Conformal Coating Strategy Comprising N-doped Carbon and

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Conformal Coating Strategy Comprising N-doped Carbon and Conventional Graphene for Achieving Ultra-high Power and Cyclability of LiFePO4 Kan Zhang, Jeong Taik Lee, Ping Li, Byoungwoo Kang, Jung Hyun Kim, Gi-Ra Yi, and Jong Hyeok Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02604 • Publication Date (Web): 21 Sep 2015 Downloaded from http://pubs.acs.org on September 22, 2015

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Conformal Coating Strategy Comprising N-doped Carbon and Conventional Graphene for Achieving Ultra-high Power and Cyclability of LiFePO4 Kan Zhang1, Jeong-Taik Lee 2, Ping Li2, Byoungwoo Kang3, Jung Hyun Kim1, Gi-Ra Yi2*, and Jong Hyeok Park1* 1

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro,

Seodaemun-gu, Seoul 120-749, Republic of Korea 2

SKKU Advanced Institute of Nanotechnology (SAINT) and School of Chemical Engineering,

Sungkyunkwan University, Suwon 440-746, Republic of Korea 3

Department of Materials Science and Engineering, Eng 1-123, Pohang University of Science and

Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 790-784, Republic of Korea

Keywords: LiFePO4 cathode, Li ion diffusion, intrinsic conductivity, electrode conductivity, conformal coating strategy

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Abstract Surface carbon coating to improve the inherent poor electrical conductivity of lithium iron phosphate (LiFePO4, LFP) has been considered as most efficient strategy. Here, we also report one of the conventional methods for LFP, but exhibiting a specific capacity beyond the theoretical value, ultra-high rate performance, and excellent long-term cyclability: the specific capacity is 171.9 mAh/g (70 µm-thick electrode with ~10 mg/cm2 loading mass) at 0.1 C (17 mA/g), and retains 143.7 mAh/g at 10 C (1.7 A/g) and 95.8 % of initial capacity at 10 C after 1000 cycles. It was found that the interior conformal N-C coating enhances the intrinsic conductivity of LFP nanorods (LFP NR) and the exterior RGO coating acts as an electrically conducting secondary network to electrically connect the entire electrode. The great electron transport mutually promoted with shorten Li diffusion length on (010) facet exposed LFP NR represents the highest specific capacity value recorded to date at 10C and ultra-long-term cyclability. This conformal carbon coating approach can be a promising strategy for the commercialization of LFP cathode in lithium ion batteries. KEYWORDS: Lithium iron phosphate, nanorods, carbonaceous layer, conformal coating, lithium battery

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Since the pioneering report by Padhi et al. in 1997, olivine-structured lithium iron phosphate (LiFePO4 or LFP) has attracted significant attention and appears to be a promising candidate for use as an LIB cathode material, due to its high theoretical capacity (170 mAh/g), safety, low toxicity, and the possibility of low cost.1-4 Recently, fabricating nanorods, nanoflakes, nanoplates

via LFP morphology control,5-7 or conductive carbon coating on LFP in various thicknesses (1~25 nm)8-10 has been shown to dramatically improve electrochemical performance; however, previous efforts, including either conductive layer coatings or controlling morphologies have not yet fully overcome the inherent drawbacks of LFP. Various carbonaceous conductive layers, primarily including amorphous carbon, carbon black, conducting graphite, ethylene black and carbon nanotubes, have been widely employed for improving electrode conductivity.11-15 Among them, reduced graphene oxide (RGO) has received a lot attentions because of its superior electronic conductivity, high mechanical strength, structural flexibility, and more importantly, high surface area (theoretical value of 2630 m2 g−1). However, achieving a uniform carbon coating on LFP using RGO is still a significant obstacle, and limits the possibility of achieving theoretical capacity. Average capacities for RGO-modified LFP cathodes range from 146 to 166 mAh/g.16-19 More recently, Li et al. recently developed RGO-modified LFP, which exhibits a capacity beyond its theoretical value (208 mAh/g at 0.1 C). In this case, the excess capacity is attributed to the reversible redox reaction between the Li+ of the electrolyte and the RGO.20 However, the excess capacity originating from interfacial Li+ storage which is due to the slow lithium ion diffusivity of LFP could not contribute to high rate-performance of the cathode electrode. In addition, regarding to RGO and/or carbon modified LFP, the outer carbon layers are coated 4


