Up-scalable synthesis, structure and charge storage properties of porous microspheres of LiFePO4@C

Up-scalable synthesis,structure and charge storage properties of porous microspheres of LiFePO4@C nanocompositesFeng Yu,ab Jing-Jie Zhang,*a Yan-Feng Yang a and Guang-Zhi Song aReceived17th August2009,Accepted30th September2009First published as an Advance Article on the web27th October2009DOI:10.1039/b916938eNovel porous micro-spherical aggregates of LiFePO4@C nanocomposites have been synthesized inlarge quantities via an improved sol–gel method combined with spray drying technology(sol–gel-SDmethod),which required no surfactants or templates.With this new procedure,a precursor wasprepared through the process of sol–gel and subsequent spray drying.A series of analyses,includingX-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),energy dispersive X-ray(EDX),scanning electron microscopy(SEM),high-resolution transmission electron microscopy(HRTEM)and a combined system focused ion beam(FIB)/SEM were performed after the as-obtainedLiFePO4@C was heat-treated at700 C for12h.The as-obtained LiFePO4@C had a large specificsurface area(20.2m2gÀ1),with an average nano-size of32nm and a main pore diameter of45nm.Contact with electrolyte occurred easily,which facilitated the electrical and lithium ion diffusion.Incomparison with nano-sized LiFePO4@C particles prepared by a sol–gel method,the current productpresented a high coulombic efficiency of97.2%,a large reversible capacity of133.8mAh/g and anexcellent capacity retention rate close to100%after50cycles.The sol–gel-SD method provides anadditional strategy to easily deal with gelatin and shows potential for use in the preparation of similarsuperstructures of other composites.1.0IntroductionSince Goodenough and co-workers1reported reversible lithium intercalation in the phosphate polyanionic compound of LiFePO4 as a positive electrode material in1997,interest has grown quickly amongst researchers in thefield of rechargeable lithium ion pared with the typical commercial layered LiCoO2 material,LiFePO4materials with olivine structures possess excellent structuralflexibility and stability.2–4It exhibits superior performance including high reversible capacity,acceptable oper-ating voltage,long cycle life,low cost,superior safety and it is environmentally benign.5,6However,further advancements in promising applications are still somewhat bottlenecked by limi-tations associated with the following obstacles:(a)the poor elec-tronic conductivity and the low Li+diffusion,which lead to initial capacity loss and poor charge/discharge capacity7,8and(b)the low tap-density,which leads to low volumetric energy density.9,10 These limitations are still to be overcome.Modifications of LiFePO4particles by minimizing particle size11,12and coating them with an electronically conductive carbon agent13,14are considered to be effective in surmounting electronic and ionic transport limitations.By combining both of these methods,many researchers have been able to prepare nano-sized LiFePO4/C composite materials.Conventional synthesis methods,such as traditional solid state methods,15hydrothermal methods16and sol–gel methods17,18have all been employed to obtain nano-sized LiFePO4/C composite materials.However,the interfacial energy of nano-sized LiFePO4/C particles is very large and the particles tend to aggregate easily.Consequently,the synthesized powders are typically composed of irregular aggre-gates,which seriously impacts the electrochemical performance of LiFePO4,making morphology an important physical property that must be considered.As has been reported in previous publications,morphology has a great influence on the properties and practical applications of LiFePO4.Morphology control is also believed to be crucial for the determination of size/structure-dependent properties and for the development of new pathways for materials synthesis. Among various materials with different morphologies,spherical morphology shows a great advantage that is superior to other pared with irregular aggregation,the spher-ical particles have lower interfacial energy,high