High performance of LiFePO4 with nitrogen-doped carbon layers for lithium ion batteries

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Original Research Paper

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High performance of LiFePO4 with nitrogen-doped carbon layers for lithium ion batteries

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Junke Ou a, Lin Yang b, Feng Jin c, Shugen Wu c, Jiayi Wang c

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a

Institute for Advanced Study, Chengdu University, Shiling Town, Chengdu 610106, China School of Medicine, Chengdu University, Shiling Town, Chengdu 610106, China c Zhanglan Honors College, Chengdu University, Shiling Town, Chengdu 610106, China b

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a r t i c l e

i n f o

Article history: Received 19 July 2019 Received in revised form 24 October 2019 Accepted 28 December 2019 Available online xxxx Keywords: LiFePO4 Nitrogen doped carbon layers Egg white Electrical conductivity Potential

a b s t r a c t Herein, we demonstrate a facile approach to fabricate a cathode material of LiFePO4 with nitrogen-doped carbon layers by applying egg white as both carbon source and nitrogen sources. The nitrogen doped carbon layers are in situ coated on the LiFePO4 particles, which effectively improves the electrical conductivity of rapid Li-ion diffusion. When evaluated as a cathode material for lithium ion batteries (LIBs), LiFePO4 material with nitrogen doped carbon shows high capacities of 164 mA h g
1 at 0.1 C, 144 mA h g
1 at 1 C and 120 mA h g
1 at 5 C. The result implies that such novel LiFePO4 material is a potential cathode material for LIBs. Ó 2020 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

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1. Introduction

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Because of the increasing demand for reducing CO2 emissions, the requirement for new energy sources to replace fossil fuels in automobiles has attracted great attention. In recent years, lithium ion batteries (LIBs) with high power and high energy densities have been recognized as new energy storages and conversion devices for electric vehicles (EVs) and hybrid electric vehicles (HEVs), which is beneficial to the improvement of the environment and energy issues [1–5]. In this case, the electrochemical performances of LIBs are mainly controlled by the quality of electrode materials, especially the cathode materials. Hence, much attention has been focused on the development of cathode materials with high capacities than commercial cathode material of LiCoO2 (a practical capacity of 130 mA h g
1), such as LiNi1/3Co1/3Mn1/3O2, LiNi0.8Co0.15Al0.05O2 and Li-rich Li1.2Ni0.13Co0.13Mn0.54O2 [6–8]. As ordered olivine lithium iron phosphate (LiFePO4) is inexpensive, nontoxic, and environmentally benign, it has received widespread attentions since Goodenough et al. discovered it as a cathode material for LIBs in 1997. [9]. However, two major drawbacks (low electric conductivity and Li-ion diffusion) may limit its battery property [10–12], such as the sharp decrease in capacity at high current densities. Therefore, improving the electrical conductivity of the material is imperative to enhance the electrochemical performance of LiFePO4. Thus far, many efforts have been devoted to

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enhancing the kinetic properties of LiFePO4, and several strategies have been explored [13–19]. Among these approaches, surface carbon coating has been regarded as one of the most effective strategies to enhance the electrochemical property of LiFePO4 [20]. A LiFePO4/C composite has been fabricated by Yang and the carbon coating content was about 1–4 wt%. This carbon coated LiFePO4 showed a capacity of about 150 mA h g
1 at the rate of 0.2 C (1 C = 170 mA h g
1) [21]. A solid-state route was used to prepare a LiFePO4/C with a specific surface area of 24.1 m2 g
1, showing a discharge capacity of 115 mA h g
1 at 5 C [22]. Carbon-coated LiFePO4 composites with small carbon particles (100–200 nm) have been reported by Huang et al. via mixing the LFP precursors with a carbon gel before pyrolysis. This LiFePO4 composite delivered superior rate performance and good cycle stability [23]. LiFePO4 particle with carbon coating can show an increase of the electric transfer, and the decrease of the LiFePO4 particles can shorten the diffusion distance of lithium ion, which can all improve the rate property [24]. Furthermore, product purity can be improved by the method of carbon coating by eliminating any Fe3+ products applying the inert gases (like N2 and Ar) under high temperatures [24,25]. Recently, N-rich carbons have been demonstrated to be potential anode materials for LIBs [26]. It was reported that additional electrons contributed by the N atom can provide electron carriers for the conduction band, which can contribute to the electrical

https://doi.org/10.1016/j.apt.2019.12.044 0921-8831/Ó 2020 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: J. Ou, L. Yang, F. Jin et al., High performance of LiFePO4 with nitrogen-doped carbon layers for lithium ion batteries, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.12.044

