synthesis of lifepo[sub 4] nanoparticles in polyol medium and their electrochemical properties

Electrochemical and Solid-State Letters, 9 �9� A439-A442 �2006� A439

Synthesis of LiFePO4 Nanoparticles in Polyol Medium andTheir Electrochemical PropertiesDong-Han Kim* and Jaekook Kim**,z

Department of Materials Science and Engineering, Chonnam National University, Bukgu, Gwangju500-757, South Korea

LiFePO4 nanoparticles were synthesized using the polyol process without any further heating as a postprocessing step. The X-raydiffraction patterns of the sample exhibited a good fit with the orthorhombic phase with no unwanted impurity phases. TheLiFePO4 nanoparticles showed a reversible capacity of 166 mAh/g, which amounts to a utilization efficiency of 98%, with anexcellent reversibility in extended cycles. The electrode shows an excellent capacity retention at high-rate current densities due toits single-crystal-like and monodispersed uniform morphology with orthorhombic shape with an average width of 20 nm andlength of 50 nm.© 2006 The Electrochemical Society. �DOI: 10.1149/1.2218308� All rights reserved.

Manuscript submitted November 7, 2005; revised manuscript received May 25, 2006. Available electronically July 11, 2006.

1099-0062/2006/9�9�/A439/4/$20.00 © The Electrochemical Society

LiFePO4 has recently received a great deal of attention due to itspotential use as a next-generation cathode in rechargeable lithiumion batteries. Padhi et al. introduced LiFePO4 as a viable alternativeto some of the transition metal oxides that are currently used incommercial batteries.1 LiFePO4 has many advantages comparedwith conventional cathode materials such as LiCoO2, LiNiO2, andLiMn2O4, namely, it is environmentally benign, inexpensive, andthermally stable in the charged state.2-4

In particular, LiFePO4 shows high thermal stability at high tem-perature because of its structure, in which the oxygen ions form ahexagonal close-packed �hcp� arrangement.1 The metal �Fe� ionsform zigzag chains of octahedrons in alternate basal planes bridgedby the tetrahedral phosphate groups �PO4�. The lithium atoms oc-cupy the octahedral sites, which are located in the remaining basalplanes. The strong covalent bonding between the oxygen and P5+

ions forming �PO4�3− units allows for the greater stabilization of thestructure compared to that observed in layered oxides, such asLiCoO2, LiNiO2, and LiMn2O4, where the oxide layers are moreweakly bound.1 This strong covalency stabilizes the antibondingFe3+/Fe2+ state through a Fe–O–P inductive effect, with the resultthat the oxygen atoms are a lot harder to extract. In addition,LiFePO4 has a high theoretical capacity of 170 mAh/g, good cycla-bility with a retention of 80% during 2000 cycles, and a flat dis-charge potential of 3.4 V vs Li/Li+.

However, the olivine LiFePO4 has the disadvantages of low elec-tronic conductivity and low lithium diffusivity. The poor rate capa-bility of LiFePO4 cathodes makes it difficult to make full use ofthem in lithium ion batteries unless modifications are made to im-prove the low electronic conductivity and slow lithium ion diffusionacross the LiFePO4/FePO4 interface.5 Improving the rate perfor-mance of the LiFePO4 cathode involves enhancing its ion/electronicconductivity by suitable preparation procedures, which can be ac-complished in two ways. The first strategy was initially suggested byArmand and co-workers, who reported an improvement in the kinet-ics of the electrochemical reaction following the addition of a con-ductive substance during the synthesis.6 The other strategy is theachievement of a small and homogenous particle size distribution.This strategy is particularly attractive when the goal is to achievefaster lithium ion diffusion and, hence, a higher rate capability inlithium cells.

In order to synthesize fine and homogenous nanoparticles ofLiFePO4, various synthetic methods have been tried under differentconditions, including coprecipitations in aqueous medium, the sol-gel method, and techniques performed under hydrothermal condi-tions with various reactants and mechanochemical activation.7-12

* Electrochemical Society Student Member.** Electrochemical Society Active Member.

z E-mail: [email protected]

address. Redistribution 128.143.23.241Downloaded on 2013-11-12 to IP

However, post heat-treatment processing at high temperature is nec-essary in order to obtain fully crystalline LiFePO4, which may resultin unwanted particle growth which is detrimental to the characteris-tics of the low electrically conductive LiFePO4 electrode. For ex-ample, the solid-state reaction requires heat-treatment and repeatedregrinding at high temperatures �500–800°C� for several hours�12–48 h�, in order to synthesize LiFePO4 with high crystallinity inflowing inert gas. Overall, LiFePO4 must have a small particle sizewith a uniform size distribution and high crystallinity, in order for itto have the excellent electrochemical properties required for its useas a next-generation cathode material. Therefore, the high cost andcomplexity of the synthetic procedures described above motivate thedevelopment of a cheaper and simpler method of preparing LiFePO4nanoparticles with high crystallinity.

