| In many energy storage technologies,the new electrochemical energy storage system represented by lithium-ion battery is favored by the market because of its high energy density,power density and excellent cycle life.However,the overall reserve of lithium resources in the world is low(the abundance in the earth’s crust is only 0.0017%)and concentrated in a few countries.Thus relying on lithium-ion batteries is not conducive to ensuring national energy security.China has abundant sodium resources than lithium resources,and sodium-ion batteries have obvious advantages in terms of production cost,low-temperature electrochemical performance and safety.Therefore,sodium-ion battery is one of the most promising systems for developing large-scale energy storage in the future,which has important economic value and strategic significance.As a key component of sodium-ion battery,cathode material directly determines the electrochemical performance of cell.The synthesis process of P2 layered oxide cathode is simple and has good rate performance.However,the irreversible phase transition of P2-O2 in high voltage range and Jahn-Teller effect caused by Mn3+ ions will lead to rapid capacity decay of the material and affect its cycle life.In this paper,P2-Na22/25Ni1/3Mn2/3O2 cathode is used as the matrix,and its crystal structure and local coordination are controlled by heterogeneous metal doping to improve its electrochemical performance.It mainly includes the following two aspects:(1)Na4/5A2/25Ni1/3Mn2/3O2(A=K,Mg,Zn)series materials were synthesized by solidstate synthesis.The results show that the Mg and Zn doped materials have a mixed phase structure of P2+O3,showing outstanding electrochemical performance in cycle stability.Among these materials,the introduction of Mg greatly improved the electrochemical performance of the material,and the first-cycle discharge capacity was enhanced to 190.85 mAh/g,and the first-time Coulombic efficiency reached 129.01%.The results show that the doping of Mg can activate the redox activity of oxygen anions and improve the theoretical capacity of materials.On the other hand,the doping of Mg can significantly increase the relative content of Mn4+/Mn3+,inhibit the Jahn-Teller effect in the cycle process,and obviously improve the stability and rate performance of the material.(2)The effects of K,Mg and Zn doping in Na22/25Ni1/3Mn2/3O2 on the structure and electrochemical properties of the materials were studied respectively.It is found that the introduction of K plays an important role in maintaining P2 structure,while Mg and Zn doped material has P2+O3 composite phase structure.Compared with Mg-K and Zn-K double doping systems,the synergistic effect of Mg and Zn in Mg-Zn system can effectively inhibit the O loss caused by anion redox and improve the capacity retention rate.Na4/5Mg4/75Zn2/75/75Ni1/3Mn2/3O2 has the best performance,and the capacity retention rate can reach 62.5%after 100 cycles,which is better than that of Mg or Zn single doped materials.To sum up,the materials doped with Mg and Zn in P2+O3 phase are beneficial to the improvement of electrochemical properties of the materials.Mg can stimulate the redox activity of anions and optimize the diffusion kinetics of sodium ions at high voltage,while improving the specific capacity and rate performance of materials.The synergistic effect of Mg-Zn double doping can effectively inhibit lattice oxygen release,ensure the stability of anion redox process,and further improve cycle life and reversible capacity.The research results in this paper will provide research direction and theoretical guidance for optimizing the cathode structure and anion redox activity of layered oxide cathodes in sodium-ion batteries. |
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