Coralline Glassy Lithium Phosphate-Coated LiFePO4 Cathodes with

Mar 8, 2013 - Man Xie , Rui Luo , Jun Lu , Renjie Chen , Feng Wu , Xiaoming Wang ..... Taolin Zhao , Li Li , Renjie Chen , Huiming Wu , Xiaoxiao Zhang...
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Coralline Glassy Lithium Phosphate-Coated LiFePO4 Cathodes with Improved Power Capability for Lithium Ion Batteries Guoqiang Tan,† Feng Wu,†,‡ Li Li,*,†,‡ Renjie Chen,*,†,‡ and Shi Chen†,‡ †

School of Chemical Engineering and Environment, Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ National Development Center of High Technology Green Materials, Beijing 100081, China ABSTRACT: Novel coralline glassy lithium phosphate-coated LiFePO4 cathodes successfully prepared by radio frequency magnetron sputtering have been studied in lithium ion batteries. These coated LiFePO4 show higher reversible capacity, stable cycle performance, and improved power capability compared to the bare one. These favorable properties are considered to be attributed to the good conductivity and stability of the glassy lithium phosphate coating. The amorphous nature of the coating reduces the anisotropy of the surface properties of LiFePO4 electrode and enhances the Li+ ionic diffusion into the LiFePO4. The glassy lithium phosphate is an effective Li+ conductor, which increases the ionic and electronic transport on the surface and into the bulk of LiFePO4 electrode, extends the electroactive zone, and facilitates the transfer kinetics. It is also a stable Li-excess material, which provides an extra lithium capacity and maintains the electrode structural integrality. Radio frequency sputtering coating of stable Li+ conductors on the surface of nanosized LiFePO4 is an attractive way to improve its power capability, and these specific LiFePO4 cathodes have great potential for application in high-power lithium ion batteries.

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INTRODUCTION Energy and environment are two themes of paramount importance in the 21st century. Renewable energy technologies are critical to realize global sustainable development. Since SONY developed the first commercial Li ion battery in the early 1990s, lithium ion batteries (LIBs) have been widely used as advanced electrochemical energy storage and conversion systems in various electronic devices and environmentally friendly vehicles including both hybrid electric vehicles (HEVs) and pure electric vehicles (PEVs).1−4 LIBs with both high energy and power density are essential to improve EVs.5,6 Currently, lithium iron phosphate (LFP) and lithium manganese oxide (LMO) are the most promising candidates for use as cathode materials in batteries for EVs.7,8 Both LFP and LMO are inexpensive, abundant, safe, and environmentally benign. LFP shows a higher gravimetric capacity and better capacity retention than LMO, but LFP exhibits a poor conductivity, resulting in a low rate performance.9,10 Various strategies to overcome the electronic and ionic transport limitations of LFP have been proposed, including improving bulk or surface electronic conductivity by doping with foreign atoms or coating with electronically conductive agents,11−13 reducing the path length of electrons and Li+ ions by decreasing the particle size14−16 or increasing diffusion of Li+ ions across the surface toward the (010) facet by coating stable Li+ conductors on the surface of nanosized LFP.17,18 Herein, radio frequency (RF) magnetron sputtering has been used to prepare coralline glassy lithium phosphate-coated © XXXX American Chemical Society

LiFePO4 (GLP-coated LFP) cathodes by depositing a thin glassy lithium phosphate coating on the surface of LiFePO4 electrodes. Amorphous lithium phosphate compounds such as Li2O−P2O5, LiPON, and LiBPO are well-known to be stable, facile Li+ conductors and have been used as solid-state electrolytes in all-solid-state LIBs.19−21 In this research, our intention is to introduce a fast, stable Li+ conductor to increase ionic and electronic transport at the surface of LFP and improve the rate capability of LIBs. The proposed mechanisms for ionic and electronic transport are shown in Scheme 1. The thin GLP coating on the surface of LFP electrode increases the rate of Li+ ionic transport along the surface as well as Li+ ionic permeation into the surface of the LFP electrode. Both of these processes promote Li+ ions migration through the electrolyte and cathode into the bulk of LFP crystals through (010) facets.22 The coralline GLP coating acts as cross-linked networks to keep LFP particles on the surface layer linked together, reducing the path length of Li+ ions and electrons transfer between LFP particles and creating more conductive paths for Li+ ions and electrons. The GLPs can be doped with transition metals to achieve electronic conduction reported in previous literature,18,23 and these transition metals include Fe, Ti, Co, Ni, Ga, Nb, et al.24 The GLP is believed to be able to dissolve partial iron ions during the charge−discharge of LFP Received: October 1, 2012 Revised: February 25, 2013

