Synthesis And Modification Of LiNi_1/3Co_1/3O₂ As Cathode Materials For Lithium-ion Batteries

Energy and environment are the two main themes of social development, researching for the new kind of energy material is an important subject that must be addressed in the 21 st century. Lithium ion batteries are the leading power sources for portable electronic devices and show great prospect in application of electric vehicles and hybrid electric vehicles, by virtue of their high energy density, good cycling stability and superior safety. Lithium ion batteries have become the first choice for energy conversion devices. Cathode material is the key of lithium ion batteries, however, such problems as its performance and price has become a bottleneck restricting the further development of lithium ion batteries. Hence, the research for cathode materials has important practical significance. Currently, most of the principal cathode materials for lithium batteries are Li Co O₂, Li Fe PO4 and Li Ni1/3Co1/3Mn1/3O₂. Unfortunately, Li Co O₂ is plagued by issues such as the poor safety and high cost of cobalt. Although Li Fe PO4 having an excellent cycle performance, its capacity is too low. Compared with them,Li Ni1/3Co1/3Mn1/3O₂ not only has higher reversible capacity, but also has superior thermal stability and relatively low cost. In this thesis, we revolve around the investigation on the best reaction conditions of Li Ni1/3Co1/3Mn1/3O₂ prepared by carbonate co-precipitation method, along with the surface coating by Mn O₂ to improve the electrochemical performance of Li Ni1/3Co1/3Mn1/3O₂, the results are as follow:1. The Ni1/3Co1/3Mn1/3CO3 precursor is synthesized by carbonate co-precipitation method, following by a high temperature solid state reaction to synthesis Li Ni1/3Co1/3Mn1/3O₂ material. By using XRD, SEM and galvanostatic charge/discharge tests, we find that different ammonia solution concentration and different amounts of lithium have great influence on the structure, morphology and electrochemical properties. When the concentration of ammonia solution is 0.5 mol L-1 and lithium excess 5 %, the cathode material shows the highest initial discharge capacity（161.99 m Ah g-1）, and after 100 cycles the capacity retention is 79.03 %.2. Li Ni1/3Co1/3Mn1/3O₂ cathode material is coated by electrochemically active Mn O₂ through a simple chemical deposition method for the first time. The results show that the material structure and the particle size before and after Mn O₂ coating are not changes, only form a dense layer on the surface. Compared with pristine Li Ni1/3Co1/3Mn1/3O₂, the Mn O₂-coated sample shows enhanced electrochemical performance. The initial discharge capacities of the pristine and modified samples at the constant current density of 15 m A g-1 in the range of 2.5⁴.5 V are 189.86 and 195.86 m Ah g-1, respectively. Even at a current density of 750 m A g-1, the discharge capacity of Mn O₂-coated Li Ni1/3Co1/3Mn1/3O₂ is 155.15 m Ah g-1, while that of the pristine electrode is only 132.84 m Ah g-1 in the range of 2.5-4.5 V. In addition, the Mn O₂-coated sample shows higher cycling stability than that of the pristine Li Ni1/3Co1/3Mn1/3O₂.