electrochemical analysis of conductive polymer-coated lifepo4
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Electrochemical Analysis of Conductive Polymer-Coated LiFePO4Nanocrystalline Cathodes with Controlled Morphology
Hung-Cuong Dinh,a Sun-il Mho,a In-Hyeong Yeo*b
a Division of Energy Systems Research, Ajou University, Suwon 443-749, South Koreab Department of Chemistry, Dongguk University, Seoul 100-715, South Korea*e-mail: [email protected]
Received: April 21, 2011;&Accepted: May 25, 2011
AbstractNanocrystalline LiFePO4 particles and donut- and dumbbell-shaped LiFePO4 microstructures hierarchically con-structed with the nanoparticles were synthesized. Coating the surfaces of the LiFePO4 particles with conductivepolymers, poly-3,4-ethylenedioxythiophene (PEDOT), improved battery characteristics, such as specific capacity, ca-pacity retention, and impedances, by increasing the ionic and electric conductivities of the cathode materials as wellas by enhancing the accessibility of lithium ions. The nanocrystalline LiFePO4 that was coated with PEDOT exhibit-ed even larger values than the theoretical value of LiFePO4 at a rate of 1 C or lower rates were observed, with itslargest specific capacities of 190 mAhg1 and 175 mAhg1 at the rates of 0.2 C and 1 C, respectively, due to thecompatibility of the redox characteristics of LiFePO4 and PEDOT. The cathode composed of nanocrystallineLiFePO4 that was coated with PEDOT exhibited the lowest value of the charge transfer resistances (Rct) from theelectrochemical impedance analysis.
Keywords: Nanocrystalline, Hydrothermal method, Conducting polymer, Hierarchical microstructure, Li-ionbattery
Lithium ion batteries are one of the great successes ofelectrochemical energy sources for modern portable elec-tronic devices and also expected to take part in large-scale applications for future electric vehicles, includinghybrid vehicles . An olivine structure material,LiFePO4, has emerged as one of the most promising cath-ode materials with good structural stability, lower toxicity,and relatively low cost, which was first reported by Padhiet al. . The strong P-O covalent bond nature of thePO4 tetrahedral polyanion in the LiFePO4 structure ren-ders good safety due to the lack of oxygen release fromthe lattices . LiFePO4 has a moderate theoretical ca-pacity of 170 mAhg1 with a stable plateau voltage of3.45 V versus Li/Li+. However, pure LiFePO4 by itselfhas relatively poor electronic (~109 Scm1) and ionicconductivities as a cathode for lithium ion battery appli-cations. In order to improve its intrinsic problem of lowion diffusivity, morphology control and particle size re-duction were attempted . Various LiFePO4 nano-structures and complex hierarchical architectures withwell-defined sizes and morphologies have been investigat-ed . Nanostructures have the benefit of enlargingthe battery capacity and increasing the charge/dischargerate, due to the resulting increase in the specific surfacearea and decrease in the diffusion length for Li+ ions.LiFePO4 microstructures hierarchically constructed with
nanostructures are also of great interest . The par-ticle size and morphology can be controlled by a carefulchoice of surfactant, solvents, and reaction parameters inhydrothermal synthesis . Surfactant molecules canbe adsorbed onto the particle surfaces during particlegrowth, and this determines the particle size and mor-phology of the product.Considerable efforts have been made to improve the
conductivity and the charge/discharge rate of LiFePO4both by doping foreign ions in the crystalline lattice andby surface coating . Improvement of the intrinsicconductivity of LiFePO4 by doping with cations, metals,or carbon atoms into the crystalline structure seems tohave been unsuccessful . On the other hand, coatingthe LiFePO4 particles with carbon powder, polymers, ormetals is reported to have improved the battery electrodeconductivity; for example, the surfaces of inorganic nano-sized particles can be coated with conductive polymers. Conductive polymers, such as polypyrrole (PPy)and poly-3,4-ethylenedioxy-thiophene (PEDOT), are at-tractive materials in covering crystallite surfaces in termsof improving not only mechanical flexibility but also elec-trical conductivity . Another advantage of conduc-tive polymers is that they can be coated under mild proc-essing conditions compared to carbon coating. In additionto their electrical conductivity and mechanical flexibility,conductive polymers can also improve lithium ion accessi-bility and diffusion pathways within battery cathodes.
