Conductivity limit of fluorine-conducting solid electrolytes for electrochemical devices operating at room temperature
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ICAL PROPERTIES OF CRYSTALS
Conductivity Limit of Fluorine-Conducting Solid Electrolytes for Electrochemical Devices Operating at Room Temperature N. I. Sorokin and B. P. Sobolev Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia e-mail: [email protected] Received November 18, 2014
Abstract—An analysis of the characteristics of operating devices and prototypes based on fluorine-conducting solid electrolytes allows for estimating the lower fluoride conductivity limits σ293 K that are sufficient for using these electrolytes in electrochemical devices operating at room temperature. The estimated σ293 K values for different electrochemical devices differ strongly: σ293 K > 10–3 S/cm for fluorine ion current sources and fluorine gas detection sensors and σ293 K > 10–5 S/cm for fluorine fluid sensors (ion-selective electrodes). The fluoride materials that are promising as solid electrolytes in fluorine-ion batteries and accumulators, fluorine-selective electrodes, and gas fluorine gas sensors operating at room temperature are listed. DOI: 10.1134/S1063774515030189
INTRODUCTION In practice, it is important to know the lower limit of ionic conductivity of fluorine-conducting solid electrolytes (FSEs) that provide the operation of electrochemical devices at room temperature. The estimation of this limit used in the literature is ~10–5 S/cm or more. However, the validity of this value for all types of solid devices appears doubtful. Primary and secondary FSE-based fluorine ion current sources described in the literature cannot operate at room temperature, i.e., without heating to 150–500°C [1–4]). Thus, one cannot directly determine the fluorine ionic conductivity σ293 K that must be provided in these devices under standard operating conditions. Fluorine gas sensors do not make it possible to objectively estimate the σ293 K value either and require heating to ~150–200°C [5, 6]. This caused the ambiguity of the σ293 K limit values used in different studies, including ours, for the FSEs based on fluorite M1 − xRxF2 + x (CaF2 structure) and tysonite R1 − yMyF3 – y (LaF3 structure) nonstoichiometric crystals (M = Ca, Sr, Ba, Cd, Pb2+, or Eu2+ and R = La–Lu, Y, Sc, or Bi3+). The conductivity limits reported in the literature are, on average, within the range σ ∈ [10–5, 10–3] S/cm [1, 6–10]. The difference in two orders of magnitude cannot be considered small for applications. In particular, for the La0.95Sr0.05F2.95 superionic conductor (σT = Aexp(–Ea/kT), where A = 4.1 × 105 S K/cm and Ea = 0.39 eV is the ion transport activation energy [11]), the conversion of the three conductivity values σ = 10–5, 10–4, and 10–3 S/cm from this range into
temperature yields significantly different temperatures: –34, –1, and 50°C, respectively, none of which can be considered operating because they are all far from 293 K. Recently, intense studies have been carried out, both in Russia and abroad, aimed at improving fully solid chemical current sources and sensors. The operation of FS
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