Carbon Nanospheres@Graphene Nanoribbons

Apr 25, 2018 - Prepared by the Biomineralization Process as the Cathode for ... N-doped graphene nanoribbons (GNRs) via biomineralization by using...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

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LiMn0.8Fe0.2PO4/Carbon Nanospheres@Graphene Nanoribbons Prepared by the Biomineralization Process as the Cathode for Lithium-Ion Batteries Yu-Kun Hou,† Gui-Ling Pan,*,‡ Yan-Yun Sun,† and Xue-Ping Gao*,† †

Institute of New Energy Material Chemistry, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China Key Laboratory of Functional Polymer Materials of the Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071, China

‡

S Supporting Information *

ABSTRACT: Biomineralization technology is a feasible and promising route to fabricate phosphate cathode materials with hierarchical nanostructure for high-performance lithium-ion batteries (LIBs). In this work, to improve the electrochemical performance of LiMn0.8Fe0.2PO4 (LMFP), hierarchical LMFP/carbon nanospheres are wrapped in situ with N-doped graphene nanoribbons (GNRs) via biomineralization by using yeast cells as the nucleating agent, self-assembly template, and carbon source. Such LMFP nanospheres are assembled by more fine nanocrystals with an average size of 18.3 nm. Moreover, the preferential crystal orientation along the [010] direction and certain antisite lattice defects can be identified in LMFP nanocrystals, which promote rapid diffusion of Li ions and generate more active sites for the electrochemical reaction. Moreover, such N-doped GNR networks, wrapped between LMFP/carbon nanospheres, are beneficial to the fast mobility of electrons and good penetration of the electrolyte. As expected, the as-prepared LMFP/carbon multicomposite presents the outstanding electrochemical performance, including the large initial discharge capacity of 168.8 mA h g−1, good rate capability, and excellent long-term cycling stability over 2000 cycles. Therefore, the biomineralization method is demonstrated here to be effective to manipulate the microstructure of multicomponent phosphate cathode materials based on the requirement of capacity, rate capability, and cycle stability for LIBs. KEYWORDS: lithium-ion battery, cathode, phosphate, nanostructure, biomineralization

1. INTRODUCTION

Recently, to promote the diffusion of Li ions and transport of electrons in the LMFP material, the combination of both reducing the size of active particles to nanoscale and coating active particles with conductive carbonaceous materials is demonstrated to be more effective. Therefore, to enhance the electrochemical performance of the LMFP material, the controllable fabrication of nanosized LMFP particles with efficient carbon coating is the key for enforcing the fast transfer of electrons on the surface and diffusion of Li ions in the bulk. Biology is instructive for discovering a new technique for the manipulative preparation of well-organized nanomaterials.11,12 Typically, biomineralization is a biological process to produce minerals with living organisms, attracting considerable biomimic interests for materials science.13−17 Many organized inorganic and composite materials with an amorphous or a crystalline structure and hierarchical morphology are prepared via the biomineralization process in the biosystem.18−21

Lithium-ion batteries (LIBs) play a key role in energy storage and conversion devices because of the advantages of both high energy density and power density.1−5 In particular, on the basis of the safety requirement, the LiFePO4 (LFP) cathode material is successfully used in LIBs for electric vehicles and hybrid electric vehicles. As an alternative cathode material, LiMnPO4 (LMP) can provide the same specific capacity but higher operating potential (4.1 V, Mn2+/Mn3+ couple) in comparison to Fe2+/Fe3+ couple (3.4 V), thus leading to about 20% higher theoretical energy density of the cathode (701 W h kg−1) than that of LFP (586 W h kg−1).6−8 However, the inherent ionic and electronic conductivity of LMP is much poor, making it challenging to achieve the large specific capacity, high rate capability, and long-term cycle stability of the cathode for LIBs.8−10 To satisfy the electrochemical performance, the LiMn0.8Fe0.2PO4 (LMFP) material, with the relatively high working potential and suitable ionic/electronic conductivity, is considered as the most promising cathode candidate for the next-generation LIBs.7−10 © 2018 American Chemical Society

Received: February 13, 2018 Accepted: April 25, 2018 Published: April 25, 2018 16500

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

Research Article

ACS Applied Materials & Interfaces

to form the N-doped GNR networks in the pyrolyzation process. As expected, the as-prepared biogenic LMFP/C cathode material exhibits remarkable electrochemical performance.