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In this work, nanocrystalline and microstructuredLiFePO4 particles with various shapes were prepared by ahydrothermal method. Rhomboidal- or rod- shaped parti-cles and hierarchical microstructures that were assembledwith the LiFePO4 nanoparticles were prepared by usingvarious surfactants and reducing agents with the startingingredients. Then, in order to increase the charge trans-port kinetics, these particles were coated with the conduc-tive polymers those were dissolved in appropriate sol-vents. The electrochemical performances of these conduc-tive polymer-coated LiFePO4 crystalline particles and mi-crostructures were examined in terms of cycle stabilityand discharge capacity with consideration for the catho-des of Li+ ion batteries. The analysis and comparison ofthe crystal structure, morphology, impedance values, andelectrochemical properties were also addressed.
2.1 Material Preparation
All chemicals and materials were obtained and used asreceived from the Sigma-Aldrich Corporation. All aque-ous solutions were prepared with ultra-pure water deion-ized by a Millipore Milli-Q system (18 MWcm1).LiFePO4 crystalline particles were synthesized by a hy-drothermal method from the stoichiometric mixture ofthe starting materials of LiOH, FeSO47H2O, and H3PO4(3 :1 :1 molar ratio). L-ascorbic acid or glucose was addedas a reducing agent to the stoichiometic mixture of thestarting materials (Fe2+ : glucose=4 :1 molar ratio). Eth-ylene glycol was added into the aqueous solution to dis-perse the inorganic salts (1 : 1 volume ratio). Citric acid orpoly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) was also added as a surfac-tant to control the particle growth and surface morpholo-gy. The resulting gray solution was transferred into a125 mL Teflon-lined stainless steel autoclave and heatedfor 12 h at 160 8C or 180 8C. After the hydrothermal reac-tion was completed, the precipitate was completelywashed and dried at 80 8C.Conductive polymers, PPy and PEDOT, were prepared
by polymerizing pyrrole and 3,4-ethylene-dioxythiophene(EDOT) monomers, respectively, by using anhydrousFeCl3 as an oxidizing catalyst [24,25]. For PEDOT, para-toluene sulfonic acid (p-TSA) was added as a dopant at a2 :1 mole ratio. Both PEDOT and PPy were dissolved inorganic solvents, such as DMSO and cresol. PEDOT andPPy were coated on the LiFePO4 particles by simply dis-persing the LiFePO4 powder in the respective conductivepolymer solution at room temperature and evaporatingthe solvent in a vacuum oven at 60 8C. For comparison,LiFePO4 was also coated with carbon (LiFePO4-C) bydispersing the LiFePO4 powder and acetylene black in N-methyl-2-pyrrolidone (NMP), and then it was heated at600 8C in a nitrogen atmosphere.
