【作者】 向兴德；

【导师】 李伟善；

【作者基本信息】 华南理工大学 ， 材料物理与化学， 2014， 博士

【摘要】 为了满足电动汽车续航里程的需求，锂离子电池的能量密度需要进一步提高，而限制其提高的主要因素就是商业化正极材料的比容量较低。锰基层状富锂氧化物在2-4.8V范围内的可逆容量超过200mAh g-1，是最有发展前景的正极材料。但是，锰基层状富锂氧化物还有很多问题需要解决，比如不可逆容量损失大、循环性能差、倍率性能低。针对这些问题，本论文提出了几种新的合成方法，制备出系列锰基层状富锂氧化物材料，用扫描电子显微镜、X-射线衍射、拉曼光谱、交流阻抗、恒电流充放电等方法，详细研究了制备材料组成、结构和电化学性能。论文的主要内容包括：（1）利用聚乙烯吡咯烷酮(PVP)和乙二醇(EG)的协同分散机制，合成出尺寸均匀的锰基层状富锂氧化物Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.3)，该氧化物在0.1C的放电容量为205mAh g-1，循环60圈后的容量保持率为87%，容量衰减的主要原因是部分过渡金属离子失去了电化学活性；（2）考察了锂前驱体用量对锰基层状富锂氧化物化学组分、晶体结构及电化学性能的影响，结果表明，适当降低锂用量可以提高材料的可逆容量，但会降低其循环稳定性，过低的锂用量会导致相分离，使材料容量降低；（3）提出了快速蒸发法，合成出具有低结构缺陷的锰基层状富锂氧化物Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.5)，该氧化物在0.05C循环50圈，几乎无容量衰减，在1C循环300圈，容量保持率在80%以上；（4）提出了自导引化学合成法，构筑出具有微-纳结构的锰基层状富锂氧化物Li[Li1/3-2x/3Mn2/3-x/3Nix]O2（x=0.5），该氧化物在0.05C的放电容量为222mAh g-1，循环50圈后的容量保持率为93%，在2C的放电容量达到110mAh g-1；（5）研究了Co3+取代对锰基层状富锂氧化物Li[Li0.2Ni0.2-x/2Mn0.6-x/2Cox]O2电化学性能的影响，结果表明，Co3+取代能够显著提高材料的不可逆失氧和充放电容量，但是会明显降低材料的容量稳定性和电压稳定性；（6）考察了Fe3+取代对锰基层状富锂氧化物Li[Li0.2Ni0.13Mn0.54Co0.13]O2电化学性能的影响，发现Fe3+取代会严重降低材料的可逆容量，但是对Co3+的取代可以抑制材料在循环过程中的电压衰减；（7）研究了化学组分与合成温度对锰基层状富锂氧化物xLi2MnO3-(1-x)LiCoO2首次充放电性能的影响，发现充电容量的最大值出现在x=0.6，而放电容量的最大值出现在x=0.4～0.5；当合成温度为800℃时，材料的首次充电过程是一个两步反应过程，而当合成温度为900～1000℃时，是一个三步反应过程；（8）研究了不同类型阳离子M（M=Co3+, Ni3+, Cr3+, Fe3+,[Ni0.5Mn0.5]3+）在锰基层状富锂氧化物Li[Li0.2Mn0.4M0.4]O2首次充放电过程中的氧化还原行为，以及对材料充放电性能的影响，结果表明，当M=Co3+时，Li[Li0.2Mn0.4Co0.4]O2的不可逆失氧很强，当M=Ni3+，Li[Li0.2Mn0.4Ni0.4]O2的充电斜区容量很高，当M=Cr3+时， Li[Li0.2Mn0.4Cr0.4]O2的失氧量很低，当M=[Ni0.5Mn0.5]3+时，Li[Li0.2Mn0.4(Ni0.5Mn0.5)0.4]O2的充电斜区容量和充电平台容量介于Li[Li0.2Mn0.4Co0.4]O2与Li[Li0.2Mn0.4Ni0.4]O2之间，当M=Fe3+时，通过溶胶凝胶法在900℃煅烧出的Li[Li0.2Mn0.4Fe0.4]O2不具有传统意义上的“锰基层状富锂氧化物”结构，在充电过程中没有表现出失氧平台，而且充放电容量很低，此外，M3+(M=Ni, Cr,[Ni0.5Mn0.5]3+)取代Co3+都可以提高Li[Li0.2Mn0.4Co0.4-xMx]O2的可逆容量。上述研究为锰基层状富锂氧化物的优化设计及性能提高提供新的研究思路和理论基础，丰富锂离子电池材料制备方法和构效关系的内容。更多还原

