Improved Electrochemical Performance of LiFePO4@N-Doped

Sep 23, 2016 - Improved Electrochemical Performance of LiFePO4@N-Doped Carbon Nanocomposites Using Polybenzoxazine as Nitrogen and Carbon ...
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Improved Electrochemical Performance of LiFePO4@N-Doped Carbon Nanocomposites Using Polybenzoxazine as Nitrogen and Carbon Sources Ping Wang, Geng Zhang, Zhichen Li, Wangjian Sheng, Yichi Zhang, Jiangjiang Gu, Xinsheng Zheng,* and Feifei Cao* College of Science, Huazhong Agricultural University, No.1 Shizishan Street, Hongshan District, Wuhan, 430070, People’s Republic of China S Supporting Information *

ABSTRACT: Polybenzoxazine is used as a novel carbon and nitrogen source for coating LiFePO4 to obtain LiFePO4@ nitrogen-doped carbon (LFP@NC) nanocomposites. The nitrogen-doped graphene-like carbon that is in situ coated on nanometer-sized LiFePO4 particles can effectively enhance the electrical conductivity and provide fast Li+ transport paths. When used as a cathode material for lithium-ion batteries, the LFP@NC nanocomposite (88.4 wt % of LiFePO4) exhibits a favorable rate performance and stable cycling performance. KEYWORDS: LiFePO4, nitrogen-doped carbon, polybenzoxazine, cathode material, lithium-ion batteries

1. INTRODUCTION Lithium-ion batteries (LIBs) have become one of the most promising power systems for potential applications in portable electronics and electric vehicles.1−3 Recently, olivine-type LiFePO4 has attracted an increasing amount of attention as a prospective candidate cathode material for the next generation LIBs owing to its multitudinous features such as environmental compatibility, acceptable operating voltage (3.4 V vs Li+/Li), superior thermal stability, and good cycling performance.4−7 However, the sluggish kinetic properties of bare LiFePO4 involving intrinsically low electronic and ionic transfer (σe− = 10−8−10−9 S cm−1, DLi+ = 10−14−10−17 cm2 s−1), hinder its wide application in practice.8−12 To address this issue, a variety of strategies have been proposed: (i) decreasing particle size to nanometer; (ii) constructing three-dimensional interlaced porous architecture; and (iii) coating conductive materials (such as carbon layers, boron/nitrogen codoped carbon layers, metal oxides, or polymers).13−24 Among them, carbon coating has been proved to be an effective way to enhance the conductivity of LiFePO4.10,17,18 Compared with traditional carbon coating techniques, the carbon coating layers doped with nitrogen can efficiently facilitate electrical conductivity and generate extrinsic defects for enhancing extra Li+ storage sites.15,22−30 Currently, two main synthetic routes are usually employed to fabricate nitrogen-doped carbon materials: (i) in situ doping nitrogen into the carbon materials using C,Ncontaining precursor; and (ii) post-treating carbon materials by nitrogen-containing precursor.21,25−30 In contrast, the former method is more easily operated and effective than the latter © 2016 American Chemical Society

one. Therefore, it is highly desired to explore a C,N-containing source for a nitrogen-doped carbon coating technique. Our previous work has demonstrated that polybenzoxazine (PBZ), a new class of high performance polymer, has high molecular design flexibility that allows for a variety of molecular structures of desired properties.31,32 The thermoset PBZ resins can be easily prepared from inexpensive raw materials, including various phenols, primary amines, and formaldehyde.33−35 Furthermore, inspired by the existence of nitrogenheteroatom in PBZ, we can effortlessly achieve in situ nitrogendoping in carbon through a carbonization process. In this work, bisphenol A as the phenol, phenylamine as the primary amine, and formaldehyde were selected to prepare monomer via a solution method. The highly cross-linked PBZs were obtained after thermally activated ring-opening of oxazine ring without adding any initiators or catalysts. Furthermore, we have successfully in situ fabricated PBZ-derived nitrogen-doped carbon coated LiFePO4 (LiFePO4@nitrogen-doped carbon, referred as LFP@NC) nanocomposites via a ball-milling route followed by solid-state reaction. These PBZ-derived nitrogendoped carbons introduced into LiFePO4 are beneficial to enhance the electrical conductivity. The electrochemical Li-ion storage performance of as-prepared LFP@NC nanocomposites was investigated. When used as cathode materials for LIBs, the LFP@NC nanocomposite (88.4 wt % of LiFePO4) can deliver a discharge specific capacity of 156.9 mA h g−1, which is 1.34 Received: August 23, 2016 Accepted: September 23, 2016 Published: September 23, 2016 26908

