Preparation And Modification Of Silicon-based Thin Film Anode Materials For Lithium-ion Batteries By Magnetron Sputtering

The Silicon(Si)is considered as one of the most promising next-generation anode materials for lithium-ion batteries,because of its high theoretical specific capacity(4200 mA h g'1)and nature abundance.However,its practical application has been restricted due to its huge volume change(-300%)during lithiation-delithiation processes and its poor electronic conductivity,which leads to pulverization of the electrode and drastic capacity fading.Alternatively,the silicon suboxide(SiOx)has high capacity and good cycle performance,but it is also suffered from large volume change(-200%)and low electronic conductivity(?10-12 S cm-1),which has limited its practical application.To solve these problems,electrochemical properties of silicon and silicon suboxide are studied from the perspective of thin film electrodes,and the silicon-based composite thin film anodes with excellent cycle performance are prepared.The research content are as follows:In the chapter 3,we prepare amorphous Si thin film anodes by magnetron sputtering.The Si film with a thickness of 179 nm exhibits an initial coulombic efficiency of 87.2%and a reversible specific capacity of 3402 mA h g'1.However,the volume effect during cycling causes destruction of the electrode structure,which leads to a drastic capacity fading after 50 cycles,and the capacity retention after 90 cycles is only 71.3%.We improve the cycle performance of the Si film anodes by three methods:(1)Introducing a Ti transition layer with stronger bonding force with the Si film,the capacity retention increases to 77.8%after 150 cycles;(2)Limiting 80%reversible capacity,reducing the delithiation potential from 1.5 V to 0.56 V,which can reduce the deformation caused by tensile stress and improve the cycle performance.The cells show a capacity retention of 96.6%after 100 cycles;(3)Preparing Si/C composite thin film anodes by co-sputtering method,the film with a carbon content of 14.9 wt.%has a capacity retention of 76.2%after 100 cycles.Because the co-sputtered carbon is able to buffer the volume effect and maintain the structural stability.We also explore the practical application of composite electrodes prepared by magnetron sputtering in the chapter 3.We prepare sputtered Si(C)graphite composite electrodes,and prove that the "current collector/graphite/sputtered Si nanoparticle"structure has an improved electrochemical performance than the "current collector/sputtered Si film/graphite" structure.The G/Si composite with a silicon content of 1 wt.%shows a specific capacity of 382 mA h g-1 at 0.1 C and a capacity retention of 96.3%after 100 cycles,which is 10%higher than the commercial graphite material,and the volume specific capacity is increased by about 10%.The graphite electrode provides excellent electronic conductivity,and the sputtered Si exists in the form of nanoparticles,which exhibit lower volume effect.However,when the active material loading increase,the electrochemical performance of the composite electrode will decline,due to an increasing volume effect of silicon particle and a large polarization of the composite electrode.In the chapter 4,we prepare amorphous SiO and SiO/C thin film anodes by magnetron sputtering.The SiO film with a thickness of 1.06 ?m shows an initial coulombic efficiency of 57.0%.Due to the poor electronic conductivity,the SiO film anode needs 60 cycles to reach the maximum reversible specific capacity of 1209 mA h g-1,and the capacity retention at 1 C after 750 cycles is 75.8%.A moderate carbon content can improve the electrochemical performance of the SiO film anode,when the carbon content is 13.9 wt.%,the film anode shows an initial coulombic efficiency of 71.8%and a capacity retention of 80%after 750 cycles.The charge transfer impedance and the volume expansion ratio are significantly reduced after cycling,and the electrode structure integrity is well maintained.However,excessive co-sputtered carbon(17.7 wt.%)will cause steric hindrance to lithium-ion diffusion and the film anode shows a worse electrochemical performance.The effect of the co-sputtered carbon can be divided into two parts:Because of the improvement of the electronic conductivity,the SiO/C film anode can be fully lithiated and delithiated during initial cycles,and the specific surface area is increased,which is beneficial to the electrolyte wetting and the Li-capacity.Furthermore,the carbon alleviates the volume expansion and maintains the electrode structure during the following cycles,which is beneficial to the cycling performance.The in-situ stress test results show that the co-sputtered carbon has the effects of mitigating stress accumulation and stabilizing the electrode structure.