Preparation And Electrochemical Performance Of Multi-metal Doped Cathode Material For Sodium Ion Batteries

As a new type of energy storage device,the sodium ion battery has become a new research hotspot in the field of energy storage in recent years because of its low cost and relatively abundant sodium resources.Among the many anode materials for sodium ion batteries,the layered transition metal oxide Na_xMeO₂ has the advantages of higher working voltage,higher theoretical specific capacity and so on,which has been attracted the attention.However,there are many deficiencies in this type of material:for example,the phase change will affect the cycle life during charging and discharging,and the weakness of kinetics has inhibited the development of high energy density.At present,the research of the new cathode materials for NIBs focuses on the Na_xMeO₂.However,it is not clear that the electrochemical reaction mechanism of the electrode materials and the migration process of sodium ions during the charging and discharging,especially for Na_xMeO₂ by multi-metal doping.Therefore,this research adopts the doping strategy to modify the electrode,and prepares the different layered transition metal oxide Na_xMeO₂ electrode materials.It has been discussed that the diffusion kinetics and phase transition process of electrode materials after doping with different elements.The main contents are as follows:?1?The layered Na_0.67Mn_0.67Ni_0.33O₂ was synthesized by hydrothermal combined calcination method.At current densities of 0.1 C and 0.5 C,it decayed to 120 mAh g^-1 and110 mAh g^-1 after 100 cycles respectively,simultaneously the rate performance and cycle stability of Na_0.67Mn_0.67Ni_0.33O₂ has been tested.The decay mechanism is the irreversible phase change process and the expand and contract of cathode,resulting in the appearance of irreversible specific capacity.At the same time,the Mn³⁺ion will also react with the electrolyte,and its production is dissolved in the electrolyte,resulting in the phenomenon of vacancies or atom misalignment between adjacent atomic layers.?2?Then,a new type of multi-metal NaMnNiAlO?NaMeO?multi-metal electrode material was successfully synthesized,and Mg and Co were selected for doping and modification.The charge-discharge specific capacities of the first cycle of Co-NaMeO electrode materials are 149 mAh g^-1 and 140 mAh g^-1,respectively.After the 100 cycle,the charge and discharge capacity decayed to 111 mAh g^-1 and 108 mAh g^-1,the retention rate can reach 76.8%,75.2%,77.7%and 74.8%at 1.0 C,2.0 C,5.0 C and 10.0 C.At the same time,the specific capacity of Mg-NaMeO at first cycle is 100 mAh g^-1 and 92 mAh g^-1,which shows the cycle stability is better than Co-NaMeO,although,less than the specific capacity is better then the former.The results show that the doping of Mg element can play a role in improving ion migration and stabilizing the structure.?3?On the basis of previous research,MgCo-NaMeO was prepared as electrode material by co-doping with Mg and Co.The charge and discharge capacities of the MgCo-NaMeO electrode at the first cycle are 127 mAh g^-1 and 126 mAh g^-1 at a rate of 1.0 C,and the capacity stabilizes at 117 mAh g^-1 and 115 mAh g^-1 after the 20 cycles.The capacity retention rates are 92.9%,93.9%,86.4%and 76.8%at 1 C,2 C,5 C and 10.0 C after the 200cycles.By comparing with the electrochemical performance of Mg-NaMeO and Co-NaMeO electrodes,MgCo-NaMeO electrode is found that the doping of Co element improves the capacity of the electrode;and Mg element could improve the capacity retention rate.According to the CV method,the sodium ion migration rate in MgCo-NaMeO is 188.23×10^-12 cm²/s,which is much higher than the sodium ion migration rate in Mg-NaMeO and Co-NaMeO electrodes.Through in-situ XRD analysis,the phase transition process of the MgCo-NaMeO electrode has been analyzed during cycling process.It shows that the MgCo-NaMeO could suppress effectively the phase change reaction during charging and discharging by Mg,Co codoping.