Densification and performance optimization of NaSICON solid electrolytes via a 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 Na_3.4Zr_1.9Zn_0.1Si_2.2P_0.8O₁₂ (NZZSP) studied here, 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 3 M 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 mA h g_NVP⁻¹ at C/2 and over 100 mA h g_NVP⁻¹ at C/10 after cycling at multiple rates. These results underscore the potential of the CSP as a sustainable, low-temperature alternative for fabricating high-performance solid-state electrolytes for application in all solid-state sodium batteries.