Complex Impedance Analyses of Li doped ZnO Electrolyte Materials

The recent studies indicate that internal point defects in solid electrolytes modify the electronic and ionic conductivity and relaxation mechanism of solid oxide fuel cells. We focused on synthesis of Lithium (Li) doped Zn1-xCoxO (x = 0.00, and 0.10) nanoparticles employing chemical synthesis technique with a reflux setup under constant Argon gas flow. The structural characterizations were performed by x-ray powder diffractometer (XRD) and x-ray photoelectron spectroscopy (XPS). Then, Rietveld refinements were performed to investigate the replacement of Li atom amount in ZnO lattice. Moreover, the variations in ionic conduction dependent on 5, 10 and 20 mol% Li doped ZnO were analysed via ac impedance spectroscopy. The complex measurements were performed in an intermediate temperature range from 100 °C to 400 °C. Ac conductivity responses of each sample were disappeared at a certain temperature due to becoming electronic conductive oxides. However, this specific temperature was tuned to high temperature by Li doping amount in ZnO lattice. Furthermore, the activation energy change by Li dopant amount implied the tuneable ionic conduction mechanism.

P-type conductivities were observed for Li doped ZnO semiconductors and high amount of Li doping (16 mol% doped ZnO) created electrically insulator samples [21][22][23] . Li doping amount in ZnO created internal point defects of Li substitutions and interstitials Li atoms, which modify the electronic and ionic conductivity and relaxation mechanism by temperature. In the literature, it is mentioned that Li atoms can be easily incorporated into ZnO lattice [21][22][23] and Li atoms reduce Zn vacancies defect amounts in ZnO lattice 21 . Li doping created a high densification, which enhancing the ionic conductivity, in a material 19 . Furthermore, Zn vacancies modifies the p-type conductivity in ZnO lattaice and low amount of Zn vacancies enhances n type conductive of ZnO nanoparticles. The studies assigned the nanosize p-n heterojunctions in composite electrolytes create low-temperature operation of solid oxide fuel cells with high ionic conductivity values [11][12][13] . Thus, the low amount of Zn vacancies causes to form n type nanoparticles and on the other side Li doped ZnO partciles perform p type conductivity. The formation of p-n heterojunctions enhances the ionic transportability of oxide semiconductor 24 . In addition to all, the researchers also prove high ionic conductivities of Li + ions, especially for glassy solid electrolytes [25][26][27] .
In this study, lithium (Li) doped Zinc Oxide (ZnO) particles were synthesized via chemical route and ionic conduction mechanism by Li amount in ZnO lattice were investigated for understanding suitability as electrolyte materials in SOFCs. Reitveld refinement were used to understand the Li atoms replacements in lattice. Furthermore, spectroscopic techniques such as x-ray photoelectron spectrometer and UV-Vis. spectrophotometer were employed to prove the calculations. The ionic conductivity, ionic transport activation energy and ionic transport relaxations mechanism by temperature were analyzed by fitting the Nyquist plots obtained via ac impedance spectrometer. The suspension was heated and optimized around 80 o C for about 180 minutes in a round bottom flask in oil bath with water cooling system and stirred, continuously, with a magnetic stirrer bar. Argon (Ar) gas flow was let into the solution by a gas inlet, and then passed through the condenser into the oil bubbler to get rid of the contaminating gases. Finally, this solid wet phase was dried in an oven at 300 °C for 8 h.
Thermal analysis were performed by synthesized particles under dry nitrogen flow with 40 ml/min in a α-Al 2 O 3 reference container to monitor chemical reactions employing DTG-60H (TGA/DTA) 6300 spectrometer (Shimadzu Corp.).
The structural analyses were performed employing a Bruker brand x-ray powder diffractometer (XRD) with Cu K α radiation (1.5418 Å) and a x-ray photoelectron spectroscopy (XPS) spectrometer equipped with AlK α monochromatic x-ray exciting radiation with a resolution ~ 0.1 eV. The surface analyses were performed via scanning electron microscopy (SEM) micrographs. Reitveld refinements were also performed on the XRD patterns by employing FullProof subprogram 28 . In order to perform Reitveld refinement, each XRD pattern was measured by from 10° to 90° with 0.008 °/min steps.
