Preparation And Modification Of Ni-rich Ternary Cathode Materials For Lithium-ion Batteries

Energy shortages and environmental pollution issues have caused world wide attention.The new energy industry that solves energy problems while also meeting environmental protection needs is gradually becoming a hot industry now.Lithium-ion batteries（LIBs）have become important new energydevices with excellent market potential and broad application prospects due to the advantages of high energy density,good safety performance and low cost.However,with the rapid development of the current electric vehicle industry,smart phone industry,smart grid and energy storage industry,the design and preparation of LIBs with higher energy density is becoming the priority of LIBs development as well as the research focus.The LIBs performance is critically determined by the cathode materials.Among the existing cathode materials,the high nickel ternary cathode materials(LiNi_xCo_yMn_1-x-yO₂,x≥0.5)is the most popular because they have the advantages of high energy density,excellent cycle performance and low cost compared with other cathode materials.For researchers,they are the most favorite cathode materials.In this thesis,we chose high-nickel ternary LiNi_0.8Co_0.1Mn_0.1O₂ material as the research object,and carry out systematic study on materials preparation,testing and characterization and electrical chemical performance.The main research contents are as follows:1.Increasing nickel content and operating voltage are two common measures to enhance the specific capacity of high nickel cathode materials(LiNi_1-x-yCo_xMn_yO₂,x≥0.5),but both of these solutions will inevitably lead to the decline of cycling stability.In this paper,a hydrothermal-assisted co-precipitation method was proposed to effectively solve the problem.Through this method,the material crystallinity was improved,and lithium nickel cation mixing was effectively alleviated.the obtained LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）electrode material delivered excellent specific capacity with improved rate performance and cycling performance.The crystallined Ni_0.8Co_0.1Mn_0.1（OH）₂precursor was successfully synthesized through the combination of reasonably designed hydroxide co-precipitation process and hydrothermal treatment process,which was converted into NCM811 cathode material via subsequent lithium mixing and high temperature calcining.The parameters of co-precipitation process and hydrothermal process,including the reaction time,hydrothermal treatment time and temperature were investigated systematically to reveal their influence on NCM811properties.The results show that the hydrothermal assisted co-precipitation method has a significant effect on the microstructure and electrochemical properties of NCM811.Compared with the NCM811 prepared by conventional co-precipitation method,the hydrothermal-assisted co-precipitation method can make the NCM811 electrode material with lower lithium-nickel mixing and higher crystallinity,thus exhibiting faster lithium ion transport rate and better electrochemical performance.The lithium ion intercalation rate was increased by 1.2 times,and deintercalation rate was increased by1.5 times.The as-prepared NCM811 achieved a high discharge capacity of 212.7mAhg^-1 at the rate of 0.5C,with capacity retention rate of 85.6%after 200 cycles at 0.5 C.2.In this paper,we report a hydrothermal/solvothermal method to prepare Ni_0.8Co_0.1Mn_0.1（OH）₂ precursor.Specificly,Ni,Co and Mn acetates were used as reagents,with hexamethylenetetramine as the precipitant,and water or ethanol as the solvent.Through adjusting the solvent composition to control the nucleation and growth rate of the precipitates,the Ni_0.8Co_0.1Mn_0.1（OH）₂ precursor with different morphologies were synthesized,and the LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials were obtained by subsequent lithium implantation at high-temperature calcination.Their electrochemical performances were studied and compared.Using water as the solvent,the prepared precursor composed of rectangular parallelepiped secondary particles was synthesized,which are assembled by plate-like primary particles.The obtained product exhibits a high specific discharge capacity of 186.94mAhg^-1 with a capacity retention of 85.71%after 200 cycles at 0.5C.Using ethanol as the solvent,the spherical secondary particle precursor was synthesized.The final product evolves into porous spherical structure and exhibits excellent cycle performance.It delivers a specific discharge capacity of 178.65mAhg^-1,but with a high capacity retention of 94.55% after 200 cycles at 0.5C.