Compared with the traditional chemical process, the biomineralization process is much more competent at the molecular control of the structure, size, shape, chemical constitution, and crystallographic orientation of inorganic materials with the highest possible accuracy, which also yields the advanced synthetic materials in an environmentally mild system.21,22 In the field of energy storage and batteries, varieties of cathode and anode materials have been developed based on biomineralization principles, and the organic compounds are also used as the carbon precursor.23−26 Park et al.23 have demonstrated a facile synthesis of nanostructured transitionmetal phosphate via biomineralization of peptide nanofibers. After heat treatment, the as-prepared FePO4-mineralized peptide nanofibers are transformed to FePO4 nanotubes with inner walls coated by a thin layer of conductive carbon by carbonization of the peptide core. Zhang et al.24 have developed a simple and inexpensive biomimetic sol−gel method to prepare high-performance mesoporous LFP based on the biomineralization assembly of Saccharomyces cerevisiae. The as-prepared mesoporous LFP is composed of densely aggregated LFP nanoparticles as well as hierarchical mesoporous biocarbon networks coating the LFP surface. Meng and Deng25 have employed waste eggshell as a multifunctional reaction system to regulate the reactants and pH value inside the reactor and used eggshell membrane as protein-based and active substrates to prepare one-dimensional (1D) Co(OH)2 nanorod arrays on protein fibers. Such biomimetic materials with a hierarchical structure through biomineralization show remarkable electrochemical performances as the cathode or anode for LIBs. Particularly, yeast cells are common unicellular microorganisms with a spherical shape, which are generally used for the bioremediation of heavy-metal contaminated industrial sewage. Here, yeast cells are capable of accumulating metal cations from aqueous solution by a series of metabolism-based physicochemical processes (such as biosorption to cell walls, phagocytosis inside the cells, entrapment into the cellular capsules, elementary absorption−assimilations, and enzymatic oxidation−reduction reactions).27 Inspired by this, the yeast cells are then chosen as bioreactors for the biomineralization process because of the abundant surface charge, large specific surface area, and ability of heavy metal adsorption within a broad range of pH value and ionic strength.28,29 Moreover, the biomass-derived advanced carbon materials, with an amorphous or a crystalline structure, hierarchical morphology, and elemental doping, are superior in electrical conductivity.30−34 In addition, graphene is the ideal substrate for anchoring and wrapping inorganic active materials for energy conversion and storage.35 N-doping can effectively adjust the electronic structure of graphene, resulting in high electron mobility.36 If both carbon coating and N-doping in graphene via the biomineralization process are introduced into LMFP to fabricate nanosized composite structures, it would be rational for realizing the optimized electrochemical performance of the LMFP cathode. In this work, we present a facile way to fabricate hierarchical LMFP/C-composite nanospheres wrapped by N-doped graphene nanoribbon (GNR) networks, by using yeast cells as the nucleating agent, self-assembly template, and advanced carbon source. During high-temperature pyrolyzation, yeast cells are transformed into active biocarbons, which are coated in situ on LMFP nanocrystals simultaneously. Meanwhile, the celluloses of cell walls and the cellular biomacromolecules are converted

