【作者】 吴彩霞；

【导师】 潘孝军； 闫鹏勋；

【作者基本信息】 兰州大学 ， 工程·材料工程（专业学位）， 2021， 硕士

【摘要】 随着便携式科技的发展,人们对可持续发展能源存储装置的需求在逐渐增加。目前市场上最主要的电化学储能装置类型是锂离子电池,其中石墨作为商用的负极材料,它的理论比容量只有374 m Ah g^-1,已不能满足日益增长的便携式电子产品等领域对储能装置的需求。为了满足逐渐增长的需求,寻找具有优异电化学性能的负极材料刻不容缓。金属有机框架（MOFs）是一种多孔材料,以金属原子和有机链分别作为节点和配体。相对于传统的多孔材料而言,MOFs有较为优异的孔隙率和孔结构,这些孔结构不仅有利于电解液的渗透,而且这有利于离子的传输,并会改善充放电过程中的体积变化问题。然而MOFs自身的导电性能较差,这导致电极材料容量较低和循环性能较差,造成MOFs在锂离子电池领域的应用受到了限制。为了提高电极材料电导性能,常用的解决方法有以下三种:（1）在合成MOFs过程中引入电活性位点,增加Li⁺储存位点;（2）MOFs通过煅烧碳化,MOFs中的有机链转化为衍生碳;（3）将MOFs与导电剂或导电基底相结合。本论文将分别采用以上三种方法来解决MOFs自身电导性差的问题,ZIF-67是以Co为金属节点,二甲基咪唑为配体的一种最常见的金属有机框架。本论文通过化学气相沉积（CVD）、水热法、高温煅烧等方法制备了高性能的有机框架材料ZIF-67@Fe复合及衍生材料并应用于LIBs的负极。主要的研究成果如下:（1）ZIF-67@Fe及复合材料作为LIBs负极材料本章工作成功制备了含有Fe元素的ZIF-67材料。由于Fe和Co离子的半径和自旋态不同,引入Fe可以改变Co的电子结构,从而提高电极材料的导电率。结果表明:电极材料ZIF-67-CoFe-12在0.5 A g^-1的电流密度下循环1000圈后,ZIF-67-CoFe-12的容量从初始的45 m Ah g^-1增加到63 m Ah g^-1,而ZIF-67的容量从初始的60 m Ah g^-1衰减到6 m Ah g^-1。ZIF-67-CoFe-12电极材料在循环后容量增加的原因可以归结为:活性物质的活化和电解液对电极较为缓慢的浸润,以及ZIF-67-CoFe-12的电荷转移电阻小于ZIF-67。为了提高电极材料的容量,通过水热法在ZIF-67@Fe上生长Co基纳米片制备复合材料电极,结果表明在电流密度为0.5 A g^-1下ZIF-67@Fe@Co-60电极材料的容量达到528 m Ah g^-1,与前驱体ZIF-67@Fe相比较,容量提升了近10倍,但是电极材料ZIF-67@Fe@Co-60的循环性能较差。因此通过添加集流体碳布,在碳布上生长Co基纳米线,改变水热时间来探索对电极材料性能影响,进一步地通过CVD法在Co基纳米线表面包覆一层ZIF-67。结果表明Co-8在电流密度为0.1 C下容量为2.9 m Ah cm^-2,Co-ZIF-40在电流密度为0.1 C下容量为3.3 m Ah cm^-2。（2）ZIF-67@Fe复合材料衍生物作为LIBs负极材料论文进一步地分别对ZIF-67@Fe@Co-60和ZIF-67-CoFe-12进行高温煅烧得到衍生物Co₃O₄-500和CoFe₂O₄-500,结果表明:Co₃O₄-500在电流密度为0.5 A g^-1下循环第500圈时容量为433 m Ah g^-1,容量保持率为56%。CoFe₂O₄-500在电流密度为0.5 A g^-1,循环500圈后,CoFe₂O₄-500的容量从初始的447 m Ah g^-1增加到488 m Ah g^-1,容量保持率为100%。原因可以归结于:CoFe₂O₄作为二元金属氧化物不仅可以提供更多的储锂位点,还可以减少在充放电过程中的体积膨胀。更多还原