DOI: 10.1021/acsami.6b10594 ACS Appl. Mater. Interfaces 2016, 8, 26908−26915

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis Process of the Benzoxazine Monomer and PBZ

spectroscopy (XPS) performed on the ESCALab 250Xi (Thermo Scientific). Scanning electron microscopy (SEM) images were observed on JEM-6701F (JEOL). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were observed on JEM-2100F (JEOL). The nitrogen absorption and desorption isotherms at 77.3 K were obtained with a MicroActive for ASAP 2460 Version 2.01 surface area pore size analyzer. Fourier transform infrared (FTIR) spectra were collected by a NEXUS 670 FTIR spectrometer using KBr disks and purging with dry air. 2.7. Electrochemical Characterizations. The electrochemical tests were carried on coin-type CR 2032 cells which were assembled in an argon filled glovebox (MBraun, Unilab, Germany). For preparing working electrodes, a mixture of LFP@NC nanocomposites or LiFePO4, super-P acetylene black, and poly(vinyl difluoride) at a weight ratio of 8:0.5:1.5 was pasted on an aluminum foil. The loading mass of the active material was 1.4−1.6 mg/cm2 on the circular working electrode. The counter electrode was a pure lithium foil. A glass fiber (GF/D) membrane from Whatman was used as a separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1, in wt %) purchased from Tianjin Jinniu Power Sources Material Co., Ltd.. The electrochemical performance was tested using a battery test system LAND CT2001A system (Wuhan Jinnuo Electronics Co., Ltd., China) in the voltage range of 2.0−4.3 V (vs Li+/Li). In this work, the specific capacity was calculated based on the mass of LiFePO4.

times that of bare LiFePO4 at 0.1 C. Even at 10.0 C, the discharging specific capacity can still retain 63.7% of the value at 0.1 C, while the retention rate is only 34.7% for bare LiFePO4.

2. EXPERIMENTAL SECTION 2.1. Materials. Phenylamine, 1,4-dioxane, formaldehyde, bisphenol A, ethanol, lithium carbonate (Li2CO3), ferrous oxalate dihydrate (FeC 2 O 4 ·2H 2 O), and diammonium hydrogen phosphate ((NH4)2HPO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All chemical agents were of analytical grade and used without any further purification. 2.2. Preparation of Benzoxazine Monomer. In a typical procedure, a mixture of 9.2 mL phenylamine and 22.0 mL 1,4dioxane was introduced into a 250 mL round-bottom flask equipped with a magnetic stirrer, then 19.6 mL of aqueous formaldehyde (wt/wt 37%) was added into the mixture and stirred for 30.0 min at 5 °C. Subsequently, a solution of bisphenol A (22.4 g) dissolved in 60.0 mL of 1,4-dioxane was added into the reaction system, and the temperature was raised to 96.0 °C refluxing for 3.0 h, after that, 1,4dioxane was removed from the reaction mixture via decompressing distillation at 55.0 °C. A sticky liquid was obtained and denoted as benzoxazine monomer. 2.3. Preparation of PBZ Films. The benzoxazine monomer was directly coated on glass slides, and subsequently monomer was evenly distributed on the surface of the glass slide, and then the thin film was formed with the thicknesses of ∼0.2 mm. Then, the glass slides with monomer were cross-linked at 180.0 °C for 2.0 h in the air-circulating oven. After cooling down to room temperature naturally, the yellow films were finally obtained and denoted as PBZ films. 2.4. Preparation of LiFePO4 Precursor. 1.9 g Li2CO3, 9.0 g FeC2O4·2H2O, and 6.6 g (NH4)2HPO4 as starting materials (Li/Fe/P molar ratio of 1:2:2) were mixed well and then ball-milled (QM3SP04, Nanjing NanDa Instrument Plant, China) at 600.0 rpm for 6.0 h with ethanol as the dispersant to obtain LiFePO4 precursor. 2.5. Preparation of LiFePO4 and LFP@NC Nanocomposites. The LiFePO4 precursor and PBZ films (the mass ratios of PBZ/ LiFePO4 precursor were 0%, 10%, 15%, 20%, and 30%, respectively) were first ground roughly in a mortar and then ball-milled at 600.0 rpm for 6.0 h with ethanol as the dispersant. After that, the mixture of LiFePO4 precursor@PBZ composites were heated to 650.0 °C with a heating rate of 5.0 °C min−1 and kept at 650.0 °C for 10.0 h under Ar atmosphere in a tubular furnace. The final obtained samples were denoted as LiFePO4 and LFP@NC-X nanocomposites (LFP@NC-I, LFP@NC-II, LFP@NC-III, and LFP@NC-IV with respect to the PBZ/LiFePO4 precursor mass ratios of 10%, 15%, 20%, and 30%, respectively). 2.6. Characterizations. X-ray diffraction (XRD) measurements were carried out using a Rigaku D/max-2500 diffractometer with filtered Cu Kα radiation (λ = 1.540 56 Å). Raman measurements were performed via a DXR from Thermo Scientific with a laser wavelength of 532 nm. Thermogravimetric (TGA) analysis was carried out with a NETZSCH TG 209 C instrument from room temperature to 800 °C at a heating rate of 20 °C min−1 under air atmosphere. The electronic binding energy of the sample was analyzed by X-ray photoelectron