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conductivity of the material through introducing N into the carbon structures. A LiFePO4 with a conductive nitrogen-doped carbon layer was fabricated and displayed a favorable initial capacity of 98 mA h g
1 at 5 C [27]. A sol-gel method was applied to fabricated a hierarchical porous LiFePO4 with nitrogen-doped carbon nanotubes and displayed a capacity of 77 mA h g
1 at 5 C (850 mA g
1) [28]. A Li4Ti5O12 composite with a nitrogen-doped carbon layers coating was reported to show a superior discharge capacity of 100 mA h g
1 at 24 C [29]. At present, two main synthesis strategies are usually applied to prepare N-doped carbons: (i) using carbon and nitrogen containing precursor for in situ doping nitrogen into the carbon structure; and (ii) treating the carbons with the nitrogen containing precursor (such as ammonia, urea, etc.) [30,31]. Typically, the former approach is more easily controlled and operated than the latter. Hence, it is meaningful to explore a cheap and available carbon and nitrogen precursor for the coating of LiFePO4. In the food, anti-microbial and cosmetic industries, yolk is in great demand, but the egg white is often discarded as waste, which not only causes environmental pollution but also wastes valuable protein resources. In this work, egg white, a widespread and low cost raw material, could be expected to in-situ grown a carbon layer with N on the surface of LiFePO4. Hence, a nitrogen-doped carbon coated LiFePO4 was fabricated via a facile calcination process by using egg white as carbon and nitrogen containing precursor. Inert Ar sintering atmosphere was introduced to eliminate the formation of Fe3+ and iron phosphides. Various contents of egg white were applied to investigate the electrochemical properties of the LiFePO4 products.

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2. Experimental sections

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2.1. Synthesis of the material of N-doped carbon coated LiFePO4

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The LiFePO4 cathode material was prepared by a hydrothermal method in our previous work [32]. The fabrication of the nitrogen doped carbon coated LiFePO4 was relatively simple and was accomplished via a simple and facile pyrolyzation treatment with egg white. Typically, the as-synthesized LiFePO4 (about 1 g) was added into egg solution and stirred to form a homogeneous solution. The as-formed mixture was then dried at 100 °C for 3 h in vacuum. The prepared material was calcined in Ar atmosphere at 750 °C for 2 h with a heating rate of 5 °C min
1. To control the content of the nitrogen doped carbon, the addition of the egg white was 0.5, 1 and 2 ml. The resulted materials are denoted as LFP0.5, LFP-1 and LFP-2, respectively. For comparison purposes, LiFePO4 without treatment by egg white was also prepared at 750 °C for 2 h and designated as Bare LFP. The synthesis of the nitrogen doped carbon coated LiFePO4 was illustrated in Scheme 1.

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2.2. Material characterization

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The as-prepared samples were subjected to various characterization methods. The crystallographic structure of the materials was studied by X-ray diffraction (XRD) carried on a TD-3500 Xray powder diffractometer (Tongda, China), scan range from 10° to 80°. Morphology structure of the material was characterized applying a Hitachi S4800 field emission scanning electron microscopy (FESEM). The transmission electron microscopy (TEM) images were obtained from a FEI Tecnai G2 20 TEM (Hillsboro, OR, USA). Raman spectrum was estimated applying a confocal LabRAM HR800 spectrometer, France. X-ray photoelectron spectra (XPS) were tested applying a Kratos XSAM 800 spectrometer (Manchester, UK). The nitrogen absorption/desorption of the asobtained LiFePO4 was conducted using Automated Gas Sorption

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Analyzer (Autosorb-IQ, USA). The thermal gravimetric analysis was carried out on the Thermo Gravimetric Analyzer (Q500, USA) with a heating rate of 10 °C min
1 in Air.

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2.3. Electrochemical measurements

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The electrochemical properties of the as-obtained N-doped carbon coated LiFePO4 were tested by using CR2032 coin cells with NEWARE-BTS-5V 5 mA battery test system. The electrode was obtained by mixing 80 wt% active materials with 10 wt% conductive acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder in N-methyl-pyrrolidone (NMP) solvent. The Li metal foil was applied as anode and the Celgard 2400 (Celgard polypropylene) as the separator. The electrolyte was 1 mol/L LiPF6 dissolved in a mixture of ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v). The mass of active materials was 2–3 mg, and was dried at 120 °C under vacuum before assembled into coin cell in an argon-filled glove box. The charge/discharge measurements were conducted between 2.2 and 4.2 V versus Li/Li+ at 25 °C. The electrochemical impedance spectroscopy (EIS) curves were carried out on the workstation (CHI650e) in the frequency range 0.01–100 kHz at a charged stage with an applied amplitude of 5 mV.

