Synthesis of Li Ion Conducting NLZSP(Na_3-xLi_xZr₂Si₂PO₁₂) Solid Electrolyte Through Novel Electroreforming Method and Its Application

Among lots of candidates of solid electrolytes, materials of NASICON type have high ionic conductivity, are inexpensive, and are stable in the atmosphere, making them suitable for various applications. History of lithium ion conducting NASICON type material starts from 1980’s and currently LATP which has Ti⁴⁺ replaced with Al³⁺ is widely used for its high ionic conductivity. However, Ti based NASICON material is unstable in contact with the Li metal due to reduction of Ti⁴⁺ to Ti³⁺. This reduction produces by-products that hinder ion conduction and cause cracks in the electrolyte, limiting the use of NASICON materials in diverse configurations.

To overcome this reduction problem, many researchers have tried replacing Ti ions with Zr ions, which have a more stable oxidative state. However, simply replacing titanium ions with zirconium led to several issues. This work suggests a novel electrochemical method to create Zr-based Li-ion conducting NASICON(Na_3-xLi_xZr₂Si₂PO₁₂) material from Na-based NASICON(Na₃Zr₂Si₂PO₁₂) material. There are two sites where mobile ions occupy in the NASICON structure. The M1 site is where the space is shared with ZrO₆ octahedra, and the M2 site is the space between two tetrahedral sites. Since the bond length between the mobile ion and oxygen in the M1 site is constant, cations in the M1 site help maintain the structural skeleton. Cations in the M2 site can be easily extracted and inserted for electrochemical activity due to the diverse ion-oxygen bond lengths.

In this work, Na in Na-based NASICON material was selectively exchanged with Li, and some ions were partially substituted to ensure structural stability. The system built for ion substitution using an electrochemical method has been named the 'electroreforming' process. To verify the characteristics of the solid electrolyte produced by this process, it was first demonstrated that Na was exchanged with Li using an electrochemical method. Structural analysis confirmed the insertion of lithium ions into the NASICON structure, showing ion conductivity three orders of magnitude higher compared to samples synthesized using conventional solid-state methods. Secondly, it was verified that it works well as a lithium-conductive ceramic membrane after modification. In a Li metal symmetric cell, it operated for over 100 hours and exhibited lower overpotential compared to conventional electrolytes. Additionally, it was applied in a new system for recovering lithium from spent batteries, the 'electrochemical lithium recycling system,' where it functioned as a stable lithium-conductive membrane in aqueous solutions.

This study proposes a simple electrochemical method to modify structurally stable Na-based NASICON-type membranes to Li-based membranes. Additionally, it presents alternative application strategies for the modified solid electrolyte beyond battery systems.