volumetric energy density and betterfluidity characteristics.When the spherical particles vibrate,they can easily move and occupy the available vacancies,so that they closely pack readily.19–22 Recently,much effort has been devoted to the realization of spherical LiFePO4products.Ying et al.23successfully prepared solid micro-spherical Li0.97Cr0.01FePO4/C powders by a co-precipitation method.However,this material often had low discharge capacity and unsatisfactory rate capability due to large amounts of inert LiFePO4at the heart of the solid LiFePO4 microsphere,which contacted poorly with electrolytes(Fig.1a). Otherwise,although hollow micro-spherical LiFePO4powders can usually be synthesized by a spray pyrolysis method,24the hollow part without active LiFePO4at the heart impacts the volumetric energy density of LiFePO4(Fig.1a).Therefore,it is still a challenge for researchers to seek out an efficient route fora Technical Institute of Physics and Chemistry,Chinese Academy ofSciences,Beijing100190,P.R.China.E-mail:jjzhang@;yufeng05@.Fax:+86-10-82543691;Tel:+86-10-82543691b Graduate University of Chinese Academy of Sciences,Beijing100049,P.R.ChinaPAPER /materials|Journal of Materials Chemistrythe synthesis of LiFePO 4with well-defined microspheres and nano-sized particles.In the present study,we developed an improved sol–gel method combined with spray-drying technology (sol–gel-SD method)to design and synthesize porous micro-spherical aggregates of LiFePO 4@C nanocomposites without employing surfactants or templates (Fig.1a).This designed LiFePO 4@C microsphere possesses outstanding morphology with nano-sized,porous and spherical distribution for achieving good electro-chemical performances (Fig.1b).Recently,increasing attention has been given to the risks of nanotechnologies which have possible impacts on human health and the environment.25–27It is worth mentioning that the LiFePO 4@C microspheres prepared by our procedure reduce the potential toxicity of pure nano-structured particles.Furthermore,the large porous particles are in principle easier to bind and to bring into electrical contact than purely nano-sized particles.28,292.0ExperimentalPorous micro-spherical aggregates of LiFePO 4@C nano-composites have been synthesized by a facile template-free process using an improved sol–gel method (sol–gel-SD method).Amounts of Li 2CO 3(AR),Fe(NO 3)3$9H 2O (AR)and H 3PO 4(AR)were dissolved in distilled water in the stoichiometric ratio n Li :n Fe :n P ¼1:1:1.The desired amount of tartaric acid was added and the aqueous solution formed a homogeneous gelatin.The gelatin was then dissolved in distilled water and made into a liquid suspension without surfactants or templates.The obtained suspension liquid was spray-dried in a spray dryer unit at a rate of 15mL min À1with inlet and outlet temperatures maintained at 200 C and 130 C.Carbothermal reduction of the obtained spray-dried precursor was performed in a tube furnaceusing a graphite crucible,heated at 10 C min À1in a flowing argon atmosphere (100mL min À1)until it reached 700 C,it was then held at this temperature for 12h to obtain the LiFePO 4@C composite sample.Scanning electron microscopy (SEM)images and energy dispersive X-ray (EDX)spectra were obtained using a Hitachi S-4300microscope and EMAX Horiba,respectively.A Nova 200Nanolab Dual-beam FIB/SEM instrument was used to gather information on the inside of the obtained LiFePO 4@C composite.This instrument is a combination of SEM and focused ion beam (FIB)with two focused beams in the same location.The FIB instrument is equipped with 30kV Ga +ionswith a beam current from 1p Ato 20n A.The minimum ion beam spot size is 7nm at 1p Abeam current.A scanning probe microscope in tapping mode was employed to obtain the topo-graphic features.X-Ray diffraction (XRD)analysis was carried out on a Rigaku D/max2200PC diffractometer with Cu K aradiation (l ¼1.5406 A).X-Ray photoelectron spectroscopy (XPS)data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W Al K a radiation.The binding energies were referenced to the C1s line at 284.8eV from adventitious carbon.Transmission electron