Na-K Alloy Anode for High-Performance Solid-State Sodium Metal Batteries

Rechargeable solid-state Na metal batteries (SSNMB) can offer high operational safety and energy density. However, poor solid–solid contact between the electrodes and the electrolyte can dramatically increase interfacial resistance and Na dendrite formation, even at low current rates. Therefore, we developed a carbon-fiber-supported liquid Na–K alloy anode that ensures close anode–electrolyte contact, enabling superior cycle stability and rate capability. We then demonstrated the first cryogenic transmission electron microscopy (cryo-TEM) characterization of an SSNMB, capturing the evolution of solid–electrolyte interphase (SEI) and revealing both crystalline and amorphous phases, which could facilitate ion transport and prevent continuous side reactions. By enhancing contact between the Na–K alloy and solid-state electrolyte, these symmetric cells are capable of cycling for over 800 h without notable increased polarization and enable an unprecedented critical current density (CCD) at 40 mA cm^–2. Our liquid Na–K alloy approach offers a promising strategic avenue toward commercial SSNMBs.