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on the surface of LFP at random, which still reduce the charge storage ability of LFP.21-27 For instance, the LFP/C/RGO composites synthesized by mixed LFP/RGO with citric acid or sucrose as carbon source have delivered a capacity of ~160 mAh/g at 0.1 C, respectively.21,24,25 The discharge capacities of one-pot hydrothermally synthesized LFP/C/RGO composites were lower than 165 mAh/g at 0.1 C respectively.22,23,27 Therefore, the unsatisfied electrochemical properties of LFP need to be improved, aided by the more advanced carbon coating strategy. Herein, we report that (010) crystal-orientated LFP nanorods (hereafter LFP NR) encapsulated with conformal double-layers comprising N-doped carbon and conventional RGO which could achieve 172 mAh/g (beyond the theoretical capacity of LFP) with ultra-high rate performances and ultra-long cyclability. Interestingly, cetyltrimethyl ammonium bromide (CTAB) not only act as surfactant to promote LFP NR growth along (010) direction, but also saver as the feedstock for intimal N-doped amorphous carbon (N-C) layer on LFP NR. The outermost RGO could be intimately coated with CTAB/LFP NR via electrostatic grafting by employing aminopropyltrimethoxysilane (APS) for the additional surface modification. We expected: 1) rapid Li ion diffusion owing to the exposed (010) facet; 2) fast electron pathways along highly conductive N-doped carbon layer on single LFP NR; and 3) high electronic conductivity throughout the entire electrode due to the 3D RGO network. These LFP NRs coated with conformal carbon double-layers are an attempt not only to compensate for the inherently poor electronic conductivity of each LFP particle, but also to increase the number of electron pathways throughout the entire electrode. More importantly, through a simple combination of 1), 2), and 3) above, we achieved the highest specific capacity of LFP cathode at 10C (1.7 A/g) with an 5


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excellent stability recorded to date. The unique synthesis method for (010) faceted LFP NR with dual carbonaceous shells is schematically depicted in Figure 1. The LFP NR synthesized with CTAB assistance was compared with that without CTAB assistance, as shown in Figure S1. It is clear that the CTAB surfactant promoted the oriented growth of LFP NR structures with lengths of up to 100 nm and widths of 20~30 nm. The high-resolution transmission electron microscopy (HR-TEM) image of the LFP NR (Figure 2a) shows a fringe spacing of 1.03 nm, which indicates the presence of the (010) plane. From Thermogravimetric analysis (TGA), it was confirmed that the residual N-C content in LFP NR from the carbonization of CTAB is ~2 wt.% (Figure S2). Therefore, the TEM image of LFP NR represents a thin layer of N-C after carbonization (Figure 2b). To form a well-organized dual carbonaceous shell for the LFP NR, the CTAB/LFP NR was modified further by APS, followed by a second carbon source, GO, was added subsequently, as shown in Figure 1. Note that surface modification of APS plays an important role in the intimate interface between LFP NR and RGO. The comparative experiment is shown in Figure S3, where it is clear that the APS modification improves the miscibility of RGO with LFP NR; this can be confirmed via the phase separation behaviour of the product solution. The HR-TEM images of Figure 2c and d show LFP NR encapsulated with a dual-carbonaceous shell (~2 wt.% N-C layer and ~2.5 wt.% of RGO content), referred to as LFP NR@N-C@RGO, demonstrating that the average thickness of the N-C layer on an LFP NR is approximately less than 3.5 nm. The full X-ray photoelectron spectroscopy (XPS) spectrum of the LFP NR@N-C@RGO displays the binding energy at 56.4 eV for Li 1s, 132.8 eV for P 2P, 190.9 eV for P 2s, 284.6 eV for C1s, 400.7 eV for N 1s, 531.3 eV for O 1s and 710.2 eV for Fe 2p, respectively (Figure 2e). The deconvolution of the N1s peak in 6