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Scheme 1. Mechanisms for Li+ Ionic and Electronic Transport in the Coralline GLP-Coated LFP Cathode

Scheme 2. Synthetic Route Used To Prepare the Coralline GLP-Coated LFP Electrodes

South Korea). The thickness of the LFP electrodes was controlled at 20 um. Li3PO4 Target Preparation. Commercially available Li3PO4 powder (99%, Acros, China) was ball-milled with dehydrated ethanol at a rotating speed of 400 rpm for 5 h using a planetary ball mill (Fritsch Puluerisette7, Germany) and then dried at 70 °C for 2 days to obtain a fine powder. The final Li3PO4 target was prepared using a conventional cold-press method. Twenty grams of dried Li3PO4 powder was pressed into a 60-mm-diameter pellet and then sintered in an electrical furnace at 600 °C for 5 h in air. Coralline GLP-Coated LFP Cathode Preparation. The coralline GLP-coated LFP cathode was prepared by RF magnetron sputtering using the Li3PO4 target under a highpurity Ar atmosphere with a specific pattern, as shown in Scheme 2. The prepared LFP electrode and Li3PO4 pellet used as the substrate and target, respectively, were placed in the corresponding positions in the sputtering chamber. The distance between the target and electrode substrate was 6 cm. The chamber was evacuated to a base pressure of 1.0 × 10−5 Pa to guarantee a clean sputtering condition. The working pressure was 1.0 Pa, and the RF power was 100 W. Before deposition,

and increase the electronic conductivity of LFP electrode for rapid mass and charge transfer. All of these factors are beneficial to enhance the power capability of the coralline GLP-coated LFP cathode. Moreover, the GLP coating with high lithium content may be able to provide extra Li+ ions for the extraction−insertion during the charge−discharge process, increasing the reversible capacity, and it maintains the electrode structural stability, improving the cycle performance. Herein, it is demonstrated that the nanosized GLP coating can significantly improve the electrochemical properties of the LFP cathodes in LIBs. The coralline GLP-coated LFP cathodes exhibit large stable reversible capacities, high rate capabilities, and excellent cycle performance.

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EXPERIMENTAL SECTION

LFP Cathode Preparation. Commercially available LiFePO4 powder (Pulead Technology Industry Co., China) was used as the cathode material. The cathode electrodes were prepared by pasting a mixture of 75 wt % LFP, 15 wt % acetylene black, and 10 wt % poly(vinylidene fluoride) (PVDF) onto an aluminum foil current collector using an AFA-III automatic film coater with cover heater (MTI corporation, B

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Figure 1. FE−SEM images of the (top row) bare LFP cathode and (middle and bottom rows) GLP-coated LFP cathodes after deposition for 10 and 20 min, respectively.

