Improvement of the rate capability of LiMn2O4 by surface coating with LiCoO2

In order to use LiMn2O4 as a cathode material of lithium-secondary battery for an electric vehicle (EV), its rate capability should be improved. To enhance the rate capability of LiMn2O4 in this work, the surface of LiMn2O4 particle was coated with LiCoO2 by a sol-gel method. Because LiCoO2 has a higher electric conductivity than LiMn2O4, it is possible to improve the rate capability of LiMn2O4. After the surface coating, LiCoO2-coated LiMn2O4 showed a higher discharge capacity of 120 mAh/g than as-received LiMn2O4 (115 mAh/g) because LiCoO2 has a higher capacity than LiMn2O4. The rate capability of the coated LiMn2O4 improved significantly. While as-received LiMn2O4 maintained only 50% of its maximum capacity at a 20C rate (2400 mA/g), the LiCoO2-coated LiMn2O4 maintained more than 80% of maximum capacity. LiCoO2-coated LiMn2O4 with 3 wt.% conducting agent (acetylene black) showed the higher rate capability than as-received LiMn2O4 with 20 wt.% conducting agent. From electrochemical impedance spectroscopy (EIS) result that the first and second semicircles of coated LiMn2O4 were reduced, the improvement of rate capability is attributed to a decrease of passivation film that acts as an electronic insulating layer and a reduced inter-particle contact resistance. Accordingly, It is proposed that the surface coating of LiMn2O4 with LiCoO2 improve the rate capability as well as the specific and volumetric energy density due to the decrease of conducting agent. (C) 2001 Elsevier Science B.V. All rights reserved.