Preparation And Electrochemical Performances Of Transition Metal Sulfur Phosphide(Phosphate)

With the depletion of readily available fossil fuels and the deepening of climate change,exploring renewable forms of energy conversion and storage has become an important research topic.Supercapacitors have attracted much attention due to their high charging and discharging efficiency,long cycle life,wide operating temperature,high specific power density.However,their energy density is relatively low,which can not meet commercial requirements for high-energy devices.Therefore,there is an urgent need to prepare electrode materials with high performance,especially high specific capacity and energy density.In this thesis,iron-based and nickel-based phosphides and phosphates were prepared and combined with graphene as electrode materials for supercapacitors.Additionally,asymmetric supercapacitors were also assembled and their electrochemical performance were also tested.The graphene/Ni-Co-S-P-0.33 composite was prepared by a two-step method.The obtained material exhibits superior electrochemical performance.The specific capacity of the electrode is as high as 1164.4 C·g^-1 at 1 A·g^-1.Graphene/Ni-Co-S-P-0.33 composite was also used as the negative electrode material of lithium ion battery,and the performance was also tested.The results show that the graphene/Ni-Co-S-P-0.33 composite has a high discharge capacity of 1098 m Ah g^-1 at 0.1 C.Graphene/Fe-Ni-S-P(graphene/Fe Ni SP)composites were prepared by solvothermal method with subsequent phosphating process.Electrochemical studies show that the graphene/NFSP composite has a high specific capacity of 762.6 C·g^-1 when the current density is 5 A·g^-1 and an excellent rate performance at 30 A·g^-1.Asymmetric supercapacitor(ASC)was assembled using graphene/Ni Co-LDH composite and graphene/Fe Ni SP composite as positive and negative electrodes,respectively.The ASC exhibits a high energy density(109 Wh·kg^-1)and good cycle stability(89%capacity retention after 80,000 cycles).Co-doped Ni₃(PO₄)₂(CNP)was synthesized by one-step hydrothermal method.Additionally,CNP/graphene composite(GCNP-0.17)composite was prepared and the electrochemical performances were also characterized.When the current density is 1 A·g^-1,the specific capacity is 734.6 C·g^-1.The capacity is still maintained 623.3 C·g^-1 when the current density is increased to 30 A·g^-1,indicating excellent rate performance.In addition,an ASC has been assembled using GCNP-0.17 composite as the positive electrode,graphene/Fe Ni SP composite as the negative electrode,which shows an energy density of 55.2 Wh·kg^-1.In addition,the ASC has extraordinary cycle stability with 88.7%retention of its initial capacity after 10,000 charge and discharge cycles at a current density of 20 A·g^-1.