In this regard, we adopted the polyol process as a new method ofsynthesizing LiFePO4 with the goal of obtaining well-defined nano-particles with high crystallinity. The polyol process consists of thereduction of metallic compounds such as oxides and salts in a liquidalcohol medium maintained at its boiling point.13 The polyol me-dium itself acts not only as a solvent in the process but also as astabilizer, limiting particle growth and prohibiting agglomeration.14

Especially, the polyol process provides a reducing environment,which is extremely advantageous for synthesizing LiFePO4, becauseit is very difficult to obtain divalent iron containing LiFePO4.

In this work, we report a new method of synthesizing LiFePO4having an olivine structure in the form of monodispersed nanopar-ticles with high crystallinity using the polyol process at low tem-perature without any further heating. The objective was to introducea new synthesis method to obtain nanoparticles and better under-standing of its electrochemical properties.

Experimental

Iron acetate �Fe–�CH3COO�2�, ammonium dihydrogen phos-phate �NH4H2PO4�, and lithium acetate �Li–CH3COO� were addedto tetraethylene glycol �TTEG� in a stoichiometric molar ratio�1:1:1�. The solution was heated at 335°C for 16 h in a round-bottom flask attached to a refluxing condenser. In order to removethe TTEG and partial organic compounds, the resulting solutioncontaining nanoparticles was washed with acetone several times.The resulting particles were separated by filtering using ceramicmembrane funnels. To evaporate the water absorbed from the atmo-sphere, the nanoparticle powder was dried in a vacuum oven at150°C for 24 h.

The crystalline nature of the LiFePO4 nanoparticles was charac-terized by X-ray diffraction. The particle morphology and size wasobserved by field emission transmission electron microscopy�TEM�. The electrochemical properties of the LiFePO4 nanopar-ticles were evaluated using lithium metal as the reference electrode.For the electrochemical measurements, the LiFePO4 materials weremixed with 30 wt % carbon black and polytetrafluoroethylene

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A440 Electrochemical and Solid-State Letters, 9 �9� A439-A442 �2006�A440

binder. This mixture was pressed onto a stainless steel mesh anddried under vacuum at 180°C for 5 h. The cell consisted of a cath-ode and lithium metal anode separated by a glass fiber. The electro-lyte used was a 1:1 mixture of ethylene carbonate �EC� and dimethylcarbonate �DMC� containing 1 M LiPF6.

Results and Discussion

Figure 1 shows the Rietveld-refined X-ray diffraction �XRD� pat-tern of the sample synthesized using the polyol process. All of thepeaks are indexed on the basis of an orthorhombic olivine-typestructure �space group: Pnma�. As shown, the fitting between thecalculated and measured data is excellent. No second phase wasfound. The unit cell parameters for the orthorhombic cell were a= 10.310, b = 5.994, and c = 4.689 Å. These values are slightlylower than those reported in the literature,1,5 probably due to thesample’s single-crystal-like characteristics. The primary particlesize, d, was calculated from the X-ray line width using the Scherrerformula, d = 0.9 �/�1/2 cos �, where � is the X-ray wavelength, �1/2is the corrected width of the main diffraction peak at half-height,and � is the diffraction angle. The d value of the sample synthesizedusing the polyol process was 40 nm. The XRD result demonstratesthat the polyol process allows for the synthesis of single-phaseLiFePO4 with no unwanted impurity phases, such as Li3PO4 and Fecompounds. Conventional synthetic methods, even though they aregenerally carried out under carefully controlled conditions, result inthe unwanted presence of impurity phases consisting of Fe2O3 andLi3Fe2�PO4�3 in the majority of cases.9,15-17 From this point of view,it can be concluded that the polyol process simplifies the preparationof LiFePO4 nanoparticles.

The capacity loss observed is known to be caused by the utiliza-tion of large particles constrained by their small surface area andthe diffusion limit of lithium ions through the decreasingLiFePO4/FePO4 interface, as described by Padhi et al., who sug-gested that the electrochemical performance of the cathode can beimproved by optimizing the powder characteristics of the LiFePO4,so that small particles with a uniform size distribution need to beproduced. In order to determine the particle size and distribution ofthe sample, field-emission TEM was used. Figure 2a and b show thefield-emission TEM images in low and high magnification, respec-tively, of the nanocrystalline LiFePO4 in the monodispersed state. Itis apparent that the LiFePO4 nanoparticles which were prepared bythe polyol process, without any further heating, exhibit high crystal-linity. The nanoparticles show orthorhombic shape morphology.

Figure 1. The Rietveld-refined XRD pat-tern of the sample synthesized using thepolyol process.


Figure 2. �a� The TEM image of the sample synthesized using the polyolprocess. �b� The FETEM magnified image of one single nanoparticle in �a�.




