Materials Research Bulletin 45 (2010) 844–849

High rate electrochemical performances of nanosized ZnO and carbon co-coatedLiFePO4 cathode

Yan Cui, Xiaoli Zhao, Ruisong Guo *

Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin University, Tianjin 300072, China

A R T I C L E I N F O

Article history:

Received 21 August 2009

Received in revised form 27 December 2009

Accepted 11 March 2010

Available online 17 March 2010

Keywords:

A. Inorganic compounds

A. Nanostructures

C. Impedance spectroscopy

D. Electrochemical properties

A B S T R A C T

The high rate electrochemical performances of ZnO and carbon co-coated LiFePO4 have been studied by

X-ray diffraction, high-resolution transmission electron microscope, electrochemical impedance

spectroscopy, cyclic voltammetry and galvanostatic measurements. The carbon coated LiFePO4 material

was prepared by a freeze-drying method, and the diffusion coefficient and exchange current of these

materials were calculated from their electrochemical impedance spectroscopy. The electrode delivered a

reversible capacity of about 90% of the theoretical capacity when cycled between 2.5 and 4.2 V and

showed stable cycle performance at high charge/discharge rates. This study showed that the co-coating

process and freeze-drying method can effectively improve the electrochemical performances of LiFePO4

materials.

� 2010 Elsevier Ltd. All rights reserved.

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1. Introduction

Lithium ion batteries have been successfully utilized in variousportable electronic devices and are considered as an idealcandidate for power sources in future electric vehicles [1]. Theolivine-type LiFePO4 is the most promising cathode material forlithium ion batteries because of its low cost, structure stability, ahigh reversible specific capacity (170 mAh g�1), a flat charge–discharge profile at intermediate voltage vs. Li/Li+ and a decentcycle life [2,3]. However, the main drawbacks of this material areits low electronic conductivity and slow lithium ion diffusion in thesolid phase, leading to the poor rate capability. Many approacheshave been developed to overcome this problem recently. One of theapproaches was to use fine LiFePO4 particles because they facilitatefast lithium extraction/insertion and enhance the ionic conduc-tivity. Another approach took advantage of the conductive coatingsuch as carbon on LiFePO4 to increase the electronic conductivity[4–6]. Doping with alien cations was another effective way toimprove the intrinsic conductivity of LiFePO4 [7,8]. In addition,several studies showed that coating the surface with electro-chemically inactive metal oxide significantly improved the cyclingstability and cell performance. For example, MgO [9,10] and La2O3

[11] were used to coat on the surface of LiCoO2 cathode materialand these coatings enhanced the cycling behavior. ZnO [12] wasused as coating of LiMn1.5Ni0.5O4 to improve its electrochemicalperformance. Chang et al. [13] reported that TiO2 coated LiFePO4

* Corresponding author. Tel.: +86 22 27404262; fax: +86 22 27404262.

E-mail address: rsguo@tju.edu.cn (R. Guo).

0025-5408/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2010.03.008

showed negligible capacity loss after 90 cycles at 1 C rate, while theuncoated LiFePO4 lost 9.0% of the capacity. Liu et al. [14] found thatthe surface modification of LiFePO4 with ZrO2 could greatlyenhance the cycle ability. The capacity of the coated LiFePO4

electrode remained at 143.4 mAh g�1 after 100 cycles at 0.1 C rate,which was higher than that of the uncoated sample (138 mAh g�1).

So far, sol–gel and freeze-drying have been widely used inpreparing composite powders with multi-element. Sol–gel processusually ensures the homogeneity of the element distribution in thereaction mixture, thus it needs only moderate conditions in termsof annealing temperature and time for the homogeneous precursorto react to form the desired composite after freeze-drying.Therefore it is expected that the particle size of the compositeprepared by this method can be greatly reduced. In the presentwork, carbon and ZnO co-coated LiFePO4 was synthesized by sol–gel and freeze-drying methods for the first time. The structure andelectrochemical properties of the synthesized material wereinvestigated.