The crystalline structure of the LiFePO4 samples wasidentified by an X-ray diffractometer (Rigaku, DMAX-2200PC, Japan) that utilized Cu-Ka radiation (l=1.54056 ). Scanning electron microscopes (FESEMJSM-6700F and SEM JSM-6380, JEOL, Japan) were usedto examine the morphology. The FTIR spectra were re-corded using an infrared spectrophotometer (NicoletiS10, USA). Samples were mixed well with KBr (approxi-mately 1 :100) and vacuum-pressed into translucent disks.The lithium batteries were assembled with LiFePO4-
PEDOT, LiFePO4-PPy, and LiFePO4-C cathodes. Themixtures of acetylene black (AB), the polyvinylidenefluoride (PVDF) binder, and the LiFePO4 that wascoated with the respective conductive polymers or carbonwere dissolved in NMP. The resulting slurry was spreadon thin aluminum foil (Doctor Blade method) and driedin an oven at 80 8C for 3 h and then pressed and dried ina vacuum for 24 h. Lithium foil was used as an anode.The electrolyte was 1 M LiFP6 that was dissolved in a 1 :1v/v mixture of ethylene carbonate (EC) and dimethyl car-bonate (DMC). The lithium batteries were assembled ina coin-type (CR2032) cell in a dry room, and the batterieswere cycled with a constant current mode (galvanostati-cally) in a potential range of 2.04.3 V using a multichan-nel battery test system (Maccor Series 4000, USA).Unless otherwise indicated, all cyclic stability tests wereconducted after initialization processes, which involvedthree cycles at 0.1 C using freshly fabricated batteries.Electrochemical impedance measurements (Schlumbergermodel SI 1260 impedance/gain phase analyzer connectedto a Schlumberger model SI 1268 electrochemical inter-face) were performed by applying an a.c. signal in the fre-quency range of 10 mHz to 1 MHz with 5 mVp-p afterthe 0.1 C cyclic initialization of the battery.
3 Results and Discussion
3.1 Structural and Morphological Characterizations
The size and morphology of LiFePO4 were examined byscanning electron microscopes. Figure 1 shows the SEMimages of the nanoparticles and the microstructuredLiFePO4 with various shapes that were synthesized by thehydrothermal method. Even though the hydrothermalmethod is a low temperature synthetic method, uniformand controlled size- and morphology-particles were ob-tained. The well-dispersed individual LiFePO4 crystalsformed with the P123 surfactant displayed rectangular- orrhombus-shapes with particle sizes that were approxi-mately 500 nm in length, and 100200 nm in width and inthickness (nanoplates), shown in Figures 1a and 1b. Thesize of the crystallites was predominantly controlled bythe reaction temperature. By increasing the reaction tem-perature to 180 8C, with the same starting solutions andwith P123 as the surfactant, the micron-sized LiFePO4rhombus particles were obtained as well-dispersed indi-
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vidual particles. The LiFePO4 particles reveal well crystal-lized rhombus shape structures with the following dimen-sions: approximately 3 mm in length, 1.3 mm in width and1 mm in thickness (Figure 1c). The surfactant has a criticalrole to play in the morphology of LiFePO4, by effectivelycontrolling the particle growth, the particle size, and mor-phology . When adding Pluronic P123, which wascomprised of block copolymers of polyethylene oxide andpolypropylene oxide, to the reactor, the product particleswere well-dispersed individually. On the other hand, themolecular surfactant, citric acid, can be adsorbed ontoparticle surfaces during particle growth, and in turn canorganize the nanoscale building blocks into complex hier-archical architectures via a self-assembly process. Whenthe surfactant was changed from P123 to citric acid, the
LiFePO4 nanoplates were hierarchically organized intomicrostructures. It should be noted that the LiFePO4 hier-archical architectures with well-defined dumbbell-like ordonut-like microstructures were successfully synthesizedby using citric acid as the surfactant. Dumbbell-like mi-crostructures of LiFePO4 with lengths of about 8 mm werehierarchically constructed with the nanoparticles, asshown in Figures 1d1f. When the amount of the surfac-tant was increased by four times (4 mol of citric acid for1 mole of Fe2+), the growth of the nanoplates of crystal-line LiFePO4 also seemed to be inhibited. The particlethicknesses were considerably smaller than those withoutthe excess amount of the surfactant added in the reactor.The thin LiFePO4 nanoplates were hierarchically organ-ized into donut-like microstructures, as shown in Figs. 1g
Fig. 1. SEM images of various shapes of LiFePO4 powders; (ab) nanocrystalline LiFePO4, (c) micron-sized LiFePO4 crystals, (df)dumbbell shape microstructure LiFePO4 assembled with nanocrystals, and (gh) donut-shaped microstructure LiFePO4 assembledwith nanocrystals.
