The Electrochemical Performance Study Of Si-based Thin Film As Anode Materials For Lithium Ion Batteries

Nowadays there is an urgent demand for secondary battery with high energy density, high power density, long life time and great reliability in industries including but not limited to electric vehicles and distributed energy storage. However, current Lithium ion battery technology cannot sufficiently satisfy this demand even it has been considered most promising and favorable by many manufacturers of mobile electronic device since its commercialization in 1990 s. As a potential cathode material, Silicon presents a possibility of greatly improvements in the energy density of Lithium ion battery with its high theoretical specific capacity(about 4200 m Ah·g^-1 for the Li₂₂Si₅ phase) and relatively low relative potential to lithium（about 0.5 V）. A proper structure design can also help to reach the requirement of high power density.However, a Silicon cathode possesses several distinct disadvantages. Its volume expands（about 400%） in the Li⁺ insertion/extraction process, which results in its pulverization and breakdown so that a stable SEI film cannot be generated on the surface. In addition, a Silicon cathode has a low Li⁺ migration rate due to its own properties as a semiconductor. In this regards, carbon nano-materials or compound materials can be applied because of their brilliant electrochemical performance.This paper introduces that applying magnetron sputtering method to prepare Si/C nanomultilayer or composite thin film, thus decreasing the volume of Silicon monomer and restraining pulverization with carbon structures as buffer layer and electrically conductive materials. Some Li⁺ transport electrolytes including Ti, TiO₂, Li₄Ti₅O₁₂, LiTi₂P₃O₁₂ are composited together as well to improve Li⁺ migration rate. Then these films are paired with Lithium plates and assembled as half cells, whose electrochemical properties are studied, in a glove box.This paper presents the results as following.1. Si/C nano-multilayer thin film: ensure that the fluid and thin multilayer film surface respectively deposition 10 nm thick Ti transition layer and C protective layer, prepared 500 nm thick modulation cycle is 4/1 and 3/1 respectively, and 1.5 μm thick modulation cycle is 4/1 of Si/C multilayer films. Compared with Silicon thin file, the multilayer structure greatly restrains the pulverization and breakdown problem. The 500 nm thick multilayer film with a modulation cycle of 4/1 shows good cycle characteristics and the rates characteristics where the initial discharge capacity is as high as 2940 m Ah·g^-1, initial columbic efficiency is about 95% and the capacity remain rate is about 95% after 166 circulations. The Carbon layer on the surface effectively protects the active Silicon material by avoiding direct contacts between Silicon and electrolyte. Because of the compactness of the thin film prepared by sputtering deposition, Li+ transportations are harder along the vertical direction which concludes the weaker electrochemical performance of the thick film. Therefore, we composited a certain amount of quick lithium ion transportation materials in the Silicon layer of the multilayer film: in the Silicon layer, certain amount of Titanium have played an important role in promoting the electrochemical performance. Rates performance is significantly better than the film of the same thickness without Ti-permeance Si/C multilayer film. Under a 2C charge and discharge rate it is still able to maintain a specific capacity of nearly 1000 m Ah·g^-1.2. Carbon transportation in the multilayer film structure is horizontal which seems unable to realize vertical Li⁺ transportation. To solve this, we apply the sputtering prepared Si（C） nanocomposite films with Silicon nanoparticles embedded in Carbon network matrix. The expectation of the structure is that it has better electronic and ionic conductivity. And compounding the right amount of Li⁺ transportation material will greatly improve the vertical transportation capacity. With Titanium to Carbon gradient transition layer and protective carbon layer, Silicon target power of 150 W, Carbon target power of 350 W, 300 nm thickness Si（C）composite film and 130 circulations, the discharge specific capacity can maintain between 2500-2000 m Ah·g^-1 which is an excellent circulation performance. After 14 A·g^-1 high current density cycle, the charge and discharge specific capacity is 900 m Ah·g^-1. And when the current density restores to 0.7 A·g^-1, circulation specific capacity can be up to 2300 m Ah·g^-1 with a stable membrane structure and a great rate rates characteristics.