| Silicon has been proposed as a potential material for next-generation lithium-ion batteries to replace the conventional carbon based anodes because its intercalation capacity is the highest known(4200mAh/g).It has been well known that the main difficulty for Si-based materials in practical application are caused by their dramatic volume expansion(>300%) and subsequent contraction during Li insertion and extraction,this lead to the pulverization of electrode materials and poor cyclic performance. Thin film anodes have the advantage for fundamental research due to their well-defined structure and the ability to not use binder and conductive additive in testing as anodes. In this work,pure Si and Si-Ti bilayer thin film with different thickness has been prepared by Mantis magnetron sputtering system and electrochemical performance was evaluated. Furthermore, a numerical model is developed to simulate the intercalation induced stress and strain, as well as the failure mechanism of Si-based thin film anodes using ANSYS.Firstly,we described the effect of sputtering power of pure Si thin film anodes on electrochemical performance and microstructure. The results indicated that higher sputtering power means lower initial irresistible capacity fade. While there is a rising after falling tendency on cyclic stability of Si thin film. The capacity retention of Si thin film anodes under the sputtering power of 100W、150W、200W is 48%、29%、54.4%,respectively.Secondly,we discussed the effect of substrate heat treatment on the electrochemical performance of Si and Si-Ti bilayer thin film anodes. The Si 290 nm 300℃ thin film delivers a stable discharge capacity of 1536mAh/g after 500 cycles,while untreated counterpart only has 538mAh/g. The diffusion coefficient of lithium ion in the Si 150 W 300℃ thin film is 6.88*10-8cm2/s,which is larger than the Si 150 W thin film(4.96*10-8cm2/s). The capacity retention of Ti 15nm-Si 290 nm thin film is 46.2%,but it increased to 62.6% when heated at 300℃. All those electrochemical performance improvement of pure Si and Si-Ti bilayer thin film is attributed to stronger interfacial adhesive and enhanced Li+ diffusion kinetic by heat treatment.Furthermore,the preparation methods、microstructure and electrochemical performance of Si-Ti bilayer thin film was studied. The Ti 15nm-Si 290 nm thin film showed a discharge capacity of 1191mAh/g at the current density of 0.5mA/cm2 after 500 cycles,and with a capacity retention of 42%,which much higher than the Si 290 nm thin film(13.7%). The Si 290 nm thin film cannot deliver any capacity at a current density of 1.2mA/cm2,while the Ti15nm-Si 290 nm thin film show a discharge capacity of 447mAh/g after 400 cycles.These good performances of Si-Ti thin film are attributed to less volume effect and stronger interfacial adhesive, as well as better electrical conductivity with Ti layer. In addition,the thickness of Ti layer has very limited effect on the cyclic stability of Si-Ti bilayer thin film,whether the substrate is the rough or the smooth face of Cu foil.Finally,based on the similarity between the heat transfer equation and the diffusion equation,the ANSYS finite element software’s thermal-structure coupling analysis module was applied to carry on the solution to the lithium ion diffusion induced stress and strain. In the lithiation process,the maximum strain and stress of Ti0Si20、Ti0.5Si20、Ti1Si20、Ti2Si20、Ti3Si20 with different thickness of Ti layer are all the same. while in the delithiation process,The maximum strain of Ti0Si20 is larger than those four Si-Ti thin film. Combined with the lithiation and delithiation process,we can conclude that the simulation results are a close match with the experiments. |
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