Preparation And Performance Research Of Three-dimensional Free-standing Electrodes For Lithium-air Batteries

Lithium-air batteries have a theoretical energy density of up to 11400 Wh/Kg and is regarded as one of the most promising energy storage systems.However,its low actual specific capacity,high charge/discharge overpotential and short cycle life still severely limit its practicality,and the research and preparation of high-performance cathode materials is one of the solutions to these problems.This paper focuses on the application of three-dimensional free-standing electrode materials in lithium-air batteries.By means of co-deposition/hydrothermal and other methods,transition metal compounds are loaded on a substrate(nickel foam,biomass)with a three-dimensional structure and then calcined at high temperature to obtain three-dimensional free-standing cathode materials,which are directly used in lithium-air batteries.Such materials have the following advantages:they facilitate the transport of oxygen and electrolyte;they can expand the three-phase reaction interface;they provide greater space for holding discharge products;and they can avoid the problem of side reactions caused by binders.Combining the above advantages,The main research in this paper is as follows:(1)Using wood as the raw material,a nitrogen source(melamine)and a sulphur source(thiourea)were mixed in a NaCl/CoCl₂molten salt system,and a nitrogen and sulphur co-doped self-supported carbonised wood precursor material was obtained in situ by high temperature carbonisation,which,after acid washing,could then be used directly as the lithium-air battery cathode.The successful doping of N and S into the carbon skeleton was confirmed by XRD and XPS characterisation,and SEM,TEM and BET characterisation revealed that the wd-NSC sample inherited the three-dimensional structure of wood,with a large number of micro-pores with a pore size of about 3.5 nm on its surface.The charge/discharge test results showed that the wd-NSC cathode had a discharge specific capacity of 13 m Ah/cm² at a current density of 0.05 m A/cm²,In addition,at a current density of 0.1 m A/cm² and a cut-off capacity of 0.25 m Ah,the cycle life of the wd-NSC positive electrode is 125 cycles.which greatly improved the performance compared to the sample with only melamine added(wd-NC).This is attributed to the multi-level pore structure of wd-NSC's three-dimensional mesh channel-microporous-mesoporous,which provides a large number of catalytic active sites while facilitating the diffusion of oxygen and electrolyte transport.(2)Using grapefruit peel as the biomass carbon material substrate,nanoparticulate Co Ni MOF/PPC precursors were synthesized in situ on the surface of grapefruit peel by co-deposition,and then the resulting precursors were mixed with melamine and carbonized at high temperature to obtain the final product Co Ni@N-CNTs/PPC materials,which were directly used in lithium-air battery cathodes.It was observed by SEM that a large number of carbon nanotubes were uniformly distributed on the surface of Co Ni@N-CNTs/PPC cathode,which was mainly due to the induction of carbon nanotubes when the cobalt-nickel metal was combined with C and N elements at high temperature.The successful doping of Co,Ni and N into the carbon skeleton was also confirmed in XRD and XPS characterization.The charge/discharge test results show that Co Ni@N-CNTs/PPC Positive pole 0.05 m A/cm² current density can discharge up to 57 m Ah,which is the maximum discharge capacity of current lithium-air battery cathode materials.In addition,at a current density of 0.1 m A/cm²,the Co Ni@N-CNTs/PPC cathode can be stably cycled for 132 cycles at a cut-off capacity of 0.56m Ah.The high discharge capacity is attributed to the uniform distribution of carbon nanotubes on the surface of the three-dimensional network of teak peel.Moreover,this loose linear tubular structure is dispersed and not agglomerated,providing a large number of transport channels for oxygen and electrolyte,and accommodating more discharge products.In addition,the introduction of transition metals greatly improves the catalytic activity of the OER/ORR(oxygen evolution reaction/oxygen reduction reaction)of the Co Ni@N-CNTs/PPC anode.(3)Surface-cleaned nickel foam(Ni Foam,NF)was used as the substrate,impregnated with an aqueous solution of cobalt nitrate,urea and ammonium fluoride,and hydrothermally reacted at 120°C for 6 h.The resulting samples were then oxidized at high temperature 350°C to obtain the final product Co₃O₄ nanoarrays.The effects of the heating rate(1?/min,5?/min and 10?/min)on the cell performance were investigated by in-situ CV,EIS and charge/discharge tests,and the optimum heating rate of 1?/min was determined by combining with the physical and chemical characterization of XRD,XPS and SEM.The charge and discharge results showed that the discharge specific capacity of the Co₃O₄nanowire cathode reached 2700 m Ah/g at a current density of 0.05 m A/cm².In addition,at a current density of 0.1 m A/cm²,it can be stably cycled 156 times at a cut-off capacity of 0.28 m Ah.The paper has 43 figures,4 tables and 125 references.