Crystallization And Electrochemical Performance Of Na_0.44Mn_1-xM_xO₂ Cathode Material For Sodium Ion Battery

Lithium-ion batteries are now dominating the electronic markets because of their higher energy density and longer cycle life compared with other rechargeable batteries.They are expanding their applications in large-scale power storage(EES)systems such as electric vehicles and renewable energy power plants.However,these large-scale applications impose stringent requirements on the battery,such as cost-effectiveness,material sustainability and environmentally friendliness.With Li as a rare metal element,lithium source and its availability,together with the needs of low cost for the upcoming large-scale EES applications,present a serious problem.Therefore,the development of new environmentally friendly and sustainable batteries is urgently needed.The sodium ion batteries(SIBs)has a similar working principle with]ithium-ion batteries,and has the advantages of Na resource-rich and low cost,known as the alternative generation of lithium-ion battery energy storage devices.Many cathode materials of high energy density and cycle stability have attracted widespread concerns.Among them,the transition metal oxide NaxMO2 is considered to be the most promising one of the next generation SIB materials,due to a variety of species,structural stability and excellent performance characteristics.However,the researches on the structural transformations and crystal regulation between these materials are still at an early stage.In this thesis,Na0.44MnO2,with a low sodium content,is investigated in terms of the structure evolution and the improvement in its electrochemical performance by doping different metal ions.In addition,we investigate the effects of different cooling systems and electrolyte additive FEC on material structure and properties.In the first chapter,we briefly introduce the working principle and development history of sodium ion battery.Secondly,we pay attention to the transition metal oxides in the cathode materials of sodium ion batteries,including tunnel type Na0.44MnO2 and layered P2 phase,P3 phase and O3 phase materials.Finally,we introduce electrolyte and addictive types of sodium ion batteries,and discuss the background and the scope of this thesis.In the second chapter,we list the experimental reagents and instruments used in this thesis.The devices for material testing and testing methods are described in detail.In the third chapter,the Na0.44MnO2 powder with tunnel phase and the Na0.44Mn1-xXFexO2 samples with different Fe doping contents are prepared by the method of thermal polymerization of acrylic acid.In our experiments,the structure of the Na0.44Mn1-xFexO2 gradually changes from the tunnel structure to the layered P2 phase structure,and finally maintains the P2 phase unchanged with increasing the Fe content.In addition,a certain amount of Fe doping can significantly improve the discharge capacity,but the cycle stability and rate performance degraded.At the same time,we also optimize the calcination time of Na0.44Mn0.89Fe0.11O2,and have found that the optimized calcination time is 10 hours to achieve the best crystallinity of the material.Different calcination temperatures also affect the structure and properties of the materials.The content of P2 phase in the material is higher at 900℃so that the specific capacity is higher.While at 950℃,the content of tunnel phase is relatively higher,so that the cycle stability is better.In the fourth chapter,we find that the structure of Na0.44Mn1-xCoxO2 changes from the T type(x = 0)to the layered P2 phase(x = 0.11)or P3(x = 0.44)with Co doping.In our experiments,the particle morphology corresponds to the crystal structure of the sample.The charge and discharge curves of Na0.44MnO2 in T-type have multiple step-like plateaus,while the layered P2 phase Na0.44Mn0.89Co0.11O2 and P3 phase Na0.44Mn0.56Co0.44O2 show a smoother charge and discharge curve.P2-type Na0.44Mn0.89Co0.11O2 displays a high discharge capacity and excellent cycling performance with 220 mAh g-1 in initial cycle and 180 mAh g-1 after 40 cycles.In the fifth chapter,the doping of divalent elements Ni and Mg was investigated.We find that they have similar relationship with previous work,that is,Ni and Mg doping can also change the structure of Na0.44Mn1-xMxO2 from the tunnel T phase(x =0)to the layered P2 phase(x = 0.11).The difference is that the relative content of P2 phase in the Ni/Mg doped samples is greater than that with Co doping.Also,Co doping level is greater than that of Fe doping(e.g.x = 0.08).These results indicate that Ni/Mg doping is easier to change the structure of Na0.44Mn1-xMxO2 to P2 phase than Co or Fe doping.And when x increases to 0.22 and 0.44,Na0.44Mn1-xMgxO2 continues to maintain P2 phase while Na0.44Mn1-xNixO2 exhibits P2/P3 mixed phase.The P2 phase Na0.44Mn0.89M0.11O2 also shows a optimized discharge capacity:at 0.1 C,2.0-4.2 V,the first discharge capacity of Na0.44Mn0.89Ni0.11O2 is 198.8 mAh g-1 and 142 mAh g-1 is remained after 70 cycles.The first discharge capacity of Na0.44Mn0.89Mg0.11O2 is 188 mAh g-1 and 153 mAh g-1 is remained after 70 cycles.In the sixth chapter,we explore the effect of different cooling procedure on the structure and properties of Na0.44Mn1-xMxO2.In addition,we investigate the effect of the electrolyte additive FEC on the electrochemical properties of the materials.We have found that a rapid cooling in liquid nitrogen can form a tunnel T phase and layered P2 mixed structure,the initial discharge capacity of Na0.44MnO2 is improved,but the cycle stability has declined simultaneously.On the other hand,since the electrolyte additive FEC can form a stable SEI film on the surface of the negative electrode so that the surface reaction and the byproduct generation are reduced.The phenomenon of the first charging abnormality as well as the cyclic stability of the material are improved.The cycling stability of the material improved significantly via the FEC.In the case of high FEC content,the capacity of the battery has declined probably due to the increased degree of the reduction of the organic electrolyte.The thicker SEI film also affects the de-intercaltion of the sodium ions in the negative electrode.Finally,the last chapter makes a brief summary of the innovation and shortcomings of this thesis,and prospects of the future research.