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Figure 2f exhibits two types of bonds: graphitic nitrogen at 400.7 eV, and oxidized nitrogen at 402.5 eV. No relative pyridinic nitrogen or pyrrolic nitrogen could be detected, implying the N doping only in first carbon layer, rather than RGO sheets. The Fe 2p spectrum in LFP NR@N-C@RGO is deconvolved into two major peaks at around 710.3 and 723.5 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 with a spin-orbit splitting of 13.5 eV, which is consistent with a olivine LFP phase (Figure 2g).28 TEM comparison between LFP NR and LFP NR@N-C@RGO shows that LFP NR@N-C@RGO has relatively rough surface with same particle size (Figure S4). The field emission scanning electron microscopy (FE-SEM) images of LFP NR@N-C and LFP NR@N-C@RGO further demonstrated that these LFP NR@C-N are strongly interconnected by RGO network as shown in Figure S5. The electrochemical properties, including charging/discharging profile, C-rate performance and long-term stability of LFP NR@N-C@RGO cathode are investigated. For comparison purpose, a series of LFP NR@RGO without N-C layer cathode materials were prepared by removing CTAB from the synthesized LFP NR surface and keeping other synthesis conditions. Figure 3a shows the voltage profiles of the first cycle with 0.1 C as the charging/discharging rate for the cathodes with LFP NR@RGO (~2.5 wt.% of RGO content) and LFP NR@N-C@RGO. The LFP NR@N-C@RGO provide a discharge capacity of 172.2 mAh/g, based on whole electrode mass including N-C and RGO, which significantly exceeds the theoretical capacity of LFPs considering the N-C and RGO contents of the cathode. The capacity of LFP NR@N-C@RGO at 3.45 V (region I) is approximately 152.8 mAh/g, corresponding to the insertion/extraction of Li from LFP at approximately 0.9 per formula unit. Region I in LFP NR@RGO is much smaller than that of LFP NR@N-C@RGO, only contributing to a capacity of 7


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~109 mAh/g even though both samples have similar RGO content. Interestingly, V versus state-of-charge slope (region II) in LFP NR@RGO represents a different shape than that of LFP NR@N-C@RGO, leading to a total capacity of up to 146 mAh/g. Generally, the region II capacity originating from the state-of-charge slope is ascribed to interfacial ion storage; the phenomenon is similar to that reported for nanometre-sized, layered, transition-metal oxides at low potential.29 From this observation, it is obvious that RGO can contribute additional charge storage in the cathode. Recent studies demonstrated that flexible and planar RGO has the potential to offer advantages over carbon (low fraction and high conductivity), representing a high capacity of approximately 170 mAh/g.16-19 To investigate the charge storage ability of RGO in the cathode, we have varied RGO contents from 0.8 to 3.8 wt.% in LFP NR@RGO, and the voltage profiles of the first cycle (0.1 C) are presented in Figure S6. A clear comparison between the LFP NR@N-C@RGO (2.5 wt.%) and the LFP NR@RGO (3.3 wt.%) with an optimized RGO content is also shown in Figure 3b, in which both samples reveal similar discharge capacity; the charge capacity of LFP NR@RGO is 10.8 mAh/g higher than that of LFP NR@N-C@RGO. A comparison of the rate performance is shown in Figure 3c, where it can be seen that LFP NR coated with RGO at 3.3 wt.% of RGO, do not meet the requirements of high-rate performance, while LFP NR@N-C@RGO do favour high-rate performance with only 16 % fading as the current density increases 500 times. This result implies that RGO can contribute additional charge storage capacity but which is irreversible at high rates. A further investigation into the cycling performance of LFP NR@N-C@RGO was carried out as shown in Figure 3d. The LFP NR@N-C@RGO revealed high stability at 0.5 C during 100 cycles and 2 C during 200 cycles. LFP NR@RGO shows poor capacity retention during the first 100 cycles, implying that the 8


