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Nanorod-Nanoflake Interconnected LiMnPO4·Li3V2(PO4)3/C Composite for High-Rate and Long-Life Lithium-Ion Batteries Xinxin Cao,† Anqiang Pan,*,† Yifang Zhang,† Jiwei Li,† Zhigao Luo,† Xin Yang,† Shuquan Liang,*,† and Guozhong Cao*,‡ †
School of Material Science and Engineering, Central South University, Changsha, Hunan 410083, China Department of Materials Science & Engineering, University of Washington, Seattle, Washington 98195, United States
‡
S Supporting Information *
ABSTRACT: Olivine-type structured LiMnPO4 has been extensively studied as a high-energy density cathode material for lithium-ion batteries. However, preparation of high-performance LiMnPO4 is still a large obstacle due to its intrinsically sluggish electrochemical kinetics. Recently, making the composites from both active components has been proven to be a good proposal to improve the electrochemical properties of cathode materials. The composite materials can combine the advantages of each phase and improve the comprehensive properties. Herein, a LiMnPO4·Li3V2(PO4)3/C composite with interconnected nanorods and nanoflakes has been synthesized via a one-pot, solid-state reaction in molten hydrocarbon, where the oleic acid functions as a surfactant. With a highly uniform hybrid architecture, conductive carbon coating, and mutual cross-doping, the LiMnPO4·Li3V2(PO4)3/C composite manifests high capacity, good rate capability, and excellent cyclic stability in lithium-ion batteries. The composite electrodes deliver a high reversible capacity of 101.3 mAh g−1 at the rate up to 16 C. After 4000 long-term cycles, the electrodes can still retain 79.39% and 72.74% of its maximum specific discharge capacities at the rates of 4C and 8C, respectively. The results demonstrate that the nanorodnanoflake interconnected LiMnPO4·Li3V2(PO4)3/C composite is a promising cathode material for high-performance lithium ion batteries. KEYWORDS: lithium-ion batteries, cathode materials, phosphates, LiMnPO4·Li3V2(PO4)3/C, hybrid nanostructure
1. INTRODUCTION Rechargeable lithium-ion batteries (LIBs), with their high energy density and durable cycle life, are currently being utilized in portable electronic devices, electric vehicles, and larger scale energy storage applications, but further improvements are needed.1,2 The breakthrough in electrode materials is the key to improve the capacity, rate performance, and cycling stability of LIBs.3−5 Among the candidate materials, lithium transition metal phosphates, such as LiMPO4 (M = Fe, Mn, Co, Ni),6,7 Li3V2(PO4)3,8 and LiVOPO4,9 have been investigated as promising cathode materials for LIBs owing to their high energy density, long lifespan, good safety, and low cost. Among these phosphates, a cathode material of nanometer-sized LiFePO4 (LFP) particles with a coating of conductive carbon has been commercialized successfully.10 Compared to LFP, LiMnPO4 (LMP) is an attractive cathode material owing to its higher Li+ intercalation potential of 4.1 V versus Li+/Li (3.45 V for LFP), providing about 20% higher theoretical energy density (701 Wh kg−1) than LFP (586 Wh kg−1) for LIBs.11,12 Such energy density is considered to be the maximum practically achievable value within the stable operation window of commercially used carbonate ester-based electrolytes.13,14 Nevertheless, LMP suffers from poor Li+ intercalation/ © 2016 American Chemical Society
