Synthesis And Modification Of Nickel-mangnese Binary Cathode Material With High Voltage Up To5V For Lithium-Ion Battery

Cathode material is one of the most important components for Lithium ion battery, itplays a crucial role for the cost and performance of the batteries. The development of highperformance cathode materials would be very important to the development of lithium ionbattery and the commercialization of electronic vehicles. High voltage nickel manganesebinary cathode material (LiNi0.5Mn1.5O4) with spinel structure, derived from lithiummanganate (LiMn2O4), possesses the merits of lithium manganate. At the same time, becauseof nickel incorporation, LiNi0.5Mn1.5O4possesses some new features. LiNi0.5Mn1.5O4cathodematerial with its superior energy density and high work voltage plateau at around4.7V, hasattracted many researchers’ attention and become one of the hottest topics in the field ofmaterials for lithium ion battery.In this thesis, we synthesized spinel LiNi0.5Mn1.5O4cathode materials with a noveltwo-step oxalate co-precipitation approach. By optimizing some of the important parametersin the preparation process, well crystallized and well distributed LiNi0.5Mn1.5O4cathodematerials with high performance and good stability were obtained. For the sample preparedwith optimal condition, its capacity is high up to136mAh g-1when charge and discharge at0.1C rate, the capacity retention after50cycles is still high up to93%. Furthermore, wesuggested the possible formation mechanism of the spinel LiNi0.5Mn1.5O4as follows, in thefirst co-precipitation stage, an oxalate precipitation of nickel and manganese was formed,then well spinel oxide precursor was obtained through calcining the oxalate powders, whichmay due to the effect of dispersion and block of the oxalic acid root. In the second step, thelithium ions were embedded into the lattice of spinel oxides precursor through calcination.Based on the previous synthesis process, cobalt doping materials were investigated andprepared. It is revealed that both the Co doped samples have well defined spinel structure,indicating cobalt can be well embedded into the spinel structure. The Co-doped materialsshowed a light capacity loss, however, their cycle stability and high rate discharge capacitywere promoted effectively. When Co doped in both Ni and Mn site simultaneously, itpossesses a capacity retention of99.7%(0.1C) and96.8%(2.0C) after80cycles, which are much better than those of Co doped in Ni site or Mn site, respectively.We attempted to coat LiNi0.5Mn1.5O4material with carbon through a simple spray dryingand pyrolysis process. Mixing the LiNi0.5Mn1.5O4with different mass fraction of sucrose andpolyvinyl pyrrolidone (PVP) dissolved in water to formulate a suspension, followed with asimple spray-drying and heat treatment process to obtain carbon coated LiNi0.5Mn1.5O4/Ccomposite material. It was found that the carbon coating does not change the spinel structure,but enhance the rate performances of the cathode material significantly. The modifiedLiNi0.5Mn1.5O4material by2wt.%sucrose exhibited a discharge capacity of about80mAh g-1at5.0C rate, and no obvious capacity loss could be observed after50cycles.