Preparation And Modification Of A New Anode Material LaFeO₃ For Ni/MH Batteries

With the growing energy and environmental crisis,the development of green energy sources and energy-saving technologies have become urgent issues.LaFeO₃as an ABO₃-type perovskite oxide is regarded as a new anode material with great potential for Ni MH batteries due to its wide source,low price,high temperature resistance and environmental friendliness.However,the problems of easy agglomeration,low electrical conductivity and poor kinetic performance,etc.limit the further application of LaFeO₃.To solve these problems,this paper systematically investigates the microstructure and electrochemical properties of LaFeO₃via modifying LaFeO₃by elemental doping as well as surface treatment to promote its practical application in special environment.The main research contents are as follows:A series of La_1-xNa_xFeO₃（x=0,0.2,0.4,0.6,0.8）and La_1-xK_xFeO₃（x=0,0.2,0.4,0.6,0.8）materials were prepared by a ball-mill solid-phase method using alkali metal elements of Na and K as doping elements,and the effect of doping with alkali metal elements on the crystal structure and electrochemical properties of LaFeO₃and its mechanism were investigated.It is found that the doping of alkali metal elements can significantly inhibit the agglomeration of the material,and appropriate amount of alkali metal element doping increases the concentration of oxygen vacancies in LaFeO₃,which in turn increases the electrical conductivity.Electrochemical tests showed that the maximum discharge capacity of doped LaFeO₃increases from 178.8 m Ah g^–1to 356.7 m Ah g^–1,the CR increases from 90.4%to 97.8%,the HRD₁₅₀₀increased from 11.2%to 50.5%,and the capacity retention after 100cycles increases from 55.11%to 73.4%.It can be seen that the doping with alkali metal elements has greatly improved the electrochemical performance of LaFeO₃.LaFe_0.8M_0.2O₃（M=Al,Co,Ni,Zn）materials with undoped LaFeO₃and B-site doped metal ions were synthesized by a high-temperature solid-phase method,and the effect of different elemental doping on the crystal structure and electrochemical properties of LaFeO₃was analysed.It is found that the doping of metal elements at the B-site enhanced the covalency of the B-O bond and the covalent bonding of the 3d orbital of the B-site transition metal with the O_2porbital,resulting in easier electron migration.The distance between-Fe-O-in LaFeO₃is also shortened,making the transport channels for electrons shorter,which in turn leads to a significant increase in conductivity.The shortening of the Fe-O bond facilitates the shortening of the rhombic length of the Fe-centered tetrahedra,shortening the path length of O^2–along the lattice to fill the oxygen vacancies in the system,which in turn increases the discharge capacity of LaFeO₃.Studies show that the maximum discharge capacity of LaFeO₃after B-site doping increases from 178.8 m Ah g^–1to 328.4 m Ah g^–1,HRD₁₅₀₀increases from11.2%to 66.1%and S₁₀₀increases from 55.11%to 77.5%.A series of A and B site co-doped La_1-xK_xFe_1-yZn_yO₃（x=0,0.2;y=0,0.4）materials were designed and synthesized,and the effect of AB-site co-doping on the crystal structure and electrochemical properties was explored.The Rietveld refinement reveals that the doping of the Lasite with K and the Fesite with Zn shorten the Fe–O bond length,shortening the electron transport channel and increasing its electrical conductivity.The A site doping can compensate for the low content of oxygen vacancies and inconspicuous dispersion between particles when only the B site is doped.The B site doping can make electron migration easier on the basis of the A site doping,shortening the electron migration.The maximum discharge capacity of LaFeO₃after AB-site co-doping increases from 178.8 m Ah g^–1to 457.2 m Ah g^–1and the HRD₁₅₀₀increases from 11.2%to 73.1%.The AB-site co-doping increases the discharge plateau pressure and the discharge capacity after 100 cycles remains 317.1 m Ah g^–1,much higher than the 98.5 m Ah g^–1before doping.LaFeO₃ materials with in-situ clad Co were prepared by a combination of co-precipitation and high temperature solidification.It is found that the in-situ coating of Co monomers not only increases the electrical conductivity of the materials,but also reduces the agglomeration of LaFeO₃and increase its specific surface area.The larger specific surface area is conducive to increasing the contact area between the active material and the electrolyte,and reducing the contact impedance between the electrolyte and the active material.At the same time,the particle size of the composite becomes smaller after Co monomer coating,which provides a large number of active sites for diffusion,thus accelerating the hydrogen absorption rate and improving its kinetic performance..La_0.8K_0.2Fe_0.6Zn_0.4O₃/C composites were synthesized by a high temperature solid phase method using glucose,sucrose,citric acid,PVP and starch as carbon sources.It is found that the coating of La_0.8K_0.2Fe_0.6Zn_0.4O₃with a conductive carbon layer after different carbon sources increases the electrical conductivity of the composite and improves its discharge performance.The maximum discharge capacity of the composite increases from 178.8 m Ah g^–1to 543.5 m Ah g^–1at an operating temperature of 60°C.The discharge capacity remains458 m Ah g^–1after 100 cycles,much higher than that of LaFeO₃at 98.5 m Ah g^–1.The composite also exhibits higher ultimate current density（I_L）,hydrogen diffusion coefficient（D）and lower charge transfer resistance(R_ct).