Fundamentals, impedance, and performance of solid-state Li-metal microbatteries

Abstract

The authors report novel results toward optimizing the electrochemical performance of high vacuum deposited lithium-based all solid-state thin film microbatteries. This study investigated hermetic encapsulation, interfacial lithium formation processes, and the role of Li-blocking and Li-nucleating layers for improved Li-metal plating on copper anodes. Photoresist was found to be an effective temporary encapsulation material, where prior to cycling, well-encapsulated Li-metal full cells yielded a total resistance reduction of nearly two orders of magnitude (282 Ω cm2) and a total capacitance increase of roughly an order of magnitude (1.35 × 10−10 F/cm2) compared with nonencapsulated Li-metal full cells. To accelerate potential failure mechanisms, high stress applied currents were used during the electrochemical formation processes. Initial cycles caused high resistance voids to form at the lithium phosphorous oxy-nitride (LiPON)/copper interface of well-encapsulated half cells. Well-encapsulated full cells, in contrast, resulted in a very low resistance composite Li-Cu anode, with a void-free LiPON interface, two orders of magnitude lower resistance (0.43 Ω cm2) and three orders of magnitude higher capacitance (6.56 × 10−8 F/cm2) compared with the half cell. Cycling performance was investigated using both Li-blocking nickel-copper and Li-nucleating gold-copper metal bilayer anodes in 100-μm diameter half cells. Nickel-copper anodes facilitated higher discharge capacity (>9 μAh/cm2) at high charge rates (>12.7 mA/cm2) due to uniform Li-metal plating on blocking electrodes. Low charge rates (<0.7 mA/cm2) displayed low discharge capacity and immediate corrosion of the cell. Gold-copper anodes displayed the opposite effect, showing sustainable cycling, minimal cell corrosion, and a discharge capacity of >6 μAh/cm2 at lower charge rates (∼0.025 mA/cm2). The work expands on fundamentals in understanding the role of the metallic anode encapsulation, interface formation, and charge storage mechanisms with respect to sustainable cell impedance for applications such as solid-state lithium metal microbatteries and microelectrochemical resistance-modulated memory devices.