Interfacial Nucleation Mechanisms in Reservoir‐Free Na All Solid‐State Batteries Using Ultrathin Interlayers and Low Stack Pressure

Controlling sodium nucleation at the current collector|solid electrolyte interface remains a key challenge for realizing “reservoir‐free” sodium all‐solid‐state batteries (RF‐ASSBs), particularly under low stack pressure. While metals have been explored as sodium hosts or current collectors, the use of ultrathin metallic interlayers capable of regulating sodium nucleation with minimal sodium inventory penalty remains largely unexplored. Here, we present a comparative interfacial study of 50 nm sputtered Sn and In interlayers deposited on NaSICON electrolytes. By combining electrochemical characterization, microelectrode experiments, and operando and ex situ time‐of‐flight secondary ion mass spectrometry, we directly probe early‐stage sodium nucleation, interfacial chemistry, and reversibility. Sn undergoes electrochemically driven Na–Sn alloy formation independent of current rate and with high sodium diffusivity, enabling homogeneous nucleation and stable plating/stripping through a persistent alloy interphase while incurring only minimal irreversible sodium loss (∼0.031 mAh cm −2 ). In contrast, In fails to sustain stable interfacial sodium transport, leading to subfilm sodium deposition, mechanical disruption of the interlayer, sodium trapping, and rapid interfacial failure. These results demonstrate that, for ultrathin interlayers operated under low stack pressure, alloy‐assisted fast sodium transport within a stabilized interlayer, rather than generic sodiophilicity, governs interfacial stability, providing mechanistic design guidelines for RF‐ASSBs.