Synthesis and characterization of Cu-doped LiFePO4 with/without carbon coating for cathode of lithium-ion batteries

Cesario AJPI, Giovana DIAZ,* Heidy VISBAL** and Kazuyuki HIRAO***,³

Laboratorio de Química del Estado Sólido, Instituto de Investigaciones Química, La Paz, Bolivia*Instituto de Investigaciones Metalúrgicas y de Materiales, La Paz, Bolivia**KRI Inc, Kyoto Research Park, 134 Chudoji Minami-machi, Shimogyo-ku, Kyoto 600–8813, Japan***Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Japan

The Cu-doped LiFePO4 with/without carbon coating was synthesized by sol­gel method. The material was characterized byX-ray diffraction, Raman spectroscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy and its electrochemicalproperties were investigated by charge­discharge tests. For the carbon coated sample presents a high uniformity and a particlesize around 50­100 nm. Electrochemical tests show that the specified capacity was 102mAh/g measured 0.25C (1.5mA) regimedischarge rate, voltage range (2.5­3.7V). Optimization of this composite would be required, but the Cu doping and carboncoating to LiFePO4 showed a promising material for use as cathode material in lithium ion battery.©2013 The Ceramic Society of Japan. All rights reserved.

Key-words : Lithium-ion batteries, Sol–gel synthesis, Cathode material, Lithium iron phosphates, Ion doping

[Received January 15, 2013; Accepted March 4, 2013]

1. Introduction

Lithium iron phosphate (LiFePO4) has attracted a great dealof attention for its applications as cathode materials for next-generation Li-ion batteries because of a relatively large theoret-ical capacity (170mAh/g), cost-effectiveness, and high cyclingstability.1) However, its low electronic conductivity (10¹9­10¹10 S cm¹1) and low lithium ion diffusivity led to a poor ratecapability that has limited its wide application.2) In order toincrease the charge capacity and to improve the conductivity ofthis material, many works on its synthesis have been performed,using different approaches as solid-state reaction,3)­5) co-precip-itation,6) hydrothermal synthesis,7) solution,8) emulsion drying.9)

To improve Li+ ion diffusion some authors have coated with anelectron-conductive layer as conductive carbon,10)­12) dispersed ametal nanosized Cu and Ag.13) Also doping the structure withions such as Mg+2, Ti+4 and others have been reported.14),15)

Croce et al.13) report the preparation and electrochemical per-formance of kinetically improved Cu-doped LiFePO4 compositecathode materials. The added Cu metal powders do not affectthe structure of LiFePO4 but clearly improve its kinetics in termsof capacity delivery and cycling life due to a reduction of theparticle size and an increase of the bulk intra- and inter-particleelectronic conductivity of LiFePO4.The objective of this study is to prepare copper (Cu)-doped

LiFePO4 with/without carbon coating by sol­gel method andevaluate its properties to look the potential of this material ascathode of lithium ion battery.

2. Experimental

2.1 SynthesisCu-doped LiFePO4 was synthesized via sol­gel method.

CH3COOLi·2H2O, FeCl2·4H2O, H3PO4, and CuCl2 were used

as reactants. For the sample with carbon coating, a mixture ofPEG and D-fructose was used as carbon source.Initially, solutions of 1M H3PO4 and 1M FeCl2·4H2O dis-

solved in ethanol were prepared and stirred for approximately 3 h.After addition of 1M CH3COOLi·2H2O and 1M CuCl2 dissolvedin ethanol, the mixture was stirred for 2 h at rt. For the samplewith carbon coating, PEG/fructose (1:1 weight ratio) mixture10% w/w was mixed in the previous step and then the mixturewas stirred for 2 h. For both samples the resulting mixture wascalcined at 700°C in an argon atmosphere for 5 h.

2.2 Analysis techniquesThe Cu-doped LiFePO4 was characterized by X-ray diffrac-

tion (XRD), Raman spectroscopy, Energy Dispersive Spectros-copy (EDS) and Scanning Electron Microscopy (SEM). TheSEM observation was performed on a JSM-6700F, the EnergyDispersive Spectroscopy (EDS) on a JSM-6700F JET 2300 andthe X-ray diffraction was carried out under the following con-ditions: CuK¡ radiation, 40 kV and 40mA.The charge and discharge measurements were conducted

using two-electrode coin-type cell (CR2016) of Li«EC:PC:LiPF6(1:1:1 in volume ratio)«LiFePO4.The cathode was prepared by mixing 85% of the Cu-doped

LiFePO4/C, 10% of an acetylene black as conducting agent and5% of an poly (vinylidene fluoride) as binder, pressing in analuminum mesh and drying in an oven at 80°C for 12 h.The cells were assembly in an Ar-filled glove box using 1.0

M/L of LiPF6 in ethylene carbonate (EC) and propylene carbon-ate (PC) with volume ratio of 1:1 as electrolyte and metalliclithium foil as counter electrode. The charge and discharge testswere performed between 3.5 and 4.3Vat 0.5C (3mA) and 0.25C(1.5mA).

