Sputtered Zero-Excess Electrodes with Metallic Seed Layers for Solid-State Sodium Batteries

Zero-excess sodium metal solid-state batteries offer improved safety, lower cost, higher energy density, and reduced resource dependency compared to today's lithium-ion technology. This study demonstrates the fabrication of zero-excess electrodes with unprecedented stability during plating/stripping cycles. The fabrication process involves the sputter deposition of 20 nm metallic seed layers – zinc, silver, indium, or tin – onto NASICON (Na_3.4Zr₂Si_2.4P_0.6O₁₂) ceramic separators, followed by the sputter deposition of a 30 μm copper current collector. The favorable influence of these seed layers on the in-situ formation of the sodium|NASICON interface is examined through nucleation and cycling experiments, with a sodium metal reservoir serving as the non-limiting counter electrode. Due to alloy formation the seed layers – particularly tin – stabilize sodium nucleation and cycling substantially and reduce dendrite formation compared to reference cells with bare copper current collectors. Sodium loss during cycling is primarily attributed to local cracking of the current collector and its partial delamination from the NASICON. Compared to polished NASICON, a roughened surface reduces the resistance e. g. of the counter electrode 200-fold to approx. 1 Ωcm² at 3 MPa and suppresses delamination further.