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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.54 n.2 Concepción jun. 2009 

J. Chil. Chem. Soc, 54, N° 2 (2009)






(1. Qinghai Institute of Salt Lakes, Chinese Academy of Sciences. Xining, China, 810008;

2. Graduate University of Chinese Academy of Sciences. Beijing, China, 100049)

* e-mail:


LiFeP04/C and LiFeP04/C+Ag samples were prepared by sol-gel method. Samples were characterized by XRD, SEM, EIS and electrochemical discharge and charge tests. Silver modification does not affect the olivine structure of LiFeP04 but enhances its electrochemical performance in terms of discharge capacity and rate capacity. Discharge capacities are improved from 130.8 mAhg-1 of LiFeP04/Cto 149.8mAhg-1 of LiFeP04/C+Ag cathodes. EIS measurements show that Silver modification slightly decreases the charge transfer resistance of LiFePOyC, and lithium ion diffusion coefficient is enhanced from 2.89 x 10-14 to 8.17 x 10-13m2s-1.

Key words: Li-ion battery; LiFePO /C; Silver modification; EIS; Li-ion diffusion coefficient



Since the pioneering work of Goodenough and co-workers1, the phosphor-olivine LiFeP04 has been recognized as a potential positive electrode material for use in lithium ion batteries. Although LiFeP04 possesses a number of advantages (high capacity, high stability during lithium extraction/insertion, environmental friendliness, etc)2, its main drawback is poor electrochemical performance due to its sluggish charge/discharge kinetics. In order to increase the charge capacity and improve the conductivity, many attempts have been performed, using approaches such as decreasing active particle size 3-5, carbon coating 6"9, ion doping 10-12 and surface modification 13-15 etc. Surface modifications are of great importance in developing electrode materials 15. In the present paper we investigate the role of silver modification in improving the performance of LiFePO4, Wherein we took silver mirror reaction to coat each particle with a thin silver coating. Particles are prepared via a sol-gel route using Fe(III) citrate as chelating agent. The impacts of silver coating on the electrochemical properties of LiFePO4-based cathodes are systematically studied.


LiFeP04/C composites were prepared by a sol-gel method with subsequent firing of xerogel in argon (purity>99.99%) atmosphere. First, Fe(HI) citrate(Sigma, Technical Grade) was dissolved at 70°C in deionized water. Second, equimolar aqueous solution of LiH2P04 was prepared from NH4H2P04(GR, sinopharm) and Li2C03 (99.84%, self-made). The solutions were mixed together and the obtained transparent sol was dryed at 70°C for at least 20h after grinding with mortar and pestle for lOmin, the obtained material was fired in argon atmosphere at 700°C for 18h the heating rate being 10K/ min.

The surface modification was performed by silver mirror reaction13-14. The LiFeP04/C powders were suspended in the AgN03 solution with stirring, and solution of glucose was added to reduce Ag+ ions. A solution of AgN03 by a weight ratio of 6(Ag):94(LiFeP04/C) was added. After stirring, the suspended powders were separated by filtration. The obtained LiFeP04/Ag+C powders were dried in argon atmosphere at 500°C for 2h. The completion of the Ag+ reduction was checked by adding 5M HC1 to the filtrate.

The powders were characterized by SEM and X-ray diffraction. The SEM observation was performed on a JEOL scanning electrode microscope(JSM-5610LV), the X-ray diffraction was carried out by using a PANalytical X' Pert PRO diffractometer with Cu Ka radiation.

Electrodes were made by dispersing appropriate 85 wt.% active materials, 8wt.% carbon black and 7wt.% polyvinylidene fluoride(PVDF) binder in 1-Methyl-2-pyrrolidone solvent to form a slurry. The slurry was then spread uniformly on to an electrode slice and dried in the vacuum oven at 65°C for at least 12h. The cells were assembled in an argon filled glove-box (MBraun, Unilab, USA). The electrolyte was 1M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC)(1:1 by volume). The cells were galvanostatically charged and discharged in the voltage range of 2.5-4.3V versus Li/Li+ counter electrode.