2. Experimental procedure

Carbon coated LiFePO4 composite materials were first preparedby sol–gel and freeze-drying method. Fe (III) citrate was dissolvedin water by stirring at 62 8C for 1 h. During this process, oxalic acidwas added to the Fe (III) citrate solution with a ratio of 2:1 (w/w) tochelate with Fe (III). Oxalic acid also serves as the carbon sourceand has reducibility to prevent the Fe2+ from converting to Fe3+.H3PO4 and Li3PO4 were dissolved in water by stirring at 70 8C for1 h. The two clear solutions were then mixed together and stirred

Y. Cui et al. / Materials Research Bulletin 45 (2010) 844–849 845

continuously at 62 8C until the sol was formed. The molar ratios ofFe (III) citrate:H3PO4:Li3PO4 were 3:2:1. The sol was subjected tothe freeze-drying process for 48 h in Freeze-Dryer (LGJ-10). Theobtained xerogel was then calcined at 600 8C for 7 h in a reductiveatmosphere (5% of hydrogen in nitrogen). After ground with amortar and pestle, the resulted powders were sieved through the280-mesh screen.

A chemical precipitation method was used to coat the carboncoated LiFePO4 with ZnO. In a typical experiment, the carboncoated LiFePO4 powder was dispersed in ethanol by sonication for0.5 h while continuously stirring at room temperature. Two dropsof dilute sulfuric acid was added to the suspension to createactivated sites on the LiFePO4 surface. A 0.1 M ZnCl2 solution wasadded drop by drop to the suspension, followed by stirring for 2 h.Zn(OH)2 precipitation was formed by adding the stoichiometricsodium hydroxide dropwise. The mixture was continuously stirredfor 8 h, then was filtered and washed with distilled water for 3times until pH was 7. The precursor was dried at 60 8C for 4 h andthen heated to 400 8C for 2 h in a reductive atmosphere (5% ofhydrogen in nitrogen) to form a thin coating layer consisted of ZnOand carbon on the surface of LiFePO4 particles. The synthesizedpowder is abbreviated as f-ZnO/C-LFP. For comparison, the ZnOand carbon co-coated LiFePO4 that was subjected to regular dryingwas also prepared, and it is designated as d-ZnO/C-LFP. In addition,the samples uncoated with ZnO are named as f-LFP/C and d-LFP/C,respectively. The abbreviations of the synthesized powders arelisted in Table 1. The carbon contents were 9.3% in all the samples,determined by dissolving the composite powder into HCl. Themeasurement procedure can be found in Ref. [15]. The weightpercent of ZnO was 2%.

The crystallographic structural characterization was performedby X-ray powder diffraction (XRD) with a Cu Ka radiation source(l = 0.15406 nm). The morphology and the surface texture of thecrystal were characterized by a high-resolution transmissionelectron microscope (HRTEM) (Philips, Tecnai G2 F20) equippedwith Energy Dispersive Spectroscopy (EDS). The specific surfacearea measurements were performed on automated surface areaanalyzer (Quantachrome BET Nova 2000).

The cathode was prepared by mixing active powders, acetyleneblack and polyfluorotetraethylene (PTFE) in a weight ratio of 80:15:5in pure ethanol to form a paste. After rolled to a thin film of 140 mmin thickness, it was cut into circular discs (0.785 cm2) as cathodes.The coin cells were assembled in an argon filled glove box. The ZnO/C-LFP electrode was used as cathode and lithium metal as anodeseparated by a Celgard 2400 separator. The electrolyte was 1 mol/LLiPF6 in a 1:1:1 mixture of ethylene carbonate/dimethyl carbonate/diethyl carbonate. The cells were charged and discharged galva-nostatically between 2.5 and 4.2 V (vs. Li/Li+) using a Neware batterytester (Neware Company, Shenzhen, China). The cyclic voltammetry(CV) curves were obtained at 0.1 mV s�1 within the range of 2.5–4.2 V. The electrochemical impedance spectroscopy (EIS) measure-ments were performed on the electrochemistry workstation(CHI660C, Shanghai Chenhua Instrument Ltd, China). The amplitudeused for impedance measurement was 5 mV and the frequency wasfrom 0.1 Hz to 100 kHz. All the electrochemical measurements werecarried out at 25 8C.

Table 1The abbreviations of the synthesized powders.

