新型锂离子电池正极材料LiFeSO4F的掺杂改性研究

LiFeSO4F was discovered as a new cathode material for lithium ion batteries in 2010. The specific energy of LiFeSO4F is similar to that of LiFePO4, with a little higher discharge voltage of 3.6 V. The material has a unique 3D framework, which facilitates the Li+ ion diffusion and transportation. Therefore, LiFeSO4F is regarded as a promising sulphate based cathode material for lithium ion batteries. However, the extremely low electronic conductivity of LiFeSO4F seriously hinders the high rate charge-discharge performance of the material. .The proposed research program aims to increase the electronic conductivity of LiFeSO4F to improve its high rate cycling performance. The first stage of the research is to study the relationships between the electronic/crystal structures, physical properties and electrochemical performance of LiFeSO4F with combinative experimental techniques and first principle calculations. This will enable us to find out the key factors those limit the electronic conductivity and Li+ ion diffusion of the material. Based on this, cation (or anion) doped LiFeSO4F will be designed by first principle calculations, and then be prepared by a novel solvent thermal synthetic method. The doped materials will show much higher electronic conductivities, which will lead to excellent high rate cycling performance. .The first principle calculations will be performed by the local spin density approximation (LSDA), generalized gradient approximation (GGA), or the LDA + U, GGA +U methods. Different experiemental techniques will be used to study the structural and physical properties of the material, such as X-ray diffraction (XRD) , scanning electron microscope (SEM), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy, and SQUID PPMS etc. The electrochemical properties of the material will be studied by charge-discharge cycling, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT) and potentiostatic intermittent titration technique (PITT)..Based on this research, we will obtain doped LiFeSO4F cathode materials which show excellent high rate performance. In parallel, the results obtained form this study will provide a model for the design of cathode materials using first principle calculations.

LiFeSO4F是2010年发现的新型硫酸盐基锂离子电池正极材料，具有重要的应用前景。该材料的工作电压适中，比能量与LiFePO4相当，其独特的三维晶体结构更有利于锂离子的扩散。但是，LiFeSO4F的电子绝缘性严重影响了材料的高倍率充放电性能。针对这一科学问题，本项目拟从材料的晶体结构和电子结构入手，结合第一性原理计算与实验研究，建立起LiFeSO4F的结构、物理性质与电化学性能之间的构效关系模型，揭示影响材料电子导电与离子扩散的关键因素。在此基础上，通过适当的离子掺杂对材料的结构进行调制与剪裁，显著提升材料的电子导电率，从而获得较高的锂离子扩散系数，提高材料的充放电倍率性能，为LiFeSO4F正极材料在锂离子电池中的应用奠定基础。

LiFeSO4F是2010年发现的新型硫酸盐基锂离子电池正极材料，该材料的工作电压适中，比能量与LiFePO4相当，其独特的三维晶体结构更有利于锂离子的扩散，但其自身的电子绝缘性严重影响了材料的高倍率重放电性能，针对这一科学问题，本项目从材料的晶体结构和电子结构入手，采用GGA+U的方法计算了LiFeSO4F/FeSO4F体系的平均插入电压为3.54 V，从LiFeSO4F/FeSO4F的电子结构上看，随着Li离子的脱出，材料从莫特哈伯德绝缘体（LiFeSO4F）转变成了电荷转移型绝缘体（FeSO4F）。通过差示扫描量热法和热重测试发现具有优异的安全性能，初始放热温度为385 oC，放出的热量为79.4 J g-1；为了改善材料的电化学性能，通过氧化石墨烯和聚多巴胺对材料进行包覆改性，改性后材料的电导率提高到1.65×10-4 S cm-1，较原始材料提高了7个数量级，使得材料的电化学性能得到明显提升，100次循环后的的放电比容量为113.2 mAh g-1，容量保持率提高到99%，在5 C下的放电比容量达到48.9 mAh g-1；此外，还分别尝试了Ti离子和Mg离子的掺杂改性，穆斯堡尔谱测试发现Ti离子掺杂在材料的Fe（2）位置，通过计算发现材料的能隙由3.9 eV降低为2.1 eV，提高了材料的电子导电率，而Mg离子作为一种同价位的惰性原子掺杂，在锂离子脱嵌过程中稳定材料的晶体结构，因而获得了较好的倍率性能和循环性能，经过掺杂后材料100次循环后的容量保持率达到95%，在2 C倍率下的放电比容量为45 mAh g-1，通过循环伏安曲线的电化学阻抗图谱测试表明，改性后材料的动力学性能远优于原始的LiFeSO4F材料，因而获得了优异的电化学性能。.通过对LiFeSO4F材料一系列的研究，改善了材料的电化学性能，获得了电化学性能优异的LiFeSO4F改性材料。LiFeSO4F作为一种高安全性能的聚阴离子型正极材料，随着材料循环稳定性和倍率性能的提升，为材料的进一步应用奠定了很好的基础。

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