Operando Imaging of Whisker Growth in NASICON Solid-State Batteries.

A central obstacle to elucidating all-solid-state sodium battery (ASSSB) failure is operando, nanoscale visualization of sodium whisker growth within solid electrolytes under realistic operating conditions. To this end, we establish an integrated framework coupling operando electrochemical electron microscopy (EM) with nanofabrication, enabling in situ tracking of structural evolution and cryogenic EM interrogation of failure hotspots. With this framework, we monitor structural dynamics and elucidate the whisker growth mechanism in a symmetric sodium cell containing a NASICON Mg-doped Na3Zr2-xMgxSi2PO12 electrolyte. During extended plating-stripping cycling, voids that develop in the vicinity of weak crystalline NaMgPO4 secondary phases of the electrolyte act as nucleation sites for whisker growth, which grow into pillar-like morphologies with cubic/hexagonal features. The explicit identification of these void-secondary phase composite nucleation sites establishes a direct mechanistic link between the chemo-physical properties of the electrolyte and whisker growth, offering fundamental design principles for dendrite-free ASSSBs.