2. EXPERIMENTAL SECTION 2.1. Preparation of the Biogenic-LMFP/C and Blank-LMFP/C Samples. All chemicals are of analytical grade and used without further purification. The biogenic-LMFP/C sample was prepared using the biotemplate and carbon thermal reduction route as described below. For cultivation of yeast bacteria, the yeast powder (2.5 g) was added into the glucose aqueous solution (5 g, 100 mL). After culturing under a constant temperature of 38 °C for 40 min, a homogeneous biosuspension was formed. Then, the active yeast cells were purified by centrifuging and washing with deionized water. The MnCl2 solution (0.8 mol L−1, 20 mL) and FeCl3 solution (0.2 mol L−1, 20 mL) were gradually added into the purified yeast solution (100 mL) and mechanically stirred for 4 h at room temperature for the deep biosorption process. Then, (NH4)2HPO4 solution (0.5 mol L−1, 40 mL) was added into the Mn2+/Fe3+/yeast mixture solution, with continuous stirring for 30 min. Then, pH was adjusted to 5 by CH3COONa, and the above mixture solution was kept overnight for the deeper biomineralization process. After centrifuging and washing with deionized water, the phosphates/yeast bioprecursor was fully dried in a vacuum freeze-drying machine. Then, the phosphate bioprecursor was mixed with Li2CO3 in the stoichiometric molar ratio of 1.03:1 (Li/P) and sintered under protection of Ar atmosphere at 350 °C for 3 h and then at 650 °C for 12 h. The heating rates were kept relatively low at 4 °C min−1 to avoid structural collapse caused by fast crystallization. The blank-LMFP/C sample was prepared without yeasts but operated in the same synthetic procedure referring to the biogenic-LMFP/C sample; additionally, glucose (2.5 g) was added as the carbon source for the sintering process to avoid the Mn2+ and Fe2+ oxidation. 2.2. Materials Characterization. Transmission electron microscopy (TEM) images were recorded on an FEI Tecnai G2F 20 microscope at an accelerating voltage of 200 kV. Morphological profiles were observed using a JSM-7600F scanning electron microscope at an accelerating voltage of 10 kV. X-ray diffraction (XRD) patterns were recorded on a Rigaku Smart Lab 3 kW diffractometer with Cu Kα radiation (λ = 1.5418 Å), with the corresponding operation voltage and current at 40 kV and 100 mA, respectively. X-ray photoelectron spectroscopy (XPS) was collected on a scanning X-ray microprobe (PHI 5000 Verasa, S4 ULAC-PHI, Inc.) using Al Kα radiation and the C 1s peak at 284.8 eV as the internal standard. Raman spectra of powder samples were obtained on LabRAM HR800 with a laser excitation wavelength of 532 nm. Fourier transform infrared (FTIR) spectroscopy was collected on a Nexus 670 spectrometer by using a KBr wafer technique. Nitrogen adsorption− desorption isotherms were measured at 77 K on Quantachrome Instruments Autosorb AS-6B. The pore size distributions were measured by the Barrett−Joyner−Halenda method. 2.3. Electrochemical Measurements. In a typical electrode preparation, active materials, Super P, and poly(vinylidene difluoride) were mixed with a mass ratio of 8:1:1 in N-methyl-2-pyrrolidone to form a slurry. Then, the slurry was pressed to a piece on an Al foil with about 0.02 mm in thickness and dried in a vacuum oven at 100 °C for 12 h. Finally, the film was cut into circular strips of 12 mm in diameter. As a result, the area of the as-prepared cathodes was 1.13 cm2, and the mass of the cathodes was precisely controlled within 1.20−1.25 mg. The coin-type cells (2032) were assembled in a high-purity argon-filled glovebox with the lithium metal serving as counter and reference electrodes and Celgard 2300 as a separator. The electrolyte comprised a solution made by 1 M LiPF6 in dimethyl carbonate/ethyl methyl carbonate/ethylene carbonate mixture (1:1:1 v/v/v). The charge/ discharge tests were carried out between 2.0 and 4.5 V (vs Li/Li+) at various rates (1 C = 170 mA g−1) with LAND-CT2001A battery testers. CV measurements were performed on a CHI600C electrochemical workstation at various scan rates from 0.1 to 1 mV/s. 16501

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

Research Article

ACS Applied Materials & Interfaces

Figure 1. Scheme of the bioinspired design and preparation. (a−c) Scheme of the adsorption, deposition, and self-assembly of the transition-metal phosphate as the bioprecursor in yeast cells via biomineralization. (d) Schematic structure of the biogenic-LMFP/C sample derived from biofabrication.

Figure 2. (a) XRD pattern of the biogenic-LMFP/C sample. (b) Comparison of XRD patterns of the biogenic-LMFP/C and blank-LMFP/C samples. The inset is the model of the crystal structure of LMFP; the lithium diffusion channels are along the [010] direction. Electrochemical impedance spectra (EIS) measurements were conducted on a Zahner IM6ex electrochemical workstation by sweeping the frequency from 100 kHz to 0.01 Hz with an ac amplitude of 5 mV. All the above electrochemical measurements were carried out at room temperature.

with the hydrophilic anion groups of biomolecules on the cell surface and inside the cells by electrostatic interactions via the metabolism processes (Figure 1a). The Mn-responsive translocators SMF1 and SMF2 and Fe-responsive translocators AFT1 and AFT2 on intracellular membranes play an important role in the regulation of Mn2+ and Fe3+ intracellular homeostasis under the control of protein metastasis and gene expression.28,29 The function of translocators in synthesizing inorganic precursors and the abundant anion groups on the surface of the cell is highly important,29 which is also worked in the preparation of mesoporous LFP based on the biomineralization assembly.24 When (NH4)2HPO4 solution is added into the Mn2+/Fe3+/yeasts mixed solution, the PO43− anion groups are electrically adsorbed to Mn2+/Fe3+ sites and further form ferromanganese−phosphate biominerals in yeast cells (Figure 1b). During biomineralization, the metabolic energy of ATP drives the nucleation process via a series of enzymatic reactions, and the cellular biomacromolecules provide nucleation sites and immobilize the ferromanganese−phosphate nanoparticles in the cells (Figure 1c). The produced biominerals are composite materials, which are consisted of the inorganic component and specific organic biomatrix. The organic