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Conductive Polymer-Coated LiFePO4 Nanocrystalline Cathodes
1h. The donut-shaped microstructures were composed ofthinner (3760 nm thick) nanoplates than those (90140 nm thick) of the dumbbell-shaped nanostructures.The XRD patterns of nano- and micro-crystalline
LiFePO4 particles and LiFePO4 microstructures that werehierarchically constructed with nanoparticles are shownin Figure 2. All diffraction peaks are well matched withthat of the standard pattern of the LiFePO4 (ICSD No.72545) of the orthorhombic structure with the Pnmaspace group. The diffraction peaks of the various mor-phologies of the LiFePO4 particles prepared by the hy-drothermal method are intense and sharp without any ad-ditional heat treatment, which indicates that the nano-and micro-structures are well crystallized.The FTIR absorption spectra of nano-crystalline
LiFePO4 powder and the conductive polymer-coatedLiFePO4 are shown in Figure 3. It is well known that thefundamental vibrations (n1n4) of PO4
3 polyanions ofLiFePO4 are split in many components due to the correla-tion effect induced by coupling with the Fe-O units in thesolid state . The n1 and n3 modes involve the symmet-ric and antisymmetric stretching vibration of the PObonds, whereas n2 and n4 involve mainly OPO symmet-ric and antisymmetric bending mode with a small contri-bution of the P vibration. The stretching and bendingmode regions are well separated from each other. Thestretching modes are identified at n1 (A1)=975 cm
1 andn3 (triplet F2) in the region of 10741136 cm
1; and thebending modes at n2 (doublet E)=471502 cm
1 and then4 (triplet F2) in the region of 551652 cm
1, as shown inFigure 3a. Since the LiFePO4 powders that were preparedin this work are in well-crystallized phases, this spectrumdisplays well-resolved peaks.
The spectra of the conductive polymers of PPy- orPEDOT-coated LiFePO4 are shown in Figures 3b and 3c.In addition to the absorbance peaks corresponding tothose of LiFePO4, the characteristic bands of the conduc-tive polymers, such as PPy and PEDOT, appear (comparewith Figures 3d and 3e). The FTIR spectra indicate thatthe respective polymers of PPy and PEDOT are coatedon the surface of LiFePO4, since no spectral changes in-cluding peak-shift or broadening of the LiFePO4 peaks,was observed. The morphologies of various sized- andshaped-LiFePO4 that were coated with conductive poly-mers also reveal that they maintain their original shapeswithout altering the pristine LiFePO4 particles inside.Smooth and complete polymer-coatings on the LiFePO4particles, even with extremely rough surfaces, results fromthe coating process of dispersing the LiFePO4 particles inthe polymer solutions.
3.2 Electrochemical Performance of the LiFePO4 andLiFePO4 Coated with Conductive Polymers
After assembling coin-type Li cells with the cathodescomposed of nanocrystalline LiFePO4 powders, their elec-trochemical performances were evaluated by a batterytest system. Figure 4A shows the voltage-capacity profilesduring the first charge/discharge cycle at a constant cur-rent rate of 1C for the LiFePO4 nanocrystals coated withPEDOT, PPy, and carbon, and the pristine nanocrystal-line LiFePO4. All these samples demonstrated a flatcharging-discharging plateau, which is one of the distinctfeatures of LiFePO4. The charge/discharge profile of thepristine LiFePO4 nanocrystals maintains the characteristicshape with a voltage plateau of 3.45 V and a specific ca-pacity of 80 mAhg1. This low specific capacity value isconsidered to be caused by the adverse effect of the low
Fig. 2. The XRD spectra of various shapes of LiFePO4 pow-ders; (a) nanocrystalline LiFePO4, (b) micron-sized LiFePO4crystals, (c) donut-shaped microstructure LiFePO4, (d) dumbbell-shaped microstructure LiFePO4.
Fig. 3. FTIR spectra of (a) nanocrystalline LiFePO4, (b) nano-crystalline LiFePO4 coated with PEDOT, (c) nanocrystallineLiFePO4 coated with PPy, (d) PEDOT, and (e) PPy.
