【作者】 刘涛；

【导师】 张大伟；

【作者基本信息】 合肥工业大学 ， 化学工程（专业学位）， 2019， 硕士

【摘要】 三元层状材料LiNi_0.8Co_0.15Al_0.05O₂（NCA）因其具有较高的能量密度和较好的循环性能被认为是极具发展前景的正极材料之一。但NCA材料的结构稳定性和倍率性能仍有待提升。本论文通过共沉淀法制备前躯体以及后续配锂烧制的条件控制探究最优合成条件,对材料进行Mg₃（PO₄）₂包覆和Li₂TiO₃包覆并探究不同包覆量对NCA材料性能的影响,对不同物质包覆机理进行研究和解释。采用共沉淀法合成NCA前驱体Ni_0.8Co_0.15Al_0.05（OH）₂,分别研究不同pH,不同氨水浓度,不同配锂量和不同煅烧温度对材料形貌结构和电化学性能的影响。通过多条件对比确定最佳合成工艺条件。XRD和SEM结果显示pH值为11,氨水浓度为1.5 M,锂过量5%,煅烧温度为850℃下所制得的样品具有最优的层状结构和球形或类球形形貌。充放电循环测试结果表明上述综合条件下所制样品电化学性能最优,0.1 C下首次放电容量可达153.64 mAh/g,100圈循环后容量保持率为89.25%。在最优合成条件下制备出NCA材料,然后利用EDTA二钠的螯合作用在NCA表面螯合附着Mg²⁺,通过加入计量比的磷源后进行离心分离和二次烧结合成Mg₃（PO₄）₂包覆的NCA样品（记为Mg₃（PO₄）₂@NCA）。探究不同包覆量对材料电化学性能的影响可以发现包覆后材料仍保持良好的六方晶系α-NaFeO₂层状结构,TEM的结果显示NCA材料表面均匀包覆一层Mg₃（PO₄）₂。电化学性能测试结果显示在不同包覆量样品中,2%Mg₃（PO₄）₂@NCA样品具有最优的循环稳定性和最好的倍率性能。EIS和CV的结果显示包覆2%Mg₃（PO₄）₂后,材料的阻抗增加得到明显抑制,极化明显减弱,有助于提升材料的电化学性能。采用乙醇超声分散和湿法研磨在NCA材料表面成功附着TiO₂和Li₂CO₃,然后高温煅烧可合成Li₂TiO₃包覆的NCA样品（记为Li₂TiO₃@NCA）。XRD和SEM结果显示不同Li₂TiO₃包覆对NCA材料的微观结构和宏观形貌并未造成影响。而由于Li₂TiO₃本身就可以作为电极材料,在充放电过程中贡献容量,从而减少了包覆对于材料整体容量损失的影响。经过电化学性能测试可以发现2%Li₂TiO₃@NCA具有最优的电化学稳定性且初始容量更高。Li₂TiO₃材料的零应变性在NCA材料的表面起到了显著有效的支撑保护作用,在经历长期充放电循环后材料可以维持很好的球形形貌,减少受到电解液的腐蚀而引起性能严重衰减。更多还原

【Abstract】 LiNi_0.8Co_0.15Al_0.05O₂（NCA）,a layered ternary material,is considered as one of the most promising cathode materials due to its high energy density and good cycling performance.However,the structural stability and rate performance of NCA materials still need to be improved.In this paper,the optimal synthesis conditions were explored through the condition control of the preparation of precursor by co-precipitation method and the subsequent calcination of lithiation.Mg₃（PO₄）₂ coating and Li₂TiO₃ coating of materials,and the effects of different coating amounts on the properties of NCA materials were explored,and the coating mechanism of different substances was studied and explained.Ni_0.8Co_0.15Al_0.05（OH）₂,a precursor of NCA,was synthesized by coprecipitation method.The effects of different pH,ammonia concentration,lithium content and calcination temperature on the morphology,structure and electrochemical properties of the materials were studied.The optimum synthetic process conditions were determined by multi-condition comparison.XRD and SEM results showed that the pH value of 11,ammonia concentration of 1.5 M,lithium excess of 5%,calcination temperature of 850℃under the above system the the sample exhibited the most optimal layered structure and spherical or class spherical morphology.The results of charging and discharging cycle test indicated that the electrochemical properties of the samples prepared under the above comprehensive conditions were optimal.The initial discharge capacity at 0.1 C could reach 153.64mah/g,and the capacity retention after 100 cycles was 89.25%.NCA materials were prepared under optimal synthesis conditions.Then,Mg²⁺was chelated on the surface of NCA using the chelation of EDTA disodium,and Mg₃（PO₄）₂-coated NCA samples（denoted as Mg₃（PO₄）₂@NCA）were synthesized by centrifugation and secondary sintering after adding phosphorus sources with stoichiometric.By exploring the influence of different cladding amounts on the electrochemical properties of the materials,it can be found that the hexagonal system ofα-NaFeO₂ layer structure remains well after coating.The TEM results showed that the surface of NCA materials is uniformly coated with a layer of Mg₃（PO₄）₂.Electrochemical performance test results showed that 2%Mg₃（PO₄）₂@NCA samples had the best cyclic stability and the best rate performance among the samples with different coating amounts.The results of EIS and CV showed that after 100cycles the increase of the material’s impedance is obviously inhibited,and the polarization is obviously weakened,which is helpful to improve the electrochemical properties of the material.TiO₂ and Li₂CO₃ were successfully adhered to the surface of NCA by ultrasonic dispersion and wet grinding with ethanol,and then the NCA samples coated with Li₂TiO₃（denoted as Li₂TiO₃@NCA）were synthesized by high-temperature calcination.XRD and SEM results showed that different Li₂TiO₃ coating amounts had no effect on the microstructure and macroscopic morphology of NCA materials.Since Li₂TiO₃ itself can be used as an electrode material to contribute capacity during charging and discharging,the influence of coating on the overall capacity loss of the material is reduced.After electrochemical performance test,it can be found that 2%Li₂TiO₃@NCA has the optimal electrochemical stability and higher initial capacity.The zero strain characteristic of Li₂TiO₃ material plays a significant and effective supporting and protection role on the surface of NCA material.After long-term charging and discharging cycles,the material can maintain a good spherical morphology and reduce the corrosion by electrolyte,resulting in serious attenuation of performance.更多还原