Study On The Potassium Storage Properties Of A_xTi₂（PO₄）₃/C Anode Materials

Lithium-ion batteries have great advantages in high-efficiency energy storage due to their advantages of repeated energy storage,high flexibility,and high energy density.However,due to the uneven distribution of lithium resources and limited reserves,it is urgent to find a new type of energy storage device to replace lithium-ion batteries in large-scale energy storage.Potassium-ion batteries and lithium-ion batteries belong to secondary alkali metal batteries,and have great potential to become a new generation of energy storage devices to replace lithium-ion batteries.And they have similar working principles and similar standard electrode potentials,which makes the research of potassium-ion batteries there are traces to follow.In addition,the abundant resources and low price of potassium make potassium-ion batteries favored by many researchers and become a research hotspot in recent years.However,due to the larger ionic radius of K⁺,higher requirements are also placed on the potassium-ion batteries electrode material.The polyanionic A_xTi₂（PO₄）₃ has the advantages of low price and harmless.Its open structural framework and three-dimensional ion transport channel are conducive to the rapid transport of K⁺.It is one of the potassium-ion batteries electrode materials with practical application potential.However,its electronic insulation and large relative molecular mass cause its poor rate performance and low theoretical specific capacity.In this paper,Ba_0.5Ti₂（PO₄）₃/C nanospheres,Mn_0.5Ti₂（PO₄）₃/C and La_0.33Ti₂（PO₄）₃/C nanofibrous membranes were prepared and their potassium storage properties were systematically studied.The electrochemical performance of A_xTi₂（PO₄）₃ is comprehensively improved from two aspects:nanometer size design and carbon coating.Firstly,Ba_0.5Ti₂（PO₄）₃/C nanospheres were in situ prepared by hydrothermal method and subsequent calcination,and the addition of urea controls the morphology and structure.Ba_0.5Ti₂（PO₄）₃/C nanospheres were matched with commercial activated carbon and assembled into potassium ion hybrid capacitors to explore its potassium storage properties.Benefiting from the large specific surface area of the composite,the K⁺transport path is shortened,and the potassium-ion hybrid capacitor exhibits excellent electrochemical performance.The maximum energy density is 566.1 Wh kg^-1,and the maximum power density is 4576.9 W kg^-1.And the capacity retention rate is almost 100%after 2000 cycles.In addition,the assembled potassium-ion hybrid capacitor can light up 20 LED lamps,which has strong application value.The density of states of Mn_0.5Ti₂（PO₄）₃ is calculated by first-principles calculations,and the results show that Mn_0.5Ti₂（PO₄）₃ has a smaller valence band and better electrical conductivity than KTi₂（PO₄）₃.A flexible Mn_0.5Ti₂（PO₄）₃/C nanofibrous membrane was fabricated by electrospinning method as a self-supporting electrode for potassium-ion batteries,providing an excellent rate capability of 87.5 m Ah g^-1 at 1 A g^-1.The mechanical flexibility test of the materials shows that the addition of selenium powder greatly improves the flexibility of the Mn_0.5Ti₂（PO₄）₃/C nanofiber film,and the film can still recover as fresh after folding,kneading,and curling.The EIS test shows that the addition of selenium powder can improve the kinetic properties of K⁺.Transmission electron microscopy was performed on the electrodes after long cycles to analyze the morphology and structural changes of the materials.The electrode was disassembled after cycling found that the electrode remained intact,and the TEM test showed that the fiber morphology was unchanged and the lattice fringes were clear,confirming the excellent cycling stability of the Mn_0.5Ti₂（PO₄）₃/C nanofiber thin film electrode.The flexible La_0.33Ti₂（PO₄）₃/C nanofibers membrane was prepared by electrospinning method,and its half-cell and full-cell performance were investigated as a self-supporting potassium-ion battery anode material.The half-cell electrode has a reversible capacity of 235.8 m Ah g^-1 at a current density of 0.02 A g^-1 in the voltage range of 0.01-3.0 V,and can be cycled stably for 2500 cycles at a large current density of 1 A g^-1.It is matched with PTCDA and assembled into a full battery,has high coulombic efficiency at 0.1 A g^-1.When fully charged,it can light up 15 LED lights,showing good practical application value.