3. RESULTS AND DISCUSSION In the biomineralization process, it is generally believed that the nucleation and growth of inorganic materials are mostly controlled by genes (DNA) and biomolecules in the biomineralization process.29 Figure 1 illustrates the adsorption, deposition, and self-assembly mechanism of the transitionmetal phosphate in yeast cells (Figure 1a−c) and the whole preparation procedure of the biogenic LMFP/C (marked as biogenic-LMFP/C) sample. During yeast propagation, the metabolic biomacromolecules of extracellular proteins and polysaccharose with abundant hydrophilic anion groups (such as −COO−, −OH−, −CONH2−, and −OPO3−) and negative charges are produced on the cell surface, which are beneficial for cation absorption/interaction.29 When the mixed solution of MnCl2 and FeCl3 is added into the yeast culture suspension, the positively charged cations (Mn2+ and Fe3+) are combined 16502

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

Research Article

ACS Applied Materials & Interfaces

Figure 3. TEM and HRTEM images of the biogenic-LMFP/C sample. (a,b) TEM images, showing the hierarchical structure of the biocarbon network around LMFP/C-composite spheres. (c) HRTEM image of the biocarbon network on the particle surface, showing the GNR structure of the biocarbon. (d) HRTEM image of the LMFP/C-composite nanosphere, showing the regular (200) lattice fringe of LMFP and uniform carboncoating surface. (e,f) HRTEM images in different regions of the LMFP/C-composite sphere, showing the nanocrystal nucleus and various antisite lattice defects of LMFP.

on the organic biosurface.22 Here, protein cages act as faster growing hosts for the guest minerals in the biomineralization process,20−22 which offers the opportunity for the formation of LMFP nucleuses with smaller primary crystallite size in the biocarbon cages. Meanwhile, the cellulate membranes, with charged head groups and intercalated proteins with a periodic secondary structure of antiparallel β-pleated sheets,28 are in favor of the oriented nucleation of LMFP along the specific [010] crystallographic direction. Such an oriented nucleation mechanism along the [010] direction in the olivine structure induced by protein biomolecules plays an important role in promoting Li-ion diffusion and the rate capability. Figure 3 shows TEM images of the biogenic-LMFP/C sample. In Figure 3a,b, LMFP particles appear as a spherical shape with the diameter of 100−300 nm on the surface of biocarbon networks derived from pyrolyzation of the cells. Such biocarbon networks are consisted of nanoribbons with the legible lattice fringes of multilayer graphene as shown in the high-resolution TEM (HRTEM) image (Figure 3c). The thickness of the GNRs is about 4.4 nm, which is further confirmed by atomic force microscopy (AFM) observation (Figure S3). In the meantime, the LMFP nanocrystals are coated by a uniform carbon layer with the thickness of about 4−5 nm to fabricate the LMFP/C spheres (Figure 3d). It is also noted that such LMFP spherical particles with the diameter of 100−300 nm are assembled by LMFP fine nanocrystals with the size of 10−30 nm by comparison with (200), (101), and (301) planes (Figure 3e), almost identical to the average crystallite size calculated from XRD analysis. In particular, the antisite lattice defects of LMFP can be observed in the HRTEM image (Figure 3f). The antisite defects with a low barrier in olivine phosphates is easily formed during the biosynthesis process, allowing for a facile cross-channel diffusion of Li ions.40−42 Therefore, some information can be refined from the TEM observation mentioned above. First, carbon exists as networks to support LMFP spherical particles and as the thin layer to coat the surface of LMFP nanocrystals. The total carbon content of the biogenic-LMFP/C sample is 10.85 wt %. Such a three-dimensional (3D) conductive architectural composite structure contributes to the fast transport of