Powder Drying methods Abbreviation

ZnO and carbon co-coated

LiFePO4 powders

Freeze-drying f-ZnO/C-LFP

ZnO and carbon co-coated

LiFePO4 powders

Regular drying d-ZnO/C-LFP

Carbon coated LiFePO4 powders Freeze-drying f-LFP/C

Carbon coated LiFePO4 powders Regular drying d-LFP/C

3. Results and discussion

3.1. X-ray diffraction

The X-ray diffraction patterns of standard LiFePO4 and f-ZnO/C-LFP sample are shown in Fig. 1. All diffraction patterns can beindexed to the ordered orthorhombic olivine crystal structure(space group: Pnma). This olivine structure with PO4 tetrahedraand distorted FeO6 octahedra produces a two-dimensional path-way for lithium ion diffusion. The Li ions occupy the octahedralsites (4a), Fe ions locate at the octahedral sites (4c) and P ions are inthe tetrahedral sites (4c) [16]. No peaks corresponding to carbonand ZnO were observed, indicating that they are amorphous or oflow crystallinity in the sample. The refined cell parameters forthe f-ZnO/C-LFP sample are a = 1.0346 nm, b = 0.6013 nm,c = 0.4698 nm, and V = 0.2923 nm3. The refined cell parametersare approximately equal to those of the standard LiFePO4, implyingthat the carbon and ZnO co-coatings did not change the crystalstructure of LiFePO4. The calculated crystallite size based on the(3 1 1) diffraction peak is 48.267 nm by the Scherer formulab cos(u) = kl/D, where b is the peak width at half-height (35.520),l is the wave length (0.154 nm), and k is a constant (0.89). The(3 1 1) peak in the XRD pattern has been marked in Fig. 1.

3.2. Surface morphology

HRTEM coupled with EDX analyses were carried out to identifythe possible existence of impurities and the results are shown inFig. 2. It is apparent from the Fig. 2a that the f-ZnO/C-LFP particlesare spherical and the particle size distribution is uniform. Theparticles have an average diameter of about 50 nm, which is ingood agreement with the XRD analysis. The specific surface areasof the four samples are listed in Table 2. The sample of f-ZnO/C-LFPshows the highest specific surface area of 37.92 m2 g�1 amongthese four samples, indicating that it has the smallest particle sizeof 44 nm. The particle size is calculated from the formula D = 6/rSw, where D is the particle size, r is the density and Sw is thespecific surface area. Bulk gravity, is the so-called tap density, is aparameter to take into account for practical applications. Thehigher tap density lead to higher energy density. It is important forpractical applications. The tap density of f-ZnO/C-LFP is0.83 g cm�3, about a quarter of 3.6 g cm�3, the theoretical densityof the compound. The relatively low temperature and shortsoaking time of firing, in addition to the large fraction of carbon,

Fig. 1. The X-ray diffraction patterns of standard LiFePO4 and synthesized f-ZnO/C-

LFP.

Fig. 2. HRTEM image and EDX spectrum of f-ZnO/C-LFP powders. (�) Area selected for EDX analysis and electron diffraction pattern.

Y. Cui et al. / Materials Research Bulletin 45 (2010) 844–849846

result in low value of bulk gravity. Electron diffraction patternindicates that the LiFePO4 particles are crystalline (Fig. 2b),whereas the ZnO and carbon coatings were too thin to be detectedwhether they are crystalline or amorphous. No impurity wasdetected based on the result of the EDX spectrum shown in Fig. 2c,except Cu which was introduced from the copper grid for TEMstudy. The carbon generated from the decomposition of oxalic acidwas detected.

3.3. Electrochemical performance

Fig. 3 shows the charge/discharge capacities in the first cycle forLiFePO4 electrodes at 0.2 C (current density 0.285 mA cm�2). Boththe f-ZnO/C-LFP and d-ZnO/C-LFP have a flat discharge plateau. Thepolarization of the f-ZnO/C-LFP between the charge and discharge

Table 2The specific surface area of all the samples.

Sample Specific surface area (m2 g�1)

f-ZnO/C-LFP 37.92

d-ZnO/C-LFP 24.49

f-LFP/C 21.47

d-LFP/C 11.02

plateau was lower than that of d-ZnO/C-LFP, indicating the betterreversibility of the sample f-ZnO/C-LFP. The charge/dischargecapacities of f-ZnO/C-LFP were slightly higher than that of d-ZnO/C-LFP at the initial cycles, but the difference in capacity became

Fig. 3. The initial charge/discharge capacities of LiFePO4 electrodes at 0.2 C.

Fig. 4. The discharge capacities vs. cycle numbers of all the samples at different

rates. Fig. 5. Cyclic voltammograms of f-ZnO/C-LFP and d-ZnO/C-LFP electrodes.

Fig. 6. The impedance spectra of all the samples.

