To better understand the nature and evolution of the silicon SEI films, model systems of SEI structures were formed using sol-gel precursors deposited over clean sputtered 50nm thick silicon films. Chemistries from Li2Si2O5 to Li4SiO4 were synthesized from starting materials of lithium hydroxide and TEOS in a solution of methanol solvent with a controlled moisture environment. These films were then studied for Li transport and stability of silicon anode cycling against a Li reference and counterelectrode using standard 2032 coin cell configurations. Cycled anodes were then recovered and studied using NR, ICP-OES, Raman, ATR IR and TOF-SIMS.
The first observations of these model films was the dependence on stoichiometry for electrochemical behavior. While electrodes with only a native oxide layer behaved normally, with expected lithiation plateaus in the silicon, when electrodes with films of different lithium silicate compositions (Li2Si2O5 or Li2SiO3) were used, these plateaus were not present or shifted. Additionally, these films showed morphological and chemical changes on lithiation.
While the reason for these electrochemical behavioral changes has not been fully understood, these model systems start to tease apart the complexity of crystal structure and compositional changes that manifest in the silicon SEI, and may give some indication as to why the instability and low cycle capability of the silicon anodes is observed in practice.
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