The Modification And Electrochemical Property Investigation On Layered Oxides Cathode For Sodium Ion Batteries

With the rapidly growing demand for energy and the anxiousness about environmental pollution and climate change induced by extreme utilization of fossil fuels,renewable clean energy has achieved rapid development in recent years.However,renewable energy sources,such as solar and wind energy,are intermittent and geographically limited,making it difficult to directly incorporate into the grid.Therefore,it is very important to develop suitable electric energy storage technology for smoothing the peak and off peak periods.Depending on the energy density and conversion efficiency,secondary batteries are most suitable as electric energy storage devices.Benefited from the high energy density and long cycle life,lithium-ion batteries are already successfully applied in portable electronic devices as well as electric vehicles.But the rising cost of lithium and cobalt makes it difficult for lithium-ion batteries to meet the large-scale application of energy storage devices.Therefore,searching alternatives to lithium-ion batteries featuring low-cost and abundant resources is crucial.Due to the abundance resource of sodium,sodium-ion batteries possessing the advantage of low cost are considered as the most potential candidates for replacing lithium-ion batteries and application in large-scale energy storage.However,the lack of suitable cathode material seriously restricts the practical progress of sodium ion battery.In recent ten years,owing to their close crystal structure,high energy density,and low cost,layered oxides have been regarded as potential cathode materials for sodium ion batteries.However,the complicated phase transition and low diffusion kinetics of sodium ions during charge and discharge process often lead to poor cycling stability and rate performance,which are the key scientific issues that limit its practicality.This dissertation focuses on improving the electrochemical properties of layered oxide cathode materials and studying its modification methods.A series of modification methods are developed and some progresses are made.The specific researches are described as follows:In Chapter One,we first summarized the development and working principle of sodium ion battery.Secondly,the classification and research progress of layered oxide cathode materials are introduced.Finally,we summarized the scientific problems faced by the layered oxide cathodes and reviewed the related modification strategies.In Chapter Two,a simple yet scalable two-step self-template method was developed to fabricate the Ni2+ doped Na0.67Ni0.15Mn0.85O2 hollow polyhedron structure.Due to the two-electron transfer characteristics of nickel ions,hollow polyhedron-induced fast diffusion kinetics of sodium ions and reduced volumetric strain,the modified sample shows higher specific capacity,better rate performance and cycle stability compared with the contrast sample.In Chapter Three and Four,we developed a co-precipitation method to synthsize the one-dimensional porous microcuboid liked Na0.67Ni0.33Mn0.67O2 cathode material with abundant exposed {010} active planes(NNMO-PMCs)and Mg2+ doped Na0.67Ni0.23Mg0.1Mn0.67O2 layered cathode featuring hierarchical one-dimensional nanostructure assembled by nanoplate subunits(NNMMO-HNA),respectively.Research shows that the well exposed {010} active planes effectively enhance the diffusion kinetics of sodium ions,and the one-dimensional porous structure offers the directional electron conduction and exposes more active substances.Compared with the contrast bulk sample,the NNMO-PMCs shows better electrochemical properties,in which the capacity retention rate of the modified samples reaches 94.6%after 1500 cycles at 5C in the potential range of 2.0-4.0 V.In addition,a sodium ion full cell based on this positive electrode can deliver a high power density of 1383.1 W/kg.As for the dual-modified NNMMO-HNA cathode,research indicates that Mg2+ doping effectively inhibits the intrinsic P2-O2 phase transition and reduces the interfacial diffusion activation energy,the one-dimensional hierarchical structure not only improves the diffusion kinetics of sodium ion but also helps to stabilize its structure during the cycling process.Therefore,the modified positive electrode material shows significantly enhanced electrochemical performance.It exhibits a capacity retention rate of 90.9%after 1000 cycles in the potential range of 2.5-4.35 V at 5C.In addition,the sodium ion full cell device based on this cathode can show an extremely high operating voltage of 3.56 V and outstanding energy density of 249.9 Wh/kg.In Chapter Five,a dual-site doping strategy was developed to prepare[Na0.67Zn0.05]Ni0.18Cu0.1Mn0.67O2(NZNCMO)cathode with Cu ion doping at the transition metal site and unique Zn ion selectively doping at the sodium site,which could achieve stable cycle at 4.35 V.Experimental characterization and density functional theory(DFT)calculation results show that the modified positive electrode inhibits the intrinsic P2-O2 phase transition due to the stable O2--Zn2+-O2-"pillar"effect.In addition,the kinetics calculation results show that the dual-site doping strategy reduces the interfacial diffusion activation energy and improves the diffusion kinetics.Therefore,NZNCMO cathode can reach a capacity retention rate of 80.6%after 2000 cycles at 10 C.More importantly,the sodium-ion full cell device assembled by this cathode and the commercial hard carbon anode can achieve a high energy density of 217.9 Wh/kg and a cycle life of 1000 cycles at 1C.In Chapter Six,a strategy was proposed to adjust the local chemical environment of O3-NaNi0.5Mn0.5O2 by Al3+ doping for improving its electrochemical performance.Combined with theoretical calculation and experimental characterization,it is confirmed that Al3+ doping enhances the bond energy of transition metal-oxygen,inhibits the complex phase transition,increases the Na layer spacing.Therefore,compared with the contrast sample,the modulated O3-NaNi0.45Al0.1Mn0.45O2 cathode raises the capacity retention rate from 40.9%to 86.2%after 200 cycles and also enhances the rate performance.In addition,the full cell device assembled by the modified cathode and commercial hard carbon anode can provide a high energy density of 213.5 Wh/kg,indicating its great potential for practical applications.Finally,the main research works of this dissertation are summarized,and the future research direction for the modification of layered oxide cathode materials for sodium ion batteries is prospected.