Y. Cui et al. / Materials Research Bulletin 45 (2010) 844–849 847

more and more obvious with the continuous cycling as shown inFig. 4. The capacities of all electrodes gradually increased in theinitial cycles. This was due to the incomplete dispersion of theelectrolyte into the electrode materials at the beginning. The orderof the electrochemical performance of these four samples is f-ZnO/C-LFP, d-ZnO/C-LFP, f-LFP/C and d-LFP/C. It is worth noting that d-ZnO/C-LFP has a better electrochemical performance than f-LFP/C,implying that the co-coating is a more effective way to improve theelectrochemical performances. The capacity retention remainedvery good at all the test rates, particularly sample f-ZnO/C-LFPmaintained the maximum discharge capacities of 147.7 mAh g�1

and 138.5 mAh g�1 at 1 C and 2 C rates, respectively. The excellentrate capacity and cycle performance were attributed to thefollowing reasons. Firstly, the sample of f-ZnO/C-LFP has a relativelower particle size owing to the freeze-drying process and the co-coating method. Therefore, the distance of the lithium ion diffusionpathway is decreased. The shorter diffusion length facilitates fasterLi+ insertion/extraction, and accordingly elevates the high ratecapacity. Secondly, the ZnO and carbon forms an integrated andcontinuous conducting coating on the surface of the LiFePO4

particles. This coating acts as a protective layer to prevent LiFePO4

particles from direct contact with the electrolyte and hencereduces the erosion of LiFePO4. Finally, the existence of ZnO andcarbon nano-layer can effectively prevent the fatigue of theparticles during the continuously cycling and keep the structuralstability. Thus, the cycle performance is prominently improved.

3.4. Cyclic voltammetry

Fig. 5 is the cyclic voltammogram of f-ZnO/C-LFP and d-ZnO/C-LFP electrodes. It can be seen that the oxidization peak and thereduction peak are highly symmetric to each other. For the samplef-ZnO/C-LFP, an oxidation peak is located at 3.60 V and thecorresponding reduction peak at 3.30 V. The separation betweenthe anodic and cathodic peaks is 0.30 V, which is lower than that ofd-ZnO/C-LFP. In addition, the peak currents of f-ZnO/C-LFP aremuch higher than those of d-ZnO/C-LFP and the ratio of Ipa/Ipc

(anode peak current/cathode peak current) is close to 1. Theseimply the outstanding reversibility of lithium intercalation intoand de-intercalation from f-ZnO/C-LFP. Furthermore, the initialthree cyclic voltammogram curves of f-ZnO/C-LFP electrode(shown in the inset of Fig. 5) are almost overlapping, demonstrat-ing that the smaller LiFePO4 particle size and continuousintegrated conductive layer between LiFePO4 particles result innegligible polarization during cycling.

The sharp oxidation and reduction peaks reveal that stronglithium intercalation and de-intercalation reactions occur for f-ZnO/C-LFP samples. In an electrochemical process of LiFePO4

electrode, the anodic process corresponds to the lithium ion de-intercalation from LiFePO4 bulk with the simultaneous transport ofan electron from the electrode to the current collector, while thecathodic process corresponds to the Li+ intercalation into FePO4,acquiring an electron from the current collector. The reaction isLiFePO4$ FePO4 + Li+ + e. The discussion of the diffusion coeffi-cient of lithium ions (D) will be presented in the following section.

3.5. Electrochemical impedance spectra

The electrochemical impedance spectra of all the samples areillustrated in Fig. 6. Each spectrum shows an intercept at highfrequency for the resistance of the electrolyte ‘‘Re’’ on the real axisZre, followed by a semicircle in the high-to-middle frequency rangeand an inclined line in the low frequency range. The inclined line isattributed to the diffusion of the lithium ions into the bulk of theelectrode material, the so-called Warburg diffusion. The impe-dance spectra can be described by the equivalent circuit presentedin the inset picture, where Rct represents the charge transferresistance, Rw and the Warburg element (Ws) represent theWarburg impedance and the constant phase element CPE1

represents the double layer capacitance.

Table 3Impedance parameters of the samples.