deintercalation kinetics caused by the intrinsically low electronic conductivity and sluggish lithium ion diffusion rate.10,12 In addition, manganese phosphate also presents other disadvantages, such as the structural instability generated by the volume change between the LiMnPO4/MnPO4 phases during charge/discharge processes, the metastable nature of the deintercalated MnPO4 phase, and the Jahn−Teller anisotropic lattice distortion in Mn3+ sites.7,10,15 Tremendous efforts have been made in recent years to overcome these limitations, such as making nanostructured materials,7,13 carbon coating,10,16 and substitutional doping.17,18 More recently, making the composites from both active components has attracted much attention. It is reported that the electrochemical properties of LMP and LFP can be remarkably improved by making composites with the fast ion conductor Li3V2(PO4)3 (LVP).12,19,20 The olivine-type structured LiMPO4 (M = Fe, Mn) only possess a curved onedimensional channel for lithium ion diffusion, which limit their high rate performance.21 Monoclinic LVP with an open threeReceived: June 2, 2016 Accepted: September 26, 2016 Published: September 26, 2016 27632
DOI: 10.1021/acsami.6b06456 ACS Appl. Mater. Interfaces 2016, 8, 27632−27641
Research Article
ACS Applied Materials & Interfaces dimensional framework has much higher lithium ion diffusion efficiency and intercalation potentials.8,22 Moreover, the mutual doping of cations (V3+ and Mn2+) in the phosphates occurs in the fabrication process, resulting in enhanced electrochemical properties. Previous reports have demonstrated that V doping in LMP17 or LFP,20,23 and Mn, Fe doping in LVP12,19,20 could enhance the electrochemical properties of the bulk materials. To date, xLiMnPO4·yLi3V2(PO4)3 (xLMP·yLVP) composites have been prepared by various approaches including solid-state reaction,12 chemical reduction and lithiation method,24 so-gel method,25 and spray drying.26 Although their electrochemical performance are improved in different extent, the homogeneity of the composites is quite low and the controllable synthesis of xLMP·yLVP nanocomposites with different morphologies is rarely reported. The difficultly may be raised from the tendency of forming large particles at high temperature during the preparation process in order to reach a more thermodynamically stable state.27−29 In order to solve this problem, the surrounding conditions for the nucleation and particle growth should be modified. Surfactants have been successfully employed to fabricate various nanostructures in the liquid solvent, which can serve as growth controller and agglomeration inhibitor.30,31 However, they are not suitable for operation at high temperature (>500 °C). Recently, a molten hydrocarbon media has been developed to replace the liquid solvent and the surfactant can function as a structural director in the molten media. Choi et al.7 and Pan et al.22 reported the surfactant-assisted synthesis of LMP nanoplates and LVP nanobelts through a one-step solidstate reaction in molten paraffin media, which may highlight the possibility of controllable synthesis of lithium transition metal phosphates at elevated temperatures. Herein, we report the synthesis of LMP·LVP/C composite with interconnected nanorods and nanoflakes via a one-pot preparation procedure in molten media. The infinitesimal LMP and LVP nanoparticles are homogeneously distributed through the hybrid architecture and the nanoparticles are coated by a carbon layer, which is in situ generated in the calcination process at high temperatures. As a cathode material for lithium ion batteries, the LMP·LVP/C composites manifest high capacity, good rate capability, and excellent cyclic stability.
105 °C for 1 h, followed by annealing in tube furnace at 700 °C for 10 h under a gas flow composed of 95% Ar and 5% H2 to yield the LMP· LVP/C composite. For comparison, the pristine LMP/C and LVP/C were synthesized by keeping the molar ratio of Li:Mn:P:oleic acid = 1:1:1:1 and Li:V:P:oleic acid = 3:2:3:3 in the milling mixture, while the other synthesis conditions were kept unchanged. Material Characterization. The crystal structure of the asprepared samples were identified using X-ray diffraction (XRD, Rigaku D/max-2500 with nonmonochromated Cu Kα radiation, λ = 1.54178 Å). The samples were scanned in the range between 10° and 80° (2θ) with a scanning rate of 0.02° per second. The Rietveld refinement/ whole profile fitting (WPF) method (Jade 9.0 