Figure 2. (A, B) Top-view and (C, D) cross-sectional FE-SEM images of the GLP-coated LFP cathode after deposition for 20 min.

the target was presputtered for 15 min. The deposition time was set in a stepwise manner of 5, 10, 15, 20, 25, and 30 min. Characterization of Properties. The morphologies of the LFP electrodes were examined by FE-SEM (Zeiss, SUPRA 55) and EDX (Phoenix). XRD was performed on a diffractometer

(Rigaku Ultima IV) using a Cu Kα radiation. FT-IR spectra were obtained on an infrared spectrometer (Nicolet 6700). Raman spectra were obtained on a Raman spectrometer (JY Labram HR 800). Electrochemical measurements were carried out by using coin-type cells. The prepared LFP electrode was C

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used as the cathode and lithium metal as the anode, while the electrolyte was 1 M LiPF6/EC + DMC (1:1 in volume). Cells were assembled in an argon-filled glovebox and then aged for 24 h before electrochemical testing. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical workstation (Chi 604D). Galvanostatic charge−discharge experiments were carried out between 2.5 and 4.2 V using a battery tester (Land CT2001A). The specific capacity of the coralline GLP-coated LFP cathode was calculated based on the weight of LFP.

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RESULTS AND DISCUSSION Morphology and Structure. The surface morphologies of the bare and coated LFP electrodes are shown in Figure 1. Generally, the bare LFP electrode exhibited a relatively rough surface that might easily cause active materials to dissolve in the electrolyte in LIBs. The morphologies of coated LFP electrodes were influenced by the thickness of the GLP film, which was controlled by varying the deposition time. As the deposition time was increased, the thickness of the GLP film gradually increased, and the electrode surface became smoother and denser. From a microscopic point of view, the GLP-coated nanosized LFP particles on the surface layer of the electrode connected with each other to form a network as the deposition time increased. The FE−SEM images also showed the charges in the pore structure of the LFP electrodes as a function of the deposition time. As the time was increased, the pore size of the LFP electrode gradually decreased. After sputtering for 20 min, the LFP electrode possessed a coralline surface morphology with sufficient porosity. The cross-linked networks could create more conductive paths for Li+ ions and electrons, and the uniform porous structure guaranteed the infiltration of the electrolyte. Figure 2 revealed that the GLP coating generated by the RF sputtering method become a thin and condensed film and was stacked well on the surface of the LFP electrode, with a homogeneous and glassy texture. The thickness of the GLP film measured by FE−SEM was about 200 nm after deposition for 20 min. The GLP film exhibited amorphous glassy behavior, as confirmed by XRD, FT−IR, and Raman measurements as shown in Figure 3. In Figure 3A, the XRD pattern of the bare LFP electrode possessed clear diffraction peaks that were consistent with the orthorhombic olivine phase of LiFePO4 (space group: Pnma).25 No obvious diffraction peaks corresponding to graphite were present; this indicated the carbon in the electrode was not well crystallized. The XRD pattern of the GLP-coated LFP electrode showed some diffraction peaks consistent with Li3PO4.11,18 The weak intensity of these peaks indicated that the layer of Li3PO4 on the surface of the LFP electrode possessed a glassy structure. In Figure 3B, the FT-IR spectrum of the bare LFP electrode was complex with a large number of bands. In the range of 800− 1200 cm−1, bands were related to the stretching modes of the (PO4)3− anion; the first two bands at 876 and 934 cm−1 corresponded to symmetric stretching modes, and those at 1034, 1098, and 1136 cm−1 corresponded to the antisymmetric stretching modes of the P−O bonds.26 Bands in the range of 420−570 cm−1 were related to the bending modes of the (PO4)3− anion.27 In the FT-IR spectra of the GLP-coated LFP electrode, the obvious changes were the appearance of two new bands at 900 and 1258 cm−1; the former was caused by the superposition of bands of crystalline LFP and glassy Li3PO4, and the latter was attributed to the stretching vibrations of

Figure 3. (A) XRD patterns, (B) FT-IR spectra, and (C) Raman spectra of the (a) bare LFP and (b) GLP-coated LFP (20 min) cathodes.

