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Research On Modification And Sodium Storage Of O3-type Ni-Mn Based Cathode For Sodium Ion Batteries

Posted on:2022-06-18Degree:DoctorType:Dissertation
Country:ChinaCandidate:M Z LengFull Text:PDF
GTID:1482306314457734Subject:Materials science
Abstract/Summary:PDF Full Text Request
With the arrival of the portable and wearable devices,electric vehicles and smart rid era,the contradiction between the explosive demand for lithium and the shortage of lithium resources is bound to become more and more intense.As a hot research direction of secondary batteries,sodium-ion batteries(SIBs)naturally comes into the vision of designers.SIBs have many comparable characteristics and unique advantages compared with lithium-ion batteries(LIBs).Firstly,the reversible storage and migration mechanisms of SIBs and LIBs are similar.Secondly,sodium is a popular substitute element for lithium,and its abundance in the earth’s crust is as high as 2.6%.At the same time,there is a large amount of sodium chloride in seawater.Third,the distribution of sodium is completely independent of resources and geographical restrictions,which is also an important factor in large-scale energy storage systems.Finally,some calculations show that the cost of SIB is about 0.37 RMB/Wh in the future,which is lower than 0.47 RMB/Wh of LIB,and there is still room for decline.At the same time,not only is the number of papers on SIBs soaring,but companies that rely on SIBs technology are also working hard to commercialize SIBs,which could have significant implications for reducing the overreliance on lithium.With the continuous expansion and deepening of the research field of SIBs,the application of cathode materials has gradually changed from single to diversified.From the perspective of technological development,it is necessary to design more new composite cathode materials to meet the commercial requirements of cost,safety performance,circulation capacity and energy density.At present,the research focus of cathode materials for SIBs mainly includes transition metal oxides NaxMO2(M=Ni,Fe,Co,Mn,Cr and other transition metals,0<X≤1),polyanion compounds and prussian blue compounds.Transition metal oxides have obvious advantages as energy storage materials,such as abundant active element centers,high reversible specific capacity and good electrochemical activity.They are very promising materials in the research history of cathode materials for SIBs.However,in the process of charge and discharge,transition metal oxides will face a series of problems.First,both P2 phase and O3 phase have phase transition problems,which is not conducive to the long-term stable cycle of the battery.Especially when charging above 4.2 V,the storage location of Na+changes from the original triangular prism vacancy to the octahedral vacancy,resulting in severe volume shrinkage and the formation of a new O2 phase(ABACAB stacking).Second,another major challenge facing layered oxides is their environmental sensitivity,especially their tendency to absorb moisture when exposed to air,which leads to poor battery performance and increased transportation costs.Third,due to the large Na+ radius,the layered oxides containing Na are usually kinetically slow.All these factors are not conducive to the long-term stable cycle of batteries,but also restrict its large-scale practical applications.In this paper,based on Ni-Mn oxides,the materials are designed and adjusted from two aspects of doping modification and surface modification to improve the electrochemical performance of the battery such as capacity,rate and cycle stability.The storage mechanism and diffusion mechanism of sodium are deeply studied from atomic/electronic,structure,surface and other aspects.