Keying of lithium chloride alkali halide salt into the interlamellar space of nacrite clay mineral leads to a stable hybrid material that after calcination under inert atmosphere at 723 - 873 K induces an amorphous metahybrid. The electrochemical impedance spectroscopy (EIS) was performed to investigate the electric/dielectric properties of the hybrid with various parameters: frequency and temperature. Equivalent circuit was proposed to fit the EIS data. The experiment results show that the ionic conduction mechanism is related to the motion of Li + cations which are thermally activated, named the hopping model. Furthermore, the resulting metahybrid obtained from dehydroxylation of the formal hybrid shows a superionic behavior with high ionic conductivity up to 10 ﹣2 S·m ﹣1, good electrochemical stability and can be used as a solid electrolyte material for Li-ion batteries.
Over the past three decades, much attention has been paid to solid electrolytes instead of liquid electrolytes because of their potential use in the electrochemical power sources (batteries, lithium ion cells, lithium batteries, fuel cells, electrochemical sensors, etc.) [
In order to improve the bulk properties of solid electrolytes, a good number of researchers are interested in the synthesis and characterization of lithium-ion conductors based on different classes of materials such as ceramics, polymers, glasses and so on [
However, there are rare studies concerning solid state electrolytes based on clay materials [
Nacrite is a 1:1 dioctahedral aluminosilicate that belongs to the kaolin group, which also includes dickite and kaolinite [
The intercalation of clay with salts of alkali halides and its correlation with ionic conduction has shown to be important phenomena to design novel hybrid materials combining good electrochemical window with stable structural properties [
This current paper is a sequel of our previous paper [
The purpose of this paper is to study the electrochemical properties of the functionalized nacrite-LiCl hybrid. In this regard, we carry out the electric impedance
Well-crystallized Tunisian nacrite [Si2Al2O5(OH)4] is used in this research (Scheme 1(a)). This layered clay mineral has been previously described and characterized [
The experimental protocol followed to synthesize the stable nacrite-LiCl hybrid material have been elucidated in our recent work [
Noting that the in-situ heat-treatment of the nacrite-LiCl hybrid at the temperature of 523 K is accompanied with the removal of the intercalated water molecules [
Knowing that the calcination of Tunisian nacrite around the temperature of 823 K leads to amorphous synthetic phase, commonly named metanacrite characterized with a disordered polymerized silicon/aluminum framework [
1) Metanacrite-LiCl hybrid:
The in-situ heat-treatment of the nacrite-LiCl hybrid at the transition-temperature of 723 K leads to the removal of structural water from the sample. The nacrite-LiCl hybrid is then converted to metanacrite-LiCl hybrid (Scheme 2) with a structural formula, deduced from TGA [
2) Metanacrite-Li2O hybrid:
Proceeding with the in-situ heat-treatment of the nacrite-LiCl hybrid from 723 to 873 K generates the complete dehydroxylation and the evaporation of chlorine anions as determined with the TG analysis [
with a structural formula
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
where,
・ The ac conductivity was calculated using the empirical relation [
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 [
where the
・ The temperature dependence of the dc conductivity was adjusted by the Mott’s equation [
where
・ The complex permittivity of the sample is given by the relation [
The dielectric permittivity characterization was interpreted according to the following expressions:
where,
The correlation between the electrical and dielectric properties of the nacrite-LiCl hybrid was the main purpose of this section starting with the electric impedance.
With the
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 [
In the temperature range 473 - 873 K, the equivalent circuit that best adequates the response of our hybrid sample is:
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
The
An Arrhenius behavior was observed for the hybrid. The
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 (
The
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 [
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 [
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 (
1) Two layers of the same sheet are bound together by the tetrahedral oxygen atom common with the edge- shared octahedra [
2) Hydrogen bonding between the OH groups of the oxide/hydroxide (Al2(OH)4) octahedrons and the intercalated anions [
3) Electrostatic interactions between the basal siloxane oxygen of the gibbsite-like layer and the intercalated cations [
Temperature range 523 - 873 [K] | ||
---|---|---|
T [K] | σdc (S·m−1) | σac (S·m−1) (1 MHz) |
523 | 4.02 × 10−6 | 1.22 × 10−4 |
548 | 8.10 × 10−6 | 1.24 × 10−4 |
573 | 1.67 × 10−5 | 1.33 × 10−4 |
598 | 3.31 × 10−5 | 1.47 × 10−4 |
673 | 5.24 × 10−5 | 9.57 × 10−5 |
698 | 7.96 × 10−5 | 1.41 × 10−4 |
723 | 1.44 × 10−4 | 2.15 × 10−4 |
748 | 2.20 × 10−4 | 3.21 × 10−4 |
773 | 3.18 × 10−4 | 4.45 × 10−4 |
798 | 5.05 × 10−4 | 6.69 × 10−4 |
823 | 6.62 × 10−4 | 8.89 × 10−4 |
848 | 8.49 × 10−4 | 1.11 × 10−3 |
873 | 0.11 × 10−2 | 0.13 × 10−2 |
Sample: nacrite-LiCl hybrid material. | ||
---|---|---|
Temperature region [K] | Ea(dc) (eV) (±0.01 eV) | Ea(ac) (eV) (±0.01 eV) (at 1 MHz) |
508 - 668 K | 0.84 | 0.51 |
673 - 873 K | 0.82 | 0.71 |
Ea(dc): dc activation energy. Ea(ac): ac activation energy.
Hybrid | σac (S·m−1) at 1 MHz | Structure type |
---|---|---|
Si2Al2O5(OH)4·(1−α)LiCl | 1.22 × 10−4 | crystalline |
Si2Al2O7·(1−α)LiCl | 2.15 × 10−4 | amorphous |
0.13 × 10−2 | amorphous |
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 [
・ 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 [
Hence, concerning Si2Al2O7·(1-α)LiCl hybrid (
Moreover, the
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 [
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 (
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
According to Leluk at al., [
At low frequencies (
temperature related to the removal of OH groups within the temperature of dehydroxylation. On the other hand, the dielectric loss (
In this work, electrical experiments are conducted in order to characterize the electrochemical properties of a new nacrite-LiCl hybrid as a function of temperature and frequency. The investigation of the ac and dc conductivity of nacrite-LiCl hybrid suggests that the suppression in the degree of crystallinity of the elaborated hybrid improves dramatically the ionic conduction which is about ten orders of magnitude larger than the low temperature conductivity value. The activation energies for conduction measured at different temperatures indicate that the conduction mechanism is driven by hopping of Li+ ions from one site to the neighboring one. The dielectric constant and dielectric loss calculations support these ac and dc results.
The “metanacrite-Li2O-hybrid” with a typical composition
N. Jaafar acknowledges Dr. Manuel Pedro F. Graça (Physics Department, University of Aveiro, Campus Universitario de Santiago 3810-193 Aveiro Portugal) for helpful discussions and corrections.