Study On The Electrochemical Properties Of LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Material Regulated By External Field

The wide application of lithium-ion batteries in small portable electronic devices and new energy electric vehicles accelerates the research process of battery cathode materials.Compared with the traditional cathode materials,the new nickel-cobalt-Manganese ternary cathode materials is widely used because of their high reversible specific capacity.Because the traditional characterization methods are limited to the study of the structure and morphology of nanoparticles under vacuum and the macroscopic electrochemical performance of the device,the atomic force microscopy（AFM）with high spatial resolution is widely used in the study of the electrode materials of lithium ion batteries in the nanometer electrochemical environment.In order to further explore the intrinsic mechanism of experimental characterization,it is an important way to conduct particle scale modeling and analysis of electrode materials through phase field simulation,which can comprehensively consider the coupling effect of multiple physical fields in electrode materials.So this paper prepares Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）cathode material experimentally,and uses the atomic force microscopy（AFM）technology to characterize the current and morphology changes of single particles under applied voltage.The phase field simulation method is used to establish the cathode material model at the particle scale and to analyze the effect of different applied voltages on the particle current and morphology by adjusting the lithium ion concentration and stress.The theoretical simulation results are used to explain the changes of the experimentally characterized phenomena.The NCM811 cathode material is prepared by sol-gel method and the powder material is ultrasonically dispersed.Under the external field conditions of 3 V lower than the phase transition voltage of NCM811 material and 5 V and 8 V higher than the phase transition voltage,the CAFM and ESM are used to characterize the internal current and morphology changes of a single particle.The results show that there is almost no current response when the applied voltage is less than 5 V,when the applied voltage exceeds 5 V to 8 V,the magnitude of current remarkably increases.There is almost no morphology changes when the applied voltage is less than 5 V,When the applied voltage exceeds to 5 V,the morphology of the particle changed obviously,it presents that the length extends by 50 nm and the height reduces by 4.6 nm with the voltage from 5 V to 8 V.The overall volume of the particles expands significantly.A three-dimensional spherical particle model with radius of 1μm is established for NCM811 particles,and studying the distribution of the concentration field and stress field inside the particle under the action of mechanical-chemical coupling.The results show that the chemical potential gradient drives the diffusion of lithium ions from the surface to the center of the particles,and the concentration and stress are distributed in a circular uniform gradient along the radial direction.Subsequently,the effects of different applied voltages on the internal current and morphology of the electrode particles are investigated,and the results show that the equilibrium current density and the maximum current density were positively correlated with the applied voltage.According to the characterization results of AFM,the above model is simplified and the parameter is optimized,and the radius of the NCM811 particle model is adjusted to 100 nm,and the current and deformation of under different applied voltages are calculated.It is compared with experimental data.The results show that the different diffusion rates of lithium ions cause the fluctuation of current in particle,and the diffusion of lithium ions is the main reason for the current response.The high applied voltage results in the increase of compressive stress inside the electrode particle,and the deformation of electrode particle is mainly caused by the stress difference between the inside and outside of the particle.In general,the lithium ion concentration and stress distribution inside the particle can represent the change rule of current and morphology to a certain extent.Through theoretical simulation,the current and morphology changes in experimental can be explained from the diffusion mechanism of lithium ions.