Study On The Diffusion Mechanism Of Lithium Ion In MoSe₂ By Using Electrochemical Strain Microscope

MoSe₂ possesses numerous advantages,such as high electronic conductivity,large interlayer distance and high initial discharge and charge capacities.As an anode of alkali metal ion batteries,MoSe₂ often subjects to unsatisfactory capacity and low capacity retention over extended cycling,far from the requirements of the commercial standards.The strain-based scanning probe microscopy is used to study the phase transition processes from intercalation to lithiation and cation migrations so that we can solve the above disadvantages.Electrochemical strain microscopy（ESM）is of small detection limitation from 2 to 10 pm level for displacement detection and direct information capture of ionic transport,and it emerges as an effective tool for visualizing the local electrochemical phenomena in electrochemical active materials.However,the contribution of ESM response is complicated under the situation of electrochemical systems involving ionic diffusion and migration,as well as possible contributions from surface reactions and space charges.Thus,it is important that the rigorous theoretical analyses and computations for ESM response under realistic experimental conditions and microscopic approximations.In this study,we combine ESM experiments and a fully coupled finite element model of electrochemomechanics to study the microscopic mechanism of Li⁺ions diffusion in MoSe₂.The specific research contents and results are as follows:1.After the mechanical exfoliated MoSe₂ nanosheets were lithiated by the chemical intercalation method,the original MoSe₂ nanosheets were converted into the lithiated MoSe₂ nanosheets with electrochemical activity.Meanwhile,by comparing with AFM morphology,X-ray photoelectron spectrum and Raman spectrum of the original and lithiated MoSe₂ nanosheets,we found that Li⁺ions has been successfully intercalated into MoSe₂ interlayer by the chemical intercalation method,and there was a phase transition from a stable semiconductor phase（2H）to a metastable metal phase（1T）.2.We visualized the diffusion coefficient of Li⁺ions in the lithiated MoSe₂nanosheets by the imaging ESM.We found that ESM response was nonuniform and diffusion coefficient was highly correlated with the surface morphology for the lithiated MoSe₂ nanosheets,so we could obtain a homogeneous diffusion coefficient by optimizing the surface morphology and structure of the sample to improve the electrochemical performance of lithium-ion batteries based on MoSe₂ cathode.In addition,the first-and second-harmonic and voltage spectroscopic ESM measurements were used to characterize the lithiated MoSe₂ nanosheets to exclude the electrostrictive and piezoelectric strain.Experiments showed that the ESM response mechanism of the lithiated MoSe₂ nanosheets originated from the electrochemical strain caused by the Vegard effect.Therefore,we can use ESM technology to characterize electrochemical performance of lithium-ion batteries at the nanoscale.3.A fully coupled finite element model of electrochemomechanics was solved to simulate ESM response of the lithiated MoSe₂ nanosheets.We confirmed that Vegard strain was a main microscopic mechanism of ESM response for the lithiated MoSe₂nanosheet by comparing the experimental and simulated hysteresis loops.Li⁺ion diffusion was limited at high frequencies,and ESM response was dominated by electromigration.However,electromigration was limited at low frequencies,and ESM response was dominated by Li⁺ion diffusion.In addition,we also conclude that ion diffusion is limited at high frequencies,and the ESM response is dominated by electromigration at this time;The diffusion-strain coupling effects can be ignored and local electroneutrality is held at high frequencies.They can be used to simplify the numerical simulation and understand the Li⁺ion diffusion mechanism in MoSe₂.