| Nowadays,the progress of global science and technology is driving the huge demand for energy,which puts forward higher requirements for energy storage technology.The development of electric vehicles’ will require more advanced electrochemical technology than current Li-ion batteries.Non-aqueous Li-O2 batteries have received considerable attention and are expected to provide energy density of approximately three to five times that of state-of-the-art Li-ion batteries,but their slow reaction kinetics on the positive electrode leads to increased overpotentials,and battery performance needs to be improved.As the key position of electrochemical reaction,the importance of cathodes of Li-O2 batteries are undoubted.The development of excellent bifunctional(OER/ORR)catalysts has become the first choice to improve battery performance.In this thesis,we develop two bifunctional catalysts:tunable cationic vacancies of cobalt oxides(CO3-xO4)and in-situ deposition of Pd/Pd4S heterostructure on hollow carbon spheres(Pd/Pd4S@HCS).The performance and catalytic mechanism of their Li-O2 batteries were further investigated.Here is the main content:(1)Co3-xO4 were used as a bifunctional cathodes catalyst for Li-O2 batteries.The electrochemical performance was tested,and further DFT theoretical calculation was carried out to explain the reasons for the enhancement of catalytic performance.Firstly,nano-sized glycerolatocobalt(GlyCo)was synthesized from the solution of glycerol and cobalt acetate by a hydrothermal method.Then,GlyCo was heat treated under air condition to successfully form spinel cobalt tetroxide with cation vacancy.The unique structure(Co-O-Co-O)of partial crystalline GlyCo with abundant cobalt and oxygen atoms provide possibility to regulate the vacancies via thermal calcination,and layered structure can promote the formation of cationic defects in oxide crystals.The prepared defected cobalt oxide exhibits a temperature-dependent evolution process on the composition.With the increase of temperature,the vacancy concentration decreases,and the crystal structure of cobalt tetroxide becomes more completed.The performance of the battery was tested when it was used as catalyst,the discharge/charge specific capacities are up to 14517/13254 mAh g" at a current density of 100 mA g-1.Even at the higher current density of 600 mA g-1,the discharge/charge specific capacities could be 8363/3120 mAh g-1.More importantly,the DFT calculations revealed that the metal vacancies resulted in the unlocalized charge distribution and decreased adsorption energy difference between intermediates,thus enhancing the electrocatalytic properties for favorable battery performance.(2)We proposed Pd/Pd4S@HCS as efficient electrocatalysts for rechargeable Li-O2 batteries and further studied the mechanism of its electrochemical reaction.The hollow porous carbon spherical shell was synthesized by hard template method,and then the Pd/Pd4S nanoparticles was successfully supported by in situ reduction on the surface of the carbon shell.The unique hollow hierarchical carbon-matrix structure with a large specific surface area and high electrical conductivity can provide enough spaces for oxygen transport and exposing active sites,as well as storing discharge products in the composite.The Pd and Pd4S nanoparticles show great catalytic activities for advancing ORR and OER during cycling,and the interfacial effect of Pd/Pd4S heterostructure could enable fast charge and ion transport,which could further boost electrocatalytic efficiency.As a result,Pd/Pd4S@HCS cathode deliver superior specific capacities of 19540 mAh g-1 at a current density of 100 mA g-1,favorable rate performance and satisfying cycling stability of 90 cycles with a fixed specific capacity of 600 mAh g-1 at a current density of 100 mA g-1. |
PDF Full Text Request