Novel multi-metal layered double hydroxides hierarchical structures as the battery type active material of battery supercapacitor hybrids: Investigations of growth mechanism and energy storage ability

Layered double hydroxides (LDHs) with high theoretical capacitance and ion-exchange properties have been widely used in battery capacitor hybrids (BSHs), but poor electrical conductivity and structural defects make it difficult to achieve theoretical capacitances. The co-precipitation method is one of the simple synthesis methods. The pH valueof the precursor solution has a great influence on the precipitation and physical and electrochemical properties of products. Using the structure-directing agent with the pH adjustment function can design novel and efficient LDH. ZIF67 is a metal-organic framework (MOF) with high specific surface area and porosity. The central metal of cobalt in ZIF67 has high conductivity and rich redox states. In the past, novel ZIF67-derived cobalt nickel LDHs was synthesized by a one-step coprecipitation method using cobalt nickel salt, 2-methylimidazole and ammonium fluoride as reactants, which has been demonstrated to achieve high energy storage properties. To extend this concept, other ammonium compounds including NH₄BF₄, NH₄HF₂, NH₄Cl, NH₄Br will be used as new additives in this project, the pH value of the precursor solution will be adjusted by changing the amount of ammonium groups, and the interlayer spacing of LDH will be modulated by migrating anions with different sizes and electronegativities. Also, novel LDH hierarchical structures will be established by mixed ammonium-based compounds, which can provide electrolyte ions with different sizes intercalate in and deintercalated from LDH to generate abundant redox reactions. This material is suitable to be applied in multi-ion energy storage systems. This study will explore the effects of the precursor concentration and adding sequence on the physical and electrochemical properties of active materials. Other metals such as Zn, Mo, Mn, Cu and Fe will replace Co and Ni to prepare bimetallic LDHs with better energy storage performances. Based on the results of the bimetallic LDH prepared using a variety of formulations, the material growth mechanism, and the related equations of material composition/LDH interlayer spacing will be summarized to provide formulation design for creating new materials.Finally, the specific capacitance, rate performance and charge-transfer resistance of bimetallic LDH electrodes will be measured. A BSH will also be assembled using the optimal bimetallic LDH positive electrode and carbon negative electrode. Its energy density, power density and cyclic stability will be investigated to understand the practical applications.