biomatrix has a great impact on the morphology and structure of the inorganic materials formed.37 The detailed interaction between biomolecules and Mn2+/Fe3+ cations and the in situ linkage between the yeast cells and biomineralized phosphates are demonstrated by FTIR spectroscopy measurement (Figure S1). As a result, the produced inorganic bioprecursor is detected to be a mixture of NH4MnPO4 and FePO4 by XRD (Figure S2). Finally, the biogenic-LMFP/C sample with the olivine structure is prepared by mixing the phosphate bioprecursor with Li2CO3 and by calcining in an inert atmosphere. To verify the superiority of yeast cells in the biomineralization process to fabricate the biogenic-LMFP/C composite, a blank sample (marked as blank-LMFP/C) is also prepared without introducing yeast in the parallel preparation condition. XRD patterns of the biogenic-LMFP/C and blank-LMFP/C samples are shown in Figure 2. Clearly, the diffraction peaks of all the samples are matched well with the olivine structure of LMFP (JCPDS no. 13-0336), and the diffraction peaks of biocarbons are not detected. However, a slight amount of Li3PO4 with weak diffraction peaks is also detected in the sample, which is beneficial to improve the electrochemical performance of the cathode as the Li-ion conductor.7,8 Meanwhile, the noticeable difference is that the diffraction peaks of (200), (101), and (301) planes in the biogenic-LMFP/ C sample are stronger because of the preferential crystalline orientation along the [010] direction, as compared with that in the blank-LMFP/C sample. It should be noted that the Li-ion conductivity is determined from the 1D diffusion pathway of the [010] direction in the LMFP lattice.8−10,38,39 The preferential crystalline orientation along the [010] direction is beneficial to the fast diffusion of Li ions in the biogenic-LMFP/ C sample. The average crystallite size of LMFP in the biogenicLMFP/C sample is calculated to be 18.3 nm (Table S1), implying that nanocrystalline LMFP materials are prepared via biomineralization because of the confinement effect of the biomolecules. Typically, the nucleation of nanocrystals with specific crystal face occurs usually at the inorganic−biomatrix interface, based on the consideration of geometric matching between lattice spacing and distance among ion-bonding sites 16503

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

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ACS Applied Materials & Interfaces

Figure 4. EDS and SEM characterization of the biogenic-LMFP/C sample. (a) Scanning TEM (STEM) image and (b−g) elemental mapping of O, P, Mn, Fe, C, and N. (h) EDS spectra on the selected region (a). (i) SEM image, showing the good electrical connection between active particles by the linkage of high-conductive GNR networks.

Figure 5. XPS spectra: O 1s core level and P 2p core level of the biogenic-LMFP/C (a,b) and blank-LMFP/C (c,d) samples.

electrons and penetration of the electrolyte among the active particles, leading to the high-efficiency electrochemical reaction.24,39 Second, LMFP spherical particles are aggregated by fine nanocrystals with the preferential crystalline orientation along the [010] direction, as well as the antisite lattice defects in the bulk of LMFP. It should be noticeable that the unique microstructure with preferential crystalline orientation, lattice defects, and more grain boundary is in favor of providing both

more effective diffusion channel for Li ions and the more active sites for the electrochemical reaction. Furthermore, the elemental mapping of the biogenic-LMFP/ C sample using X-ray energy-dispersive spectrometry (EDS) under high-angle annular dark-field (HAADF) mode is presented in Figure 4. Here, O, P, Mn, and Fe elements are evenly distributed in the active particle, and C and N elements are well-distributed at the particle surface. The mapping signals 16504

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a) Raman spectra of the biogenic-LMFP/C and blank-LMFP/C samples. XPS spectra of C 1s (b) and N 1s (c) of the biogenic-LMFP/C sample. The C 1s peak is split into three Lorentzian peaks at 284.7, 285.9, and 288.6 eV. The N 1s peak can be divided into three Lorentzian peaks at 397.0, 399.6, and 402.5 eV. (d) Schematic illustration of the graphitic-N, pyridinic-N, and pyrrolic-N doped in the graphitic carbon lattice.

graphene walls could be formed through continuous sequential steps, such as the formation, transformation, and carbonization from glucose-based polymers, as reported previously.32 Therefore, it seems that the formation of graphene-based carbon from the biomass could be feasible in biomineralization and subsequent carbonization processes. Figure 5 shows the XPS spectra (O 1s and P 2p core levels) of the biogenic-LMFP/C sample and blank-LMFP/C sample. There are three different chemical states for oxygen as shown in the O 1s core level of the biogenic-LMFP/C sample. The O1 species at 529.3 eV can be attributed to lattice oxygen atoms (O−M−O, M = Mn, Fe), whereas the O2 species at 531.5 eV is closely related to the oxygen vacancy.18,43,44 The O3 species at 530.2 eV is ascribed to the OP chemical state. The core level is fitted to the P 2p doublet (2p1/2 and 2p3/2), which is divided by ∼0.5 eV with the integrated intensity ratio of 1.2:1, showing two different chemical states of phosphorus. Contrastively, it is found that the atomic percentage (at.%) of the O2 species and P 2p3/2 species from the biogenic-LMFP/C sample is higher than that of blank-LMFP/C (Figure 5c,d), further confirming the existence of antisite lattice defects and oxygen vacancy in the fine LMFP crystal structure of the biogenic-LMFP/C sample.40−44 Raman spectra are the most direct and sensitive technique to reveal the form and quality of carbon materials.45 As shown in Figure 6, the peaks at 1350 and 1580 cm−1 in Raman spectra are assigned to the D band and G band, respectively.46 The G band is caused by the high-frequency E2g vibration mode of sp2 carbon domains, which is related to the graphitization of carbon atoms. Whereas the D band is associated with the disordered structure and structure defects of sp2 carbon domains.47 The intensity of the G band in the biogenic-LMFP/C sample is stronger than that of the D band, indicating the good graphitization in the biocarbon,46 which is mainly attributed

