Effect Of Interface And Cu On Electrochemical Performance Of Si/C Multilayer Films Electrodes

Lithium-ion batteries（LIBs）have been widely used in energy storage due to their high energy density,high power and long life.In recent years,with increasing concern about the environment and the progress of science and technology,high energy density is required for energy storage devices in fields such as solar and wind energy conversion,electric vehicles and portable intelligent electronic products.In order to increase the energy density of LIBs,it is necessary to develop electrodes with high specific capacity.Silicon（Si）possesses the highest specific capacity(3579 mAh g^-1 for Li₁₅Si₄ alloy at room temperature)among the currently known LIBs anode materials.In addition,it has abundant crustal reserves and low potential relatively to lithium（⁰.34 V vs.Li/Li⁺）.Si has been considered to be one of the most attractive and promising anode materials.However,Si severely suffers from the huge volume change（about 400%）during the process of lithium intercalation/deintercalation,which leads to the pulverization of Si particles,the failure of the electrical contact with the current collector,the formation of unstable SEI and the extremely low cycle stability.In addition,the low electronic and ionic conductivity of Si result in poor rate performance of Si-based anodes,which seriously limits the use of Si in commercial LIBs on market.Previous solutions to these problems are mainly focused on scaling Si particle down to nano size,coating Si particle to stabilizing the structure,creating extra space for volume expansion and making strong stable layer for local separation.Such strategies inevitably introduce multi-phase composite structure.Although the property of composite anodes can be optimized by taking advantages of each individual material,the volume ratio of composite materials and the diversity of interface will bring many unpredictable effects.1.In this paper,the lithium ions（Li-ions）diffusion kinetics before and after the cracking of electrodes,which is made of amorphous Si/C multilayer films of about400 nm,is studied.The influencing mechanisms of Si-C interfaces on the transport kinetics of Li-ions were discussed.The results show that:1)Li_xSi alloy conductor formed at the Si-C interfaces is polarized under the external electric field,which results in an enhancement of the electric field around the alloy locally and promotes the Li-ions transportation.2)Defects existing at the Si-C interfaces lead to a high diffusion ability of Li-ions along the interface.With suitable stacking thickness of Si and C（for instance,10 nm Si and5 nm C）,even if Si contacts directly with the electrolyte after the film cracks into small pieces of about 1μm,a very stable SEI can be formed on the surface.The battery presents high cycle stability and residual specific capacity.A specific capacity of 2729 mAh g^-1,which is 102.1%of its first lithiation capacity and 101.8%of its theoretical specific capacity,is retained after 200 cycles.The retained specific capacity is higher than the theoretical specific capacity,which further demonstrates that a stable SEI is formed on the surface of the film.Based on these results,a micron-sized hierarchical multi-layer block design with appropriate amorphous Si and C stacking thickness is proposed,which can be used as a reference for the design of thin film electrodes and high performance core-shell electrode structures.2.The Si/C multilayer film with appropriate stacking thickness shows good electrochemical properties.However,due to the dense nature of the films prepared by physical vapor deposition（PVD）method,the related electrode is prone to crack during the process of Li-ions intercalation/deintercalation,which results in the formation of unstable SEI.Therefore,the preparation of looser film electrodes by PVD method is crucial.Copper（Cu）has excellent electrical conductivity and ductility.It is widely employed as the current collector of anodes.It is also used to combine with Si to alleviate the volume expansion of Si and improve the electronic conductivity and ionic conductivity of the corresponding anode.Cu is highly volatile at high temperature when it is mixed with other elements.We co-deposite Cu and Si to prepare the films,in which Cu diffuses out of the films after heat treatment to reduce the density of the films,providing extra space for Si expansion/contraction in the process of Li-ions intercalation/deintercalation.We prepare Si（Cu）/C（Cu）composite multilayer film and conduct a post-treatment at 800°C.After the heat treatment,a large amount of Cu particles are formed on the surface of the films.Electrochemical measurements show that the film electrode has good electrochemical performance and a high retained specific capacity of 2036 mAh g^-1 after 300 cycles.The morphology of the electrode after cycling shows few electrode cracks,which indicates that our prepared composite films,which can provide large expansion/contraction space for Si,can be used as a potential anode.