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conductivity of the pristine material itself. The LiFePO4nanoparticles coated with conductive materials on thesurface show much improved performance in terms ofspecific capacity. The specific capacity of the nanocrystal-line LiFPO4 with the carbon coating is measured to be152 mAhg1, which is higher than that of pristine LiFPO4nanocrystals. The charge/discharge capacities of theLiFePO4 coated with conductive polymers also revealmuch improvement compared to the pristine particles.We believe that the conductive polymers on the surfacesof LiFePO4 particles enhanced the conductivity as well asimproved the ion movement through the conductive poly-mer chains. Nanocrystalline LiFePO4-PPy has a specificdischarge capacity of 164 mAhg1. Particularly, the nano-crystalline LiFePO4-PEDOT reveals its largest specific ca-pacity to be 167 mAhg1. The high specific capacity ofthe PEDOT-coated LiFPO4 is close to the theoreticalvalue (170 mAhg1) of LiFePO4.The first discharge/charge curves of the lithium batter-
ies assembled with cathodes that had four different mor-phologies of LiFePO4 crystals, each coated with PEDOT,are shown in Figure 4B. All the curves show reversiblelithium extraction/insertion during the charging/discharg-ing processes with varying specific capacities. TheLiFePO4 nanoparticle cathode shows the highest specificcapacity approaching the theoretical value of170 mAhg1, the micron-sized rhombus-shaped crystallineLiFePO4 cathode shows the lowest specific capacity, andthe dumbbell- and donut-shaped microstructure LiFePO4cathodes show medium capacity values. The increase inthe specific capacity certainly reflects the increase in thespecific surface area. In addition, the smaller the particleis, the more densely the particles can be packed withsmaller interstitial volumes in the cathode. High packingdegree with small void volume results in high tap density.High tap density leads to a high volumetric specific ca-pacity. The tap densities of nanocrystals, rhombus micro-crystals, donut- and dumbbell-shaped hierarchical micro-structurs are 1.56, 1.05, 1.06, and 1.20 gcm3, respectively.As the nanocrystalline LiFePO4 possesses the highest tapdensity (1.56 gcm3) among the various morphologies, ahigh volumetric specific capacity (260.5 mAhcm3) wasachieved.The impedance measurement can probe and detect Li-
ion movement within electrode lattices and electrodeelectrolyte interfaces at various charged states of the bat-tery. In order to understand the effect of coating theLiFePO4 nanoparticles with conductive polymers, we per-formed an ac impedance measurement during the charg-ing and discharging processes. Figure 5 shows Nyquistplots of the impedance data obtained of the lithium bat-teries with nanocrystalline LiFePO4 cathodes that hadfour different types of coating, and the data was obtainedwhen they are fully charged (100%; C100). From this im-pedance data, the series ohmic resistances (Rs) andcharge transfer resistances (Rct) can be estimated fromthe real part of the graph (Zreal, in-phase value; x-axis)[3, 19,27]. The Zreal value at the highest frequency (Rs)
corresponds to the total cell resistance of the electrodes,electrolyte, separator, and electrical contacts. The ohmicresistance (Rs) was about 5W for all the Li cells assembledwith the cathodes of nanocrystalline LiFePO4 coated withconductive materials, such as PEDOT, Ppy, or C, as wellas for the cathode of pristine LiFePO4 nanocrystals with-out any coating of conductive materials.The diameter of the semicircle in the high frequency
range is related to the charge transfer resistance (Rct) atthe interface of the crystalline LiFePO4 particles. Thecharge transfer resistances (Rct) are estimated to be 33W,65 W, 70 W, and 392W for the cathodes of nanocrystallineLiFePO4 coated with PEDOT, PPy, C, and pristineLiFePO4 without coating any conductive materials, re-spectively. The values of Rct for the nano-LiFePO4 elec-trode coated with conductive materials are much smallerthan that of the pristine nano-LiFePO4 electrode, and Rctfor the PEDOT-coated LiFePO4 nanocrystal electrode is
Fig. 4. A) The first charge and discharge curves recorded at a1 C rate for the cathode composed of nanocrystalline LiFePO4(a) coated with PEDOT(10 wt%), (b) coated with PPy(10 wt%),(c) coated with C(10 wt%), and (d) pristine particles. B) Thefirst charge and discharge curves recorded at a 1 C rate for thecathode composed of the PEDOT(10 wt%)-coated (a) nanocrys-talline LiFePO4, (b) micron-sized LiFePO4 crystals, (c) donut-shaped microstructure LiFePO4, (d) dumbbell-shaped micro-structure LiFePO4.
