| Due to the high abundance of sodium sources,sodium ion batteries are expected to apply in large-scale energy storage system,such as national and smart grids.Among all cathodes of sodium ion batteries,the layered sodium transition metal oxides(NaxTmO2),especially Na0.67MnO2 and Na0.67Ni0.33Mn0.67O2,have the advantages of low-cost,easy preparation,environmental sustainability,and high capacity,which makes them one of the most promising cathode materials for sodium ion batteries.However,they still face multiple challenges,including irreversible phase transformations during the repeated charge-discharge process,instable electrodeelectrolyte interface,poor air-stability,and complex charge-compensation mechanisms.The solution to these critical issues is of great benefit for the development of structureperformance relationships as well as solid-state electrochemistry.In this thesis,we adopted a series of spectroscopic,electron microscopic,and electrochemical methods to investigate air-stability,phase transformations,electrodeelectrolyte interface,charge-compensation mechanisms,and full-cell assembling of NaxTmO2 materials.Firstly,the chemical and structural evolution mechanisms of NaxTmO2 in moist atmospheres have been studied.The results indicate that when exposed in moist atmospheres,the sodium ions will be extracted from the bulk of NaxTmO2 due to their high formation energy.If the remaining sodium content in NaxTmO2 is lower than critical sodium content nc,the water molecules insert into the sodium layers and form the hydration phases.The outer layer of some NaxTmO2,decomposes into the corresponding transition metal oxides or hydroxides,once these oxides exposed to moist air or immersed in water.Moreover,based on the study of a variety of NaxTmO2,we proposed that the initial redox potential,i.e.the active redox couples in the first charging process,can be used as a practical principle for evaluating the air-stability of layered sodium transition metal oxides.Secondly,to solve the multiple phase-transformations of Na0.67MnO2,we designed a combined strategy of Zn2+ substitution and Al2O3 coating to improve the cycling performance of Na0.67MnO2 from 70 cycles(initial Na0.67MnO2 electrode)to 400 cycles(Al2O3@Na0.67Zn0.1Mn0.9O2 electrode).In situ X-ray diffraction(XRD)and ex situ solid-state nuclear magnetic resonance(ss-NMR)demonstrate that the phase transformations in Na0.67MnO2 electrode can be mitigated by appropriate amount of Zn2+ substitution.X-ray photoelectron spectroscopy(XPS)and scanning electron microscopy(SEM)shows that the Al2O3 nano-layer can protect the electrode from the corrosion of acid species in the electrolyte and form a uniform and dense CEI film-on the electrode surface to protect the electrolyte from fast decomposition.Then,by combining X-ray absorption spectroscopy(XAS)and density functional theory(DFT)calculations,we investigated the charge-compensation mechanisms of P2-type Na0.67Ni0.33Mn0.67O2.The results demonstrate that Ni2+/3+,lattice oxygen redox reaction,and a small amount of surface Mn ions participate in the charge compensation mechanism of P2-type Na0.67Ni0.33Mn0.67O2.The DFT calculations further show that the released electrons during the lattice oxygen reactions come from Ni-O eg*orbital.This is the reason that the lattice oxygen redox reactions of P2-type Na0.67Ni0.33Mn0.67O2 are highly reversible and no oxygen gas is released during the charge-discharge process.In addition,DFT calculation results also show that the O 2p band and Ni 3d band locate very close and compete with each other at low sodium contents.When too many electrons have been removed from lattice oxygen,the oxidized oxygen ions would attract the electrons on Ni ions and the Ni3+/4+ redox couple participate in the chargecompensation mechanisms.This might be the reason for the different experimental results of the valence-state changes of nickel and oxygen during the charge-discharge process of Na0.67Ni0.33Mn0.67O2.At last,by using Zn2+ substituted P2-Na0.66Ni0.33Mn0.67O2 as models,the influence of different substitution sites on the electrochemical performances of NaxTmO2 has been investigated.The results demonstrate that the substitution sites make a great difference in the electrochemistry of NaxTmO2.Then,based on Na0.67ZnxNi0.33xMn0.67O2 cathodes,we investigated the electrochemical performance of NaxTmO2 and assembled full sodium ion batteries in diglyme-NaPF6 electrolyte.The results show that the layered P2-Na0.67Zn0.07Ni0.28Mn0.67O2 exhibits outstanding chemical/electrochemical stability with a cycle life of more than 200 cycles.The electrochemical performance of a series of assembled petroleum coke/1 M NaPF6(diglyme+2%FEC)/Na0.67ZnxNi0.33-xMn0.67O2(x=0,0.07 and 0.14)batteries were investigated and the full cell based on Na0.67Zn0.07Ni0.28Mn0.67O2 has a long cycle life. |
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