Encapsulation of LiFePO4 Nanoparticles into 3D Interpenetrating

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Encapsulation of LiFePO Nanoparticles into 3D Interpenetrating Ordered Mesoporous Carbon as a High-Performance Cathode for Lithium-Ion Batteries Exceeding Theoretical Capacity Diganta Saikia, Juti Rani Deka, Chieh-Ju Chou, Chien-Hua Lin, Yung-Chin Yang, and Hsien-Ming Kao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01682 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Energy Materials

Encapsulation of LiFePO4 Nanoparticles into 3D Interpenetrating Ordered Mesoporous Carbon as a High-Performance Cathode for Lithium-Ion Batteries Exceeding Theoretical Capacity Diganta Saikia,† Juti Rani Deka, ‡ Chieh-Ju Chou,† Chien-Hua Lin,† Yung-Chin Yang‡,* and Hsien-Ming Kao†,*

†

Department of Chemistry, National Central University, Chung-Li, 32054, Taiwan,

R.O.C.

‡

Institute of Materials Science and Engineering, National Taipei University of

Technology, Taipei 106, Taiwan, R.O.C.

ACS Paragon Plus Environment

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KEYWORDS. Ordered mesoporous carbon; LiFePO4; cathode; cyclic voltammetry; lithium-ion battery

ABSTRACT A nanocomposite cathode based on LiFePO4 (LF) nanoparticles embedded 3D cubic ordered mesoporous carbon CMK-8 for lithium-ion batteries is synthesized by a facile impregnation method followed by further modification with carbon coating. The effects of variation of carbon contents on electrochemical performances of cathodes are investigated. The well crystalline nanophase of LiFePO4 particles is confirmed by X-ray diffraction and TEM analysis. Nitrogen adsorption-desorption isotherms reveal persistence mesoporosity after encapsulation of LiFePO4 nanoparticles. The graphitic phase in LF/C@CMK-8-X (X = amount of CMK-8) nanocomposites is detected by


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analyzing the Raman spectrum of the matrix carbon due to CMK-8 and the coated carbon (C). The electrochemical properties of the LF/C@CMK-8-X nanocomposites are evaluated with cyclic voltammetry, impedance spectroscopy and charge-discharge cycling. The excellent rate capability with a discharge capacity value of 184.8 mA h g-1 is obtained for LF/[email protected] nanocomposite electrode at a current rate of 0.05C, which is higher than the theoretical capacity of LiFePO4 (170 mA h g-1). The discharge capacity (178.3 mA h g-1) is higher than the theoretical capacity up to the current rate of 0.2C. The long cycle stability test at a higher current rate of 10C exhibits remarkable discharge capacity of 120 mA h g-1 with 96.7% capacity retention after 1000 cycles and demonstrating the great potential of LF/[email protected] nanocomposite cathode for use in lithium-ion batteries.


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1. INTRODUCTION Lithium-ion batteries (LIBs) are widely used in many electronic devices, and powertrain of electric and hybrid electric vehicles due to their high energies and power densities.1-3 Recently, Tesla has unveiled lithium-ion battery packs with a capacity of 100 MW to store energy from wind turbine and deliver it to the national power grid in Australia.4 Therefore, necessity for high performance lithium-ion batteries is increasing rapidly in day-to-day life. To be a competent cathode material in modern LIBs, the materials should perform well in high charge-discharge rates, and possess good reversibility at higher capacities of intercalation of lithium-ions, high electropositive potentials with respect to anode, compatibility with electrolyte, lower volume changes during the charge-discharge process and good electronic conductivity.5,6 Currently, the majority of cathode materials used in commercial LIBs are LiCoO2, LiMn2O4, LiNixMnyCo1-x-yO2 and LiFePO4.6,7-9 Among them, LiFePO4 has attracted much attention due to low cost, abundant resources, environmental friendliness, safety, nontoxicity, high thermal stability and relatively higher capacity.10,11 However, low electronic conductivity (10-9‒10-10 S cm-1) restricts its application in high power batteries.12


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Extensive research has been conducted to enhance the electronic and ionic conductivity of the LiFePO4 material by reducing the particle size, doping with aliovalent cations, dispersing metals and coating with carbon or conductive polymers.13-17 Downsizing the LiFePO4 particles is a salient approach to increase the electronic conductivity of the material. Nanostructured electrodes have considerably improved the storage capacity, rate capability and cyclic stability of rechargeable batteries.18,19 The surface area of the particles increases when the particle size is smaller, leading to an increase in surface reactivity and reduction in the diffusion path of lithium ions.20 The relationship between particle size and discharge capacity was investigated by Gaberscek et al. and found that the capacity decreased almost linearly with the increase in average particle size.21 Different synthesis methods, such as solid-state synthesis, hydrothermal, sol-gel, co-precipitation, emulsion drying, and spray pyrolysis have been employed to synthesize the nanostructured LiFePO4 particles.11,22-26 Normally, the drawback of the solid-state synthesis is particle growth and agglomeration, which could result in a lower surface area and hence not beneficial in electrochemical performance.27 On the other hand, the solution based methods such as hydrothermal


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and sol-gel are more effective for producing nanostructured materials. Hydrothermal synthesis can effectively control the particle size from a few nanometers to hundreds of nanometers by tweaking the reaction time, temperature and pH values of the precursor solution. Ordered mesoporous carbons (OMCs) with large surface areas and periodically arranged pore structures are considered as the promising materials to improve the electrical conductivity and charge transport of the LiFePO4 materials.12,28 LiFePO4 nanoparticles synthesized with the help of OMCs as the support may have controllable and uniform size and distributed evenly in the OMC matrix as the particles are embedded inside the mesopores. The large surface area of mesopore carbon frameworks enhances electrolyte-electrode contact area and helps in the infiltration of electrolyte to the mesopores. This results in the easy migration of Li+ and e− around the embedded LiFePO4 nanoparticles and helps in the improvement of rate capability. Moreover, agglomeration of nanoparticles can be avoided with the use of OMCs as the particles grow inside the mesopores and are separated from each other by carbon walls.


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Carbon coating on the nanostructured LiFePO4 particles is one of the efficient ways to increase the electronic conductivity, which helps to improve the performances of LIBs with respect to specific capacity, rate capability and cycle stability.29-31 In addition, it helps to reduce the particle growth and aggregation.32 Moreover, carbon can acts as a reducing agent to prohibit the formation of ferric impurities during the calcination process.33 With the help of carbon coating on the LiFePO4 particles, researchers are able to reduce the charge transfer resistance and achieve capacity value close to its theoretical capacity at lower current rates.34-36 However, the high rate capability and cycling stability of the LiFePO4/C composite is still needed to be improved. In the present work, ordered cubic mesoporous carbon CMK-8 was used as the support to fabricate the LiFePO4@CMK-8 nanocomposite cathode. CMK-8 has the highly branched interwind three dimensional (3D) channel networks and open porous structure that can accommodate the LiFePO4 particles or charged ions and thus facilitate rapid diffusion. In addition, CMK-8 possesses higher Ia3d symmetry than many other ordered mesoporous carbons to allow an isotropic graphitized structure with higher conductivity, which can effectively enhance the heterogeneous electron


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transfer.37 Moreover, electrolyte can easily infiltrate into the 3D porous architecture of CMK-8 to increase the contact with the electrode surface and thus assists in the faster interfacial charge transport between electrode and electrolyte. Furthermore, CMK-8 can accommodate the large volume change during charge-discharge by providing void spaces. To the best of our knowledge, this is the first time that the ordered mesoporous carbon CMK-8 is employed as the support to confine LiFePO4 nanoparticles and the resulting nanocomposite is used as the cathode in lithium-ion batteries, although various mesoporous carbons have been used as supports to fabricate composite cathodes. Additionally, carbon is coated on the surface of the LiFePO4 nanoparticles to increase the electronic conductivity. In this study, a new approach is developed to have the LiFePO4 based cathode material with high rate capability and coulombic efficiency as well as lower irreversible capacity with long cycle stability for LIBs. With the hydrothermal and incipient wetness impregnation methods, LiFePO4 nanoparticles can be encapsulated in the CMK-8 matrix, followed by carbon coating by using citric acid as the precursor. The specific capacity, rate capability and cycling stability of the resulting cathode material were investigated by varying the amount of CMK-8. The physical and


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electrochemical characterizations of the synthesized materials were carried out by small and wide angle XRD, nitrogen adsorption-desorption isotherm, TGA, XPS, Raman, SEM/TEM, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and charge-discharge testing.

2. EXPERIMENTAL SECTION 2.1. Preparation of KIT-6 and CMK-8 In order to prepare the support CMK-8, mesoporous silica KIT-6 was first synthesized by following the reported procedures shown in Scheme S1 (Supporting Information, SI).38 Briefly, 4 g of Pluronic P123 (EO20PO70EO20, Aldrich) was dissolved in 0.5 M HCl (7.44 g, 37% HCl, Aldrich) and 139.92 g of DI H2O, and stirred at 35 °C for 6 h. Then, 4 g of butanol (J. T. Baker) was added dropwise and stirred for 1 h at the same temperature. Afterwards, 10.4 g of tetraethyl orthosilicate (TEOS, Aldrich) was added dropwise to the solution and stirred at 35 °C for 24 h. The mixture was then transferred to an oven and hydrothermally treated at 100 °C for 24 h. The obtained solid product was filtered and washed with DI H2O and subsequently dried at 100 °C for 24 h.


