All-Solid-State Ion-Selective Electrode Inspired from All-Solid-State Li-Ion Batteries

Abstract

Solid electrolytes employed in all-solid-state Li-ion batteries (ASSBs) electronically isolate the positive and negative electrodes, while allowing the carrier ions, Li⁺, to pass through. Inorganic solid-state electrolytes, which typically exhibit a Li⁺-transference number of 1, are theoretically applicable as ion-sensitive membranes of potentiometric ion-selective electrodes (ISEs). Inspired by the ASSB architecture, an all-solid-state Li ISE was developed in a two-layer stacking configuration using a redox-active material (LiFePO₄) and a solid electrolyte (Li_1+x+yAl_x(Ti, Ge)_2–xSi_yP_3–yO₁₂) as inner and outer layers, respectively, on the substrate (i.e., current collector). The solid electrolyte acts as an ion-selective membrane because the Donnan membrane potential obeys a Nernstian response to Li⁺ activity in the analyte solution. The fabricated ASSB-inspired ISE selectively responds to Li ions, exhibiting a Nernstian slope of 60.8 ± 0.5 mV dec^–1, limit of detection of 10^–4.9±0.4, and minimal potential variation (−3 to +6 mV over 17 d). Using a two-phase LiFePO₄/FePO₄ layer with a highly stable potential as the inner reference electrode significantly minimizes the deviations in the response potential. Moreover, applying Li_1+x+yAl_x(Ti, Ge)_2–xSi_yP_3–yO₁₂ as a durable and highly ion-conductive inorganic solid electrolyte enables remarkable long-term stability.