Understanding Coupled Electro-Chemo-Mechanics during I n Situ Li Metal Anode Formation in Anode-Free Solid-State Batteries

Solid-state batteries (SSBs) are potentially disruptive for a range of applications owing to their promise of high energy density, improved safety, and long cycle life. Their ability to enable Li metal anodes is a major advantage for energy density, but Li presents challenges for manufacturing due to its reactivity and the difficulty of fabricating thin Li films and high-quality Li/Electrolyte interfaces. Recently, in situ anode formation has shown significant promise for overcoming these challenges. In this approach, the cells are assembled without an anode (“Anode-free”), and Li metal is plated out from the cathode after fabrication. This reduces the need for inert atmospheres and reduces cell complexity, potentially lowering cost. As Li is plated out for the first time, mechanical stresses evolve at the Li/Electrolyte interface due to the volumetric changes in the electrodes. These stresses and the coupling between mechanics and electrochemistry play an important role in the resulting uniformity and quality of the in situ formed Li electrode. This work leverages 3D operandooptical video microscopy to observe morphology changes during in situ anode formation on one of the most promising solid electrolytes, Li7La3Zr2O12 (LLZO). These morphology changes are linked to the electrochemical signatures and the dynamic evolution of mechanical stresses at the Li/LLZO interface. A mechanistic framework is built to understand these factors, which is then used to provide guidance on what parameters control uniformity, and how systems can be designed to improve the resulting electrode properties. The role of stack pressure and the importance of stack pressure uniformity is highlighted. The impact of interfacial toughness, current collector properties, and cell geometry are discussed. Based on this understanding, the areal Li coverage is improved by more than 50%, providing insight for future works to enable in situ anode formation in a range of material systems and cell architectures.