Densification and Performance Optimization of NaSICON Solid Electrolytes via Low-Temperature Cold Sintering Process with Sodium Ionic Salt Doping

Abstract

Sodium-ion batteries (SIBs) have emerged as a sustainable alternative to lithium-ion systems, offering cost-effective and environmentally friendly energy storage solutions. Solid-state electrolytes (SSEs), particularly NaSICON-type materials such as the studied here Na3.4Zr1.9Zn0.1Si2.2P0.8O12 (NZZSP), are critical for enhancing the safety and stability of SIBs. However, conventional high-temperature sintering methods for fabricating these electrolytes are energy-intensive and environmentally impactful. In this work, we employed the Cold Sintering Process (CSP) to densify NZZSP at a low temperature of 150 °C under 720 MPa with the aid of a transient liquid phase (TLP), achieving a sustainable electrolyte production with competitive performance. The effects of milling time and two different TLP media were evaluated, with 3M acetic acid solution (HAc) being more effective than 25 mM sodium hydroxide solution (NaOH) in preserving particle integrity and yielding higher ionic conductivity (0.50 mS cm-1). Doping with NaPF6 and NaTFSI further enhanced performance, with 20% NaPF6 -doped samples achieving the highest densification (94.3%) and conductivity (0.80 mS cm-1). Optimized 2-hour-milled, 20% NaPF6 electrolytes demonstrated suitable cycling stability in symmetric cells (over 500 hours) and specific capacity in half cells, with Na metal and Na3V2(PO4)3 (NVP) as electrodes, of about 85 mAh g-1NVP at C/2 and over 100 mAh g-1NVP at C/10 after cycling at multiple rates. These results underscore the potential of CSP as a sustainable, low-temperature alternative for fabricating high-performance solid-state electrolytes for application in all solid-state sodium batteries.