Carbon Nanospheres@Graphene Nanoribbons

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LiMn0.8Fe0.2PO4/Carbon Nanospheres@Graphene Nanoribbons Prepared by Bio-Mineralization Process as Cathode for Lithium-Ion Batteries Yu-Kun Hou, Gui-Ling Pan, Yan-Yun Sun, and Xue-Ping Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02736 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Applied Materials & Interfaces

LiMn0.8Fe0.2PO4/Carbon Nanospheres@Graphene Nanoribbons Prepared by Bio-Mineralization Process as 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. E-mail: [email protected], Tel/Fax: +86-2223500876. ‡

Key Laboratory of Functional Polymer Materials of the Ministry of Education, College of

Chemistry, Nankai University, Tianjin 300071, China. E-mail: [email protected] Keywords: lithium-ion battery; cathode; phosphate; nanostructure; bio-mineralization.

Abstract

Bio-mineralization technology is a feasible and promising route to fabricate phosphate cathode materials with hierarchical nanostructure for high performance lithium-ion batteries. In this work, in order to improve the electrochemical performance of LiMn0.8Fe0.2PO4, hierarchical LiMn0.8Fe0.2PO4/carbon nanospheres are wrapped in-situ with N-doped graphene nanoribbons via bio-mineralization by using yeast cells as nucleating agent, self-assembly template and carbon source. Such LiMn0.8Fe0.2PO4 nanospheres are assembled by more fine nanocrystals with average size of 18.3 nm. Moreover, the preferential crystal orientation along [010] direction and certain anti-site lattice defects can be identified in LiMn0.8Fe0.2PO4 nanocrystals, which promote rapid diffusion of Li ions and generate more active sites for the electrochemical reaction. Moreover,

such

N-doped-graphene

nanoribbon

1 Environment ACS Paragon Plus

networks,

wrapped

between

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

LiMn0.8Fe0.2PO4/carbon nanospheres, are beneficial to the fast mobility of electrons and good penetration of electrolyte. As expected, the as-prepared LiMn0.8Fe0.2PO4/carbon multi-composite presents the outstanding electrochemical performance, including the large initial discharge capacity of 168.8 mAh g-1, good rate capability, and excellent long-term cycling stability over 2000 cycles. Therefore, the bio-mineralization method is demonstrated here to be effective to manipulate the microstructure of multi-component phosphate cathode materials based on the requirement of capacity, rate capability and cycle stability for lithium-ion batteries.

1. Introduction Lithium-ion batteries (LIBs) play a key role in energy storage and conversion devices due to the advantage of both high energy density and power density.1-5 In particular, based on the safety requirement, LiFePO4 (LFP) cathode material is successfully used in lithium-ion batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs). 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, 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


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Recently, to promote the diffusion of Li ions and transport of electrons in 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 LMFP material, the controllable fabrication of nano-sized 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 new technique for the manipulative preparation of well-organized nanomaterials.11,12 Typically, the bio-mineralization is a biological process to produce minerals with living organisms, attracting considerable bio-mimic interests for materials science.13-17 Many organized inorganic and composite materials with amorphous or crystalline structure and hierarchical morphology are prepared via bio-mineralization process in the bio-system.18-21 Compared with traditional chemical process, bio-mineralization 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 bio-mineralization principles, and the organic compounds are also used as the carbon precursor.23-26 C. B. Park et al.23 have demonstrated a facile synthesis of nanostructured transition metal phosphate via bio-mineralization 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. X. Zhang et al.24 have developed a simple and inexpensive biomimetic sol-gel method to prepare high-performance mesoporous LiFePO4 based on the bio-mineralization assembly of



saccharomyces cerevisiae. The as-prepared mesoporous LiFePO4 are composed of densely aggregated LiFePO4 nanoparticles as well as hierarchical mesoporous biocarbon nets coating the LiFePO4 surface. D. Deng et al.25 have employed waste eggshell as a multifunctional reaction system to regulate the reactants and pH value inside the reactor and using eggshell membrane as protein-based and active substrates to prepare 1-D Co(OH)2 nanorod arrays on protein fibers. Such biomimetic materials with hierarchical structure through bio-mineralization show remarkable electrochemical performances as cathode or anode for lithium-ion batteries. Particularly, yeast cells are common unicellular microorganisms with a spherical shape, which are generally used to 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 bio-sorption 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 bio-reactors for the bio-mineralization process, due to the abundant surface charge, large specific surface area, and ability of heavy-metals adsorption within a broad range of pH value and ionic strength.28,29 Moreover, the biomass-derived advanced carbon materials, with amorphous or 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 bio-mineralization process are introduced into LiMn0.8Fe0.2PO4 to fabricate nanosized composite structure, it would be rational for realizing the optimized electrochemical performance of LiMn0.8Fe0.2PO4 cathode.


