(Invited) ALD and MLD of Functional Thin-Film Coatings for Enhanced Performance in Li-Ion and Li-Metal Solid-State Batteries

Abstract

The key figures of merit for batteries are Energy density, charging time (C-rate) and cycling Life time. In this invited talk we will focus on the application of continuous closed thin films as artificial interfaces in large capacity batteries or for the fabrication of functional battery components in micro-batteries. We will deal mostly with the perspective of device performance where the ALD and MLD processes merely provide the films and enable their ultrathin, continuous and conformal nature. Nanoscale film thickness allows for low interface and cell resistance even when the materials themselves are poor conductors. For example. LiPON (N-doped Li3PO4) is a solid electrolyte interesting because it is stable against metallic lithium but, because of its poor ionic conductivity (<10-6 S/cm) only practically useful for thicknesses under few hundred nanometers. Even further, materials which are not solid electrolytes by their own merit (e.g. TiO2) still have ion-transparent properties up to several tens of nanometer and can be used as ion transparent artificial interfaces in contrast to alumina which is an insulator for Li-ions. For coating of individual particles, ideally coatings which have both electronic and ionic conducting (or transparent) properties. Doping is one approach to enhance either conductivity. When scaling down of the film thickness also finite size effects have to be considered, for example, the thickness of the electrical double layer, diffusion layer thickness and the electric field over the dielectric. ALD and MLD can be used also to fabricate functional nanomaterials; i.e. by harvesting nanoscale effects. For example, nanocomposite solid electrolyte films of a few tens of nanometers were fabricated by a combination of MLD of inorganic-organic hybrid alucone thin-films, etching the organic fraction in water and functionalization by ALD of Li2CO3 and Li3PO4. For the first time, enhanced ion conductivity is shown by harvesting the enhanced ion conductivity at oxide/ion conductor interfaces. Finally, volume changes during charge and discharge are limiting the cycle life-time of batteries, especially for rigid solid-state batteries. Also ALD thin-films suffer from the mechanical strain and the benefits of the “closed” protective film are lost. Therefore, MLD of hybrid organic/inorganic coatings is explored to enable more elastic coatings for improved cycle life time.