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The final product KIT-6 was obtained by calcining the sample at 550 °C for 6 h under an air environment. The ordered mesoporous carbon CMK-8 was synthesized by the nanocasting technique as shown in Scheme S2 (SI). First, 1 g of KIT-6 and 1.25 g of sucrose (Riedel-de Haën) were dissolved in DI H2O (3.2 mL). Then, 0.14 g of H2SO4 (Aldrich) was added to the solution in a dropwise manner. The mixture was stirred at room temperature for 2 h and then dried at 100 °C for 24 h, followed by another 24 h at 160 °C. The same impregnation process was repeated again but with an aqueous solution containing 0.75 g of sucrose, 3 mL of DI H2O and 0.08 g of H2SO4. Then, the resulting mixture was thermally treated at 100 °C for 1 h and subsequently at 160 °C for another 4 h. The carbonization process was conducted by initially treating the resulting product at 150 °C for 1 h, and later at 900 °C for 3 h under Ar/H2 (95%/5%) atmosphere, and the carbon-silica composite was obtained. To obtain the pure carbon material, i.e., CMK-8, the carbon-silica composite was etched in 2 M NaOH (50 vol% DI H2O/50 vol% ethanol) at 95 °C for 24 h to remove the silica template. After filtration, the solid sample was washed with DI H2O several times, and dried at 70 °C to finally obtain the CMK-8. The


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CMK-8 powder (1 g) was dispersed in concentrated HNO3 acid (30 mL,  65%, Honeywell Fluka) solution and stirred at 80 °C for 1 h to induce hydrophilicity. 2.2. Preparation of Carbon Coated LiFePO4@CMK-8 Cathodes The preparation procedures of the cathode material are illustrated in Scheme 1. For the preparation of LiFePO4, 4.04 g of iron nitrate [Fe(NO3)3∙9H2O, Showa] and 2.04 g of lithium acetate (CH3COOLi·2H2O, Alfa Aesar) were dissolved in 20 mL ethanol (99%, J.T. Baker) with stirring. The color of the solution became dark red. Then, 0.98 g of H3PO4 (85%, Aldrich) was added to the solution slowly. The solution was vigorously stirred while the color changed from orange to yellow-white and became clear. The molar ratio for Li:Fe:P was 2:1:1. Afterwards, the transparent solution was poured over various amounts of CMK-8 (either 0.3, 0.5 or 0.7 g, depending on the choice) and continuously stirred for 5 h. Then, the mixture was dried at 100 °C for 24 h and grounded to powder. The impregnation process was repeated one more time to increase the loading of LiFePO4 particles. The resulting mixture was hydrothermally treated in Scheme 1. Schematic illustration of synthesis of LF/C@CMK-8-X nanocomposites.


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an air environment by putting in an autoclave at 160 °C for 3 h. The final product was collected after filtration, washed and dried at 100 °C. The dried product was grounded into a fine powder and heat treated at 150 °C for 1 h and then at 650 °C for 3 h under Ar/H2 (95%:5%) environment to obtain the LiFePO4@CMK-8 powder. For carbon coating, the pre-synthesized LiFePO4@CMK-8 material was uniformly mixed with 40 wt.% citric acid (Sigma-Aldrich) as the carbon source in ethanol and then let it dry in an oven at 100 °C. The sample was annealed under Ar/H2 (95%:5%) environment initially at 150 °C for 1 h and then at 700 °C for 12 h to obtain the final carbon coated LiFePO4


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nanoparticles encapsulated in CMK-8. The samples are denoted as LF/C@CMK-8-X, where LF stands for LiFePO4 and X represents the amount of CMK-8 (X = 0.3, 0.5 and 0.7 in grams). 2.3. Fabrication of the Cell for LIBs The nanocomposite cathode was prepared by thoroughly mixing the active material with conductive carbon black (10% Super P, 5% KS6, Timcal) and polyvinylidene fluoride (PVdF) in a weight ratio of 80:15:5 in N-methyl-2-pyrrolidone (NMP). The mixture was stirred slowly for 6 h. The resulting slurry was spread onto an aluminum foil and dried in vacuum oven at 100 °C for 12 h. The cells were assembled using the coin type cell (CR2032) in an argon filled glove box with lithium metal (Sigma-Aldrich) as anode and polypropylene (PP) membrane as separator. The chosen electrolyte was 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC = 1:1 vol.%, Tomiyama Pure Chemicals). 2.4. Characterization Methods The structure and crystallinity of the LF/C@CMK-8-X nanocomposites were analyzed by the small and wide angle X-ray diffractometers (SXRD, Wiggler-A


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beamline, λ = 1.321712 Å, at National Synchrotron Radiation Research Center in Taiwan; WXRD, Shimadzu LabX XRD-6000 with Cu Kα radiation, λ = 1.5406 Å) at room temperature. The nitrogen adsorption-desorption measurements of the samples were carried out in a Micromeritics ASAP 2020 analyzer at 77K and the isotherms were obtained after degassing the samples at 150 °C for 10 h. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method in the relative pressure range of P/P0 = 0.05‒0.3, while the pore volume was calculated at or in the vicinity of P/P0 = 0.99. The pore size distribution was determined by the Barrett-Joyner-Halenda (BJH) method based on the analysis of the desorption branch of the isotherm. The microstructure and morphology of the samples were explored by transmission electron microscope (TEM, JEM-2000FX II)/high resolution transmission electron microscope (HR-TEM, JEOL JEM2100F) and field-emission scanning electron microscope (FESEM, Hitachi S-800), respectively. The thermal behavior of the LF/C@CMK-8-X samples was analyzed by a thermogravimetric analyzer (TA instrument Q50) at a heating rate of 10 °C min-1 in an air environment. X-ray photoelectron spectroscopy (XPS) was recorded in a Thermo VG Scientific Sigma Probe spectrometer equipped


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with a monochromated Al K radiation with spot size of 400 m. Raman spectroscopy measurements were carried out on DONGWOODM500i spectrometer. The galvanostatic charge-discharge performance was measured with WonATech WBCS3000 automatic battery cycler at room temperature in the voltage range of 2.6– 4.2 V. Cyclic voltammetry (CV) measurement was conducted between 2.6 and 4.2 V at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 105‒10-2 Hz using an Autolab PGSTAT302 potentiostat/galvanostat at room temperature. After the test cells were disassembled, the cathode was recovered and washed with pure EC to remove the electrolyte. Then it was dried in vacuum oven to remove the residual EC. SEM and WXRD measurements were carried out to investigate the morphology and crystallinity of the cathode material after cycles.

3. RESULTS AND DISCUSSION 3.1. Structural Analysis by X-ray Diffraction


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The structural ordering of mesoporous materials and the phase identification of crystalline materials can be analyzed by small and wide angle X-ray diffraction techniques, respectively. Figure 1(A) shows the small angle XRD (SXRD) patterns of the pristine CMK-8 and LF/C@CMK-8-X nanocomposites. The SXRD pattern of CMK-8 displayed one prominent peak around 2 = 0.95 and another weak peak around 2 = 1.1, corresponding to (211) and (220) diffraction planes of a three-dimensional cubic mesoporous structure (Ia3d space group),


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Figure 1. (A) SXRD of (a) CMK-8, (b) LF/[email protected], (c) LF/[email protected] and (d) LF/[email protected] nanocomposites. (B) WXRD of (a) LiFePO4 (JCPDS #83-2092) (b) CMK-8, (c) LF/[email protected], (d) LF/[email protected] and (e) LF/[email protected] nanocomposites.

suggesting a regular and highly ordered nature of the mesoporous CMK-8 structure.38 After impregnation of the LiFePO4 nanoparticles into CMK-8, the peak intensity of the LF/C@CMK-8-X samples was significantly decreased, suggesting the filling of pores by the LiFePO4 nanoparticles although complete regular ordered structure was somewhat disturbed or partly destroyed during the loading process. The wide angle XRD (WXRD) patterns of the LF/C@CMK-8-X nanocomposites in Figure 1(B) show intense diffraction peaks that are well indexed to orthorhombic LiFePO4 phase (space group Pnma) and consistent with standard JCPDS no. 83-2092. A small amount of the impurity phase belonging to Li3PO4 is also present in the nanocomposites. This minor nanocluster impurity may form because of a highly reducing environment at the time of the annealing process of the samples.39 It has been


