Bi2O3-Assisted Sintering of Na3Zr2Si2PO12 Electrolyte for Solid-State Sodium Metal Batteries

Solid-state sodium metal batteries using non-flammable solid-state electrolytes are recognized as next-generation energy storage technology in view of their merits of high safety and low cost. However, the lower ion conductivity (below the application requirements of 10−3 S cm−1) and interface issues that exist in electrolytes/electrodes for most solid-state electrolytes hinder their practical application. In this paper, NASICON-type Na3Zr2Si2PO12 (NZSP) electrolytes with enhanced ion conductivity are synthesized by the Bi2O3-assisted sintering method. The influence of the Bi2O3 sintering agent content on the crystalline phase, microstructure, density and ion conductivity as well as the electrochemical performances applied in batteries for the obtained NZSP electrolytes are investigated in detail. With the presence of Bi2O3, the formed Na3Bi(PO4)2 impurity increased the Si/P ratio in the NASICON structure with higher Na+ occupancy, then enhanced the ionic conductivity to a level of 1.27 × 10−3 S cm−1. Unfortunately, the Bi2O3-assisted sintered NZSP shows a degradation in the cycling stability when applied to solid-state sodium batteries because of the decreased interfacial stability with Na anodes. The formation of a Bi-Na alloy during cycling might be conducive to Na dendrite growth in electrolytes, degrading the cycling performance. This work presents a facial method to improve the ion conductivity of NASICON-type electrolytes and gives insight into the interface issues of solid-state sodium metal batteries.