Research On Fluorine-doped Layered Cathode Material For Sodium/Potassium Ion Batteries

Lithium-ion batteries have achieved great success in commercial applications.However,the lack of lithium metal leads to a higher cost of lithium-ion batteries.Sodium and potassium with abundant reserves are expected to replace lithium-ion batteries in large energy storage devices.Among many cathode materials,layered transition metal oxides have the advantages of high theoretical specific capacity,high voltage plateaus and low cost.P-type layered oxide is one of the proming cathode materials for sodium-ion batteries because it has high ionic conductivity and low diffusion barrier.However,most P-type layered metal oxide cathode materials with one transition element suffer from rapid capacity fading usually.In this paper,three electrode materials,including P2-Na_0.67MnO₂,P2-Na_0.7CoO₂,and P3-K_0.45MnO₂,were prepared by the solid-state reaction,and then modified by fluorine doping.The effects of fluorine doping on the structure,morphology,and electrochemical performance of the there electrode materials were investigated by using various analytical methods.The main research contents of this paper include the following aspects:1.P2-Na_0.67MnO₂ was synthesized by the solid-state reaction.Samples with fluorine doping amounts of 5%and 10%were obtained by changing the addition ratio of sodium fluoride.It shows that fluorine doping has little effect on the morphology by XRD,SEM,TEM,XPS,ICP,etc.Since the electronegativity of fluoride is strongter than oxide,theO-O plate spacing is increased after fluorine doping and the structure collapse is avoided,illustrating a better capacity delivery after cycling.At 0.5 C rate,it can be observed that the original P2-Na_0.67MnO₂just kept 66.7%reversible capacity(99.05 m Ah g^-1)after 100 cycling.At the same condition,5%fluorine doped material can deliver 134.1 m Ah g^-1 reversible specific capacity and the capacity retention increased to 90%.Except for that,at a large current density of 2 C,the capacity retention of this sample increased from 71.0%to 90.9%after 100 cycling.2.P2-Na_0.7CoO₂ was synthesized by a simple solid-state reaction method.Electrode material with fluorine doping amount of 8%was designed by ajusting the content of sodium fluoride.It shows that fluorine doping has little effect on the morphology by XRD,SEM,etc.Similarly,theO-O plate spacing is increased after fluorine doping,making it can adapt to volume expansion.At 0.5 C rate,it can be observed that the original P2-Na_0.7CoO₂ just kept71.0%reversible capacity after 100 cycling and the capacity retention of 8%fluorine-doped material has improved to 95.7%.This result evidenced that F doping can effectively improve the cycle stability of the original P2-Na_0.7CoO₂.3.P3-K_0.45MnO₂ was synthesized by the solid-state reaction.By trying to adjust the amount of potassium fluoride,samples with fluorine doping amounts of 10%and 20%were obtained.It shows that fluorine doping has no effect on the morphology by XRD,SEM,etc.But it illustrates a better capacity delivery after cycling.At 20 m A g^-1 current density,it can be observed that the original P3-K_0.45MnO₂ just kept 74.4%reversible capacity after 50 cycling.But the capacity retention of 20%fluorine-doped material has improved to 94.0%.