Sc/Zn Co-Doped NASICON Electrolyte with High Ionic Conductivity for Stable Solid-State Sodium Batteries
Abstract
Sodium-ion batteries (SIBs) have emerged as a sustainable alternative to lithium-ion batteries (LIBs), benefiting from the natural abundance and low cost of sodium resources. However, the inherent safety risks of conventional organic liquid electrolytes necessitate the development of robust solid-state electrolytes (SSEs). NASICON-structure Na3Zr2Si2PO12 (NZSP) represents a promising SSE candidate due to its superior ionic conductivity, wide electrochemical stability window, and excellent chemical compatibility. Nevertheless, polycrystalline NZSP suffers from high grain boundary resistance and interfacial instability, limiting its practical application. Herein, a Sc³⁺/Zn²⁺ co-doped strategy was developed to synthesize Na3.2+2xZr1.8-xSc0.2ZnxSi2PO12 (NZSSP-Znₓ, x=0-0.20), the incorporation of Sc³⁺ stabilizes the highly conductive rhombohedral phase and expands Na⁺ transport channels, while Zn²⁺ doping lowers the sintering temperature of monoclinic-to-rhombohedral transition in NZSSP-Zn₀.₁₀. The synergistic co-doped approach employing NZSSP-Zn₀.₁₀ demonstrates remarkable enhancement in both bulk and grain boundary conductivity. During thermal processing, the precursor materials undergo in-situ transformation to generate a Na₃PO₄ secondary phase, exhibiting a high ionic conductivity of 2.41×10⁻3 S cm⁻1, and a low activation energy of 0.20 eV. The distribution of relaxation times (DRT) method was applied to analyze the time-resolved electrochemical impedance spectroscopy (EIS), elucidating the correlation between dynamic interfacial changes and electrochemical processes at the Na/NZSSP-Zn₀.₁₀ SSE interface. The Na|NZSSP-Zn₀.₁₀|Na symmetric cell achieves a high critical current density (CCD) of 0.9 mA cm⁻2 and stable Na plating/stripping for 550 h at 0.1 mA cm⁻2. The assembled Na|NZSSP-Zn0.10|NVP/C demonstrates excellent cycling stability (0.029% decay per cycle over 400 cycles at 0.5 C). This work provides a feasible co-doped approach to optimize NASICON electrolytes for high-performance solid-state sodium batteries.