(Invited) Atomic Layer Deposition for Interface Engineering of (Thin-Film) Lithium-Ion Battery
Authors: Mattelaer Felix , Timmermans Marina Y. , Vereecken Philippe M. , Dendooven Jolien , Detavernier Christophe 
Published: February 2020

Atomic layer deposition (ALD) is a gas-phase deposition technique based on sequential self-limiting surface reactions [1]. The self-limiting nature of these gas-surface interactions give ALD several advantages over other deposition techniques if exploited properly: excellent conformality on and in 3D-structured substrates (powders, pores, trenches [2]), smooth and pinhole-free films and ultimate thickness control (~0.1nm precision). These near-unique advantages can be exploited to advance energy storage devices, lithium-ion batteries foremost amongst them [3,4].
On the one hand the excellent step coverage inherent to atomic layer deposition can be exploited to construct complex engineered micro- and nano-structured batteries. These 3D thin-film nano- and microbatteries are interesting storage modes for electronics thanks to their relatively high capacity and very high power densities [5-6]. However, ALD is not without disadvantages. The slow deposition rate (0.1nm/s-0.1nm/min) and high cost compared to for example wet chemical deposition methods typically prevent a full battery stack grown with ALD from being commercially viable.
The real strength of ALD lies not in the micron-thick films needed for the electrodes in a thin-film battery, but rather in thin films in the range of 0.1nm to 100nm. An electrochemical storage system consists of many interfaces, and most of the important phenomena occur at those interfaces. Tuning or engineering of these interfaces can be of paramount importance to enhance battery characteristics or unlock novel battery chemistries. In that respect, ALD can be used to deposit conductive coatings as current collector on non-conductive scaffolds, seed layers to enable adhesion or prevent current collector oxidation, protective coatings at the electrode/liquid electrolyte interface and solid electrolytes to prevent the liquid electrolyte all together [3,4]. In this talk, two examples will be highlighted.
Firstly, ALD has been heavily investigated for electrode/electrolyte interface engineering since 2010, with the main focus on ALD of Al2O3, given the ease of deposition and proven scalability of this process. However, using model-system thin film electrodes, we can show that ALD Al2O3 films beyond 3nm are completely blocking towards Li+ ions, while thinner coatings impose a significant impedance of >1012 Ω cm. ALD TiO2 on the other hand is shown to be not blocking towards Li+ ions, and can alleviate solvent decomposition or metal dissolution, two important capacity fading mechanisms found at this interface [5,7].
Secondly, electrolytic manganese oxide (EMD) and its converted form to LiMn2O4 (LMO) are considered interesting cathode materials. However, the electrodeposition of thick films of EMD is challenged by film adhesion and current collector oxidation of non-noble current collectors (eg TiN or Ni), limiting the deposition of and electrochemically active film in acidic electrolytes to approx. 50nm. It is shown that using an ALD MnO2 seed layer of only a 2-4 nm, the adhesion was improved and substrate oxidation was alleviated, allowing electrochemically active EMD films up to 1µm. This could also be demonstrated on 3D-structured substrates, thanks to the conformal nature of both electrodeposition and ALD [8].
In conclusion, despite the slow deposition rates typical for ALD, it can be used as a powerful tool in engineering of interfaces in lithium-ion battery development.
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