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology Characterizations. In order to obtain nitrogen-doped carbon coating layers, the PBZ films based on bisphenol A (carbon source) and phenylamine (nitrogen source) were synthesized. According to Scheme 1, the benzoxazine monomer was first synthesized through Mannich condensation of bisphenol A, phenylamine, and formaldehyde without the addition of any catalyst. After that, a ring opening/curing polymerization process of benzoxazine monomer took place to form polymerization of benzoxazine at 180 °C.33−35 The oxazine ring in the benzoxazine could be further transferred to dioxazines through polymerization reactions during the curing procedure. The benzoxazine monomer and PBZ films are analyzed by FTIR spectra (Figure S1 of the Supporting Information), respectively. The broad peak around 3385 cm−1 is attributed to the OH stretching mode of the phenolic hydroxyl group,33−35 and the absorption peaks of the bands at 2959 and 2854 cm−1 are attributed to the stretching vibrations of CH2 groups.34 The characteristic peaks at 1508 and 940 cm−1 correspond to the oxazine ring.33−35 The bands at 1231, 1038, and 1178 cm−1 can be assigned to asymmetric and symmetric stretching of COC and asymmetric stretching of CNC of the benzoxazine structure, respectively.31,32,35 It is noted that the disappearance of peaks at 940, 1038, and 1178 cm−1 and the decrease of the integrated intensity for the bands at 1231 and 1508 cm−1 after curing at 180 °C, indicate that highly cross-linked PBZ was 26909

DOI: 10.1021/acsami.6b10594 ACS Appl. Mater. Interfaces 2016, 8, 26908−26915

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Figure 1. Schematic illustration for the preparation of LFP@NC nanocomposites (LiFePO4 is denoted as LFP).

