Surface coverage and SEI induced electrochemical surface stress changes during Li deposition in a model system for li-ion battery anodes

We report electrochemical surface stress and potential dependent matrix assisted laser desorption ionization (MALDI)-time of flight (TOF) mass spectrometry (MS) results combined with detailed density functional theory (DFT) analysis of Li deposition on a Au model system for Li ion battery anodes. Deposition of Li on Au surfaces at potentials >0.2 V vs. Li/Li⁺ occurs through the formation of a Li-Au surface alloy, a result that is predicted by DFT calculations. As the Au surface potential becomes more cathodic, compressive stress develops on the surface, a result again predicted from calculation. The compressive stress is completely removed by cycling the potential back to 2.0 V vs. Li/Li⁺ through delithiation of the surface alloy. Lithiation of the Au electrode during Li bulk alloy formation at potentials <0.2 V vs. Li/Li⁺ results in compressive stress, as expected. However, in this case residual tensile stress is observed following delithiation, the magnitude of which increases with increasing lithiation/delithiation cycles. Potential dependent MALDI-TOF MS analysis shows that solid electrolyte interphase (SEI) oligomers are formed during delithiation following Li bulk alloy formation and that these oligomers are the likely origin of the observed residual tensile stress. This residual tensile stress is not present when the carbonate solvent is replaced with an ionic liquid. These results show that surface stress is determined by Li-host atomistic interactions as well as the nature of the SEI.