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Enhancement of ionic conductivity by addition of LiAlO2 in Li6.6La3Zr1.6Sb0.4O12 for lithium ion battery
Solid State Ionics 345 (2020) 115185
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Enhancement of ionic conductivity by addition of LiAlO2 in Li6.6La3Zr1.6Sb0.4O12 for lithium ion battery Pritee Wakudkar, A.V. Deshpande
T
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Department of Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur, Maharashtra 440010, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Li7La3Zr2O12 Ionic conductivity Solid electrolytes Modulus studies Sintering additives
Li6.6La3Zr1.6Sb0.4O12 has been synthesized by solid state reaction and effect of addition of LiAlO2 on density, structure and ionic conductivity has been studied. XRD pattern is studied for phase confirmation. Density has been calculated and SEM analysis has been done for morphological studies and elemental distribution in the sample. Complex impedance analysis was used to measure ionic conductivity of all samples and transport number measurement has been carried. Highest conductivity of 3.16 × 10−4 Scm−1 at 25°C is observed for 1 wt % LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramic sintered at 1050 °C for 8 h.
1. Introduction Lithium ion batteries have gained incredible popularity nowadays because they are widely used in most of the portable consumer electronics. Recently much focus has been drawn by solid state lithium ion secondary batteries in which particularly solid state lithium ion conducting electrolytes having some superiority over liquid organic electrolytes in terms of leakage problem, dendrite formation and flammability are used [1]. So, many solid state electrolytes have been extensively studied for solid state lithium ion batteries such as Perovskite Titanate [2], LISICON Sulfides [3] and Garnet Oxides [4–8]. With nominal chemical formula Li7La3Zr2O12 from garnet family has been studied well because of its high conductivity (> 10−04 Scm−1), large electrochemical window and stability against lithium anode [9]. Li7La3Zr2O12 as solid state electrolyte has been reported to have mainly two conductive phases, i.e. tetragonal and cubic, where the cubic phase is having a higher ionic conductivity by two orders of magnitude than tetragonal one hence this phase has been studied broadly. To improve ionic conductivity of cubic Li7La3Zr2O12, doping of various elements was reported by various groups [10,11] also. For forming cubic phase sintering temperature is high around 1230 °C [12], which may result in loss of lithium from sample which can lead to formation of lithium deficient pyrochlore as well as poor density. So there is need to inhibit loss of lithium during annealing. This can be achieved by adding of certain additives as sintering aids. Various groups have studied the effect of sintering additives on the Li7La3Zr2O12 since additives help to reduce the sintering temperature besides formation of cubic phase at low temperature [13]. Janani and
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group [14] have studied the effect of addition of Li3BO3, Li3PO4 and Li4SiO4 on density and conductivity of Al doped Li7La3Zr2O12 where they found that conductivity has increased up to 6.1 × 10−4 Scm−1 by 1 wt% addition of Li4SiO4. Nataly et al. [15] also have observed that addition of 4 wt% of Li2O:B2O3:SiO2:CaO:Al2O3 (LBSCA) glass in Nb doped Li7La3Zr2O12 gives conductivity of 0.8 × 10−4 Scm−1 at room temperature by liquid phase sintering. Li3BO3 added Li7La3Zr2O12 gives 86.4% relative density by sintering at 1100 °C for 8 h [16]. Il'ina E.A. and group [17] has reported that, 3 wt% addition of 65Li2O:27B2O3:8SiO2 in Li7La3Zr2O12 has increased conductivity by two orders of magnitude. Masashi et al. [18] have reported that addition of Al2O3 in Li7La3Zr2O12 lowers the sintering temperature by 230 °C by suppressing the formation of La2Zr2O7 impurity phase with comparable value of ionic conductivity. In the present study we have synthesized highest conducting Sb doped sample Li6.6La3Zr1.6Sb0.4O12 [19] and investigated the effect of addition of LiAlO2 on its density, structure and ionic conductivity where LiAlO2 is used as sintering aid which lowers the sintering temperature as well as acts as conducting medium for Li ion which helps to reduce grain boundary resistance. 2. Experimental 2.1. Material preparation LiAlO2 was synthesized by adding Li2CO3 (Merck) in Al2O3 (Fluka) in an appropriate molar ratio. Mixing was done using agate mortar pestle and then mixed powder was heated at 900 °C for 6 h in alumina
Corresponding author. E-mail address: [email protected] (A.V. Deshpande).
https://doi.org/10.1016/j.ssi.2019.115185 Received 29 August 2019; Received in revised form 3 December 2019; Accepted 9 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
3.2. Density measurement
crucible. A conventional solid state reaction method was used for the preparation of Li6.6La3Zr1.6Sb0.4O12 in which chemicals of high purity Li2CO3 (Merck), La2O3 (Sigma Aldrich), ZrO2 (Merck) and Sb2O3 (Merck) were used. All powders were mixed in Stoichiometric amounts in agate mortar. To suppress the lithium losses during the heating process, 10 wt% of excess Li as Li2CO3 was added. The mixture was heated in an open alumina crucible at 900 °C for 6 h and then cooled to room temperature. Calcined powder was then mixed with 0.5, 1 and 1.5 wt% of LiAlO2 and uniaxially pressed with a hydraulic press into pellets having dimensions of 1.5 mm thickness and 10 mm diameter. Closed alumina crucible containing mother powder was used to sinter pressed pellets at 1050 °C for 8 h.
