Plasma-Assisted Atomic Layer Deposition of Thin Film Li2CO3 for All-Solid-State Li-Ion Batteries

Abstract

All-solid-state Li-ion batteries offer the advantage of a higher power density, a wider temperature window and an improved safety with respect to batteries containing liquid electrolytes. However, the lack of solid electrolytes with sufficient ion conductivity and wide electrochemical window is currently inhibiting the implementation of solid state batteries. Although the limited ionic conductivity can be overcome by employing very thin solid-state electrolyte films which shorten the ionic diffusion distance, the capacity will still be insufficient. This makes the use of non-planar devices of particular high interest as the application of 3D-(nano)structured materials will increase the total effective battery area. Moreover, the area enhancement will result in an ever higher energy density, faster (dis)charging, better stability, etc. Conventional deposition techniques, such as PVD and PE-CVD, do not generally result in conformal thin layers on 3D structures. Therefore, novel deposition techniques leading to enhanced control in thin film properties and conformality are being introduced in the field of battery research. Atomic layer deposition (ALD), which is based on sequential and self-limiting reactions, has emerged as a powerful tool since it offers exceptional conformality on high-aspect ratio structures, thickness control at sub-nm level, and tunable film properties. In this work, thin film lithium carbonate (Li₂CO₃) is being deposited by ALD. Li₂CO₃ is widely used as a building block for the fabrication of Li-ion battery active materials, such as lithium cobalt oxide (LiCoO₂), which is used for most lithium-ion battery cathodes [1]. It has also been reported that Li₂CO₃, without combining it with other materials, has a pure ion conductive behavior and a good electrochemical stability [2]. The high electrochemical stability of Li₂CO₃ makes it useful as an artificial solid-electrolyte interface (SEI) or buffer layer [3]. The SEI and buffer layer need to be conformal and very thin, for which the use of ALD would be beneficial. In addition, ultra-thin Li₂CO₃ can also improve the stability of known solid electrolyte layers, such as lithium lanthanum titanate or lithium phosphate. More specifically, we focus on the optimization of the plasma-assisted Li₂CO₃ ALD process by (in situ) growth studies and we investigate the influence of the process parameters on the electrical and electrochemical properties. Using plasmas in combination with ALD opens up the possibility to tune the properties of the layer by varying the plasma exposure time and also to deposit at lower temperatures. Although the plasma-assisted ALD process for Li₂CO₃ has been reported in literature before [4], to our knowledge, the optimization of the process was not reported, and also an electrochemical analysis of Li₂CO₃ layers fabricated with this method is still lacking. The ALD process employed, consists of a combination of LiOtBu as lithium precursor and an O₂ plasma as oxidant source, alternated by Ar purges. A growth rate of ± 0.8 Å/cycle was obtained at a process table temperature of 150 °C on TiN substrates in the linear growth regime (figure). TiN substrates are used as an electrical contact for electrical and electrochemical testing of the layers. The film stoichiometry was investigated with Elastic Recoil Detection (ERD) and X-ray Photoelectron Spectroscopy (XPS). The results are in good agreement with the stoichiometry and density of Li₂CO₃. Excellent electronic insulation properties were confirmed by current-voltage and DC-polarization measurements. This observation is in agreement with impedance measurements at open circuit potential (figure), which showed no apparent electronic conductivity of the film. The dependence of ALD process parameters such as temperature and O₂ plasma exposure time on the electrochemical properties is investigated. Moreover, half cells employing Li₂CO₃ with TiO₂, LiMn₂O₄ and Si electrodes were studied for cyclability and rate performance. These and other results will be presented in this contribution. Acknowledgements: This project is financially supported by the Dutch program “A green Deal in Energy Materials” ADEM Innovation Lab. M. E. Donders et al., Journal of The Electrochemical Society, 160 (5), 2013J. Mizusaki et al., Solid State Ionics, 53-56, 1992P. Verma et al., Electrochimica Acta, 55, 2010A. C. Kozen et al., The Journal of Physical Chemistry C, 118, 2014 Figure caption: On the left: growth curve for the plasma-assited ALD process of Li₂CO₃ measured by in-situ spectroscopic ellipsometry on TiN at 150 °C and the corresponding process steps (insert). On the right: Electrochemical impedance spectroscopy measured at room temperature showing the presence of ionic conductivity. Figure 1