Doping Modification And Electrochemical Performance Study Of Sodium Manganese Oxide As Cathode Materials For Sodium-Ion Batteries

Currently,lithium-ion batteries(LIBs)have been widely applied in portable devices,electronic vehicles due to the advantages of high energy density,long cycle life and environmental friendliness.Nevertheless,the low abundance and uneven distribution of lithium resources will limit the application of LIBs in large-scale electrical energy storage(EES)systems.In contrast,the renewed sodium-ion batteries(SIBs),with extraordinary superiorities of inexhaustible and ubiquitous of sodium sources,as well as similar chemical storage mechanism to LIBs,are expected to be used in EES systems.However,there are still great challenges for finding suitable host materials to accommodate the Na⁺insertion/extraction due to its larger ion radius than that of Li⁺.Among various cathode materials for SIBs,the tunnel-structured Na_0.44MnO₂ is regarded as a promising cathode material due to its unique three-dimensional crystal structure,which can not only facilitate the Na⁺diffusion rate but also well accommodate the structural change during the charge/discharge process.At present,the preparation process of Na_0.44MnO₂ material usually involves a long time pretreatment and calcination process to obtain the final material,which has the disadvantages of tedious preparation process,time-consuming and energy-consuming.Moreover,its rate performance and cycle stability are not satisfactory,which limits its application and development.Therefore,it is of great theoretical and practical significance to further optimize the preparation process and enhance the electrochemical performance of Na_0.44MnO₂ material.In this paper,the tunnel phase Na_0.44MnO₂ is used as the research target as the cathode material for SIBs.By optimizing the preparation process and investigating the reaction mechanism during the synthesis process,Na_0.44MnO₂cathode material with excellent electrochemical performance was prepared.Subsequently,a series of doping modification strategies were proposed to further enhance its rate performance and cycle stability.And the influence of ion-doping on the structure,morphology,electrochemical properties and diffusion rate,as well as the doping mechanism were systematically explored and investigated.The main works and results include the following aspects:Firstly,Na_0.44MnO₂ electrode material was prepared by oxalic acid coprecipitation method.The influences of different preparation conditions on the structure,morphology and sodium storage performance of the material,as well as the chemical reaction mechanism of the material synthesis and the structural evolution within the electrode redox process were systematically investigated.The experimental results showed that the Na_0.44MnO₂ prepared at 900? for 3 h(denoted as NMO-9003)possessed superior electrochemical performance.A high reversible capacity of 118 m Ah g^–1 could be obtained at 0.2 C,which was close to the theoretical specific capacity(120 m Ah g^–1).In addition,NMO-9003showed excellent rate performance and long cycle life.At a high rate of 5 C,the specific capacity maintained 71 m Ah g^–1 after 600 cycles,corresponding to a capacity retention of 84%.At the same time,the changes of lattice parameters and volume of NMO-9003 during charge/discharge process were calculated.In the voltage range of 2.0–3.8 V,the volume change is only about 7%,indicating that the material has outstanding structural stability.The preparation method used in this experiment has the advantages of facile and efficient,saving time and energy,so it is conducive to industrial application.Secondly,to further optimize the electrochemical properties of Na_0.44MnO₂electrode material,Ti doping strategy was proposed.In this part of work,a series of Ti substituted Na_0.44Mn₁–_xTi_xO₂(x=0,0.11,0.22 and 0.33)materials were prepared by oxalic acid co-precipitation method.And the influences of Ti doping on the structure,morphology and sodium storage performance,as well as Na⁺diffusion coefficient were systematically investigated.The structure characterization showed that the appropriate amount of Ti doping did not change the crystal structure and morphology of Na_0.44MnO₂.Electrochemical performance tests demonstrated that the cathode material with improved electrochemical performance could be obtained when the doping content of Ti was 0.11(Na_0.44Mn_0.89Ti_0.11O₂).The optimized cathode delivered a practically usable capacity of 119 m Ah g^-1 at 0.1 C.Moreover,the specific capacity could reach 96 m Ah g^-1 at high rate of 5 C.After 1000 cycles,the remained specific capacity was 71 m Ah g^-1,which corresponded to 74%capacity retention rate.In addition,galvanostatic intermittent titration technique measurement(GITT)indicated that the Na⁺diffusion rate could be greatly enhanced after Ti introduction.In situ XRD test results showed that the optimized Na_0.44Mn_0.89Ti_0.11O₂ electrode material undergone a lowered volume change of only 5.26%during the Na⁺insertion/extraction process.These results show that Ti introduction can improve the structural stability and reversibility of the Na_0.44MnO₂ material,so it exhibits excellent sodium storage performance.Thirdly,in order to explore the influence of 4d transition metals on Na_0.44MnO₂ electrode material,the Zr doping strategy was proposed.In this part of work,a series of Zr-substituted Na_0.44Mn1-x Zr_xO₂(x=0,0.02,0.04 and 0.06)materials were prepared by oxalic acid co-precipitation method.And the influences of Zr doping on the structure,morphology,electrochemical properties and sodium ion diffusion coefficient were systematically investigated.Structural characterization showed that appropriate Zr doping did not change the crystal structure and morphology of Na_0.44MnO₂.Electrochemical performance tests indicated that the cathode materials with excellent electrochemical performance could be obtained when the doping content of Zr was 0.02(Na_0.44Mn_0.98Zr_0.02O₂).At low rate of 1 C,the discharge specific capacity of Na_0.44Mn_0.98Zr_0.02O₂ was112 m Ah g^-1.Its long cycle lifespan could be supported by the maintained 78m Ah g^–1 after 1000 cycles with a capacity retention of 80%at 5 C.In addition,GITT indicated that the Zr-introduction could also effectively improve the Na⁺diffusion rate.In situ XRD test results demonstrated that the optimized Na_0.44Mn_0.98Zr_0.02O₂ electrode material exhibited a smaller volume change of only 4.25%during the initial charge/discharge process.These results show that Zr introduction can improve the structural stability and reversibility of the material,so it exhibits excellent sodium storage performance.Fourthly,it was found that anion-doping strategy represents one efficient route to adjust the structure of electrode material for advanced reversible batteries by consulting the literatures.Therefore,F-doping strategy was proposed in this part of the work.The effects of F doping and preparation conditions on the structure,morphology,electrochemical performance,and sodium ion diffusion coefficient were systematically explored and investigated.Structural characterization indicated that F doping changed the crystal structure and morphology of the Na_0.44MnO₂ material,which made it gradually evolve from a single-phase tunnel phase to a layered-tunnel hybrid phase.Electrochemical performance tests demonstrated that the optimized cathode material(denoted as NMOF_0.07-9003)with F doping content 0.07 prepared at900? for 3 h exhibited superior electrochemical performance.At 0.5 C and 1 C,NMOF_0.07-9003 delivered an extraordinary discharge capacity of 149 m Ah g^-1and 138 m Ah g^-1,respectively.In addition,NMOF_0.07-9003 also exhibited excellent rate performance and cycle stability.At 5 C,the specific capacity of NMOF_0.07-9003 was 109 m Ah g^-1 and maintained 86 m Ah g^-1 after 400 cycles,corresponding to 79%capacity retention rate.The reason why NMOF_0.07-9003shows excellent electrochemical performance is that the layered-tunnel hybrid phase exhibits synergistic effect in the electrochemical performance,which not only improves the reversible specific capacity of the pure tunnel phase material,but also enhances the capacity retention rate of the single P2 phase material.