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incorporation of RGO in LFP NR is not effective way for cyclability. From these results, we can confirm that the electronic conductivity of intra- and inter-particle is crucial for the high-rate and cycling performance of LFP. To further evaluate their cycling performances at high current density, the LFP NR@N-C@RGO was performed upon prolonged 1000 charge–discharge cycles at 10C and 50 C, respectively. As shown in Figure 3e, the LFP NR@N-C@RGO electrodes show impressive cycling performance at 10 C over 1000 charge/discharge cycles, which retains 95.8% of their capacity at the end of 1000 cycles, corresponding to a small average capacity decay of 0.0042% per cycle. The average Coulombic efficiency is calculated to be 99.8%. Further increasing the current density to 50 C, the LFP NR@N-C@RGO electrodes still retains 77.1% of initial capacity after 1000 cycles. We have also demonstrated the possibility of mass-production, as shown in Figure S7. The precursor dosage was scale up to 25 times, and the total mass of the resulted LFP NR@N-C@RGO was about 23.86 g. Moreover, the rate-performance of the mass produced LFP NR@N-C@RGO revealed a discharge capacity of 170.5 mAh/g at 0.1 C which could be significantly retained to 163.3 mAh/g when the current density was increased to 1 C. To fully understand the capacity properties of the LFP NR@N-C@RGO, the charge/discharge profiles of LFP synthesized without CTAB, LFP NR and LFP NR@N-C are shown in Figure S8 a. The capacity of region I, which originates from LFP, was maintained better by N-C coating on individual LFP NR. However, the LFP NR@N-C does not exhibit high capacity and high-rate performance, as shown in Figure S8 b. The results demonstrated that the exterior RGO layer might facilitate electron transport and collection. The LFP NR coated with dual-carbonaceous shells achieved a capacity matching that of its theoretical value; the mechanisms were carefully studied. Due to two key obstacles which 9


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severely restrict the performance of LFP NR, lithium ion diffusivity and electronic conductivity, the morphology and method of carbon coating greatly affect electrochemical properties of LFP. Figure 4a shows the X-ray diffraction (XRD) patterns of LFP NR, LFP NR@N-C@RGO, and LFP particles synthesized without using CTAB. All reflection lines can be indexed to an orthorhombic space group, Pnma (JCPDS card No. 81-1173), indicative of the perfect crystalline structure of olivine LFP. No characteristic lines of impurity phases are observed, indicating the high purity of the LFP NR. The exposed facet can be deduced from the intensity ratio of I(020)/I(200). Kanumara et al. suggested that the ratio of I(020)/I(200) for (010) faceted LFP is higher than that of the standard.30 According to their XRD analysis protocol, the I(020)/I(200) ratios of LFP NR, LFP without CTAB, and standard are 3.5, 2.2, and 2.1, respectively. The easy lithium diffusion direction along exposed (010) is perpendicular to the (001) direction of the nanorods, which is particularly effective for achieving fast lithium diffusion and high-rate capability.31 Figure 4b shows the Raman spectra of the LFP NR, LFP NR@RGO, and LFP NR@N-C@RGO; the two broad peaks in the range of 1300 ~ 1340 cm-1 and 1595 ~ 1598 cm-1 for carbonaceous-coated LFP are attributed to disordered graphite (D-band) and crystalline graphite (G-band), respectively. A serial of peaks at 447, 513, 595, 989, and 1043 cm-1 can be assigned to the internal stretching modes (v2, v3, and v4) of the PO43− anion,32 while other peaks below 400 cm-1 are assigned to the external stretching modes or lattice vibrations. Most of the external modes cannot be assigned with any degree of confidence without the aid of calculations, still, this region is expected to contain lithium-ion ‘‘cage’’ modes. In comparison of LFP NR and LFP NR coated with carbonaceous materials, these peaks below 400 cm-1 become clear after carbonaceous coating, implying a change of 10


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stretching modes or lattice vibrations. The intrinsic conductivity of (010) exposed LFP is initially investigated using UV-vis absorption spectra. As shown in Figure 4c, the absorption edges of LFP NR coated with carbonaceous materials show an obvious red-shift with an intense and broad background absorption in the high-wavelength region. The bandgaps are established using the Tauc plots of (αhv)1/2 versus hv for the LFP composites. As list in Table 1, the indirect bandgap of LFP NR is approximately 3.36 eV, while the bandgap of LFP synthesized without CTAB is as large as 3.45 eV. Significantly, the bandgap of LFP NR is dramatically decreased to 2.56 and 2.32 eV after being coated with RGO and N-C/RGO, respectively. The intimal N-C is believed that could be doped into the LFP lattices, hence forming new N 2p orbital above C 2p orbital in LFP NR/N-C/RGO, which is better than individual C 2p orbital directly from RGO, as shown in Figure 4e. In general, the electrical conductivity can be determined primarily by the free carrier concentration which depends exponentially on temperature and the band gap energy (Eg). σ = exp (-Eg/2kBT) The distinction between semiconductors and insulators essentially lies in the magnitude of the band gap. At room temperature, materials with Eg above 3.5 eV are generally termed insulator. The value of pure LFP NR (3.36 eV) is closed to that of insulator, which accounts for the low electronic conductivity. After being coated with N-C@RGO, the band gap energy decreases to 2.32 eV due to newly formed N 2p orbital. Such a narrow band gap could result in an improvement of surface electronic conductivity of LFP NR, which is favorable to the electrochemical performance, especially at high C rates. Such results demonstrate that coating with N-C materials mediates the intrinsic electronic conductivity of LFP due to the 11


