Defect Engineering Of Transition Metal-Based Materials For High Performance Supercapacitors

With the increasing demand for various portable electronic devices,supercapacitors have been developed because of their longer cycling performance and higher power density than their counterparts of rechargeable batteries.However,their relatively lower energy density impedes their practical applications.According to the equation of E_A=1/2CV²(E_A,energy density;C,capacitance;V,operating voltage),the energy density for supercapacitors can be achieved by increasing the specific capacitance and/or maximizing the operating voltage.In general,the effective methods that enrich the energy density mainly include the strategies to explore high-capacity transition metal-based electrode materials and/or to construct various kinds of asymmetric supercapacitors.Nevertheless,these electrode materials easily suffer from some disadvantages,such as low electrical conductivity,slow charge transfer capability,and poor electrochemical stability.All these limits their implementation in real applications.To address these issues,in this thesis,some strategies including composition control,structural/morphological design,and defect engineering are proposed to upgrade electrochemical performance of supercapacitors.The main research contents are summarized as follows:Firstly,a hierarchical Mn O₂ nanowires-Mn₃O₄ nanoparticles nanocomposite was fabricated by an effective two-step synthesis strategy.This nanocomposite features that Mn O₂ nanowire decorated by Mn₃O₄ nanoparticles.The Mn₃O₄ nanoparticles were formed in situ by controllably etching the surface of?-Mn O₂ nanowires under the reduction condition by Na BH₄ aqueous solution.The mass specific capacitance of the optimized Mn O₂-Mn₃O₄ nanocomposite electrode reaches 278.2 F g^-1 at 20 A g^-1,and the specific capacitance retaines at 99.3%even after 5000 cycles.The asymmetric supercapacitors assembled by optimized Mn O₂-Mn₃O₄ and activated carbon(AC)could deliver an excellent energy density of?41.3 W h kg^-1at?1153 W kg^-1.And the superior cycling stability achieves as well.The excellent capacitance properties of the Mn O₂-Mn₃O₄ nanocomposite can be attributed to the hybrid structure,in which the one-dimension?-Mn O₂ nanowires provide a super highway for electron and ion transfer and the Mn₃O₄ nanoparticles attached offer the abundant pseudocapacitive sites for charge transportation.Secondly,an efficient strategy was proposed to boost the electrochemical performance of F-Cu Co₂S_4-x through the synergetic incorporation of F dopants and S vacancies.The induced effects of F dopant and S vacancy on the physical characteristics and the electrochemical behaviors of F-Cu Co₂S_4-x series were investigated systemically.Experimental results reveal that the introduction of F dopants and S vacancies in dominant lattice can boost the low oxidation state concentrations of Cu and Co species effectively.This leads to the improved electric conductivity and the enhanced interfacial activities of F-Cu Co₂S_4-x,and facilitates the reaction kinetics.The as-synthesized F-Cu Co₂S_4-x exhibits the ultrahigh specific capacity of 2202.7 C g^-1 at 1 A g^-1,and the excellent capacity retention of 96.7%after 5000 cycles at 20 A g^-1.The asymmetric supercapacitors assembled with F-Cu Co₂S_4-x and AC as the positive and the negative electrodes,respectively,delivers the favorable energy density of 49.8 W h kg^-1 at897.39 W kg^-1,as well as the long-term cycling lifetime.Thirdly,P-Co₃O₄@PNC hybrid nanosheets have been prepared by a surface and structural engineering strategy,in which the metal-organic framework(MOF)acts as a template and in situ grown phosphorus-doped Co₃O₄ nanoparticles are uniformly embedded into the conducting P-N co-doped carbon matrix.The hybrid architecture offers some features like the shortened ion diffusion distance,the expanded surface-to-volume ratio,more newly generated active sites,and enrichened structural defects.The high availability of electrochemical active sites/interfaces along with the strong intercomponent synergy of heteroatom-doped Co₃O₄ and carbon enable the fast charge/mass transfer kinetics required for superior charge-storage capabilities.The P-Co₃O₄@PNC hybrid nanosheets deliver a high area capacity of 614 m C cm^-2 at1 m A cm^-2 and an extraordinary cycling stability.Flexible solid-state asymmetric supercapacitors constructed with self-supported P-Co₃O₄@PNC and PNC materials exhibit a high energy density of 69.6?W h kg^-1 at 750 W kg^-1,and display excellent cycling stability with a capacitance retention of 96.8%even after 10000 cycles at 20A g^-1.Moreover,the fabricated P-Co₃O₄@PNC//PNC devices present superior performance uniformities and high flexibilities with no significant capacitance changes under different flexing conditions.Fourthly,defect engineering at the atomic level is adopted to improve charge storage kinetics for P-Ni Co₂S_4-x,through the incorporation of P dopants and S vacancies onto the surface.Experimental results reveal that the introduction of these defects effectively increases the electrical conductivity and induces the formation of low oxidation state Ni and Co species,accelerating the charge transfer kinetics enabling rich Faradaic redox chemistry.Moreover,the partial substitution of S sites with P improves the covalent nature of P-Ni Co₂S_4-x,facilitating surface electroactivity.The as-prepared P-Ni Co₂S_4-xpossesses a high mass specific capacity of 1806.4 C g^-1 at 1 A g^-1 and a 95.5%capacity retention after 5000 cycles at a high current density of 30 A g^-1.Flexible solid-state asymmetric supercapacitors with P-Ni Co₂S_4-x and AC as the positive and negative electrodes,respectively,deliver a high energy dneisty of 68.2 W h kg^-1 at800 W kg^-1and excellent cycling stability.Moreover,the P-Ni Co₂S_4-x//AC devices exhibit good mechanical flexibility with negligible capacitance decay under different bending states.Finally,the strategies of composition control,structural/morphological design,and defect engineering on transition metal-based materials has been proved to effectively improve their electrochemical properties for supercapacitor applications.