| The energy issue is the basic problem for human survival.At present,the fossil energy such as coal and oil occupies the leading position in the application of energy.But it is well known that the pollution and non renewable nature of fossil energy will lead to its elimination by human society eventually,and the development and application of new sustainable energy is imminent.New energy resources such as wind energy,solar energy and tidal energy can meet the human demand for energy completely.However,the discontinuity of this kind of energy in time and space leads to the inability of large-scale industrial application,advanced energy storage facilities must be built to match it.At present,the more mature energy storage equipment is the lithium ion battery.Lithium ion battery have entered into human production and life as the secondary energy storage facilities in the 20st century.However,the storage of lithium element in the crust is rare and its distribution is very uneven,which results in that the lithium ion battery will not be the end of the development of human energy storage equipments.Sodium,as an element with similar properties to lithium,ranks the sixth in the crustal storage.Sodium ion battery is one of the most potential energy storage batteries in the future.Manganese is a kind of element with rich storage and friendly environment,so there is a good application prospect for the Na-M-Mn-O type cathode materials.The paper mainly studied the Na-M-Mn-O type cathode materials and its doping modification with Fe,Co,Ni elements.Firstly,Li and Zn elements were doped at the M sites in Na-M-Mn-O type materials to prepare Na0.6Li0.4Mn0.75O2 and Na0.6Zn0.2Mn0.75O2,which was for exploring the influence of Zn and Li elements on electrochemical performance.Secondly,Na1-xLixMn0.75O2(x=0.25,0.35,0.45)were prepared to discuss the effects of lithium content on the properties of Na-Li-Mn-O type materials.Finally,the metal elements Fe,Co,and Ni,were doped to prepare Na0.7Li0.2Fe0.1Mn0.7O2,Na0.7Li0.2Co0.1Mn0.7O2,and Na0.7Li0.2Ni0.31Mn0.62O2,and their electrochemical performances were discussed.The conclusions are as follows:(1)For the Zn-doped material Na0.6Zn0.2Mn0.75O2,the specific charge capacity was 184.3 mAh·g-1,and the specific discharge capacity reached 193 mAh·g-1.The capacity retention rate was 72.2%after 30 cycles,at the voltage of 1.5-4.4V,and the current density of 17 mA·g-1.The discharge specific capacity of the Li-doped material Na0.6Li0.4Mn0.75O2 reached 149.4 mAh·g-1,and the capacity retention rate was 94.5%after 30 cycles at the same current density and voltage range.The EIS results revealed that the diffusion coefficient of Na+ in the Zndoped material was larger than that of the Li-doped material.The XPS showed that there were valence changes for Mn and O elements before and after the materials were charged.There were changes between O2-and O2n-(n<4),Mn3+ and Mn4+.(2)For materials Na1-xLixMn0.75O2 with different Li content,when x=0.25,the discharge specific capacity reached 207.2 mAh·g-1,and the discharge specific capacity retention rate was 62.4%after 40 cycles,at the voltage of 1.5-4.4V,and the current density of 17 mA·g-1.When x=0.35,the material had a better cycle performance.The discharge specific capacity was 167.3 mAh·g-1 at the same voltage and current density,and the discharge specific capacity retention rate was 71.7%after 40 cycles.When x=0.45,the discharge specific capacity was 183.9 mAh·g-1,and the retention rate was 62.0%after 40 cycles.There was no much difference in cycle performances of the three materials when increasing the current rate to 0.2C,and their retention rates were about 75.0%after 50 cycles.It was found that the cycle performances of the three materials were improved after reducing the voltage range to 2-4.4V,and that the higher Li content was,the more stable materials were.(3)As for doping Fe,Co,Ni metal elements,the discharge specific capacity of Fe-doped material Na0.7Li0.2Fe0.1Mn0.7O2 reached 181.2 mAh·g-1 at the voltage of 1.5-4.4V,and the current density of 17 mA g-1.The discharge specific capacity was 140.7 mAh·g-1 and the capacity retention rate was 77.6%after 50 cycles.The discharge specific capacity of the Codoped material Na0.7Li0.2C0.1Mn0.7O2 was 167.2 mAh·g-1,and the capacity retention rate was 78.7%after 50 cycles.The capacity retention rates of the two materials were 75.0%after 100 cycles,at the voltage of 2-4.4V,and the current density of 34 mA·g-1.The specific discharge capacity of the Ni-doped material Na0.7Li0.2Ni0.31Mn0.62O2 was 97.4 mAh·g-1 at the voltage of 2-4V,and the current density of 17 mA g-1.The capacity retention rate was 92.1%after 150 cycles. |
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