Enhanced Electrochemical Performance of LiMn0.75Fe0.25PO4

Apr 20, 2017 - Combined with the FFT pattern (inset, Figure 5b), one can confirm the large exposed crystal face is (010), and the thinnest crystal dir...
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Research Article pubs.acs.org/journal/ascecg

Enhanced Electrochemical Performance of LiMn0.75Fe0.25PO4 Nanoplates from Multiple Interface Modification by Using FluorineDoped Carbon Coating Xiao Yan,† Deye Sun,† Yanqing Wang,†,‡ Zengqi Zhang,† Wenchao Yan,† Jicheng Jiang,†,‡ Furui Ma,†,‡ Jian Liu,§ Yongcheng Jin,*,† and Kiyoshi Kanamura*,∥ †

CAS Key Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, 189 Songling Road, Qingdao 266101, P. R. China ‡ University of Chinese Academy of Sciences, 19A Yuquanlu Road, Beijing 100049, P. R. China § School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China ∥ Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: We report a novel composite of fluorine-doped carbon-decorated LiMn0.75Fe0.25PO4 (LMFP) nanoplates synthesized via a facile method by using hybrid sucrose and polyvinylidene fluoride as carbon and fluorine sources. In the composite, the thin LMFP nanoplates expose large amounts of (010) crystal face which shortens the Li+ ion diffusion distance. Also, the fluorine-doped carbon coating layer can provide a sufficient pathway for rapid electron transport, and the partially formed metal fluorides in the interface between the LMFP nanoplates surface and fluorine-doped carbon coating layer will help reduce charge transfer resistance. Because of this unique structure, the resulting product exhibits a superior discharge capacity of 162.2 mA h g−1 at the 1 C current rate, and the capacity is retained 94.8% over 200 cycles. Furthermore, this material also can deliver a reversible capacity of 130.3 mA h g−1 at an ultrahigh current rate of 20 C, in which the discharge procedure can be accomplished only in 144 s. The celerity and cycling capability of the prepared material endow it with great potential for application in high performance lithium-ion batteries. KEYWORDS: Lithium-ion battery, Cathode, Lithium manganese phosphate, Fluorine-doped carbon, Electrochemistry

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INTRODUCTION

cost, and safety capability. However, the low electronic conductivity (∼10−10 S cm−1) of LMP leads to inferior electrochemical performances.5,6 Hence, a strategy of partial substitution of Mn in LMP conducted by other transition metals to form LiMnxM1−xPO4 composites (M = Mg, Zn, Fe, etc.)7−10 was proposed to enhance the electrochemical kinetics during the charge−discharge process and, consequently, improve the electrochemical properties. Among the above-

The lithium-ion battery (LIB) is regarded as one of the most promising candidates for energy storage owing to its excellent energy density. It has dominated the market of portable electronic products, and its application fields are expanding to electric vehicles (EVs) and large energy storage systems.1−3 The electrode as a critical part of LIBs has a significant effect on the electrochemical performance of devices. In recent decades, olivine-structure LiMnPO4 (LMP) as the cathode material has been attracting wide attention and has been deeply studied because of its high theoretical capacity (170 mAh g−1), superior energy density (684 Wh kg−1),4 environmental friendliness, low © 2017 American Chemical Society

Received: December 26, 2016 Revised: March 22, 2017 Published: April 20, 2017 4637

DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644

Research Article

ACS Sustainable Chemistry & Engineering

suspension under stirring for 5 min, and then, the mixture was transferred into a Teflon-lined stainless steel autoclave, followed by solvothermal treatment at 200 °C for 12 h. After the autoclave was cooled to room temperature, the obtained sample was collected and washed with water and ethanol each three times and dried in a vacuum oven under 60 °C for 10 h. Preparation of Carbon-Coated LMFP (LMFP/C) and FluorineDoped Carbon-Decorated LMFP (LMFP/C−F). For carbon-coated LMFP, LMFP and sucrose (weight ratio of LMFP/sucrose is 4:1) were mixed and then heated at 600 °C for 4 h under Ar/H2 (95/5 in volume %). For fluorine-doped carbon-decorated LMFP, the process was similar to carbon coating except for hybrid sucrose and polyvinylidene fluoride (PVDF) (weight ratio of LMFP/(sucrose +PVDF) is 4:1, and weight ratio of sucrose/PVDF is 1:2) as coating agents. The above resultant products were denoted as LMFP/C and LMFP/C−F, respectively. Materials Characterization. Crystallographic structures of the prepared samples were detected by X-ray diffraction (XRD) (Bruker AXS D8 diffractometer, Cu Kα radiation). The morphology and microstructure of the samples were characterized using a field emission scanning electron microscope (FESEM, Hitachi S-4800) equipped with an energy dispersive X-ray (EDX) detector, transmission electron microscopy (TEM, Hitachi H-7650), and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2). Raman spectra were recorded on a Thermo DXR with a 532 nm Nd-line laser beam. The surface state of the composite was obtained by an X-ray photoelectron spectrometer (XPS, PHI-5400, Mg Kα light source). Thermogravimetric analysis (TGA) was determined by a thermal analyzer (SDTO600) in air flow with temperature rising of 10 °C min−1. Electrochemical Measurement. Here, 2032-type coin cells were used to evaluate the electrochemical performances of the synthesized samples. The cathode slurry was prepared by dispersing 75 wt % of active material, 10 wt % of polyvinylidenefluoride (PVDF), and 15 wt % of super P in N-methyl-2-pyrrolidone (NMP). Then, the slurry was uniformly casted onto pure Al foil and dried at 120 °C in a vacuum oven for 12 h. The half cell was assembled in an Ar-filled glovebox using a prepared cathode, Celgard 2500 membrane, and lithium foil as the working electrode, separator, and counter electrode, respectively. The electrolyte was 1 M LiPF6 solution with ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v = 1:2). The charge−discharge measurement was performed on an automatic battery test instrument (LAND-2010) at a specific C rate (1 C = 170 mA h g−1) using a constant-current constant-voltage (CC−CV) protocol in a voltage of 2.0−4.5 V (vs Li/Li+). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a CHI660 electrochemical workstation. The frequency of the EIS test is from 100 kHz to 10 mHz using an AC voltage of 5 mV amplitude. All the experiments for electrochemical performances were performed at a constant temperature of 25 °C.

mentioned LiMn x M 1−x PO 4 materials, it is found that LiMnxFe1−xPO4 is an extremely promising composite.11−13 However, LiMnxFe1−xPO4 also suffers from poor electronic conductivity and inferior lithium transportation. Extensive efforts have been used to enhance the electrochemical performance of LiMnxFe1−xPO4, such as nanomodification,14−16 surface decoration by coating with Li+ ion and e− conductive agents,17−19 doping with alien ions, and so on.20,21 Among all these explored methods, coating with conductive carbon is recognized as the most common and effective method,17 in which the quality of the carbon coating layer is a key factor on the performance of the material; however, the excellent properties of materials cannot be achieved through pristine single-carbon coating. Recently, nonmetal-doped carbon has been deemed as a promising coating for electrode materials due to the fact that a nonmetal atom can provide electron carries for the conduction band and further enhance electronic conductivity for active materials.22,23 For example, Zhang et al. 24 synthesized uniformly nitrogen-doped carbon-coated LiFePO4 (LFP) by using a one-step solid state method, and its discharge capacity was up to 124.5 mA h g−1 even at a current rate of 10 C. LFP covered with a phosphorus-doped carbon layer was reported by Zhang et al.25 The material displayed highly reversible capacity of 124.0 mA h g−1 at 20 C. Xu et al.26 obtained sulfur-doped carbon-decorated LFP using a sol−gel route, which demonstrated a superior rate capability and long cyclic property. Fluorine as the element with the largest electrical negativity has similar properties compared with other nonmetal elements, such as nitrogen, phosphor, sulfur, etc. Previous literatures have proved that fluorinated carbon as an anode material can expedite Li+/e− conductivity and, consequently, higher capacities and superior cyclic performance.27,28 To the best of our knowledge, there are few reports on fluorine-doped carbon as the coating on LiMnxFe1−xPO4 cathode materials to improve the electrochemical property of LIBs. Controlling the olivine-structure material-exposed (010) crystal face has been confirmed as a valid strategy to decrease the Li+ ion diffusion length and, consequently, improve the electrochemical reaction.29 In this paper, we spared no effort to coat a large number of (010) crystal facet-exposed LiMn0.75Fe0.25PO4 (LMFP) nanoplates with multiple interface modifications using fluorine-doped carbon. The structure of this composite has three main merits: (i) Nanoplates shortened the solid-state ion transport distance in the LMFP nanoplates, therefore enhancing the rate performance. (ii) Fluorine-doped carbon coating provides sufficient pathways for rapid electron transport. (iii) Partially formed metal fluorides in the interface between the LMFP nanoplates surface and fluorine-doped carbon coating layer will help reduce charge transfer resistance. Electrochemical test results demonstrate that the composite manifests excellent rate capability and cycling life.

