Recently Solid-State Battery has received increased attention as the next generation energy storage device due to its high energy density, improved safety, and thermal stability. Among various types of solid electrolytes (SE), lithium (Li)-stuffed garnet-type Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZT) delivered high ionic conductivity (~10^-4 S/cm) and wide electrochemical stability window. Despite such promising properties, there are few technical challenges that prevent the implementation of LLZT in practical battery cells. First, LLZT suffers from a high interfacial resistance due to its poor wettability in contact with Li metal anode. Second, LLZT suffers from Li-dendrite penetration through grain boundaries during cycling. The grain boundaries have high electrical and low ionic conductivities which attracts Li-ions and electrons and nucleates metallic Li. Additionally, poor wetting between Li-anode and LLZT causes their limited contact at the Li/SE interface. Electrical current will be concentrated on those limited interfacial areas and accelerate the Li nucleation and subsequent dendritic growth.

To address these issues, various strategies have been proposed including elemental doping, in-situ solid-electrolyte interface (SEI) formation, and interlayer engineering. Among these strategies, amorphous glass layer between Li-anode and LLZT has advantages of simple processing and manufacturing friendliness, which involves heating to melt a glass followed by a fast cooling. Therefore, moderate melting temperature (e.g., T < 800 ^oC) is an important selection criteria for suitable glass material candidate. In addition, the coating layer should have reasonably good ionic conductivity .

In this study, we selected and investigated Li₃BO₃ (LBO) as the glass interlayer based on its low melting temperature (~700 ^oC), good stability between Li metal and LLZT, and moderate ionic conductivity (~ 10^-6 S/cm). A systematic design of experiments was performed to understanding the synthesis parameter – microstructure – performance relationship. First, we examined the effect of temperature programs (e.g., heating/cooling rate, temperature, time) and slurry composition on the microstructure of the LBO glass layer coated onto LLZT pellet. Scanning electron microscopy (SEM) revealed that homogeneous LBO layer without cracks or pinhole could be obtained by optimizing the parameters.

Secondly, we investigated the effect of LBO glass interlayer on the electrochemical properties of the solid-state battery cells. Introducing LBO interlayer to a Li/SE/Li symmetrical cell (i) increased cell interfacial conductivity from 1.32*10^-4 to 1.06*10^-3 S/cm as evidenced by electrochemical impedance spectroscopy (EIS) and (ii) increased critical current densities (CCD) from 0.1 mA/cm² (baseline) to 0.4 mAh/cm². Finally, we performed extended plating/stripping cycling test from the symmetric cells at a specific current of 0.1 mA/cm². Although the bare LLZT cell failed after 10 cycles, the cell with LBO interlayer operated without short-circuit for 93 cycles. The electrochemical characterization test clearly demonstrated that the LBO layer stabilize the Li/SE interface by improving a tolerance against the Li-dendrite penetration. Our promising results will offer a useful guideline for the future researchers on the design and optimization of the solid-state battery interfaces.