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suggested that the presence of the Li3PO4 phase in the nanocomposites enhances the charge-discharge rate by providing an ion conductive surface.40,41 The crystallite size of the LiFePO4 particles is calculated from the Scherrer equation (i.e., cos = k/D, where : the FWHM of the peak, : Bragg angle, k = 0.9, : X-ray wavelength) by considering the diffraction peaks of (200), (101) and (311). The mean crystallite size of LiFePO4 was found to be 36.5, 25.5 and 32.7 nm for the LF/[email protected], LF/[email protected] and LF/[email protected] nanocomposites, respectively. No peaks due to CMK-8 could be recognized separately in the nanocomposites as its peaks were not very intense and might be overlapped with the peaks due to LiFePO4. The variation in the size of the nanocomposites could be attributed to the hydrophilic/hydrophobic interactions of the surfaces of CMK-8 with the LiFePO4 precursors. Initially, the CMK-8 material was treated with HNO3 to induce hydrophilicity. Although most of the carbon surfaces become hydrophilic, there may still some surfaces that are hydrophobic after the acid treatment. After acid treatment, the surface become oxidized and negatively charged. At lower and medium carbon concentrations (in the cases of CMK-8-0.3/CMK-8-0.5), the negatively charged surfaces are able to repel certain amounts of negatively charged


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precursors (e.g., PO43-) from entering the mesopores. At the same time, the remaining hydrophobic surfaces restrict the formation of the particles inside the mesopores. However, attraction of hydrophilic surfaces and the LiFePO4 precursor is stronger than repulsion and forms many particles inside the mesopores. These particles are smaller than the particles formed on the surfaces, and thus the particle size decreases with the increase in the carbon amount as the attractive interaction dominates. Therefore, LF/[email protected] possesses the smallest particle size. However, with the increase in the CMK-8 amount (CMK-8-0.7) the hydrophilic surfaces increase along with the hydrophobic surfaces. The repulsive interaction become stronger with more negatively charged surfaces along with hydrophobic surfaces at a higher carbon concentration. Under this condition, more particles are formed on the surfaces than inside the mesopores. Therefore, the average particle size increases for the LF/[email protected] nanocomposite with high carbon content.

3.2. Nitrogen Adsorption-Desorption Analysis


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Nitrogen adsorption-desorption measurements were carried out to investigate the pore structure, pore volume and surface area of the mesoporous support CMK-8 and the LF/C@CMK-8-X nanocomposites. Figure 2 depicts the adsorption-desorption isotherms of the pristine CMK-8 and LF/C@CMK-8-X nanocomposites, which reveal the type IV isotherms of mesoporous materials. The textural properties of the CMK-8 and LF/C@CMK-8-X nanocomposites are shown in Table 1. The pristine CMK-8 exhibited a high surface area of 1028 m2 g-1, pore volume of 1.54 cm3 g-1 and pore size of 3.8 nm. After impregnation of LiFePO4 nanoparticles into CMK-8, the surface area and pore volume were significantly decreased to 30‒83 m2 g-1 and 0.05‒0.17 cm3 g-1 for the LF/C@CMK-8-X (X = 0.3‒0.7) nanocomposites. However, the pore size of the three nanocomposites remained unchanged as it was in the pristine CMK-8 since there was no change in the synthesis condition, such as addition of swelling agents, co-surfactant, salts, etc. during the addition of nanoparticle precursors to the CMK-8 support. In addition, the wet impregnation process was followed to synthesize the nanocomposites, where there was little chance of change in the pore size. The reduction in surface areas and pore


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Figure 2. Nitrogen adsorption-desorption isotherms and pore size distribution curves (as inset) of (a) CMK-8, (b) LF/[email protected], (c) LF/[email protected] and (d) LF/[email protected] nanocomposites.

volumes clearly suggests partial filling of pores by LiFePO4 nanoparticles as well as blockage of channel cross-section due to the wall coverage by the LiFePO4


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nanoparticles. The surface area may also decrease because of the additional carbon coating of the LiFePO4 nanoparticles using citric acid as the carbon source. The observation of adsorption over P/P0 = 0.8 suggests the presence of interparticles pores or voids in CMK-8 and LF/C@CMK-8-X nanocomposites.42 Table 1. Textural properties of CMK-8 and LF/C@CMK-8-X nanocomposites. Surface area, ABET

Pore volume, Vp

Pore size, Dp

(m2 g-1)

(cm3 g-1)

(nm)

CMK-8

1028

1.54

3.8

LF/[email protected]

30

0.05

3.8


72

0.15

3.8


83

0.17

3.8

Samples

These interparticle voids generate large pores of around 11 nm in CMK-8 as shown in the pore size distribution curve. In the LF/C@CMK-8-X nanocomposites, the size of interparticle pores reduces to around 6 to 9 nm, suggesting partial filling of these pores


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also by LiFePO4 nanoparticles. The presence of the hysteresis loop, despite the reduction in steepness, suggests some mesopores are still open in the LF/C@CMK-8-X nanocomposites. These remaining mesopore volumes would be beneficial for infiltration of electrolytes, ionic diffusion as well as accommodation of volume change during the charge-discharge cycles.

3.3. Thermogravimetric Analysis The carbon content of LF/C@CMK-8-X nanocomposites was evaluated from thermogravimetric analysis (TGA). Figure 3 shows the TGA curves of the LF/C@CMK8-X nanocomposites under an air atmosphere. In pristine CMK-8 sample, the weight loss of around 12% up to 150 C was attributed to physisorbed water. The sample lost almost all weight due to the oxidation of carbon in the temperature range of 350 and 450 C. The remaining weight of 2.7% after 900 C might be the impurities present in the sample. In the cases of LF/C@CMK-8-


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Figure 3. TGA curves of (a) CMK-8, (b) LF/[email protected], (c) LF/[email protected] and (d) LF/[email protected] nanocomposites.

X nanocomposites, the initial weight loss was less than 1% (~0.6‒0.8 wt%) up to 300 C. However, there was a weight gain for all the nanocomposite samples in the temperature range of 330 to 520 C due to the oxidation of LiFePO4 (i.e., 12LiFePO4 + 3O2  4Li3Fe2(PO4)3 + 2Fe2O3). Theoretically, the pristine LiFePO4 exhibits a weight gain of 5.07% for this oxidation reaction and should be considered the weight in the calculation of carbon content from the TGA data.43,44 However, the presence of the


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coating carbon and matrix carbon (CMK-8) in the nanocomposites, the observed weight gains were limited to 1.72 wt% for LF/[email protected] and 0.61 wt% for both LF/[email protected] and LF/[email protected] samples. After 450 C, the carbon mixed with the LF/C@CMK-8-X nanocomposite samples started to oxidize leading to release of CO2 gas and loss in weight. Therefore, the percentage of weight gain is lower than the theoretical value for the LF/C@CMK-8-X nanocomposites. The remaining weight at higher temperatures above 700 C was attributed to LiFePO4 only. The total weight losses were found to be 8.26, 9.25 and 15.08 wt% for LF/[email protected], LF/[email protected] and LF/[email protected] nanocomposites, respectively. Therefore, the carbon contents

in

the

LF/[email protected],


and


nanocomposites were calculated to be 13.33, 14.32 and 20.15 wt%, respectively.

3.4. Morphology and Microstructure Analysis The morphology of ordered mesoporous carbon CMK-8 and its nanocomposite LF/[email protected] are analyzed by FESEM and presented in Figure 4(a,b). Some tiny particles aggregated to irregular shaped spherical particles of size around 1 m are


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observed for CMK-8. After incorporation of LiFePO4 nanoparticles, the morphology of the LF/[email protected] nanocomposite was apparently changed to sub-micron sized irregular shaped particles. It appeared that the LiFePO4 nanoparticles were well connected by the carbon network of CMK-8. The TEM and HRTEM images of the pristine CMK-8 and LF/C@CMK-8-X nanocomposites are presented in Figure 4(c-f). A long-ranged ordered cubic mesostructure was observed for CMK-8 (Figure 4c). The pore sizes of CMK-8 were found to be around 5 nm. The TEM images of the LF/C@CMK-8-X samples with different carbon amounts showed that the LiFePO4 nanoparticles were distributed both inside and outside of the mesopores. The particle sizes of the LiFePO4 nanoparticles varied from a few nm to a few tens of nm. While the TEM image of the LF/[email protected] nanocomposite showed most of the particles with a larger size of around 30 nm on the surface, some smaller particles also could be observed inside the mesopores. However, most of the nanoparticles were encapsulated in the mesoporous structure for


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Figure 4. SEM images of (a) CMK-8 and (b) LF/[email protected] nanocomposite. TEM images of (c) CMK-8, (d) LF/[email protected], (e) LF/[email protected] and (f) LF/[email protected] nanocomposites. Some particles are encircled in red color for better viewing.

