Plasma - Assisted ALD of Lipo(N) for Solid State Batteries

Abstract

Present-day planar thin-film solid state Li-ion batteries provide an excellent power density and improved safety over their liquid counterparts. However, their energy density remains typically low. For this reason the transition to a 3D thin film battery is presently being explored. This preserves the extreme power characteristics associated with the usage of thin films while increasing the amount of active material and therefore the energy density. Key to the realization of such battery is the possibility to coat these large surface area structures in a conformal and defect free way1. This aspect is especially critical for the solid electrolyte which provides electronic insulation between the two electrodes. Nitrogen-doped lithium phosphate glass or LiPON, is the solid state electrolytes of choice for planar thin-film batteries. This is attributed to its good electronic insulating properties, its wide electrochemical window (0 - 5.5V vs Li+/Li) and reasonable ionic conductivity (~10-6 S/cm). The ionic resistance of the LiPON is still below 100 Ω.cm2 when the film thickness is kept below 1 μm. It was shown recently that LiPON layers can provide electronic insulation down to 15 nm.2 Atomic layer deposition (ALD), which is a vapor based technique with sequential and self-limiting reactions, is known to provide excellent conformality and thickness control (down to sub-nm level). Hence, it is ideally suited for the deposition of the solid electrolyte in thin film battery applications. Recently both thermal and plasma assisted ALD of LiPON was shown3,4. In this work we report the deposition of Li3PO4 and LixPOyNz using LiOtBu and TMPO as precursors and H2O (or O2 plasma) and N2 plasma as reactants. The effect of these different processing conditions on the stoichiometry is studied by a combination of Elastic Recoil Detection (ERD) and X-ray Photoelectron Spectroscopy (XPS). As expected, nitrogen is incorporated in the Li3PO4 layers by sequentially exposing the layers to a N2plasma. For the first time the electrical and electrochemical properties were investigated for a range of different material compositions and thicknesses in detail. Large varieties in ionic conducitivity (between 10-14 – 10-7 S/cm) and activation energy were measured by impedance spectroscopy for the different processing conditions. DC-polarization measurements showed good electronic insulating properties for films below 50 nm (ρ ~ 1015 Ω.cm). Such thin films lead to a minimal ionic resistance (on the order of 1.5 Ω.cm2for the thinnest layer), which can facilitate extreme charging and discharging kinetics. Finally, the deposition of these layers was tested in high aspect ratio pillar arrays and in Li-ion half cells comprising TiO2/LiPO(N) and LiMn2O4/LiPO(N). The layers fabricated here can serve as electrolyte in the all solid state 3D batteries. However, they also hold potential as buffer layer to stabilize electrodes in a classical wet battery. Vereecken et al., ECS transactions, 58, 111-118, 2013Put et al., ACS Applied materials & interfaces, 8, 7060–7069, 2016Kozen et al., Chemistry of materials, 27, 5324-5331, 2015Nisula et al., Chemistry of materials, 27, 6987–6993, 2015 Figure Caption: Figure 1: Complex plane plot of a Li3PO4 layer showing the characteristic semi-circle associated with Li-ion conductivity through the layer. The inset shows the conformal coating of a high aspect-ratio pillar array. Figure 1