【Abstract】 To meet the demands of electric vehicles running in long mileage, the energy density oflithium ion batteries should be further improved. The energy density of currently commerciallithium ion battery is mainly limited by the cathode materials of low capacity density. In thisregard, manganese-based layered lithium-rich oxides would be a promising cathode candidateas they offer a high capacity of more than200mAh g-1in the range of2.0-4.8V. However,they still suffer from several problems such as huge irreversible capacity, poor cyclicperformance and low rate capability. With an aim to solve these problems, several newmethods were proposed in this thesis to synthesize manganese-based layered lithium-richoxides, and the composition, structure and electrochemical performances of the resultingproducts were investigated by SEM, XRD, Raman, EIS and charge/discharge test.The contents of this thesis include:(1) Synergic dispersion of PVP and EG is proposed tosynthesize Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.3) with size-uniform particles. It is found that thesynthesized oxide can deliver a capacity of205mAh g-1at0.1C, and retain87%of thecapacity after60cycles. The decayed capacity is attributed to inactivation of partialtransition-metal ions.(2) Influence of Li concentration on chemical composition, crystalstructure and electrochemical performance of manganese-based layered lithium-rich oxides isinvestigated. It is demonstrated that lower Li concentration increases the reversible capacitybut decreases the cyclic performance. Additionally, too low Li concentration would lead tophase separation and reduce the capacity of the material.(3) A fast-evaporation approach isproposed to synthesize manganese-based layered lithium-rich oxide Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.5) with low structural defects. The synthesized oxide exhibits good cyclic performance.No capacity degradation is found after50cycles at0.05C, and the capacity retention after300cycles at1C is higher than80%.(4) A self-directed chemical method is proposed tofabricate Li[Li1/3-2x/3Mn2/3-x/3Nix]O2(x=0.5) with a micro-nano structure. The synthesizedoxide delivers a capacity of222mAh g-1at0.05C, with a capacity retention of93%after50cycles. Besides, its capacity at2C is up to110mAh g-1.(5) The influence of Co3+substitution on electrochemical properties of Li[Li0.2Ni0.2-x/2Mn0.6-x/2Cox]O2cathodes isconsidered. With increasing the substitution, the oxygen loss and reversible capacity arelargely enhanced, but the capacity stability and voltage stability are severely weakened.(6)The influence of Fe3+substitution on electrochemical properties of high capacityLi[Li0.2Ni0.13Mn0.54Co0.13]O2cathode is investigated. It is found that the substitution would lead to the decrease of the reversible capacity, but the substitution for Co3+is able to suppressthe voltage decay with extended cycling.(7) The influence of chemical composition andsynthesis temperature on electrochemical properties of xLi2MnO3-(1-x)LiCoO2cathodes isconsidered. The results show that the maximum charge capacity appears at x=0.6, but themaximum discharge capacity locates at x=0.4～0.5. The materials synthesized at800℃have a two-step reaction process at the first charge, but those synthesized at900℃～1000℃possess a three-step reaction process.(8) Redox behavior of different cations M (M=Co3+,Ni3+, Cr3+, Fe3+,[Ni0.5Mn0.5]3+) in Li[Li0.2Mn0.4M0.4]O2, and their influences oncharge/discharge performance of Li[Li0.2Mn0.4M0.4]O2cathodes are investigated.Li[Li0.2Mn0.4Co0.4]O2displays a long charge plateau, Li[Li0.2Mn0.4Ni0.4]O2shows a longcharge slope, and Li[Li0.2Mn0.4Cr0.4]O2exhibits a short charge plateau. The slope and plateaulengths of Li[Li0.2Mn0.4(Ni0.5Mn0.5)0.4]O2are located between those of Li[Li0.2Mn0.4Co0.4]O2and Li[Li0.2Mn0.4Ni0.4]O2. The Li[Li0.2Mn0.4Fe0.4]O2synthesized with a sol-gel method at900℃does not show a traditional “manganese-based layered lithium-rich oxide” structure.Additionally, substitution of M (M=Co3+, Ni3+, Cr3+,[Ni0.5Mn0.5]3+) for Co3+is able toimprove the reversible capacity of Li[Li0.2Mn0.4Co0.4-xMx]O2These investigations provide new pathways and theoretical fundamentals for optimizingmanganese-based layered lithium-rich oxides or improving their electrochemical performance,and enrich the synthesis approaches and structure-performance relation of materials forlithium ion batteries.更多还原