Uncovering the Effects of Alloy Interfacial Layers in Anode Free Solid-State Batteries
Authors: Sandoval Stephanie Elizabeth , Nelson Douglas Lars , Lewis John A , McDowell Matthew 
Published: February 2024

To achieve high energy density and high specific energy in solid state batteries (SSBs), “anode-free” battery configurations have garnered a great deal of interest. Anode-free systems substantially increase volumetric energy density compared to standard Li-ion batteries and lithium metal batteries by removing the graphite anode and excess lithium, respectively. Additionally, manufacturing is simplified in anode-free configurations by removing the need to process lithium metal. However, to advance this technology, it is critical to understand how lithium deposits and strips on current collectors throughout cycling while also investigating methods to spatially control these processes. Previous work which probed the effects of alloy interlayers in liquid electrolyte systems using operando optical microscopy has found that silver thin films enable higher Coulombic efficiencies (CEs) than bare current collectors and cause different deposition and stripping dynamics1. Silver layers have also shown beneficial effects in enabling long-term cycling of SSBs2, 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 CEs 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, operando EIS measurements show distinct signatures due to contact loss for the bare copper while contact is retained for alloy-modified interfaces. 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.

Sandoval, S. E. et al. Understanding the Effects of Alloy Films on the Electrochemical Behavior of Lithium Metal Anodes with Operando Optical Microscopy. J. Electrochem. Soc.
168, 100517 (2021).

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