software, MDI, USA) was used to refine the X-ray diffraction patterns to analyze the crystal structures and phase percentages of the samples. The general sizes and morphology of the samples were characterized by field-emission scanning electron microscope (FESEM, FEI Nova Nano SEM 230, 20 kV), and transmission electron microscope (TEM, FEI Tecnai G2 F20, 200 kV). The carbon content of the samples were determined by carbon−sulfur analyzer (CS-2000, Eltar, Germany) and the property of carbon layer was analyzed by Raman spectrometer (LabRAM HR800). Battery Fabrication and Electrochemical Measurements. The cathode electrodes for electrochemical measurements were composed of 75% active materials, 15% acetylene black, and 10% polyvinylidene fluoride (PVDF) binder. N-methyl-2-pyrrolidone (NMP) was used as solvent. The mixed slurry was spread on aluminum foil, which served as current collector in the cathode, and then was dried in a vacuum oven at 100 °C overnight prior to cells assembly. The CR 2032 coin cells were fabricated in an argon-filled glovebox (Mbraun, Germany) using 1.0 M LiPF6 in ethylene carbon (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC) (1:1:1, by volume) as electrolyte, a metallic lithium negative electrode, and Celgard 2400 polypropylene separator. The loading mass of each electrode ranges from 1.0 to 1.5 mg cm−2. Galvanostatic charge/ discharge cycling behavior was investigated at ambient temperature in a potential range of 2.5−4.5 V versus Li/Li+ with a multichannel battery testing system (LAND CT2001A, China). And the calculation of specific capacity of the LMP·LVP/C cathode is based on the mass of active materials (LMP·LVP/C composite) only. The cyclic voltammetry (CV) measurement (3−4.5 V vs Li/Li+) was performed using an electrochemical workstation (CHI604E, China) at different scan rates. The electrochemical impedance spectroscopy (EIS) data of the electrodes were tested with a ZAHNER-IM6ex electrochemical workstation (ZAHNER Co., Germany) on a cell under the assembled condition.
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION Figure 1a shows the powder X-ray diffraction (XRD) pattern and Rietveld refinement of as-prepared material. All diffraction reflexes can be well indexed to orthorhombic LMP (space group Pbnm, PDF#97−003−8208) and monoclinic LVP (space group P21/n, PDF#97−016−1335), with no evidence of secondary phases nor crystalline carbon diffraction reflexes presented.32,33 In addition, the intense and sharp diffraction peaks suggest the good crystallinity of the as-synthesized composite material. Rietveld refinement (Jade 9.0 software, MDI, USA) of the XRD pattern was performed to further explore the crystal structures and phase contents of the material. The Rietveld refinement of the LMP phase and LVP phase were based on the orthorhombic (space group Pbnm) and monoclinic (space group P21/n) crystalline structure, respectively. For example, the characteristic peaks located at 25.451°, 29.565°, and 35.439° are attributed to orthorhombic LMP phase (space group Pbnm), and the characteristic peaks located at 20.673°, 24.525°, and 29.339° are attributed to monoclinic LVP phase (space group P21/n). The results are summarized in Table 1. As shown, the observed and calculated
Synthesis of LMP·LVP/C, LMP/C, and LVP/C. All the chemical reagents used in our experiments were of analytical grade and were used directly without further purification. The LMP·LVP/C composite was prepared by a facile one-pot synthesis in molten paraffin media. Vanadium pentoxide (V2O5, ≥ 99.0%), oxalic acid dehydrate (H2C2O4·2H2O, ≥ 99.5%), ammonium dihydrogen phosphate (NH 4 H 2 PO 4 , ≥ 99.0%), manganese(II) acetate tetrahydrate (C 4 H 6 MnO 4 ·4H 2 O, ≥ 99.0%), lithium acetate dehydrate (CH3COOLi·2H2O, ≥ 99.0%), oleic acid (C18H34O2, ≥ 98.0%) and paraffin wax (melting point 50−54 °C) were used as raw materials. First, V2O5 and H2C2O4·2H2O in a molar ratio of 1:3 were dissolved in deionized water under