Doping Modification Of P2-Type Layered Transition Metal Oxide Cathode Materials For Sodium-Ion Batteries

Low-cost sodium-ion batteries(SIBs)have recently attracted entensive attention due to the scarcity and high cost of lithium resources.As the main energy and power output,the cathode materials determine the electrochemical performance and cost of SIBs.Among various cathode materials,P2-type layered transition metal oxides are considered as one of the most promising cathode materials for SIBs owing to the high theoretical capacity,simple synthesis,and environmental friendliness.However,several problems,such as irreversible phase transition during cycling,poor air stability,and inferior electrochemical properties,overwhelmingly hinder their practical applications.Therefore,developing structurally stable and high-performance cathode materials is the key to improve the performance of SIBs.This thesis mainly focuses on the modification of P2-type layered cathode materials by transition metal doping strategy to improve their electrochemical properties.In addition,the charge compensation mechanism and structural evolutions of the synthesized cathode materials were investigated by synchrotron-based soft and hard X-ray absorption spectroscopy(XAS),which could provide better understanding for the modification mechanism.The main contents are as follows:(1)Fe-doped P2-type Na0.6Ni0.25M0.65Fe0.1O2 cathode material was synthesized by high temperature solid phase reaction,and the mechanism of doping effect on the electrochemical performance was systematically investigated.The results show that Fe doping significantly improves the capacity retention and rate performance of the electrode.XAS results show that Ni,Fe,and Mn participate in charge compensation through reversible redox reactions,providing high reversible capacity.Further studies show that Fe3+was oxidized to Fe4+during charging,and the Jahn-Teller effect of Fe4+induced the elongation of the Fe4+O6 octahedra in the z-axis direction.On the one hand,this deformation can promote partial Fe migration to the Na layer,which in turn suppresses the irreversible P2-O2 phase transition and reduces the volume change during cycling,conferring excellent cycling stability.On the other hand,it can accelerate the kinetics of Na+migration and thus improve the multiplicative performance.The research results provide deeper insight into the design of high-performance layered transition metal oxide cathode materials using the Jahn-Teller effect of transition metals.(2)The P2-type Na0.6Ni0.2Mn0.7Cu0.1O2 cathode material was prepared by Cu doping into Na0.6Ni0.3Mn0.7O2,and the improvement mechanism of its electrochemical performance and air stability was comprehensively investigated.It is discovered that Cu dopants with a strong electronegativity could stabilize the crystal structure by inhibiting the common P2-O2 phase transition,leading to the improved cycling stability.The expanded interlayer spacing after Cu doping is facilitated for the charge transfer kinetics,which ensures the excellent rate performance.Furthermore,the XAS results demonstrate that all Ni,Mn,Cu,and O participate in the charge compensation upon sodiation and desodiation through reversible redox reactions.More importantly,Cu substitution improves the moisture stability of the cathode materials because the Cu2+/Cu3+redox couple increases the initial charging potential.This work provides a new theoretical basis for the design of low-cost,high-performance,and air-stable cathode materials with synergistic redox activity of anions and cations.(3)In view of the limitations of single-element doping on the performance enhancement of P2-type cathode materials,we synthesized P2-type Na0.6Fe0.1Cu0.1Mn0.8O2 cathode material using the Cu/Fe co-doping strategy and thoroughly investigated the synergistic enhancement effect of dual-element doping on the electrochemical performance and air stability of the materials.Compared with undoped and single-element doped materials,the discharge specific capacity,cycle stability,and air stability of Cu/Fe co-doped cathode materials are have been significantly improved.In addition,Cu/Fe co-doping can increase the layer spacing and promote the transmission of Na+,thus improving the rate performance of the cell.This work explored the synergistic mechanism of polymetallic doping,providing a new research idea for the design of high-performance SIBs cathode materials.