LF/[email protected] (also see Figure S1, SI) and LF/[email protected] nanocomposites, which is in agreement with the N2 sorption results. Encapsulation of some larger


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nanoparticles inside the mesopores was also observed in the LF/[email protected] nanocomposite due to deformation of some pore walls. Also, void spaces of CMK-8 particles can accommodate some larger LiFePO4 nanoparticles. The observed particles in the TEM measurements were smaller than the crystallite


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Figure 5. (a) LiFePO4 particle with carbon coating from LF/[email protected] sample, (b) SAED pattern of LiFePO4 crystallite, (c-f) elemental mapping of LiFePO4 and (g) EDS spectrum of LiFePO4 from LF/[email protected] nanocomposite.

size determined by the Scherrer equation from XRD. This is because the synthesized LiFePO4 nanoparticles are not completely spherical. Moreover, X-ray diffraction is sensitive to the size of coherent scattering domains which may vary from the particle size significantly if any lattice defects or amorphous surface layers present.45 In addition, small crystallites may exclude from the analysis due to the limitation in the peak profile analysis. As TEM delivers direct images and local information on the particle shapes and sizes, it allows the best estimation of the degree of homogeneity of the sample. The uniform coating layer of carbon on the LiFePO4 particle is shown in Figure 5a with the thickness measured to be around 4 nm. The uniform carbon coating helps to enhance the electrical conductivity as well as lithium-ion diffusion. A highly crystalline nature with a distinct lattice spacing of 0.27 nm corresponding to the (301) plane of orthorhombic LiFePO4 was observed in the image (Figure 5a). The selected


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area electron diffraction (SAED) showed in Figure 5b provided a well-defined diffraction pattern of olivine structure and suggested the formation of single crystal LiFePO4 particles. Energy-dispersive X-ray spectroscopy (EDXS) elemental mapping and spectrum (Figure 5(c-g)) showed the presence of Fe, P, C and O in the nanocomposite sample, confirming successful synthesis of LF/C@CMK-8-X nanocomposites. The mapping also revealed the well dispersed LiFePO4 nanoparticles in the CMK-8 matrix. The tap density (TD) of cathode materials is directly related to the volumetric energy density with higher tap density leads to higher volumetric energy density and thus crucial for high energy storage devices. The morphology and size distribution of the LiFePO4 particles play a significant role in enhancing the tap density. The tap density of the LF/C@CMK-8-X nanocomposite was measured and found to be 1.23, 1.25 and 1.21 g cm-3 for LF/[email protected], LF/[email protected] and LF/[email protected], respectively. These values are comparatively higher than the commercial LiFePO4 particles provided by Targray (0.95 ± 0.15 g cm-3) and Aleees (1.0 ± 0.2 g cm-3).46,47

3.5. X-ray Photoelectron Spectroscopy


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X-ray photoelectron spectroscopy was carried out to verify the elemental composition and chemical state of the atoms in the LF/[email protected] nanocomposite. The core level spectra of C 1s, O 1s, P 2p and Fe 2p along with the deconvoluted peaks are presented in Figure 6. The C 1s

Figure 6. XPS core level spectra of C 1s, P 2p, O 1s and Fe 2p in LF/[email protected] nanocomposite.

spectrum could be deconvoluted into four peaks at 284.2, 285, 286.3 and 288.2 eV, which can be attributed to C−C, C−O, C=O and O−C=O, respectively.48,49 The stronger


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C−C peak at 284.2 eV was related to the carbons from citric acid and CMK-8. The P 2p spectrum displayed two components because of the spin-orbit coupling with the 2p3/2 component at 132.7 eV and the 2p1/2 component at 133.8 eV. The observation of the P 2p doublet suggested the existence of the PO43- moiety.50 The P 2p peak might contain contribution from Li3PO4 (133.6 eV) as both peaks overlapped in this binding energy region.51 The O 1s spectrum could split up to three peaks located at 531.5, 532.9 and 533.9 eV. The peak at 531.5 eV corresponded to oxygen atoms of the (PO4)3- unit of LiFePO4.50 The other two peaks at 532.9 and 533.9 eV were related to oxygencontaining groups attached to the carbon (O−C and C=O). The Fe 2p spectrum consisted of two components, namely Fe 2p3/2 and Fe 2p1/2, due to the spin-orbit coupling together with two satellite peaks. The peak due to the Fe 2p3/2 component was located at 711.8 eV, while it was at 725.1 eV for the Fe 2p1/2 component. The satellite peaks were observed at 714.3 and 726.8 eV. The energy separation between the two main peaks was 13.3 eV, confirming the presence of Fe2+ oxidation state in the olivine type LiFePO4.52 The results demonstrate the presence of the crystalline phase of LiFePO4 and carbon in the LF/[email protected] nanocomposite.


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3.6. Raman Spectroscopy The structure and properties of the matrix carbon and coated carbon were analyzed by Raman spectra. Figure 7 shows the Raman spectra of the pristine CMK-8 and LF/C@CMK-8-X nanocomposites. The pristine CMK-8 exhibited two strong broad bands at 1370 and 1640 cm-1, which were related to D (disordered carbon) and G (graphitic carbon) bands of carbon, respectively.53 The band at 1640 cm-1 could be assigned to the sp2 graphite-like carbon (E2g mode), suggesting its presence within the CMK-8 framework. In LF/C@CMK-8-X nanocomposites, the band at 1090 cm-1 was attributed to the symmetric PO4 stretching vibration of LiFePO4.53,54 Similar D and G bands were also observed for the nanocomposites due to the coating carbon and matrix carbon. Normally, the intensity ratio of D and G band (ID/IG) indicates the degree of graphitization.55 The ID/IG ratio of all the samples were measured and found to be 1.0, 1.0, 0.99 and 1.002 for the pristine CMK-8, LF/[email protected], LF/[email protected] and LF/[email protected], respectively. The lower the ID/IG value, the higher the electronic


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Figure 7. Raman spectra of (a) CMK-8 and LF/C@CMK-8-X nanocomposites with X = (b) 0.3, (c) 0.5 and (d) 0.7.

conductivity. Therefore, all samples have partial graphitic nature. This will boost the electronic conductivity which will be beneficial for good electrochemical performance of


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the electrode. The intensity ratio of PO4 band to carbon bands suggests the nature of surface coating of deposited carbon on LiFePO4 particles with a smaller ratio representing more uniform coating of carbon.56 The intensity ratio (𝐼𝑃𝑂4/(ID+IG)) was found to be 0.15, 0.19 and 0.51 for LF/[email protected], LF/[email protected] and LF/[email protected], respectively. The smaller ratios for LF/[email protected] and LF/[email protected] in comparison to LF/[email protected] suggested more uniform coating of carbon over the LiFePO4 particles in these samples.

3.7. Electrochemical Performances The cyclic voltammograms (CVs) of the LF/C@CMK-8-X nanocomposites are measured at scan rate of 0.1 mV s-1 and depicted in Figure 8(A). Two phase reactions of LiFePO4, mainly Fe2+/Fe3+ redox couple reaction corresponding to the extraction of lithium from LiFePO4 and the insertion of lithium into FePO4 phase were observed from the CV results. Smaller separation values between the anodic and cathodic peaks (~0.19 to 0.22 V) and almost overlapping of cycles from the second cycle onwards for the LF/C@CMK-8-X nanocomposites suggested good reaction reversibility during


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lithium ion insertion and extraction process.57 Moreover, almost symmetrical peaks of LF/C@CMK-8-X indicated lower polarization during the charge-discharge process. The higher peak current for LF/[email protected] in comparison to LF/[email protected] and LF/[email protected] nanocomposites suggests that the LF/[email protected] sample has better kinetic performance than the other two. Figure 8(B) shows the initial charge-discharge voltage profiles of the LF/C@CMK-8X nanocomposites at current rates varies from 0.05C to 5C. At a lower current rate of 0.05C, the discharge capacities reached 144.9, 184.8 and 166 mA h g-1 for the LF/[email protected],


and


nanocomposite

electrodes, respectively. As the LF/C@CMK-8-X nanocomposites possessed a higher tap density (1.21–1.25 g cm-3), the specific volumetric capacity reached 178.2, 231.0 and 200.8 mA h cm-3 for LF/[email protected], LF/[email protected] and LF/[email protected], respectively, which could enhance the energy


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Figure 8. (A) Cyclic voltammetry and (B) charge-discharge profiles of (a) LF/[email protected], (b) LF/[email protected] and (c) LF/[email protected] nanocomposites.

density of the batteries. The discharge capacity value of the LF/[email protected] electrode was relatively higher than the theoretical value of LiFePO4 (170 mA h g-1) and closer to the value for LF/[email protected] electrode. The value higher than theoretical


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capacity prevailed for LF/[email protected] electrode up to the current rate of 0.2C. It is suggested that the unique 3D interpenetrating bicontinuous mesoporous structure of CMK-8 helps to achieve such a high discharge capacity value for the LF/[email protected] electrode. However, the discharge capacity of the carbon coated LiFePO4 (LF/C) without CMK-8 was found to be 124.6 mA h g-1 at a current rate of 0.05C (Figure S2(A), SI). Therefore, it is obvious that mesoporous CMK-8 play an important role in the enhancement of capacity. Although the discharge capacity decreased with the increase in the current rates, the LF/[email protected] electrode still delivered higher discharge capacities of 149.8 and 126.9 mA h g-1 at current rates of 1C and 5C, respectively. In comparison, the LF/[email protected] and LF/[email protected] electrodes delivered discharge capacities of 101.5 and 83.1 mA h g-1 and 135.1 and 104.8 mA h g-1 at current rates of 1C and 5C, respectively. The results showed good stability of the electrodes at different charge and discharge current rates. The rate capability measurement was carried out to explore the effectiveness of CMK-8 modification on improving the electrochemical performance. The rate capability


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data of the LF/C@CMK-8-X nanocomposite electrodes are measured at different current rates by running the cell for 10 cycles at each current rate and depicted in Figure 9(A). It was observed that the LF/[email protected] nanocomposite possessed the highest capacities among the other two composites and its capacities were almost stable in all the current rates employed. The discharge capacity values of 184.8, 180.7, 178.3, 165.4, 149.8, 138.1, 126.9 and 120.1 mA h g-1 were obtained at current rates of 0.05C, 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 10C, respectively, for the LF/[email protected] nanocomposite. After prolonged cycles, the current rate was again reverted to 0.1 C with a slightly lower capacity value of 171.5 mA h g-1 than the previous value at the same current rate. This suggested the good structural stability of the LF/[email protected] nanocomposite electrode even after cycled at high charge-discharge current rates. The LF/[email protected] and LF/[email protected] nanocomposites also demonstrated similar behaviors at different current rates, but with lower capacity values than the LF/[email protected] nanocomposite. On the other hand, the discharge capacity values of 124.6, 121.1, 112.1, 100.8,


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Figure 9. (A) Rate capability and (B) long-term cycling performance of (a) LF/[email protected], (b) LF/[email protected], (c) LF/[email protected] nanocomposites.