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In this work, we present a very facile way to fabricate hierarchical LiMn0.8Fe0.2PO4/Ccomposite nanospheres wrapped by N-doped-graphene nanoribbon networks, by using yeast cells as nucleating agent, self-assembly template and advanced carbon source. During hightemperature pyrolyzation, yeast cells are transformed into active biocarbon, which are coated insitu on LiMn0.8Fe0.2PO4 nanocrystals simultaneously. Meanwhile, the celluloses of cell walls and the cellular bio-macromolecules are converted to form the N-doped-graphene nanoribbon networks in the pyrolyzation process. As expected, the as-prepared biogenic LiMn0.8Fe0.2PO4/C cathode material presents remarkable electrochemical performance. 2. Experimental Section 2.1 Preparation of the biogenic-LMFP/C and blank-LMFP/C samples All chemicals are analytical grade and used without further purification. The biogenicLMFP/C sample were prepared using the bio-template 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 constant temperature of 38 ºC for 40 min, a homogeneous bio-suspension was formed. Then the active yeast cells were purified by centrifuging and washing with de-ionized 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 deep bio-sorption process. Then, (NH4)2HPO4 solution (0.5 mol L-1, 40 mL) was added into the Mn2+/Fe3+/yeast mixture solution and continued stirring for 30 min. Then adjusted pH to 5 by CH3COONa and stilled the above mixture solution overnight for the deeper bio-mineralization process. After centrifuging and washing by de-ionized water, the phosphates/yeast bio-precursor was fully dried in a vacuum freeze drying machine. Then mixed the phosphate bio-precursor with Li2CO3 in the



stoichiometric molar ratio of 1.03:1 (Li:P), and sintered under protection of Ar atmosphere at 350 ℃ for 3 h, then sintered at 650 ℃ 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 biogenicLMFP/C sample, additionally glucose (2.5g) was added as carbon source for the sintering process to avoid the Mn2+ and Fe2+ oxidation. 2.2 Materials characterization TEM images were carried out on FEI Tecnai G2F-20 microscope at an accelerating voltage of 200 kV. Morphological profiles were observed using JSM-7600F scanning electron microscope (SEM) at an accelerating voltage of 10 kV. X-Ray diffraction (XRD) patterns were recorded on Rigaku Smart Lab 3 kW diffractometer with Cu Kα radiation (λ=1.5418 Å), the corresponding operation voltage and current is 40 kV and 100 mA, respectively. X-ray photoelectron spectroscopy (XPS) was collected on scanning X-ray microprobe (PHI 5000 Verasa, S4 ULAC-PHI, Inc.) using Al Kα radiation and the C1s peak at 284.8 eV as internal standard. Raman spectra of powder samples were obtained on Lab-RAM HR800 with a laser excitation wavelength of 532 nm. Fourier transform infrared spectroscopy (FTIR) was collected on a Nexus 670 spectrometer and by using a KBr wafer techique. Nitrogen adsorption-desorption isotherms were measured at 77K on a Quantachrome Instruments Autosorb AS-6B. The pore size distributions were measured by the Barrett-Joyner-Halenda (BJH) method. 2.3 Electrochemical measurements In a typical electrode preparation, active materials, super P and PVDF were mixed with a mass ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) to form a slurry. Then, the slurry was