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RESULTS AND DISCUSSION Structure and Morphology Analysis. Figure 1 shows the XRD patterns of as-synthesized products LMFP/C and LMFP/

EXPERIMENTAL SECTION

Materials Preparation. Preparation of LiMn0.75Fe0.25PO4 (LMFP). LMFP nanoplates were synthesized by a facile solvothermal method. First, three mixture solutions were prepared: 10 mmol H3PO4 dispersed in 50 mL of diethylene glycol solvent, 30 mmol LiOH· H2O dispersed in 10 mL of deionized water, 7.5 mmol MnSO4·H2O and 2.5 mmol FeSO4·7H2O dispersed in 15 mL of deionized water in the presence of 1 mmol ascorbic acid as the reducing agent. Then, the above LiOH solution was added dropwise into an H3PO4 solution to prepare Li3PO4, and the resultant suspension was stirred for 1 h. After that, the Mn/Fe mixing solution was added dropwise into the Li3PO4

Figure 1. XRD patterns of LMFP/C and LMFP/C−F. 4638

DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644

Research Article

ACS Sustainable Chemistry & Engineering C−F. Distinct diffraction peaks indicate that both samples have good crystallization and are well assigned to the orthorhombic structure with a Pnma space group. Compared with the standard card of LiMnPO4 (JCPDS: 74-0375), the diffraction peaks slight shift to a higher angle due to the substitution of partial Mn by Fe results in the decrease of lattice parameters.11 In addition, it is obviously observed that the highest diffraction peak in the prepared product is vested in the (020) crystal facet rather than (311) in the standard card. Specifically, the intensity ratio of I(020)/I(200) are 5.16 and 5.60 for LMFP/C and LMFP/ C−F, respectively, which are much stronger than that of the standard card ratio (2.64), indicating exposure of a large amount of (010) crystal face.30 It is known that the Li+ diffusion pathway in an olivine-structure material is parallel to the [010] direction.5 The (010) facet is exposed to the surface of the olivine structure samples, which could reduce the Li+ ion diffusion distance and provide a great deal of active sites for Li+ ion diffusion. Specifically, compared with LMFP/C, LMFP/C− F possesses a higher intensity ratio of I(020)/I(200), indicating faster Li+ ion transport capability during the charge−discharge process. Moreover, the absence of a carbon or F-doped carbon peak in the XRD pattern indicates the amorphous nature of carbon or F-doped carbon. The weight fractions of carbon and F-doped carbon detected by TGA in the LMFP/C and LMFP/ C−F samples are 5.08% and 6.43%, respectively (Figure S1). In view of the limited information obtained from XRD results, Raman spectroscopy was used to further confirm the structural information on the products shown in Figure 2. After

