Modification Of Silicon Anode And The Solid Electrolyte Interphase Study For Lithium-ion Batteries

The development of lightweight,long-lasting Li-ion batteries(LIBs)is of great technological importance for critical applications,such as low-or zero-emission hybrid electrical and electrical vehicles,energy-efficient cargo ships and locomotives,aerospace and power-grid applications.Increasing the specific capacity of Li-ion battery anodes is considered an attractive route to lower battery weight,volume and ideally cost.In the case of anodes,silicon based materials provide high specific theoretical capacity(Li4.4Si : 4200 m Ah/g)which is more than ten times than commercial carbon anode(graphite:372 m Ah/g).Extremely high theoretical specific capacity makes silicon one of the potential anode materials.In addition,Si has the advantages of low working potential,natural abundance and well-established manufacturing technologies.However,low intrinsic electrical conductivity and enormous volume expansion(>300%)during lithium alloy/dealloy processes limit the commercial use of Si in LIBs.Such huge volume change leads to capacity loss with cycling and is mainly due to particle pulverization,breakdown of electric conductive network,loss of contact with the conductive network or the current collector,and continual formation of stable solid electrolyte interface(SEI)on the fresh Si surfaces.Thus,doping and coating are useful methods to improve the conductivity of Si-based anodes and suppress huge volume change;SEI layer plays an important role on LIBs’ s cycling performance,so it is also necessary to learn the formation process of SEI layer.Herein,we report a two-step ball milling method to prepare nanostructured P-doped Si/graphite composite as anode for Li-ion batteries,and study the size effect on SEI layer.The results are as follows:(1)We present a two-step ball milling method to prepare P-doped Si/graphite composite using the abundant and inexpensive micro-Si as high-performance anode in LIBs.The first step ball milling process created P-doped Si which had a lower charge transfer resistance and provided significant electrons and lithium ions transport channels in the Si bulk.A graphite skin formed after the second ball milling suppressed the huge volume change.The as-prepared composite(named as PSG55)anode exhibited outstanding cyclability with a specific capacity of 883.4 m Ah/g after 200 cycles at the current density of 200 m A/g.PSG55 showed good cycling performance using PVDF or CMC as binder.Furthermore,the graphite skin also improved the conductivity and prevented the direct contact between Si and the electrolyte,which can lead to a stable SEI layer.The cost-effective materials and scalable preparation method make it feasible for large-scale application of the P-doped Si/graphite composite as anode for Li-ion batteries(2)We report the SEI morphological evolution on micro-and nano-sized Si anode during CV process.In both Li PF6/EC/DMC and Li PF6/FEC/DMC electrolyte systems,the SEI layer formed on the micro-sized Si anode was thick and soft while the SEI layer formed on the nano-sized Si anode was thin and stable.XPS analysis showed that SEI formed on nano-sized Si anode contained more inorganic component like Li F while SEI formed on micro-sized Si anode contained more organic component like ROCO2 Li.The thick SEI layer formed on the micro-sized Si anode was due to the huge volume change of Si during the charge/discharge process.The larger specific surface area of nano-sized Si contained more Si Ox on the surface,which was much easier attacked by HF in the electrolyte and formed more Li F component.This work revealed the formation process of SEI layer on different sized Si and its properties.