89.3, 78.6, 63.4 and 42.8 mA h g-1 were obtained for LF/C (without the support CMK-8) at current rates of 0.05C, 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 10C, respectively (Figure


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S2(B), SI). To get an idea about the contribution of carbon into the theoretically exceeded capacity, the capacity of CMK-8 was also measured in the voltage range of 2.6‒4.2 V at 0.05C and found to be 46.7 mA h g-1 in the first cycle (Figure S3, SI). It was found from the TGA analysis that the carbon content in the LF/[email protected] nanocomposite was 14.3%. Therefore, multiplication of the carbon content in LF/[email protected] with the discharge capacity of CMK-8 can provide the capacity contribution of carbon to the total capacity and found to be 6.6 mA h g-1 at a current rate of 0.05C.58 Although the coating carbon contributes to the total carbon content of the nanocomposite, it was not separately considered for the capacity calculation since its weight was merely 1~2 wt.% in the nanocomposite. Similarly, the discharge capacity values for [email protected] (without carbon coating) were 135.6, 130.4, 121.8, 110.1, 96.6, 85.7, 70.1, 49.8, 128.7 mA h g-1 at current rates of 0.05C, 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C and 0.1C, respectively (Figure S4, SI). These values were remarkably lower than the LF/C@CMK-8-X nanocomposite electrodes. The present discharge capacity values are higher than the previously reported values in the literature on mesoporous carbon based LiFePO4 electrodes at lower current rates and comparable


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at higher current rates.12,28,53,59-61 Therefore, the role of ordered cubic mesoporous carbon CMK-8 as the support played in enhancing the capacity of the electrode is undeniable. At the same time, carbon coating also assists in improving the capacity of the LF/C@CMK-8-X nanocomposite electrodes. The good rate capability of the LF/C@CMK-8-X nanocomposite was ascribed to the well crystallized nature of LiFePO4 nanoparticles, carbon coating as well as the bicontinuous interpenetrating 3D mesopore networks of CMK-8 for rapid Li+ and electron transportation. The long-term cycling performances of the LF/C@CMK-8-X nanocomposite electrodes were carried out at a current rate of 10C for up to 1000 cycles. The discharge capacity and coulombic efficiency of the LF/C@CMK-8-X and LF/C electrodes are presented in Figure 9(B) and Figure S2(C) (SI), respectively. The cells delivered a discharge capacities of 67.1, 120 and 93.8 mA h g-1 at the first cycle for the LF/[email protected],


and


nanocomposites,

respectively, at 10C current rate. After 1000 cycles, the capacity slightly reduced to 61.6, 116 and 90.5 mA h g-1 corresponding to the LF/[email protected], LF/[email protected] and LF/[email protected] nanocomposites. Almost 91.8, 96.7 and 96.5% capacities can be


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retained

by

the

LF/[email protected],


and


nanocomposite electrodes after 1000 cycles. Although the coulombic efficiency (CE) values of LF/C@CMK-8-X were in the range of 94 to 98% for a few initial cycles, the CE values enhanced consecutively and stayed above 99.7% for the remaining cycles. On the other hand, the LF/C without CMK-8 delivered a discharge capacity of 46 mA h g-1 at the first cycle and reduced to 34.8 mA h g-1 after 800 cycles at a current rate of 10C, indicating the discharge capacity retention of only 75.7% after 800 cycles (Figure S2(C), SI). The superior capacity of the LF/C@CMK-8-X nanocomposites at higher current rates is ascribed to the 3D cubic bicontinuous mesoporous architecture of CMK-8. The 3D bicontinuous cubic framework behaved as a conductive network for the embedded LiFePO4 nanoparticles and contributed to the superior electrochemical performances.53 Moreover, the presence of mesoporosity in the CMK-8 framework helped to absorb a sufficient amount of liquid electrolyte and retained it inside the pores, and thus facilitated the interfacial contact between the LiFePO4 nanoparticles and the electrolyte for rapid movement of Li+ ions.62 In addition, aggregation of LiFePO4 nanoparticles can be


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prevented and volumetric changes in the transformation of LiFePO4/FePO4 phases can be accommodated with the use of 3D mesoporous CMK-8 support. These characteristics of CMK-8 OMC make significant contribution to the higher capacity for the LF/C@CMK-8-X nanocomposites in comparison to the LF/C sample. Among the LF/C@CMK-8-X nanocomposites, the LF/[email protected] possessed a higher discharge capacity than the other two compositions of the nanocomposite. Normally, the electrochemical performance of the cathode depends on the factors such as particle size, crystallinity, surface area, morphology, carbon content, porosity, etc. As observed in the WXRD patterns, all samples were well crystallized with the presence of a minor Li3PO4 phase. As the Li3PO4 phase provides ion conductive surfaces, the higher peak intensity of Li3PO4 in LF/[email protected] suggests improved chargedischarge rate in this nanocomposite.40,41 In addition, the particle size was smaller for LF/[email protected] in comparison to LF/[email protected] and LF/[email protected]. From the diffusion formula, t = L2/2D (t: diffusion time, L: diffusion length, D: diffusion coefficient), it is clear that the reduced particle size shorten the lithium intercalation and deintercalation length in LiFePO4, thereby expediting the lithium ion transfer and


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improving the performance of the LF/[email protected] sample.53 As the particle size of LF/[email protected] was larger than the other two samples, its electrochemical performance was inferior to the other two nanocomposite samples. Carbon coating also influences the electrochemical performance of the cathode. As reported previously, if the carbon layer is too thin, then the performance cannot be improved effectively, whereas a thicker carbon coating will hamper the lithium ion diffusion.63 From the TEM image of LF/[email protected], a carbon coating layer of 4 nm was observed on the surface of the LiFePO4 nanoparticles. This coating layer improves the interconnecting network of carbon and enhances the electronic conductivity, and thus leads to better charge-discharge performances.64 It was observed from Raman analysis that the ID/IG values were almost 1.0 for all the nanocomposite samples, suggesting partial graphitic nature and higher electronic conductivity. On the other hand, the intensity ratio 𝐼𝑃𝑂4 /(ID+IG) showed that carbon coating was more uniform in LF/[email protected] and LF/[email protected] than LF/[email protected]. In addition, the amount of carbon was higher for LF/[email protected] (20.2 wt%) than LF/[email protected] (13.3 wt%) and LF/[email protected] (14.3 wt%) as confirmed by TGA. The higher percentage of carbon lengthened the


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Li+-ion diffusion through the carbon layers during charge-discharge and reduced the diffusion coefficient resulting in the decrease in the capacity.65 On the other hand, the LF/[email protected] sample with a lower content of carbon exhibited poor electrochemical performance due to poor electronic conductivity.36 Therefore, the LF/[email protected] sample with the optimum carbon content delivered the highest capacity in comparison to the LF/[email protected] and LF/[email protected] samples. The electrochemical impedance spectroscopy (EIS) measurements were further carried out to evaluate the lithium diffusion coefficients. The Li+-ion diffusion coefficient is measured by using the following equation66,67 R2T2

(1)

DLi + = 2A2n4F4C2𝜎2

𝑤

where R is the gas constant, T is the absolute temperature (in K), A is the area of the positive electrode, n represents the number of transferred electron (n = 1, for Fe3+/Fe2+ redox reaction), F is the Faraday constant, C is the concentration of lithium ions and w is the Warburg factor which is related to Zre and can be calculated using the equation Zre = Re + Rct + σwω ―

12

(2)


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where Re is electrolyte resistance, Rct is the charge transfer resistance and  is the angular frequency. The impedance spectra of the LF/C@CMK-8-X nanocomposites after 1000 cycles in the frequency range from 105 to 10-2 Hz are shown in Figure S5 (SI). All the three electrodes showed similar plots with a depressed semi-circle in the high frequency region and an inclined line in the lower frequency region. An equivalent circuit model is fitted and shown in the inset of Figure S5 to examine the impedance spectra. An intercept at Zre axis at high frequency gives the electrolyte resistance (Re), while Rf and Rct are related to the interphase resistance and charge transfer resistance, respectively. The constant phase element (CPE) represents the double layer capacitance and passivation film capacitance. The depressed semi-circle was associated with Rf and Rct, whereas the inclined linear part determined the Warburg impedance (Zw) related to the Li+-ion diffusion.66 The relationship between Zre and inverse square root of angular frequency (-1/2) at the low frequency region is shown in the inset of Figure S5 (SI). The slopes of the linear fitting determined the w and were found to be 22.02, 21.2 and 21.64  cm2 s-1/2 for LF/[email protected], LF/[email protected] and LF/[email protected], respectively. Accordingly, the lithium diffusion coefficients were


