Study On Layered Oxide Na₂[Mn₃(□_Mn)_1/2]O₇ As Anode Material For Sodium Ion Batteries

Among many secondary battery technologies,lithium-ion batteries have attracted wide attention due to their high energy density,long cycle life and no memory effect.However,due to their high cost,lithium-ion batteries cannot be applied in large-scale energy storage technologies.Compared with lithium-ion batteries,sodium-ion batteries have obvious advantages in large-scale energy storage technology due to the abundant sodium resources,wide distribution and low cost.The sodium ion layered oxide anode material Na₂Mn₃O₇ named as Na₂[Mn₃(□_Mn)_1/2]O₇(□_Mn is a transition metal vacancy),is a monoclinic crystal system,and its space group is P-1.As a sodium-ion battery anode,it can reversibly embed two sodium ions,and deliver an average potential of about 2.1 V v.s.sodium,which is just within the working voltage window of water.Therefore,Na₂Mn₃O₇ shows great potential in both aqueous electrolyte and non-aqueous electrolyte.However,previous studies found that this material has poor cycling and rate performance.In this work,Na₂Mn₃O₇ was optimized by changing the sintering atmosphere and adjusting the stoichiometric proportion.The Na₂Mn₃O₇ material sintered in air atmosphere has good electrochemical performance in non-aqueous electrolyte and aqueous electrolyte,but the cycling and rate performance is still not very good.On this basis,the crystal structure of Na₂[Mn₃(□_Mn)_1/2]O₇ was investigated in detail.This material has a lamellar structure.Compared to other lamellar materials,Vacancies distributed uniformly in the transition metal layer,and the effect of these vacancies on the electrochemical properties is unknown.Therefore,we boldly proposes to change the local structure by doping the vacancy with transition metal ions,and then study the influence on the electrochemical performance.Two series of materials,Na₂Mn_3+xO₇ and Na₂Mn₃Ti_xO₇,were obtained by doping the vacancy with Mn element and Ti element,respectively.Among them,Na₂Mn_3.4O₇ and Na₂Mn₃Ti_0.4O₇ showed good rate and cycling performance in both non-aqueous electrolyte and aqueous electrolyte.The Na₂Mn₃Ti_0.4O₇ material in the non-aqueous electrolyte sodium ion half battery was recycled for 200 weeks at a 2C rate,and the remaining capacity was66m Ah/g with a retention rate of 83.75%.By doping the vacancy in the transition metal layer,the electrochemical properties of the material are improved,which provides a new direction for the study of cathode materials.At the same time,Mn element in the transition metal layer of the material was partially replaced by Ti in this paper,but the electrochemical performance of the obtained material was not ideal.This also confirmed that the previous study was vacancy doping rather than transition metal element doping.