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pressed to a piece on Al foil with about 0.02 mm in thickness and dried in 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 glove box with lithium metal served as counter and reference electrodes, and Celgard 2300 as a separator. The electrolyte was comprised of a solution made by 1 M LiPF6 in dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC)/ethylene carbonate (EC) 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 CHI600C electrochemical workstation at various scan rate from 0.1–1 mV/s. EIS measurements were conducted on a Zahner IM6ex electrochemical workstation by sweeping the frequency from 100 kHz to 0.01 Hz with AC amplitude of 5 mV. All the above electrochemical measurements were carried out at room temperature. 3. Results and discussion In bio-mineralization process, it is generally believed that the nucleation and growth of inorganic materials are mostly controlled by genes (DNA) and bio-molecules in the biomineralization process.29 Fig. 1 illustrates the adsorption, deposition and self-assembly mechanism of the transition metal phosphate in yeast cells (Fig. 1a–c), and the whole preparation procedure of the biogenic LiMn0.8Fe0.2PO4/C (marked as biogenic-LMFP/C) sample. During yeast

propagation,

the

metabolic

bio-macromolecules

of

extracellular

proteins

and

polysaccharose with abundant hydrophilic anion groups (such as –COO-, –OH-, –CONH2-, and – OPO3- etc.) 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 with the hydrophilic anion groups of biomolecules on the cell surface and inside the cells by electrostatic interactions via the metabolism processes (Fig. 1a). The Mn-responsive translocator SMF1 and SMF2, and Fe-responsive translocator 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 precursor and the abundant anion groups on the surface of cell are highly important,29 which is also worked in the preparation of mesoporous LiFePO4 based on the bio-mineralization assembly.24 When (NH4)2HPO4 solution is added into the Mn2+/Fe3+/yeasts mixed solution, the PO43- anion groups electrically are adsorbed to Mn2+/Fe3+ sites and to further form ferromanganese-phosphate bio-minerals in yeast cells (Fig. 1b). During bio-mineralization, the metabolic energy of ATP drives the nucleation process via a series of enzymatic reaction, and the cellular bio-macromolecules provide nucleation sites and immobilize the ferromanganesephosphate nanoparticles in the cells (Fig. 1c). The produced bio-minerals are composite materials, consisted of the inorganic component and specific organic bio-matrix. The organic bio-matrix has a great impact on the morphology and structure of the inorganic materials formed.37 The detailed interaction between bio-molecules and Mn2+/Fe3+ cations, and the in-situ linkage between the yeast cells and bio-mineralized-phosphates, are demonstrated by Fourier transform infrared spectroscopy (FT-IR) measurement (Fig. S1). As a result, the produced inorganic bio-precursor is detected to be mixture of NH4MnPO4 and FePO4 by X-ray diffraction (XRD) (Fig. S2). Finally, the biogenic-LMFP/C sample with the olivine structure is prepared by mixing the phosphate bio-precursor with Li2CO3 and calcining in an inert atmosphere. To verify the superiority of yeast cells in bio-mineralization process to fabricate the biogenic-LMFP/C


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composite, a blank sample (marked as blank-LMFP/C) is also prepared without introducing yeast in the parallel preparation condition.

Fig. 1 Scheme of the bio-inspired design and preparation. (a–c) Scheme of the adsorption, deposition and self-assembly of the transition metal phosphate as bio-precursor in yeast cells via bio-mineralization. (d) Schematic structure of the biogenic-LMFP/C sample derived from biofabrication. XRD patterns of the biogenic-LMFP/C and blank-LMFP/C samples are shown in Fig. 2. Clearly, the diffraction peaks of all the samples are matched well with the olivine structure of LiMn0.8Fe0.2PO4 (JCPDS No. 13-0336), and the diffraction peaks of biocarbon 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 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 due to 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 1D diffusion pathway of the



[010] direction in 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 LiMn0.8Fe0.2PO4 in the biogenic-LMFP/C sample is calculated to be 18.3 nm (Table S1), implying that nano-crystalline LMFP materials are prepared via biomineralization due to 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 ionbonding sites on the organic bio-surface.22 Here, protein cages act as faster growing hosts for the guest minerals in the bio-mineralization 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 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 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 as well as the rate capability.

Fig. 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. Inset is model of the crystal structure of LMFP, the lithium diffusion channels are along the [010] direction.