vigorous stirring at 70 °C until a transparent blue solution was formed, which indicates the formation of VO2+. After evaporating the water for several hours with vigorous stirring, the VOC2O4·nH2O was obtained for further use. For LMP·LVP/C composite synthesis, NH4H2PO4 was milled initially with oleic acid for 1 h using a QM-3B high-energy milling machine. Then, the paraffin wax was added and milled for another 30 min. Finally, C4H6MnO4· 4H2O, VOC2O4·nH2O, and CH3COOLi·2H2O were added and milled further for 1 h. The overall molar ratio of the milling mixture is Li:Mn:V:P:oleic acid = 4:1:2:4:4. The paraffin wax addition amount is twice the weight of oleic acid. The precursor paste was dried in oven at 27633
DOI: 10.1021/acsami.6b06456 ACS Appl. Mater. Interfaces 2016, 8, 27632−27641
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expected molar ratio of 1:1. XRD patterns of the prepared pristine LMP/C and LVP/C samples are illustrated in Figure S1. All diffraction reflexes can be well indexed to orthorhombic LiMnPO4 (space group Pbnm, PDF#97−003−8208) and monoclinic Li3V2(PO4)3 (space group P21/n, PDF#97−016− 1335), respectively, and no evidence of secondary phases or crystalline carbon diffraction reflexes is present. The existence and content of coated carbon in the LMP· LVP/C composite were investigated using Raman measurements and C−S analysis. The Raman spectrum of the prepared LMP·LVP/C composite (Figure 1b) displays two characteristic bands of carbonaceous materials located around 1361 and 1589 cm−1, which are attributed to the A1g vibration mode of the disordered carbon (D-band) and the E2g vibration mode of the ordered graphitic carbon (G-band),36,37 respectively. In the previous reports,38 the peak intensity ratio of D and G bands (ID/IG) generally provides a useful index for detecting the degree of crystallinity for various carbon materials, i.e., smaller the ratio of ID/IG, higher the degree of ordering in the carbon material. In our case, an ID/IG value around 0.91, indicating a relatively high degree of graphitization, thereby implying good electronic conductivity.39−41 Moreover, a weak peak of the LMP·LVP/C composite can be distinguished around 953 cm−1, which assigns to the stretching vibration of PO43− anion.36,41 These results are reasonable since the interaction between the PO43− anion with the Raman laser was concealed by the in situ generated carbon coating layer on the surface of the primary LMP·LVP particles.42 Furthermore, the actual carbon content of the as-prepared sample is 7.37%, which was determined by carbon−sulfur analyzer. The morphology and microstructure of the products were observed by using FESEM and TEM, as shown in Figure 2. The FESEM images (Figure 2a,b) reveal that the LMP·LVP/C composite is constructed by randomly oriented nanorods and nanoflakes. The diameter of the nanorods ranges from 40 to 100 nm. The thickness of the nanoflakes is estimated to be tens of nanometers, and have an in-plane extension as long as 300− 500 nm. In addition, neighboring nanorods and nanoflakes are loosely interconnected with abundant open spaces existing between them, which is beneficial to the electrolyte penetration, fast ion, and electron transportation. The low-magnification TEM image (Figure 2c) unambiguously reveals that the nanoflakes are continuous and highly flexible with transparent feature, demonstrating the ultrathin nature. The nanorods and nanoflakes are interconnected with each other to form their inner empty space, which is in agreement well with the FESEM observations. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images (Figure 2d) and elemental mapping results confirm the homogeneously distribution of C, V, and Mn elements in the prepared sample, indicating the high efficiency of making LMP and LVP composite, rather than a
Figure 1. (a) X-ray diffraction pattern with Rietveld refinement and (b) Raman scattering spectrum of the prepared LMP·LVP/C composite.
