Wang Qianchen, Gao Hongcai, Li Jingbo, Liu Gui-Bin, Jin Haibo
Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China.
School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China.
ACS Appl Mater Interfaces. 2021 Mar 31;13(12):14312-14320. doi: 10.1021/acsami.1c01663. Epub 2021 Mar 22.
The V/V (3.4 V) redox couple has been well-documented in cathode material NaV(PO) for sodium-ion batteries. Recently, partial cation substitution at the vanadium site of NaV(PO) has been actively explored to access the V/V redox couple to achieve high energy density. However, the V/V redox couple in partially substituted NaV(PO) has a voltage far below its theoretical voltage in NaV(PO), and the access of the V/V redox reaction is very limited. In this work, we compare the extraction/insertion behavior of sodium ions from/into two isostructural compounds of NaVGa(PO) and NaVAl(PO), found that, by DFT calculations, the lower potential of the V/V redox couple in NaVM(PO) (M = Ga or Al) than that in NaV(PO) is because of the extraction/insertion of sodium ions through the V/V redox reaction at different crystallographic sites, that is, sodium ions extracting from the Na(2) site in NaVM(PO) while from the Na(1) site in NaV(PO), and further evidenced that the full access of the V/V redox reaction is restrained by the excessive diffusion activation energy in NaVM(PO).
V/V（3.4V）氧化还原对在钠离子电池的阴极材料NaV(PO)中已有充分记载。最近，人们积极探索在NaV(PO)的钒位点进行部分阳离子取代，以利用V/V氧化还原对实现高能量密度。然而，部分取代的NaV(PO)中的V/V氧化还原对的电压远低于其在NaV(PO)中的理论电压，并且V/V氧化还原反应的利用率非常有限。在这项工作中，我们比较了钠离子在两种同构化合物NaVGa(PO)和NaVAl(PO)中的脱出/嵌入行为，通过密度泛函理论计算发现，NaVM(PO)（M = Ga或Al）中V/V氧化还原对的电位低于NaV(PO)中的电位，这是由于钠离子通过V/V氧化还原反应在不同晶体学位点脱出/嵌入，即钠离子从NaVM(PO)中的Na(2)位点脱出，而从NaV(PO)中的Na(1)位点脱出，并且进一步证明了NaVM(PO)中过高的扩散活化能限制了V/V氧化还原反应的充分利用。
