DEVELOPMENT OF VAPOR-PHASE DEPOSITED THREE DIMENSIONAL ALL-SOLID-STATE BATTERIES

Abstract

Thin film solid state batteries (SSBs) are an attractive energy storage technology due to their intrinsic safety, stability, and tailorable form factor. However, as thin film SSBs are typically fabricated only on planar substrates by line-of-sight deposition techniques (e.g. RF sputtering or evaporation), their areal energy storage capacity (< 1 mWh/cm2) and application space is highly limited. Moving to three dimensional architectures provides fundamentally new opportunities in power/energy areal density scaling, but requires a new fabrication process. In this thesis, we describe the development of the first solid state battery chemistry which is grown entirely by atomic layer deposition (ALD), a conformal, vapor-phase deposition technique.

We first show the importance of full self-alignment of the active battery layers by measuring and modelling the effects of nonuniform architectures (i.e. does not reduce to a one-dimensional system) on the internal reaction current distribution. By fabricating electrochemical test structures for which generated electrochemical gradients are parallel to the surface, we directly quantify the insertion of lithium into a model cathode material (V2O5) using spatially-resolved x-ray photoelectron spectroscopy (XPS). Using this new technique, we show that poorly electrically contacted high aspect ratio structures show highly nonuniform reaction current distributions, which we describe using an analytical mathematical model incorporating nonlinear Tafel kinetics. A finite-element model incorporating the effects of Li-doping on the local electrical conductivity of V2O5, which was found to be important in describing the observed distributions, is also described.

Next, we describe the development of a novel solid state electrolyte, lithium polyphosphazene (LPZ), grown by ALD. We explore the thermal ALD reaction between lithium tert-butoxide and diethyl phosphoramidate, which exhibits self-limiting half-reactions and a growth rate of 0.09 nm/cycle at 300C. The resulting films are primarily characterized by in-situ XPS, AFM, cyclic voltammetry, and impedance spectroscopy. The ALD reaction forms the amorphous product Li2PO2N along with residual hydrocarbon contamination, which is determined to be a promising solid electrolyte with an ionic conductivity of 6.5 × 10-7 S/cm at 35C and wide electrochemical stability window of 0-5.3 V vs. Li/Li+ . The ALD LPZ is integrated into a variety of solid state batteries to test its compatibility with common electrode materials, including LiCoO2 and LiV2O5, as well as flexible substrates. We demonstrate solid state batteries with extraordinarily thin solid state electrolytes, mitigating the moderate ionic conductivity (< 40 nm).

Finally, we describe the successful integration of the ALD LPZ into the first all-ALD solid state battery stack, which is conformally deposited onto 3D micromachined silicon substrates and is fabricated entirely at or below 250C. The battery includes ALD current collectors (Ru and TiN), an electrochemically formed LiV2O5 cathode, and a novel ALD tin nitride conversion-type anode. The full cell exhibits a reversible capacity of ~35 μAh cm-2 μmLVO -1 with an average discharge voltage of ~2V. We also describe a novel fabrication process for forming all-ALD battery cells, which is challenging due to ALD’s incompatibility with conventional lithography. By growing the batteries into 3D arrays of varying aspect ratios, we demonstrate upscaling the areal capacity of the battery by approximately one order of magnitude while simultaneously improving the rate performance and round-trip efficiency.