Investigation Of Si And Cu Nanomultilayers Anode Materials For Lithium-ion Batteries

There are many Li-Si alloy phases in silicon anode materials for lithium-ion battery,Li_4.4Si alloy phase with the theoretical capacity 4212 mAh/g,which is more than ten times that of graphite materials.The Li-Si alloy has a potential range of 1.0-0.3 V?vs.Li/Li⁺?.Therefore,the large volume effect of the silicon material in the alloy/dealloying process easily destroys the overall structure of the material and reduces the conductivity of the electrode,thereby restricting the silicon-based material.In order to solve the problem of structural fragmentation caused by the expansion of silicon,structural nanomaterials and composite materials are generally adopted,such as formation of silicon nanoparticles,silicon nanowires,nanocrystalline films,or Si-Mg,C/Si,and Si-Cu composite materials.The Si-Cu composite negative electrode materials,Cu plays an important role in increasing the electrical conductivity of Si and the ductility of Cu can buffer the cracking of Si during charge and discharge.Our lab focus on the solar cells and lithium-ion batteries integrated devices,backside silicon of crystalline silicon solar cells as an anode for lithium-ion batteries.In this common anode integrated device,electrons are collected from a crystalline silicon solar cell and stored in a negative electrode of a lithium ion battery.The Si-Cu multilayer film can adjust the conductivity of the silicon electrode.In this dissertation,prepared Si-Cu multilayer films with different layered structures to research the performance of Si-Cu multilayer films as anode materials for lithium-ion batteries,and established experimental foundation for solving test problems of common anode integrated devices.Firstly,pure Si nano-film materials with different thicknesses?56.8-192 nm?were prepared on the Cu foil collector by controlling the magnetron sputtering time;then different layered structural materials were prepared:Cu-Si-Cu-Si?Multilayer A?Cu-Si-Cu-Si-Cu?Multilayer A@Cu?,I-Si-I-Si-I?Multilayer B,where structure I=Cu-Si-Cu?.Assemble a CR2025 button cell and test its charge-discharge and cycling performance.The thickness of the Si nanofilm was calibrated with an ellipsometer.The thickness of pure Si nano-film has an effect on the decay of the charge and discharge cycle performance.At a current density of 0.5 A/g,the specific capacity retention rate of Si nano-films with different thicknesses was maintained in the range of20-43%after 100 cycles;at the current density of 1 A/g,the initial discharge specific capacity of 192nm Si nanofilms reached 4790.2 mAh/g,631.8 mA and 36.7 mAh/g after the 100th and 200th cycles respectively,and the the battery lost its activity after 200cycles.This may be due to the fragmentation of silicon.After annealing at 500°C for 2h,the specific capacity decay rate of Si nanofilms was significantly reduced,and the capacity retention rate reached 52.73%.Annealing makes pure Si nano-film samples have better contact at the interface of Cu foil and Si,and the stronger adhesion ensures that the capacity retention rate during the cycle is improved.Si-Cu multilayer nano-films improves the cycle performance of pure silicon nano-films.Multilayer A,Multilayer A@Cu,and Multilayer B,contain¹28 nm pure silicon film,have a much smaller specific capacity than pure silicon,their cycle performance has been greatly improved.The 128 nm pure silicon film has a retention rate of only 40.15%,and the cycle retention rate of Multilayer A,Multilayer A@Cu and Multilayer B has increased to over 53%.Moreover,the retention rate of Multilayer A@Cu?400°C?and Multilayer B?400°C?prepared at 400°C during the sputtering process,were increased to 60%after 100 cycles.They are better than that prepared at room temperature.The Cu/CNTs structure was electrochemically co-deposited on the Cu foil as a structured current collector,and the perfoemance of the loaded Si nano-film were initially explored.Three Cu/CNTs current collectors were prepared by electrochemical co-deposition with three different current densities at the same charge?1.6 C?,and then RF magnetron sputtering Si nano-film?64 nm?on Cu/CNTs current collectors.Electrochemical co-deposition of Cu/CNTs at current density of 7mA/cm² optimized the performance of Si nanofilms well.The capacity retention rate was 48.22%after 100cycles at 0.5A/g charge and discharge test current density.But the coulomb efficiency only 96.72%.However,the range of material charge and discharge platforms is too large?0.3¹.3V?,and many kinds of phase transitions cause the volume effect to significantly affect the cycle performance.In this thesis,the Si-Cu multilayer films material can improve the cycle performance,and the cycle performance can also be improved by different heat treatment.In addition,the structure of Cu/CNTs as a current collector also improves the cycle stability of Si nanofilm.