal of Physical Chemistry C c = 4.7039 Å, the result illuminates that the crystalline growth of embedded LiFePO4 particles has been constrained in the constructed rigid carbon structure, indicating intimate incorporation between lithium iron phosphate and carbon framework, which can favor electron/ion transfer through the two-phase interface. The carbon content in the LiFePO4/C composite is calculated of 36.14% (Figure S1 of the Supporting Information)36,37, and the broad peaks corresponding to carbon are not visible, which can be probably attributed to its amorphous phase and the high pack of LiFePO4 nanospheres in the established carbon structure.18 As shown in parts a and b of Figure 2, LiFePO4 nanospheres with an average diameter of ∼300 nm are densely embedded in the 3D carbon framework and ornamented on its whole surface. Part c of Figure 2 displays the high-resolution TEM image of LiFePO4/C composite, indicating that numerous smaller LiFePO4 nanospheres are fully filled in interspaces of the larger spheres in the continuous porous carbon matrix, which has been exhibited from a broken profile in LiFePO4/C particles (insert

Figure 1. XRD pattern of the prepared LiFePO4/C powders.

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part I of part a of Figure 2). This abundant distribution of LiFePO4 nanospheres has contributed to the favorable tap density of 1.0 gcm-3 for this cathode material to guarantee the high energy density in working electrodes. From the deep observation in part d of Figure 2, a refined carbon coating layer is simultaneously elaborated on the surface of LiFePO4 spheres with a thickness of ∼5 nm. The fabrication of 3D carbon framework in combination with the perfect carbon coating layer can substantially facilitate charge transfer for lodged LiFePO4 nanospheres. The enhanced electronic conductance of ∼10-2 Scm-1 has been achieved to ensure the excellent high-rate capabilities in electrochemical performance. The homogeneous distribution of the continuous guest carbon framework and host LiFePO4 lodgers is demonstrated by the related elemental mappings of C, Fe, P, and O (Figure S2 of the Supporting Information). When the carbon component was absolutely burned in air at 700 °C, the corresponding residual compounds from oxidated LiFePO4 nanospheres are Li3Fe2(PO4)3 and Fe2O3, and preserve well in spherical morphology in parts a and b of Figure S3 of the Supporting Information36,37. On the other hand, the remaining circular trails can also be observed on the surface of separated carbon framework when LiFePO4 nanospheres were completely dissolved in concentrated HCl solution (parts c and d of Figure S3 of the Supporting Information). The typical hysteresis in nitrogen adsorption/desorption isotherms in Figure 3 reveals the porous characteristic of this intriguing LiFePO4/C composite with abundant macro/mesopores. The amazing BET surface area is arrived at 200.5 m2 g-1, which is obviously higher than that of previous LiFePO4/C products.14,16-18,26,30,31 The remarkable high surface area is probably derived from the porous carbon framework and highdispersed LiFePO4 nanospheres (Figure 2). The sufficient porosity in the carbon structure is attributed to the vigorous

Figure 2. Morphology and structure of the LiFePO4/C composite. (a) FESEM and (b) TEM micrographs of LiFePO4/C particles, (c) High-resolution TEM observation in the continuous porous carbon matrix, and (d) high-resolution lattice image of LiFePO4 nanospheres. 2890

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Figure 3. Nitrogen adsorption/desorption isotherms of the LiFePO4/ C composite.

Figure 4. Schematic diagram of the LiFePO4/C composite in the 3D structure (left) and the 2D profile (right).