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measured from the equation 1 and found to be 6.22 × 10-12, 7.1 × 10-12 and 6.6 × 10-12 cm2 s-1, respectively. The higher diffusion coefficient for LF/[email protected] in comparison to other nanocomposites facilitates faster kinetics to achieve the higher rate performance of this electrode. Consequently, the factors discussed above such as smaller particle size, crystallinity, shorter lithium diffusion distance, carbon coating thickness and carbon content, the presence of Li3PO4 phase and higher lithium diffusion coefficient acted in a combined fashion to deliver the higher capacity for the LF/[email protected] nanocomposite electrode. From the above discussion, it is possible to specify some important features which help in the improvement of the capacity of LF/[email protected] beyond the theoretical capacity of LiFePO4. Based on these points, a conduction mechanism is proposed to explain the higher capacity of the present nanocomposite material as illustrated in Scheme 2. The main factor that leads the capacity to exceed the theoretical limit of LiFePO4 can be attributed to the contribution from the mesoporous carbon CMK-8. As mentioned above, there is a contribution of 6.6 mA h g-1 from CMK-8 to the total capacity of 184.8 mA h g-1 of the LF/[email protected] electrode.


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Scheme 2. Schematic representation of the conduction mechanism of ions and electrons in LF/C@CMK-8-X cathode material.

Therefore, it is clear that CMK-8 can store some Li+-ions to contribute in the chargedischarge process. However, the exceeded capacity is still higher than the capacity obtained from the CMK-8. Therefore, it suggests that capacity contribution cannot be estimated directly by combining the capacity of individual components. There may be a synergistic effect between LiFePO4 and CMK-8 with regard to both structures of the material and energy storage.68,69 The highly interconnected 3D cubic bicontinuous


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mesoporous carbon structure of CMK-8 with uniform pore size and regular pore arrangement allows an even distribution of electrolytes and higher electrode/electrolyte interfacial contacts, offering a large quantity of active sites for Li+ ion intercalation and deintercalation. As part of the LiFePO4 nanoparticles is encapsulated inside the mesopores, these nanoparticles can easily come into the contact of the electrolyte. Consequently, the Li+-ions move faster from the electrolyte to LiFePO4 and vice versa, during the lithium intercalation/deintercalation process and thus shorten the diffusion distance of Li+-ions to boost the capacity.28 Furthermore, the interconnected 3D porous carbon framework of CMK-8 provides multiple pathways for rapid electron conduction in the electrode (Scheme 2). The size of the LiFePO4 nanoparticles also participates in the enhancement of the capacity. The ordered structure of mesoporous carbon CMK-8 can provide spatial confinement to restrict the growth of the LiFePO4 nanoparticles and prevents their aggregation. As the particle size of LF/[email protected] nanocomposite is smaller than those of LF/[email protected] and LF/[email protected] nanocomposites, the Li+-ion diffusion would be faster in LF/[email protected] during charging and discharging due to short solid state diffusion paths, which leads to improvement in kinetics and


50

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thereby capacity. Another important factor that improves the capacity of the nanocomposite electrode is the carbon coating on LiFePO4 nanoparticles. The uniform carbon coating lowers the charge transfer resistance, protects the nanocomposite electrode from direct contact with the electrolyte and improves the electronic conductivity and cycle life of batteries. This carbon coating is believed to provide electronic tunnel to balance the charge equilibrium during Li+-ions intercalationdeintercalation.70 In addition, this carbon coating is permeable for the Li+-ions from the electrolyte to diffuse rapidly through the lattice of LiFePO4. The Raman and TEM analyses show that carbon is coated uniformly on the surface of the LF/[email protected] nanocomposite, which may lead to faster electron transfer since thinner and uniform carbon coating can reduce the charge transfer resistance for surface reactions and improve the rate performances. The carbon coating and smaller particle size thus increase the electronic and ionic conductivity and enhance the kinetics. This facilitates more Fe2+ to Fe3+ oxidation for LF/[email protected] in comparison to LF/[email protected] and LF/[email protected] as observed from the CV results of Figure 8(A), leading to higher capacity than the theoretical capacity of LiFePO4.71 In a nutshell, the capacity


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contribution from CMK-8, faster electronic and ionic conduction from the double carbon structure of 3D interpenetrating mesoporous conducting networks of CMK-8 and uniform carbon coating, shortening of Li+-ion diffusion length owing to the smaller particle size of LiFePO4 nanoparticles, and higher Fe2+ to Fe3+ oxidation enhance the capacity of LF/[email protected] nanocomposite beyond the theoretical capacity of LiFePO4.

3.8. Post Characterizations of the Cycled Cathode After 1000 cycles, the cycled electrode was disassembled in order to explore any changes in the morphology and crystalline structure of the cathode. The SEM images of the LF/[email protected] nanocomposite electrode before and after cycle are shown in Figure S6(a,b) (SI). Some particle aggregation and pulverization were observed after 1000 cycles, although some similar size particles were still present in the electrode after cycles. There was no any cracking observed on the surface, suggesting good electrical contacts and low polarization of the electrode. The discharge capacity also decreased minimally from the first cycle to the 1000th cycle because of good morphology of the


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cathode. The WXRD pattern (Figure S6(c), SI) showed that particles were in a well crystallized form, further confirming good conductivity and excellent structural stability of the electrode after a long cycle test.

4. CONCLUSIONS In summary, LiFePO4 nanoparticles partly encapsulated in three dimensional bicontinuous and interconnected network of ordered mesoporous carbon CMK-8 were prepared by a facile impregnation method. The synthesized nanocomposites were further coated with carbon to enhance the performance for LIBs. The close contact between the LiFePO4 nanoparticles and the support carbon framework provides rapid transfer of electrons during lithium insertion/extraction as well as restricted the particle growth and aggregation. The mesoporosity of CMK-8 helps to absorb and retains sufficient electrolyte inside the mesopores and permits faster lithium-ion diffusion to enhance the rate capability. Among the nanocomposites, the LF/[email protected] nanocomposite cathode exhibits excellent rate capability with a discharge capacity reaching 184.8 mA h g-1 at a current rate of 0.05C, which is higher than the theoretical


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capacity of LiFePO4. The LF/[email protected] nanocomposite delivers higher than the theoretical capacity of LiFePO4 up to a current rate of 0.2C with a discharge capacity value of 178.3 mA h g-1. At a higher current rate of 10C, the LF/[email protected] nanocomposite reveals a remarkable discharge capacity of 120 mA h g-1 with the capacity retention of 96.7% after 1000 cycles. The excellent rate capability, cyclability and higher coulombic efficiency of above 99.7% demonstrate the great potentials of the present LF/[email protected] nanocomposite as the cathode for applications in lithium-ion batteries.

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. Synthesis schemes of KIT-6 and CMK-8. TEM image of [email protected] nanocomposite Charge-discharge profile, rate capability and long-term cycle performance of LF/C composite.


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Cycle performance of CMK-8 in the voltage range of 2.6‒4.2 V at 0.05C. Rate capability of [email protected] nanocomposite. EIS plots of LF/C@CMK-8-X nanocomposites. SEM and WXRD of LF/[email protected] nanocomposite after 1000 cycles.

AUTHOR INFORMATION

Corresponding Authors * Hsien-Ming Kao, E-mail: [email protected] * Yung-Chin Yang, E-mail: [email protected] ORCID ID Diganta Saikia: 0000-0003-0256-5884 Juti Rani Deka: 0000-0003-3710-2281 Hsien-Ming Kao: 0000-0002-4144-3890

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources Ministry of Science and Technology, Taiwan. Grant number: MOST 105-2119-M-008012.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The financial support for this work from the Ministry of Science and Technology of Taiwan (Grant number: MOST 105-2119-M-008-012) is gratefully acknowledged.

REFERENCES (1) Nitta, N.; Wu, F.; Lee, J.T.; Yushin, G. Li-ion Battery Materials: Present and Future, Mater. Today 2015, 18, 252‒264. (2) Blomgren, G. E. The Development and Future of Lithium Ion Batteries, J. Electrochem. Soc. 2017, 164, A5019‒A5025.


