双金属MOFs碳化材料的结构对锂硫电池正极性能的影响

Effect of the structure of bimetallic MOFs carbonized material on the cathode performance of Li-S batteries

利用硝酸钴和铁氰化钾在水溶液中沉淀反应以及ZIF-8在水溶液中的部分分解,分别制备了2种组成相近但结构不同的双金属有机框架前驱体Co-Fe precursor和ZCF precursor。经过多巴胺包覆,对2种产品在氩气气氛下退火获得碳化产物。将碳化产物与纳米硫粉混合得到的碳/硫复合材料(E-CoFeCN@C/S、E-ZCF@C/S)分别作为正极,组装扣式锂硫电池并测试电化学性能。利用X射线衍射仪、扫描电子显微镜、透射电子显微镜、热重分析仪进行形貌和结构分析。在组成相似的情况下,核壳结构碳载体(E-CoFeCN@C)与发散式结构碳载体(E-ZCF@C)对锂硫电池性能的影响呈现出“此消彼长”的特点:E-ZCF@C/S在循环伏安测试中表现出更小的极化以及更强的电流响应,在电化学阻抗测试中表现出更低的电荷转移阻抗,表明该材料有利于促进正极电荷传递过程,即加快电极反应动力学。E-ZCF@C/S在0.2C倍率下放电初始比容量为1211.3mAh/g,在2C倍率下放电初始比容量为794mAh/g,均优于E-CoFeCN@C/S。而核壳结构的优势主要体现在容量衰减方面,E-CoFeCN@C/S在0.2C倍率下经过100次循环后平均衰减率为0.074%(E-ZCF@C/S为0.26%),在2C倍率下循环300次后平均衰减率为0.047%(E-ZCF@C/S为0.13%),说明核壳结构对活性物质的锚固作用明显而对电荷转移不利。

Two bimetallic organic framework precursors,Co-Fe precursor and ZCF precursor,with similar compositions but different structures,were obtained by partial decomposition of ZIF-8 in an aqueous solution and the precipitation reaction of Co(NO_3)_(2) and K_3Fe(CN)_(6) in an aqueous solution.After dopamine coating,the carbonized products were obtained by annealing of the two products in argon atmosphere.The carbon/sulfur composites(E-CoFeCN@C/S,E-ZCF@C/S) prepared by mixing the carbonized products with sulfur nanopowder were used as cathodes,respectively,for assembling button lithium-sulfur batteries and testing the electrochemical performance.The morphology and structure of the samples were characterized and analyzed by X-ray diffractometer(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM) and thermal gravimetric analyzer(TG).With similar composition,the influence of the core-shell structure(E-CoFeCN@C) and the divergent structure of carbon carriers(E-ZCF@C) on the performance of the lithium-sulfur battery was opposite;when one was rising,the other was falling.E-ZCF@C/S showed smaller polarization and stronger current response in cyclic voltammetry test,and lower charge transfer impedance in electrochemical impedance test,indicating that this material was beneficial to promote the positive charge transfer process,i.e.,to accelerate the electrode reaction kinetics.E-ZCF@C/S exhibited an initial discharge specific capacity of 1211.3 mAh/g at 0.2C rate and 794 mAh/g at 2C rate,both of which were significantly better than E-CoFeCN@C/S.The advantage of the core-shell structure was mainly reflected in the capacity attenuation.The average decay rate of E-CoFeCN@C/S after 100 cycles at 0.2C was 0.074%(E-ZCF@C/S was 0.26%),and after 300 cycles at 2C was 0.047%(E-ZCF @ C/S was 0.13%),indicating that the core-shell structure could anchor the active material significantly but was unfavorable for charge transfer.