PREPARATION AND SILVER MODIFICATION OF LIFEPO4/C FOR LI-ION BATTERIES

Abstract

LiFePO 4 /C and LiFePO 4 /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 LiFePO 4 but enhances its electrochemical performance in terms of discharge capacity and rate capacity. Discharge capacities are improved from 130.8 mAhg-1 of LiFePO 4 /C to 149.8mAhg-1 of LiFePO 4 /C+Ag cathodes. EIS measurements show that Silver modification slightly decreases the charge transfer resistance of LiFePO 4 /C, and lithium ion diffusion coefficient is enhanced from 2.89×10-14 to 8.17×10-13 m2 s-1. 1 , the phosphor- olivine LiFePO 4 has been recognized as a potential positive electrode material for use in lithium ion batteries. Although LiFePO 4 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 LiFePO 4 , 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(Ш) citrate as chelating agent. The impacts of silver coating on the electrochemical properties of LiFePO 4 -based cathodes are systematically studied. EXPERIMENT LiFePO 4 /C composites were prepared by a sol-gel method with subsequent firing of xerogel in argon(purity≥99.99%) atmosphere. First, Fe(Ш) citrate(Sigma, Technical Grade) was dissolved at 70°C in deionized water. Second, equimolar aqueous solution of LiH 2 PO 4 was prepared from NH 4 H 2 PO 4 (GR, sinopharm) and Li 2 CO 3 (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 10min, 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 reaction 13-14