The Study Of Na_2/3Ni_1/3Mn_2/3O₂-based Layered Oxide Cathodes For Sodium-Ion Batteries

Driven by the ever-increasing demand for replacing fossil fuels with clean energy and sustainable energy storage,extensive research has been conducted on secondary batteries.Sodium-ion battery（SIB）are emerging as a promising next-generation technology for stationary energy storage because of their similar“rocking chair”working principle to lithium-ion batteries,the sustainable resource supply and competitive cost benefit.Designing advanced electrode materials is an effective way for development of the SIB.Specifically,layered transition metal oxides Na_xTMO₂（TM=transition-metal ions,0<x≤1）is one of the most attractive research fields in cathode materials for SIB owing to their abundant resources,facile synthesis and environmental benignity,as well as their available 2D space which delivers superior Na-storage capacity.However,the application of the Na_xTMO₂ is restricted by the three major challenges（undesirable phase transition,storage instability,and capacity attenuation）in practice.In this thesis,based on site substitution in the P2-type Na_2/3Ni_1/3Mn_2/3O₂（NNMO）,several cathode materials with superior structural stability and decent electrochemical performance were successfully prepared.Meanwhile,their Na⁺storage properties,electrode kinetics and Na⁺（de）insertion mechanism have been investigated intensively.The major contents are listed below:（1）The electrochemically active iron ions(Fe³⁺)are introduced into the NNMO crystal structure,forming Na_2/3Ni_1/3Mn_2/3-xFe_xO₂（x=0,1/24,1/12,1/8,1/6）to accelerate the step toward superior reversible capacity and structural stability.The Rietveld refinement manifests that the iron ions have been successfully incorporated into the transition-metal oxide layer.In such Fe-substituted materials,the inhibition of P2-O2 phase transition and the amelioration of Na diffusion are achieved,as confirmed by the studies of ex-situ X-ray diffraction（XRD）and electrode kinetics,respectively.In result,the optimized Na_2/3Ni_1/3Mn_7/12Fe_1/12O₂（1/12-NNMF）exhibits much improved electrochemical performance in terms of long-term cycling stability（over 300 cycles,the fading rate per cycle is only 0.05%at 5 C）,outstanding high-rate capability(65 mA h g^-1 at 25 C)and excellent low-temperature energy storage performance（94%capacity retention over 80 cycles at-25 ^oC）.Furthermore,1/12-NNMF also delivers superior sodium ion full-cell properties as coupling with LS-Sb@G anode,thus promoting the practicability of SIB.（2）All sodium ions were substituted with larger ionic radii potassiumion ions to obtain a novel potassiumion-intercalated cathode material for SIB(K_2/3Ni_1/3Mn_2/3O₂,short as KNMO).The strategy could expand the Na⁺diffusion channel and improve the electrochemical properties.On the basis of ex situ XRD analysis,we identify that KNMO maintains a P2/OP4structure when cycled to high voltages and provides a stable skeleton structure upon Na de-/intercalation.As a result,potassiumion-intercalated material KNMO displays more excellent performances than sodium-intercalated material NNMO,as a cathode for SIB.A high Na-storage capacity 197 mA h g^-1 is obtained between 2.2 and 4.4 V.After the rate tests of 35 cycles,the cell can recover all capacity when the rate is returned to 0.1 C,owing to its faster Na-mobility,which is testified by the studies of electrode kinetics.Moreover,KNNO also delivers high K-storage capacity(116 mA h g^-1 at 0.1 C)and remarkable electrochemistry performance,as the cathode in potassium ion battery.（3）A high sodium content P2-type Na_0.9Ni_0.5Mn_0.5O₂（NNM）is synthesized by a facile sol-gel method,aiming at promoting progress toward SIB commercialization.The prepared NNM delivers a high Na-storage capacity of 175 mA h g^-1 with average voltage up to 3.7 V,corresponding to a high energy density of⁶48 W h kg^-1.However,it suffers from insufficient cycling stability induced by the Na⁺/vacancy-ordered and phase transition.When the cut-off voltage is reduced to 4 V,cycling stability will be effectively improved,combine with decreased energy density and limited practical applications.Herein,we accomplish the modulation for NNM by the cosubstitution of Fe³⁺and Cu²⁺.The as-obtained Na_0.9Ni_0.25Cu_0.08Mn_0.59Fe_0.08O₂retained stable crystal structure during cycling（85%capacity retention over 50 cycles at 0.1C）.