Ionic transportation in synthesized samples were obtained employing a two-probe AC impedance spectrometer, Solartron 1260 and 1286, with an electrochemical interface. Electrochemical Impedance spectra (EIS) measurements were performed from 0.01 Hz to 13 MHz frequency range and 100 mV AC bias voltage amplitude under air atmosphere in temperature range of room temperature and 600 o C. Before measurement, each powder was pressed under 300 MPa pressure to create 10 mm in diameter and 1 mm thick pellets. Each side of pellet was covered with silver paste to create Ohmic contacts with platinum electrodes of electrochemical interface.

Results
Li atom amount depending ionic transport ability of ZnO semiconductors was targeted to synthesis via combination of chemical route and thermal treatment at high temperatures. After chemical route, a gel state mixture was obtained, and the temperature dependent chemical formations were analysed by a differential thermal and thermogravimetric spectrometer (DT-TGA). DT-TGA spectra of a mixture, prepared for 10 mol% Li doped ZnO, was demonstrated in Fig. 1. The spectra in figure was recorded from 30 to 1000 o C, at which temperature no chemical reactions were observed.
The evaporations of possible organic chemicals were occurred by a mass loss with four different endothermic reactions in temperature ranges from 30 to 200 °C, from 290 to 315 °C and from 300 to 500 °C due to evaporation of water 29 , losing pure citric acid 30 , losing weakly bonded functional groups such as COOH on the surface 30 , and completing chemical reactions, respectively. The last endothermic reaction without mass loss was measured in temperature range of 500 and 1000 °C due to finalize the chemical reactions as Li doped ZnO.
According to thermal analyses, all samples with different Li doping molar ratios were annealed at 1000 o C. The XRD patterns of samples with different Li doping molar ratios were shown in Fig. 2. The patterns were almost performed similar diffractions, which are accordance with Zinc Oxide (ZnO) pattern, ICDS card of PDF#00-036-1451. No additional diffraction patterns were related that the samples were produced without an impurity phase or unreacted element. In order to identify Li atom replacement in ZnO lattice, the Reitveld refinements were performed on the patterns shown in Fig. 2.
ZnO has a hexagonal crystal structure with P6 3 mc Space group 31 . Zn 2+ ions settle at ( 1 Li atom replacements into ZnO crystal lattice were also modified the lattice parameters as identified from a shift at diffraction peak positions 34,35 . The shift at diffraction peak positions of (100), (002) and (101) planes were illustrated on Fig. 4. As seen on Fig. 4, while the peak positions were shifted to low two theta positions for Li   www.nature.com/scientificreports www.nature.com/scientificreports/ doping amounts lower than 3 mol%. However, high Li amount, more than 3 mol%, in ZnO lattice caused a shift at the peak positions to high 2 theta positions.
Moreover, XPS measurements were employed for the structural analyses. XPS measurements provided a comparison between expected positions of Li atoms in ZnO lattice and Reitveld refinements. C 1 s binding energy level, 284.5 eV, were used as a reference energy level. To get rid of charging dependent shift at the spectra, all XPS spectra were shifted according to C 1 s energy level. Figure 5a    After completing structural analyses and replacement of Li atom dependent point defects in ZnO lattice, ionic transportation properties of particles were analyzed performing ac impedance measurements. The measurements were shown in Fig. 6a-d. The conductivity of ionic transportation of each pellet was measured in temperature range of 100 and 500 o C. In order to specify the change of relaxation time by Li amount in ZnO lattice, frequency versus imaginer resistivity were also demonstrated in Fig. 7a-d.

Discussion
Li doped ZnO particles were produced employing sol-gel technique. Firstly, XRD patters were determined to understand Li atoms replacement in ZnO lattice. On XRD patterns, no peak position was observed related to Li atoms or Li compounds which showed that Li atoms were replaced inside of ZnO lattice. In order to understand Li atom positions in lattice, Reitveld refinements were performed on the patterns as shown on the Table 1. The Reitveld refinements were performed according to the two assumptions which were assigned to Li atoms instead of Zn atoms and interstitial Li atoms in ZnO lattice. The hexagonal wurtzite structure of ZnO were not changed by increase in Li amount in lattice. As reported in the literature, if Li atoms prefer to be in Zn positions the lattice parameters should be decrease due to smaller ionic radius of Li 1+ ions (0.060 nm) than Zn 2+ ions (0.074 nm) 36,37 . In contrast the lattice parameters of ZnO should be enhanced in case of replacement of Li 1+ ions as the interstitial defect states in ZnO lattice. As reported on Table 1, the Reitveld refinements for both conditions were fit very well. In case of Li 1+ ions replacement in Zn 2+ ions a small decrease in the lattice parameters, a and c, values as expected. On the other hand, no change in lattice parameters were observed for the Li 1+ ions as the interstitial defects. According to the change in lattice parameters, Reitveld refinements indicated that Li 1+ ions substituted with Zn 2+ ions. In the literature, it was also mentioned that Li substituted Zn atoms at moderate concentrations doping amount, even if Li 1+ ions were known high diffusion ability due to having low ionic radius than Zn 2+ ions 21 .
