Synthesis And Modification Of Lithium-and Manganese-enriched Li_1.2Ni_0.2Mn_0.6O₂as Cathode Material For Lithium Batteries

In order to satisfy the requirement of the development of mobile electronic devices, electricvehicles and smart grid etc., high energy, high power lithium-ion batteries are required and excellentcathode material is the key to the problem. Lithium-and manganese-enriched layered oxides havebeen considered as the potential alternative of the traditional cathode material LiCoO₂due to its highdischarge specific capacity, high working voltage, low cost and environmental friendly. The relatedresearch on these materials has atracted more attention in recent years. Howerver, as its complexstructure and disadvantage such as high initial irreversible capacity and poor rate performance makethe preparation and modification work very important. In this thesis, Li_1.2Ni_0.2Mn_0.6O₂, as belong tothe lithium-and manganese-enriched layered oxides has been prepared through the carbonatecoprecipitation methode, and then improved the electrochemical performance of the as-preparedcathode materal by surface modification with different oxides. Specific contents are as follows:（1） Li_1.2Ni_0.2Mn_0.6O₂cathode material has been successfully prepared through the carbonatecoprecipitation method following with high temperature solid state synthesis method. Influence of pHon the shape appearance and structure of coprecipitation precursor was particularly researched, andobtained that the best pH value for coprecipitation reaction is7.8. Influences of the calciningtemperature, sintering time and lithium content in solid state synthesis on the morphology, structureand electrochemical performance of material were discussed. And finally it is found that when calcine12h at900°C with5%extra lithium content can obtain the Li_1.2Ni_0.2Mn_0.6O₂cathode material withnormal structure and the best electrochemical performance.（2） Li_1.2Ni_0.2Mn_0.6O₂was coated by different content of Al₂O₃which is not electrochemicalactive. Al₂O₃coated samples keep the original good layered structure of Li_1.2Ni_0.2Mn_0.6O₂, and thecathode particle is evenly coated by uniform Al₂O₃. Electrochemical tests show that1.5wt.%Al₂O₃coated sample exhibits the highest capacity and the best improvements on cycle performance and rateproperties: the initial discharge capacity at0.1C is up to266.8mAh g^-1from232.5mAh g^-1compared with the pristine Li_1.2Ni_0.2Mn_0.6O₂and achieves a capacity retention of86.8%after30cycles, while the high current discharge capacity at5C increases by113.4%in comparison with thepristine sample.（3） Li_1.2Ni_0.2Mn_0.6O₂was modified by the nickel manganese compound oxide Ni_0.5Mn_1.5O_x, andcompare the structure and electrochemical performance with the pristine and equivalent spinelLiNi_0.5Mn_1.5O₄blended Li_1.2Ni_0.2Mn_0.6O₂as positive electrode material. The core-shell composite structure Li_1.2Ni_0.2Mn_0.6O₂-Li1-yNi_0.5Mn_1.5O_xof incorporation by the high capacity layeredLi_1.2Ni_0.2Mn_0.6O₂and high voltage stable spinel LiNi_0.5Mn_1.5O₄was successfully prepared. TheNi_0.5Mn_1.5O_xsurface modified materials achieves256.8mAh g^-1in wide voltage window range of2.0⁵.0V at0.1C and the capacity still retain94%after50cycles. It also shows good rate property as itsdischarge capacity at5C increases by1.6times in contrast with the pristine sample. After surfacemodification by Ni_0.5Mn_1.5O_x, electrochemical performance of the sample is much improved than boththe pristine and LiNi_0.5Mn_1.5O₄simply blended Li_1.2Ni_0.2Mn_0.6O₂.