Direct Evidence of a Chemical Conversion Mechanism of Atomic-Layer-Deposited TiO2 Anodes During Lithiation Using LiPF6 Salt

Titanium dioxide has been identified as a prospective anode material for use in lithium ion batteries. The higher lithiation potential of TiO2 versus other common anodes is more electrochemically compatible with most organic electrolytes, thus leading to reduced solid electrolyte interphase formation and overall more stable battery systems. However, in this study TiO2 has exhibited poor cycling stability with common electrolytes containing lithium hexafluorophosphate (LiPF6) salt. Combined electrochemical and spectroscopic analyses have revealed this to be due to the onset of reversible fluorination of the anode and chemical conversion of TiO2 to TiOF2 during lithiation. Comparison of electrochemical cycling of atomic-layer-deposited TiO2 anodes using LiPF6 with and without a hydrofluoric acid scavenger, tributylamine, to cycling using a non-fluorinated lithium perchlorate (LiClO4) salt indicates that the in situ formation of hydrofluoric acid in the electrolyte from decomposition of the PF6- anion alters the lithiation electrochemistry. X-ray photoelectron spectroscopy (XPS) analysis and time-of-flight secondary-ion mass spectrometry (ToF SIMS) depth profiling measurements confirmed the presence of TiF2 and TiOF surface and bulk phases, respectively, in the electrode upon lithiation.