(1)In view of the unstable factors such as the distortion of crystal structure which is prone to occur in the charging/discharging process of O3-NaNi0.5Mn0.5O2,a small amount of electrochemical inert elements Ti and Zr were introduced into the transition metal layer to improve the stability of the structure without sacrificing the energy density and safety of the battery.The experiments showed that the specific capacity of O3-NaNi0.45Mn0.3Ti0.2Zr0.05O2 in 2-4 V voltage range,under the ratio of 0.05 C,increased from 115.2 mA h g-1(NaNi0.5Mn0.5O2)to 141.4 mA h g-1.After 200 cycles,the capacity retention increased from 57%to 70%.When the electrochemical inert elements Ti4+ and Zr4+ entered the lattice of NaNM,the lattice parameter c and the transition metal layer spacing expanded without changing the original lamellar structure and space group type,which is conducive to the diffusion of Na+.The electron delocalization was improved,and the further accumulation of valence electrons around Ni Fermi level was promoted.The increase of mixing entropy after doping was also beneficial to the stability of crystal structure.(2)In view of the insufficient capacity of O3-NaNi0.5Mn0.3Ti0.2O2 cathode material and its inability to withstand high voltage and other shortcomings,the chemical composition and lattice structure were optimized through element doping,so as to effectively improve the storage capacity of Na/Li.The experiments showed that the specific capacity of O3-NaNi0.45Mn0.3Ti0.2Ru0.05O2,in 1.5-4.5 V voltage range,under the ratio of 0.05 C,increased from 144.4 mA h g-1(NaNi0.5Mn0.3Ti0.202)to 155.3 mA h g-1.After 50 cycles,the capacity retention can reach 70%.When the Ru4+entered the lattice of NaNMT,the transition metal layer spacing and lattice parameter c expanded,which were conducive to the diffusion of Na+,and the conductivity was also improved.The diffusion coefficient was changed from 4.3×13-13 cm2 s-1 to 5.68×10-11 cm2 s-1。without changing the original layered structure and space group type,the crystal structure is more stable and the rate performance is better.(3)In view of wet chemical method and mechanical polishing method for surface coating limited in powder material,the magnetron sputtering method directly on the surface of O3-NaNi0.45Mn0.3Ti0.2Zr0.05O2 composite electrode will reduce the contact area between the electrode and the electrolyte,and inhibit the interface side effects.When the sputtering time is about 10 s,the thickness of MgO film is about 6 nm.The electrochemical test showed that the specific capacity of NaNMTZ-MgO10 increased from 130.7 mA h g-1(NaNi0.45Mn0.3Ti0.2Zr0.05O2)to 140.7 mA h g-1 at 0.05 C,in the voltage range of 2-4 V.After 300 cycles,the capacity retention can reach 71.6 mA h g-1 at 1 C.Different from the wet chemical method and mechanical grinding method,MgO thin layer directly inhibited the dissolution of the active substance NaNi0.45Mn0.3Ti0.2Zr0.05O2 in the electrolyte,and increased the stability of the electrode structure with promoting the Na+diffusion and electron transport.At 5 C and 8 C,the discharge capacities of NaNMTZ-MgO10 can also reach 80.3 and 70.3 m A h g-1.(4)In view of the transition metal elements,there are still many gaps and unclear mechanisms in the application of Nb/Mo/Cr in the cathode materials of SIBs.After introducing Nb/Mo/Cr into O3-NaNi0.5Mn0.3Ti0.2O2 as doping elements,the relationship between material composition,crystal structure and battery performance was further explored.NaNi0.45Mn0.3Ti0.2M0.05O2(M=Nb/Mo/Cr)compounds have carried on the electrochemical tests in the voltage range of 2-4.2 V.The results showed that the specific capacity of NMTCr increased from 147.5 mA h g-1(NaNi0.5Mn0.3Ti0.2O2)to 185.7 mA h g-1 at 0.05 C.The capacity retention increased from 41.8%to 57%at 1 C after 100 cycles.For NMTNb and NMTMo,the Jahn-Teller effect of Mn3+/Ni3+and the SOJT effect of Nb5+and Mo6+are all important factors that lead to the instability of the crystal structure of doped materials and the deterioration of battery performance.Cr3+doping can minimize the Mn3+content,thus reducing the adverse effects of related Jahn-Teller effect,which is conducive to the stability of the layered structure.The diffusion coefficient of NMTCr is.3.65×10-11 cm2 s-1,and the diffusion barrier is only 0.37 eV.
Keywords/Search Tags:Sodium ion battery, Layered oxide, Doping modification, Surface modification, Crystal structure
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