Dynamic Electrode-Electrolyte Intermixing in Solid-State Sodium Nano-Batteries

Nanostructured solid-state batteries (SSBs) are poisedto meetthe demands of next-generation energy storage technologies by realizingperformance competitive to their liquid-based counterparts while simultaneouslyoffering improved safety and expanded form factors. Atomic layer deposition(ALD) is among the tools essential to fabricate nanostructured deviceswith challenging aspect ratios. Here, we report the fabrication andelectrochemical testing of the first nanoscale sodium all-solid-statebattery (SSB) using ALD to deposit both the V2O5 cathode and NaPON solid electrolyte followed by evaporation of athin-film Na metal anode. NaPON exhibits remarkable stability againstevaporated Na metal, showing no electrolyte breakdown or significantinterphase formation in the voltage range of 0.05-6.0 V vsNa/Na+. Electrochemical analysis of the SSB suggests intermixingof the NaPON/V2O5 layers during fabrication,which we investigate in three ways: in situ spectroscopicellipsometry, time-resolved X-ray photoelectron spectroscopy (XPS)depth profiling, and cross-sectional cryo-scanning transmission electronmicroscopy (cryo-STEM) coupled with electron energy loss spectroscopy(EELS). We characterize the interfacial reaction during the ALD NaPONdeposition on V2O5 to be twofold: (1) reductionof V2O5 to VO2 and (2) Na+ insertion into VO2 to form Na (x) VO2. Despite the intermixing of NaPON-V2O5, we demonstrate that NaPON-coated V2O5 electrodes display enhanced electrochemical cycling stabilityin liquid-electrolyte coin cells through the formation of a stableelectrolyte interphase. In all-SSBs, the Na metal evaporation processis found to intensify the intermixing reaction, resulting in the irreversibleformation of mixed interphases between discrete battery layers. Despitethis graded composition, the SSB can operate for over 100 charge-dischargecycles at room temperature and represents the first demonstrationof a functional thin-film solid-state sodium-ion battery.