gas release generated from decompositions of nitrate and ammonium dihydrogen phosphate and the pyrolysis of organic compounds during sintering treatment25,30,32, and the high surface area also benefits from LiFePO4 particles’ spherical type, nanometer size and dense distribution in/on the 3D carbon monolith. The separated porous carbon framework delivers the considerably large surface area of 720.9 m2/g with the removal of LiFePO4 member (part a of Figure S4 of the Supporting Information), and the corresponding 7.9 m2/g from oxidated LiFePO4 nanospheres is obtained when the carbon crust was burned in air condition (part b of Figure S4 of the Supporting Information). Overall, the significant porosity in cathode materials can facilitate the access and accommodation of electrolyte and shorten the diffusion length of lithium ions to achieve high power density in electrode materials.25,26 As discussed above, the LiFePO4/C entity has exhibited favorable tap density, excellent electronic conductivity, and notable porosity. It is visible in schematic diagram in Figure 4 that LiFePO4 nanospheres are tightly lodged in the 3D porous carbon framework. Nanospherical LiFePO4 can contribute to shorten diffusion path of lithium ions and offer more active sites for electrochemical reactions,33-35 and furthermore the established carbon fabric can provide various advantages in terms of serving as a conductive network for ultrafast electron transfer, a

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capacious container for sufficient electrolyte penetration and a rigid scaffold to release the negative volumetric force from delithiation and lithiation procedures.18,19 Therefore, the LiFePO4/C composite possesses potential capability as a promising cathode material for improved high-rate performance and related reversible storage capacities. It is suggested that the mechanical activation from ball-milling for reactive precursors and subsequent aging in tanks creates the charming morphology and configuration of LiFePO4/C composite, because the elementary structure of this composite has been fabricated before calcinations (part e of Figure S3 of the Supporting Information). The precursors can be homogenously mixed in ethanol solvent by ball-milling, and the generated high temperature and pressure in sealed stainless steel tanks after operations provide the solvothermal condition for the initial nucleation of spherical LiFePO4 nanoparticles and formation of the carbon framework.28 The electrochemical performance and properties of three different working electrodes, T-LEP/C, B-LEP/C, and P-C, were carried out. The rate characteristics of T-LEP/C are performed in part a of Figure 5, and the C-rates and storage capacities are calculated based on the mass of LiFePO4 with the amount of carbon being subtracted. The initial charge/discharge reactions deliver specific capacities of 157.4 and 155.0 mAhg-1 at 0.1 C, respectively. A high Coulombic efficiency of 98.5% is achieved in accordance with a pair of flat voltage plateaus positioned at near 3.4 V in symmetric charge/discharge curves. The remarkable reversibility is probably attributed to the enhanced electronic and ionic conductivities. As mentioned above, the outstanding electrochemical performance of T-LEP/C profits from the refined LiFePO4 nanospheres in combination with the created carbon framework. The spherical morphology of LiFePO4 particles has been demonstrated to create the more isotropic active sites for lithium ion transportation as comparison to other shapeless structures.3 Furthermore, LiFePO4 spheres in nanometer size can significantly shorten the path way for lithium ion diffusion, and its full distribution in composite ensures high energy storage capacities (parts a-c of Figure 2). In addition to taking advantage of LiFePO4 nanospheres, the constructed carbon framework simultaneously contributes to the superior electrochemical performance as follows: (1) the carbonaceous configuration acts as a 3D continuous conductive network for embedded LiFePO4 nanospheres, and the key limitation of sluggish electron transfer between LiFePO4 can be overcome for high power capability.14,18 (2) the abundant porosity in carbon framework allows for sufficient penetration and saturated lodging of lithium ion electrolyte to guarantee the intimate twophase interface contact between immersed LiFePO4 nanospheres and circumambient electrolyte for facile delivery of lithium ions.25 (3) the carbon skeleton can also serve as a solid scaffold for supporting active material, so that the harmful aggregation in usage can be restrained and the destructive volumetric force from LiFePO4/FePO4 two-phases transformation can be released in long-term cycling performance.19,38 Therefore, the high-rate discharge capacity of 132.9 mAhg-1 is accomplished at the current density of 1 C rate, and 69.5 mAhg-1 at 20 C. The prominent capacity retentions of approximate 100% are achieved at various rates lower than 10 C as shown in part d of Figure 5. The reproducibility in CV measurements also proves the outstanding reversibility of LiFePO4/C cathode material (Figure S5 of the Supporting Information). The lithium ion diffusion coefficient is calculated of ∼10-15-10-14 cm2 s-1 (Figure S6 and Table S1 of the Supporting Information),39 which 2891