of Fe (Figure 4e) are weaker than that of Mn (Figure 4d), which provides a rational response to the stoichiometric ratio of Fe/Mn in LMFP. Besides, both Mn and Fe show a relatively homogeneous distribution among the active particle, implying that Fe is successfully introduced into certain Mn sites in LMFP. The corresponding EDS spectra (Figure 4h) also reveal that the atomic ratio of Mn, Fe, P, and O in the biogenicLMFP/C sample is very close to the stoichiometric ratio of the target materials. Particularly, the molar ratio of Mn/Fe in the biogenic-LMFP/C sample is tested to be 8.06:1.98 by X-ray fluorescence (XRF) spectroscopy. The scanning electron microscopy (SEM) image of the biogenic-LMFP/C sample is given in Figure 4i, where the composite networks can be clearly observed. It is noticeable that the biogenic-LMFP/C sample shows a loose and porous presentation with well-defined GNR networks adhering on the surface of active spherical particles, where GNRs are in situ formed and intimately linked together among active spherical particles during pyrolyzation of the yeast cells. Accordingly, the electrical conductivities of the biogenicLMFP/C sample and blank-LMFP/C sample are 1.13 × 10−2 S cm−1 and 7.62 × 10−4 S cm−1, respectively. The improved electrical conductivity of the biogenic-LMFP/C sample is mainly attributed to the good connection between active particles by the linkage of high-conductive GNR networks. The GNRs are converted from the long-chain crystalline structured celluloses on cell walls during the pyrolyzation process, similar to the formation mechanism of graphene from the biomass in the pyrolysis process of the polysaccharides (such as chitosan, cellulose, peptidoglycan, and starch), as reported previously.31−34 For instance, N-doped graphene with few layers can be prepared by the pyrolysis of chitosan films under moderate temperatures (600−800 °C) in inert atmosphere and without acid or catalyst assistance as the prerequisite.31 In addition, 3D macroporous graphene-based carbon with thin 16505

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

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ACS Applied Materials & Interfaces

Figure 7. Electrochemical characterization of the biogenic-LMFP/C and blank-LMFP/C samples. (a) CVs of the biogenic-LMFP/C and blankLMFP/C cathodes (0.3 mV s−1). (b) Initial charge/discharge curves of the biogenic-LMFP/C and blank-LMFP/C cathodes at the C/20 rate between 2.0 and 4.5 V. (c) Rate capability of the biogenic-LMFP/C and blank-LMFP/C cathodes at various rates (C/20, C/10, 1, 2, 5, and 10 C). (d) Cycle performance of the biogenic-LMFP/C and blank-LMFP/C cathodes at the 1C rate. (e) Long cycle performance of the biogenic-LMFP/C cathode at the 2C rate. The inset (f) in (e) shows charge/discharge curves of different cycles from the 1st to the 2000th. Note: the carbon content in all the samples is eliminated when the capacity is calculated.

the biogenic-LMFP/C sample. The main peak appears at 284.7 eV with the graphitic sp2 carbon (CC), suggesting that most of the carbon atoms in the biogenic-LMFP/C sample are arranged in a conjugated honeycomb lattice.48 The two weak peaks at 285.9 and 288.6 eV can be recognized, which are assigned to the N-sp2 C (C−N) and N-sp3 C (CN) bonds, respectively.47 In the case of the N 1s core level (Figure 6c), the pyridinic N (397.0 eV), pyrrolic N (399.6 eV), and graphitic N (402.5 eV) can be assigned, which are typical bonding characteristics in the N-doping graphene.49,50,52,53 It means that nitrogen is doped in GNR networks of the biogenicLMFP/C sample via the in situ pyrolyzation from the N-rich biomolecules. To get an insight into the electrochemical performance of the biogenic-LMFP/C and blank-LMFP/C cathodes, cyclic voltammograms (CVs) at the scan rate of 0.3 mV s−1 with the potential range of 2.0−4.5 V (vs Li/Li+) are presented in Figure 7a. The well-defined reversible redox couple peaks located around 3.6 and 4.1 V are ascribed to the Fe2+/Fe3+ and Mn2+/

