Understanding Alloy Interlayer Evolution in Anode-Free Solid-State Batteries

To achieve high energy density and high specific energy in solid state batteries, “anode-free” battery configurations have garnered a great deal of interest. With no active material at the anode current collector, anode-free systems substantially increase volumetric energy density compared to standard Li-ion batteries and lithium metal batteries with excess lithium. The anode-free configuration additionally simplifies manufacturing by removing the need to process lithium metal. To advance this technology, however, it is critical to understand Li growth and stripping on current collectors while also investigating methods to spatially control these processes. 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, but there is a lack of knowledge regarding the mechanisms by which they improve performance. Here, we investigate the structural and morphological evolution of alloy interlayers in SSBs using cryogenic and plasma focused ion beam (cryo-FIB, PFIB) methods correlated with electrochemical measurements. Improved Coulombic efficiency and cycling stability are observed when using silver and gold interlayers compared to bare copper. The origin of these performance improvements are investigated using cryo-FIB and PFIB to uncover morphology evolution at the solid-state interface. We observe non-uniform lithium growth on bare copper throughout cycling, while uniform lithium growth is observed in silver- and gold- modified interfaces. Interestingly, the silver and gold interlayers undergo different morphological evolution, which affects cycling behavior. Electrochemical impedance spectroscopy (EIS) is used to understand and investigate the influence of the alloy interlayers on interfacial impedance during cycling, which is correlated to morphology evolution. Over the first five cycles, bare copper electrodes exhibit a relatively large increase in impedance while the alloy-modified interfaces exhibit only minor increases in impedance throughout cycling. Furthermore, <i>operando</i> EIS measurements show distinct signatures due to contact loss for the bare copper. This work provides new understanding of the mechanisms governing the behavior of alloy interlayers in anode-free SSBs, which is important for engineering high-performance and high-energy devices.<br/><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).