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Figure 5. Electrochemical performance and properties of working electrodes in the voltage range of 2.5-4.2 V. (a) Rate capabilities of T-LEP/C, (b) galvanostatic charge/discharge of B-LEP/C, (c) galvanostatic charge/discharge of P-C, and (d) discharge capacity retentions of the T-LEP/C at various rates and the B-LEP/C at the corresponding 0.1 C (insert).

is comparable to that of the published researches.9,40,41 Despite that the carbon layer coating and the established carbon structure are not essential to improve the chemical diffusion of lithium ions within the crystal of LiFePO442, the enhanced lithium ion diffusion in LiFePO4/C composite can be ascribed to the very convenience for electron and lithium ion transport between LiFePO4 nanospheres and electrolyte, both of which are dwelled in the 3D porous and continuous carbon framework. The excellent electrochemical performance indicates that the host LiFePO4 nanoshperes within the guest tridimensional carbon structure as cathode material can afford high energy and power densities in lithium ion batteries. It is unquestionable that nanospherical electrode materials lodged in 3D porous carbon framework can considerably contribute to the enhanced high-rate electrochemical performance and properties. However, the specific storage capacities are always calculated on the mass of LiFePO4 active material in LiFePO4/C composite with the weight of carbon being subtracted.17,18 In our investigation, the practical effect of carbon member as cathode material for lithium storage has been estimated. The P-C was fabricated with the separated carbon as cathode material mixed with conductive carbon and PVDF binder at a weight ratio of 75:15:10 in consistence with that of T-LEP/C. As shown in part c of Figure 5, the first charge of P-C yields a specific capacity of 31.1 mAhg-1 at the related current density of 0.1 C for T-LEP/C, and the corresponding discharge capacity is 26.0 mAhg-1 in dot curves. The initial charge/ discharge capacities are presumably attributed to the extraction and insertion of lithium ions from the saturated infiltration of electrolyte in the porous carbon framework. The irreversible

capacity can be assigned to the formation of the solid electrolyte interphase (SEI) layers on the surface of the electrode.3 The reversible capacity of ∼25.0 mhAg-1 is achieved after five cycles, denoting the quasi-theoretical capacity of P-C as cathode material. Therefore, the actual discharge capacity of LiFePO4/ C cathode material delivers 96.1 mAhg-1 at 0.1 C indeed as comparison to the nominal capacity of 155.0 mAhg-1 when the carbon content is considered. Based on the weight ratio of 36.14/ 63.86% for carbon/LiFePO4 components, the carbon member in LiFePO4/C composite contributes to a real capacity of 9.0 mAhg-1 to the energy density cooperating with the actual 136.4 mAhg-1 for LiFePO4 active particles at 0.1 C. It is suggested that the mass of carbon in LiFePO4/C entity should be taken into account for the real storage capacity in composite electrode materials. More efforts are ongoing to reduce the carbon content in final products but maintain the 3D structure with remarkable high surface area and substantially enhanced electronic conductivity. To preliminarily evaluate the electrochemical properties of pristine LiFePO4/C composite in nature, the B-LEP/C was prepared without conductive carbon but with the same 10 wt % of inert PVDF binder for contrast. The original five cycles of galvanostatic charge/discharge are displayed in part b of Figure 5 at the related current density of 0.1 C for T-LEP/C. The first cycle possesses specific capacities of 134.9 and 127.8 mAhg-1 respectively, and the flat voltage plateaus are located at 3.3 and 3.5 V in the symmetric patterns. The outstanding electrochemical performance of the B-LEP/C can be attributed to the high electronic conductivity in bulk LiFePO4/C powders together with saturated lodging of lithium ion electrolyte in porous carbon 2892

