| In recent years,aqueous magnesium ion battery system with high theoretical capacity,low production cost and high safety has received more and more attention.Among them,the development of high-performance cathode materials is the key to promote the practical application of aqueous magnesium ion batteries.Sodium manganese oxides are commonly used as cathode materials for lithium-ion batteries and sodium-ion batteries,and are currently less reported as cathode materials for magnesium ion batteries.It was found that sodium manganese oxides have problems such as less storage space and poor structural stability,leading to their low specific capacity and poor cycling stability.To solve the above problems,this work achieves effective modulation of the phase structure and microscopic morphology of the material by controlling the hydrothermal synthesis time and doping with titanium elements to further improve the reversible capacity and capacity retention of sodium manganese oxides.On the one hand,the sodium manganese oxide nanomaterials were prepared by hydrothermal synthesis,and the effective modulation of the phase structure and microscopic morphology of the materials was achieved by regulating the hydrothermal reaction time.It was shown that a small amount of Na4Mn9O18 phase structure appeared in the Na0.55Mn2O4·1.5H2O phase structure with the increase of hydrothermal time.At the same time,the microscopic morphology changed from nanosheets to nanowires.Cyclic voltammetry tests showed that the magnesium ion storage process of the sodium-manganese oxide material was mainly divided into two phases,corresponding to two discharge platforms.The microscopic morphology of the sample（NMO-72）with a reaction time of 72 h is the best mixture of nanowires and nanosheets,and has the best electrochemical performance.The specific capacity of NMO-72reached 213.5 mAh g-1 at 50 mA g-1.After 50 cycles at high rate,the capacity did not decay significantly.Also,the diffusion coefficient of magnesium ions of NMO-72 is enhanced to 1.34×10-10 cm~2 s-1.On the other hand,based on the above optimal hydrothermal synthesis time,the amount of titanium element doping was further controlled to improve the structural stability of the sodium manganese oxide during the charging and discharging process.XRD,SEM,TEM and XPS were used to characterize the phase structure,morphology,and elemental composition of the materials.The results show that the Na0.55Mn2O4·1.5H2O phase gradually transforms into Na4Mn9O18 phase with the increase of titanium doping.The sample with a titanium-manganese atomic ratio of1:20（NMTO-5）has the best electrochemical performance with a uniform distribution of the two phases.the discharge specific capacity of NMTO-5 reaches 231 mAh g-1 at50 mA g-1,and remains 122 mAh g-1 when it rises to 1000 mA g-1,which is 1.5 times higher than that of the pure sample NMO-60 material.After 50 cycles of stable cycling at high magnification,the capacity retention rate reached 97.4%when returning to low magnification again,which is much higher than other materials.In addition,the maximum specific energy of NMTO-5 reached 50 Wh kg-1,the charge transfer resistance was reduced to 2.2Ω,and the diffusion coefficient of magnesium ions was improved to 1.80×10-10 cm~2 s-1,which has excellent multiplicative performance and cycling stability. |
