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.

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A solid-state sodium metal battery has cut a striking figure in next-generation large-scale energy storage technology on account of high safety, high energy density, and low cost. Nevertheless, the large interfacial resistance and sodium dendrite growth originating from the poor interface contact seriously hinder its practical application. Herein, a modified ultrasound welding was proposed to atomically bond Na anodes and Au-metalized Na 3 Zr 2 Si 2 PO 12 electrolytes associated with the in situ formation of Na-Au alloy interlayers. Thereupon, intimate Na 3 Zr 2 Si 2 PO 12 -Au/Na interfaces with a low interfacial resistance (~23 Ω cm 2 ) and a strong dendrite inhibition ability were constructed. The optimized Na symmetric battery can cycle steadily for more than 900 h at 0.3 mA cm −2 under a low overpotential (<50 mV) of Na electroplating/ stripping and deliver a high critical current density of 0.8 mA cm −2 at room temperature. By incorporating the above interface into the solid-state Na metal battery, taking three-dimensional Na 3 V 2 (PO 4 ) 3 as the cathode, the full battery offers a high energy density of 291 Wh kg −1 at a high power density of 1860 W kg −1 . A pouch-type solid-state sodium metal full battery based on a ceramic electrolyte was assembled for the first time, and it lit a 3 V LED lamp. Such a strategy of the ultrasound welding metalized solid-state electrolyte/Na interface by engineering the Na-Au interlayer would pave a new pathway to engineer a low-resistance and highly stable interface for high-energy/density solid-state sodium metal batteries.

NASICON-type Li[Formula: see text]Al[Formula: see text]Ti[Formula: see text](PO4)3 (LATP) solid electrolyte features low cost of raw materials, high stability against O2/H2O, high ionic conductivity and high compatibility with cathode. Despite these merits, the direct contact of Li with LATP would trigger the reduction of Ti[Formula: see text] into Ti[Formula: see text], and thereupon decrease ion conductivity and enhance electron conductivity, thus limiting its application. In this work, the garnet Li[Formula: see text]La3Zr[Formula: see text]Ta[Formula: see text]O[Formula: see text] (LLZTO) particles were chosen to disperse in NASICON-type Li[Formula: see text]Al[Formula: see text]Ti[Formula: see text](PO4)3 matrix in order to strengthen stability against Li metal. During sintering, the matrix/particle reaction occurs, where LLZTO reacts with LATP, inducing the formation of LaPO4 particles. By optimization of LLZTO content, an enhanced ionic conductivity of 1.03 × 10[Formula: see text]S cm[Formula: see text] can be harvested. Moreover, introducing LLZTO particles into LATP matrix can slow down the degradation of the electrolyte to a certain extent and provide better interfacial stability of symmetrical battery than pure LATP solid electrolyte.