A441Electrochemical and Solid-State Letters, 9 �9� A439-A442 �2006� A441

Their orthorhombic crystalline shape was confirmed by tilting theimage, which allows the observation of the variations of latticealignment together with corresponding crystallographical directionchanges. The nanocrystals show relatively uniform morphology withan average width of 20 nm and a length of 40 nm of a very narrowparticle size distribution, as shown in Fig. 2b. The interplanar dis-tance is estimated to be 2.813 Å, which is in good agreement withthe �301� planes of the olivine LiFePO4, suggesting that the pre-pared nanoparticles were grown in �100� direction. This result isparticularly interesting because the established, solution-based syn-thetic methods generally produce either poorly crystalline nanopar-ticles or much larger sized particles, with diameters at least on themicro scale, following the postheating process, which is necessaryin order to obtain a desirable phase with sufficient crystallinity. Thepolyol process used in this study has a clear advantage over othermethods in the production of highly crystalline nanoparticles. Fur-thermore, the nanocrystalline olivine LiFePO4 provides a relativelylarger area of �010� plane at its surface to facilitate an easy lithiumion diffusion through 1D �010� direction, which is clearly beneficialto improve its high rate performance. The elemental analysis usinginductively coupled plasma atomic emission spectrometry confirmedthe 1:1:1 molar ratio of Li, Fe, and P.

Figure 3 shows the voltage profile of the carbon-mixed LiFePO4cathode in the first cycle at a current density of 0.1 mA/cm2 in thevoltage range of 2.5–4.0 V. The observed reversible first dischargecapacity of 166 mAh/g, which amounts to a utilization efficiency of98% of its theoretical capacity, is comparable to that of highly con-ductive LiFePO4 reported in previous studies.7 Figure 4 shows thecyclability of the LiFePO4 during 50 cycles at a current density of0.1 mA/cm2 in the voltage range of 2.5–4.0 V. The reversible ca-pacity of the LiFePO4 cathode prepared in the present study remainsat approximately 163 mAh/g from the 2nd to 50th cycle, showingexcellent cyclability. This can be attributed to the structurally fullycrystalline nanoparticles, which are quite stable and mechanicallyrobust. The high rate performance of the sample is shown in Fig. 5.It seems that the nanocrystalline nature of LiFePO4 prominentlyassists the electrode to exhibit an excellent rate performance. Alsoworth noting is that the capacity retention rate at high rate of 30 and60 C is remarkable, because no surface treatment on a sample withelectrically conducting substances such as carbon was performed onthe electrode materials.

It is crucial to obtain a uniform nanoscale-size distribution withhigh crystallinity to overcome the low electronic conductivity andlow lithium diffusivity of LiFePO4. We believe that the cathodematerial consisting of LiFePO4 nanoparticles with high crystallinitysynthesized using the polyol process allow the diffusion ions to have

Figure 3. The initial discharge profile of the sample synthesized using thepolyol process at a current density of 0.1 mA/cm2 in the voltage range of2.5–4.0 V.


a shorter path through the lattice from the core of the particles to thesurface, which leads to excellent electrochemical properties withgood capacity retention.

Conclusion

LiFePO4 nanoparticles were prepared from the polyol medium ofFe–acetate �Fe–�CH3COO�2�, ammonium dihydrogen phosphate�NH4H2PO4�, and Li–acetate �Li–CH3COO� by refluxing them at335°C for 16 h. The sample showed orthorhombic shape with anaverage width of 20 nm and a length of 40 nm with a monodis-persed state and high crystallinity. The XRD pattern was indexed onthe basis of an orthorhombic olivine structure type �space group:Pnma�. The LiFePO4 sample showed an initial capacity of166 mAh/g at a current density of 0.1 mA/cm2, which amounts to autilization efficiency of 98%. In the cyclability test, the LiFePO4showed a reversible capacity of 166 mAh/g in the first cycle andapproximately 163 mAh/g from the 2nd to 50th cycle. Especially, itexhibits an excellent rate performance at high current rates of 30 or60 C by providing around 58 and 47% of capacity retention, respec-tively. The orthorhombic-shaped nanoparticle resembled its buildingblock of LiFePO4 unit cell, indicating that about 40 unit cells arealigned three-dimensionally to each direction. Especially, this mor-phological characteristic allows the accommodation of fasile lithium

Figure 4. Cyclability of the sample at a current density of 0.1 mA/cm2.

Figure 5. The rate performance of the sample at 1–60 C rates. The samplewas cycled 3 times at each rate and forwarded to the next step.




A442 Electrochemical and Solid-State Letters, 9 �9� A439-A442 �2006�A442

diffusion through 1D �010� direction by exposing larger area of fa-vorable crystallographical plane of �010�.

Most solution-based synthetic methods produce either poorlycrystalline nanoparticles or much larger sized particles, with diam-eters at least on the micro level, due to the use of a post-heat-treatment step. However, the polyol process used in this work en-ables the production of nanoscale powder with high crystallinity,while preventing the formation of unwanted impurities by providingeffective reducing environment. In addition, we believe that thepolyol process has a powerful advantage in that it allows for thepreparation of LiFePO4 nanoparticles without any further heating asa postprocessing step, thereby rendering it less expensive and easierto carry out.

Chonnam National University assisted in meeting the publication costs ofthis article.

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synthesis of lifepo[sub 4] nanoparticles in polyol medium and their electrochemical properties

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