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Conductive Polymer-Coated LiFePO4 Nanocrystalline Cathodes
the smallest among the cathode materials with variouscoatings. The cathode composed of nanocrystallineLiFePO4 that was coated with PEDOT exhibits the lowestcombined value of Rs and Rct, and that is followed byPPy-coated, C-coated, and pristine LiFePO4 in the in-creasing order, which is consistent with the decreasingtrend of the specific capacities of the first dischargingprocess shown in Figure 4A. The charge transfer throughthe electrode-electrolyte interface apparently becomeseasier with the conductive films coated on the crystallineparticles.During reversible charge/discharge of the LiFePO4
cathodes, Li-ion is extracted from/inserted into the latti-ces forming a Li1xFePO4 composition with 0x1. Ny-quist plots of the impedance data of the Li cell assembledwith the cathode of nanocrystalline LiFePO4 coated withPEDOT during the discharging process from the fullycharged state (C100) to the fully discharged state (C0)are shown in Figure 6. The impedance values change con-tinuously depending on the state of charge (SoC) of thesame battery. The estimated series ohmic resistances (Rs)and charge transfer resistances (Rct) of the impedancedata indicate that the value of Rct increases continuouslyfrom 33W to 340W during the discharging process, whileRs is almost constant at about 5W. For cells assembledwith the pristine LiFePO4 nanocrystals, Rct increases from392W to 597 W during the discharging process, with thelimitation of discharging only half of the full range(LixFePO4; 00.5). The cathode of nanocrystallineLiFePO4 coated with PEDOT exhibits much smaller Rctvalues for the full range from C100 to C0 than the pris-tine LiFePO4 nanocrystals.In addition, the impedance data in the low frequency
region were analyzed to study the Li-ion diffusion at vari-ous SoCs during the discharging and charging processeswith the cathodes composed of the nano-crystalline
LiFePO4 coated with PEDOT. The impedances at lowfrequencies are expected to be dominated by the War-burg impedance, which is associated with the Li-ion diffu-sion through the crystalline LiFePO4 particles. A straightline with an angle of 458 with respect to the real (Z) axisis observed in the low frequency region (0.011 Hz) ofthe Nyquist plot (Figure 6). The diffusion coefficient ofthis cathode was calculated using the equation proposedby Ho et al. .
DLi 1=2 Vm=SFAdE=dx2 1
Where Vm is the molar volume of LiFePO4(44.11 cm3mol1), S is the contact area between the elec-trolyte and the cathode (1.961 cm2), F is Faradays con-stant (96485 C mol1), dE/dx is the variation of the open-circuit voltage (OCV) accompanying the change of theLi-contents (x) in the crystalline LixFePO4, and A can becalculated from the Warburg impedance. In the case ofsemi-infinite diffusion, both the real (Z) and imaginary(Z) parts of the Warburg impedance are proportional tow1/2 and the slope is A (Zw=Aw
1/2jAw1/2), which con-tains a concentration-independent diffusion coefficient[27,28]. Hence, the Li-ion diffusion coefficients (DLi+)were able to be calculated using equation 1, with parame-ters such as the estimated A values and voltage changesupon discharging (dE/dx) obtained from the analysis ofthe impedance data in the low frequency region at vari-ous SoCs.The diffusion coefficient (DLi+) calculated in the initial
fully charged state is 4.01010 cm2s1, which is consid-ered to be the value for Li0FePO4 nanocrystals. The diffu-sion coefficients decreased to 2.31013~4.61012 cm2s1 in the middle of the phase transition regionfrom Li0FePO4 to Li1FePO4 (voltage plateau; the inter-mediate SOCs). In the fully discharged region where the
Fig. 5. Impedance spectra (Nyquist plots) during the discharg-ing process for the lithium battery assembled with the nanocrys-talline LiFePO4 (a) coated with PEDOT, (b) coated with PPy, (c)coated with C, and (d) pristine particles. Frequency range: 1 Hz1 MHz.
