To get maximum information about the electrical properties of the new nacrite-LiCl hybrid, the electrical impedance and the dielectric permittivity provide a wide scope of graphical analysis concerning these properties.

Electrochemical Impedance Spectra (EIS) were obtained using a Hewlett-Packard (HP) 4192 analyzer. The impedance measurements were taken in an open circuit using two electrode configurations with signal amplitude of 50 mV and a frequency band ranging from 10 Hz to 13 MHz at different temperatures. The examined sample is pressed into pellet using a hydraulic press. To ensure good electrical contact between the sample and the electrical junctions, the pellet was sandwiched between two platinum electrodes to form a symmetrical cell. The cell was placed into a programmable oven coupled with a temperature controller from ambient to 873 K. The resulting data were fitted using the equivalent circuit of the Z view software.

The analytical background used in the electrical data analysis was the following:

・ The total conductivity can be expressed using Jonscher’s law [21] [22] as:

where, is the ac conductivity due to the hopping conduction, is the conductivity due to the direct current, A is the temperature dependent parameter which determines the strength of polarization, while the exponent s represents the degree of interaction between mobile ions with the sample matrix.

・ The ac conductivity was calculated using the empirical relation [23] :

where g is the geometric factor, d corresponds to the thickness of the pellet, r the radius of the pellet, S the area of the pellet and R the resistance.

・ The temperature dependence of the ac conductivity was determined using the Arrhenius expression [24] :

where the can be calculated from the slope of.

・ The temperature dependence of the dc conductivity was adjusted by the Mott’s equation [23] [25] :

where is the activation energy for dc conductivity, k_B is the Boltzmann’s constant and T the temperature in Kelvin, is called the pre-exponential factor.

・ The complex permittivity of the sample is given by the relation [23] :

The dielectric permittivity characterization was interpreted according to the following expressions:

where, is the real part of dielectric permittivity and is the imaginary part of the dielectric permittivity, is the real part of the electric impedance, is the imaginary part of the electric impedance and. is the capacitance of the empty cell, is the angular frequency, is the dielectric permittivity of free space (8.85418782 × 10⁻¹² F·m⁻¹), S is the electrode surface and d is the distance between the two plane electrodes.

With the versus representations (Figure 1(a) and Figure 1(b)), we observed the existence of semicircles in the complex plane from 548 K to 873 K. At high temperature, these semi-arcs shift towards higher frequencies with a significant reduction of their size. We point out, that nacrite-LiCl hybrid becomes more conductive at high temperature. We think that this phenomenon is attributed to the existence of a deformation (destruction) of some physical characteristics of the host clay material framework and to some chemical cha- racteristics of LiCl alkali halide.

Mathematically, the impedance diagrams for the nacrite-LiCl hybrid are studied using a fit procedure. The equivalent circuit allows the establishment of correlations between electrochemical parameters and characteristic impedance elements [26] .

In the temperature range 473 - 873 K, the equivalent circuit that best adequates the response of our hybrid sample is:. R_s represents the grain resistance connected in series and R_p the grain resistance connected in parallel to an intuitive element, called capacity of the fractal interface (CPE) (Figure 2). The (CPE)

element accounts for the observed depression of semicircles and also the non-ideal electrode geometry. The impedance of this constant phase element is represented in Equation (8):

where is the admittance at ω =1 rad/s and β a value between 0 and 1.

The of nacrite-LiCl hybrid increases from (4.02 × 10⁻⁶ S·m⁻¹) at 523 K to the (0.11 × 10⁻² S·m⁻¹) at 873 K (Table 1).

An Arrhenius behavior was observed for the hybrid. The was determined from the slope of log (Figure 3(a) and Figure 3(b)) and were found 0.84 eV and 0.82 eV, respectively below and above 673 K (Table 2). The obtained values suggest an ionic conduction process.

The ac conductivity measurements at high frequencies (1 MHz) show that increases from 1.22 × 10⁻⁴ S·m⁻¹ at 523 K to 0.13 × 10⁻² S·m⁻¹ at 873 K (Table 1). This remarkable increase is also related to the increase of the number of free ions in the hybrid matrix in terms of temperature.

The (Table 2), calculated in agreement with Equation (3), corresponds to the free energy barrier an ion has to overcome for a successful jump from one site to another. These values designate that the ionic transport mechanism can be interpreted by the thermally activated hopping process [27] .

Subsequently, these electrical measurements are classified in two domains: 1) before 723 K, nacrite-LiCl hybrid behaves as a good ionic conductor and 2) after 723 K, the amorphous nacrite-LiCl hybrid acts as a fast ionic conductor. According to these experimental results, we deduce that the disordered hybrid bears easier motion than the ordered one. We conclude that contribution of disorder and defects in the hybrid framework are res- ponsible for the motion of charge carriers; this result is in agreement with the previous publication of Kumar & Yashonath [2] .

In general, both cations and anions can be carriers of electric current in ionic solids. Applying the simple logic that “smaller ions diffuse faster” and since cations have smaller ionic radii than anions, the majority of superionic solids discovered are cation conductors [2] . Another factor that influences the diffusivity of ion is the magnitude of charge it carries. When the charge of an ion is large, it is likely to be confined to its site by stronger Coulombic attraction of its neighboring ions (which are of opposite charge) [2] . That is why the predominant current conducting species in known superionic solids are monovalent cations [2] .

