Plasma-assisted and thermal atomic layer deposition of electrochemically active Li2CO3

Abstract

Thin-film lithium carbonate (Li₂CO₃) has applications in various electrochemical devices, like Li-ion batteries, gas sensors and fuel cells. ALD of Li₂CO₃ is of interest for these applications as it allows for uniform and conformal coating of high-aspect ratio structures and particles with very precise thickness control. However, there are few studies that focus on its fabrication and characterization. In this work, plasma-assisted and thermal ALD were adopted to grow ultra-thin, conformal Li₂CO₃ films between 50 and 300 °C using lithium tert-butoxide as a precursor and O₂ plasma or H₂O/CO₂ as co-reactants. More specifically, we focus on the plasma-assisted process by film growth, stability and conductivity studies and emphasize the differences from its more extensively adopted thermal counterpart. Plasma-assisted ALD allows for higher growth per cycle values (0.82 vs. 0.60 Å), lower substrate temperatures and shorter cycle times. The stoichiometry of the films, ranging from Li₂CO₃ to Li₂O, can be controlled by substrate temperature and O₂ plasma exposure time. The ionic conductivity for both plasma-assisted and thermal ALD is measured for the first time and is in the order of 10⁻¹⁰ S cm⁻¹ after normalizing to the different effective surface areas. The Li-ion conductivities found here are in line with literature values predicted by simulation studies.