Effect of the Li₂O–B₂O₃–Li₂SO₄ Amorphous Boundary Layer on the Ionic Conductivity and Humidity Stability of the LiTa₂PO₈ Solid Electrolyte

Recently, a Li-ion solid electrolyte material LiTa₂PO₈ (LTPO) which exhibits a high bulk ionic conductivity of 1.6 × 10^–3 S/cm and a total ionic conductivity of 2.5 × 10^–4 S/cm was developed. In a previous study, we sintered the LTPO pellet, which has a high relative density of 82% and a total ionic conductivity of 1.05 × 10^–5 S/cm at room temperature via a cold sintering process (CSP). In this study, to achieve the ionic conductivity comparable to LTPO ceramic electrolytes obtained via high-temperature sintering, a Li₂O–B₂O₃–Li₂SO₄ amorphous layer was formed at the interface between LTPO particles via the CSP, and the microstructure and electrochemical properties of LTPO with the Li₂O–B₂O₃–Li₂SO₄ amorphous layer were investigated. Moreover, humidity acceleration tests were conducted to confirm the chemical stability of the pellet under ambient humidity conditions. It was found that pellets of LTPO prepared via the CSP exhibited a relative density of 85–87%, which is comparable to the density of high-temperature sintered pellets, and high adhesion between LTPO particles was observed due to the Li₂O–B₂O₃–Li₂SO₄ amorphous layer forming a particle interface. LTPO pellets with the Li₂O–B₂O₃–Li₂SO₄ amorphous boundary layer exhibited a high grain boundary ionic conductivity of 7.47 × 10^–5 S/cm, a total ionic conductivity of 1.07 × 10^–4 S/cm, and an extremely low activation energy of 0.215 eV. After humidity acceleration testing, the pellets showed good chemical stability against humidity, and the grain boundary and total ionic conductivities were increased by approximately 1.3 times to 9.21 × 10^–5 and 1.38 × 10^–4 S/cm, respectively. These results provide evidence that introducing an amorphous layer at the particle interface is a solution to the issues associated with low grain boundary ionic conductivity in ceramic-based solid electrolytes.