Optimization And Energy-Storage Mechanism Of O₃-Type Layered Oxide Cathode Materials For Sodium Ion Batteries

The traditional and non-renewable sources of energy are,however,becoming depleted,and the environmental pollution caused by their use is becoming a pressing concern.Therefore,the development of clean and renewable energy has become increasingly urgent.The emergence of Lithium-ion batteries（LIBs）has revolutionized traditional energy storage,but the limited availability of lithium is a constraint to its wider utilization in large-scale energy storage systems.Consequently,Sodium-ion batteries（SIBs）are expected to replace LIBs as a new widely used energy storage technology due to its abundant reserves and low price.In terms of cathode materials for SIBs,layered oxides have been attractive as cathode because of their simple preparation,high theoretical capacity,and stable structural stability.However,the charging and discharging process of layered cathode materials present certain challenges,such as structural changes induced by the inserted and extracted large radius of Na⁺between layers that impede cyclic stability and kinetic performance.This paper discusses the synthesis of P2/O3 composite phase and abnormal sodium deficiency structure O3 layered positive electrode materials utilizing composite structure regulation and cation doping.The structure and electrochemical performance of the materials were studied by using XRD,TEM,XPS and XAS.The detailed content of the thesis is presented in the following two parts.（1）Optimization of layered oxide design and sodium storage mechanism for composite phase sodium ion batteriesThe biphasic P2/O3-Na_2/3Li_1/18Ni_5/18Mn_5/18Ti_5/18Fe_2/18O₂was synthesized using a high-temperature solid-phase reaction method.To investigate the influence of calcination time on the electrochemical properties and structure of the material,a series of experiments were performed.The results revealed that there is a certain relationship between the proportion of P2 and O3 phases and the calcination time in the material at the same temperature.In particular,the O3 phase structure content gradually increased with the increasing of calcination time,and reached the maximum value with P2 phase accounting for 23.53%and O3 phase accounting for 70.27%,when the calcination time was increased to 15 h.The existence of the composite phase structure of the material was well characterized by the XRD refinement and HRTEM results.Furthermore,In-situ XRD analysis identified a reversible phase transition process among P2/O3-P2/P3-P2/OP2-P2/P3-P2/O3 phases,indicating excellent structural stability of the material.Consequently,the P2/O3-biphase electrode achieves a discharge capacity of 97.8 m A h g^-1at 0.1C in the voltage range of 2.5-4.15 V,with a capacity retention rate of 85.5%after 500 cycles.Moreover,the assembled full battery,which employed hard carbon as the anode,demonstrates an energy density of238.74 Wh kg^-1.（2）Construction and electrochemical performance of an abnormal O3-Na_2/3Li_1/18Ni_5/18Mn_5/18Ti_4/18Fe_3/18O₂via cation dopingTheabnormalsodiumdeficiency O3-Na_2/3Li_1/18Ni_5/18Mn_5/18Ti_4/18Fe_3/18O₂was synthesized by calcination at high temperature with element doping.The content of Na in the material is 2/3,which is theoretically a P2 phase structure.The XRD Rietveld refinement results indicate that the doping of Li and Fe elements gives rise to the phase transition from P2 to pure O3 phase.The HRTEM results show that the resulting O3-type material is highly crystalline.The X-ray absorption spectra under different charging and discharging states elucidates the charge compensation mechanism,wherein the redox reactions of Ni and Fe elements provide charge compensation during Na⁺insertion and extraction.In-situ XRD,GITT and DSCV measurements demonstrate that the material undergoes reversible phase transitions of O3-P3-OP2-P3-O3 during the charging and discharging process,while maintaining remarkable sodium ion diffusion kinetics.Consequently,the material shows excellent structural stability even after long-term cycling.The material exhibits a reversible specific capacity of 106 m A h g^-1in the2.5-4.15 V voltage range at 0.1C current density,with 82.3%of the initial capacity maintained after 500 cycles at 1C.