Superionic Conductivities in Li1.7Al0.3Ti1.7Si0.4P2.6O12 Made Using LF-FSP (Liquid- Feed Flame Spray Pyrolysis) Processing to Free Standing Thin Films

Although ceramic materials with superionic conductivities (> 10^-3 S/cm) have been reported for pellets or sheets, no such examples exist for thin films (< 100 µm). Also, almost all superionic conductivities are observed in materials produced by the glass-ceramic processing method. However, to convert glass-ceramic sheets (1-2 mm) to thin films (< 100 µm), they are ball-milled, tape cast, and sintered to give conductivities of > 10^-4 S/cm. Alternately, similar conductivities (> 10^-4 S/cm) have been obtained for thin films produced using powders derived from sol-gel processing. However, sol-gel processing requires calcining powders producing hard agglomerates that can only be eliminated using high-energy milling. We demonstrate here that liquid-feed flame spray pyrolysis (LF-FSP) processing provides non-agglomerated nanopowders that can be used immediately to tape cast. Li_1+x+yAl_xTi_2-xSi_yP_3-yO₁₂ (x = 0.1, 0.3/y = 0.2, 0.4) nanopowders were prepared by LF-FSP with a primary focus on the effects of Al_0.3/Si_0.4 doping on conductivities. Furthermore, the effects of excess Li₂O on Al_0.3/Si_0.4 doped materials were studied. Li_1.7Al_0.3Ti_1.7Si_0.4P_2.6O₁₂ pellets sintered to 93-94 % of theoretical density and samples with varying excess Li₂O contents all show superionic conductivities of 2-3×10^-3 S/cm at room temperature. Li₂O lowers both the crystallization temperatures and temperatures required to sinter. Total conductivities range from 2×10^-3 to 5×10^-2 S/cm in the temperature span of 25° to 125 °C. Small grain sizes of 600±200 nm were produced consistently. Initial attempts to make sturdy thin films gave films with thicknesses of 52±1 µm and conductivities of 3-5×10^-4 S/cm at sintering temperatures of only 1000 °C; attributed to final densities of only ≈ 88 %. Thin films of LAGP materials where Ge substitutes for Ti of LATP, reported by other groups, show comparable conductivities but may not be suitable for mass production due to the high cost of Ge. Typical liquid electrolytes used today offer conductivities of 10^-3-10^-2 S/cm at room temperature and can tolerate temperatures ≈ 60 °C. Beyond this point, permanent damage is done to the battery due to electrolyte decomposition coincident with gas evolution. This, in turn, results in pressure build-up leading to mechanical damage within the battery pack, fire, or explosions in extreme cases. Replacement with the ceramic thin film electrolytes as developed here could offer much improved safety as they will be stable to much higher temperatures.