3. Results and discussion

The Cu-doped LiFePO4 powders were analyzed by XRD toverify phase purity. As shown in Fig. 1, all of the peaks for both

³ Corresponding author: K. Hirao; E-mail: hirao@bisco1.kuic.kyoto-u.ac.jp

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samples in the X-ray diffraction patterns can be assigned to thoseof orthorhombic LiFePO4 (space group: Pnma) as reported inthe literature.16) For the sample Cu-doped LiFePO4 with carboncoating no evidence of diffraction peaks for crystalline carbon(graphite) appeared in the diffraction pattern, which indicatingthat the carbon generated from the carbon source (PEG/fructose)mixture was amorphous and that its presence did not influencethe structure of Cu-doped LiFePO4. Additionally, in some pre-vious work, the introduction of carbon or a carbon precursor intoa lithium iron phosphate has led to produce iron or iron phos-phides during the thermal treatment by carbothermal reduction,15)

but no copper neither as iron relative peak was detected in theXRD pattern in the present work.

The presence of 1 atomic % copper was confirmed from EDSmeasurements.Figure 2 shows the scanning electron micrograph images

corresponding to the Cu-doped LiFePO4 with/without carboncoating. In the case of Cu-doped LiFePO4 without carbon coatingrounded particles with high crystallinity and a size distributionof 1­5¯m were observed. However, the Cu-doped LiFePO4 withcarbon coating has a particle size between 50­100 nm with moreuniform and narrow size distribution (50 to 100 nm). Hence,the carbon as an additive inhibits the particle growth yielding ahomogeneous particle size distribution with an average particlesize of under 100 nm. The carbon had played an important rolein controlling the particle size. The nanosize particles wouldinfluence (shorten) the diffusion length of Li ion in the dischargeand charge to improve ion conductivity of the material.The Cu-doped LiFePO4/C powder presents a dark color

suggesting total carbonization of carbon source. The carboncontent determined by TD-DTA was below the 10% targeted.The residual carbon amount in the final product of Cu-dopedLiFePO4/C was around 5%. The Raman spectra at various pointsof the Cu-doped LiFePO4/C powder are presented in Fig. 3. Thespectra are dominated by the intense carbon D- and G-bands at1360 and 1600 cm¹1. The weak PO4

3¹ ¯1 symmetric stretch bandat 945 cm¹1 was observed still visible together with a band at 340cm¹1. As shown in Fig. 3, the carbon coating is well homoge-

Fig. 1. XRD patterns of the samples calcined at 700°C before the charge.

Fig. 2. Scanning electron micrographs of Cu-doped LiFePO4

a) without carbon coating b) with carbon coating prepared by sol­gelmethod.

Fig. 3. Raman spectra of Cu-doped LiFePO4 with carbon coating.

Ajpi et al.: Synthesis and characterization of Cu-doped LiFePO4 with/without carbon coating for cathode of lithium-ion batteriesJCS-Japan

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nized for the present sample. From Raman measurement it can beassumed that the carbon coating is in the nm order.Figure 4 shows the charge­discharge curves of Cu-doped

LiFePO4 with/without carbon coating at rate of 0.5C (3mA)and 0.25C (1.5mA) regime discharge for 1st and 2nd cycle,respectively, at voltage range of 2.5­3.7V. The capacity valuesfor both samples are resumed in Table 1. For the sample withoutcarbon coating the 1st and 2nd discharge capacity were 23 and37mAhg¹1 for 0.5 and 0.25C, respectively. For the sample withcarbon coating, the 1st and 2nd discharge capacity were 92.3 and102.5mAhg¹1 for 0.5 and 0.25C, respectively. The increase incapacity when carbon coating is clearly shown; a change from23 to 92mAh/g for 0.5C, and a change of 37 to 102mAh/g for0.25C. This difference is associated with the presence of carbon,confirming its role in optimizing the morphology of the Cu-

doped LiFePO4 and thus enhancing the kinetics of its elec-trochemical process.The Cu-doped LiFePO4 with carbon coating proved to be far

better compared to the Cu-doped LiFePO4 without coating as itenhanced electronic conductivity. Decrease in particle size for theCu-doped LiFePO4 with carbon coating would be beneficial inreducing the diffusion length of the lithium ion inside, resultingin fast reaction and diffusion kinetics.

4. Conclusions

The olivine type Cu-doped LiFePO4 with/without carboncoating composite was prepared by sol­gel method usingCH3COOLi·2H2O, FeCl2·4H2O, H3PO4, and CuCl2 as raw mate-rials. The particle size obtained by this method presented a highuniformity size around 50­100 nm for carbon coating sample.Electrochemical tests show that the specified capacity was

102mAh/g measured 0.25C (1.5mA) regime discharge rate,voltage range (2.5­3.7V). Optimization of this composite wouldbe required, but the Cu doping and carbon coating to LiFePO4

showed a promising material for use as cathode material inlithium ion battery.

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Fig. 4. Charge­discharge curves of the sol­gel synthesized Cu-dopedLiFePO4 A) without carbon coating B) with carbon coating 1st cycle at0.5C and 2nd cycle at 0.25C rate.

Table 1. Capacity values of the Cu-doped LiFePO4

CycleNo.

Chargecapacity

Dischargecapacity Efficiency

mAh mAh/g mAh mAh/g

Carbon coating1 4.083 95.0 3.970 92.3 0.972 4.395 102.2 4.408 102.5 1.00

Without carboncoating

1 0.77 23 0 0 ®

2 1.25 37 1.24 36 0.99

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