graphite are observed in the XRD pattern of LFP@NC nanocomposites due to its amorphous state.5 Additionally, Raman spectroscopy is conducted to verify the existence of a carbon layer in LFP@NC nanocomposites which is a technique with higher sensitivity for structural changes of disordered carbon in comparison with XRD. As shown in Figure S3, the bands at 100−500 cm−1 and 520−1120 cm−1 are ascribed to the Raman vibrations of FeO and PO43− of LiFePO4, in addition, two other prominent peaks at ∼1330 cm−1 and ∼1590 cm−1 are assigned to the D-band (disordered portions of carbon) and G-band (ordered graphitic crystallites of carbon), respectively.5,14,16,18 The peak intensity ratio (ID/IG) in Raman shifts is calculated to be an index of the degree of disordered carbon.11,15,16 As the ratio of PBZ/LiFePO4 precursor increase, the ID/IG values of LFP@NC nanocomposites are increased gradually from 0.970 to 1.021. It is worth noting that the defect introduced by PBZ-derived nitrogen-doped carbon could reduce the degree of graphitization and facilitate more electrochemical active sites for Li+ diffusion, thus resulting in the improvement of the electrochemical properties of [email protected],16 The LiFePO4 weight percentage in the LFP@NC-III composite is 88.4%, measured by means of a TGA analysis (see the details in Figure S4). To demonstrate the structure evolution of LFP@NC nanocomposites, bare LiFePO4 was synthesized for comparison. The as-synthesized LFP@NC nanocomposites exhibit a black powder appearance (Figure 3a), while the bare LiFePO4 is a light green powder (Figure S5a), indicating that PBZcarbonized nitrogen-doped carbon was successfully introduced into the LiFePO4. The bare LiFePO4 exhibits micrometer-sized bulk morphology with good crystallinity (Figure S5b,c). After being coated by nitrogen-doped carbon, the size of LiFePO4 is decreased to the nanometer scale (Figure 3b and Figure S6). The nitrogen-doped carbon networks derived from the carbonization of the PBZ films are coated on the surface of the generated LiFePO4 particles, which could impede their crystalline growth, thus providing shorter Li+ diffusion distance and enhancing the ionic diffusivity.6,9,13,14,38 The fine architecture of LFP@NC nanocomposites is further characterized by TEM. As revealed in Figure 3c and Figure S6d−f, the graphene-like carbon wrapped LiFePO4 nanoparticles are obviously observed, which can partly prevent the agglomeration of the LiFePO4 particles, provide good grain-to-grain electronic contact, reduce the resistance between the grain interfaces, and offer a favorable path for the rapid flow of electrons throughout the composite.21,22,39−41 From the HRTEM image (Figure 3d), it is also clearly seen that there is an amorphous carbon layer coated on the surface of the LiFePO4, which is consistent with the above XRD and Raman results. Thus, this PBZ-derived nitrogen-doped carbon builds a conductive network for fast electron transport throughout the electrodes, which significantly decreases the internal resistance, leading to efficient utilization of LiFePO4.42 Additionally, the d-spacing of 0.25 nm

acquired through the thermally activated ring-opening polymerization of the benzoxazine monomer.33−35 These results prove the formation of the benzoxazine monomers and corresponding PBZ. The approach for LFP@NC nanocomposites is schematically illustrated in Figure 1. First, Li2CO3, FeC2O4·2H2O, and (NH4)2HPO4 as starting materials with ethanol as the media were mixed well and then ball-milled at 600.0 rpm for 6.0 h to obtain LiFePO4 precursor. Next, using PBZ films as novel C,Ncontaining sources, LiFePO4 precursor@PBZ composites were obtained through a ball-milling process. Finally, the LFP@NC nanocomposites were achieved after calcination under inert atmosphere, during which LiFePO4 was obtained through solidstate reaction of LiFePO4 precursor as well as an electrically conducting nitrogen-doped carbon was formed from carbonization of PBZ films. Unlike other mixing or coating LiFePO4 with a conducting material, in our work, the conductive coating and LiFePO4 formation take place simultaneously which is more favorable for limiting the LiFePO4 particle growth and coating with nanoscale nitrogen-doped carbon. The fine coated carbon layer and well controlled particle size are crucial to enhance electrical conductivity and Li-ion diffusivity of LiFePO4. The XRD spectrum of the LiFePO4 precursor (Figure S2) indicates it is a mixture of LiPO3 (JCPDS card No. 26-1177) and Fe2O3 (JCPDS card No.21-0920). After subsequent high temperature solid-state reaction, the LiFePO4 precursor was transformed to olivine-structured LiFePO4. All the XRD patterns of bare LiFePO4 and LFP@NC nanocomposites show intense diffraction peaks which perfectly matches the LiFePO4 phase with orthorhombic olivine structure (JCPDS card No.40-1499) in space group Pmnb (No.62) (Figure 2 and Figure S2) without impurity phases, indicating that the introduction of PBZs do not influence on the formation of LiFePO4.5,11,36,37 No significant peaks corresponding to