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interfacial interaction between N-C and LFP at the interface (Figure 4d).33 Apart from intrinsic electronic conductivity, the charge transfer in the electrodes is key for efficient electron collection, electrochemical impedance spectroscopy (EIS) could therefore gain an intuitional understanding of this influence. Figure 4f presents typical Nyquist plots of the LFP NR@RGO, and LFP NR@N-C@RGO electrodes measured in the fully discharged state after two cycles. The intercept impedance on the Zreal axis represents the ohmic resistance, which consists of the resistance of the electrolyte and electrode. According to recent research by Murugan and Liu, the high-frequency region of the semicircle is more likely to be related to the particle-particle contact resistance and current collector.34,35 There is an obvious and remarkable order in the charge transfer resistance, LFP NR@RGO (180.6 Ω) ＞ LFP NR@N-C@RGO (80.2 Ω), which suggests that the combination of conductive RGO, for improving inter-particle electronic conductivity, and a thin carbon layer, for compensating for the poor intrinsic electronic conductivity of individual LFP particles, is more effective than RGO coating alone for improving the kinetics of the electrochemical reaction.17 Notably, the rate performance of our LFP NR@N-C@RGO is significantly better among recent reports regarding LFP cathode materials (Figure 4g),20-27 which could be ascribed to the well-organized carbonaceous layers offering an excellent synergistic effect for electron transport and collection. We have also synthesized LFP NR/N-C/RGO by one-pot solvothermal method, which presented a rod on sheet structure, as shown in Figure S9. As shown in Figure S10, its electrochemical properties exhibited poor rate performance and low capacity at 0.5C compared to well-organized LFP NR@N-C@RGO. Furthermore, we have also synthesized LFP sheets@N-C@RGO by following Figure 1 except using 2D LFP sheets 12


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instead of 1D LFP NR (Figure S11). Its electrochemical properties of LFP sheets@N-C@RGO were also investigated where the LFP sheets with the (100) facet are prominent. As expected, the LFP sheets@N-C@RGO could not have a good rate performance and high capacity (Figure S12). These results indicated that shorter Li+ diffusion length combined with fast charge transport in LFP cathode and interfacial Li+ storage in RGO is also key point to achieve their theoretical storage capability. To explore the influence of Li+ diffusion and electron transfer on the efficiency of LFP NR@N-C@RGO, cyclic voltammetry and a polarization curve were obtained, as shown in Figure 5a and b. The diffusion coefficient is proportional to the square of the peak current,

Ip, according to the Randles–Sevcik equation36:

Ip= 2.69×105n3/2AD1/2v1/2C (Equation 1),

where A is the electrode area (cm2), n is the number of electrons involved in the redox process (1 in our case), C is the shuttle concentration (mol·cm-3), v is the potential scan rate (V·s-1), Ip is in units of amperes, and D is in units of cm2·s-1. According to the above equation, the Li+ diffusion coefficient of LFP@N-C@RGO is much higher than LFP@RGO. The result can reasonably explain that the thin N-C layer on the LFP could not interfere with Li+ diffusion, and that highly-porous RGO connecting individual LFP NR may promote Li+ diffusion from the high electrolyte uptake. The efficiency of the electrical energy storage in a rechargeable battery is