56

Page 57 of 66 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Gong, C.; Xue, Z.; Wen, S.; Ye, Y.; Xie, X. Advanced Carbon Materials/Olivine LiFePO4 Composites Cathode for Lithium Ion Batteries, J. Power Sources 2016, 318, 93‒112. (4) Mogg, T. Digital Trends, https://www.digitaltrends.com/cool-tech/elon-musk-australianbattery-record/ (Accessed September 2018). (5) Liu, C.; Neale, Z. G.; Cao, G. Understanding Electrochemical Potentials of Cathode Materials in Rechargeable Batteries, Mater. Today 2016, 19, 109‒123. (6) Eftekhari, A. LiFePO4/C Nanocomposites for Lithium-ion Batteries, J. Power Sources 2017, 343, 395‒411. (7) Self, E. C.; McRen, E. C.; Wycisk, R.; Pintauro, P. N. LiCoO2-based Fiber Cathodes for Electrospun Full Cell Li-ion Batteries, Electrochim. Acta 2016, 214, 139‒146. (8) Duan, L.; Zhang, X.; Yue, K.; Wu, Y.; Zhuang, J.; Lü, W. Synthesis and Electrochemical Property of LiMn2O4 Porous Hollow Nanofiber as Cathode for Lithium-ion Batteries, Nanoscale Res. Lett. 2017, 12, 109(1‒8). (9) Ren, D.; Shen, Y.; Yang, Y.; Shen, L.; Levin, B. D. A.; Yu, Y.; Muller, D. A.; Abruña, H. D. Systematic Optimization of Battery Materials: Key Parameter Optimization for the Scalable Synthesis of Uniform, High-Energy, and High Stability LiNi0.6Mn0.2Co0.2O2 Cathode Material for Lithium-ion Batteries, ACS Appl. Mater. Interfaces 2017, 9, 35811−35819. (10) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho‐Olivines as Positive‐Electrode Materials for Rechargeable Lithium Batteries, J. Electrochem. Soc. 1997, 144, 1188‒1194.


57


Page 58 of 66

(11) Yamada, A.; Chung, S. C.; Hinokuma, K. Optimized LiFePO4 for Lithium Battery Cathodes, J. Electrochem. Soc. 2001, 148, A224‒A229. (12) Yang, M,; Gao, Q. LiFePO4/C Composite Cathode Material with a Continuous Porous Carbon Network for High Power Lithium-ion Battery, J. Alloys Compd. 2011, 509, 3690–3698. (13) Yang, Z.; Dai, Y.; Wang, S.; Yu, J. How to Make Lithium Iron Phosphate Better: A Review Exploring Classical Modification Approaches In-depth and Proposing Future Optimization Methods, J. Mater. Chem. A 2016, 4, 18210–18222. (14) Mi, Y.; Yang, C.; Zuo, Z.; Qi, L.; Tang, C.; Zhang, W.; Zhou, H. Positive Effect of Minor Manganese Doping on the Electrochemical Performance of LiFePO4/C Under Extreme Conditions, Electrochim. Acta 2015, 176, 642‒648. (15) Park, K. S.; Son, J. T.; Chung, H. T.; Kim, S. J.; Lee, C. H.; Kang, K. T.; Kim, H. G. Surface Modification by Silver Coating for Improving Electrochemical Properties of LiFePO4, Solid State Commun. 2004, 129, 311‒314. (16) Xia, J.; Zhu, F.; Wang, G.; Wang, L.; Meng, Y.; Zhang, Y. Synthesis of LiFePO4/C Using Ionic Liquid as Carbon Source for Lithium Ion Batteries, Solid State Ionics 2017, 308, 133‒138. (17) Levin, O. V.; Eliseeva, S. N.; Alekseeva, E. V.; Tolstopjatova, E. G.; Kondratiev, V. V. Composite LiFePO4/Poly-3,4-Ethylenedioxythiophene Cathode for Lithium-ion Batteries with Low Content of Non-Electroactive Components, Int. J. Electrochem. Sci. 2015, 10, 8175–8189. (18) Bruce, P. G. Energy Storage beyond the Horizon: Rechargeable Lithium Batteries, Solid State Ionics 2008, 179, 752–760.


58

Page 59 of 66 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Liu, D.; Cao, G. Engineering Nanostructured Electrodes and Fabrication of Film Electrodes for Efficient Lithium Ion Intercalation, Energy Environ. Sci. 2010, 3, 1218– 1237. (20) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries, Nature 2001, 414, 359‒367. (21) Gaberscek, M.; Dominko, R.; Jamnik, J. Is Small Particle Size More Important than Carbon Coating? An Example Study on LiFePO4 Cathodes, Electrochem. Commun. 2007, 9, 2778‒2783. (22) Ellis, B.; Kan, W. H.; Makahnouk, W. R. M.; Nazar, L. F. Synthesis of Nanocrystals and Morphology Control of Hydrothermally Prepared LiFePO4, J. Mater. Chem. 2007, 17, 3248‒3254. (23) Wang, G. X.; Shen, X. P.; Yao, J. One-Dimensional Nanostructures as Electrode Materials for Lithium-Ion Batteries with Improved Electrochemical Performance, J. Power Sources 2009, 189, 543‒546. (24) Yang, M. R.; Ke, W. H.; Wu, S. H. Preparation of LiFePO4 Powders by Co-Precipitation, J. Power Sources 2005, 146, 539‒543. (25) Cho, T. H.; Chung, H. T. Synthesis of Olivine-Type LiFePO4 by Emulsion-Drying Method, J. Power Sources 2004, 133, 272‒276. (26) Konarova, M.; Taniguchi, I. Synthesis of Carbon-Coated LiFePO4 Nanoparticles with High Rate Performance in Lithium Secondary Batteries, J. Power Sources 2010, 195, 3661–3667.


59


Page 60 of 66

(27) Takahashi, M.; Tobishima, S.; Takei, K.; Sakurai, Y. Characterization of LiFePO4 as the Cathode Material for Rechargeable Lithium Batteries, J. Power Sources 2001, 97–98, 508–511. (28) Wang, G.; Liu, H.; Liu, J.; Qiao, S.; Lu, G. M.; Munroe, P.; Ahn, H. Mesoporous LiFePO4/C Nanocomposite Cathode Materials for High Power Lithium Ion Batteries with Superior Performance, Adv. Mater. 2010, 22, 4944–4948. (29) Doeff, M. M.; Wilcox, J. D.; Kostecki, R.; Lau, G. Optimization of Carbon Coatings on LiFePO4, J. Power Sources 2006, 163, 180‒184. (30) Chang, Q.; Yao, G.; Pan, F.; Chen, D.; Wang, W.; Yan, W. Investigation on Interfacial Interaction and the Origin of Fe3+ in LiFePO4/C, Electrochim. Acta, 2013, 108, 211‒218. (31) Yuan, L. -X.; Wang, Z. -H.; Zhang, W. -X.; Hu, X. -L.; Chen, J. -T.; Huang, Y. -H.; Goodenough, J. B. Development and Challenges of LiFePO4 Cathode Material for Lithium-Ion Batteries, Energy Environ. Sci. 2011, 4, 269–284. (32) Lin, Y.; Gao, M. X.; Zhu, D.; Liu, Y. F.; Pan, H. G. Effects of Carbon Coating and Iron Phosphides on the Electrochemical Properties of LiFePO4/C, J. Power Sources 2008, 184, 444‒448. (33) Julien, C. M.; Mauge, A.; Ait-Salah, A.; Massot, M.; Gendron, F.; Zaghib, K. Nanoscopic Scale Studies of LiFePO4 as Cathode Material in Lithium-Ion Batteries for HEV Application, Ionics 2007, 13, 395‒411. (34) Ma, Z.; Fan, Y.; Shao, G.; Wang, G.; Song, J.; Liu, T. In Situ Catalytic Synthesis of High-Graphitized Carbon-Coated LiFePO4 Nanoplates for Superior Li-Ion Battery Cathodes, ACS Appl. Mater. Interfaces 2015, 7, 2937−2943.


60

Page 61 of 66 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Paolella, A.; Bertoni, G.; Marras, S.; Dilena, E.; Colombo, M.; Prato, M.; Riedinger, A.; Povia, M.; Ansaldo, A.; Zaghib, K.; Manna, L.; George, C. Etched Colloidal LiFePO4 Nanoplatelets Toward High-Rate Capable Li-Ion Battery Electrodes, Nano Lett. 2014, 14, 6828−6835. (36) Talebi-Esfandarani, M.; Savadogo, O. Improvement of Electrochemical and Electrical Properties of LiFePO4 Coated with Citric Acid, Rare Met. 2016, 35, 303‒308. (37) Maiyalagan, T.; Alaje, T. O.; Scott, K. Highly Stable Pt–Ru Nanoparticles Supported on Three-Dimensional Cubic Ordered Mesoporous Carbon (Pt–Ru/CMK-8) as Promising Electrocatalysts for Methanol Oxidation, J. Phys. Chem. C 2012, 116, 2630–2638. (38) Kleitz, F.; Choi, S. H.; Ryoo, R. Cubic Ia3d Large Mesoporous Silica: Synthesis and Replication to Platinum Nanowires, Carbon Nanorods and Carbon Nanotubes, Chem. Commun. 2003, 17, 2136‒2137. (39) Salah, A. A.; Mauger, A.; Julien, C. M.; Gendron, F. Nano-sized Impurity Phases in Relation to the Mode of Preparation of LiFePO4, Mater. Sci. Eng., B 2006, 129, 232‒244. (40) Herle, P. S.; Ellis, B.; Coombs, N.; Nazar, L. F. Nano-Network Electronic Conduction in Iron and Nickel Olivine Phosphates, Nature Mater. 2004, 3, 147‒152. (41) Ellis, B. L.; Makahnouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F. A Multifunctional 3.5 V Iron-based Phosphate Cathode for Rechargeable Batteries, Nature Mater. 2007, 6, 749‒753. (42) Chen, Y.; Li, X.; Park, K.; Song, J.; Hong, J.; Zhou, L.; Mai, Y. -W.; Huang, H.; Goodenough, J. B. Hollow Carbon-Nanotube/Carbon-Nanofiber Hybrid Anodes for LiIon Batteries, J. Am. Chem. Soc. 2013, 135, 16280‒16283.