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Fig. 3 shows TEM images of the biogenic-LMFP/C sample. In Fig. 3a and b, LiMn0.8Fe0.2PO4 particles appear as 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 HRTEM image (Fig. 3c). The thickness of the graphene nanoribbons is about 4.4 nm, which is further confirmed by AFM observation (Fig. S3). In the meantime, the LiMn0.8Fe0.2PO4 nanocrystals are coated by uniform carbon layer with the thickness about 4–5 nm to fabricate the LMFP/C spheres (Fig. 3d). It is also noted that such LiMn0.8Fe0.2PO4 spherical particles with the diameter of 100–300 nm are assembled by LiMn0.8Fe0.2PO4 fine nanocrystals with the size of 10– 30 nm by comparison with (200), (101) and (301) planes (Fig. 3e), almost identical to the average crystallite size calculated from XRD analysis. In particular, the anti-site lattice defects of LiMn0.8Fe0.2PO4 can be observed in HRTEM image (Fig. 3f). The anti-site 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 above TEM observation. Firstly, carbon exists as networks to support LiMn0.8Fe0.2PO4 spherical particles, and as thin layer to coat the surface of LiMn0.8Fe0.2PO4 nanocrystals. The total carbon content of the biogenic-LMFP/C sample is 10.85 wt %. Such 3D conductive architectural composite structure contributes to the fast transport of electrons and penetration of electrolyte among the active particles, leading to the high-efficiency electrochemical reaction.24,39 Secondly, LiMn0.8Fe0.2PO4 spherical particles are aggregated by fine nanocrystals with the preferential crystalline orientation along the [010] direction, as well as the anti-site lattice defects in the bulk of LiMn0.8Fe0.2PO4. 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 electrochemical reaction.

Fig. 3 TEM and HRTEM images of the biogenic-LMFP/C sample. (a, b) TEM images, showing hierarchical structure of the biocarbon network around LMFP/C-composite spheres. (c) HRTEM image of the biocarbon network on the particle surface, showing graphene nanoribbon structure of the biocarbon. (d) HRTEM image of the LMFP/C-composite nanosphere, showing regular (200) lattice fringe of LMFP and uniform carbon-coating surface. (e, f) HRTEM images in different region of the LMFP/C-composite sphere, showing nanocrystal nucleus and various antisite lattice defects of LMFP. Furthermore, the elemental mapping of the biogenic-LMFP/C sample using X-ray energydispersive spectrometry (EDS) under high angle annular dark field (HAADF) mode is presented in Fig. 4. Here, O, P, Mn and Fe elements are evenly distributed in the active particle and C, N elements are well distributed at the particle surface. The mapping signals of Fe (Fig. 4e) is weaker than that of Mn (Fig. 4d), which is in a rational response to the stoichiometric ratio of


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Fe/Mn in LiMn0.8Fe0.2PO4. 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 LiMn0.8Fe0.2PO4. The corresponding EDS spectra (Fig. 4h) also reveal that the atomic ratio of Mn, Fe, P, and O in the biogenic-LMFP/C sample is very close to the stoichiometric ratio of target materials. Particularly, the molar ratio of the Mn:Fe in the biogenic-LMFP/C sample is tested to be 8.06:1.98 by X-ray fluorescence spectroscopy (XRF). SEM image of the biogenicLMFP/C sample is given in Fig. 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 welldefined graphene nanoribbon networks adhering on the surface of active spherical particles, where graphene nanoribbons are in-situ formed and intimately linked together among active spherical particles during pyrolyzation of the yeast cells. Accordingly, the electrical conductivity of the biogenic-LMFP/C sample and blank-LMFP/C sample is 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 highconductive graphene nanoribbon (GNR) networks. The graphene nanoribbons 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 pyrolysis of chitosan films under moderate temperatures (600-800 ºC) in inert atmosphere and without acid or catalyst assistance as prerequisite.31 In addition, 3D macroporous graphene-based carbon with thin 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 bio-mineralization and subsequent carbonization processes.