Sample Re (V) Rct (V) sw (V cm2 s�0.5) D (cm2 s�1) s (S cm�1) i0 (mA cm�2)

f-ZnO/C-LFP 4.3 195.7 28.2 6.1�10�11 9.2�10�5 1.2�10�4

d-ZnO/C-LFP 5.4 256.7 37.5 3.4�10�11 6.9�10�5 9.2�10�5

f-LFP/C 6.2 341.9 50.2 1.9�10�11 5.2�10�5 6.9�10�5

d-LFP/C 8.7 432.8 155.4 2�10�12 4.1�10�5 5.4�10�5

Y. Cui et al. / Materials Research Bulletin 45 (2010) 844–849848

All the parameters of the equivalent circuit are recorded inTable 3. The plot of Zre vs. the reciprocal root square of the lowerangular frequencies (v�1/2) is shown in Fig. 7. The linearrelationship of the fitted line is governed by equation:

Zre ¼ Re þ Rct þ swv�0:5 (1)

where v is the angular frequency in the low frequency range. Theslope of the fitted line is the Warburg coefficient sw. Among all thesamples, sample f-ZnO/C-LFP has the lowest Warburg coefficient(sw) of 28.2 V cm2 s�0.5. The diffusion coefficient values of thelithium ions (D) were calculated from the equation:

D ¼ 0:5RT

AF2swC

� �2

(2)

where R is the gas constant (8.314 J mol�1 K�1), T is thetemperature (298.5 K), A is the area of the electrode surface, F isthe Faraday’s constant (96,500 C mol�1) and C is the molarconcentration of Li+ ions. The sample f-ZnO/C-LFP shows thehighest lithium ion diffusion coefficient of 6.1 � 10�11 cm2 s�1,three orders of magnitude higher than that of the pure LiFePO4

(1.8 � 10�14 cm2 s�1) [17]. The enhancement of the diffusioncoefficient is mainly attributed to the shorter diffusion path in f-ZnO/C-LFP particles. In addition, the conductivity (s) werecalculated from the equation:

s ¼ 1=Rct

t=A(3)

where t is the thickness of the electrode [18]. It is found that thevalue of Rct for the f-ZnO/C-LFP sample is lower than that of others.Accordingly, the conductivity increases from 4.1 � 10�5 S cm�1 ofLFP/C to 9.2 � 10�5 S cm�1 of f-ZnO/C-LFP. The improvement of theconductive behavior is obvious. Since the f-ZnO/C-LFP sample hasthe smallest particle size, the diffusion length of Li+ is muchshorter, which facilitates fast Li+ insertion/extraction and improves

Fig. 7. The relationship between Zre and v�1/2 at low frequencies.

lithium ion diffusion rate. Accordingly, the carrier concentration isgreatly enhanced, so is the conductivity of f-ZnO/C-LFP.

The exchange current density is a parameter to indicate thereversibility of the electrode. The exchange current density (i0) ofthe synthesized LiFePO4 was calculated from equation:

i0 ¼ RT

nFRct(4)

where n is the number of electrons transferred per molecule duringthe intercalation and is 1 for LiFePO4. It can be seen that sample f-ZnO/C-LFP has the highest i0 among all the four samples, implyingits best reversibility, which agrees well with the CV results.

4. Conclusion

ZnO and carbon co-coated LiFePO4 particles were synthesizedby sol–gel process and freeze-drying method. HRTEM analysisshowed that the f-ZnO/C-LFP particles were spherical and particlesize distribution was uniform, with an average particle size ofabout 50 nm. The f-ZnO/C-LFP powder had the highest specificsurface area of 37.92 m2 g�1, indicating that the co-coated methodand freeze-drying process can remarkably reduce the particle size.The small particle size enhanced the Li+ diffusion capability due tothe short diffusion path of lithium in LiFePO4 particles. Conse-quently, the lithium ion diffusion coefficient of f-ZnO/C-LFPcalculated from the EIS was much higher than that of othersamples. Sample f-ZnO/C-LFP also had the lowest Warburgcoefficient sw and highest current exchange density i0 among allthe samples, which were beneficial to its electrochemicalperformances, including the discharge capacity, rate capabilityand cycle performance. The maximum discharge capacities of147.7 mAh g�1 and 138.5 mAh g�1 at 1 C and 2 C rates have beendemonstrated by f-ZnO/C-LFP. It also exhibited good cyclability atdifferent charge/discharge rates.

Acknowledgements

Professors Liang Guangchuan (School of Materials Science andEngineering, Hebei University of Technology) and Yuan Xubo(School of Materials Science and Engineering, Tianjin University)are gratefully appreciated for their helps.

This work is supported by the National Natural ScienceFoundation of China (50872090).

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