Synthesis and electrochemical behavior of Li[Li0.1Ni0.35-x/2CoxMn0.55-x/2]O2 cathode materials

Abstract

A layered LiNi0.5Mn0.5O2 material prepared by co-precipitation method showed promising capacity of around 150 mAh/g in the voltage range of 2.5-4.3 V. Kang et al. have reported increased discharge capacity and electrical conductivity by doping Al, Ti, and Co in LiNi0.5Mn0.5O2. Among those materials, Co doping increased electrical conductivity most successfully. J. R. Dhan have studied layered Li[Li(1-2x)/3NixMn(2-x)/3]O2 material, considered to be a solid solution of LiNi0.5Mn0.5O2 and Li2MnO3. Li[Li(1-2x)/3NixMn(2-x)/3]O2 is very promising cathode materials due to its non-toxicity and large capacity. In this work, we investigated structure and electrochemical properties of Li[Li0.1Ni0.35-x/2CoxMn0.55-x/2]O2 (x=0-0.3) compounds synthesized by a sol-gel method. Li[Li0.1Ni0.35-x/2CoxMn0.55-x/2]O2 compounds are considered to be a solid solution of LiNi0.5Mn0.5O2, LiCoO2, and Li2MnO3. Also, Li[Li0.1Ni0.35-x/2CoxMn0.55-x/2]O2 can be understood as Co doped compound for Ni and Mn in Li[Li(1/3-2x/3)NixMn(2/3-x/3)]O2. By increasing Co-doping amount in Li[Li0.1Ni0.35-x/2CoxMn0.55-x/2]O2, the calculated lattice based on R m space group decreased linearly and the ratio of c/a also increased monotonously. Furthermore, the gradual decrease in the c/a ratio means that Co doping resulted in much layer-like structure. These indicate that Ni and Mn were well replaced by Co and the solid solution of LiCoO2, Li2MnO3, and LiNi0.5Mn0.5O2 was formed. The Co doping of x=0.1 showed improved capacity and rate capability comparing to undoped Li[Li0.1Ni0.35Mn0.55]O2. Area specific impedance (ASI) and ac impedance measurement show that Co doped electrode have the lower impedance value during cycling. It is considered that the higher capacity and superior rate capability of Co doping sample, especially for the Li[Li0.1Ni0.3Co0.1Mn0.5]O2, would be ascribed to the reduced resistance of the electrode during cycling.