Paper TF+EM-MoM6
Three-Dimensional Solid State Batteries Grown Via Atomic Layer Deposition

Monday, October 30, 2017, 10:00 am, Room 20

Thin film solid state batteries have multiple attractive properties, including low interfacial impedance, exceptional cycling stability, and intrinsic safety, but have not seen significant implementation because their overall capacity is very low (on the order of 0.1 mAh/cm²). This is a consequence of their growth by physical vapor deposition, which limits them to planar substrates. In this configuration, increasing areal capacity necessarily sacrifices rate performance due to slower ionic diffusion in thicker electrodes. Realizing practical energy or power densities in thin film SSBs will involve the development of entirely new fabrication processes in order to allow the integration of thin film SSBs with 3D substrates, such as micromachined silicon or conductive fabrics. In particular, 3D designs allow the decoupling of capacity and rate performance for area-normalized metrics.

We describe the development and synthesis of a solid state lithium-ion battery film stack grown entirely by atomic layer deposition (ALD), which allows us to successfully integrate solid state energy storage with 3D structures. The entire device is fabricated at temperatures at or below 250C, allowing for compatibility with a broad variety of substrates and processing environments. We utilize a simple electrochemical process for inserting free lithium ions into a crystalline cathode, avoiding the need to integrate lithiation into the ALD growth process itself. The solid electrolyte is a lithium polyphosphazene compound (a variant of LiPON) grown by a 2-precursor thermal process (Pearse et al. Chemistry of Materials 2017). The battery itself is comprised of a LiV₂O₅ – Sn_xN_y couple, providing a reversible capacity of approximately 35 μAh/cm₂ μm_LVO with an average discharge voltage of 2V. By growing these batteries into micromachined silicon structures with aspect ratios of up to 10, we are able to increase areal capacity by nearly one order of magnitude while simultaneously improving capacity retention at high rates- a type of scaling not possible for planar devices. We also discuss challenges associated with interface chemistry in the ALD growth environment and with patterning highly conformal films, and compare device performance to finite element electrochemical simulation.