Feasibility study on high-energy-density almost-solid-state sodium batteries with thin ceramic Na _3.4 Zr ₂ Si _2.4 P _0.6 O ₁₂ separators

This study investigates the feasibility and limitations of almost-solid-state sodium batteries (Na-aSSBs) as novel energy storage solutions. The cell concept comprises a sodium metal anode, a tape-cast Na3.4Zr2Si2.4P0.6O12 solid electrolyte, and a Na3V2(PO4)3 cathode with liquid electrolyte. The impact of the Na3.4Zr2Si2.4P0.6O12 separator and sodium electrode on total cell resistance is evaluated in symmetric Na| Na3.4Zr2Si2.4P0.6O12|Na cells, demonstrating an ultra-low Ohmic resistance below 10 Ωcm2. The Na-aSSB achieved (85 ± 1) % of the theoretical cathode capacity and energy densities up to (239 ± 10) Wh/l at the cell level, among the highest reported for similar concepts. Cycling stability shows a Coulombic efficiency exceeding 99% over 70 cycles at a 2-h discharge rate. Five performance-limiting factors were identified: initial cathode resistance, degrading cell resistance during cycling, insufficient mechanical strength of the separator, dendrite formation, and non-optimized energy density. Suggested approaches to address these limitations highlight the technological potential of Na-aSSBs.