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3. Results and discussion

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XRD patterns were conducted to determine the phase compositions of the as-obtained samples. As presented in Fig. 1a, all the asprepared samples display narrow and strong diffraction peaks, which were ascribed to an orthorhombic structure with a space group of Pnma (JCPDS no. 40-1499) [33]. It should be pointed out that no other diffraction peaks were found, indicating that all the LiFePO4 samples possess a high crystallinity and phase purity. Clearly, the crystalline structure of LiFePO4 was not independent on the introduction of nitrogen into the carbon layers. Raman spectroscopy based on the limited penetration depth of the laser into the sample was used to detect the surface phase components. As displayed in Fig. 1b, the peaks about 1345 and 1598 cm
1 were ascribed to the D-band and G band, respectively. As shown in Fig. S1, without the protein treatment, the LFP sample presents no peaks, revealing no carbon coating. The D-band corresponds to the defect-induced mode whereas the G-band to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice [34]. These two peaks can be clearly seen in all of the three samples, indicating that all LFP samples have been successfully coated by the carbon layer. Generally, compared with disordered carbon, it is accepted that the materials with graphite carbon display higher electric conductivity, which is essential to the Li-ion insertion and extraction [22], i.e., higher the amount of graphite carbon on the surface of materials, better the electrochemical performances of the materials. The intensity ratio of the D and G bands (ID/IG) was generally applied to determine the degree of graphitization of the materials [35]. Remarkable, the sample of LFP-1, with an ID/IG ratio of 1.01 (1.09 for LFP-0.5 and 1.02 for LFP-2), delivers the highest graphitization degree, which is mainly favorable for the superior electrochemical property of the sample, as studied in the later discussion. Nitrogen adsorption/desorption isotherms were conducted to study the pore structure of the materials. As depicted in Fig. 2a, the LFP-1 presents type IV isotherms with a specific surface area of 23.2 m2 g
1, which is much larger than that of Bare LFP (5.8 m2 g
1). The pore size distribution of the LFP-1 was centered in mesopores and macropores (seen in Fig. 2b). The relatively larger surface area of LFP-1 may be attributed to the amorphous carbon coating layer forming a network framework. The more pore structure may derive from the gas release during the pyrolysis of egg white, which could promote the contact between active

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Please cite this article as: J. Ou, L. Yang, F. Jin et al., High performance of LiFePO4 with nitrogen-doped carbon layers for lithium ion batteries, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.12.044

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Scheme 1. Schematic illustration of the fabrication process for LFP@NC materials.

Fig. 1. (a) XRD pattern and (b) Raman spectra of the as-obtained samples.

Fig. 2. (a) Nitrogen adsorption/desorption isotherms of the Bare LFP and LFP-1 and (b) pore size distribution of the LFP-1.

Please cite this article as: J. Ou, L. Yang, F. Jin et al., High performance of LiFePO4 with nitrogen-doped carbon layers for lithium ion batteries, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.12.044

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Fig. 3. SEM images of Bare LFP (a), LFP-0.5 (b), LFP-1 (c), LFP-2 (d) and HRTEM images of LFP-1 (e, f).

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material and electrolyte and shorten the distance of Li-ion diffusion to enhance the electrochemical property of the LiFePO4 [36,37]. FESEM was applied to observe the morphologies of all the LiFePO4 samples (Fig. 3a–d). The Bare LFP shows the average particle size ranging from 300 to 500 nm. It can be obviously seen that with the addition of egg white, the particle size of the material gradually decreased (about 200–300 nm), which may be ascribed to the limitation of crystal growth of LiFePO4 by carbon coating [20,38]. For further studying the structure of the coating layer, N doping carbon coated LiFePO4 particle was employed to be observed by HRTEM (Fig. 3e and f). Fig. 3f clearly shows that the LiFePO4 particles were well-covered by a uniform N-doped carbon layer with a thickness of about 5 nm, suggesting a highly crystallinized LiFePO4 in the cores of each particle. In order to compare the group of particles and the carbon layers thickness, the TEM of the LFP-1 on lower magnification scale were presented (Fig. S2). It can be clearly seen that the amorphous carbon layer derived from the pyrolysis of the egg white was covered on the surface of the LiFePO4 material (the size of the LFP was about 150–200 nm). And the thickness of amorphous carbon layer was about 5 nm, which can be seen in Fig. 3f. This uniform carbon layer on the surface of the LiFePO4 is mainly ascribed to production of carbon on

