【作者】 高超；

【导师】 刘海萍；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2021， 博士

【摘要】 新能源电动汽车及手机等电子设备的发展与普及,迫切要求开发一种低成本、高能量密度的锂离子电池正极材料。高电压Li Ni_0.5Mn_1.5O₄（LNMO）材料具有高能量密度（650 Wh/kg）、低成本等优点,但该材料存在大倍率及高温环境下循环稳定性差等问题,制约了其商业化应用。为改善LNMO材料的循环稳定性,本论文采用高温固相法构筑了八面体结构LNMO材料、共沉淀法构筑了中空球状结构LNMO材料、一锅法构筑了中空球状-纳米棒状混合形貌LNMO材料,并通过石墨烯表面包覆、Ti⁴⁺离子体相掺杂、全锂电解质改性包覆等方法对其进行改性研究,在此基础上,系统地研究了改性处理对LNMO材料电化学性能的影响,并借助于第一性原理计算（Density functional theory,DFT）、扫描电镜（Scanning Electron Microscope,SEM）、透射电镜（Transmission Electron Microscopy,TEM）等材料分析方法研究了改性处理后LNMO材料循环性能的提升机制及复合材料的嵌锂机制。采用高温固相法构筑了八面体结构LNMO材料,通过退火处理优化了LNMO材料颗粒尺寸,提高了循环过程中材料的晶体结构稳定性,从而改善了材料的电化学性能,如退火处理后LNMO材料在10 C放电倍率下第1000次循环的比容量为87.8 m Ah/g,容量保持率高达97.4%。在此基础上,通过添加分散剂NMP制备了纳米尺寸LNMO材料,并以NMP为N源在材料表面引入了Ni-N结构,提高了材料的电子导电性,进一步改善了该材料的电化学性能,如在0.2 C和2 C放电倍率下的比容量分别为125 m Ah/g和115.3 m Ah/g,远高于未处理LNMO材料的比容量（115.6 m Ah/g和92.5 m Ah/g）。采用共沉淀法构筑了中空球状结构LNMO材料,石墨烯表面包覆改善了该材料的高温循环稳定性,如55°C下100次循环后石墨烯-LNMO复合材料在2 C放电倍率下比容量（87.8 m Ah/g）和容量保持率（98.8%）,远高于LNMO材料（41.8 m Ah/g,87%）。SEM、TEM、EIS阻抗谱及原位XRD等研究结果表明:石墨烯包覆改善了LNMO材料的电子导电性,减缓了LNMO材料与电解液之间的界面反应,缓解了充放电过程中LNMO材料表面微裂纹的产生,提高了LNMO材料的大倍率长循环和高温循环稳定性。采用共沉淀法构筑了Ti⁴⁺离子体相掺杂中空球状结构Li_1-xNi_0.5Mn_1.5-xTi_xO₄（LNMTx O,x=0.01,0.02,0.03）材料。DFT研究结果表明,与Mn-O、Ni-O键结合能相比,Ti⁴⁺离子掺杂引入了较强的Ti-O键,降低了LNMO材料体系的能量。Ti⁴⁺离子掺杂缓解了LNMO材料充放电过程中Mn³⁺离子溶出和二次相Ni_0.25Mn_0.75O₂产生,提高了LNMO材料的热扩散速率,从而改善了LNMTx O材料的高温循环稳定性,如55°C下100次循环后LNMTx O（x=0.02）材料在2 C放电倍率下的比容量和容量保持率分别为114.4 m Ah/g和98.4%。采用一锅法构筑了中空球状-纳米棒状混合形貌LNMO材料,在此基础上,对其进行全锂电解质改性包覆处理。电化学研究结果表明,全锂电解质改性包覆处理后LNMO材料（LNMO-CG）在20 C放电倍率下1000次循环后比容量和容量保持率分别为120.6 m Ah/g和96.2%,远高于未处理LNMO材料的比容量和容量保持率（60.6 m Ah/g和79%）,在以Li₂Ti O₃（LTO）为负极的全电池中,LNMO/LTO和LNMO-CG/LTO在0.1 C放电倍率下比容量为124.5 m Ah/g和149.1 m Ah/g,2 C放电倍率下比容量为53.1 m Ah/g和97 m Ah/g。SEM、TEM及In-situ-XRD等研究结果表明:中空球状-纳米棒状结构有助于改善LNMO材料的循环稳定性,全锂电解质改性包覆进一步改善了该材料在大倍率及高温下的电化学性能。分散在LNMO材料表面和内部颗粒边界处的全锂电解质减缓了液态电解液向材料内部渗透,缓解了循环过程中材料结构坍塌,在材料表面和内部提供了锂离子快速传输通道,弥补了循环过程中锂损失,从而提高了LNMO材料的循环稳定性。更多还原

