Li Deposition Mechanism Of Silicon Anode And Its Application In Fast Charging Li-ion Batteries

High energy density and fast charging are two key performance indicators for lithium-ion batteries（LIBs）.However,the low capacity of graphite anode leads to the limited energy density of the LIBs(<300 Wh kg^-1).In addition,Li-plating easily occurs on the graphite anode during fast charging process,causing short circuit of the LIBs.Silicon anode has ultrahigh theoretical capacity(3579 m Ah g^-1)and good lithiophilicity.Therefore,it is of great significance to study the fast charging performance and Li deposition mechanism of the silicon anode for the development and application of next-generation LIBs.In this thesis,the silicon thin film（Si）and copper foil（Cu）were used as the substrates for Li deposition.The Li deposition behaviours on different substrates were investigated by combing ex-situ scanning electron microscopy（SEM）,in-situ optical measures,and electrochemical tests.The effects of pressure,current,capacity,and temperature on the Li deposit morphologies were also detailly studied.The tendency of whisker growth was quantified and its dependence on current density was determined.It is shown that all the Li deposits show whisker shape as the current is over a critical value（J_w）.The tendency of Li growth is dependent on the Li-diffusion behaviours of the substrates and the Si substrate can efficiently suppress the Li whisker formation.The correlations between the whisker growth and the internal short-circuit tendency as well as the coulombic efficiency（CE）were revealed,which indicated that the Li whisker is always corelated with faster short-circuit tendency and lower CE.The fast-charging performance of the traditional silicon anode and graphite anode were further explored in this thesis.The effects of current,capacity,and temperature on the Li deposit behaviours within the anodes were also detailly studied by ex-situ SEM,in-situ optical measures,and electrochemical tests.The traditional silicon anode showed better electrochemical performance due to its good lithiophilicity which inhibited the formation of the Li whisker.Based on the above results,we designed a gradient anode composed of silicon and graphite for LIBs.The silicon component with gradient distribution guides even growth of Li deposits,thus suppressing the Li whisker formation.As a result,the LIBs using the gradient anode exhibit good fast-charging performance（with a capacity retention of85.9%after 100 cycles at 4 C）.