A Dendrite-Resistant Sodium/Porous-Carbon Anode for Solid-State Batteries: Strategies and Challenges for Low-Pressure...

Sodium solid-state batteries (Na-SSBs) are attracting growing interest due to the abundance of Na over Li, but they still tend to fail under practical current densities and cycling capacities due to dendrites, and are therefore frequently evaluated under impractically high stack pressures. Here, a porous carbon interfacial layer is utilized in conjunction with Na-ß″-Al₂O₃ solid electrolytes (BASE) to enable Na-cycling at milder cell pressures. This sodium/porous carbon layer enables improved solid-state Na cycling in symmetric cells, up to a current density of 10 mA cm^-2 at 25 °C. 1 mAh cm^-2 capacity can be reliably cycled at 1 mA cm^-2 at an elevated temperature of 60 °C in symmetric cells. Interfacial evolution is investigated via cryogenic ion milling and cross-sectional imaging, revealing void formation, Na extraction from porous carbon, and/or delamination of the porous carbon matrix at the Na-metal/BASE interface, depending on temperature, pressure, current density, and areal capacity. Despite these interfacial changes, excellent dendrite resistance is maintained. A quasi-solid-state full cell using a Na-transition-metal-oxide cathode delivers an areal capacity of ∼2.7 mAh cm^-2 at 0.125 mA cm^-2. This work demonstrates an alternative pathway toward an Na-metal anode in Na-SSBs without excessive stack pressure.