Application And Modification Of Layered Oxide Cathode Materials For Sodium-ion Batteries

To meet global energy needs,the use of renewable energy requires effective and efficient energy storage technology.In the secondary batteries,although the lithium-ion batteries(LIBs)have the advantage of high energy density,the reduction of lithium resources and the increase in cost highlights the need of a next generation energy storage batteries to replace LIBs.Therefore,sodium-ion batteries(SIBs)have received extensive attention due to the rich natural abundance and wide distribution of sodium resources.Among many SIB cathode materials,the layered transition metal oxide cathode materials have become a hot research filed because of their high energy densities.However,these materials would experience complex phase transitions during cycling,which leads to the degradation of electrochemical performance.In this thesis,P2 type Na2/3Ni1/3Mn2/3O2 is investigated in terms of the structure stability and electrochemical properties.In addition,the ternary material LiNi0.80Co0.15Al0.05O2 originally for LIBs is investigated for possible application in SIBs.In the first chapter of the thesis,the working principle,development history and key components of SIBs are briefly introduced.Besides,the cathode and anode materials are summarized in detail.We mainly focus on transition metal oxide cathode materials,including tunnel phase,P2 phase and 03 phase.Moreover,the background of research is also provided based on the recent research.In the second chapter,we list the experimental equipment and regents used in this thesis and explain in detail the procedures of coin-cell assembly and electrochemical test methods.In the third chapter,the P2 type Na2/3Ni1/3Mn2/3O2 and Nat/3Ni1/3Mn2/3-xMgxO2 samples with different Mg doping contents are prepared by a sol-gel method.The crystal structure cannot be changed by the doping of small amount of Mg2+.Owing to the larger ionic radius of Mg2+than Mn4+,the c-axis layer spacing of this material is enlarged and the Na+ diffusion coefficient is significantly improved by the GITT method.In addition,it can be clearly seen from charge/discharge profiles that Mg2+doping can inhibit the P2-O2 phase transition and improve electrochemical performance.After 300 cycles,the capacity retention of Na2/3Ni1/3Mn5/8Mg1/24O2 sample can reach 86%.Furthermore,we have coupled Na2/3Ni1/3Mn5/8Mg1/24O2 with Sb/C to make full cells,which show excellent electrochemical properties and low temperature performance.In the fourth chapter,the P2 type Nat/3Niii3Mn2/3O2 is prepared by the same method as for Chapter three.We propose a facile oxidization method by treating Na2/3Nii/3Mn2/3O2 with an(NH4)2S2O8 aqueous solution.During the oxidization process,the Na+ions in the structure are partly withdrawn and the valence of Ni increases,which leads to the formation of a transition metal oxide layer on the surface of the material.Electrochemical tests show that the sodium-storage performance is improved by the oxidization and the sodium ion diffusion coefficient obviously increases around 4.2 V based on GITT measurement In addition,it is found by ex-situ XRD analysis that the surface oxidation treatment helps to inhibit the P2-02 phase transition.In the fifth chapter,the LIB layered mental oxide cathode material NCA is investigated as a possible SIB cathode material for the first time.Specifically,NCA is used as the cathode,sodium metal as the anode and 1 M NaClO4 dissolved in EC/DMC as the electrolyte.From ICP testing under different charge/discharge conditions,it can be concluded that both Li+and Na+can be reversibly intercalated/de-intercalated into NCA structure.The results demonstrate that the hybrid Li+/Na+ battery shows outstanding electrochemical performance.In the voltage range of 1.5 to 4.1 V and 12 mA g-1 current density,the initial discharge capacity is 174 mAh g-1 with an initial coulombic efficiency of 95%.Furthermore,even at a high current density of 1200 mA g-1,the capacity reaches 92 mAh g-1,which shows better performance than the conventional 03 phase cathode materials in SIBs.In the last chapter,we give a brief summary of the innovation and drawbacks of this thesis,and prospects of the future research are also pointed out.