Preparation And Performance Of Si-Ag Thin Films Anodes By Magnetron Sputtering For Lithium Ion Batteries

With a high specific capacity(4200 mAh g-1),silicon based materials have become the most promising anode materials and will replace graphite as the next generation lithium-ions batteries.However,pristine silicon anode suffers severe volume change(>300%)during lithium insertion/extraction processes,resulting in the electrode pulverization and subsequent loss of electrical contact between active material and current collector,eventually leading to poor reversibility and rapid capacity fading.In addition,its low intrinsic electrical conductivity(10-3 S/cm)impedes electrochemical kinetics of the charge/discharge processes,which lead to moderate rate performance.The cycling performance Si anodes can be effectively improved by the synthesis of Si-M(M=Mg,Cr,C etc.)structures or composites,because they effectively robust against stress resulting from volume changes,as well as display an increased electronic conductivity compared to pure silicon.In this work,silver layer was used as an additional modified layer to fabricate Si-Ag thin film electrodes by magnetron sputtering method to stabilize the cycling performance of silicon thin film layer electrodes.The main innovative results are listed as follows:1.In order to study the influence of silver thin film layers on the electrochemical characteristics,Si/Cu,Ag/Si/Cu,Si/Ag/Si/Cu,and Si/Ag/Cu electrodes are directly deposited on copper foil by magnetron sputtering technique.The results show that silver thin film incorporated between the amorphous Si thin film layers(Si/Ag/Si/Cu)not only displays a high and stable capability(with the highest discharge areal capacity of 111.7?Ah cm-2 and optimal capacity retention of 80%compared to other electrodes after 150 cycles at 65.2?A cm-2),but also exhibits the best rate performance compared to other electrodes.It also possesses lower charge-transfer impedances due to excellent electron conductivity of the silver.2.Based on the results mentioned above,Si/Ag/Si electrode was selected to study the optimum thickness of the Ag layer as well as the influence of the Ag layers on the electrochemical performance.The cycling performance of a-Si/Ag/a-Si electrodes with different thicknesses of silver layers(5,15,30,and 60 nm)is compared.a critical thickness of silver layer(30 nm)is found,in this case,it exhibits the highest capacity retention of 70%after 200 cycles.The RCT value of the multilayer Si/Ag/Si electrodes decreases with the increase of the thickness of the silver layer except for the silver layer thickness of 5 nm.The above results are explained by the interaction of silver content,mutual compatibility and crack size of Si based film during lithium insertion/extraction processes.It is demonstrated that,for multilayer Si/Ag/Si(140/30/140 nm)electrode,the proportion value contributed by Ag layer is 59.1%after 200 cycles.3.Based on the multilayer Si/Ag/Si structure,the relationship between annealing temperature and cyclic stability was investigated,and the internal mechanism was preliminarily analyzed.The annealing-temperature effect on the cycling performance in the range from 100 to 400 ? is studied and 30 min is set as the annealing time.Of all the annealed Si/Ag/Si thin films,the 300 ? sample stands out with a capacity retention of 71.3%(versus the 10th cycle)after 500 cycles with a corresponding delithiation capacity just slightly above the target of 1343.7 mAh g-1,,which is more than 3.6 times higher than the theoretical capacity of a graphite anode(372 mAh g-1).Further analysis shows that the reason for the capacity decay of Si/Ag/Si thin film anodes annealed at 300 ? relates to the cracks of the electrode structure,loss of the electrical contact,rather than rupture of the SEI layer and forming new SEI layer.