patterns match well, and the reasonably small R factor (7.24%) suggests that the refinement results are convincible. It is clear that the unit cell volume of LMP in prepared LMP·LVP/C composite decreases obviously, compared with that of pristine LMP, which may be attributed to the V3+ doping on the Mn2+ sites during the crystal growth process because the ionic radius of V3+ (0.74 Å) is smaller than that of Mn2+ (0.80 Å).24,34 However, the cell volume of LVP in the LMP·LVP/C composite increases compared with the pure phase LVP, indicating that Mn2+ also entered into the LVP host lattice.34 As a result, the possible mutual cross-doping between LMP and LVP is beneficial to improve their conductivity, lithium ion diffusion coefficient and to catalyze the intercalation reactions.17,23,35 The mass content of LMP and LVP determined by multiphase refinement is 27.7 ± 1.0 wt% and 72.3 ± 1.4 wt%, respectively, which is consistent with the
Table 1. Lattice Parameters and Phase Contents of the Prepared LMP·LVP/C Composite Obtained from XRD Rietveld Refinement and the Standard Data of LMP (PDF#97-003-8208) and LVP (PDF#97-016-1335) lattice parameters sample
a (nm)
b (nm)
c (nm)
γ (deg)
V (nm3)
phase content (wt%)
R (%)
LMP in LMP·LVP/C LMP LVP in LMP·LVP/C LVP
0.4706 0.4711 0.8596 0.8608
1.0372 1.0374 1.2063 1.2045
0.6031 0.6038 0.8602 0.8599
90.0000 90.0000 90.4165 90.5000
0.2944 0.2951 0.8919 0.8915
27.7
6.63
72.3
7.84
27634
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Figure 2. (a,b) FESEM images, (c) TEM image, (d) STEM-HAADF images, (e,f) HRTEM images of the prepared LMP·LVP/C composite; inset of (e) is the corresponding Fourier transform (FFT) image, inset of (f) are the Fourier transform (FFT) images of the corresponding area.
anticipated to dramatically increase the electronic conductivity and maintain structural integrity thus to boost the rate performance and cycling stability in LIB applications.27,45,46 Figure 3 schematically illustrates the proposed formation mechanism of the nanorod-nanoflake interconnected LMP· LVP/C composite via a solid-state method in molten hydrocarbon media where oleic acid is used as surfactant, and paraffin serves as a nonpolar solvent.7 Oleic acid is a kind of typical anionic surfactant with carboxyl group, long alkyl chain, and unsaturated bond. During the sintering process, the carboxyl group in oleic acid could anchor to the surface of nanoparticles and the lower polar tail section extends into the paraffin solvent.7,47 Besides, the presence of the long alkyl chain
simple macroscopical mixing. Figure 2e and f show the HRTEM images of the prepared sample. The coated carbon layer with a thickness of about 3 nm can also be observed (Figure 2e). Figure 2f shows the obvious interface of crystalline LMP and LVP. The fast Fourier transform (FFT) images of corresponding regions (inset of Figure 2f) show the LMP, and LVP diffraction spots, respectively. Two types of lattice fringes can also be observed in Figure 2f. The interplanar spacing is 3.017 Å between the (002) set of planes in LMP structure, and 3.24 Å between (122) set of planes in LVP structure. The results further indicate that the LMP and LVP coexist in the composite material, and agree well with those reported recently in the literature.43,44 This unique hybrid architecture is 27635
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Figure 3. Schematic illustration of the proposed formation mechanism of the nanorod-nanoflake interconnected LMP·LVP/C composite.