Fig. 6. Impedance spectra (Nyquist plots) during the discharg-ing process from fully charged state (SoC 100%; C100) to fullydischarged state (SoC 0%; C0) for the lithium battery assembledwith the nanocrystalline LiFePO4 coated with PEDOT(10%).Frequency range: 1 Hz1 MHz.
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phase transition to LiFePO4 is completed, DLi+ again in-creased to 3.31010. Values of the diffusion coefficientare dependent on the state of charge and on the composi-tion of LixFePO4. In the monophase regions of the fullycharged (FePO4) and fully discharged (LiFePO4) states ofthe cathode, the diffusion coefficients are 23 orders ofmagnitude larger than those in the intermediate states(binary phases) of charge, which is in good agreementwith the results obtained with different techniques .Under the condition of PEDOT coating of this work, thediffusion coefficients both at the fully charged state andthe fully discharged state were able to obtain. Similarvalues of the diffusion coefficients in both regions of fullycharged and fully discharged states are speculated thestructural similarities between LiFePO4 and FePO4. Fur-ther in depth investigations of the diffusion process, mi-gration, and charge transfer mechanism are in progress aspart of our future work.Figure 7A contains the discharge specific capacities of
fifty completed charge/discharge cycles of the batteriesassembled with the various cathodes coated with conduc-tive materials: nanocrystalline LiFePO4 particles coatedwith PEDOT, with PPy, and with carbon, micron-sizedrhombus-shaped LiFePO4 particles coated with PEDOT,and hierarchical microstructures assembled with LiFePO4nanoparticles coated with PEDOT. The conductive poly-mer plays an important role in improving the battery per-formance. All the LiFePO4 particles and microstructurescoated with PEDOT reveal excellent cycling stability witha degradation rate of approximately 6%. Both the well-crystallized LiFePO4 and the conductive material coatingresult in excellent rate capabilities and cycling stabilities.The LiFePO4 nanoparticles coated with PEDOT revealthe highest specific capacities close to the theoreticalvalue. The hierarchical LiFePO4 microstructures with dif-ferent shapes coated with PEDOT exhibit somewhatsmaller specific capacities than the LiFePO4 nanoparti-cles, which reflect the smaller specific surface areas of thehierarchically assembled microstructures than the individ-ual nanoparticles. The excellent cycle-stability of theLiFePO4 nanoparticles coated with carbon is also ob-served under repeated charge/discharge cycling(152 mAhg1). The nanocrystalline LiFePO4-PPy exhibitsa high specific capacity during the first few cycles (about164 mAhg1), and the capacity fades rapidly with cycling(127 mAhg1 at the 50th cycle). The inferior cyclic stabili-ty with the PPy coating is ascribed to the degradation ofPPy at the higher biasing voltage region above 4 V.The amount of the conductive polymer coated on the
LiFePO4 nanoparticles is also an important factor thatneeds to be controlled. As shown in Figure 7B, the cycleperformances under various discharge/charge rates, rang-ing from 0.2 to 10 C with five cycles at each rate, werealso tested for the LiFePO4 nanocrystals coated with dif-ferent amounts of PEDOT 10 and 20 wt%. In general,the specific capacities decrease as the charge/dischargerate increases for the cathodes. The cathode composed ofthe LiFePO4 nanoparticles with 20% PEDOT coating ex-
hibited larger specific capacities than that with the 10%PEDOT coating. For example, the discharge capacity was190 mAhg1 at 0.2 C for the 20% PEDOT coating, and itdecreased to 124 mAhg1 at a high current rate of 10 C.The discharge capacity for a cathode with less coating(10%) was 167 mAhg1 at 0.2 C and 119 mAhg1 at arate of 10 C. Cathodes that were coated with morePEDOT exhibited larger specific capacities, and evenlarger values than the theoretical value of LiFePO4 at arate of 1 C or lower rates were observed. The best perfor-mance (190 mAhg1 at 0.2 C; 175 mAhg1 at 1 C) of thenanocrystalline LiFePO4 coated with PEDOT (20%)might be due to the synergistic effect of the inorganic andorganic electroactive materials, in addition to the compat-
