pH-sensitive and reference behavior of all-solid-state LLTO electrodes controlled by processing

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Abstract
Lithium lanthanum titanium oxide (LLTO) was studied as an all-solid-state electrode exhibiting either pH-sensitive or reference behavior depending on processing conditions. LLTO ceramics were prepared by solid-state reaction and densified by spark plasma sintering (SPS) using different temperatures and dwell times. Electrochemical impedance spectroscopy shows that the bulk ionic conductivity remains close to 10-3 S.cm-1 at room temperature, while grain-boundary conductivity strongly depends on SPS conditions and controls the overall ionic transport. X-ray photoelectron spectroscopy reveals variations in surface hydroxylation without changes in the Ti4+ bulk valence. Densely sintered samples exhibit a pH-dependent response with slopes up to -26 mV.pH-1, whereas fine-grained ceramics display a stable, pH-independent potential of ~154 mV vs. SHE, over the pH range 4-13. The reference behavior remains stable under low oxygen partial pressure (~10-6 atm) and in sulfide-containing media (10-7-10-3 M). These results demonstrate that processing-controlled transport and surface chemistry enable switching between pH-sensitive and reference regimes in LLTO all-solid-state electrodes.

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