Electro-Chemo-Mechanics of “Anode-Free” Solid-State Batteries

Solid-state batteries have the potential to be a disruptive technology because of their ability to improve safety and increase energy density by incorporating Li metal anodes. However, all solid-state interfaces present unique challenges, including high interfacial impedances, accommodation of mechanical stresses due to solid-solid interfacial contact, and (electro)chemical instabilities that can evolve during dynamic cycling conditions. Furthermore, a significant challenge facing the scale-up of SSBs is to improve our understanding of processing science needed to enable manufacturing. An emerging strategy to address some of these limitations is the elimination of excess metallic Li at the negative electrode during cell fabrication, which overcomes many of the challenges in working with bulk Li foils, while also minimizing volumetric and gravimetric energy density.

Despite the potential advantages of “anode-free” cell configurations, the removal of excess Li metal during the initial manufacturing process also presents unique challenges and phenomena. In particular, the nature of the direct interface between a metal current collector and solid electrolyte depends significantly on both the material systems and fabrication processes employed. Furthermore, owing to the less compliant nature of this interface compared to a soft Li metal foil, the physical and electrical contact along the interface is often discontinuous, resulting in “hot spots” for nucleation, growth, and void formation during stripping. While the application of an external stack pressure is often used to compensate for some of these challenges, the ability to maintain a uniform and continuous stack pressure is also a significant challenge in anode-free configurations.

In this talk, I will present recent work from our group in understanding and engineering interfaces in “anode-free” solid-state battery configurations. First, I will describe our efforts to understand the dynamic evolution of carbon interlayers between the solid electrolyte and current collector, which have a profound impact on the location, onset, and morphological evolution of Li plating. A series of electrochemical, mechanical, and operando microscopy analyses will be presented, revealing the unique dynamics of these carbonaceous interlayer systems. Next, I will describe the critical role of stack pressure in not only maintaining contact along the anode-free interface, but also in determining the nucleation and growth behavior under different geometric boundary conditions. Finally, I will provide a perspective on the manufacturing challenges and opportunities to scale-up this technology into technologically relevant cell formats and geometries.