Controlled Preparation And Energy-Storage Properties Of FeCo Phosphides/Sulfides Composite Electrode Materials

The rapid development of modern society has put forward higher requirements for new energy sources.Supercapacitors（SCs）are as a new energy device between conventional capacitors and chemical batteries and have been widely concerned by scholars and industry for the advantages of high charge and discharge efficiency and long service life.Low energy density is still the main problem that restricts the application of SCs.Design and preparation of new electrode materials are the keys to address the issue.Transition metal phosphides/sulfides are considered to be ideal pseudocapacitive materials to replace traditional metal oxides due to their high electrical conductivity and high theoretical capacitances,but low specific surface area makes their actual specific capacitance much lower than the theoretical value.The poor structure stability makes that the rate performance and long-term cycling under high current densities do not meet the practical requirements.The growth mechanism and performance optimization of FeCo phosphides needs a deep investigation.In addition,composite materials often show better performance than single-component materials.However,few reports focused on the composite electrode materials consisting of metal sulfides and phosphides.Control over the composite structure is a challenge,the molar ratio between the sulfides and phosphides as well as their synergistic enhancement effects on electrochemcial performance are still unclear.In this dissertation,uniform arrays of FeCo precursors are firstly prepared,and the mechanism of surface phosphating reaction is studied by analysis of phosphating degree on structure and performance.The electrochemical performance of bimetallic phosphides is also optimized.Based on this,sulfidation of the precursor under the phosphide surface layer is further studied.Porous nanosheet arrays（phosphide on the top）are obtained.Finally,using graphene as the negative electrode,an asymmetric all-solid-state SCs are assembled to achieve an increase in energy density and power density.The main contents are summarized as follows:（1）Uniformly distributed FeCo precursors are grown on the ligaments of three-dimensionally conductive nickel foam（NF）via a hydrothermal process.Then,FeCo phosphides with different phosphation degrees are prepared by adjusting mass ratio of sodium hypophosphite to the precursor.Based on microstructure and component characterizations,the involved mechanism of phosphating reactions is discussed in detail:the reactions of the PH₃（gas phase）with the precursor mainly occurs on the outer surface,which leads to the phosphide nanometer layers with a rough surface because the newly formed phosphide covers the precursor.The electrochemical performance shows that the specific capacitance is the highest when the mass ratio of sodium hypophosphite to the precursor is 20:1,the electrode has a specific capacitance of 2275 F g^-1at a current density of 5 m A cm^-2,and the rate performance is only 55.3%(from 5 m A cm^-2to 40 m A cm^-2).After charge-discharge for 10,000 cycles,the capacitance retention is 81.3%.Using this material as the positive electrode and porous nitrogen-doped graphene as the negative electrode,an asymmetric all-solid device is assembled with an energy density of 47.3 Wh kg^-1(power density of 900 W kg^-1).After charging-discharging 5000 cycles,the specific capacitance remains 80.2%of the initial value.（2）On the basis of FeCo phosphide,FeCo phosphide/sulfide composite electrode materials are further prepared.The vulcanization reaction makes a three-dimensional porous structure composed of many nanosheets interwoven with each other during the composition change of the precursor under the phosphide surface layer.This structure provides more active area and active sites which facilitates the transport of ions during charging and discharging.The phosphide is"squeezed"to the top of the nanosheet.The structural advantages of the external phosphides and internal sulfides make the composite material exhibit high-quality specific capacitance and high cycle stability:the capacitance value reaches 3692 F g^-1at a current density of 5 m A cm^-2,much higher than the corresponding single-component material.At a high current density of 30 m A cm^-2,the capacitance loses for 10,000 cycles of charge and discharge is as low as 9.1%.The energy and power density(82.1 Wh kg^-1,750 W kg^-1)of the prepared asymmetric all-solid supercapacitor is about twice that of the device using FeCo phosphide as a positive electrode.Even at a high power of 6000 W kg^-1,the energy density reaches up to 51.7 Wh kg^-1.After charge-discharge for 5000 cycles,the capacitance retention rate of the device is 93.2%.In addition,we use two devices in series to successfully light up 16 LEDs in parallel,proving the practical application ability of the electrode material for SCs.