【作者】 高静；

【导师】 杜春雨；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2023， 硕士

【摘要】 与商业化的多晶富镍三元正极材料(PCNCs)相比,单晶富镍三元正极材料(SCNCs)具有机械强度高、能量密度高、循环性能优异、热稳定性好等优势引起了广泛的关注,然而不成熟和复杂的合成路线、Li+扩散动力学缓慢和阳离子混排等问题是目前所面临的主要挑战。SCNCs的合成方法如熔盐合成和多步锂化合成由于工艺复杂、能耗高、合成成本高、产率低,设备安全性差等问题限制了其大规模商业化应用。本文采用共沉淀-高温固相法制备SCNCs,大大简化了制备工艺,适合大规模工业化生产。针对SCNCs的Li+扩散动力学缓慢和阳离子混排两大关键性问题,本文通过异质元素掺杂改性策略来提升材料的倍率性能和循环稳定性,并对材料的形貌、结构和电化学性能的构效关系进行了深入分析。具体研究工作如下:本文采用共沉淀-高温固相法制备了单晶型Li[Ni0.83Co0.11Mn0.06]O2(SNCM)正极材料,首先优化正极制备工艺,包括烧结温度、锂配比和保温时间,研究发现,在850℃纯氧气氛下,锂配比为Li:TM=1.04,烧结12 h制备的样品具有稳定的层状结构,较好的单晶形貌,平均粒径2.0 μm,初始放电比容量达到192.3 mAh g-1,在0.5 C倍率下循环100周后容量保持率为82.8%。针对单晶型富镍三元正极材料Li+扩散动力学缓慢这一问题,基于扩展锂层间距,降低Li+扩散势垒这一思路,在锂层引入与Li+具有相似化学性质的大半径Na+作为支柱离子,系统探究了 Na掺杂对于材料的形貌、结构和电化学性能的影响。当 Na 掺杂量为 2%mol 时,Li0.08Na0.02[Ni0.83Co0.11Mn0.06]O2(SNCM-Na2)正极的Li+扩散系数DLi+约为SNCM材料的1.3倍。在3.0～4.3 V电压区间内表现出209.9 mAh g-1的首次可逆容量,在5 C的高倍率下仍有159.5 mAh g-1的高倍率性能,比原始材料提高了 31.1 mAh g-1,在0.5 C倍率下循环100周后容量保持率为92.5%。结果表明,Na+掺杂大幅提升了 Li+扩散动力学。在脱锂过程中,绝大多数Na+作为柱离子稳定存在于主体晶格结构中起支撑作用,缓解了充放电过程中c轴的变化,增强了 SNCM-Na2材料层状结构的稳定性。针对单晶型富镍三元正极材料阳离子混排这一问题,通过掺杂非磁性阳离子Y3+对材料进行改性,系统探究了 Y掺杂对于材料理化性能的影响。当Y掺杂量为 3%mol 时,Li[Ni0.80Co0.11Mn0.06Y0.03]O2(SNCM-Y3)正极表现出 215.8 mAh g-1的首次可逆容量,在5 C的高倍率下仍有153.1 mAh g-1的高倍率性能,比原始材料提高了 23.8 mAh g-1,在0.5 C倍率下循环100周后容量保持率从原始材料的82.6%提升至94.4%。结果表明,一方面,Y3+没有未成对电子,可以缓解过渡金属层的磁阻挫,极大的增加了 Li+/Ni2+混排形成的难度;另一方面,Y-O键的强相互作用可以极大地抑制晶格氧的释放并提高化学稳定性,从而进一步产生优异的循环稳定性。更多还原

【Abstract】 Compared to commercial polycrystalline nickel-rich ternary cathode materials（PCNCs）,single crystal nickel-rich ternary cathode materials（SCNCs）have attracted a lot of attention due to their high mechanical strength,high energy density,excellent cycling performance and good thermal stability,but immature and complex synthesis routes,slow Li⁺diffusion kinetics and cation mixing are the main challenges.The synthesis methods such as molten salt synthesis and multi-step lithiation synthesis have limited their large-scale commercial application due to complex processes,high energy consumption,high synthesis costs,low yields and poor equipment safety.In this paper,SCNCs are prepared by co-precipitation-high temperature solid phase method,which greatly simplifies the preparation process and is suitable for large-scale industrial production.To address the two key problems of slow Li⁺diffusion kinetics and cation mixing in SCNCs,this paper uses a heterogeneous elemental doping modification strategy to enhance the multiplicative performance and cycling stability of the materials,and provides an in-depth analysis of the conformational relationships between the morphology,structure and electrochemical properties of the materials.The specific research work is as follows:In this paper,single-crystal Li[Ni_0.83Co_0.11Mn_0.06]O₂（SNCM）cathode materials were prepared by co-precipitation and high-temperature solid-phase method.Firstly,the cathode preparation process was optimized,including sintering temperature,lithium ratio and holding time.The samples were sintered for 12 h at 850°C in a pure oxygen atmosphere with a Li:TM ratio of1.04 and had a stable layered structure,good single crystal morphology,an average particle size of 2.0μm.The initial discharge capacity reached 192.3m Ah g^-1 and the capacity retention rate was 82.8%after 100 c ycles at 0.5 C.To address the problem of slow Li⁺diffusion kinetics in single-crystal Ni-rich ternary cathode materials,the effects of Na doping on the morphology,structure and electrochemical properties of the materials were systematically investigated based on the idea of expanding the spacing of the lithium layer and lowering the Li⁺diffusion barrier by introducing a large radius Na⁺with similar chemical properties to Li⁺as a pillar ion in the lithium layer.When the Na doping amount was 2%mol,the D_Li+of Li_0.08Na_0.02Ni_0.83Co_0.11Mn_0.06O₂（SNCM-Na2）cathode is approximately 1.3 times that of SNCM material.It exhibited a first reversible capacity of 209.9 m Ah g^-1 in the voltage range of 3.0to 4.3 V and still had a high multiplicity performance of 159.5 m Ah g^-1 at a high multiplicity of 5 C,an improvement of 31.1 m Ah g^-1 over the pristine material,and a capacity retention of 92.5%after 100 cycles at a multiplic ity of0.5 C.The results show that Na⁺doping substantially enhances the Li⁺diffusion kinetics.During the de-lithiation process,the majority of Na⁺is stably present in the main lattice structure as column ions to play a supporting role,alleviating the c-axis changes during charging and discharging and enhancing the stability of the laminar structure of the SNCM-Na2 material.To address the problem of cation mixing in single-crystal nickel-rich ternary cathode materials,the material was modified by do ping with the non-magnetic cation Y³⁺,and the effect of Y doping on the physicochemical properties of the material was systematically investigated.When the Y doping amount was 3%mol,Li[Ni_0.80Co_0.11Mn_0.06Y_0.03]O₂（SNCM-Y3）cathode exhibited a first reversible capacity of 215.8 m Ah g^-1 and still had a high multiplicity performance of 153.1 m Ah g^-1 at a high multiplicity of 5 C,which was 23.8 m Ah g^-1 higher than the pristine material,and the capacity retention at a multiplicity of 0.5 C The capacity retention rate after 100 cycles was increased from 82.6%to 94.4%of the pristine material.The results show that,on the one hand,the absence of unpaired electrons in Y³⁺can alleviate the magnetoresistance frustration and greatly increase the difficulty of Li⁺/Ni²⁺hybrid arrangement formation;on the other hand,the strong interaction of Y-O bonds can greatly inhibit the release of lattice oxygen and improve chemical stability,which further produces excellent cycling stability.更多还原