to the abundant GNRs as shown in Figures 3c and 4i. In addition, the 2D band in Raman spectra is the valuable structure characteristics of graphene materials, which are sensitive to the number of layers of graphene.48 The 2D band of the biogenic-LMFP/C sample appears nearby 2800 cm−1 as the upshifted and broader peak, in line with the structure characteristics of the multilayer graphene47,49,50 and in consistent with the observation of multilayered lattice fringes of the biocarbon in the HRTEM image (Figure 3c). Meanwhile, the sharp Raman peak at 958 cm−1 can be detected for the blank-LMFP/C sample, which is assigned to the stretching vibration mode of PO43− anions.51 Whereas the weak Raman signal of PO43− anions appears for the biogenic-LMFP/C sample because of the interference of the biocarbon on the surface layer and matrix. The XPS spectrum is also presented to probe the nitrogen doping on the graphene structure in the biogenic-LMFP/C sample (Figure 6b). Both well-defined C 1s peak and N 1s peak appear. There are three components in the C 1s core level of 16506

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

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ACS Applied Materials & Interfaces

Figure 8. TEM images (a,b) of the biogenic-LMFP/C sample after 2000 cycles at the 2 C rate. STEM image (c) recorded by the HAADF detector and (d−i) EDS elemental mapping of O, P, Mn, Fe, C, and N on the selected region. (j) HRTEM image of the LMFP nanocrystal of the biogenicLMFP/C sample after 2000 cycles.

Figure 9. (a) Nyquist plots of the biogenic-LMFP/C and blank-LMFP/C cathodes before cycling. (b) Nyquist plots of the biogenic-LMFP/C and blank-LMFP/C cathodes after cycling at 1C. (c) CVs of the biogenic-LMFP/C cathode at various scan rates of 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mV s−1. (d) Relationship between peak current and the square root of the scan rate for the corresponding redox peaks in (c).

168.8 mA h g−1 at C/20 rate, very close to the theoretical capacity (170 mA h g−1) of the LMFP materials.7−10,35,51 Furthermore, the biogenic-LMFP/C cathode exhibits the excellent rate performance with reversible capacities of 162.1, 144.5, 130.2, 118.6, and 109.4 mA h g−1 at various rates of C/ 10, 1, 2, 5, and 10 C, respectively (Figures 7c and S7). Even after 70 cycles at various rates, the discharge capacity is still recovered to 167.6 mA h g−1 at the C/20 rate with the capacity

Mn3+ redox couples, accompanied with the Li extraction/ insertion in LMFP. In comparison with the blank-LMFP/C sample, the biogenic-LMFP/C sample delivers larger current density of redox peaks in CVs, suggesting a good electrochemical activity. The superiority is further proved by the larger discharge capacity of the biogenic-LMFP/C sample in the initial charge/discharge curves (Figure 7b). Specifically, the initial discharge capacity of the biogenic-LMFP/C sample is 16507

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

Research Article

ACS Applied Materials & Interfaces

ion deintercalation (anodic process, A and C) and intercalation (cathodic process, B and D) peaks are well-defined at the different scan rates, illustrating good reversibility of the biogenic-LMFP/C cathode. Especially, the good linear relation of the peak current with the square root of the scan rate is obtained for all the cathodic and anodic processes (Figure 9d), suggesting that the rate determined step is dominant by the diffusion process based on the Randles−Sevcik equation.58,59 The electrochemical performance is more sensitive to the particle morphology, surface modification, and microstructure of phosphate cathode materials. In the biogenic-LMFP/C sample, the dense and spherical particle morphology is good for obtaining the high tap density (1.57 g cm−3) and large volumetric capacity.10,58 The combination of the surface modification by biocarbons on LMFP nanocrystals with the average size of 18.3 nm and the wrapping on the N-doped GNR networks facilitate the formation of a hierarchical LMFP/ C composite structure. Such a 3D structure is unique for the fast transport of electrons and good penetration of the electrolyte. Furthermore, inside LMFP nanocrystals, the preferential crystalline orientation and lattice defects can be observed clearly, which are demonstrated to be favorable for promoting the diffusion of Li ions and for providing the active sites for the electrochemical reaction. Therefore, to achieve the superior electrochemical performance, a rational structure design of phosphate cathode materials can be realized via the biomineralization process.

