Self-forming Na3P/Na2O interphase on a novel biphasic Na3Zr2Si2PO12/Na3PO4 solid electrolyte for long-cycling solid-state Na-metal batteries

A novel biphasic Na₃Zr₂Si₂PO₁₂/Na₃PO₄ solid electrolyte is proposed to effectively address critical anode interface challenges for solid-state Na-metal batteries (SSSMBs). The Na₃PO₄ phase, at an optimal composition of ∼20 mol%, transforms the interface chemistry throughout the NZSP electrolyte, which results in dense electrolytes with high Young's modulus, rapid ion transport (6.2 × 10^-4 S cm^-1) at low activation barrier (0.19 eV), negligible electronic conductivity, and excellent sodiophilicity. The AIMD/DFT calculations and XPS analysis reveal a self-formed, (electro)chemically stable mixed Na⁺/electron-conducting interphase, comprising Na₃P and Na₂O, at the Na anode interface. The interphase not only homogenizes the Na⁺ flux distribution and accelerates the interfacial charge transport, but prevents continuous interfacial reactions, thereby stabilizing the anode interface against dendrite formation. Benefiting from the low-impedance, dendrite-free anode interface, Na symmetric cells demonstrate a low interface resistance of 12.7 Ω cm² and exceptional cyclability of 3000 hours. Additionally, full cells with Na₃V₂(PO₄)₃ cathodes achieve 93% capacity retention after 550 cycles at 0.5 C. This research comprehensively elucidates and leverages the critical advantages of Na₃PO₄ in enhancing the bulk and interface properties of Na₃Zr₂Si₂PO₁₂ solid electrolytes. The design strategy of biphasic solid electrolytes presented here offers new insights into the developing high-performance solid electrolytes for advanced SSSMBs.