Influence of the Degree of Doping on the Efficiency in a Battery LiFePO4 - Yttrium Used As Cathode Material

Abstract

LiFePO4 has been proposed as a potential cathode materials for the next generation of lithium ion batteries.A good cathode material must have the following properties: high cycling stability (300 cycles); thermodynamic stability during energy process, high charge storage capacity per mole of material and compatible with the environment. LiFePO4 electrode, which has a structure of type "olivine” mixed with a conductive material, as reported by Goodenough et al. in 1997 [1], appears to meet many of these requirements. However, its low electronic conductivity has prevented their widespread use. Several studies have investigated the effect of doping agents [2] in the structure, and consequently in its electrochemical response.The LiFePO4 can be synthesized at high temperature reactions [1] or by using hydrothermal conditions [2]. As this material has low conductivity at room temperature, it can reach the theoretical capacity only when a low current density is used [3] or at elevated temperatures [4], as a consequence of the low diffusion of lithium on the electrode / electrolyte interface. Ravet et al. [5] demonstrate that a carbon covering significantly improves the electrochemical performance of the material.In this work we have synthesized LiFePO4 doped yttrium by hydrothermal method, using yttrium oxide as a dopant source. The synthesis was carried out in an acid digestion bomb, Parr. The imposed temperature was 170°C for 5 hours and then increased to 180°C for another 5 hours. The sample was subsequently dried under vacuum at 80°C for 24 hours. Under these experimental conditions should present a theoretical capacity of 169 mAhg-1. Both LiFePO4 and LiFePO4-Y were characterized by XRD and TEM. The working electrode was prepared by mixing active material (LiFePO4) with poly(vinylidene fluoride) (PVDF) and acetylene black at a weight ratio of 20:10:70 in N-methyl-2- pyrrolidone (NMP) to form slurry. Then, the resultant slurry was uniformly pasted on aluminium foil and dried to make the cathode. The electrochemical response of four samples were tested, pristine LiFePO4 and three different doping ratios 0,01, 0,5 and 1% named A1 A2 and A3 respectively. The obtained results indicate an improvement when LiFePO4 is doped, being A2 the sample with the better performance. Electrochemical properties were recorded in the potential range 2.0-4.0 V versus Li/Li+ at a C/20 current density. The curve of specific capacity versus cycle number shows the stability over time. Moreover, potential versus specific capacity and efficiency versus rate cycle performance were also tested. Agradecimientos : The authors acknowledge financial support from Fondecyt grant N° 1131019.[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [2] S. Yang, P.Y. Zavalij, M.S. Whittingham, Electrochem. Commun. 3 (2001) 505. [3] A. Yamada, S.C. Chung, K. Hinokuma, J. Electrochem. Soc. 148 (2001) A224 [4] A.S. Andersson, J.O. Thomas, B. Kalska, L. Häggström, Electrochem. Solid-State Lett. 3 (2000) 66