Figure 2. XRD patterns of LFP@NC nanocomposites. 26910

DOI: 10.1021/acsami.6b10594 ACS Appl. Mater. Interfaces 2016, 8, 26908−26915

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190.9, 284.9, 400.4, 531.6, and 711.9 eV, corresponding to Li 1s, P 2p, P 2s, C 1s, N 1s, O 1s, and Fe 2p, respectively.25,27 Deconvolution of the Fe 2p XPS spectrum illustrates the presence of Fe 2p doublet (Fe 2p3/2 and Fe 2p1/2), which is characteristic for Fe2+ (Figure 5b).7,8,14 The N 1s spectrum of the LFP@NC-III nanocomposite can be resolved into three peaks at 398.7, 400.3, and 401.6 eV, attributed to pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen, respectively, which indicates that nitrogen is successfully doped into carbon molecular skeletons (Figure 5c).44 Additionally, the C 1s XPS spectrum of the core level peak can be resolved into three components centered at 284.7, 285.4, and 286.5 eV, assigned to sp2 C-sp2 C, N-sp2 C, and N-sp3 C bonds, respectively (Figure 5d).22,26−28,44,45 The N-sp2 C and N-sp3 C bonds further confirm the presence of nitrogen covalently bonded to the carbon framework.26−28,44,45 It is confirmed that we effortlessly achieve nitrogen-doped carbon by utilizing PBZ films as carbon source and nitrogen source, which is beneficial to enhance the electrochemical reactivity and electrical conductivity owing to the higher electronegativity of nitrogen than carbon, generating defects and withdrawing electrons from carbon atom to promote charge transfer reactions.15,26,28,44,45 3.2. Electrochemical Performances. In order to examine the effectiveness of PBZ-derived nitrogen-doped carbon coating on improving the electrochemical performance of LiFePO4, the Li+ insertion/extraction properties of LFP@NC nanocomposites as cathode material for LIBs were investigated. According to the galvanostatic charge−discharge profiles of LFP@NC nanocomposites and bare LiFePO4 at 0.1 C (one lithium per formula unit in 10 h), it can be seen that all samples have a very flat voltage plateau around 3.4 V vs Li+/Li corresponding to the typical Fe2+/Fe3+ redox reaction during Li+ extraction and insertion process (Figure 6a,b and Figure S7).6,8,11 After PBZderived nitrogen-doped carbon coating, it can also be seen that the polarization between the charge and discharge plateaus is greatly reduced from 35 mV to 17−26 mV compared to the sample without PBZ-derived nitrogen-doped carbon coating (Figure S8) at 0.1 C, which confirms that the kinetics of LiFePO4 is significantly improved after PBZ-derived nitrogendoped carbon modification.41,46 Additionally, LFP@NC-III nanocomposite exhibits minimal polarization among all LFP@NC samples, especially at higher rates, which is ascribed to the optimum content of PBZ-derived nitrogen-doped carbon modification. The specific capacities of LFP@NC-X (I, II, III, IV) nanocomposites at 0.1 C are 137.3, 142.1, 156.9, and 141.1 mA h g−1, respectively, which are much higher than those of

Figure 3. (a) The optical image, (b) SEM image, (c) TEM image, and (d) HRTEM image of LFP@NC-III nanocomposite.

corresponds to (131) plane of orthorhombic olivine-type LiFePO4, further demonstrating their pure crystalline phase. N2 adsorption−desorption isotherms are employed to investigate the possible porous structure of the LFP@NC-III nanocomposite, which exhibits type IV isotherms with a hysteresis loop (Figure 4a). The Brunauer−Emmett−Teller (BET) specific surface area of LFP@N−III nanocomposite is 47.3 m2 g−1 with a pore size concentrated at ∼4.0 nm (Figure 4b), which is much higher than that of bare LiFePO4 (2.9 m2 g−1). The relatively higher surface area of LFP@N−III nanocomposite may be ascribed to the amorphous carbon layer forming a network framework and its tiny crystal grains.38,39 The porosity in the carbon structure is from vigorous gas evolution during degradation of the PBZ polymer, which could effectively facilitate the access, accommodation of electrolyte and shorten the diffusion distance of Li+ to improve the electrochemical performance for LiFePO4.38−43 The XPS analysis is employed to further understand the surface elemental compositions and the chemical states of elements. As illustrated in Figure 5a, the XPS full survey of the LFP@NC-III nanocomposite shows seven peaks at 56.4, 133.8,

Figure 4. (a) Nitrogen adsorption/desorption isotherms of LiFePO4 and LFP@NC-III nanocomposite and (b) the BJH pore-size distributions of LFP@NC-III nanocomposite. 26911

DOI: 10.1021/acsami.6b10594 ACS Appl. Mater. Interfaces 2016, 8, 26908−26915

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Figure 5. (a) XPS full survey spectra of LFP@NC-III nanocomposite and the corresponding high-resolution XPS spectra with Gaussian fitting of (b) Fe 2p, (c) N 1s, and (d) C 1s.