carbon atom. The disorder of graphitic materials can be estimated by the intensity ratio of the D and G bands (ID/IG). LMFP/C−F shows a higher ID/IG (0.907) than that of LMFP/ C (0.888), implying a more disordered structure of F-doping as well as the fluorine element successfully doped into the carbon layer, which is consistent with the previous report about Fdoped carbonaceous materials.33 The enhanced rate performance of LMFP/C−F discussed below shows that the F-doping carbon coating provides sufficient electronic pathways, although the increased disorder degree of graphitic materials in LMFP/ C−F will reduce the electronic transfer pathway. The surface oxidation state and composition of the LMFP/ C−F composite were qualitatively determined by using XPS technology. All of the binding energies were calibrated according to the reference peak at 284.6 eV for C 1s. Figure 3a shows the survey spectrum. The binding energies centered at

Figure 2. Raman scattering of PVDF, LMFP/C, and LMFP/C−F.

Figure 3. XPS patterns of (a) survey, (b) Mn 2p, (c) Fe 2p, (d) C 1s, and (e) F 1s of LMFP/C−F.

the calcination process, all of the wavenumbers assigned to the PVDF precursor disappear, indicating that PVDF is fully transported to F-doped carbon. For LMFP/C and LMFP/C−F samples, the wavenumbers observed between 200 and 1100 cm−1 are attributed to the vibrational motions of the LMFP nanoplates,31 in which the peak under 400 cm−1 is caused by lattice vibrations. Peaks between 400 and 700 cm−1 are attributed to the antisymmetric stretching vibrations of PO43−, and peaks between 900 and 1100 cm−1 are vested in partial antisymmetric and intermolecular stretching vibrations of PO43−. LMFP/C−F has similar peaks in this region with LMFP/C, suggesting they have almost the same crystal structures, and these peaks fit well with the olivine structure, which is in accord with the analytic XRD result. Furthermore, another two bands located at 1338 and 1596 cm−1 are assigned to the D and G peaks of carbonaceous materials, respectively.32 The D peak is vested in edges, disorder, and other defects in the carbon atom in the graphite-based structure, whereas the G peak is attributed to the orders sp2 hybridization-bonded

54.6, 133.6, 531.6, and 686.6 eV beloing to Li 1s, P 2p, O 1s, and F 1s, respectively. The Mn 2p high resolution XPS spectrum is represented in Figure 3b. The spectrum has a spin−orbit splitting component of Mn 2p1/2 at 653.6 eV and Mn 2p3/2 with a “shake-up” satellite at 641.3 and 644.1 eV, respectively, which fit well with those of Mn2+ ion.34,35 Figure 3c exhibits the Fe 2p high resolution XPS spectrum. The peaks at 724.4 and 710.8 eV are ascribed to FeΠ 2p1/2 and FeΠ 2p3/2 respectively, demonstrating that partial Mn has been replaced by Fe.36 The C 1s high resolution XPS spectrum can be fitted into three peaks positioned at 284.6, 286.4, and 288.8 eV (Figure 3d). The dominant binding energy located at 284.6 eV is ascribed to the graphite-like sp2 C. The peak located at 286.4 eV is vested in the C−O bond configuration. In addition, the peak centered at 288.9 eV is attributed to the C−F chemical bond.37 Figure 3e displays the high revolution XPS spectrum of F 1s. The fitted spectrum has two peaks, in which the peak located at 687.0 eV belongs to the F chemical bonded to the C 4639

DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644

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ACS Sustainable Chemistry & Engineering atom, and the tiny peak centered at 684.7 eV is attributed to the F−Mn bond.37,38 The results suggest the F atom not only doped into the carbon coating but also combined with LMFP through the F−Mn bond. Compared to LMFP/C−F, the LMFP/C material shows similar spectra except for the absence of the fluorine atom in the carbon coating and the formation of metal fluorides (Figure S2). Therefore, the XPS results further verify that the high purity of LMFP/C−F and the carbon layer have been successfully doped with the F atom as well as the formation of Mn fluorides. The morphology and microstructure were observed by using SEM. SEM images with different magnifications (Figure 4a1

Figure 5. (a) TEM image. (b) HRTEM image and corresponding FFT pattern of LMFP/C−F.

size and morphology for LMFP/C−F, suggesting that the partial PVDF substitute of sucrose as the carbon and fluorine source exerted a minor effect on the crystal growth of LMFP during the sintering process. In order to confirm the large exposed face of LMFP/C−F nanoplates, the range perpendicular to the length and width direction of the nanoplates were characterized by HRTEM. From Figure 5b, two sets of lattice fringes can be seen with interplanar spacing of 2.36 and 3.76 Å, corresponding to the (002) and (101) crystal planes of LMFP. Combined with the FFT pattern (inset, Figure 5b), one can confirm the large exposed crystal face is (010), and the thinnest crystal direction of nanoplates is the [010] direction. In addition, it can be seen clearly that the edges of LMFP are consecutively coated by a F-doped carbon layer with a thickness of ∼2 nm, which endows the material with excellent electronic conductivity. Thus, the F-doped carbon layer and large exposed (010) facet of LMFP nanoplates could expedite electron transport and lithium ion diffusion, which would enhance the electrochemical properties of the material. Electrochemical Characterization. The detailed electrochemical performances of the LMFP/C and LMFP/C−F samples as cathodes for LIBs were carried out by galvanostatic charge−discharge and CV measurements. Figure 6a shows the typical charge−discharge curves in a voltage of 2.0−4.5 V (vs Li/Li+) at a current rate of 0.1 C, where 1 C is equal to 170 mA g−1. The specific capacities are computed based on the weight of the LMFP in electrodes. Both samples display two typical discharge potential plateaus located at 4.06 and 3.54 V, corresponding to the redox potentials of Mn2+/Mn3+ and Fe2+/ Fe3+, respectively.39 By comparison with LMFP/C (162.4 mA h g−1), LMFP/C−F has slightly higher specific discharge capacities (165.2 mA h g−1) at 0.1 C. Some other minor but vital distinctions including the polarization potentials and voltage plateaus were characterized by CV with a scan rate of 0.1 mV s−1, as illustrated in Figure 6b. LMFP/C−F exhibits a lower peak-to-peak separation (Epp) between oxidation and reduction peaks (0.29 V for Mn2+/Mn3+) than LMFP/C (0.37 V for Mn2+/Mn3+), In addition, the anodic peak profile of LMFP/C−F is more sharp and symmetric than that of LMFP/ C, indicating it has better ion transport ability. It is noteworthy that both the oxidation and reduction peaks current (Ip) of LMFP/C−F are stronger than LMFP/C. According to the Randles−Sevcik formula,40 Ip = kn1.5ADn0.5C0v0.5, the apparent Li+ diffusion coefficient is proportion to Ip−1/2. Therefore, it is qualitatively deduced that the F-doped carbon layer and the metal fluorides can accelerate Li+ insertion/extraction into/ from LMFP. Figure 7a represents the rate performances of the two samples. As expected, the LMFP/C−F exhibits higher rate capability. It presents discharge specific capacities of 161.6,

Figure 4. SEM images of LMFP/C (a1, a2) and LMFP/C−F (b1, b2). (c1−c6) SEM and EDS mapping images of LMFP/C−F.

