Impact of Composite Cathode Architecture Engineering on the Performance of All-Solid-State Sodium Batteries

The electrode/electrolyte interfacial contact, ionic percolation pathways, and charge transfer resistance within the cathode significantly impact the performance and lifespan of all-solid-state sodium batteries (AS³Bs). Addressing these issues requires optimization of the composite cathode architecture and the electrode/electrolyte interface in AS³Bs. One major challenge in developing composite cathodes with oxide solid electrolytes is selecting the appropriate thermal processing temperature to ensure intimate contact between the cathode active material and solid electrolyte while ensuring a sufficient ionic percolation pathway inside the composite cathode. In this study, we present an approach for fabricating a composite cathode by cofiring sodium vanadium fluorophosphate (Na₃V₂(PO₄)₂F₃, NVPF) and the sodium superionic conductor (Na₃Zr₂Si₂PO₁₂, NZSP) at 700 °C using an optimized weight ratio. This method ensures reduced interfacial resistance between NVPF and NZSP while establishing an efficient ionic percolation pathway. To further enhance ionic percolation within the composite cathode, residual voids are filled with a polymer electrolyte composed of PEO/NaClO₄. Benefits of the dense composite cathode structure, a stable NVPF/NZSP interface, negligible pores in the composite cathode, and a three-dimensional electronic and ionic percolation network facilitate the greater utilization of cathode active material with almost no capacity degradation upon long-term cycling. The full cell, with the optimized composite cathode, delivers an initial discharge capacity of 114 mA h g^-1 at 0.1 C, retaining 85% of its capacity after 500 cycles with a 99% Coulombic efficiency and excellent rate capability at 1 C.