Uncovering the Effects of Alloy Interfacial Layers in Anode Free Solid-State Batteries Through Cryogenic Focused Ion Beam Characterization

In both liquid and solid-state electrolyte (SSE) battery systems, there is growing interest in developing “anode-free” architectures. Anode-free systems feature no active material at the anode current collector and therefore substantially increase the volumetric energy density compared to standard Li-ion batteries. This configuration additionally simplifies manufacturing by removing the need to process lithium metal. During charging of an anode-free cell, Li⁺ ions are removed from the lithiated cathode and electrodeposited as lithium metal on the anode current collector. Upon discharge, lithium is stripped and intercalated back into the cathode. Since there is no excess lithium at the anode, it is essential that deposited lithium is entirely removed to achieve high Coulombic efficiencies (CE). Thus, it is critically important to understand the nucleation and growth behaviors of lithium in anode-free configurations while exploring methods to spatially control lithium growth. Previous work which probed the effects of alloy interlayers in liquid electrolyte systems using <i>operando</i> optical microscopy has found that silver thin films enable higher CE than bare current collectors and cause different deposition and stripping dynamics¹. Silver layers have also shown beneficial effects in enabling long-term cycling of solid-state batteries (SSBs)². Here, we investigate the structural and morphological evolution of alloy interlayers in SSBs using cryogenic focused ion beam (cryo-FIB) methods correlated with electrochemical measurements. Improved CEs are observed using silver and gold thin films compared to bare copper. Performance improvements are investigated using cryo-FIB to probe the Cu| electrodeposited Li| SSE interface. Samples are cooled to -145 °C to stabilize the electrodeposited lithium and minimize interactions between lithium and the gallium ion beam. Once cooled, ~25 mm x ~25 mm x ~25 mm trenches are milled to reveal the solid-solid interface of interest. We observe non-uniform growth on bare copper while uniform lithium growth is observed in silver- and gold- modified interfaces. Interestingly, we observe distinct morphological evolution comparing both alloy interlayers, which also affects cycling behavior. Electrochemical impedance spectroscopy during deposition and stripping is further used to understand and investigate the influence of the alloy interlayers and correlate to morphology evolution. This work provides new understanding of interfacial modification at solid-solid interfaces in SSBs, which will be important for engineering anode-free SSBs.<br/><br/>1. Sandoval, S. E. <i>et al.</i> Understanding the Effects of Alloy Films on the Electrochemical Behavior of Lithium Metal Anodes with Operando Optical Microscopy. <i>J. Electrochem. Soc.</i> <b>168</b>, 100517 (2021).<br/>2. Lee, Y. G. <i>et al.</i> High-Energy Long-Cycling All-Solid-State Lithium Metal Batteries Enabled by Silver–Carbon Composite Anodes. <i>Nat. Energy</i> <b>5</b>, 299–308 (2020).