In the following part, we investigate the charge carriers responsible for conduction in our hybrid before and after the temperature of dehydroxylation.

・ Before 723 K, nacrite-LiCl hybrid displays well-packed layers (Table 3) [15] . Vibrational spectra of the hybrid recorded using infrared spectroscopy provide valuable insights about the interactions between the nacrite host matrix and the intercalated species [15] :

1) Two layers of the same sheet are bound together by the tetrahedral oxygen atom common with the edge- shared octahedra [28] .

2) Hydrogen bonding between the OH groups of the oxide/hydroxide (Al₂(OH)₄) octahedrons and the intercalated anions [15] [29] [30] .

3) Electrostatic interactions between the basal siloxane oxygen of the gibbsite-like layer and the intercalated cations [15] [29] [30] .

E_a(dc): dc activation energy. E_a(ac): ac activation energy.

So, owing to these strong interactions, the structural O²⁻, H⁺ and OH⁻ ions of the alumino-silicate fairly rigid framework are localized around their equilibrium sites, i.e., they cannot escape from their lattice sites. Besides, H⁺ and OH⁻ ions of the intercalated water molecules are eliminated due to the dehydration starting from 373 K as demonstrated by TG analysis [15] . Furthermore, Li⁺ is known as an excellent current carrying ion in superionic solids motivated by its small ionic radius of 0.076 nm, lower weight and ease of motion [2] but Cl⁻ anion appears to be rather less mobile than the cation due to its great ionic radius equals to 0.181 nm [2] . To conclude, Li⁺ is the predominant current carrier in Li-hybrid conductor at low temperatures.

・ In the temperature range between 723 - 873 K, calcination occurs, nacrite-LiCl hybrid converts to metanacrite- LiCl hybrid and then to metanacrite-Li₂O hybrid containing large amount of amorphous silico-aluminates; therefore disorder and/or defects arise. TG investigations indicated that, in this range of temperature, dehydroxylation is accompanied by the removal of the inner-surface hydroxyls and the inner hydroxyls from the basic nacrite structure [15] . Therefore, OH⁻ and H⁺ ions do not contribute to the ionic conduction process. Besides, a new Si-O-Al bond is created during dehydroxylation, which prevents O²⁻ anions to contribute to the transport mechanism.

Hence, concerning Si₂Al₂O₇·(1-α)LiCl hybrid (Table 3), the associated ionic conductivity is found to increase significantly in terms of temperature, which is reasonable since the amorphous structure is so suited for easy motion of Li⁺ ions. Besides, a clear conductivity jump is found at 723 K, which could be ascribed as the transition for the hybrid from the crystalline phase to the amorphous one.

Moreover, the hybrid showed higher conductivity (Table 3). This phenomenon can be

explained by the presence of Li₂O content in its framework. The presence of lithium oxide cleaves the structure and disturbs the bonding between the hybrid forming cations and oxygen anions. This alkali metal oxide increases the number of non-bridging oxygen than bridging oxygen in the amorphous hybrid matrix [31] - [33] . This en- hanced conductivity might be associated with the increase in the non-bridging oxygen (NBOs) and the resulting improved Li⁺ ion mobility [34] .

Finally, Li⁺ is the common current carrier via hopping from one site to the next for both “metanacrite-LiCl hybrid” and “metanacrite-Li₂O hybrid”. Since the “metanacrite-Li₂O-hybrid” phase is more amorphous than the “metanacrite-LiCl-hybrid” phase therefore it produces greater ionic conductivity (Table 3). This shows that the conductivity was preferably affected by the amorphicity of the metanacrite framework through which mobile lithium ions may migrate.

The study of the dielectric permittivity in ion conducting clay composite materials is very interesting to understand the ionic transport properties of materials. The real and imaginary part of dielectric permittivity for nacrite-LiCl hybrid material at different temperatures and frequencies are shown in (Figure 4 and Figure 5), respectively.

According to Leluk at al., [28] , heating of kaolin-type clay minerals from room temperature to 873 K results in a gradual decrease in the dielectric permittivity [28] . A significant volume of the structural water was removed from the aluminium silicate and gibbsite layers. The decomposition of the material produces defects that liberated a small distance movement. Consequently, the dielectric permittivity decreases considerably.

At low frequencies (Figure 4), nacrite-LiCl hybrid exhibits a decrease in the dielectric permittivity in terms of

temperature related to the removal of OH groups within the temperature of dehydroxylation. On the other hand, the dielectric loss (Figure 5) increases in terms of temperature, this result corroborates the ac conductivity investigation of the examined sample and can be attributed to the increase in charge carrier density due to the increase of free ions in the hybrid matrix [35] . This result confirms the thermal behavior of the studied hybrid consisting in the destruction of the crystalline structure of the hybrid as a function of temperature and thus a faster movement of the lithium cations should be expected. However, at high frequencies (Figure 4), the dielectric constant remains invariant with temperature, whereas the dielectric loss (Figure 5) drops and becomes almost temperature independent for f > 10³ Hz. This behavior may be due to the fact that in the high frequency range, the charge carriers cannot follow the electric field.

hybrid’s highly conductivity (σ_ac ~ 10⁻² S·m⁻¹) makes it excellent candidate as electrolyte solid for lithium-ion batteries. Furthermore, this amorphous-type Li-ion conductor offers several advantages such as: low cost and ease of handling. Its use instead of the conventional superionic conductors can drastically improve the safety aspects of lithium batteries.