terminal PO3 units of glassy Li3PO4.28,29 In Figure 3C, the Raman spectrum of the bare LFP electrode exhibited two intense bands at 1355 and 1593 cm−1 that were attributed to the D-band (disorder-induced phonon mode) and G-band (graphite band) of highly disordered carbon, respectively.10 Other bands in the range of 100−500 and 500−1100 cm−1 corresponded to the Raman vibrations of Fe−O and (PO4)3− in LFP, respectively. The intramolecular vibrational bands of the (PO4)3− anion were observed at 582, 985, and 1030 cm−1.26,27 These were consistent with the presence of well-crystallized LFP. The Raman spectrum of the GLP-coated LFP electrode could almost be superposed on that of the bare LFP electrode except for a new weak band at 636 cm−1, and the intensity of the bands at 445 and 985 cm−1 were increased. These changes were attributed to the glassy Li3PO4 film, the band at 636 cm−1, and the increased band at 445 cm−1 corresponding to the bending modes of the (PO4)3− anion; the other increased band at 985 cm−1 was attributed to the stretching modes of the (PO4)3− anion in glassy Li3PO4.29,30 D

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Electrochemical Properties. The electrochemical properties of the bare and GLP-coated LFP cathodes were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge−discharge testing. Figure 4 shows the charge−discharge profiles and cycle

resistance (Rct) and double-layer capacitance (Cdl). The sloping line represents the Warburg impedance (Zw) related to the effect of the diffusion of lithium ions at the electrode/ electrolyte interface. Rs is also referred to as the “ohmic impedance”, and the combination of Rct and Zw is called the “faradaic impedance”, which reflects the kinetics of the cell reactions.31,32 The Rct values of the bare LFP and GLP-coated LFP cathodes after 2 cycles were 158.6 and 50.4 Ω, respectively. The smaller Rct of the GLP-coated LFP cathode suggested that the transfer of Li+ ions and electrons was more feasible on this electrode, indicating that the GLP coating improved the electrochemical activity of the LFP electrode. When after 51 cycles, the Rct of the bare LFP and GLP-coated LFP cathodes became 520.6 and 240.4 Ω, respectively. The slow increase in Rct with increased cycles confirmed the good cycle performance of the LFP cathodes at the low rate. Interestingly, the impedance spectra of the GLP-coated LFP cathode after 51 cycles exhibited two partially overlapping semicircles and a straight sloping line. As our consideration, the appearance of the first depressed semicircle at high frequency was attributed to the GLP coating, and the subsequent semicircle represented the charge transfer process on the LFP particles. The parameters Rct1 and Cdl1 were responsible for the resistance and capacitance of the GLP coating, respectively, whereas Rct2 and Cdl2 represented the charge transfer resistance and double-layer capacitance for the LFP, respectively. In the cells, the GLP coating acted as a transition layer between the electrolyte and LFP electrode. The GLP film was very thin and possessed a fast Li+ ionic conductibility; it could reduce the resistance of Li+ ions transfer from the electrolyte into the LFP electrode. It was also proposed that the disorder nature of the coating modified the surface potential of lithium to facilitate the Li+ ionic adsorption from the electrolyte by providing different lithium sites with a wide range of energies that can be matched to the energy of lithium in the electrolyte.18 All of these processes resulted in a decreased charge transfer resistance. Meanwhile, the GLP might be able to dissolve partial iron ions during the charge−discharge to increase its electronic conductivity, facilitating the mass and charge transfer. The rate performance of the LFP cathodes was also investigated at different test conditions. Figure 6A shows the cycle performance of the LFP cathodes discharged at a rate of 1 C. The cells were first discharged at a rate of 0.1 C for 10 cycles to activate them and then discharged at a rate of 1 C for 90 cycles. The initial discharge capacities of the bare LFP and GLP-coated LFP at 1 C became 152.6 and 165.8 mAh g−1, respectively. After 90 cycles, the bare LFP exhibited a relatively low capacity of 135.7 mAh g−1 with average Coulombic efficiency of 99.1% for 100 cycles. However, the corresponding values for the GLP-coated LFP were 159.1 mAh g−1 and 99.7%, respectively. These results indicated that the cycle performance of the GLP-coated LFP was much better than that of the bare LFP at 1 C rate. The improved cycle performance of the GLPcoated LFP was primarily ascribed to its structural stability. The stable GLP coating prevented the dissolution of active materials from the electrode into the electrolyte. Figure 6B shows the impedance spectra of the GLP-coated LFP cathode discharged to 2.5 V during cycles. After 2 cycles, the impedance spectra showed no obvious resistance characteristic of the GLP layer, with a low Rct of 48.4 Ω. But after 51 cycles, two partially overlapping semicircles were observed, and the total Rct became 282.6 Ω. Figure 6C shows the CV curves of the LFP cathodes obtained at different scan rates. In contrast to the bare LFP

