Studies On Layered 0.5Li₂MnO₃·0.5LiMO₂ （M=Co、Mn、Ni） Lithium-rich Cathode Materials

With development of Li-ion batteries,traditional commercial LiCoO2 is expensive,toxic and has a practical capacity of only about 150mAh/g, which are severely restricted the scope of its application. Layered structural lithium-rich metal oxides with rhombohedral α-NaFeO2 crystal structure have been proven to be particularly suitable for application as cathode materials in lithium-ion batteries. Compared with LiCoO2, Li-excess layered Mn based oxides xLi2MnO3·(1-x)LiMO2 (M=Mn, Ni,Co) are promising, inexpensive, nontoxic, and have high thermal stability and high capacity.thus, they are extensively studied as alternative cathode electrode materials to the commercial LiCoO2 electrode. However,a lot of work needs to be done to reduce the reversible discharge capacities and extend the effective lifetime and decrease the voltage drop.In this thesis,The effects of synthesizing technological parameters on the performance of 0.5Li2MnO3·0.5LiMn1/3Co1/3Ni1/3O2 material were investigated.Using different methods to synthesis high tap density spherical materials and With the Al-doping modification to decrease the voltage drop and improve cyclic performance.Firstly,The effects of synthesizing technological parameters on the performance of 0.5Li2MnO3·0.5LiMn1/3Co1/3Ni1/3O2 material were investigated, the optimum conditions were obtained as the molar ratio of Lii.2/(Ni+Co+Mn)0.8 was 1.05 and calcination temperature was 900°C,calcination time was 10 h.The results showed primary irregular nanoparticles with an average diameter similar to 300 nm. The material presented layered a-NaFeO2 structure and has the characteristic peaks of the Li2MnO3;the initial discharge specific capacity was 270 mAh/g and the coulombic efficiency was 82% at the rate of 0.1C and the voltage range of 2.0-4.8V, the capacity retention ratio was 88% after 100 cycles.Secondly, Investigation on synthesizing technological parameters about co-precipitation method and spray drying method to preparation of 0.5Li2MnO3·0.5LiMn1/3Co1/3Ni1/3O2 microsphere. Pure phase microspheres were synthesized via a facile co-precipitation method with a solution of Na2CO3,which had a high tap density and low surface area.It had a good electrochemical performance in terms of discharge capacity. The initial discharge specific capacity was 266 mAh/g and the coulombic efficiency was 77% at the rate of 0.1C and the voltage range of 2.0-4.8V.The capacity retention ratio was 80% after 100 cycles at the rate of 1C.So it had a huge potential to apply to the industry.The effects of Al-doping modification on the performance of Li-rich material were carried out,which efficiently decreased the voltage drop and improved cyclic performance.XRD results indicated that Al-doping did not change the layered a-NaFeO2 structure of the material with good crystallization and pure phase. When doping quantity was 0.01,it had a good electrochemical performance. The initial discharge specific capacity was 230 mAh/g at the rate of 0.1C and the voltage range of 2.0-4.8V.The capacity retention ratio was 96.9% and the voltage drop was about 0.02V after 100 cycles at the rate of 1C,which was higher than other materials.