A Study On Plasma Regulation And Refilling Strategy Of Cobalt Tetroxide Oxygen Vacancy In Lithium Anode Materials

With the rapid development of modern society,people continue to put forward higher performance requirements for widely used lithium-ion batteries.The theoretical capacity of transition metal oxides（TMOs）is 2-3 times higher than conventional graphite,making them one of the most promising anode materials for lithium-ion batteries.However,the low intrinsic electronic conductivity and volume expansion problems of TMOs limit their applications.Since the electrochemical reaction rate and charge transfer process are affected by the electronic structure of electrode materials,point defect（vacancy and doping）control strategies have been widely used to improve the electronic structure of TMOs,thereby achieving improved electrochemical performance.Unfortunately,excessive defect concentration can also lead to problems such as structural instability and increased diffusion resistance.Therefore,rational design of the composition and concentration of point defects is of great significance to optimize the performance of electrode materials.In this paper,taking transition metal oxide Co₃O₄nanosheets as the research object,combined with plasma oxygen vacancy preparation and heat treatment of heteroatom doping method,the oxygen vacancy concentration,refilling heteroatoms（N,P,S）on the structural stability,electrical conductivity and electrochemical properties were studied by first theory computation and experimental research methods.The specific research content and results are as follows:（1）The effect of oxygen vacancy concentration on the lithium storage performance of Co₃O₄was investigated.Co₃O₄nanosheets were prepared by hydrothermal method,and the effect of plasma treatment temperature（100℃,150℃,200℃,250℃）on the concentration of Co₃O₄oxygen vacancies was studied.XRD,SEM and TEM showed that after plasma treatment at low temperature（100℃,150℃,200℃,250℃）,the phase and morphology of Co₃O₄did not change significantly.After plasma treatment,the ratio of oxygen vacancy（O_V）to lattice oxygen（O_L）increases from 0.056 to 0.692.The increase in the O_V/O_Lratio indicated that the oxygen vacancy concentration also increased with the increase of the plasma treatment temperature.When evaluated as an anode material for LIBs,the electrochemical performance showed that an appropriate concentration of oxygen vacancies could help improve the lithium storage performance,while an excessively high oxygen vacancy would destroy the structural stability of the material and lead to a decrease in the electrochemical performance of the material.Among them,the sample treated with plasma at 150℃（O_V/O_L=0.467）has the best performance,with 50 cycles at the current density of 0.1 Ag^-1,the reversible specific capacity is as high as 1040.2 m Ahg^-1;at the current density of 0.5 Ag^-1,the capacity remained at 560.7 m Ahg^-1after 300 cycles.（2）The effect of refilling phosphorus atoms on the lithium storage performance of Co₃O₄was studied.The samples enriched with different oxygen vacancies were doped with phosphorus atoms by heat treatment,and Co₃O₄with phosphorus atoms refilling oxygen vacancies was prepared.XRD,SEM and TEM showed that the phase and morphology of Co₃O₄did not change significantly after heat treatment.XPS test showed that the ratio of O_V/O_Lafter heat treatment was between 0.11 and 0.23,which was much lower than that before heat treatment,indicating that phosphorus atoms had been successfully filled into oxygen vacancies,and the higher the concentration of oxygen vacancies,the higher the concentration of refilled phosphorus atoms.Phosphorus refilling not only stabilizes the structure,but also exhibits excellent electrochemical performance.The plasma treated sample（Vo-Co₃O₄-200）at 200°C showed the best electrochemical performance after phosphating.After 300 cycles at a current density of 0.5 Ag^-1,the sample still had a capacity of 751.5m Ahg^-1,and the capacity retention rate was 66.6%.The capacity remains at 438.8 m Ahg^-1even at the ultra-high current density of 2 Ag^-1.In addition,the density functional theory（DFT）calculation results also show that the refilling of phosphorus atoms can maintain the structural stability,significantly reduce the band gap,and improve the adsorption energy of lithium ions on the surface.（3）The effects of different refilling heteroatoms（N,P,S）on the lithium storage performance of Co₃O₄were compared.XRD,SEM and TEM showed that the structure and morphology of the material did not change significantly.XPS results showed that the concentration of oxygen vacancies decreased and the content of heteroatoms increased,indicating that different atoms can refill oxygen vacancies.The electrochemical performance shows that backfilling heteroatoms can effectively improved electrochemical performance(PVo-Co₃O₄-200:the capacity of 751.5 m Ahg^-1after 300 cycles at 0.5 Ag^-1;N-Co₃O₄-200:the capacity of 696.2 m Ahg^-1after 300 cycles at 0.5 Ag^-1;SVo-Co₃O₄-200:the capacity of 689.9m Ahg^-1after 300 cycles at 0.5 Ag^-1).At the same time,DFT calculations also show that the formation energy of refilling heteroatoms is lower（compared to oxygen vacancies）,so the structure is more stable;refilling heteroatoms can not only maintain the structure stability,but also reduce the band gap and improve the adsorption energy of lithium ions.Among them,refilling phosphorus atoms can reduce the band gap and increase the adsorption energy to a greater extent,so its electrochemical performance is more superior.