Study On The Electrical Transport Properties Of LiMn₂O₄ Under High Pressure

Lithium-ion batteries have the advantages of light weight,good safety performance and high energy density,so they are widely used in automotive,energy,and other fields.Among them,LiMn₂O₄is an important cathode material in lithium-ion batteries.Pressure is an effective means of regulating the structure and properties of matter,bringing a new dimension to the study of matter.In this paper,we use diamond to measure LiMn₂O₄with high-pressure in-situ AC impedance spectroscopy,and use the CASTEP module in Materials Studio software to calculate the first-principles band structure and state density of LiMn₂O₄,and the results are as follows:1.Through high-pressure in-situ AC impedance spectroscopy,the electrical transport properties of LiMn₂O₄under high voltage were studied,and it was found that LiMn₂O₄was electronically conductive in the range of 0～20.45 GPa,and the conduction mechanism did not change in the experimental pressure range.Under high pressure,the resistance value of LiMn₂O₄decreases with the increase of pressure,and the relaxation peak is moving towards high frequency,and the peak strength has been decreasing.2.The AC impedance spectrum under high pressure was fitted by equivalent circuit method,and the relationship between resistance,relaxation frequency and conductivity of LiMn₂O₄with pressure was obtained.The results show that the rate of decrease of resistance value,the rate of increase of relaxation frequency and conductivity change discontinuously at 8.8 GPa and 12.53 GPa,and we attribute this discontinuous change to the phase transition of the compressive structure.LiMn₂O₄begins to undergo a phase transition at 8.8 GPa,from a cubic Fd3m structure to a quad I41amd structure,and this structural phase transition lasts until the end of 12.53GPa.In the range of 0～20.45 GPa,the resistance value of LiMn₂O₄in both cubic phase and tetragonal phase continues to decrease with the increase of pressure,which may be caused by the compression of the distance between atoms under the action of pressure,and the band structure changes.Using first-principles calculations,the state density and band structure of LiMn₂O₄samples under high pressure were simulated.The calculation results of the band structure turn out that the position of the Fermi energy level in the conduction band increases with the increase of pressure of both cubic structure and quadrangular structure of LiMn₂O₄,indicating that the conductivity of the material is getting better and better,which is consistent with the conclusion that the resistance decreases with the increase of pressure in Chapter 3.The state density calculation results show that the 2p state density peak of O and the 3d state density peak of Mn overlap strongly in the conduction band,indicating that there is a strong hybridization between the 2p electron orbital of O and the 3d electron orbital of Mn,and the interaction of O atom and Mn atom determines the electronic structure properties of LiMn₂O₄sample,and forms a stable skeleton.