microscopy (TEM)and high-resolution TEM (HRTEM)images and the selected area electron diffraction (SAED)were performed with a Philips TECNAI-20transmission electron microscope.The specific surface area and pore structure were examined with a Quantachrome NOVA 4000BET apparatus.For fabrication of the working electrodes,the obtained LiFePO 4@C composite was mixed with acetylene black and polyvinylidene fluoride (PVDF)in a weight ratio of 80:15:5in N -methyl-2pyrrolidinone (NMP).The obtained slurry was coated on to Al foil and dried at 80 C for 4h.The dried tape was punched into round discs with a diameter of 10.0mm as the cathode electrodes.The electrodes were dried again at 120 C for 5h in a vacuum prior to use.Finally,the prepared cathodes and Celgard2400separator (diameter of 16.0mm)were placed into an argon filled glove box (H 2O and O 2<1ppm)and assembled into a coin cell (CR2032)with lithium anode,electrolyte of 1M LiPF 6in EC-DEC-DMC (1:1:1vol.%)and the other compo-nents of the coin-type cell.The cells were examined with capacity retention studies performed with various rates between 2.0and 4.3V.The cells were retained for ten minutes at 4.3V in charging.3.0Results and discussion3.1Schematic illustration of the sol–gel-SD methodIn order to obtain the designed porous micro-spherical aggre-gates of LiFePO 4@C nanocomposites,a sol–gel method combined with spray drying technology (sol–gel-SD method)was employed.Fig.1c demonstrates the formation mechanism of this designed product.The main processes were as follows:(1)the sol–gel method was employed to produce a gelatin,in which multiple reactants were homogeneously mixed at the molecular level,resulting in good particle size control.30(2)In the spray-drying step,the obtained gelatin was dissolved in distilled water and made into a liquid suspension without surfactants or templates.Subsequently,the obtained suspension was spray-dried with a spray dryer unit.The evaporation of the gelatinwasFig.1(a)Degrees of active LiFePO 4at different interfaces of electrolyte and various LiFePO 4microspheres:a solid microsphere with inert LiFePO 4(left),a hollow microsphere with a hollow part without LiFePO 4(middle)and a porous microsphere with sufficient interface with electrolyte (right).(b)Advantages of the designed LiFePO 4@C porous microspheres:outstanding morphology with nano-sized,porous and spherical distribution for achieving good electrochemical performance.(c)Synthesis of the porous macro-spherical aggregates of LiFePO 4@C nanocomposites using the sol–gel-SD method.more rapid and yielded finer homogeneous and micro-spherical aggregates of precursor particles compared to the routine evap-oration technique.Spray-drying technology is considered an attractive route that exhibits superior performance including energy savings,cost effectiveness,continuous preparation and environmental safety.22,31(3)In the carbothermal reduction step,the carbon from degradation and carbonization of tartaric acid provides a special environment that is favourable for the reduc-tion of Fe(III )and for the formation of the nanocrystalline composite LiFePO 4@C powders.3.2Structural characterizationThe crystallinity and the phase information for the as-obtained product,have been confirmed with the X-ray diffraction (XRD)method,as shown in Fig.2.All of the reflections can be attrib-uted to the orthorhombic phase LiFePO 4(triphylite)(JCPDSno.40-1499)that lacks any impurity phase.The profiles of the reflection peaks are quite narrow and symmetric,indicating the high crystallinity of the LiFePO 4sample.Although the result of the energy dispersive X-ray (EDX)analysis confirms the presence of carbon (Fig.3c),this carbon is not detected in the XRD pattern since residual carbon is amorphous.A space group of Pnma was selected as the refinement model.The absolute lattice parameters for the as-obtained product were as follows:a ¼6.0320nm,b ¼10.3588nm,c ¼4.7063nm and the cell volume was V ¼294.07nm 3,similar to the previous reports.1,10The estimated crystallite size,as deduced using the Debye-Scherrer equation and five crystal lattice indexes of (011),(111),(121),(031)and (131),is 32nm,which is consistent with the SEM observations.Therefore,a pure,homogeneous and well-crystallized LiFePO 4@C composite is indicated.X-Ray photoelectron spectroscopy (XPS)analysis of the as-obtained product has been carried out to examine the oxidation state of Fe in the sample (Fig.4).Two distinct peaks at binding energies of 710.9eV and 724.4eV were observed in the high resolution spectrum of Fe2p.The two peaks could be ascribed to Fe2p3/2and Fe2p1/2,which is characteristic of Fe 2+in LiFePO 4.Otherwise,the binding energy of Li 1s,P 2p,and O 1s are determined to be 55.8eV,133.6eV and 531.8eV respectively,with the reference binding energy at 284.8eV for the C 1s peak.Thus,the XPS analysis further confirms the purity of the LiFePO 4sample.3.3Morphological characterizationThe scanning electron microscopy (SEM)images at different magnifications of the spray-dried precursor microspheres are shown in Fig.5a and b.In Fig.5a,a panoramic image of the precursor microspheres without any dispersion treatment,shows the micro-spherical morphology of the precursor with an average centre diameter of 10m m and good uniformity of morphology.The SEM image of a single precursor microsphere clearly demonstrates that the micro-spherical aggregates are proposed of nano-sized precursor particles (Fig.5b).A close observation of the precursor reveals the nano-sized particles as thebuildingFig.3(a)A TEM image of the as-obtained porous microspheres of LiFePO 4@C nanocomposites.(b)An enlarged TEM image of the surface of one individual LiFePO 4@C microsphere and the corresponding SAED pattern (inset in panel a).(c)An EDX spectrum of as-obtained porous microspheres of LiFePO 4@C nanocomposites.(d)A HRTEM image of A area (indicated by a rectangle in panel b).(e)A HRTEM image of B area (indicated by a rectangle in panelb).Fig.4An XPS pattern of the as-obtained porous microspheres of LiFePO 4@C nanocomposites and a high-resolution XPS Fe2p spectrum (insert).units possess a mean diameter of 30nm.(Fig.5b,insert).Then,the as-obtained title product was prepared by carbothermal reduction of the spray-dried precursor at 700 C for 12h.A combined system focused ion beam (FIB)/SEM provided further insight into the morphology and the structural state of the as-obtained title product,which had a spherical agglomerate structure.From the FIB image shown in Fig.5c,the 3D cross section of the as-obtained title product can be seen to have a porous structure both inside and on the surface.As shown in Fig.5d and e,there are obviously inter-connected pores and surface-open pores in the as-obtained title product,a scheme of which is shown in Fig.5f.The specific BET surface of the as-obtained title product is 20.2m 2g À1and the pore diameter is essentially 45nm.When filled with electrolyte,the pores greatly aid the solid-state diffusion kinetics and are responsible for the immediate supply of lithium ions.The morphology and microstructure details of the as-obtained title product superstructures have been further examined by transmission electron microscopy (TEM)accompanied by selected area electron diffraction (SEAD).As shown in Fig.3a,the TEM image of two LiFePO 4@C microspheres indicated that both were spherical in morphology,which agrees with the SEM result.A closer TEM observation of the surface of a single microsphere,as shown in Fig.3b,further confirms that the as-obtained LiFePO 4@C sample consists of large-scale nano-sized microstructures with sizes ranging from 20–40nm.The corre-sponding SEAD pattern taken from the individual particle clearly suggests a single-crystal nature of the as-obtained title product (inset of Fig.3a).It is worth mentioning that these microstructures are adequately stable;consequently,they cannot be disrupted into fragments or dispersed into nano-sized particles even after ultrasonic treatment.In order to further reveal the fine structure of the LiFePO 4@C superstructures,high-resolution TEM (HRTEM)analysis wasalso carried out.In Fig.3d and e,the corresponding HRTEM images of the different areas marked by rectangles (A and B),are shown.The clear lattice image