Effect of carbon coating process on the structure and electrochemical performance of LiNi_(0.5)Mn_(0.5)O2 used as cathode in Li-ion batteries

摘要

LiNi_(0.5)Mn_(0.5)O2 powder was synthesized by a coprecipitation method. LiOH.H2O and coprecipitated [(Ni_(0.5)Mn_(0.5))C2O4] precursors were mixed carefully together and then calcined at 900°C. Surface modified cathode materials were obtained by coating LiNi_(0.5)Mn_(0.5)O2 with a thin layer of amorphous carbon using table sugar and starch as carbon source. Both parent and carbon-coated samples have the characteristic layered structure of LiNi_(0.5)Mn_(0.5)O2 as estimated from X-ray diffractometry measurements. Transmission electron microscope showed the presence of C layer around the prepared particles. TGA analysis emphasized and confirmed the presence of C coating around LiNi_(0.5)Mn_(0.5)O2. It is obvious that the carbon coating appears to be beneficial for the electrochemical performance of the LiNi_(0.5)Mn_(0.5)O2. A capacity of about 150 mAh/g is delivered in the voltage range 2.5-4.5 V at current density C/15 for carbon coated LiNi_(0.5)Mn_(0.5)O2 in comparison with about 165 mAh/g obtained for carbon free LiNi_(0.5)Mn_(0.5)O2 at the same current density and voltage window. About 92% and 82% capacity retention was obtained at 50th cycle for coated LiNi_(0.5)Mn_(0.5)O2 using sucrose and starch, respectively; whereas, 75% was retained after only 30th cycle for carbon free LiNi_(0.5)Mn_(0.5)O2. This improvement is mainly attributed to the presence of thin layer of carbon layer that encapsulate the nanoparticles and improve the conductivity and the electrochemical performance of LiNi_(0.5)Mn_(0.5)O2.