the particle surfaces through the thermal decomposition of the egg white during the calcinations process. It should be pointed out that the egg yolk was applied as a co-coating carbon, nitrogen and phosphorus source to fabricate the N, P doped carbon coated LiFePO4 composite [39]. The obtained carbon layer was about 4– 6 nm. The nitrogen doped carbon layer (egg white used in this work) was covered on the surface of LiFePO4 more evenly and contributed to the enhancement of the electronic conduction, which can be seen in Fig. 3 and Fig. 1 (HRTEM image and Raman analysis). Such a unique carbon coated conductive network is expected to significantly facilitate the transportation of Li-ion and electrons, and thus improve the rate performances. To further investigate the chemical states and the surface elemental compositions of the elements, XPS spectra of the sample of LFP-1 was performed (Fig. 4a–d). As displayed in Fig. 4a, the XPS full survey of the LFP-1 sample shows seven peaks at 710.9, 533.2, 401.8, 285.7, 133.8, 195.7 and 56.8 eV, corresponding to Fe 2p, O 1s, N 1s, C 1s, P 2s, P 2p and Li 1s, respectively. The highresolution Fe2p spectrum (Fig. 4b) shows two obvious peaks at about 711.3 and 724.3 eV, assigned to the Fe2p3/2 and Fe2p1/2, respectively [40,41], suggesting the element state of Fe2+ and the purity of the as-prepared LiFePO4. Fig. 4c displays the deconvoluted C 1s spectra of the LFP-1, which can be deconvoluted into

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Fig. 4. XPS full survey spectra of LFP-1 (a) and corresponding high-resolution XPS spectra of Fe2p (b), C1s (c) and N1s (d).

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three peaks located at around 284.6, 285.7, and 288.6 eV, corresponding to CAC, CAN, and OAC@O bonds, respectively [42]. The N 1s spectrum (Fig. 4d) can be deconvoluted into three peaks centered at 398.4, 399.7, and 400.9 eV, ascribed to pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), and graphitic nitrogen (N-Q), respectively [43], suggesting that the nitrogen was successfully introduced into the carbon skeletons. The N content from the XPS analysis was about 1.68%. Furthermore, the EDS mapping of LFP1 (Fig. S3) further confirms the existence of nitrogen. Such nitrogen doped carbon structure in LiFePO4 is beneficial for the improvement of electrochemical conductivity and electrochemical reactivity [44,45]. As shown in Fig. S4, a 3.1% weight increase was observed in the sample of bare LFP, which mainly attributed to the oxidation of the Fe2+. As for the other LFP samples with carbon layers, the carbon was oxidized to the carbon monoxide, carbon dioxide, etc., leading to a weight loss. Clearly from the TG curves, the carbon contents of LFP-0.5, LFP-1 and LFP-2 can be determined to be about 6.5, 10 and 16.2%, respectively. This result further indicated that N-doped carbon layer coating was derived from the pyrolysis of the egg white. Furthermore, the sample of bare LFP was not treated by egg white and does not have N doped carbon layer, so N 1s and C 1s was not founded in the XPS analysis. The Li 1s, Fe 2p and P 2p spectra of the bare LFP and LFP-1 were compared as follows (Fig. S5). As shown in the bare LFP (a, b, c), the binding energies of 55.5, 710.9 and 133.2 eV, corresponding to Li 1s, Fe 2p and P 2p, respectively. For LFP-1, the Li 1s, Fe 2p and P 2p show the binding energies located in the 55.6, 711.2 and 133.4 eV, respectively. The N 1s and C 1s spectrum of the LFP-1 can be seen in Fig. 4. This result further indicates that the N doped carbon derived from the decomposition of the egg white was coated on the surface of the LFP. The galvanostatic charge/discharge property of as-prepared materials was estimated in Fig. 5. As shown in Fig. 5a, after 10 cycles, the sample of Bare LFP displayed a discharge capacity of 94 mA h g
1 at the current density of 170 mA g
1 (1 C). When the egg white is introduced into the material, the LiFePO4 with