【Abstract】 With the development of portable electronics and electric vehicles,the demand for sustainable energy storage devices is increasing.Now,lithium-ion batteries（LIBs）,as one of the most important types of electrochemical energy storage devices,have received more and more attentions.The theoretical specific capacity of current commercial graphite anode materials is only 374 m Ah g^-1,which not meet the growing demand for energy storage equipment in the fields of portable electronics and electric vehicles.In order to meet the growing demand,it is urgent to find high-performance anode materials for LIBs.Metal-organic frameworks（MOFs）are porous materials with metal atoms as nodes and organic chains as ligands.The MOFs possess the excellent porosity and pore structures.These pore structures are conducive to the penetration of electrolyte and the transmission of ions will inhibit the volume expansion problem during charging and discharging.However,the conductivity of MOFs is poor,which makes the electrode material low in capacity and poor cycle performance,and limits its use in LIBs field.In order to improve the electrical conductivity of electrode materials,there are some frequently-used methods:（1）Introduce electroactive sites in MOFs to increase Li⁺storage sites;（2）Calcined MOFs to convert into derived carbon;（3）Combine MOFs with conductive agents or conductive substrates.In this thesis,the ZIF-67 composite and derived materials are prepared and applied in anodes of LIBs.The main research results are as follows:（1）ZIF-67@Fe and composite materials as LIBs anode materialsThe ZIF-67@Fe was successfully prepared.Due to the different radii and spin states of Fe and Co ions,Fe could change the electronic structure of the Co,thereby increase the conductivity of the electrode materials.The results show that after cycling1000 cycles at a current density of 0.5 A g^-1,the capacity of ZIF-67-CoFe-12 changes from the initial 45 m Ah g^-1 increased to 63 m Ah g^-1,while the capacity of the ZIF-67decreased from the initial 60 m Ah g^-1 to 6 m Ah g^-1.The reasons could be attributed to the activation of the active material and the slower infiltration of the electrode by the electrolyte,and the charge transfer resistance of the ZIF-67-CoFe-12 smaller than that of the ZIF-67.In order to improve electrochemical properties,the Co-based nanostructures were grown on surface of the ZIF-67 by the hydrothermal method.The results showed that the capacity of the ZIF-67@Fe@Co-60 electrodes reach to 528m Ah g^-1 at a current density of 0.5 A g^-1,which is 10 times to the ZIF-67@Fe electrodes.Unluckily,the cycle performance of the ZIF-67@Fe@Co-60 is poor.Therefore,by adding a current collector of carbon cloth,then the Co-based nanostructures were grown on the carbon cloth,and the ZIF-67 was further coated on the surface of the Co-based nanostructures by a CVD method.The results show that the Co-ZIF-40 presents a capacity of 3.3 m Ah cm^-2at a current density of 0.1 C.As a contrast,the Co-8 presents a capacity of 2.9 m Ah cm^-2 at a current density of 0.1 C.（2）ZIF-67@Fe composites derived materials as LIBs anode materialsThe ZIF-67@Fe composites were calcinated to generate derived materials.The ZIF-67@Fe@Co and ZIF-67-CoFe-12 were synthetized,and then calcinated at a high-temperature to form Co₃O₄and CoFe₂O₄,respectively.The results show that the Co₃O₄-500 present a capacity of 433 m Ah g^-1 at a current density of 0.5 A g^-1 after 500 cycles with a retention rate of 56%.The capacity of CoFe₂O₄-500 increases from the initial447 m Ah g^-1to 488 m Ah g^-1 and the capacity retention rate of 100%after 500 cycles of current density of 0.5 A g^-1.The reasons could be explained as followed:the CoFe₂O₄as a binary metal oxide could provide more lithium storage sites,also could reduce the volume expansion in charging and discharging processes.更多还原