Electrochemical Lithium Insertion into TiO ₂ Anatase ALD Thin Films for Li‐Ion Microbatteries: An Atomic‐Scale Picture Provided by Raman Spectroscopy

Abstract Electrochemical lithium (de)intercalation in an atomic layer deposited (ALD) TiO 2 anatase thin film deposited on a planar Si /Al 2 O 3 /Pt substrate is investigated by Raman spectroscopy. An initial discharge capacity of 63 µAh cm −2 µm −1 (0.5 Li + mole −1 ) is reached at C/10 rate, which increases up to 77 µAh cm −2 µm −1 upon further cycles. An excellent capacity retention is achieved over at least 100 cycles, showing the good adherence of the ALD thin film. Raman spectra of Li x TiO 2 (0 ≤ x ≤ 0.5) thin film electrodes point to the nucleation of the orthorhombic lithiated titanate (LT) Li 0.5 TiO 2 phase from x = 0.1. This LT phase coexists with tetragonal TiO 2 in the 0.1 ≤ x ≤ 0.4 composition domain to be pure for x = 0.5. A fully reversible transformation from orthorhombic LT to tetragonal TiO 2 is observed upon the charge. The high quality of the Raman spectra allows identifying for the first time 12 modes in the 100–800 cm −1 region for the electrochemically formed LT phase. Furthermore, an appropriate Raman spectra analysis allows a reliable and quantitative determination of the thin film composition during discharge and charge. These results illustrate Raman spectroscopy is a powerful probe to scrutinize the Li insertion/extraction mechanism in TiO 2 thin films.