Figure 6. (a) Charge−discharge profiles of LFP@NC-III nanocomposite electrode at 0.1 C for initial 10 cycles. (b) Charge−discharge profiles of LFP@NC-III nanocomposite electrode at different rates. (c) Rate performance of bare LiFePO4 and LFP@NC at different rates. (d) Cycling performance and corresponding Coulombic efficiency of the LFP@NC-III nanocomposite electrode at 2.0 C. All the electrodes were tested between 2.0 and 4.3 V (vs Li+/Li).

bare LiFePO4 (117.0 mA h g−1). In the case of the LFP@NCIII nanocomposite, it stabilizes at about 156.9 mA h g−1, which is close to the theoretical capacity (170 mA h g−1) of LiFePO4. The improved electrochemical performance of LFP@NC nanocomposites results from the PBZ-derived nitrogen-doped carbon and the favorable morphology of nanometer-sized LiFePO4 particle which can effectively enhance the electrical conductivity and provide shorter transport distance for both Li+ and e−.15,25−28,41,47 When the charge/discharge rate is increased

to 5.0 C, the LFP@NC-III nanocomposite can still maintain 75.8% of its initial discharge specific capacity, which is much higher than other LFP@NC-I (II, IV) nanocomposites and bare LiFePO4 (Figure 6c). Note that the LFP@NC-III nanocomposite could still maintain 63.7% of its initial discharge specific capacity even at very high charge/discharge rate of 10.0 C, whereas the LFP@NC-I (II, IV) and bare LiFePO4 only can maintain 45.8%, 53.8%, 47.1%, and 34.7% of their initial values, respectively. It may be concluded that less carbon content does 26912

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ACS Applied Materials & Interfaces not cover the entire LiFePO4 surface, leading to nonuniform coating or unsatisfactory surface conductivity forming an insufficient electronically conducting network.1,17 However, the high carbon content would affect the contact between the electrolyte and the electroactive materials, resulting in the restriction of easy penetration of Li+.1,13 Therefore, the LFP@ NC-III nanocomposite exhibits the best electrochemical performance among these LFP@NC nanocomposites ascribed to the optimum content of PBZ-derived nitrogen-doped carbon, which could make full usage of the active materials and balance the electronic conductivity and easy Li + penetration without appreciable polarization.1,13,17 Moreover, the LFP@NC-III nanocomposite electrode could retain a reversible discharge specific capacity of 151.3 mA h g−1 as the charge/discharge rate lowered to 0.1 C again, demonstrating that the PBZ-derived nitrogen-doped carbon in situ coating LiFePO4 particles’ architecture is tolerant to varied charge and discharge currents. Another excellent property of the LFP@NC-III nanocomposite is the superior cycling performance which is measured by a continuous charge−discharge test at 2.0 C (Figure 6d). The initial Coulombic efficiency (CE) of the LFP@NC-III nanocomposite electrode is 83.3%, and then quickly jumped to higher than 98.2% after the first cycle. The initial irreversible capacity is attributed to the decomposition of electrolyte and the corresponded formation of the solid electrolyte interphase layers.44 Additionally, a reversible discharge specific capacity of the LFP@NC-III nanocomposite could be delivered 132.0 mA h g−1 after 180 cycles, which is 93.2% of the initial value. It is worth noting that the LFP@NCIII nanocomposite shows stable cycling properties and excellent reversibility, implying that the conductive network constructed by PBZ-derived nitrogen-doped carbon can effectively provide sufficient Li+ and e− transport paths among LiFePO4 particles, as well as an elastic buffer for releasing the strain on/from LiFePO 4 during the lithium insertion/extraction process.5,15,22,25−28

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (F.C.). Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Wuhan Chenguang Science and Technology Project for Young Experts (2015070404010192), National Natural Science Foundation of China (NSFC 21303064), Specialized Research Fund for the Doctoral Program of Higher Education (20130146120013) and Fundamental Research Funds for the Central Universities of China (2662015PY163, 2662015QC046).