demonstrates the high crystallinity and single-crystal features of the LiFePO 4@C superstructures,which is in good agreement with the XRD and the SEAD results.The typical d-spacing of 0.30nm and 0.35nm are consistent with the (200)and (021)planes of orthorhombic phase LiFePO 4,respectively.In addition,it also can be clearly seen from the HRTEM images that an amorphous carbon layer in the inter-stitial particle/boundary region covered the surface of LiFePO 4.This amorphous carbon layer,with a thickness of 2–3nm,was generated by carbonization of the tartaric acid precursor.3.4Electrochemical characterizationFig.6shows initial charge-discharge profiles and the corre-sponding cyclic performance of LiFePO 4@C samples prepared by the sol–gel method and the sol–gel-SD pared with the nano-sized LiFePO 4@C powders (sample LFPCb)prepared by traditional sol–gel method,the as-obtained title product (sample LFPCa)prepared by the sol–gel-SD method had a high reversible discharge capacity,high coulombic effi-ciency and excellent capacity retention rate at close to 100%cycled at a current rate of 0.1C in the voltage range of 2.0–4.3V at room temperature.The initial charge–discharge profiles of both samples had the perfect plateau voltage of 3.4V (versus Li +/Li),indicating a typical two-phase reaction between LiFePO 4and FePO 4.1However,the polarization between the charge and discharge plateaus of sample LFPCa was less than that of sample LFPCb,due to its excellent Li +diffusion rate across the two-phase interface.For this reason,the sample LFPCa showed a higher specific discharge capacity and a lower polarization.The initial specific discharge capacities of sample LFPCa and sample LFPCb were 106.7mAh g À1and 137.5mAh g À1respec-tively.The sample LFPCa electrodes delivered a charge capacity of 141.5mAh g À1,and calculation showed that the coulombic efficiency during the first cycle of sample LFPCa wasaboutFig.5(a and b)SEM images of the micro-spherical aggregates of precursor particles at different magnifications.(c)FIB images showing 3D information of the as-obtained porous micro-spherical aggregates of LiFePO 4@C nanocomposites.(d)A SEM image of area A (indicated by a rectangle in panel c).(e)SEM image of area B (indicated by a rectangle in panel c).(f)A scheme showing the structure of LiFePO 4@C nano-composites in porousmicrospheres.Fig.6Initial charge-discharge profiles of LiFePO 4@C samples prepared by the sol–gel method (----)and the sol–gel-SD method (——)cycled at a current rate of 0.1C and the corresponding cyclic performance (insert).Galvanostatic tests were carried out in the voltage range of 2.0–4.3V at room temperature.97.2%,which was higher than the91.5%obtained for sample LFPCb.There was no capacity fading and a slight increase in capacity for sample LFPCa in thefirst10cycles(inset of Fig.6), which demonstrated the excellent cycling stability of the as-obtained title product.4.0ConclusionsIn summary,the sol–gel-SD method was a novel and facile route for preparing porous micro-spherical aggregates of LiFePO4@C nanocomposites without employing surfactants or templates. The as-obtained LiFePO4@C possessed outstanding morphology with nano-sized,porous and spherical distribution for achieving good electrochemical performance.In addition, this work provides a novel strategy for solving the evaporation problem of gelatin,especially in large pared with the traditional sol–gel method,this novel and facile route is a tempting prospect,as it enables quick evaporation of the gelatin and exhibits superior performance,including energy savings,cost effectiveness,continuous preparation and envi-ronmental safety.We believe that this synthesis route illuminates a new way to prepare the title product without employing surfactants or templates and holds the potential to be extended for the preparation of similar superstructures of many other composites.AcknowledgementsThe authors thank Dr Kai-Fu Peng(National Center for Nanoscience and Technology,China)for his kind help with the FIB/SEM measurements.References1A.K.Padhi,K.S.Nanjundaswamy and J. 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