Figure 4. (A) Initial charge−discharge profiles and (B) cycle performance of the bare and GLP-coated LFP (20 min) cathodes discharged at a rate of 0.1 C; inset: EIS of the Li/LFP cells discharged to 2.5 V after selected cycles.

performance of the LFP cathodes at a constant current density of 16 mA g−1 (0.1 C). In Figure 4A, the capacity of the GLPcoated LFP cathode was obviously increased, and its potential plateau was slightly decreased compared to that of the bare LFP cathode. As our consideration, these were attributed to the GLP coating, which improved the kinetics of the LFP electrode and also provided partial additional capacity for the LFP electrode. In Figure 4B, the bare LFP cathode delivered an initial discharge capacity of 156.4 mAh g−1, with capacity retention of 95.5% after 51 cycles. Comparatively, the GLP-coated LFP cathode exhibited a higher initial discharge capacity of 167.3 mAh g−1 and capacity retention of 98.6% after 51 cycles, demonstrating its ultrahigh cyclability at the low rate. The electrochemical kinetic performance of the LFP cathodes was analyzed by the EIS measurement. Figure 4B (inset) shows the impedance spectra of the bare LFP and GLPcoated LFP electrodes discharged to 2.5 V after selected cycles, and Figure 5 shows the corresponding test circuits and equivalent circuits used to explain the impedance spectra. Generally, for the bare LFP electrode, the impedance spectra of the Li/LFP cell is composed of a semicircle at high frequency and a straight sloping line at low frequency. An intercept at the Z′real axis at high frequency corresponds to the cell electrolyte resistance (Rs). The semicircle indicates the charge transfer E

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Figure 5. Nyquest plots, equivalent circuits, and test circuits for the bare LFP and GLP-coated LFP (20 min) cathodes.

Figure 6. (A) Cycle performance of the LFP cathodes discharged at a rate of 1 C; (B) EIS of the Li/LFP cell discharged at a rate of 1 C to 2.5 V after selected cycles; (C) cyclic voltammograms of the Li/LFP cells at different scan rates; (D) initial discharge curves and specific capacity versus cycle number (inset) for the GLP-coated LFP cathode at different discharge rates. All cells were charged at a rate of 0.1 C at room temperature, and the GLP-coated LFP cathode in this figure was deposited for 20 min.

electrode, the GLP-coated LFP showed sharper redox peaks with reduced separation. Even at a high rate of 5 mV s−1, the sharp redox peaks were still maintained. This suggested that the GLP-coated LFP electrode showed better electrochemical

reversibility and kinetics than the bare one. This was believed to be attributed to the increased ionic and electronic conductibility. Figure 6D illustrates the discharge profiles of the GLP-coated LFP electrode at various rates. At a rate of 0.1 F

