<i>Operando</i> Acoustic Analysis of Formation Parameter Coupling in Lithium Metal Batteries

The formation conditions of anode-free lithium metal batteries establish long-term cyclability. In this work, spatiotemporally-resolved ultrasound transmission is used to study the coupling between temperature, stack pressure, and current density during formation on Li plating dynamics in multilayered, anode-free Li metal batteries. Additionally, formed batteries undergo accelerated-rate cycling with simultaneous ultrasound transmission to determine the impact of formation protocols on electrochemical and chemo-mechanical performance. These operando techniques are validated with ex-situ optical and scanning electron microscopy imaging. Electrochemical analysis and X-ray photoelectron spectroscopy provide further insight into the relationship between electrochemical performance, Li mechanics, and the solid-electrolyte interphase. Our results indicate that increased temperature, stack pressure, and C-rate produce more uniform morphology across the anode during formation. Furthermore, improvements gained during formation lead to more stable chemo-mechanical behavior during cycling, though cathode dynamics and electrolyte side reactions convolute electrochemical performance.