Fig. 7. A) Specific capacities of lithium batteries assembledwith cathodes of the nanocrystalline LiFePO4 coated with (a)PEDOT (20%), (b) PEDOT (10%), (c) PPy (10%), and (d) C(10%). Specific capacities of the (e) dumbbell-shaped and (f)donut-shaped microstructure LiFePO4 that were coated withPEDOT. Specific capacities of the (g) micron-size LiFePO4 crys-tals coated with PEDOT (10%). All of the batteries were dis-charged at a 1 C rate for 50 discharge-charge cycles after the ini-tialization step consisting of the first 3 cycles at 0.1 C rate. B)The cycle stability test by sequentially increasing the charge-dis-charge rates from 0.2 C to 10 C for the nanocrystalline LiFePO4coated with (a) PEDOT (20%) and (b) PEDOT (10%). Fivecycles at each rate were recorded and the rate was returned tothe initial rate of 0.2 C for the last step.
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ibility of the redox characteristics of LiFePO4 andPEDOT. The specific capacity of the battery assembledwith the PEDOT-only film cathode was about 30 mAhg1
at 1 C. It is also worthy to note that it cannot be simplyexplained by the conductivity of PEDOT, which is rough-ly estimated to be more than 27 S cm1 based on theactual battery assembly in this work. The conductivepolymer also helps to improve the performance by in-creasing the ionic and electric conductivities along theinter-LiFePO4 particle paths within the cathode. Furtherinvestigation to find an optimal condition of the nano-sized LiFePO4-PEDOT seems promising and is in prog-ress.
In order to overcome the inherently low conductivity ofthe LiFePO4 cathode material, the crystallite particle sizeof LiFePO4 was minimized and these nanoparticles werecoated with conductive polymers. The nanocrystallineparticles with rods, plates, and rhomboid shapes wereformed and LiFePO4 microstructures hierarchically con-structed with these nanoparticles were also synthesizedby properly choosing the surfactant. Nanosized particleshave large surface-to-volume ratios and this is desirablefor lithium ions to have access to LiFePO4 during thecharge/discharge cycling of rechargeable batteries. Com-plete polymer coating on the LiFePO4, even on the roughsurfaces of the hierarchically assembled microparticles,was performed by simply dispersing the particles in poly-mer solutions. The combination of the highly conductiveand mechanically flexible conductive polymers, such asPPy and PEDOT, with LiFePO4 nanoparticles can offerlarger capacities because of the increased surface accessi-bility, improved diffusion pathways, and shortened Li+
diffusion lengths. In addition, the conductive polymers onthe surfaces of the isolated LiFePO4 particles can offervalid conductive networks in the electrode with reversibleredox behavior.The PEDOT-coated LiFePO4 exhibited excellent elec-
trochemical performance showing the full theoretical spe-cific capacity of LiFePO4. The nanocrystalline LiFePO4coated with PEDOT (20%) exhibited the best cycle per-formance with the largest specific capacity, which is evenlarger than the theoretical value of LiFePO4; 175 mAhg
at a rate of 1 C and 190 mAhg1 at the 0.2 C rate. Thecharge transfer resistances (Rct) estimated from the impe-dance analysis decrease significantly due to coating theparticles with conductive materials. The nanocrystallineLiFePO4 that was coated with PEDOT exhibits the small-est value of Rct, followed in increasing order by PPy-coated, carbon-coated, and pristine nanocrystallineLiFePO4 .
This work was supported by the Korea Research Founda-tion (20100029617).
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