Four corresponding plateaus are also observed on the charge curves. The potentials of the charge−discharge plateaus are in good agreement with the CV results (Figure 4a). In addition, the initial Coulombic efficiency of the as-prepared composite cathode material is 88.5%, and a high initial discharge capacity of 145 mAh g−1 can be delivered within the voltage range of 2.5−4.5 V at a current density of 0.1 C. Figure 4c displays the cycling performance of the LMP·LVP/ C composite electrode at the rates of 0.5, 1, and 2 C in the voltage range of 2.5−4.5 V versus Li+/Li. The maximum specific discharge capacities are 144.4, 132.1, and 123 mAh g−1, respectively, with a corresponding capacity retention of 86.3, 90.8, and 92.3% after 100 cycles. Interestingly, all the three cycling profiles exhibit a gradual increase in the initial cycles and then reach a stable value, which may be caused by the activation process in initial cycles.51,52 Similar behavior was also shown in the previous literatures.53,54 For further evaluation of the electrochemical behavior, rate performance at progressively increased rates (ranging from 0.1 C to 16 C) was measured as shown in Figure 4d and Figure S6. The LMP·LVP/C composite electrode displays a reversible capacity of 145, 144.4, 133.7, 122.4, 114.8, 107.6, and 101.3 mA h g−1 at the current rate of 0.1, 0.5, 1, 2, 4, 8, and 16 C, respectively, indicating excellent rate capability. Even at a high rate of 16 C, it still delivers a high reversible capacity of 101.3 mA h g−1. When the current is reset to 0.5 C, a specific capacity of 135.9 mA h g−1 can be restored. The rate performance of the LMP·LVP/C composite electrode is much better than the pristine LMP/C and LVP/C electrodes (Figure S4c and S5c). When discharged at low current density, surprisingly, the capacity of the composite electrode declines rapidly. However, the capacity remains stable at higher current density (Figure S7). According to the diffusion equation (τ = L2/2D),55 when electrode is discharged at low current density (e.g., 0.5 C), the diffusion time τ is so long that electrolyte permeates into interior of the material and a higher discharge capacity is obtained. However, it also causes huge volume expansion/ contraction and aggravates the structure degradation of the host materials, which leads to the rapid capacity decay in subsequent cycles.54 But, higher current density would shorten the diffusion time τ and the intercalation/deintercalation of Li+ occurs near the surface of electrode material, which leads to the increasing
and unsaturated bond in oleic acid provides significant hydrophobicity to the nanoparticles and dictates the shape of nanocrystals by thermodynamic means.7,48,49 Furthermore, to reduce the interfacial energy, the LMP nanocrystals first selfassemble into nanoflake morphology and the LVP nanocrystals self-assemble into nanorod morphology (Figures S2).7,22 With a further increase of reaction time, the LMP·LVP nanoclusters further direct in situ crystallization on the surface of the nanoflakes and nanorods (Figures S3). Moreover, the LMP and LVP nanocrystals are carbon coated with the decomposition and in situ carbonization of oleic acid, which may be beneficial for the electron transportation in lithium ion batteries.20,29,50 The as-prepared LMP·LVP/C composite, pristine LMP/C and LVP/C were assembled into CR 2032 coin cells to evaluate their electrochemical performance. The cyclic voltammetry (CV) technique is first used to evaluate the Li+ ions intercalation and deintercalation behavior in LMP·LVP/C composite at a scan rate of 0.05 mV s−1. As Figure 4a shows, the first three consecutive CV curves are almost identical and no obvious decay is observed, which indicates good reversibility in the electrochemical processes. Obviously, the curves show three main oxidation peaks (3.61, 3.69, and 4.11 V) and four main reduction peaks (4.03, 3.97, 3.64, and 3.56 V), which can be attributed to multiple phase transformations for LixMnPO4 (x = 1 and 0) and LixV2(PO4)3 (x = 3.0, 2.5, 2.0, and 1.0).21,44 The redox peaks for the composite electrode are in good accordance with those for pristine LMP/C and LVP/C (Figure S4a and S5a). It should be noted that the anodic peaks of the extraction of the Li+ from LixMnPO4 (x from 1 to 0) and the extraction of the second Li+ from LixV2(PO4)3 (x from 2 to 1) appear in the way of superimposition around 4.11 V.44 Moreover, compared with the strong and sharp redox peaks of LVP, the redox peaks of LMP are very weak and broad, which may be attributed to the intrinsically poor kinetic features of the LMP.25 Figure 4b shows the first charge− discharge profiles of the LMP·LVP/C composite electrode at a C-rate of 0.1 C (1 C = 170 mA g−1) within a voltage window of 2.5−4.5 V. The charge plateaus located around 3.56, 3.64, and 3.97 V can be identified as the extraction of lithium ions from LVP (Figure S4b),33 and the plateau at 4.03 V is attributed to the lithium extraction from LMP (Figure S5b), which indicates the multistep Li+ intercalation process in the active materials. 27636
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Figure 4. Electrochemical performance of the prepared LMP·LVP/C composite: (a) the first three successive cyclic voltammograms curves at a scan rate of 0.05 mV s−1 in a voltage range of 3−4.5 V; (b) the first galvanostatic charge−discharge profiles at a current density of 0.1 C; (c) cycling performance at the current density of 0.5, 1, and 2 C; (d) rate performance at various current rates from 0.1 to 16 C; (e) long-term cycling performance at 4 C and 8 C.
