Composition Of Ionic Liquid Electrolytes And Its Effect On The Electrochemical Hydrogen Storage Performance Of A-Si Thin Film Electrodes

Nickel-metal hydride batteries,as a mature technology and the most industrialized secondary battery,have been widely used in new energy vehicles.However,they are limited by the low capacity of traditional hydrogen storage alloy electrodes and the narrow electrochemical window（1.23V）of water-based electrolytes,resulting in a lower energy density compared to lithium-ion batteries.Therefore,there is an urgent need to find a new type of electrolyte with a wide electrochemical window and a high-capacity negative electrode material that matches it to solve these problems.Ionic liquids（ILs）have become an ideal electrolyte due to their wide electrochemical window and good chemical stability,and have tremendous development potential.In this study,1-ethyl-3-methylimidazolium acetate（EMIMAc）was selected as a non-protonic ionic liquid,and a series of protonic ionic liquids were prepared by adding organic acids（acetic acid,citric acid,and oxalic acid）as proton sources to EMIMAc.In order to address the issue of high viscosity and low conductivity of EMIMAc,the effect of acetic acid concentration on the conductivity of EMIMAc was studied,and the microstructure and interaction of the EMIMAc+acetic acid system were investigated using FTIR,TG,DSC,conductivity meter,and electrochemical testing.Based on this,amorphous silicon thin film electrode materials were prepared by magnetron sputtering as negative hydrogen storage electrodes,Ni（OH）₂was sintered as the positive electrode,Ag⁺was used as the quasi-reference electrode,and EMIMAc+organic acid was used as the electrolyte solution.The electrochemical reaction behavior and hydrogen storage performance of the a-Si thin film electrode in EMIMAc+different organic acid electrolyte solutions were studied and analyzed through Swagelok cell assembly and preliminary testing.Finally,hydrogenated amorphous silicon film a-Si:H was prepared by gas-phase hydrogenation of the sputtered a-Si thin film.The electrochemical hydrogen storage performance of the a-Si:H film in EMIMAc+x M CH₃COOH（x=1M,2M,3M,4M）ionic liquids was studied,and the influence and correlation of hydrogenation treatment and changes in electrolyte solution properties on the charge-discharge cycle life of the film electrode were analyzed.The following main conclusions were obtained from the study:（1）Using magnetron sputtering and preparation process parameters（power 300 W,bias voltage 50 V,deposition time 75 min）,active material Si with a mass between 0.09～0.11 mg and an average film thickness of 940.5 nm of a-Si film were respectively deposited on nickel and glass substrates by sandblasting.The prepared film material was analyzed to be amorphous silicon through relevant characterization methods.（2）Under the same charging and discharging conditions,the maximum discharge capacities of a-Si film electrodes in three sets of protonic ionic liquids EMIMAc+0.1M CH₃COOH,EMIMAc+0.1M C?H?O?,and EMIMAc+0.1M H?C?O?were 585.5 m Ah/g,515.9m Ah/g,and 312.6 m Ah/g,respectively.The discharge rate performance of EMIMAc+0.1M CH3COOH was better,indicating that a-Si film electrodes have the best electrochemical performance in the protonic ionic liquid EMIMAc+0.1M CH₃COOH.（3）The increase in acetic acid concentration can increase the conductivity of the EMIMAc+CH₃COOH system,and the conductivity significantly increases with temperature.Protonic ionic liquids have good thermal stability,only undergoing glass transition.The increase in acetic acid concentration weakens the hydrogen bonding between the cations and anions in EMIMAc,causing changes in the structure and absorption peak intensity.The electrochemical performance of a-Si film electrodes in EMIMAc+x M CH₃COOH（x=1M,2M,3M,4M）electrolytes was analyzed and compared.Under the same charging and discharging conditions,the maximum discharge capacity of a-Si film electrodes was 1615.3 m Ah/g in EMIMAc+4M CH₃COOH,and the higher the concentration,the better the high-rate discharge performance.This indicates that the increase in acetic acid concentration can improve the performance of ionic liquids,and the corresponding electrochemical performance has also been improved.（4）The electrochemical performance of a-Si thin film electrodes in EMIMAc+x M CH₃COOH（x=1M,2M,3M,4M）electrolytes after hydrogenation treatment was analysed and compared.The discharge capacity of the thin film electrodes was significantly increased after the hydrogenation treatment.The maximum discharge capacity（2057 m Ah/g）is available in EMIMAc+3M CH₃COOH.However,from the cycling results,the hydrogenation treatment did not improve the cycle life of the films,with the capacity retention of the hydrogenated film electrodes after the 10th,30th and 50th cycles being S₁₀=28.89%,S₃₀=23%and S₅₀=23.8%,respectively.Analysis of the IR results concluded that the gradual reduction of acetic acid within the EMIMAc+4M CH₃COOH electrolyte resulted in changes in the structure of the ionic liquid and the intensity of the absorption peak.Thermogravimetric analysis showed that the mass loss of acetic acid before cycling and after the 10th,30th and 50th cycles corresponded to 24.63%,13.46%,11.94%and 8.4%respectively.This indicates that the acetic acid in the electrolyte gradually decreases as the number of cycles increases.ICP-OES testing of the post-cycle electrolyte showed that Si and Ni elements were detected in the electrolyte,resulting in a change in colour due to contamination of the electrolyte and therefore a decrease in cycle life.