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REFERENCES

(1) Wang, J.; Sun, X. Understanding and Recent Development of Carbon Coating on LiFePO4 Cathode Materials for Lithium-Ion Batteries. Energy Environ. Sci. 2012, 5, 5163−5185. (2) Cao, F. F.; Guo, Y. G.; Wan, L. J. Better Lithium-Ion Batteries with Nanocable-Like Electrode Materials. Energy Environ. Sci. 2011, 4, 1634−1642. (3) Zhou, X. S.; Guo, Y. G. A PEO-Assisted Electrospun SiliconGraphene Composite as an Anode Material for Lithium-Ion Batteries. J. Mater. Chem. A 2013, 1, 9019−9023. (4) Hu, Y. S.; Guo, Y. G.; Dominko, R.; Gaberscek, M.; Jamnik, J.; Maier, J. Improved Electrode Performance of Porous LiFePO4 Using RuO2 as an Oxidic Nanoscale Interconnect. Adv. Mater. 2007, 19, 1963−1966. (5) Wu, X. L.; Jiang, L. Y.; Cao, F. F.; Guo, Y. G.; Wan, L. J. LiFePO4 Nanoparticles Embedded in a Nanoporous Carbon Matrix: Superior Cathode Material for Electrochemical Energy-Storage Devices. Adv. Mater. 2009, 21, 2710−2714. (6) Bazzi, K.; Nazri, M.; Naik, V. M.; Garg, V. K.; Oliveira, A. C.; Vaishnava, P. P.; Nazri, G. A.; Naik, R. Enhancement of Electrochemical Behavior of Nanostructured LiFePO4/Carbon Cathode Material with Excess Li. J. Power Sources 2016, 306, 17−23. (7) Jiang, J.; Liu, W.; Chen, J. T.; Hou, Y. L. LiFePO4 Nanocrystals: Liquid-Phase Reduction Synthesis and Their Electrochemical Performance. ACS Appl. Mater. Interfaces 2012, 4, 3062−3068. (8) Shu, H. B.; Wang, X. Y.; Wen, W. C.; Liang, Q. Q.; Yang, X. K.; Wei, Q. L.; Hu, B. A.; Liu, L.; Liu, X.; Song, Y. F.; Zho, M.; Bai, Y. S.; Jiang, L. L.; Chen, M. F.; Yang, S. Y.; Tan, J. L.; Liao, Y. Q.; Jiang, H. M. Effective Enhancement of Electrochemical Properties for LiFePO4/ C Cathode Materials by Na and Ti Co-doping. Electrochim. Acta 2013, 89, 479−487. (9) Toprakci, O.; Toprakci, H. A.; Ji, L. W.; Xu, G. J.; Lin, Z.; Zhang, X. W. Carbon Nanotube-Loaded Electrospun LiFePO4/Carbon Composite Nanofibers as Stable and Binder-Free Cathodes for Rechargeable Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 1273−1280. (10) Ilango, P. R.; Gnanamuthu, R.; Jo, Y. N.; Lee, C. W. Design and Electrochemical Investigation of a Novel Graphene Oxide-Silver Joint Conductive Agent on LiFePO4 Cathodes in Rechargeable Lithium-Ion Batteries. J. Ind. Eng. Chem. 2016, 36, 121−124.

4. CONCLUSIONS In summary, using PBZ as a novel nitrogen and carbon source, we have developed a facile ball-milling method combined with a high-temperature carbonization technique to fabricate the LFP@NC nanocomposites. In this hybrid nanostructure, the optimum PBZ-derived nitrogen-doped carbon matrix can serve as an electrolyte container for high-rate operation as well as an elastic buffer to relieve the strain during Li+ uptake/release. As a result, when used as a cathode material for LIBs, the LFP@NCIII nanocomposite exhibits higher specific capacity and excellent rate performance in comparison with bare LiFePO4. In view of its facility in terms of synthesis, PBZ can also be used to coat other cathode or anode electrode materials for LIBs, such as LiCoO2, LiNiO2, LiMn2O4, TiO2, Li4Ti5O12, and Si to obtain improved electrochemical performance, which may open a new prospect of using novel nitrogen sources and carbon sources for LIBs.

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SEM images of LiFePO4 precursor@PBZ, LiFePO4, and LFP@NC nanocomposites; and the electrochemical results of LiFePO4 and LFP@NC composites (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10594. FTIR spectra of polybenzoxazine and benzoxazine monomer; XRD patterns, Raman spectra, TEM images, 26913

DOI: 10.1021/acsami.6b10594 ACS Appl. Mater. Interfaces 2016, 8, 26908−26915

Research Article

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DOI: 10.1021/acsami.6b10594 ACS Appl. Mater. Interfaces 2016, 8, 26908−26915

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DOI: 10.1021/acsami.6b10594 ACS Appl. Mater. Interfaces 2016, 8, 26908−26915