Fig. 4 EDS and SEM characterization of the biogenic-LMFP/C sample. (a) 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 graphene nanoribbon (GNR) networks. Fig. 5 shows 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


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to the oxygen vacancy.18,43,44 The O3 species at 530.2 eV is ascribed to 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 those of blank-LMFP/C (Fig. 5c and d), further confirming the existence of anti-site lattice defects and oxygen vacancy in the fine LMFP crystal structure of the biogenic-LMFP/C sample.40-44

Fig. 5 XPS spectra: O 1s core level and P 2p core level of the biogenic-LMFP/C (a and b) and blank-LMFP/C (c and d) samples. Raman spectra are the most direct and sensitive technique to reveal the form and quality of carbon materials.45 As shown in Fig. 6, the peaks at 1350 cm-1 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 highfrequency E2g vibration mode of sp2 carbon domains, which is related to the graphitization of carbon atoms. While the D band is associated with the disordered structure and structure defects of sp2 carbon domains.47 The intensity of G band in the biogenic-LMFP/C sample is stronger than that of D band, indicating the good graphitization in the biocarbon,46 which is mainly attributed to the abundant graphene nanoribbons as shown in Fig. 3c and Fig. 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 up-shifted and broader peak, in line with the structure characteristics of the multilayer-graphene,47,49,50 and in consistent with the observation of multilayered lattice fringes of the biocarbon in the HRTEM image (Fig. 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 PO43anions appears for the biogenic-LMFP/C sample due to 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 (Fig. 6b). Both well-defined C 1s peak and N 1s peak appear. There are three components in the C 1s core level of the biogenic-LMFP/C. The main peak appears at 284.7 eV with the graphitic sp2 carbon (C=C), suggesting that the most of carbon atoms in the biogenic-LMFP/C is 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 Nsp3 C (C=N) bonds, respectively.47 In the case of the N 1s core level (Fig. 6c), the pyridinic N (397.0 eV), pyrrolic N (399.6 eV) and graphitic N (402.5 eV) can be assigned, which are typical


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bonding characteristics in the N-doping graphene.49,50,52,53 It means that nitrogen is doped in graphene nanoribbon networks of the biogenic-LMFP/C sample via the in-situ pyrolyzation from the N-rich bio-molecules.

Fig. 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 splited 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 graphitic-carbon lattice. To get an insight into the electrochemical performance of the biogenic-LMFP/C and blankLMFP/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 Fig. 7a. The well-defined reversible redox couple



peaks located around 3.6 V and 4.1 V are ascribed to the Fe2+/Fe3+ and Mn2+/Mn3+ redox couples, accompanying with the Li extraction/insertion in LiMn0.8Fe0.2PO4. In comprison with the blank-LMFP/C sample, the biogenic-LMFP/C sample delivers larger current density of redox peaks in CVs, suggesting the 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 (Fig. 7b). Specifically, the initial discharge capacity of the biogenic-LMFP/C is 168.8 mA h g-1 at C/20 rate, very close to the theoretical capacity (170 mA h g-1) of the LiMn0.8Fe0.2PO4 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 C, 2 C, 5 C, and 10 C, respectively (Fig. 7c and Fig. S7). Even after 70 cycles at various rates, the discharge capacity is still recovered to 167.6 mA h g-1 at C/20 rate with the capacity 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 (Fig. 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 1C rate is shown in Fig. 7d. Obviously, no distinct decay of the discharge capacity can be seen for the biogenic-LMFP/C cathode even after 600 cycles at 1 C rate. While, the discharge capacity is lower at 1 C rate for the blank-LMFP/C cathode, and the capacity retention is about 86.2 % after 400 cycles. When cycled at 2 C rate (Fig. 7e), the biogenic-LMFP/C cathode present the relatively high initial discharge capacity of 129.1 mAh g-1, and the 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


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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 N-doped graphene nanoribbon networks, and as well as more active sites for electrochemical reaction in LiMn0.8Fe0.2PO4 nanocrystals with certain lattice defects. Although after 2000 cycles (Fig. 8), the graphene nanoribbon networks on the surface of LMFP/C spherical nanoparticles are still observed, implying the stable covalent interaction between graphene nanoribbons and 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 graphene nanoribbons. Furthermore, LiMn0.8Fe0.2PO4 nanocrystals maintain the fine structure (Fig. 8j), suggesting the good structure stability of the biogenic-LMFP/C cathode during the long-term cycling.



