| Due to the depletion of fossil fuels and the change of greenhouse environment,the use and storage of renewable clean energy have been widely concerned and strongly supported.In terms of large-scale energy storage,sodium ion batteries with abundant resources and low price will have broad prospects and strong competitiveness as a supplement and substitute for lithium ion batteries in the future practical application.The working principle of lamellar electrode materials for sodium ion battery is very similar to that of lithium ion battery,and they have the advantages of stable structure,high energy density and easy preparation.Therefore,they have become one of the main research objects of electrode materials for sodium ion battery at present.However,the main reason for the poor rate performance and cycle stability is that the large-size sodium ions are difficult to migrate between the layers.In this paper,the effects of structural design and element doping on the improvement of transition metal layered oxide as sodium anode and negative electrode materials in the rate and long cycle performance were studied,and the practical application of sodium layered oxide cathode materials in soft-coated full battery was explored.This paper is divided into the following three parts.(1)Study on the structure change and sodium storage performance of Tidoped and P3/03 layered intergrowth cathode materialsP type nickel manganese system of layered cathode material with appropriate working voltage and cobalt free environment friendly,no phase change within the range of proper voltage,but the large size of sodium ions in the process of the layer board embedded off the presence of sodium space orderly structure not only show clearly in the process of charging and discharging voltage platform,but also accompanied by sodium space orderly rearrangement ratio caused by poor performance,at the same time in the process of long cycle capacity attenuation rapidly.P3-Na1/3Ni1/3Mn2/3 O2,P3-Na1/3Ni1/3Mn1/2Ti1/6O2,P3/O3Na1/3Ni1/3Mn1/3Ti1/3Ti1/3O2 cathode materials with different proportions and structures were obtained by adjusting the content of Ti by introducing Ti4+ in the transition metal layer which is different from Mn4+ Fermi level.Density functional theory calculations show that when the content of Ti is 1/3,the difference between the thermodynamic stability energy of the P3 structure and the thermodynamic stability energy of O3 is the smallest(1.1 eV).The XRD refinement and spherical aberration correction electron microscopy results both prove that the existence of P3/O3 twophase symbiotic structure.P3/O3-NNMT maintains 78%capacity retention rate after 1000 cycles at 1C,which is much higher than P3NNM(36%);especially,after 2000 cycles of charge and discharge,P3/O3-NNMT still has 68.7%of initial capacity.Ex-situ XRD,GITT,and CV test results respectively revealed the highly reversible structural transformation of P3/O3—P3—P3/O3 and the rapid Na+ diffusion kinetics between the P3/O3 phases,which significantly improved P3/O3NNMT.The electrochemical performance provides a theoretical basis.This work not only explores the effect of transition metal doping on the material structure,but also studies the structure and performance of composite phase cathode materials,providing new ideas for optimizing the kinetics and cycle stability of cathode materials.(2)Stability and kinetics of the abnormal sodium deficiency structure O3Na2/3Ni1/3Ti2/3O2 layered materialThe layered oxide can not only be used as a positive electrode material to provide capacity for the entire battery,but also can be used as a negative electrode material to store sodium ions.Because titaniumbased compounds have good sodium storage capacity,good safety and stability and other advantages,they have developed rapidly in recent years.In batteries,the oxidation-reduction potential of titanium is usually～0.7 V,which ensures high safety during large-scale energy storage.In view of the current low rate performance of titanium-based materials,the 03-Na2/3Ni1/3Ti1/3O2 layered anode material was successfully synthesized by reducing the sodium content and replacing Ti4+ with Ni2+with higher ionic conductivity.The prepared material has a reversible capacity of 112 mA h g-1,and the capacity retention rate can still reach more than 90%after 1400 cycles under 2C conditions.The variable-speed CV test results show that the Na+ diffusion coefficient is 2.0624×10-10 cm2 s-1,and the DFT calculation results confirm that the migration barrier of sodium ions is 0.59 eV,showing high diffusion kinetics.The in-situ XRD method revealed that the main reason for the excellent electrochemical performance is the close to zero structural strain(ΔV=1.1%)during cycling. |
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