Electrochemical properties of LiFe_(0.9)Mn_(0.1)PO_4/Fe_2P cathode material by mechanical alloying

摘要

LiFePO_4, olivine-type LiFe_(0.9)Mn_(0.1)PO_4/Fe_2P composite was synthesized by mechanical alloying of carbon (acetylene back), M_2O_3 (M = Fe, Mn) and LiOH·H_2O for 2 h followed by a short-time firing at 900℃ for only 30 min. By varying the carbon excess different amounts of Fe_2P second phase was achieved. The short firing time prevented grain growth, improving the high-rate charge/discharge capacity. The electrochemical performance was tested at various C/x-rate. The discharge capacity at 1C rate was increased up to 120 mAhg~(-1) for the LiFe_(0.9)Mn_(0.1)PO_4/Fe_2P composite, while that of the unsubstituted LiFePO_4/Fe_2P and LiFePO_4 showed only 110 and 60 mAhg~(-1), respectively. Electronic conductivity and ionic diffusion constant were measured. The LiFe_(0.9)Mn_(0.1)PO_4/Fe_2P composite showed higher conductivity and the highest diffusion coefficient (3.90 × 10~(-14) cm~2 s~(-1)). Thus the improvement of the electrochemical performance can be attributed to (1) higher electronic conductivity by the formation of conductive Fe_2P together with (2) an increase of Li~+ ion mobility obtained by the substitution of Mn~(2+) for Fe~(2+).