0.5 ml egg white displayed a capacity of 135 mA h g
1 after 10 charge-discharge cycles (Fig. 5b), which is better than that of Bare LFP. With the increase of the weight of egg white, the electrodes of LFP-1 and LFP-2 delivered discharge capacities of 144 and 141 mA h g
1, respectively. The LFP-1 shows the best electrochemical property, which may be ascribed to the appropriate egg white introduced. Interestingly, from the Fig. 5a–d, we can clearly see that the gap between charge and discharge became narrower with increasing the contents of egg white, indicating the enhancement of the reversibility of the insertion/extraction process of lithium ions. Fig. 5e presents the galvanostatic charge/discharge profiles of the LFP-1 for the 10th cycle between the voltages of 2.2 and 4.2 V at current rates ranging from 0.1 C to 5 C. The discharge capacity decreases with the increase of rate, suggesting that the capacity loss is limited by Li-ion diffusion. At 0.1 C (17 mA g
1), the electrode shows a reversible capacity of 164 mA h g
1, which corresponds to the 96.5% of the theoretical capacity of LiFePO4. It was important to be pointed out that the LFP-1 delivers a highrate performance of 120 mA h g
1 at 5 C, which is higher than those of reported LiFePO4 previously (about 113 mA h g
1 at 5 C) [22,28,46]. In addition, the as-obtained LFP-1 and commercial graphite were employed as the cathode and the anode, respectively, the full lithium ion battery was assembled. As shown in Fig. 5f, the full battery delivers a high capacity of 158 mA h g
1 at the current density of 17 mA g
1 after 5 cycles, suggesting its potential commercial application. Fig. 6 compares the C-rate influence and cycling performance of the four LFP samples. For the rate performances, the LFP-1 shows the best performance (from the Fig. 6a). Fig. 6a shows the specific capacities of LFP-1 electrode evaluated sequentially at rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C, charging and discharging for 10 cycles at each rate and then evaluated at 0.1 C for 10 cycles. Clearly, the electrode of LFP-1 delivered a good electrochemical property, remarkably at high rates, with discharge capacities of about 164, 155, 149, 144, 136 and 120 mA h g
1 at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C, respectively. When the C rate was recovered to 0.1 C,

Please cite this article as: J. Ou, L. Yang, F. Jin et al., High performance of LiFePO4 with nitrogen-doped carbon layers for lithium ion batteries, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.12.044

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Fig. 5. Galvanostatic charge-discharge curves of Bare LFP (a), LFP-0.5 (b), LFP-1 (c), LFP-2 (d) at 1C (1 C = 170 mA g
1), galvanostatic charge-discharge profiles of LFP-1 for the 10th cycle at different charge-discharge rates (e) and the charge/discharge curves of the full lithium ion battery at 0.1 C (f).

Fig. 6. The multi-rate capability (a) and cycling capability (b) of LiFePO4 samples.

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the discharge capacity of LFP-1 can maintain at 160 mA h g
1, revealing that the working electrode and the formed SEI film were quite stable during cycling. Meanwhile, this result suggests that the appropriate nitrogen doping into the carbon layers not only enhances the reversible capacities at different rates, but also improves the cycling property of the materials. The long cycling performance of the electrode of LFP at rates of 1 C was performed in Fig. 6b. The LFP-1 shows the superior cycling performances. After 100 cycles, the capacities of the bare LFP, LFP-0.5, LFP-1 and LFP-2 were about 83, 137, 141 and 133 mAh g
1, respectively.

It can be seen that the capacity losses of the 100th cycle are only 2.77% from the first cycle at 1 C, revealing that the nitrogen doping into the carbon layers of LiFePO4 materials would be beneficial for achieving a superior cycling performance. The Cyclic voltammetry (CV) profiles of Bare LFP and LFP-1 in the first 10 cycles at a scan rate of 0.1 mV s
1 between 2.2 and 4.2 V was presented in Fig. 7a. For the LFP-1 electrode, the oxidation/reduction peaks at 3.36 V and 3.50 V are attributed to the Fe2+/Fe3+ redox pair, assigned to the reduction of Fe3+ to Fe2+ (Li-ion intercalation) and the oxidation of Fe2+ to Fe3+ (Li-ion

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Fig. 7. CV profiles of Bare LFP and LFP-1 at 0.1 mV s
1 (a), CV profiles of LFP-1 at various scan rates (b), linear responses of peak current (Ip) versus square root of scan rate (v1/ ) (c) and Electrochemical impedance spectra (EIS) of the four electrodes (d).