In order to prove and understand the calculated values by Reitveld refinements, the XPS patterns were taken. On the XPS patterns Li 1 s and O 1 s binding energy levels were analyzed. The both bind energy levels, Li 1 s and O 1 s, were fit into two peak positions and the maximum values of each fit were demonstrated on Table 2. The observed peak at 55.7 eV related to Li 1+ ions replaced to Zn 2+ ions and bonded with O atoms as mentioned in references 23,37 . On the other hand, the peak at 54.9 eV may related to Hydroxides (Li-OH bonds on the surface) as mentioned in reference 38 . In our study no binding energy positions due to metallic Li cluster (at 52.3 eV 23,37 ) and interstitial Li atoms (at 52.9 eV 23,37 ) were measured in the XPS spectra. After confirming the Li atom positions in ZnO lattice, Li atom amount depending ionic transportation performances of ZnO semiconductors were measured by ac impedance spectrometer. Activation energy of ionic transportation, ionic conductivity and relaxation mechanisms of ionic transportation by Li atom amount in ZnO lattice were calculated employing Nyquist plots and imaginer resistivity versus frequency curves. The frequency dependent ionic conductivity of each sample was analyzed at the constant temperature values. In addition, electronic conductivity response was identified the maximum temperature values for measurable ionic conductivities. The temperature range for ionic conductivity measurements were determined at fixed frequency (0.01 Hz) and under 100 mV voltage. The ionic conductivity measurements were performed for the high dc electrical resistivity values which was mostly out of measurement limits of the current system. And right after maximum temperature electrolyte became electrical conductor, therefore ionic conductivity was not able to be recorded. Moreover, Li doping amount was also tuned the ionic conductivity response by temperature. While ZnO pellets gave response in temperature range of 25 o C and 250 o C, Li doped ZnO pellets ionic conductivity responses reached in range of 100 o C and 450 o C. ZnO and Li doped ZnO behaved as electronically conductive above the 250 o C and 450 o C, respectively, at which temperature ionic conduction performance were not useful. As seen on Fig. 8, the activation energy of ionic transportation (E A ) were obtained employing Arrhenius plots shown in Eq. (1). σ, T, σ o , k B and E A symbols were assigned for the ionic conductivity, temperature, pre-exponential constant, Boltzmann constant (-8.617×10 -5 eV.K -1 ) and the activation energy of ionic transportation, respectively. The ionic conductivity values were obtained from Nyquist plots.
A B 0 E A values of ZnO, 5 mol% Li doped ZnO, 10 mol% Li doped ZnO and 20 mol% Li doped ZnO were calculated as 390 ± 3 meV, 371 ± 1 meV, 561 ± 2 meV and 479 ± 3 meV, respectively (shown in Table 3). A decrease in E A value of ZnO was observed by 5 mol% Li doping. Similarly, increase in Li amount to 10 mol% was also increase the E A values of ZnO. However, the E A value of 20 mol% Li doping was lower than 10 mol% Li doped ZnO, but still higher than E A value of undoped ZnO. Generally, the E A values associated with ionic conductivity values and relaxation mechanisms of ionic conductivity. Therefore, the relaxation time changes by Li doping amount was www.nature.com/scientificreports www.nature.com/scientificreports/ evaluated by Eq. (2). In Eq. (2), the relaxation times of ionic carriers and the maximum peak frequency of Z″(f) curve were symbolized by τ c and f max , respectively.
The calculated values were plotted by temperatures on Fig. 9. Obtained values at maximum temperature were demonstrated on Table 3. As seen in Fig. 9a