and a2) illustrate that LMFP/C has a typical rectangle-like nanoplate structure with a particle size of ∼150 nm. Compared with the hydrothermal resultant LMFP (Figure S3), it is noteworthy that the edges of particles become blurred and a layer of stuff between the particles can be observed, which is due to the recombination between the LMFP nanoplates and the pyrolysis carbon during the sintering process. LMFP/C−F exhibits similar morphology but excellent dispersion compared with LMFP/C (Figure 4b1, b2). The alleviative agglomeration between LMFP nanoplates is beneficial to electrolyte accessibility. The elements distributions of LMFP-C/F were obtained from energy dispersive X-ray (EDX) (Figure 4c1−c6 and Figure S4). It illustrates that the composites are composed of C, F, P Mn, and Fe elements. and they are homogeneously distributed in the products. The atom ratio of Mn/Fe is ∼3. which was in good agreement with the raw material ratio. The F-doping level is ∼1.2 wt % in the composite. The TEM image shown in Figure 5a indicates that the LMFP/C−F nanoplates have lengths of 50−200 nm, widths of 20−100 nm, and thicknesses of 10−20 nm. Compared with LMFP/C (Figure S5), no obvious changes can be observed in 4640

DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644

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161.3, 160.2, 155.7, 148.3, and 130.3 mA h g−1 at 0.5, 1, 2, 5, 10, and 20 C rate, respectively, while LMFP/C only delivers 158, 153.7, 148.2, 135.4, 113.4, and 71.8 mA h g−1 at the same current rates. Notably, after charging/discharging at a high current rate of 20 C, the specific capacity of LMFP/C−F could still recover to 161.5 mA h g−1 when the rate was regained at 0.1 C, revealing it has superior current rate tolerance capability. Moreover, stability of LMFP/C−F has outperformed that of LMFP/C (Figure 7b and c). Even at 10 C current rate, an obvious discharge plateau of Mn2+/Mn3+ can be detected. In addition, the reversible capacity of LMFP/C−F can remain 94.8% of the original capacity at 1 C after 200 cycles. Most importantly, the reversible capacity could achieve 120.1 mA h g−1 even over 500 cycles at a 10 C high rate (Figure 7e). The excellent rate and cycling stability of LMFP/C−F are partially attributed to the desired (010) crystal orientation of the LMFP nanoplates, which reduces Li+ ion diffusion distance. On the basis of insight from the similar morphology and structure of nanoplates between LMFP/C and LMFP/C−F and the increased disorder degree of graphitic materials of LMFP/C− F compared to that of LMFP/C mentioned above, the excellent rate performance of LMFP/C−F strongly indicates that the partially formed metal fluorides (Mn−F bond) may have an important role in promoting the charge transfer process in the interface between the LMFP nanoplates surface and fluorinedoped carbon coating layer. The schematic diagram displaying the difference in structure between LMFP/C−F and LMFP/C is shown in Figure 8. LMFP/C−F demonstrated outstanding

Figure 6. (a) First charge−discharge profiles at 0.1 C. (b) CV curves at a scan rate of 0.1 mV s−1 of LMFP/C and LMFP/C−F.

Figure 7. Rate performance of LMFP/C and LMFP/C−F (a). Charge−discharge profiles of LMFP/C (b) and LMFP/C−F (c). Cycling performance of LMFP/C−F at 1 C rate (d) and 10 C rate (e). 4641

DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644

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of Re and Rct are 1.58 and 151.9 Ω for LMFP/C−F, respectively, which are obviously lower than those of LMFP/ C (1.73 and 276.7 Ω), suggesting that the F-doped carbon layer (including the Mn−F bond) not only reduces the ohmic resistance of the electrode but also improves the charge transport kinetics at the surface/interface region of the active material. The apparent Li+ diffusion coefficient of the materials (DLi+) can be determined based on the following equation:40

electrochemical performance in terms of rate capability. This improvement may ascribe to the unique structure of LMFP/ C−F.