Fig. 7 Electrochemical characterization of the biogenic-LMFP/C and blank-LMFP/C samples. (a) CVs of the biogenic-LMFP/C and blank-LMFP/C cathodes (0.3 mV s-1). (b) Initial charge– discharge curves of the biogenic-LMFP/C and blank-LMFP/C cathodes at C/20 rate between 2.0 and 4.5 V. (c) The rate capability of the biogenic-LMFP/C and blank-LMFP/C cathodes at various rates (C/20, C/10, 1 C, 2 C, 5 C and 10 C). (d) The cycle performance of the biogenicLMFP/C and blank-LMFP/C cathodes at 1C rate. (e) Long cycle performance of the biogenicLMFP/C cathode at 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.


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Fig. 8 TEM images (a and b) of the biogenic-LMFP/C sample after 2000 cycles at 2 C rate. STEM image (c) recorded by the high angle annular dark field detector, and (d-i) EDS elemental mapping of O, P, Mn, Fe, C and N on the selected region. (j) HRTEM image of the LiMn0.8Fe0.2PO4 nanocrystal of the biogenic-LMFP/C sample after 2000 cycles. To get a further understanding with the reaction mechanism, electrochemical impedance spectra (EIS) and CVs are measured with a focus on the surface charge transfer and diffusion of Li ions. As shown in Fig. 9a and 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, meaning that the electrochemical reaction is mainly controlled by a mixed processes 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 Ω and 193 Ω, respectively, for the biogenic-LMFP/C and blank-LMFP/C cathodes before cycling (Fig. 9a). The extremely low Rct value suggests the remarkable electrochemical activity on the surface of the biogenic-LMFP/C cathode. After 600 cycles at 1 C, the Rct value of the biogenic-LMFP/C cathode is increased to 118 Ω due to the interface passivation during the long-term cycling.10,35 While, the interface passivation (Rct=526 Ω) is more serious for the blank-LMFP/C cathode after 400 cycles at 1 C. 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 LiMn0.8Fe0.2PO4 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 biogenic-LMFP/C cathode, still much lower than that (1847 Ω) of the blank-LMFP/C cathode. Moreover, CVs of the biogenicLMFP/C cathode at various scan rates from 0.1 to 0.6 mV s-1 are measured as shown in Fig. 9c. The symmetric Li 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 (Fig. 9d), suggesting that the rate determined step is dominant by the diffusion process based on the Randles-Sevcik equation.58,59


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Fig. 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). 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 biocarbon on LiMn0.8Fe0.2PO4 nanocrystals with the average size of 18.3 nm, and the wrapping on the N-doped graphene nanoribbon networks facilitates the formation of the hierarchical LiMn0.8Fe0.2PO4/C composite structure. Such 3D structure is unique for fast transport of electrons and good penetration of electrolyte. Furthermore, inside LiMn0.8Fe0.2PO4 nanocrystals, the preferential crystalline orientation and lattice defects can be observed clearly, which are



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demonstrated to be favorable for promoting the diffusion of Li ions and providing the active sites for electrochemical reaction. Therefore, in order to achieve the superior electrochemical performance, a rational structure design of phosphate cathode materials can be realized via the bio-mineralization process.

4. Conclusions In

summary, the bio-mineralization is facile and versatile way to

fabricate

LiMn0.8Fe0.2PO4/C-multi-composite by using yeast cells as the nucleating agent, self-assembly template and carbon source. LiMn0.8Fe0.2PO4 nanocrystals with the average size of 18.3 nm are coated by biocarbon, which are further assembled to form dense and spherical LiMn0.8Fe0.2PO4 nanoparticles. Meanwhile, spherical LiMn0.8Fe0.2PO4 nanoparticles are wrapped with N-doped graphene nanoribbon networks to fabricate hierarchical 3D structure of the biogenic-LMFP/C sample. Moreover, the preferential crystallographic orientation along the [010] direction and facile anti-site lattice defects in the bulk of LiMn0.8Fe0.2PO4 nanocrystals are favorable for accelerating diffusion of Li ions and generating more active sites for electrochemical reaction. Based on 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. Supporting Information FTIR spectra, Nitrogen adsorption-desorption isotherms, XPS spectra and structure feature of all the samples, AFM image and initial charge–discharge curves of the biogenic-LMFP/C sample at various rates.

Acknowledgments


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TOC:


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Carbon Nanospheres@Graphene Nanoribbons

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