Structure Control And Performance Study Of Cathode Materials For Layered Transition Metal Oxide Based Sodium Ion Batteries

With the commercialization of lithium-ion batteries by SONY in the 1990s and the high specific energy and excellent cycling performance of lithium-ion batteries,lithium-ion batteries have been widely used in various fields.However,with the rapid development of lithium-ion batteries,the demand for lithium resources is increasing,and lithium-ion batteries have gradually exposed some of their limitations.Sodium-ion batteries have a similar energy storage mechanism as lithium-ion batteries,and they are abundant and lower in cost.They also have a high voltage platform and are more secure.Finding the suitable cathode material is the key to the development of sodium-ion batteries,and sodium ion cathode materials are the key to the commercialization of sodium-ion batteries.This article is mainly about structure regulation and modification research of P2 cathode materials.The main research content of this paper is showed as follows.1.We propose a strategy to optimize the electrochemical performances of P2-Na0.7Mn0.75Fe0.25-x-yNixCoyO2 cathode materials for SIBs through modulating the crystal structure with a synergistic substitution of Ni and Co for Fe.The structure-performance relationship and the synergistic improvement mechanism have been unraveled by means of x-ray diffraction,neutron diffraction and electrochemical techniques.The synergistic substitution of Ni and Co for Fe leads to the enlargement of the interlayer spacing and the Na-0 bond length,and shrinks the TM-O and O-O bonds,which enhances the sodium ion diffusion coefficient,the rate capability and the cycling stability.Ni and Co co-doped Na0.7Mn0.75Fe0.15Ni0.05Co0.05O2(FNC)cathode shows a relatively smooth charge-discharge curve in 1.5-4.2V and demonstrates a much better rate performance in compared to Na0.7Nn0.75Fe0.25O2(MF),Na0.7Mn0.75Fe0.15Ni0.1O2(FN)and Nao.7Mno.75Feo.15Co0.1O2(FC).At a high current density of 1C,2C,5C and IOC,FNC can still deliver a reversible capacity of 127,109,83 and 58 mAh g-1,respectively.FNC also provides a higher reversible capacity of 181 mAh g-1 at 0.1 C with a high cycling stability.This study offers some new insights into designing high performance cathode materials for SIBs through cooperatively modulating the crystal structure with multi-elements doping.2.Fe and Mn based P2-type layered oxides have attracted great interest as cathode materials for sodium-ion batteries(SIBs)owing to their inexpensive metal constituents and high theoretical capacity.But the rapid capacity fade and the complicated phase transformations is still a big challenge.Herein,we propose to modulate the crystal structure and optimize the electrochemical performances of Na0.67Mn0.5Fe0.5O2 by Al doping for Mn or Fe,and the roles of Al have been unraveled.A1 doping for Mn or Fe leads to the decrease of both lattice parameter a and c but enlarges d-spacing and Na-0 bond length which largely enhances Na+diffusion and rate capability especially for Na0.67Mn0.5Fe0.47Al0.03O2.Al doping reduces the thickness of slab TMO2 and strengthens TM-O/O-O bonding,which benefits to the layered structure stability.A1 doping can mitigate Mn3+and Jahn-Teller distortion which also improves the layered structure stability and cycling performance.Al doping also alleviates the lattice volume variation(AV)and the structure strain during discharge-charge cycles particularly for the substitution of AI for Fe,which further benefit to the layered structure stability.This study not only unravels the roles of Al doping for Fe or Mn but also offers some insights into optimizing the electrochemical performance of layered cathode materials for SIBs through lattice doping.