Sythesis And Property Investigation Of Cathode Materials Based On P2 Type Layer Metal Oxides For Sodium Ion Batteryes

Due to the abundant sodium reserves and wide distribution,the competitive edge of sodium ion batteries in the field of large-scale energy storage is obvious.In order to promote the industrialization of sodium ion batteries,it is the key to develop the low-cost,long-cycle and high-capacity cathode materials.Although P2-Na2/3Mn1/2Fe1/2O2 cathode material has high specific capacity and low cost,it still has the shortcoming of structural instability under high voltage.Based on this,we investigate the effect on the electrochemical properties of Na2/3Mn1/2Fe1/2o2 by different amounts of Mg doping and the modification mechanism;The P2/03 composite phase ratio of Na0.85Mn0.5Fe0.25Nio.25O2 can be controlled by Li doping,so we prepare different ratios of P2/03 composite phase materials and observe the effect of phase proportion on the electrochemical performances.The specific research are shown as below:(1)The Na0.67Mn0.5Fe0.5-xMgxO2 materials(x=0,0.05,0.10,0.15)with different Mg doping amounts are synthesized by a solid state method.We find that the materials synthesized by solid-state method have uniform distribution of elements and Na0.67Mn0.5Fe0.4Mg0.1O2 shows the best electrochemical performance(The discharge capacity is 159.6 mAh·g-1 at 20 mA·g-1 current density,and the cycle retention rate is 85.2%after 50 cycles).Further study shows that the phase transition from P type to Z type will appear if the pristine material is charged above 4.0 V,which leads to structural instability and cycle performance degradation.However,Doping Mg can suppress the phase transition originated from iron migration to alkali metal layer to maintain the structure stability(The capacity retention rate increased from 61.4%to 85.2%after 50 cycles).It can be a reference for suppressing P2-Z phase transition and improving the cycle stability of P2 type metal oxides under high voltage in sodium ion batteries.(2)We successfully prepare Na0.85Liy(Mn0.5Fe0.25Ni0.25)1-yO2(y=0,0.02,0.05,0.10,0.15)with Li doping by co-precipitation assisted solid-phase method.This study shows that the synthesis of P2/03 composite phase materials with different ratios can be achieved by adjusting Li doping amounts.And the proportion increasement of 03 type can significantly improve special discharge capacity and cycling stability.The discharge capacities at 20 mA·g-1 current density are 106.2 mAh·g-1,103 mAh·g-1,110.3 mAh·g-1,116.6 mAh·g-1 and 120.9 mAh·g-1,respectively.And the capacity retention rates are 24.20%,24.66%,30.47%,47.60%and 76.51%after 50 cycles,respectively.Furthermore,we find that the O3 type enable to inhibit P2-O2 phase transition when it reaches a certain level in the P2/O3 composite phase materials.It is instructive for the design of the suitable P2/O3 composite phase cathode materials in sodium ion batteries.