61


Page 62 of 66

(43) Belharouak, I.; Johnson, C.; Amine, K. Synthesis and Electrochemical Analysis of Vapor-Deposited Carbon-Coated LiFePO4, Electrochem. Commun. 2005, 7, 983–988. (44) Cao, Y. L.; Yu, L. H.; Li, T.; Ai, X. P.; Yang, H. X. Synthesis and Electrochemical Characterization of Carbon-Coated Nanocrystalline LiFePO4 Prepared by PolyacrylatesPyrolysis Route, J. Power Sources 2007, 172, 913–918. (45) Borchert, H.; Shevchenko, E. V.; Robert, A.; Mekis, I.; Kornowski, A.; Grübel, G.; Weller, H. Determination of Nanocrystal Sizes: A Comparison of TEM, SAXS, and XRD Studies of Highly Monodisperse CoPt3 Particles, Langmuir 2005, 21, 1931‒1936. (46) Targray

LFP

cathode

product,

https://www.targray.com/li-ion-battery/cathode-

materials/lfp (Accessed date January 7, 2019). (47) Aleees

lithium

ferrous

phosphate

nano

co-crystalline

compound,

http://www.aleees.com/en/product/lfp-nco/m12.html (Accessed date January 7, 2019). (48) Tu, X.; Zhou, Y.; Tian, X.; Song, Y.; Deng, C.; Zhu, H. Monodisperse LiFePO4 Microspheres Embedded with Well-Dispersed Nitrogen-Doped Carbon Nanotubes as High-Performance Positive Electrode Material for Lithium-Ion Batteries, Electrochim. Acta 2016, 222, 64‒73. (49) Ding, Y.; Pan, P.; Chen, L.; Fu, Z.; Du, J.; Guo, L.; Wang, F. LiFePO4 Composites Decorated with Nitrogen-Doped Carbon as Superior Cathode Materials for Lithium-Ion Batteries, Ionics 2017, 23, 3295‒3302. (50) Castro, L.; Dedryvère, R.; El Khalifi, M.; Lippens, P. -E.; Bréger, J.; Tessier, C.; Gonbeau, D. The Spin-Polarized Electronic Structure of LiFePO4 and FePO4 Evidenced by In-Lab XPS, J. Phys. Chem. C 2010, 114, 17995–18000.


62

Page 63 of 66 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Rho, Y. -H.; Nazar, L. F.; Perry, L.; Ryan, D. Surface Chemistry of LiFePO4 Studied by Mössbauer and X-ray Photoelectron Spectroscopy and Its Effect on Electrochemical Properties, J. Electrochem. Soc. 2007, 154, A283‒A289. (52) Wang, B.; Abdulla, W. A.; Wang, D.; Zhao, X. S. A Three-Dimensional Porous LiFePO4 Cathode Material Modified with a Nitrogen-Doped Graphene Aerogel for High-Power Lithium Ion Batteries, Energy Environ. Sci. 2015, 8, 869–875. (53) 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. (54) Kravchenko, V. V.; Michailov, V. I.; Sigaryov, S. E. Some Features of Vibrational Spectra of Li3M2(PO4)3 (M = Sc, Fe) Compounds Near a Superionic Phase Transition, Solid State Ionics 1992, 50, 19‒30. (55) Shen, L. F.; Zhang, X. G.; Uchaker, E.; Yuan, C. Z.; Cao, G. Z. Li4Ti5O12 Nanoparticles Embedded in a Mesoporous Carbon Matrix as a Superior Anode Material for High Rate Lithium Ion Batteries, Adv. Energy Mater. 2012, 2, 691–698. (56) Hong, J.; Wang, C.; Dudney, N. J.; Lance, M. J. Characterization and Performance of LiFePO4 Thin-Film Cathodes Prepared with Radio-Frequency Magnetron-Sputter Deposition, J. Electrochem. Soc. 2007, 154, A805‒A809. (57) Kim, J. -K.; Choi, J. -W.; Chauhan, G. S.; Ahn, J. -H.; Hwang, G. -C.; Choi, J. -B.; Ahn, H. -J. Enhancement of Electrochemical Performance of Lithium Iron Phosphate by Controlled Sol–Gel Synthesis, Electrochim. Acta 2008, 53, 8258‒8264.


63


Page 64 of 66

(58) Cheng, F.; Wang, S.; Wang, C. -Y.; Li, W. -C. Interconnected Porous Carbon with Tunable Pore Size as a Model Substrate to Confine LiFePO4 Cathode Material for Energy Storage, Microporous Mesoporous Mater. 2015, 204, 190–196. (59) Zhao, J.; He, J.; Zhou, J.; Guo, Y.; Wang, T.; Wu, S.; Ding, X.; Huang, R.; Xue, H. Facile Synthesis for LiFePO4 Nanospheres in Tridimensional Porous Carbon Framework for Lithium Ion Batteries, J. Phys. Chem. C 2011, 115, 2888–2894. (60) Sun, S.; Ghimbeu, C. M.; Janot, R.; Le Meins, J. -M.; Cassel, A.; Davoisne, C.; Masquelier, C.; Vix-Guterl, C. One-Pot Synthesis of LiFePO4–Carbon Mesoporous Composites for Li-Ion Batteries, Microporous Mesoporous Mater. 2014, 198, 175–184. (61) Wu, G.; Ran, R.; Zhao, B.; Sha, Y.; Su, C.; Zhou, Y.; Shao, Z. 3D Amorphous Carbon and Graphene Co-modified LiFePO4 Composite Derived from Polyol Process as Electrode for High Power Lithium-Ion Batteries, J. Energy Chem. 2014, 23, 363–375. (62) Dominko, R.; Bele, M.; Goupil, J. -M.; Gaberscek, M.; Hanzel, D.; Arcon, I.; Jamnik, J. Wired Porous Cathode Materials: A Novel Concept for Synthesis of LiFePO4, Chem. Mater. 2007, 19, 2960–2969. (63) Cho, Y. -D.; Fey, G. T. -K.; Kao, H. -M. The Effect of Carbon Coating Thickness on the Capacity of LiFePO4/C Composite Cathodes, J. Power Sources 2009, 189, 256–262. (64) Liu, Y.; Liu, D.; Zhang, Q.; Yu, D.; Liu, J.; Cao, G. Lithium Iron Phosphate/Carbon Nanocomposite Film Cathodes for High Energy Lithium Ion Batteries, Electrochim. Acta 2011, 56, 2559–2565. (65) Yang, S. T.; Zhao, N. H.; Dong, H. Y.; Yang, J. X.; Yue, H. Y. Synthesis and Characterization of LiFePO4 Cathode Material Dispersed with Nano-Structured Carbon, Electrochim. Acta 2005, 51, 166–171.


64

Page 65 of 66 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(66) Zhou, Y. K.; Wang, J.; Hu, Y. Y.; O'Hayre, R.; Shao, Z. P. A Porous LiFePO4 and Carbon Nanotube Composite, Chem. Commun. 2010, 46, 7151–7153. (67) Qin, G. H.; Ma, Q. Q.; Wang, C. Y. A Porous C/LiFePO4/Multiwalled Carbon Nanotubes Cathode Material for Lithium Ion Batteries, Electrochim. Acta 2014, 115, 407–415. (68) Wang, B.; Wang, Q.; Xu, B.; Liu, T.; Wang D.; Zhao, G. The synergy effect on Li storage of LiFePO4 with activated carbon modifications, RSC Adv. 2013, 3, 20024–20033. (69) Wang, B.; Xu, B.; Liu, T.; Liu, P.; Guo, C.; Wang, S.; Wang, Q.; Xiong, Z.; Wang D.; Zhao, X. S. Mesoporous carbon-coated LiFePO4 nanocrystals co-modified with graphene and Mg2+ doping as superior cathode materials for lithium ion batteries, Nanoscale 2014, 6, 986–995. (70) Cushing, B. L.; Goodenough, J. B. Influence of carbon coating on the performance of a LiMn0.5Ni0.5O2 cathode. Solid State Sci. 2002, 4, 1487‒1493. (71) Zhang, Q.; Huang, S. -Z.; Jin, J.; Liu, J.; Li, Y.; Wang, H. -E.; Chen, L. -H.; Wang B. -J.; Su, B. -L. Engineering 3D bicontinuous hierarchically macro-mesoporous LiFePO4/C nanocomposite for lithium storage with high rate capability and long cycle stability, Scientific Reports 2016, 6, 25942 (1-12).


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Table of Contents Graphic


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Encapsulation of LiFePO4 Nanoparticles into 3D Interpenetrating

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