【Abstract】 The popularization and development of electric vehicles and mobile phone have promoted the research in pirsing cathode material with low cost and high energy density.High voltage spinel Li Ni_0.5Mn_1.5O₄（LNMO）has the advantages of high energy density,low cost,etc.However,the poor cycling stability under high rate and high temperature limited its commercial application.To improve the cycling stability of LNMO,LNMOs with octahedral structure,hollow-sphere-like structure and sphere-nanorod-like structure were prepared by solid-state method,co-operation method and one-pot hydrothermal method,and modified it with surface coating,doping and modified surface coating,the electrochemical properties of the samples mentioned above were systemtically studied,the enhancement mechanism and lithium intercalation mechanism of the composite material were systematically studied by DFT calculation,TEM,HRTEM,In-situ-XRD,etc.LNMO with octahedral structure was prepared by solid-state method.The particle size was optimized and crystal structure stability in cycle process was improved by anneal process,leading to the superior electrochemical properties with the 1000th discharge capacity of 87.8 m Ah/g and capacity retention of 97.4%under 10 C.Furthermore,nano-sized spinel LNMO was synthesized with the addition of N,N-dimethylpyrrolidone（NMP,LNMO-N）,which exhibits larger discharge capacity under low rate with discharge capacity of 125 m Ah/g udner 0.2 C and 115.3 m Ah/g after 100 cycles under 2 C much larger than that of untreated LNMO（115.6 m Ah/g and 92.5 m Ah/g under 0.2 C and 2 C,respectively）,which is ascribed to the better electronic conductivity of LNMO-N originated from the Ni-N structure on LNMO-N surface.Hollow-sphere-like structure LNMO was synthesized by co-operation method.The electrochemical property of LNMO was significantly improved by graphene coating,with the 100th discharge capacity of 87.8 m Ah/g under 2 C at 55°C,corresponding to capacity retention of 98.8%,while that of LNMO was 41.8 m Ah/g and 87%,respectively.XRD,SEM,TEM,and in-situ-XRD results reveal that RGO coating is beneficial for improving electronic conductivity of LNMO,suppressing the side reactions between electrode and electrolyte,suppressing the volume and lattice strain changes during charge and discharge process,leading to better electrochemical property of RGO-LNMO under high discharge rate and high temperature.Li_1-xNi_0.5Mn_1.5-xTi_xO₄（LNMTx O,x=0.01,0.02,0.03）,synthesized by co-operation method,exhibits better thermal stability and superior cycling stability under high rate and high temperature,such as,under 2 C the 100th discharge capacity of LNMTx O（x=0.02）was 114.4 m Ah/g with high capacity retention of 98.4%at 55°C.The superior crystal structure stability and electrochemical property are ascribed to the stronger Ti-O bands compared with Mn-O and Ni-O bands,as revealed by DFT result,the suppressed Mn³⁺dissolution,heat aggregation and two phase reaction generation during charge and discharge process.Furthermore,sphere-nanorod-like micro nano structured LNMO was prepared by one-pot hydrothermal method,and then it was modified by modified surface coating with solid state electrolyte.Electrochemical result exhibits that LNMO with sphere-nanorod-like micro nano structure shows superior electrochemical property,and the electrochemical property is further improved by modified surface coating,with the1000th discharge capacity of 120.6 m Ah/g and high capacity retention of 96.2%in half cells,and the 1st discharge capacity of 149.1 m Ah/g under 0.1 C and 97 m Ah/g under 2C in LNMO-CG/Li₂Ti O₃（LTO）full cells,while that of LNMO was 60.6 m Ah/g after 1000cycles under 20 C with capacity retention of 79%,124.5 m Ah/g under 0.1 C and 53.1m Ah/g under 2 C in LNMO/Li₂Ti O₃（LTO）full cells.The superior electrochemical property of LNMO-CG is ascribed to the special micro struature,solid state electrolyte contributes to prevent electrolyte from penetrating into the material along grain boundary,suppress structure collapse,as confirmed by SEM,TEM and in-situ-XRD.Solid state electrolyte can also provide fast lithium ion transport channel on material surface and compensate for lithium deficiency during cycling process,leading to larger discharge capacity and better cycling stability of LNMO.更多还原