| Sodium-ion batteries(SIBs)have been regarded as promising alternatives to lithium-ion batteries(LIBs)for future large-scale energy storage systems owing to the low cost,wide availability,and abundant Na resources.However,sodium ions have slower reaction kinetics due to their larger ionic radius than lithium ions.Thus,to find efficient electrode materials is a key to realize the commercialization of SIBs.Compared with the anode materials which have higher energy density for SIBs,the development of cathode materials is relatively slow.Cathode material is the core and demerit of sodium ion battery,which determines the energy density and cost.Designing and constructing cathode materials for SIBs with high specific capacity,long cycle life and excellent rate performance is the focus of current research.During cathode materials,Mn-based layered oxides(NaxMO2)have been considered as one of the most promising cathode materials due to their high specific capacity,environmental friendliness,low cost and easy synthesis.However,these materials also have some disadvantages.The first is the Jahn-Teller effect caused by the presence of Mn3+ ions,resulting in the distortion of the local structure and the loss of specific capacity.Normally,the layered NaxMnO2 has three crystal structures including P2,O3 and P3.Each crystal structure has its advantages and disadvantages and will undergo phase structure transition(such as P2-O2)during the charge/discharge process,resulting in the decayed specific capacity and hindering diffusion of sodium ions.In addition,the Na+/vacancy ordering process will occur during the Na+extraction process which will reduce the Na+ diffusion coefficient.In order to solve the above problems,a series of multiphase materials were synthesized by controlling the cooling rate or ion doping.However,the control of synthesis temperature is complex.And there are few reports on the effect of phase ratio on the properties of multiphase materials.In this paper,a series of multiphase materials were synthesized by controlling the cooling rate or ion doping,and the phase ratio was controlled.A series of microstructure and morphology characterization and electrochemical performance tests were carried out.Finally,the mechanism was studied by in-situ X-ray diffraction/Raman(XRD/Raman)technique The research content and achievements are as follows:1.The solid-state sintering method combined with different cooling methods was used to manipulate the phase structure.P2-Na0.7MnO2-LL,P2/O'3-Na0.7MnO2-KL and P2/O'3-Na0.7MnO2-YD cathodes were prepared by solid-state sintering combined with furnace cooling,air cooling and liquid nitrogen cooling,respectively.When air cooling method is adopted,O'3 phase is induced in the system.The O'3 phase has high specific capacity but poor cycling and rate performance.When the annealed samples were cooled in liquid nitrogen,the proportion of O'3 phase in the system was further increased.Electrochemical test results show that P2/O'3-Na0.7MnO2-KL sample prepared by cooling at air combines the advantage of P2 and O'3 phase,exhibiting high specific capacity(192 mA h g-1 at 20 mA g-1),excellent cycling stability(83%capacity retention after 100 cycles at 1000 mA g-1)and rate performance(93 mA h g-1 at 1000 mA g-1).The in-situ XRD results show that the high voltage phase transition of the P2 phase in the P2/O'3-Na0.7MnO2-KL composite electrode is effectively suppressed,which ensures the relatively stable crystal structure.2.A novel dealloying-annealing strategy was proposed to fabricate Na0.67Mn1-x-yCoxAlyO2(x=0.1,0.2,0.3;y=0.06-0.08)composite cathode for SIBs.The Co substitution could effectively tailor the phase composition of layered NaxMnO2,and the dealloying-annealing strategy is a controllable method to synthesize multiphase composites.The optimized P2/P3-Na0.67Mn0.64Co0.30Al0.06O2(MCA-3)cathode exhibits outstanding electrochemical performance,in terms of the attractive reversible capacity(160 mA h g-1 at 20 mA g-1),superior rate capability(83 mA h g-1 at 1700 mA g-1),and excellent cycling performance(81%capacity retention after 200 cycles at 1000 mA g-1).EIS,GITT and CV methods were used to study the diffusion kinetics of sodium ions.The diffusion coefficients of sodium ions calculated by GITT and CV methods were between 10-11-10-10 cm2 s-1,indicating that the electrode has fast diffusion rate of Na+.Additionally,the operando XRD results clearly unveil the structural evolution of the involved P2/P3 phases during the charge/discharge processes of the MCA-3 cathode.No new phase can be detected,and the P2-P'2 transition is effectively suppressed,which is attributed to the good structural stability of MCA-3.Furthermore,the operando Raman results demonstrate that the P2/P3 biphases exhibit better structural reversibility compared to pure P2 and P3 phases.3.Na0.7Mn1-xAlxO2(x=0.08,0.23)cathode materials were prepared by dealloying-annealing strategy combined with cooling at air.The Al doping can effectively tailor phase composition.The prepared sample is belonging to P2 phase with 8%doping of Al,while the P2 phase change to P3 phase with 23%doping of Al.The optimized Na0.7Mn0.92Al0.08O2(MA-1)cathode exhibits outstanding electrochemical performance,in terms of the attractive reversible capacity(181 mA h g-1 at 20 mA g-1),excellent rate capability(83 mA h g-1 at 1000 mA g-1)and cycling performance(73%capacity retention after 300 cycles at 1000 mA g-1).The excellent electrochemical performances are attributed to relatively stable crystal structure of P2 phase.When the doping amount of Al increases to 23%,the cyclic stability of P3-Na0.7Mn0.77Al0.23O2 sample is also improved compared with that of P2/O'3-Na0.7MnO2 sample prepared by solid-state sintering combining with cooling at air but the cost is a serious loss of capacity.In situ XRD results show that MA-1 samples didn't undergo high voltage phase transformation,only highly reversible P2-P'2 phase transformation is observed,and the crystal structure is relatively stable.This provides guidance and basis for the development of low cost,environmental friendly and high performance cathode materials for SIBs. |
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