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protected, and active material particles were mutually connected through a GLP layer, forming a stable network structure. The GLP had a good oxidation resistivity because (PO4)3− anion formed a strong covalent bond between P and O, and it was not easily oxidized.33 The EDX spectra of the GLP-coated LFP electrode also showed a decreased C content in contrast to that of the bare LFP electrode. Therefore, it could preserve electrode structural integrity at a high current rate with fast lithium ion conduction at the interface between the electrode and electrolyte. These results confirmed that the GLP-coated LFP cathode maintained a good structural stability and exhibited an excellent rate performance. The rate performance versus cycle performance of all LFP electrode samples with different deposition times was also obtained, as shown in Figure 9. We could see that the reversible capacity and cycle performance were gradually improved as the deposition time increased until 20 min. However, the corresponding performance was no more increased but decreased when the deposition time was sequentially increased to 25 and 30 min. This suggested an optimization amount of the GLP coating on the LFP electrode. The longer deposition time introduced a thicker GLP coating, which reduced the pore structure on the LFP electrode to retard the infiltration of the electrolyte, resulting in the decreased charge transfer efficiency. As expected, improved electrochemical properties including reversible capacity, power capability, and cycle performance were obtained for the GLP-coated LFP electrode, which were ascribed to the presence of the GLP coating. On the one hand, the amorphous nature of the GLP removed the anisotropy of the surface properties of the LFP electrode and enhanced the delivery of Li+ to the LFP. It acted as a fast ionic conducting surface, providing rapid Li+ ionic transport along the surface and permeation into the bulk of LFP. It also acted as a porous Li+ absorbing agent and modified the surface potential of lithium to facilitate the Li+ ionic adsorption from the electrolyte. Increased Li+ ionic diffusion across the surface was known to facilitate Li insertion into the bulk of the LFP crystal in the (010) direction. All of these mechanisms were important for the LFP cathode because it could exchange Li+ ions with the electrolyte over its whole surface. On the other hand, the GLP acted as cross-linked networks that reduced the Li+ ionic and electronic transfer path length between LFP particles and created more conductive paths for the Li+ ions and electrons, significantly increasing the size of the electroactive zone. The GLP was also believed to be able to dissolve partial iron ions during the charge−discharge of the LFP electrode, forming a new Fe-doped phosphate compound, which could increase the electronic conductivity of the LFP electrode, further improving the kinetics of the LFP electrode. All of these factors improved electrochemical properties of the LFP electrode, especially its rate capability. Moreover, the GLP had a good oxidation resistivity, so it could well protect the structural integrity of LFP electrode to ensure an excellent cycle performance. In a word, the coralline LFP cathodes exhibited improved electrochemical properties, and these were mainly attributed to the increased ionic and electronic conductivities and stable network structure.

C, the initial discharge capacity of the GLP-coated LFP electrode was 168 mAh g−1, which was very close to the theoretical capacity of LFP. Although the capacity decreased with increasing rate, the GLP-coated LFP electrode exhibited excellent rate capability. A high capacity of 158 mAh g−1 was achieved at a rate of 2 C; even at the highest rate of 10 C, a capacity of 138 mAh g−1 was still obtained. Its discharge capacity remained stable upon cycling at each rate, as confirmed by the cycle performance shown in the inset of Figure 6D. The capacity recovered to its original value when the initial current rate of cycling was restored. This also suggested that the GLPcoated LFP electrode showed excellent kinetics and structural integrity. The rate performance of the LFP cathodes at various charge and discharge rates was also investigated. In Figure 7A,

Figure 7. Rate performances of the (A) bare LFP and (B) GLP-coated LFP (20 min) cathodes at various current densities (the charge current is the same as the discharge current at each current density).

the bare LFP cathode showed an acceptable rate capability with discharge capacities of 155 (0.1 C), 148 (0.2 C), 139 (0.5 C), 122 (1 C), and 82 mAh g−1 (2 C). In contrast, in Figure 7B, the GLP-coated LFP cathode exhibited an improved rate capability with discharge capacities of 168 (0.1 C), 165 (0.2 C), 156 (0.5 C), 143 (1 C), and 125 mAh g−1 (2 C). The charge and discharge capacity maintained stable upon cycling for each current rate. This indicated that the GLP coating was tolerant to the various current rates, and the integrity of the GLP-coated LFP electrode was well maintained. In order to investigate the interface information between the LFP electrode and electrolyte, the surface morphology of the LFP electrodes after the rate test was analyzed by SEM, as shown in Figure 8. The bare LFP cathode after the rate test showed a badly destructive surface morphology. The LFP particles on the electrode surface were obviously cracked after the high rate test. However, the surface morphology of the GLP-coated LFP after the rate test illustrated its structural integrity. The electrode was well