EIS was measured on an IM6ex electrochemical workstation (ZAHNER electric, Germany) at 3.4V, the amplitude of the AC voltage was 5mv over the frequency range 10-2 to 106 Hz



The X-ray diffraction profiles of LiFeP04/C and LiFeP04/C+Ag are shown in Fig. 1. Ag diffraction patterns are detected and numbered in the X-ray diffraction profile of LiFeP04/C+Ag. The LiFeP04/C is pure single phase indexed with the orthorhombic Pnmb space group. The lattice parameters of LiFeP04/C and LiFeP04/C+Ag are calculated separately by MDI Jade 5.0. The results are a = 6.005(5) A, b = 10.32(11) A, c = 4.687(5) A, V = 290.6(5) A3 (LiFeP04/C) and a = 6.01(11) A, b = 10.32(18) A, c = 4.689(9) A, V = 291.1(3) A3 (LiFePO /C+Ag).



Fig.2 shows the SEM photos of prepared LiFePOyC and LiFeP04/C+Ag. After sintering at 700°C for 18h, well crystallized powders are obtained with an average crystal size of l-5um. LiFePOyC particles have smooth surface, whereas a lot of flocculi cover the surface of LiFeP04/C+Ag particles, which may well be the elementary silver produced by silver mirror reaction.

The electrochemical performance of LiFePOyC and LiFeP04/C+Ag is displayed in Fig.3. The reversible capacity of silver modified LiFeP04/C is 149.8mAhg-1 at a current density of 60mAg-1, as opposed to 126.0mAhg-1 for LiFePOyC at the same current density. The introduction of silver obviously enhances the electrochemical properties of the active material.


In Fig. 4, the impendence spectra are combinations of a depressed semicircle at high frequencies and a straight line at low frequencies. In the high frequency region, the intercepts with real impendence axis of LiFeP04/C and LiFePO / C+Ag composite are 115Í2 and 110Í2 respectively. This value is believed to be the total electric resistance of the electrode materials, electrolyte resistance and electrode leads. The minor decrease of the total electric resistance may be attributed to the silver modification to LiFePO /C. In the low frequency region, faraday reaction is the main effect. The slope of impendence of the LiFePOy C+Ag composite is higher than that of LiFePOyC, indicating that silver is able to enhance the electrochemical activity of LiFeP0416.

The lithium ion diffusion coefficients (D) of LiFePO4/C and LiFePO4/C C+Ag are calculated according to the following equation17.

Where R is the gas constant, T is the absolute temperature, n is the number of electrons per molecule during oxidation, F is the Faraday Constant, C is the concentration of lithium ion, and a is the Warburg factor which has relationship with ZIm :

Fig. 5 shows the relationship between -ZIm and square root of frequency in the low frequency region for LiFeP04/C and LiFePO4/C+Ag. The diffusion coefficients of lithium ion are calculated based on equation (1) and (2). The calculated diffusion coefficients for LiFePO,/C andLiFePO4/C+Ag are 2.89xl0-14 and 8.17x10° m2 s-1 respectively. The lithium ion diffusion coefficient increases markedly by the silver modification.



The cycle life of the LiFeP04 /C and LiFeP04/C+Ag samples is shown in Fig.6. It is found that discharge capacities of LiFeP04/C+Ag are higher than that of LiFeP04/C. The highest discharge capacity is 130.8mAhg-1 at the first circle for the latter and 149.8mAhg-1 at the third circle for the former. Table 1 presents the statistics of the cycling properties of LiFePO4/C and LiFePO4/C+Ag cathodes at various discharging rates. At the discharging rates of 0.4C and l.OC, better electrochemical performance can be achieved by silver modification.





LiFePO4/C and LiFeP04/C+Ag were prepared by a sol-gel process. Silver modification improves its electrochemical performance in terms of discharge capacity and rate capability. Discharge capacities are improved from 130.8 mAhg1 of LiFePO/C to 149.8mAhg4 of LiFePO4/C+Ag cathodes. EIS measurements show that the Ag modification slightly decreases the charge transfer resistance of LiFePO4/C, and enhances its lithium ion diffusion rate.



This work is supported by Qinghai province science and technology key project fund (No. 2006-G-168 China).



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(Received: September 3, 2008 - Accepted: May 13, 2009)

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