The Structure Design And Electrochemical Performance Study Of Cobalt-based Phosphide

Transition metal phosphates(TMPs)are very promising anode materials for lithium-ion capacitors(LICs)due to their high theoretical specific capacity and low lithium insertion potential.However,low electrochemical activity,electronic conductivity,and ion transport are the key factors that limit its development.On the other hand,the kinetic and capacity mismatch between different electrodes limits its wide application in the LICs.This topic takes CoP electrode material as the research object,starting from the point of view of structure design and performance control.Meanwhile,a significant research mainline has been carried out of the composite-heterojunction phase-bimetallic phosphide-dopant phase.The modification of cobalt phosphide is realized by compounding r-GO conductive network,constructing heterogeneous structure,hard template method,doping second phase atom,and nano-structure.These efforts are to study the influence of electronic conductivity and intrinsic structure of materials on its electrochemical performance and to explore the relationship between the structure and electrochemical performance.The main research contents and results are as follows:1.The CoP nanocomposites were prepared with high surface electrochemical reaction activity by introducing the r-GO conductive network.The results show that the CoP/r-GO composite exhibits fast kinetic characteristics dominated by the surface process with the b value is as high as 0.93.It still maintains a specific capacity of510m Ah g^-1 at 0.1A g^-1 after 500 cycles.At the same time,the CoP/r-GO//AC LIC can provide a specific capacitance of 70.3F g^-1 at a current density of 0.1A g^-1,and the maximum energy density and power density are 119.3Wh kg^-1 and 8400W kg^-1,respectively.2.The cobalt phosphide nanorods with various morphologies were synthesized by controlling the phosphating time controllably,including CoP,Co₂P and CoP@Co₂P.The CoP@Co₂P electrode material obtaining excellent rate performance and cycle stability.These results were attributed to the CoP@Co₂P hierarchical heterostructure combine the advantages of CoP and Co₂P,and reasonably designs electrode materials with nanostructure and high conductivity to adapt to the volume changes,and effectively improves electron-ion transport and fast kinetic characteristics.The CoP@Co₂P nanorods provide high reversible specific capacity of 621m Ah g^-1 at 0.1A g^-1after 300 cycles.The CoP@Co₂P//AC LIC provides a specific capacitance of 97.7F g^-1 and achieves high energy densities of 166.2 and 36Wh kg^-1 at power densities of175 and 17524W kg^-1(the capacity retention rate after 12000 cycles is 83.7%).3.Bimetallic phosphide Ni CoP microspheres with excellent cycle performance and high-rate performance were prepared by the hard-template method.Meanwhile,a sustainable strategy was proposed to prepare biomass-derived carbon WPBC-x with high reversible specific capacity.Due to the synergistic effect between the bimetallic atoms and the unique structural integrity,the Ni CoP microspheres show high reversible specific capacity(567m Ah g^-1 at 0.1A g^-1)and excellent rate performance(300m Ah g^-1 at 1A g^-1),outstanding cycle performance(specific capacity 58m Ah g^-1 after more than 15000 cycles at 5A g^-1).Through the combination of Ni CoP anode and WPBC-6cathode,the LIC device provides a specific capacitance of 33.4F g^-1 at 9.6A g^-1 and shows high energy densities of 127.4Wh kg^-1 and 67Wh kg^-1at power densities of 190W kg^-1and 18240W kg^-1,and excellent cycle stability.4.The morphologically controllable Ni_xCo_2-xP(x=2,1.8,1.6,1.4,1.2,1,0.8)electrode materials were prepared by the Co-doping Ni₂P.The results show that the introduction of defect engineering by Co-doping can shorten the transmission path of electrons,and improve the electronic conductivity of the electrode materials from the essential structure.At the same time,the bulk doping of the Co atom will reduce the binding energy between the metal atoms M(Ni,Co)and P atoms,thus improving the intrinsic electrochemical reaction activity and possessing high electrochemical kinetic characteristics.As a result,the Ni_xCo_2-xP electrode materials exhibit ultra-high electrochemical performance,especially the Ni_1.2Co_0.8P which can deliver a reversible specific capacity of 800m Ah g^-1 after 400 cycles at 0.1A g^-1.The results show that the surface electrochemical reaction characteristics,electronic conductivity,Li⁺diffusion ability,electrochemical activity and cycle stability of CoP electrode material can be effectively improved by various modification strategies.At the same time,these strategies provide important significance for the application of other phosphides in energy storage.In addition,the Co Ni P₂O₇ with wide potential window and high chemical stability was prepared to explore the application of pyrophosphate in LICs.The Co Ni P₂O₇//AC LIC can realize a wide potential window of 0-4.5V and can provide a specific capacitance of 52.3F g^-1 within a stable voltage range of 0-4V.