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The Journal of Physical Chemistry C structure. However, the perceptible enlarged polarization occurs in long-term cycling trials with discharge capacity fade of 40.5% after 10 cycles (insert of part d of Figure 5), which is probably ascribed to the lower ability of electron transfer in B-LEP/C than that in T-LEP/C with mixture of the acetylene black additive, a perfect conductive carbon. Provided that the graphitic degree of 3D carbonaceous framework is progressed to approach the quality of conductive additives, extra carbon additives can be highly reduced or even avoided. In that case, the carbon framework will become the tridimensional refined graphitic carbon web with perfect electronic conductivity and can serve as the wonderful conductive network and the current collector substrate. Our work will focus on developing an effective route to enhance the graphitic degree in carbon component for advanced electrochemical performance of LiFePO4/C composite.

’ CONCLUSIONS The extremely low electronic conductivity of LiFePO4 cathode material severely prevent from its practical applications in high power lithium ion batteries. Here, the construction of tridimensional carbon framework in abundant porosity for lodging LiFePO4 nanospheres has been proved as an effective approach to improve high-power electrochemical performance and properties of LiFePO4. In this article, the LiFePO4/C composite have been facilely prepared using glucose as carbon source. The mechanical activation of reactive precursors by ball milling and consequent aging procedure in tanks are essential to obtain the LiFePO4 nanospheres and the established carbon framework. This LiFePO4/C cathode material reveals various advantages in terms of amazing high surface area of 200.5 m2 g-1, enhanced electronic conductivity of ∼10-2 S cm-1 and calculated lithium ion diffusion coefficient of ∼10-14 to 10-15 cm2 s-1 in combination with inexpensive raw materials and simple preparation. The excellent high-rate capacities of 132.9 and 69.5 mAhg-1 at 1 and 20 C are accomplished, and capacity retentions of approximate 100% are achieved at rates lower than 10 C. The LiFePO4/ C composite delivers the practical discharge capacity of 96.1 mAhg-1 at 0.1 C in comparison with a nominal capacity of 155 mAhg-1 calculated with the carbon content being subtracted. It is worthy to notice that the pristine LiFePO4/C composite has shown outstanding electrochemical performance without conductive carbon additives, motivating us to pursue the lower weight ratio and higher graphitic degree of carbon framework. ’ ASSOCIATED CONTENT

bS

Supporting Information. Calculation of carbon content in LiFePO4/C composite; elemental mappings of the LiFePO4/ C composite; FESEM images, XRD patterns, and nitrogen adsorption/desorption isotherms of separated LiFePO4 and carbon members from the LiFePO4/C composite; morphology of the precursor of the LiFePO4/C composite; CV measurements of T-LEP/C; calculations of lithium ion diffusion coefficient (DLiþ) of the LiFePO4/C composite. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86-25-52112900, Fax: þ86-25-52112626, E-mail: jianph@ nuaa.edu.cn.