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Efficiency % = ( ) where Vdis and Vch are the voltages during discharge and charge, respectively. A high-efficiency LIB should have a Vdis and Vch values as close to each other as possible over the entire state of charge. The Figure 5b shows that the polarization of LFP@N-C@RGO between Vdis and Vch is reduced to 20 mV from 49 mV of LFP@ RGO. The results demonstrate that LFP coated with N-C and RGO achieves high energy efficiency during the charge/discharge step, because of the aforementioned synergistic effects, as illustrated in Figure 5c. In conclusion, we have demonstrated a promising strategy to address the poor electronic conductivity and lithium ion diffusion typically found in LFP, using (010) facet exposed nanorods (LFP NR) coated with N-C and RGO with a double-shell architecture. The surfactant-assisted synthesis of LFP NR with a high content of exposed (010) facet endowed high lithium ion diffusivity through the 1D channel, which minimizes the resistance for Li extraction and insertion. The first conformal coating layer of N-C on the LFP NR led to significant enhancement of the intrinsic electronic conductivity which compensated for the insulating LFP cathode material, while the second carbon layer of RGO functioned as a bridge, building an electronic conductive network across the entire electrode to provide an efficient electron transport pathway with additional capacity. As a result, our materials not only achieved the theoretical capacity of 170 mAh/g with excellent cyclical performance, but also presented a capacity of 147.3 mAh/g at 10 C which is a higher value than any found in carbonaceous-modified LFP reports.

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ASSOCIATED CONTENT Supporting Information. Additional SEM images, TGA analysis, photographs, TEM images, rate and cycling performances. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION * Corresponding author: [email protected] (Prof. J. H. Park) ACKNOWLEDGMENT This work was supported by the NRF of Korea Grant funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A2A1A09014038, 2011-0006268). K. Zhang and J. Lee contributed equally to this work.

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(31) Kang, B. W.; Ceder, G. Battery materials for ultrafast charging and discharging.

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Table 1 Calculated bandgaps for LiFePO4 synthesized without CTAB, LiFePO4 NR, 3.3 wt.% LiFePO 4 NR@RGO and LiFePO4 NR@N-C@RGO. Samples

Bandgap (eV)

LFP without CTAB

3.45

LFP NR

3.36

LFP NR@RGO

2.56

LFP NR@N-C@RGO

2.32

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Figure 1. Schematic representations of the formation process and the microscale structure of LiFePO4 NR@N-C@RGO.

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Figure 2. (a) HR-TEM of LiFePO4 NR, (b) HE-TEM of LiFePO4 NR@N-C, (c) and (d) TEM and HE-TEM of LiFePO4 NR@N-C@RGO. (e) Full XPS spectra of LiFePO4 NR@N-C@RGO, (f) High resolution of N 1s and (g) High resolution of Fe 2p.

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Figure 3. (a) Charge/discharge profiles of the 2.5 wt.% LiFePO4 NR@RGO and LiFePO4 NR@N-C@RGO. (b) Charge/discharge profiles of the optimizing 3.3 wt.% LiFePO4 NR@RGO and LiFePO4 NR@N-C@RGO. (c) Rate performances of 3.3 wt.% LiFePO4 NR@RGO and LiFePO4 NR@N-C@RGO. (d) Cycling performances of 3.3 wt.% LiFePO4 NR@RGO and LiFePO4 NR/N-C/RGO at 0.5C and 2C. (e) Cycling performances of LiFePO4 NR/N-C/RGO upon prolonged 1000 charge–discharge cycles at 10 C and 50 C and its Coulombic efficiency at 10 C.

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Figure 4. (a) XRD pattern of the LFP synthesized without CTAB, LFP NR, and LFP NR@N-C@RGO (b) Raman shift of the LFP NR, LFP NR@RGO and LFP NR@N-C@RGO (c) UV absorption spectra of the LiFePO4 synthesized without CTAB, LFP NR, LFP NR@RGO and LFP NR@N-C@RGO. (d) Diagram of (010) surfaces of the LiFePO4 coated with carbonaceous. (e) Schematic illustration of surface energy band diagram of pure LFP NR and LFP NR@carbonecous. (f) The Nyquist plots of the LFP NR@RGO and LFP NR@N-C@RGO electrodes. All of RGO content in LFP NR@RGO is 3.3 wt.%. (g) Rate capacity retention of the LiFePO4 NR@N-C@RGO compared with previous published studies on LiFePO4@C@RGO. 24


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Figure 5. (a) Polarization curves of the 3.3 wt.% LiFePO4@RGO and LiFePO4@N-C@RGO at

0.1

C.

(b)

Cyclic

voltammograms

of

the

3.3

wt.%

LiFePO4@RGO

and

LiFePO4@N-C@RGO at scanning rate of 0.1 mV/s. (c) Schematic diagram of the LiFePO4@N-C@RGO electrode with reasonable electron transport mechanism for the superior properties.

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Conformal Coating Strategy Comprising N-doped Carbon and

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