Preparation Of Hierarchical Structured Transition Metal Phosphate And Phosphide And Their Application In All-solid-state Supercapacitors

Supercapacitors have received widespread attention for their long cycle life and fast charging and discharging speed,but their low energy density cannot meet the needs of real life.In order to increase the energy density,many researches have been emphasised on the strategies,like selection,design of electrode materials and construction of asymmetric devices.Transition metal phosphate（TMP）has the advantages of multiple valence state, good cycling stability and high theoretical capacitance.It is considered as a potential material to increase the energy density of supercapacitors,but this type of material is mainly caused by the electrochemical behavior of diffusion control,so it has poor rate performance and power density.The energy density of TMP-based supercapacitors can be improved by reducing the size of electrode materials,combining with conductive substrates,and combining with pseudocapacitive materials to construct the hierarchical structure.In this paper,transition metal phosphates with high theoretical capacitance are selected,and the energy density of supercapacitors is improved by constructing a hierarchical porous structure.The specific content and conclusions are as follows:（1）KNi_0.67Co_0.33PO₄·H₂O-reduced graphene oxide composite hydrogel（KNCP-GH/NF）supported on nickel foam（NF）was prepared by one-step hydrothermal method,which attains notable improvements in the mass loading and active sites of active materials.KNCP-GH/NF electrode possesses a hierarchical open-porous structure,where KNCP-GH composite hydrogel fills in the voids of NF while the porous graphene hydrogel（GH）provides the large support sites for growing active KNCP nanoflowers.Accordingly,KNCP-GH/NF electrode exhibits striking high capacitance of 6480 m F cm^-2/1751 F g^-1 at 2 m A cm^-2 and satisfactory rate performance with 78.3%retention at 100 m A cm^-2.Further,an all-solid-state asymmetric supercapacitor,constituted by using KNCP-GH/NF and Fe₂P/GH/NF of hierarchical structure and high capacitance as the positive and negative delivers high energy density of 69.2 Wh kg^-1/3.9 m Wh cm^-3 and power density of 13229 W kg^-1/720 m W cm^-3 as well as notable cyclability with 81.2%capacitance retention after 10000 charge/discharge cycles..（2）NCOH@CP/NF with Ni Co double hydroxide and cobalt phosphate supported on NF framework were prepared by a two-step hydrothermal method.The combination of pseudocapacitive material and battery type material effectively realized the regulation of electrochemical behavior of electrode.The NCOH@CP/NF electrode exhibits an excellent specific capacitance of 29590 m F cm^-2/2690 F g^-1at 2m A cm^-2.When the current density is increased to 200 m A cm^-2,the initial capacitance can be maintained at 62.5%,demonstrating good rate performance.The electrode can still retain 72%of the original capacitance after 6000 cycles at a current density of 30 m A cm^-2,showing excellent cycling stability.In addition,in order to avoid the collapse of pore structure in Fe₂P/GH/NF in（1）,this article synthesizes iron phosphide（nickel）nanosheets firstly,and then composites the graphene network with iron（nickel）phosphide nanosheets.The hierarchical structure of Fe Ni P@G/NF electrode is obtained,which has a high specific capacitance of 2236 m F cm^-2/972.2 F g^-1 at a current density of 2 m A cm^-2.When the current density increases to 30 m A cm^-2,the initial capacitance can retain51.7%.After 10,000 cycles,it still has a capacitance retention of 73.9%,showing good rate performance and excellent stability.The all-solid asymmetric device assembled with CP@NCOH/NF and Fe Ni P@G/NF with high capacitance as positive and negative electrodes has a maximum energy density of 92.35 Wh kg^-1 and a maximum power density of 16720.4 W kg^-1.