retention of 99.3%, enduring the abuse use of various rates to retain high stability upon cycling. However, the discharge capacity is lower for the blank-LMFP/C cathode at various rates (Figure 7c), and the capacity retention is 93.5% after 70 cycles at various rates. Long-term cycle stability is more critical for phosphate cathode materials.54,55 The cycle performance of the biogenic-LMFP/C and blank-LMFP/C cathodes at the 1C rate is shown in Figure 7d. Obviously, no distinct decay of the discharge capacity can be seen for the biogenic-LMFP/C cathode even after 600 cycles at the 1 C rate, whereas the discharge capacity is lower at the 1 C rate for the blank-LMFP/ C cathode and the capacity retention is about 86.2% after 400 cycles. When cycled at the 2 C rate (Figure 7e), the biogenicLMFP/C cathode presents a relatively high initial discharge capacity of 129.1 mA h g−1 and a low capacity decay of 10.5% after 2000 cycles. It should be noticeable that no distinct polarization can be observed for the biogenic-LMFP/C cathode within 1000 cycles as shown in charge/discharge curves in the different cycles, while the slight polarization occurs from 1400th cycle to 2000th cycle. The enhancement in discharge capacity and cycle stability can be ascribed to the unique 3D architectural structure of LMFP/C nanospheres wrapped by Ndoped GNR networks, as well as more active sites for the electrochemical reaction in LMFP nanocrystals with certain lattice defects. Although after 2000 cycles (Figure 8), the GNR networks on the surface of LMFP/C spherical nanoparticles are still observed, implying a stable covalent interaction between GNRs and the biocarbon layer coated on particles. The O, P, Mn, and Fe elements are well-distributed among spherical active particles, and C and N elements are crossed over GNRs. Furthermore, LMFP nanocrystals maintain a fine structure (Figure 8j), suggesting the good structure stability of the biogenic-LMFP/C cathode during the long-term cycling. To get a further understanding with the reaction mechanism, EIS and CVs are measured with a focus on the surface charge transfer and diffusion of Li ions. As shown in Figure 9a,b, all the Nyquist plots are consisted of a depressed semicircle in the high-frequency region and a slope line in the low-frequency region for the corresponding cathodes before and after cycling, indicating that the electrochemical reaction is mainly controlled by a mixed process of both the charge-transfer process on the surface and semi-infinite Warburg diffusion of Li ions in the bulk.35,56−58 The charge-transfer resistances (Rct) calculated from EIS are 40 and 193 Ω, respectively, for the biogenicLMFP/C and blank-LMFP/C cathodes before cycling (Figure 9a). The extremely low Rct value suggests a remarkable electrochemical activity on the surface of the biogenic-LMFP/ C cathode. After 600 cycles at 1 C, the Rct value of the biogenicLMFP/C cathode is increased to 118 Ω because of the interface passivation during the long-term cycling.10,35 Whereas after 400 cycles at 1 C, the interface passivation (Rct = 526 Ω) is more serious for the blank-LMFP/C cathode. Besides, the Warburg diffusion impedance of the biogenic-LMFP/C cathode is low (283 Ω) as compared with that (776 Ω) of the blank-LMFP/C cathode, implying the relatively fast diffusion of Li ions due to the short diffusion pathway in LMFP nanocrystals and the formation of more effective diffusion channels with certain defects in the biogenic-LMFP/C cathode. After different cycles, the Warburg diffusion impedance is increased to 616 Ω for the biogenic-LMFP/C cathode, still much lower than that (1847 Ω) of the blank-LMFP/C cathode. Moreover, CVs of the biogenic-LMFP/C cathode at various scan rates from 0.1 to 0.6 mV s−1 are measured as shown in Figure 9c. The symmetric Li

4. CONCLUSIONS In summary, biomineralization is a facile and versatile way to fabricate LMFP/C multicomposite by using yeast cells as the nucleating agent, self-assembly template, and carbon source. LMFP nanocrystals with an average size of 18.3 nm are coated by a biocarbon, which are further assembled to form dense and spherical LMFP nanoparticles. Meanwhile, spherical LMFP nanoparticles are wrapped with N-doped GNR networks to fabricate a hierarchical 3D structure of the biogenic-LMFP/C sample. Moreover, the preferential crystallographic orientation along the [010] direction and facile antisite lattice defects in the bulk of LMFP nanocrystals are favorable for accelerating the diffusion of Li ions and generating more active sites for the electrochemical reaction. On the basis of the synergistic effect as demonstrated above, the biogenic-LMFP/C sample presents excellent electrochemical performance, including large discharge capacity, high rate capability, and long-term cycle stability.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02736. FTIR spectra, nitrogen adsorption−desorption isotherms, XPS spectra and structure feature of all the samples, and AFM image and initial charge/discharge curves of the biogenic-LMFP/C sample at various rates (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-L.P.). *E-mail: [email protected]. Phone/Fax: +86-22-23500876 (X.-P.G.). 16508

DOI: 10.1021/acsami.8b02736 ACS Appl. Mater. Interfaces 2018, 10, 16500−16510

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Xue-Ping Gao: 0000-0001-7305-7567 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the 973 Program (2015CB251100) and NFSC (21421001) of China is gratefully acknowledged.

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