kinetic performance and keeps the structural stable.54 Thus, the cycling stability is much better. A similar phenomenon occurs in other electrode materials.25,54,56 Long-term cycling performance of the LMP·LVP/C composite electrode at high rates is subsequently shown in Figure 4e. The initial specific discharge capacities of the composite electrodes were 117 and 107.5 mA h g−1 at rates of 4 and 8 C, respectively. After 4000 cycles, the capacity at 4 C decreased to 92.9 mA h g−1, which corresponds to a capacity retention of 79.39% and to a capacity fading rate of 0.0052% per cycle. The capacity decreased to 78.2 mAh g−1 at 8 C after 4000 cycles, which corresponds to a capacity retention of 72.74% and to a capacity fading of 0.0068% per cycle. It is
worth noting that the overall average Coulombic efficiencies under both rates are around 100%, indicating the good stability of the electrodes. The results demonstrate the excellent longterm cyclic stability of the LMP·LVP/C composite compared with the pristine LMP/C (Figure S4e) and LVP/C (Figure S5e). To reveal the reasons of the enhanced electrochemical performance for LMP·LVP/C composite electrodes, the CV curves at various scanning rates from 0.05 to 0.8 mV s−1 were performed. As shown in Figure 5a, the anodic peaks shift right and the corresponding cathodic peaks move left with the increase of the scanning rate, resulting in the aggravated polarization.43 The intensities of all redox peaks are enhanced 27637
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Table 2. Diffusion Coefficients of Lithium-ion in Composite Electrode Calculated from CV Based on the Classical Randles Sevchik Equation anodic oxidation process state LiMnPO4·Li2.5V2(PO4)3 LiMnPO4·Li2V2(PO4)3 MnPO4·LiV(PO4)3 MnPO4·Li2V2(PO4)3
peak A1 A2 A3
Dse (cm2 s−1) −10
7.17 × 10 1.19 × 10−9 3.82 × 10−10
cathodic reduction process peak
Dse (cm2 s−1)
A1′ A2′ A3′ B1′
1.32 1.16 8.16 3.89
× × × ×
10−9 10−9 10−9 10−11
Figure 5. (a) The CV curves at various scan rates, and (b) the line relationship of the peak current (Ip) versus square root of scan rate (ν1/2) of the prepared LMP·LVP/C composite.
as the increase of scanning rate. From Figure 5b, the peak current (Ip) has a linear relationship with the square root of scan rate (ν1/2), indicating a diffusion-controlled process.57 The apparent diffusion coefficients of lithium ions are calculated based on the Randles Sevcik equation:58 Ip = 2.69 × 105n3/2AC0*D1/2ν1/2
(1)
Where Ip is the peak current (A), n is the number of electrons per species reaction (for Li+, n = 1), A is the active surface area of the electrode (here 1.131 cm2 is used for simplicity), C0* is the is the concentration of lithium ions (mol cm−3), D is the apparent diffusion coefficient of lithium ion (cm2 s−1), and ν is the scanning rate (V s−1). Because the interface reaction here is very complex, we just calculate the effective diffusion coefficients (Dse) herein for the solid-state diffusion in electrode,43 also we used ne and C*0e as effective values, respectively (Table S1). Based on eq 1 and the slope of Ip versus ν1/2 plots in Figure 5b, the effective diffusion coefficients (Dse) are calculated and listed in Table 2. The values of Dse for solid state electrode are up to the order of magnitude of 10−11 to 10−9 cm2 s−1 and close to each other. EIS were further measured to provide thorough investigation. As shown in Figure 6a and Table S2, the charge transfer resistance (Rct) value is decreased from 73.62 Ω after the first cycle to 66.53 Ω after 10 cycles, which consist well with the capacity increase in the initial cycles.59,60 Even after 1000 cycles, only a small amount of Rct increase is discovered, indicating the
Figure 6. (a) Nyquist plots of the prepared LMP·LVP/C composite after different cycles at the voltage of 4.5 V, and (b) the corresponding profile of the relationship between Z′ and ω−1/2 at low frequency region.
