Plasma-enhanced atomic layer deposition of titanium phosphate as an electrode for lithium-ion batteries

Abstract

Titanium phosphate thin films were deposited by a new plasma-enhanced atomic layer deposition process. The process consisted of sequential exposures to trimethyl phosphate (TMP, Me₃PO₄) plasma, O₂ plasma and titanium isopropoxide (TTIP, Ti(OCH(CH₃)₂)₄) vapor, and it was characterized by in situ spectroscopic ellipsometry and ex situ X-ray reflectometry. The growth linearity, growth per cycle (GPC), and density of the resulting thin films were investigated as a function of the pulse times and the substrate temperature. The conformality of the process was characterized by deposition on micropillars. At a substrate temperature of 300 °C and using saturated pulse times, linear growth with a GPC of 0.66 nm per cycle and without nucleation delay was achieved. The as-deposited films were amorphous, while crystalline TiP₂O₇ was formed upon annealing in air or helium atmospheres. In lithium-ion test cells, the as-deposited films showed insertion and extraction of Li⁺ around a potential of 2.7 V vs. Li/Li⁺. Charge/discharge measurements revealed a volumetric capacity of 330 mA h cm⁻³, together with a good rate capability and minimal capacity fading.