Alpha a

EIS
was performed in the frequency range from 10⁶ to 0.01∼1
Hz, depending on the specific cells. The experiments were conducted
in the potentiostatic mode, with an amplitude of 10 mV, using Solartron
1260 and Novocontrol Alpha-A devices. Temperature-dependent EIS measurements
(in the range between 5 and 50 °C) were performed using an external
thermostat (Lauda RC6CP). In this setup, two thermocouples were employed:
one was placed in the vicinity of the coin cell for the precise sample
temperature measurement and the other was located inside a water/oil
bath for control purposes. Temperature-dependent EIS measurements
were carried out for 30 min for each temperature (25 min to reach
the equilibrium and 5 min for EIS measurement) to minimize the changes
of the SEI during the experiment. Analysis of the impedance spectra
was performed with ZView software (Scribner Associates, version 3.5c).
Galvanostatic polarization was performed using a Keithley Current
Source (Model 220). Galvanostatic stripping–plating tests were
performed in Li/electrolyte/Li and Li/electrolyte/Cu cells by a Neware
Battery Testing System (BTS V.5.3 by Neware Technology Limited) with
a constant current of 0.1 mA cm^–2. Other parameters
affecting the stripping–plating behavior such as the thickness
of the electrode, the amount of liquid electrolyte, and the thickness
of the separator were kept constant in every cell.

Impedance spectra were recorded on a Novocontrol PotGal electrochemical test station and a Novocontrol Alpha A impedance analyzer in potentiostat mode, with applied DC voltages between the working and reference electrode from 0 to –600 mV. An AC voltage of 10 mV rms and a frequency range of 1 MHz to 10 mHz were used. Each frequency point was measured for at least one second and one period. Measurements were performed between 500 °C and 650 °C with different oxygen/nitrogen mixtures (0.25 mbar to 1 bar O₂) (Alphagaz, 99.995%) in a closed apparatus of fused silica. For the three-electrode samples, working and counter electrodes were contacted between two platinum sheets, the reference electrode was contacted with platinum thread. Microelectrode samples were contacted by using a platinum sheet (counter electrode) and a platinum needle (microelectrode).

Complex permittivity ε_r^* of the injection molded composite samples was measured in the frequency range of 0.01 Hz to 1 MHz using Novocontrol Alpha-A dielectric analyzer (Novocontrol Technologies, Montabaur, Germany) with a ZG4 test interface (two-wire mode). For the sample capacitance range considered in this study, the absolute loss factor (tan δ) measurement accuracy was approximately ± 10⁻⁴ (absolute phase accuracy of 6 m°). The measurements were performed at room temperature using an AC measurement voltage of 1 Vrms. The samples were the same as those used for TSDC; the permittivity measurements were performed right before and after the TSDC measurement from the same samples.

The obtained NH₄(Mg_1-xLi_x)F_3-x and (NH₄)₂(Mg_1-xLi_x)F_4-x powders were pelletized at 200 MPa by a cold isostatic pressing method. Au thin film electrodes were sputtered on the both sides of the dense pellets. Electrical conductivities were evaluated from AC electrochemical impedance spectroscopy (EIS) at 303–343 K in N₂ gas with 30–50 mV of amplitude with frequency of 4.0 × 10⁷ to 1 Hz by using the impedance analyzer (Alpha-A, Novocontrol Technologies GmbH & Co. KG, Germany).
The powders of K(Mg_0.9Li_0.1)F_2.9 and K₂(Mg_0.9Li_0.1)F_3.9 were pelletized at 200 MPa by a uniaxial pressure, and sintered at 1073 or 873 K for 10 h. Electrical conductivities of K(Mg_0.9Li_0.1)F_2.9 and K₂(Mg_0.9Li_0.1)F_3.9 were evaluated from AC EIS at room temperature − 788 K in Ar atmosphere by using a potentiostat (VersaSTAT, Princeton Applied Research, USA).
To confirm the dominant fluoride ion conduction, DC polarization measurements were performed by using the blocking cell consisting of Pb/PbSnF₄/sample/PbSnF₄/Pb at room temperature − 423 K under vacuum. Schematic illustration of the blocking cell was given in Fig. S9. The current for DC polarization measurements was 10 or 20 mA.