Syntheses And Modification Of NaCrO₂-based Cathode Materials For Sodium Ion Batteries

Lithium-ion batteries（LIBs）have been widely applied to portable electronic equipment and electric vehicles.However,the limited lithium resources and its uneven distribution in the earth’s crust trigger the concerns about the application of LIBs in large-scale energy storage grids.Due to the abundant sodium resources,sodium-ion batteries（SIBs）have been considered as the most appropriate candidates with the similar electrochemical reaction mechanism to LIBs.The cathode is the essential component of a battery to determine its energy density.Considering the large radii of Na⁺,it is very important to develop suitable cathode materials for the optimization of SIBs.The layered sodium transition-metal oxides have the advantages of facile synthesis route and high ionic conductivity,which can achieve high specific capacity and high energy density in SIBs.As a typical layered SIB cathode material,O₃ type NaCrO₂ has a flat potential plateau and provides good electrochemical performances in SIBs when tested between 2.0-3.6 V.Nevertheless,NaCrO₂ suffers from poor rate and cycle performances caused by the unstable interface and the side reactions.Besides,when the charging potential is above 3.6 V,the layered structure of NaCrO₂ transforms into rock salt structure,which is an irreversible process.Hence it is difficult to achieve larger specific capacity and higher energy density by elevating the working potential.This thesis mainly aims at above problems of NaCr02 cathode material,and explore the effect of morphology design,surface coating and ion doping strategies on its electrochemical performances and phase evolution in SIBs.In Chapter 1,the working mechanism and main characteristics of sodium ion battery are firstly introduced.Secondly,the research progress of typical cathode and anode materials for SIBs is reviewed,especially on the research status of NaCr02.Finally,the background and research content of this thesis are put forward.In Chapter 2,the drugs,synthetic methods,characterization instruments and battery test methods used in this thesis are introduced in detail.In Chapter 3,03-NaCr02 sub-microspheres are fabricated through a facile self-template strategy by combining hydrothermal and solid state method.When tested between 2.0-3.6 V in SIBs,s-NaCr02 shows excellent electrochemical performance which delivers 90 mAh g^-1 at 50 C and maintains 87%of the initial capacity after 1500 cycles at 20 C.In addition,the ex-situ XRD reveals the phase evolution in the first charge between 2.0-3.6V:03→O’3→P3,which is reversible during discharge.In Chapter 4,NaCrO₂ is in-situ coated by non-electrochemically active Cr203 through sol-gel method.The ultra-thin Cr₂O₃ layer reduces the direct contact between the active material and the electrolyte,hence effectively inhibits the side reaction in the interface during cycling.Therefore,the cyclic stability of NaCr02 is greatly improved between 2.0-3.6 V in SIBs.After 1000 cycles at 10 C,a specific capacity of 100 mAh g^-1 can still be maintained.In Chapter 5,Na_1-2yCr_1-yNb_yO₂ samples with different ratios of Nb⁵⁺ doping are obtained by sol-gel method,and their phase structure and electrochemical behavior are characterized and analyzed in SIBs.The sodium storage behavior of Na1-2yCr1-yNbyO₂ is investigated between 2.0-3.6 V,2.0-3.7 V and higher charging potential.NCO-Nb3 exhibits higher initial coulomb efficiency and better electrochemical performance when charging potential is above 3.6 V.Between 2.0-3.7V,2.0-3.8V,2.0-4.0V and 2.0-4.4V,the initial coulombic efficiency of NCO-Nb3（97%,57%,29%and 10%）are higher than those of NCO（93%,26%,20%and 6%）.In Chapter 6,Ca²⁺and Ti⁴⁺are doped of Na⁺ and Cr3+in NaCrO₂ respectively by solid state reaction,and Na0.82Ca0.06Cr0.94Ti0.06O₂（NCO-CT）is obtained.The electrochemical performances of the samples in SIBs（2.0-3.8V）are investigated.It is found that dual ion doping can effectively improve the performance of sodium storage of NaCr02.NCO-CT shows specific capacities of 57 and 40 mAh g^-1 at 3 and 4 A g^-1 respectively in SIBs.But NCO has almost no capacity contribution.In Chapter 7,on the basis of Chapter 5 and Chapter 6,NaCrO₂ is doped by Ca²⁺,Ti⁴⁺ and Nb⁵⁺ through solid state reaction,and the interface is optimized by carbon coating method.In addition,the phase transition process of the materials during sodium/potassium storage is investigated by XRD.Through multiple elements doping and carbon coating,the generation of rock salt structure is significantly suppressed in the optimal sample Na0.76Ca0.06Cr0.91Ti0.06Nb0.03O₂@C（NCO-C6T6N3@C）when charged to 3.8 V in SIBs respectively,thus higher initial coulombic efficiencies are achieved.The initial coulombic efficiency of NCO-C6T6N3@C（95%）is much higher than that of NCO（49%）in SIBs,respectively.Chapter 8 summarizes the thesis,points out the innovation and deficiency of this thesis and presents a plan for the future research.In addition,appendix A describes the electrochemical properties of pyrophosphate Cr2P2O7 as a new cathode material for SIBs and LIBs.At the same time,in-situ carbon coating was employed to improve the electronic conductivity and optimize the interface.The optimal sample Cr₂P₂O₇-20PVP provides a specific capacity of 230 mAh g^-1 when tested at 100 mA g^-1 in SIBs.In appendix B,in-situ rGO composite KFeF₃ cathode material is synthesized by solvothermal method,and then KFeF₃/rGO-PVA-500 is obtained after secondary carbon coating.KFeF₃/rGO-PVA-500 shows excellent cycling properties:It maintains 94%of its initial capacity after 1000 cycles at 200 mA g^-1.Besides,XRD,XPS and ICP results reveal the mixed potassium storage mechanism which mainly composes of solid solution and partial conversion reaction.