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’ ACKNOWLEDGMENT The authors acknowledge the financial support from the National Natural Science Foundation (50871053). Doctor Huang Pang from Nanjing University is greatly appreciated for TEM and FESEM determinations. The assistance of Yu Guan in measuring of electronic conductivity and Junming Lu in drawing schematic diagram is sincerely thanked. ’ REFERENCES (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188–1194. (2) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359–367. (3) Fergus, J. W. J. Power Sources 2010, 195, 939–954. (4) Ellis, B. l.; Makahnouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F. Nat. Mater. 2007, 6, 749–753. (5) Delmas, C.; Maccario, M.; Croguennec, L.; Le Cras, F.; Weill, F. Nat. Mater. 2008, 7, 665–671. (6) Ramana, C. V.; Mauger, A.; Gendron, F.; Julien, C. M.; Zaghib, K. J. Power Sources 2009, 187, 555–564. (7) Barker, J.; Saidi, M. Y.; Swoyer, J. L. Electrochem. Solid-State Lett. 2003, 6, A53–A55. (8) Li, Y. Z.; Zhou, Z.; Ren, M. M.; Gao, X. P.; Yan, J. Electrochim. Acta 2006, 51, 6498–6502. (9) Chung, S. Y.; Bloking, J. T.; Chiang, Y. M. Nat. Mater. 2002, 1, 123–128. (10) Thackeray, M. Nat. Mater. 2002, 1, 81–82. (11) Kang, B.; Ceder, G. Nature 2009, 458, 190–193. (12) Wagemaker, M.; Ellis, B. L.; L€utzenkirchen-Hecht, D.; Mulder, F. M.; Nazar, L. F. Chem. Mater. 2008, 20, 6313–6315. (13) Lai, C. Y.; Xu, Q. J.; Ge, H. H.; Zhou, G. D.; Xie, J. Y. Solid State Ionics 2008, 179, 1736–1739. (14) Chen, J. M.; Hsu, C. H.; Lin, Y. R.; Hsiao, M. H.; Fey, G. T.-K. J. Power Sources 2008, 184, 498–502. (15) Liu, J.; Wang, J. W.; Yan, X. D.; Zhang, X. F.; Yang, G. L.; Jalbout, A. F.; Wang, R. S. Electrochim. Acta 2009, 54, 5656–5659. (16) Doherty, C. M.; Caruso, R. A.; Smarsly, B. M.; Drummond, C. J. Chem. Mater. 2009, 21, 2895–2903. (17) Doherty, C. M.; Caruso, R. A.; Smarsly, B. M.; Adelhelm, P.; Drummond, C. J. Chem. Mater. 2009, 21, 5300–6306. (18) Wu, X. L.; Jiang, L. Y.; Cao, F. F.; Guo, Y. G.; Wan, L. J. Adv. Mater. 2009, 21, 1–5. (19) Cheng, F. Y.; Tao, Z. L.; Liang, J.; Chen, J. Chem. Mater. 2008, 20, 667–681. (20) Croce, F.; D’ Epifanio, A.; Hassoun, J.; Deptula, A.; Olczac, T.; Scrosati, B. Electrochem. Solid-State Lett. 2002, 5, A47–A50. (21) Song, G. M.; Wu, Y.; Xu, Q.; Liu, G. J. Power Sources 2010, 195, 3913–3917. (22) Cui, Y.; Zhao, X. L.; Guo, R. S. Electrochim. Acta 2010, 55, 922– 926. (23) Huang, Y. H.; Goodenough, J. B. Chem. Mater. 2008, 20, 7237– 7241. (24) Meethong, N.; Kao, Y. H.; Speakman, S. A.; Chiang, Y. M. Adv. Funct. Mater. 2009, 19, 1060–1070. (25) Dominko, R.; Bele, M.; Goupil, J. M.; Gaberscek, M.; Hanzel, D.; Arcon, I.; Jamnik, J. Chem. Mater. 2007, 19, 2960–2969. (26) Qian, J. F.; Zhou, M.; Cao, Y. L.; Ai, X. P.; Yang, H. X. J. Phys. Chem. C 2010, 114, 3477–3482. (27) Ferrari, S.; Lavall, R. L.; Capsoni, D.; Quartarone, E.; Magistris, A.; Mustarelli, P.; Canton., P. J. Phys. Chem. C 2010, 114, 12598– 12603. (28) Qin, X.; Wang, X. H.; Xiang, H. M.; Xie, J.; Li, J. J.; Zhou, Y. C. J. Phys. Chem. C 2010, 114, 16806–16812. (29) Vadivel Murugan, A.; Muraliganth, T.; Manthiram, A. J. Phys. Chem. C 2008, 112, 14665–14671. (30) Kim, J. K.; Choi, J. W.; Chauhan, G. S.; Ahn, J. H.; Hwang, G. C.; Choi, J. B.; Ahn, H. J. Electrochim. Acta 2008, 53, 8258–8264. 2893

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dx.doi.org/10.1021/jp108363y |J. Phys. Chem. C 2011, 115, 2888–2894