Core-shell And Doping Modification Of O3 Phase In Layered Cathode Materials For Sodium Ion Batteries

Lithium-ion batteries have been widely used in electronic devices such as mobile phones?camera and laptops duo to their high energy density.However,there are serious concerns about the cost of lithium because of its scarcity and concentrated distribution.Under these circumstances,sodium-ion batteries are considered to be a promising alternative,especially when the cost and supply are taken into account.Na-based transition metal layered oxides Na TMO₂?TM=transition metal?,as an important class of cathode materials for sodium ion batteries,have attracted widespread attention since their simple synthetic process and excellent electrochemical performance.There are two important branches in Na TMO₂,O3-type and P2-type,which are distinguished by occupancy of sodium ion and stacking method of oxygen atom.Compared with P2-type,O3-type cathode materials have attracted wide attention because of high sodium ions content and superior reversible capacity.As one of the most promising O3-type cathode materials,Na Ni_0.5Mn_0.5O₂has further cost-effective and environmentally friendly advantages.However,there are still some problems that restrict its development,such as poor cyclic stability,inferior rate performance and complex phase transition.In this dissertation,we focus on O3-Na Ni_0.5Mn_0.5O₂ and introduce the core-shell structure into it.We propose two simple synthetic methods for forming the core-shell structure and further doping to obtain Na Ni_0.5Mn_0.5O₂with high capacity?excellent rate performance and good cycle stability,and the structure-effect relationship between the battery performance and the microstructure of the cathode material Na Ni_0.5Mn_0.5O₂ was deeply studied.First,using melamine as a bridge,a shell with a high concentration of manganese ions was formed on the surface of Ni_0.5Mn_0.5CO₃ by electrostatic attraction,and then mixed with sodium hydroxide at a certain stoichiometric ratio,fully ground and sintered to obtain a series of core-shell cathode materials with high manganese in surface and high nickel in core.The capacity retention rates of CS-0.05 is 48.01%while that of pristine is 39.5%after 105 cycles at 1 C in the voltage range of 2.0-4.3V.In order to further optimize the electrochemical performance of the material,on this basis,we have further studied the modification of elemental doping.By consulting the literature and reports,the doping of lithium element can play a role in inhibiting the phase change.The zirconium oxygen bond has a strong binding energy,so it can be concluded that the incorporation of zirconium can play a role in stabilizing the structure.In summary,we have selected lithium and zirconium for doping.The initial capacities of CLS-0.05 and CZS-0.05 are 95.10?103.59 m Ah?g^-1,while that of the pristine is 82.07 m Ah?g^-1at a high current density of 3 C in the voltage range of 2.0-4.3 V.The capacity retention of CLS-0.05,CZS-0.05 improved by 1.16 times and 1.26 times compared with that of the pristine one.Secondly,we directly use nickel oxide as the center,and precipitate manganese ions on its surface through the principle of heterogeneous nucleation,and then mix and grind with sodium salt to synthesize the core-shell structure with high manganese on the shell?hereinafter referred to as CS-NM?.Test the rate performance under different current densities?0.2 C?1 C?2 C?5 C?8 C?10 C?.The corresponding CS-NM discharge specific capacity is 130,102,80,53,42,31 m Ah g^-1,while the corresponding capacity of the pristine sample is 102,78,63,37,22,18 m Ah g^-1,and severe cracks appear on the surface of the original sample through scanning electron microscopy,but the sample structure of the core-shell material remains almost unchanged.These works can provide more possibilities for the study of the stability of sodium ion batteries.