Solid Electrolyte Densification for High Performance All-Solid-State Sodium Batteries

Abstract

Advances in lithium-ion battery (LIB) technology have taken center stage over the past decade due to their high energy densities compared to other battery chemistries. However, resource limitations are expected to increase production costs, prompting interest in alternative technologies. Sodium-ion batteries (SIBs), which share similar chemistry with LIBs but rely on more abundant elements, have emerged as promising next-generation energy storage solutions. In this context, solid electrolyte materials are crucial for enabling safe and efficient sodium batteries by mitigating the risks associated with flammable liquid electrolytes. Sodium-Super-Ionic-Conductor (NASICON) structures like Na₃Zr₂Si₂PO₁₂ (NZSP) have attracted considerable attention for their excellent ionic conductivity and thermal stability. However, their practical use is often hindered by inherent porosity and low density, which degrade ionic conductivity and electrochemical stability, ultimately limiting battery performance. In this study, we significantly enhance the performance of NZSP solid electrolyte materials tailored for All-Solid-State batteries (ASSBs) through a successful densification process. By introducing a densification agent that interacts effectively with the sodium-ion conducting matrix, we achieved notable improvements in both material compactness and ionic transport properties. The densified sample exhibited an impressive room-temperature ionic conductivity of 8.4 × 10⁻⁴ S/cm and attained ~98% of the theoretical density, resulting in a robust and mechanically stable electrolyte structure. This process also reduced porosity and grain boundary resistance, enabling faster and more efficient sodium-ion conduction. Comprehensive electrochemical evaluations were conducted to assess the performance of the optimized solid electrolyte. The densified sample exhibited a high critical current density (CCD) of 6 mA cm⁻², which is three times greater than that of the pristine counterpart. This high CCD signifies the ability of the electrolyte to support high rates of sodium-ion transport without significant polarization, a crucial parameter for high-power applications. To further validate the effectiveness of our densification approach, full cells were assembled using the densified NZSP electrolyte paired with a sodium vanadium phosphate (NVP) cathode. These cells exhibited a marked increase in capacity, reflecting the improved ionic conductivity and reduced interfacial resistance achieved through densification. The results demonstrate the practical viability of the optimized electrolyte in real battery systems. Such advancements are crucial for the commercialization of high-energy-density sodium batteries, paving the way for safer and more sustainable energy storage technologies. Figure 1