Preparation Of MOF Derived Carbon-based Nanocomposites And Research On Key Issues Of Lithium Storage

Lithium ion batteries(LIB)have dominated the field of portable electronic equipment power sources with the advantages of long lifetime,high energy density and environmental protection.However,with the gradual development of lightweight and multifunction smart devices,the traditional LIB cannot satisfy the increasing energy requirements due to the limited specific capacity of anode which is merely 372 mAh/g corresponding to commercial graphite anode of LIB.The growing anxiety drives researchers to explore the advanced anode materials with high specific capacity for LIB.The carbon-based nanocomposites derived from MOF as a novel coordination polymer have been extensively applied in energy storage field due to high capacity and large specific surface area.Nevertheless,some key issues relating to lithium storage behavior are still be concerning,such as low coulombic efficiency in the first circle and inferior cycle stability.In this thesis,these key issues associating with nanoporous cabon nanocomposites and its phosphates derived from MOF were comprehensively explored.The detailed contents are summarized as follows:(1)The N-doped nanoporous carbon NC-ZIF-8 was synthesized from direct pyrolysis of ZIF-8 in nitrogen atmosphere,and the microstructures,components and capacity fading of nanoporous carbon anode was clearly characterized.When employed as anode for LIB,NC-ZIF-8 owns outstanding initial discharge capacity of 3164 mAh/g in the first circle.The charge capacity fades to 1347 mAh/g and the coulomb efficiency in the first circle is only 42.6%.The undesirable coulombic efficiency in the first circle of residual ZnO characterized by using XRD and XPS is less than 50%.The specific surface area of the sample reached 1275.8 g/cm2 indicated by nitrogen adsorption and desorption curve,and excessive specific surface area increased the possibility of side reactions between the electrode material and the electrolyte.Comprehensively considering ZnO and excessive specific surface area result in lower coulomb efficiency in the first circle of NC-ZIF-8.Secondly,fragile structure resulting from long-time pyrolysis at high temperature are responsible for structural collapse during cycling and undesirable cycle stability.(2)A modified phosphorization approach was utilized to prepare the PC@CNT@CoP nanocomposite via parkerising porous carbon material derived from ZIF-67.The microtopography,structural evolution process during preparation and growth mechanism of carbon nanotubes were investigated by TEM.As a result,PC@CNT@CoP electrode exhibits high initial discharge capacity of 2410.9 mAh/g at current density of 100 mA/g in the first circle and reversible charge capacity of 1187.5 mAh/g.Even the current density increased to 5.0 A/g,the electrode still possesses capacitance of 155.4 mAh/g.The excellent capacitance remaining and rate capability highlight remarkable lithium storage behavior of nanoporous cabon and transition metal phosphate composite electrode derived from MOF.(3)Nickel foam was chosen to in situ form aligned ZIF-67 nanoarrays as current collector.The nanoarrays transformed free-standing electrode composed of CoP3/CP/CoO by employing modified phosphorization approach.The CoO nanowire acts as bridge to link nickel foam and active materials,which not only reinforces the mechanical stability but also enhances the capacity of whole electrode.CoP3 is evenly coupled with carbon polyhedron,which can effectively buffer the volume expansion of CoP3 during the charge/discharge process.Moreover,the novel hierarchical architecture of CoP3/CP/CoO/NF is beneficial to improve the electronic conductivity of electrode.As a result,the CoP3/CP/CoO/NF anode delivers an ultrahigh specific capacity and reliable rate capability.At current density of 500 mA/g,the reversible capacity is 1715 mAh/g in the first circle,which can remain at 1150 mAh/g after 80 cycles and possess specific capacity of 572.3 mAh/g when current density increased to 5.0 A/g.