Structural Regulation And Stability Mechanism Exploration Of Layered Oxide For Sodium-ion Cathode Materials

Layered sodium-ion cathode materials are considered promising candidates for sodium-ion battery cathodes due to their high theoretical capacity,simple synthesis,low cost,and similar crystal structure and energy storage mechanism to commercial layered sodium-ion oxides.Especially,the lower operating voltage,complex phase transitions during charge and discharge progress,and poor cycling performance of Na Mn_0.5Ni_0.5O₂ layered cathode material make it difficult to meet the high energy density,high-rate performance,and long cyclic life requirements of modern energy storage devices.To address these challenges,this thesis employed first-principles calculations to comprehensively study and evaluate the correlation between the phase structure stability,electrochemical performance,and mechanical properties of sodium-ion layered oxides with various active elements and designed superior element doping schemes based on a comprehensive analysis of performance data.Specifically,the thesis investigated the effects of element doping on the crystal structure and electrochemical performance of Na Mn_0.5Ni_0.5O₂layered cathode material,and elucidated the correlation between material chemical composition,phase transition direction,and electrochemical performance,while exploring the mechanism of element doping in enhancing the structural stability and electrochemical performance of the material at high voltage.The details are summarized as follows:（1）The structural stability,energy density,diffusion property,electronic conductivity,mechanical properties and elastic anisotropy for O3-type Na_xTMO₂（TM=Sc,Ti,Cr,Mn,Fe,Co,Ni,Cu,Zn,and Al）systems were systematically investigated through first-principles calculations.The outcomes derived from the computational analyses suggest that Sc,Ti,and Al serve as superior doping agents.The systems in which they are incorporated demonstrate robust structural strength,superior structural stability,minimal lattice distortion throughout phase transitions,and outstanding mechanical performance,except for their electrochemical performance.Optimized elemental combination strategies for Na Mn_0.5Ni_0.5O₂ layered cathode material are proposed to uphold the electrochemical performance whilst enhancing the mechanical properties.Based on the comprehensive analysis and evaluation results,Sc and Ti elements are selected for the following doping research for Na Mn_0.5Ni_0.5O₂layered cathode.（2）A Sc-doped strategy of introducing charge polarons into the crystal structure was explored to regulate the internal electronic structure and overall properties for Na Mn_0.5Ni_0.5O₂ materials.It is found that the electron localization within crystal structure induced by charge polaron can effectively enhance the structural strength,electrochemical and kinetic properties of the original material.The formation of the OP2 phase at high voltage during the cycling process is also induced to alter its phase transition path and thus significantly inhibits the capacity loss and structural degradation caused by irreversible phase transitions in the original material.Experimental results show that the doped material has excellent reversible specific capacity,rate performance,and cycling performance.It delivers a reversible specific capacity of 194.4 m Ah g^-1 at the rate of 0.1 C and 104.6 m Ah g^-1 at 10 C,and a capacity retention rate after 500/1000 cycles of 76.9%and 63.3%,respectively,which is better than that of the original material.（3）A Mg/Ti co-doping strategy has been proposed to tune the electronic structure and ordered arrangement of Na⁺/vacancy during the cycling process of Na Mn_0.5Ni_0.5O₂ materials.Co-doping with Mg/Ti can promote the formation of O3-type Na Mn_0.5Ni_0.5O₂ material,while increasing the layer spacing of the TMO₂ layer and reducing the spacing of the Na O₂ slab in the doped structure,thereby improving the structural stability of the doped sample.In addition,Mg/Ti co-doping is demonstrated to effectively suppress the irreversible P3-O1 phase transition of Na Mn_0.5Ni_0.5O₂ material at high voltage,improving its structural reversibility and cycling stability in the cycling process.Experimental results show that Mg/Ti co-doping can improve the electronic conductivity,and enhances the Na⁺diffusion kinetics and rate performance at high voltage of the doped material.It delivers a reversible specific capacity of177.7 m Ah g^-1 at 0.1 C rate and 104.1 m Ah g^-1 at 5 C rate,and a capacity retention rate after 200 cycles at 0.2 C rate of 72.65%,which is better than that of the original material.（4）A Na Ni_0.5Mn_0.5O₂ layered cathode material with high-performance and high sodium-content P2/O3 intergrowth structure was constructed by boron-doping via manipulating the interlayer interaction.Incorporating strong covalent B-O bonds into the crystal structure through boron-doping effectively stabilizes the oxygen framework of Na Ni_0.5Mn_0.5O₂,promoting oxygen covalent electron localization,and thus facilitating charge transfer and Na⁺migration.Additionally,the interstitial[BO₄]³⁺electriferous groups within the crystal structure can regulate interlayer electrostatic repulsion and van der Waals attraction,inducing a high sodium-content P2/O3 intergrowth structure composite with O3 as the primary phase.Experimental results show that the P2/O3 composite material can release a reversible capacity of 171.5 m Ah g^-1 at a current density of 0.1 C and of75.3 m Ah g^-1at a high rate of 5 C,and exhibit a capacity retention of 74.1%after 300 cycles at 1 C rate,which is better than that of the original material.