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de-intercalation), respectively. Moreover, the potential difference between the two redox peaks is 140 mV for LFP-1, whereas that of Bare LFP is 370 mV. Meanwhile, LFP-1 with a sharper peak shape and a higher peak current may show a favorable electrochemical reactivity and lower polarization of the electrode, suggesting faster lithium ions diffusion by the introduction of the nitrogen doped carbon layer [5,47]. To further investigate the Li-ion diffusion kinetics, CV measurements at various scanning rates from 0.1 to 1.6 mV s
1 for LFP-1 electrode were carried out, as depicted in Fig. 7b. The symmetrical redox peaks further confirms the good reversibility of the Li-ion insertion-extraction in the LiFePO4 lattice. Randles-Sevcik equation was employed to calculate the Liion diffusion coefficients (D) of the materials during charge and discharge process:

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Ip ¼ 2:69  105 n3=2 AD1=2 Cv1=2

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where n is determined to be 1 for the Fe2+/Fe3+ redox couples, A is the surface area of the electrodes (1.54 cm2), C is the molar concentration of lithium ion of LiFePO4 and v is the scan rate (V s
1). Fig. 7c displays the relationship between the current peak (Ip) and scan rate (v). According to the Randles-Sevcik equation, the Li-ion diffusion coefficients for LFP-1 was estimated to be 1.66  10
10 and 1.23  10
10 cm2 s
1 for the charge and discharge processes, respectively, which are similar with that of the reported LiFePO4 materials [48,49]. This result demonstrates that the nitrogen doped carbon coated LiFePO4 can be beneficial for the Li-ion diffusion in the electrode, which is beneficial to the enhanced rate property of LiFePO4. Fig. 7d displays the electrochemical impedance spectroscopy (EIS) of the four electrodes after 50 cycles at 1 C. According to the equivalent circuit displayed in the inset of the Fig. 7d, the EIS data was analyzed, where Rs corresponds to the solution resistance. Rct corresponds to the charge transfer resistance, which refers to the electrochemical reaction between the interfaces of electrode/electrolyte. Constant phase element (CPE) is used to more precisely simulate the capacitance provided by the surface of the active material [50,51]. It can be clearly observed that the impedance spectrum

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contains an intercept at high frequency, a depressed semicircle at medium frequency and a straight line in the low frequency region. The semicircle of the high and middle frequency regions describes the charge transfer resistance, whereas the sloping line in the lower frequency displays the Warburg impedance (Zw), which is ascribed to the diffusion of Li-ion in the bulk of the electrode [27]. Remarkably, the LFP-1 shows smallest semicircle among these four samples, revealing a favorable electrochemical property. Based on the fitting results in Table S1, the Rct value for LFP-1 was 20.5 X, which is smaller than that of other three samples, suggesting its fast charge transfer kinetics. These above results all reveal that the introduction of egg white plays a positive and important role in enhancing the electrochemical property of LiFePO4 which can be ascribed to three main factors. Firstly, the highly efficient and stable conductive network can offer superior electronic transportation between LiFePO4 active particles and thus improve the electronic conductivity. Secondly, a facile electron transport from/to LiFePO4 owing to the N-doping carbon coating the LiFePO4 nanoparticles results in high discharge capacities. Furthermore, Owing to the N-doped carbon layer derived from the decomposition of the egg white, the particle size of the LiFePO4 is effectively controlled, which is beneficial for reducing the diffusion distance of the lithium ion, resulting in the high-rate capability.

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4. Conclusions

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In summary, we have successfully fabricated nitrogen-doped carbon coated LiFePO4 materials by applying egg white as a novel carbon and nitrogen source. The result showed that the LiFePO4 within the nitrogen-doped carbon layer effectively promotes the transportation of electron and lithium ions. When tested as a cathode material for LIBs, especially the LFP-1 exhibits higher specific capacities (144 mA h g
1 at 1 C), better rate performance (120 mA h g
1 at 5 C) and favorable lithium ion diffusion coefficient (1.66  10
10 cm2 s
1). The nitrogen-doped carbon coated

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LiFePO4 materials are determined to be promising for highperformance LIBs. Such a unique strategy with nitrogen-doped carbon encapsulation may be further used to the fabrication of other electrode materials for advanced batteries.

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Acknowledgements

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We greatly appreciate the National Natural Science Foundation of China (21606024) and the Sichuan Science and Technology Program (2019YJ0665) for supporting this work.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.12.044.

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