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CONCLUSION In conclusion, a kind of GLP-coated LFP cathode was prepared by RF magnetron sputtering. The GLP coating was a thin, homogeneous, glassy film and was stacked well on the LFP cathode. The GLP-coated LFP cathodes possessed coralline G

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Figure 8. SEM images and corresponding EDX spectra of the (A, a) bare LFP and (B, b) GLP-coated LFP (20 min) cathodes after discharged at a rate test for 51 cycles to 2.5 V. The discharge procedure was shown in Figure 6D.

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ACKNOWLEDGMENTS This study was supported by the National Key Program for Basic Research of China (No. 2009CB220100), the International S&T Cooperation Program of China (2010DFB63370), the National 863 Program (2011AA11A256), New Century Educational Talents Plan of Chinese Education Ministry (NCET-10-0038), and Beijing Novel Program (2010B018).

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(1) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271−4301. (2) Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652−657. (3) Tollefson, J. Charging Up the Future. Nature 2008, 456, 436− 440. (4) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Batteries. J. Power Sources 2011, 196, 6688−6694. (5) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243−3262. (6) Scrosati, B.; Hassoun, J.; Sun, Y.-K. Lithium-Ion Batteries. A Look into the Future. Energy Environ. Sci. 2011, 4, 3287−3295. (7) Li, H.; Wang, Z.; Chen, L.; Huang, X. Research on Advanced Materials for Li-Ion Batteries. Adv. Mater. 2009, 21, 4593−4607. (8) Park, O. K.; Cho, Y.; Lee, S.; Yoo, H.-C.; Song, H.-K.; Cho, J. Who Will Drive Electric Vehicles, Olivine or Spinel? Energy Environ. Sci. 2011, 4, 1621−1633. (9) Herle, P. S.; Ellis, B.; Coombs, N.; Nazar, L. F. Nano-Network Electronic Conduction in Iron and Nickel Olivine Phosphates. Nat. Mater. 2004, 3, 147−152. (10) Wu, X.-L.; Jiang, L.-Y.; Cao, F.-F.; Guo, Y.-G.; Wan, L.-J. LiFePO4 Nanoparticles Embedded in a Nanoporous Carbon Matrix: Superior Cathode Material for Electrochemical Energy-Storage Devices. Adv. Mater. 2009, 21, 2710−2714. (11) Chung, S.-Y.; Bloking, J. T.; Chiang, Y.-M. Electronically Conductive Phospho-Olivines as Lithium Storage Electrodes. Nat. Mater. 2002, 1, 123−128. (12) Huang, Y.-H.; Goodenough, J. B. High-Rate LiFePO4 Lithium Rechargeable Battery Promoted by Electrochemically Active Polymers. Chem. Mater. 2008, 20, 7237−7241.

Figure 9. Rate performances of all GLP-coated LFP cathodes with different deposition times at different discharge current densities. All cells were charged at a rate of 0.1 C at room temperature.

surface morphology with uniform microporous structure after an optimization deposition time. The nanosized GLP coating, as a fast and stable ionic conductor, improved the electronic and ionic transport in the LFP cathode, facilitating the mass and charge transfer and enhancing the reversible capacity and power capability of the LFP cathode. It also acted as a stable protective layer to maintain the structural stability of the LFP cathode, improving the cycle performance. This coralline GLPcoated LFP cathode possessing excellent properties is a promising candidate cathode for high-power lithium ion batteries.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel. +86-10-68912508; fax +86-10-68451429; e-mail chenrj@ bit.edu.cn, [email protected] (R.C.); tel. +86-10-68912508; fax +86-10-68451429; e-mail [email protected] (L.L.). Notes

The authors declare no competing financial interest. H

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp309724q | J. Phys. Chem. C XXXX, XXX, XXX−XXX