Elemental Doping And Core-shell Structure Synergistically Construct ZnCo-based LDHs For Supercapacitors

Supercapacitors have important application value in the field of energy storage due to their high power density,fast charging and discharging speed,environmental protection,and good cycle stability,and electrode materials are one of the key factors to obtain excellent superelectric performance.Layered metal hydroxides（LDHs）show excellent electrochemical performance in the field of energy storage due to their high theoretical capacitance,unique layered structure,low cost,abundant resources,and various oxidation states.The chemical reaction kinetics is relatively slow,the electronic and ionic conductivity is poor,and it is easy to agglomerate during synthesis,so it is difficult to achieve its theoretical capacitance in practical applications.In this thesis,elemental doping and core-shell structure are used to synergistically construct ZnCo-based LDHs with a hierarchical 3D network structure to improve their electrochemical properties.Combined with various characterization methods,the optimal preparation parameters were obtained,and the electrochemical performance enhancement mechanism was studied,and finally ZnCo-based transition metal hydroxide electrode materials with high specific capacitance and high-performance supercapacitors were obtained.The main research results are as follows:（1）N-doped ZnCo-LDH electrode materials（N_y-ZnCo-LDHs）were prepared on nickel foam（NF）by hydrothermal method,and the relationship between N-doping amount and the morphology,structure and electrochemical performance of electrode materials was investigated.the optimal N doping amount(the molar ratio of methylimidazole/(Zn²⁺+Co²⁺)is 0.5)was determined,and the optimal electrode material(N_0.5-ZnCo-LDH)was obtained.It exhibits a high specific capacitance of 943.6 C g^-1(2096.8 F g^-1)and excellent cycling stability at 1 A g^-1.The assembled N-ZnCo-LDH//AC device possesses an energy density of 28.3 Wh k g^-1 at a power density of776.1 W kg–1,and still has 84.04%energy density after 10 000 cycles at 5.0 A g^-1capacitance retention.（2）The N_0.5-ZnCo-LDH electrode material obtained above was further modified and optimized,and the N/Ni co-doped ZnCo-LDH(N_0.5-Ni_y-ZnCo-LDHs)ternary cathode material was prepared by a hydrothermal method,and explore the effect of Ni doping amount on the morphology,structure and electrochemical performance of electrode materials.Under the optimal Ni doping amount（Ni/Co molar ratio=2.0）,N_0.5-Ni_2.0-ZnCo-LDH exhibits 1405.8 C g^-1(3124.0 F g^-1)at 1 A g^-1 high specific capacitance and excellent cycling stability.The N-Ni-ZnCo-LDH//AC device assembled with it as the cathode has an energy density of 31.9 Wh k g^-1 at a power density of 750.6 W k g^-1,and after 10 000 cycles at 5.0 A g^-1,Its capacitance retention rate is still 87.84%,and the coulomb efficiency is 99.50%.（3）Using NiC₂O₄ grown on NF as the core,N_0.5-Ni_y-ZnCo-LDH was coated on its surface to construct a layered NiC₂O₄@N_0.5-Ni_y-ZnCo-LDHs core-shell material.The layered core-shell structure not only provides a high specific surface area that can store more charges,but also acts as a fast diffusion channel and buffer storage site.At the same time,the introduction of heteroatoms modulates the electronic structure of LDH to improve the electrochemical reaction kinetics and electrical conductivity.The assembled NiC₂O₄@N-Ni-ZnCo-LDH//AC device possesses an energy density of 56.9 Wh kg^–1 at a power density of 800.2 W kg^–1.After 10 000 cycles at 5.0 A g^–1,the capacitance retention rate is still 91.53%and the Coulombic efficiency is 99.36%.