Preparation And Electrochemical Properties Of Silicon Thin Film Anode Materials For Lithium-Ion Batteries By Electrodeposition

Lithium-ion batteries are widely used in the portable electronic devices and new energy vehicles,while the commercial graphite anode materials can no longer meet the requirements for specific capacity and cycle life of lithium-ion batteries.Due to higher specific capacity and lower embedded lithium potential,silicon has become the most promising replacement material for graphite anode.Silicon thin film anode materials have higher average output voltage,lower electrode quality,and longer cycle life,which can rapidly undergo lithium removal and intercalation reactions,significantly improving the rate performance of lithium-ion batteries.The current laboratory preparation of silicon thin film for lithium-ion batteries is mainly magnetron sputtering,with higher energy consumption and higher temperature.While the energy consumption of electrodeposition technology is relatively lower,and the electrodeposition can be compeleted at lower temperature.Meanwhile,the amorphous silicon thin film prepared by electrodeposition can effectively alleviate the volume expansion problem during the charging and discharging of Lithium-ion batteries.At present,the ionic liquid is usually used to prepare silicon thin film,however,its high viscosity and easy absorption of water will lead to poor electrical conductivity and lower electrodeposition efficiency.Moreover,when using direct current electrodeposition to prepare silicon thin film,the ions in the solution are limited by diffusion,which will lead to uneven silicon thin film deposition and poor cycling performance of the batteries.To solve the above problems,the pulsed electrodeposition technology is used to prepare the dense amorphous silicon thin film on the surface of copper foil in this paper,with propylene carbonate as solvent and silicon tetrachloride as silicon source.Firstly,the nucleation mechanism,growth mechanism and growth process of silicon on the surface of copper foil were investigated by the chrono-current method.The effects of time,temperature,voltage,tetrabutylammonium chloride concentration and silicon tetrachloride concentration on the microscopic morphology and corrosion resistance of silicon thin film were investigated through the orthogonal experiments,optimizing the electrodeposition process.The dense silicon thin film was prepared on the copper foil surface by the optimal process,and the composition and phase composition of silicon thin film were analyzed by XRD and TEM,et al.Then,the prepared silicon thin film anode was assembled into batteries,and the electrochemical performance was tested.The main research contents and conclusions are as follows:（1）The cyclic voltametric curve of the electrolyte was tested by the electrochemical workstation,and it is found that the deposition potential of silicon on the copper foil surface is-1.74 V.The cyclic voltammetric curve with different sweep rates was measured,and the results show that the silicon deposition reaction is controlled by diffusion and the reduction reaction of silicon in the solution is irreversible.The nucleation mechanism,growth mechanism and growth process of silicon were investigated by chronoamperometry and chrono-Coulomb method.The growth process of silicon is divided into two stages,the first stage is the transient nucleation of silicon,and the second stage is the three-dimensional growth of silicon.（2）The optimal process parameters for the preparation of silicon thin film by electrodeposition were obtained by the orthogonal experiments,which were 2 h,45℃,-1.74 V,0.05 mol/L TBAC,0.75 mol/L Si Cl₄.The effect of electrodeposition time and temperature on the morphology and corrosion resistance of silicon thin film were investigated.The results show that the thickness of the silicon thin film gradually increased with the increasing of electrodeposition time.When reaching a certain thickness,the stress of the silicon thin silicon increases,which eventually leads to the cracking or peeling of the silicon thin film.When the electrolyte temperature is relatively lower,there is not enough mass transfer power for the silicon ions,resulting in insufficient silicon ions on the copper foil surface and uneven deposition.When the electrodeposition temperature was 45℃,the electrodeposition effect is the best.As the electrodeposition temperature increasing,the electrolyte undergoes oxidation,leading to the uneven deposition of silicon thin film on the copper foil surface.The amorphous film prepared on the copper foil surface with the optimal process is composed of silicon and its oxides,which is caused by the oxidation of the silicon thin film when exposed to the air during the testing process.（3）The mass of the deposited silicon prepared by the electrodeposition technology is0.065 mg.The cycling performance of the batteries was tested by the battery cycling system.The results show that silicon thin film anode has more excellent cycling stability compared to the graphite anode.When the current density is 200 m A g^-1,silicon thin film anode exhibits the capacity of 1250 m A h g^-1 and the coulomb efficiency of 98%over 100 cycles,and the capacity decay rate is slow.Silicon thin film anode has more excellent rate capability and it exhibits the capacity of 1400 m A h g^-1 at a current density of 100 m A g^-1 for the second time,which is still about 3 times higher than that of graphite anode.The diffusion coefficient of lithium ions in silicon thin film anode and graphite anode was measured through impedance testing and constant current intermittent titration method.The results show that the diffusion coefficient of lithium ions in silicon thin film anode is higher,and the silicon thin film anode has better electrochemical performance.