Preparation Of O3-NaFe_0.5Mn_0.5O₂ As Cathode Material For Sodium Ion Batteries And Study Of Electrochemical Performance

In recent decades,lithium-ion battery technology has been widely used in mobile electronicequipment and electric vehicles.However,due to the tension between the high load levels of renewable energy sources and smart grids and the restricted lithium resources,the sustainability of lithium-ion battery is increasingly worrying.In order to alleviate these problems,research has focused on alternative energy storage systems,and sodium-ion batteries are expected to be applied to large-scale energy storage systems considering the abundance of sodium resources.Among sodium-ion battery cathode materials,layered transition metal oxides are a promising cathode materials for sodium ion batteries due to their lower molecular and higher theoretical specific capacity.It is crucial to explore the layered oxides with excellent performance without expensive elements shuch as nickel and cobalt.The layered cathode materials containing iron/manganese have the advantages of rich elements and environmentally friendly,as well as higher working voltage and specific capacity,but still have some significant drawbacks that limit its further applications.Therefore,this paper takes the O3-Na Fe_0.5Mn_0.5O₂ cathode materials as the research object,and improves the inherent defects via the method of element doping.The main research contents of this paper are as follows:（1）The oxalate precursors combined with high temperature sintering were used as a preparation method.Na Fe_0.5Mn_0.5O₂ and Na_0.9Fe_0.5Mn_0.5O₂ are synthesized,and meanwhile doped with different content of Cu to improve the elctrochemical performance and air stability.The experimental results show that the Na_0.9Fe_0.5Mn_0.5O₂ material achieves a discharge specific capacity of 137.1 m Ah g^-1 at 0.1 C and a voltage range of 2-4.1 V;Sodium vacancies speeds up the kinetic reaction process,which is reflected in the rate performance.At a high rate of 5 C,Na Fe_0.5Mn_0.5O₂ and Na_0.9Fe_0.5Mn_0.5O₂ cathode materials achieve the discharge capacity of 10.6m Ah g^-1 and 24.2 m Ah g^-1,respectively.After the Cu doping,the electrochemical performance is further improved.When the Cu doping content is 0.1,Na_0.9Fe_0.35Mn_0.55Cu_0.1O₂ electrode material with excellent electrochemical performance can be obtained,which achieves a discharge capacity of 131.9 m Ah g^-1 at 0.1 C.The discharge capacity of Na_0.9Fe_0.35Mn_0.55Cu_0.1O₂ material is 40.3 m Ah g^-1 at 5 C,and the capacity retention rate was69.3%after 100 cycles at 0.2 C,which is greatly improved compared with the raw material.In addition,the materials exhibits improved air-stability.After the Na Fe_0.5Mn_0.5O₂ material has been immersed in water,more Na₂CO₃.H₂O and Na HCO₃ impurity phases seriously suppress its electrochemical performance.In contrast,the major structure of the Na_0.9Fe_0.35Mn_0.55Cu_0.1O₂material shows insignificant change,and after 200 cycles at 0.5 C,it still maintains a relatively high capacity retention rate.（2）Considering that the introduction of Cu²⁺has sacrificed part of the specific capacity,basedon the work of Cu doping,an attempt was made to further enhance the reversible specific capacity and electrochemical properties.Therefore,the Cu/Zr double doping strategy was tried to synthesize Na Fe_0.4Mn_0.5-xCu_0.1Zr_xO₂（x=0.005,0.01,0.015）materials,and it was found that the introduction of trace amounts of Zr significantly promoted the electrochemical performance.When the Zr doping content is 0.01,it is the most optimized component.The Na Fe_0.4Mn_0.49Cu_0.1Zr_0.01O₂ material achieves a discharge capacity of 147.5 m Ah g^-1 at 0.1 C and a voltage range of 2-4.1 V;the discharge capacity of Na Fe_0.4Mn_0.49Cu_0.1Zr_0.01O₂ material at0.2 C is about 142.1 m Ah g^-1.The capacity retention rate reaches more than 69.6%after 100cycles;even at 5 C,the discharge capacity of Na Fe_0.4Mn_0.49Cu_0.1Zr_0.01O₂ still reaches 70.8 m Ah g^-1;In addition,the material exhibits a highly reversible phase transition process,improving the sodium storage performance of raw materials;similarly,the Na Fe_0.4Mn_0.49Cu_0.1Zr_0.01O₂ shows improved air stability,and it also exhibits similar charge-discharge curves and cycle stability at0.2 C before and after water immersion treatment.