DLi + = R2T 2/2A2 n 4F 4C 2σ 2

(1)

where R represents the gas constant (8.314 J mol−1 K−1), T is the absolute temperature (298 K), A is the surface of working electrode, n is the relevant number of electrons in the redox reaction (n = 1 for Mn2+/Mn3+ or Fe2+/Fe3+ redox pair), F is the Faraday constant (96485 C mol−1), C is the shuttle concentration of the Li+ ion in LMFP (0.0223 mol cm−3), and σ is the Warburg coefficient, which adheres to the following relationship:

Figure 8. Hypothetical diagram displaying the difference in structure between LMFP/C−F and LMFP/C.

Z′ = σω−1/2

EIS is one distinct evidence to insight into the outstanding electrochemical properties. The curves were obtained at the state of discharge of 2.0 V after the first cycle. As shown in Figure 9a, two EIS curves demonstrate the same character-

(2)

The value of σ can be gained from the slope of the linear fitting of Z′ vs ω−1/2 plots as exhibited in Figure 9b. According to eqs 1 and 2, the apparent Li+ diffusion coefficients of LMFP/ C and LMFP/C−F are 9.52 × 10−15 and 1.61 × 10−14 cm2 s−1, respectively, suggesting that LMFP/C−F has fast Li+ ion transport kinetics. The fast lithium diffusion capability of LMFP/C−F should be closely related with the partially formed metal fluorides in the interface between LMFP nanoplates surface and fluorine-doped carbon coating layer which expedites the Li+ ions transfer through a robust internal electrical field. Thus, it could be concluded that the unique structure formed in LMFP/C−F (Figure 8) will help reduce charge transfer resistance and therefore improve the electrochemical performance of LMFP/C−F.

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CONCLUSION In summary, a composite consisting of a large number of (010) exposed LMFP nanoplates coated by F-doped carbon with partially formed metal fluorides, LMFP/C−F, was successfully synthesized via a facile solvothermal reaction followed by the sintering process. The special crystal direction in combination with excellent coating and partially formed metal fluorideendowed material has fast lithium-ion diffusion and electron transfer capability; thus, it outperforms excellent electrochemical properties in specific capacity, cycling life, and rate capability. It exhibits a discharge capacity of 120.1 mA h g−1 over 500 cycles at a high current rate of 10 C. Such a LMFP/ C−F cathode material demonstrates potential applications in high energy/power lithium-ion batteries.

Figure 9. (a) Experimental and fitted Nyquist plots. (b) Linear fitting of Z′ vs ω−1/2 relationship of LMFP/C and LMFP/C−F. Inset in panel (a): equivalent circuit model.

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ASSOCIATED CONTENT

S Supporting Information *

ization: an intercept at the Z′ axis in high frequency corresponds to the ohmic resistance (Re), the semicircle in the high frequency region is attributed to the charge transfer resistance (Rct), and the sloping line in the low frequency is ascribed to the Warburg impedance, which indicates the Li+ ion diffusion resistance through the active material. The exact kinetic parameters of Re and Rct of LMFP/C and LMFP/C−F are calculated by modeling the Nyquist plots according to the equivalent circuit exhibited in the inset of Figure 9a. The values

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03163. TGA curves of LMFP, LMFP/C, and LMFP/C−F samples; XPS patterns of survey; SEM, TEM, and HRTEM images of LMFP/C sample, EDS image and element content of LMFP/C−F sample. (PDF) 4642

DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Tel. and Fax: +86-532-80662703 (Y. Jin). *E-mail: [email protected] (K. Kanamura). ORCID

Yongcheng Jin: 0000-0002-9429-4489 Notes

The authors declare no competing financial interest. Xiao Yan and Deye Sun are cofirst authors, who contributed equally to this manuscript.

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ACKNOWLEDGMENTS The authors appreciate the “100 Talents” program of Chinese Academy of Sciences and the Think-Tank Mutual Fund of Qingdao Energy Storage Industry Scientific Research.

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DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644

Research Article

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DOI: 10.1021/acssuschemeng.6b03163 ACS Sustainable Chem. Eng. 2017, 5, 4637−4644