Investigation of the Performance Improvement of Silicon Electrodes Cycled with Electrolyte Containing FEC or VC

Silicon is one of the most promising candidates for an anode material in LIBs due to the high theoretical capacity, 3580 mAh/g.  This theoretical capacity is ~10 times that of commercial graphite (372 mAh/g) currently used in lithium ion batteries. However the silicon electrodes have a very large volume expansion (300–400%) during lithiation resulting in instability of the solid electrolyte interphase (SEI) and poor capacity retention.   The two most frequently utilized SEI stabilizing additives are vinylene carbonate (VC) and fluoroethylene carbonate (FEC).   A systematic comparison of the effects of added FEC or VC at multiple concentrations is being conducted with uniform silicon nano-particle electrodes.

Capacity retention of Li/silicon nano-particle cells with different concentrations of VC and FEC in 1.2 M LiPF₆ in 1:1 EC/DEC have been investigated.  The capacity fades very rapidly for the baseline electrolyte.  Incorporation of FEC at any of the concentrations investigated (5, 10, 15, or 25 %) results in significant improvements in capacity retention.  Interestingly, intermediate concentrations of FEC 10-15 % give the best capacity retention suggesting that lower concentrations do not generate a sufficiently stable SEI while higher concentrations may results in increased cell resistance.  Cells containing added VC do not have significantly better performance than the cells containing the baseline electrolyte.  Incorporation of 3 % VC results in cells with very similar capacity fade to the baseline electrolyte, while cells containing 6 % VC have an odd intermittent behavior which may be due to high cell impedance as evidenced by electrochemical impedance spectroscopy.  The cycling efficiencies correlate very well with the capacity retention.  Cells containing 10-15 % FEC have the best efficiencies (~99 % for cycles 10-50), while cells containing the baseline electrolyte or electrolyte with added VC have lower efficiencies (<98 % for cycles 10-50).  

Ex-situ surface analysis of the electrodes after cycling via a combination of SEM, XPS and FT-IR will be reported.  Structural characterization of the SEI will lead to a better understanding of the source of performance enhancement due to the incorporation of added FEC.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No 6879235 under the Batteries for Advanced Transportation Technologies (BATT) Program.