TiO2 as Second Phase in Na3Zr2Si2PO12 to Suppress Dendrite Growth in Sodium Metal Solid-State Batteries

Solid-state sodium–metal batteries will not achieve reasonable power density without electrolytes that solve the dendrite (filamentation) problem. Metal-filament formation during plating at ceramic/metal interfaces can cause electrical failure by internal short-circuit or mechanical failure by electrolyte fracture. Herein, an Na 3 Zr 2 Si 2 PO 12 (NZSP) sodium-ion-conducting NASICON electrolyte in which TiO 2 is incorporated as an additive is presented, leading to a two-phase composite NZSP(TiO 2 ) with improved density, Young's modulus, hardness, grain structure, and bulk permittivity. These features of NZSP(TiO 2 ) suppress dendrite growth along grain boundaries, microcracks, and micropores. As well as demonstrating ultralow ceramic/Na kinetic resistance with electrochemical measurements, X-ray photoelectron spectroscopy is performed to probe interfacial reaction mechanisms. The TiO 2 phase forms within grain boundaries and along NZSP surfaces. This modifies the two-phase material's microstructure and improves its electrochemical performance, while also increasing the critical current density for dendrite formation. Design guidelines are discussed to mitigate microscopic defects and dendrites in two-phase ceramic electrolytes.