fast electronic mobility of the LMP·LVP/C composite. Besides, the Nyquist plots (Figure S4d and S5d) show that the charge transfer resistance (Rct) of both pristine LMP/C and LVP/C electrodes are much bigger than the LMP·LVP/C composite electrode (Table S2 and S3). This indicates that making the composites with novel nanostructures indeed improves the charge transfer kinetics. According to the linear relationship between Z′ versus ω−1/2 at low-frequency region (Figure 6b) and eq S1 and S2, we calculate the lithium ion diffusion coefficients of the LMP·LVP/C composite electrode after different cycles, and the results are listed in Table S4. The values are at least 2 orders of magnitude larger than that of LiFePO4 (10−13 to 10−15 cm2 s−1),61 LiMnPO4 (10−13 cm2 27638
DOI: 10.1021/acsami.6b06456 ACS Appl. Mater. Interfaces 2016, 8, 27632−27641
Research Article
ACS Applied Materials & Interfaces s−1),16 and LiFexMn1−xPO4 (10−15 to 10−17 cm2 s−1),35 and are comparable with that of Li3V2(PO4)3,29 xLiFePO4·Li3V2(PO4)3 composite,19,20 and xLi3V2(PO4)3·LiVPO4F composite.43 The diffusion coefficient and EIS results indicate that the ionic transport kinetics and electronic conductivity have been greatly improved by making LMP·LVP/C composite with interconnected nanorods and nanoflakes. Table S5 lists the electrochemical performance of many previous published works on lithium transition metal phosphates and our LiMnPO4·Li3V2(PO4)3/C composite. According to the results, the LMP·LVP/C composite prepared here demonstrate superior electrochemical properties. The good performance can be attributed to the interconnected nanorod-nanoflake structures, which provide (1) easy electrolyte penetration due to their large inner empty space; (2) reduced lithium ion diffusion and electron transportation distance; (3) the improved electron conductivity due to the cation mutual-doping effect; and (4) the enhanced structural integrity and electron conductivity due to the in situ generated carbon layer on the surface.
tion of China (No. 51374255, 51302323), Program for New Century Excellent Talents in University (NCET-13-0594), Research Fund for the Doctoral Program of Higher Education of China (No. 201301621200), and Natural Science Foundation of Hunan Province, China (14JJ3018).
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4. CONCLUSION In summary, we have successfully synthesized nanorodnanoflake interconnected LMP·LVP/C composites by a onepot, solid-state method in molten media. The structures of the composite is well characterized and a possible formation mechanism is proposed. When used as cathode for LIBs, the LMP·LVP/C composite exhibit superior electrochemical performance, including good rate capability and excellent cycling stability. The composite electrode can deliver a high reversible capacity of 101.3 mAh g−1 at the rate up to 16 C. Moreover, it can retain 72.74% of its maximum specific discharge capacities after 4000 cycles at the rates of 8 C. The superior electrochemical performance are attributed to the synergistic effects of nanorod-nanoflake interconnected structures, mutual cations doping and the in situ generated carbon coating layer of the LMP·LVP/C composite. The synthesis strategy can be used to explore other nanocomposites, especially for high temperature synthesis.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06456. Supplementary figures including TEM, STEM-HAADF images, electrochemical performance, and EIS fitting data of LMP·LVP/C electrodes. XRD patterns, electrochemical performance of pristine LMP/C and LVP/C (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*
[email protected]. *
[email protected]. *
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (863 Program) (No. 2013AA110106), the National Natural Science Founda27639
DOI: 10.1021/acsami.6b06456 ACS Appl. Mater. Interfaces 2016, 8, 27632−27641
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