Archimedes' principle was used to measure density of all ceramics where toluene acted as immersion medium. The bulk density and relative density values of ceramics are given in Table 2. From the table it can be observed that with increasing LiAlO2 up to 1 wt%, the bulk density and relative density increases. The density has been found to be maximum for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. This can be understood as the LiAlO2 acts as sintering aid and helps to hold the particles of Li6.6La3Zr1.6Sb0.4O12 together. Density of 1.5 wt% LiAlO2 was observed to be decreasing as lithium aluminate phase has grown enough in the pellet since the density of the LiAlO2 phase is 2.62 g/cc which is lesser than parent phase [21]. 3.3. Morphological studies
2.2. Characterization
Fig. 3(a, b, c and d) shows typical cross sectional SEM images of LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 samples. Fig. 3(a) shows that the grains are having pores in between them. The presence of such pores surely obstructs the diffusion of Li+ ions over the grain boundaries. From Fig. 3(b), (c) and (d) it can be seen that with addition of LiAlO2 the microstructure becomes denser as compared to pure (without addition of LiAlO2), grains are in contact with each other as LiAlO2 phase has grown within the grain boundary area. Fig. 3(c) exhibits the growth of grains with regular size, which are spherical in shape to some extent in 1 wt% LiAlO2 added sample as compared to 0, 0.5 and 1.5 wt% addition of LiAlO2. Fig. 3(d) shows that particles have grown in irregular sizes which might be one of the reasons of decrease in conductivity. Fig. 4 shows elemental mapping of 1 wt% LiAlO2 added fractured pellet where grain boundaries can be seen significantly. La, Zr, Sb and O elements have been observed to be uniformly dispersed over the entire specimen as seen in figure which are main components of Li6.6La3Zr1.6Sb0.4O12. Whereas Al is distributed distinctly in grain boundaries and more intensity has been observed for O in grain boundaries which suggests that formation of Li rich LiAlO2 phase has taken place. Such ceramic additives can enhance the Li ion conductivity by stabilizing the amorphous region within grain boundaries [22]. From the SEM elemental mapping it can be observed that LiAlO2 resides in grain boundary eventually helps to facilitate migration of Li+ ions in between grains. LiAlO2 additive functions as an adhesive to bind the grains within the sample. LiAlO2 also acts as sintering aid which helps to lower down sintering temperature, which eventually helps to control loss of lithium, which generally occurs at high temperature such as 1230 °C. It assists in the formation of pure cubic phase at temperatures about 1050 °C and also helps to inhibit the formation of other unreacted phases within the sample. The average size of particle is found to be 11.13 μm for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramic sample.
To investigate the obtained phase, pellets sintered at 1050 °C were characterized by X-ray Diffraction using Bruker AXS D8 Advance with step size of 0.019 degrees. Diffraction data were collected in the range from 10 to 100 of 2theta with 29.1 s scan step. For the calculation of density of prepared samples Archimedes' principle was employed using K-15 Classic (K-Roy) where toluene is used as immersion medium. Microstructural analysis and composition determination were done by using scanning electron microscope (JSM-7600 F/JEOL) with EDX. To determine conductivity, Novocontrol impedance analyzer was used to measure impedance and modulus in the frequency range of 20 Hz to 20 MHz from 180 °C to 280 °C in an air furnace with 20 °C interval. Before measurement pellets were polished with SiC paper. For plating both surfaces of the sintered pellets, silver paste was used and plated pellets having approximately 1.5 mm thickness and 10 mm diameter were held between silver electrodes (as Li+ blocking electrodes) for AC and DC conductivity measurements. For transport number measurement, the sample holder arrangement was used as that for conductivity measurements having silver electrodes as irreversible electrodes. A DC potential of 1 V was applied and the current through the sample was measured as function of time with digital nanoammeter. AC conductivity which equals the total conductivity (σac = σe + σi) has been considered as 0 time conductivity. 3. Results and discussion 3.1. X-ray diffraction Fig. 1(a) shows the recorded Powder X-ray diffraction spectra of pure and 0.5, 1.0 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics. The XRD peaks show good agreement with the JCPDS file no. 45–109. XRD peaks were indexed to the garnet-type cubic structure and along with main phase, there are no other reflections from LiAlO2 and its derivable phases have been observed which therefore suggests the most of the raw materials were utilized. The Rietveld refinement of sample was performed using Fullprof software and fitted profile has been shown in Fig. 2. The crystal structure and structural parameters of 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramic was refined by Rietveld fitting to cubic symmetry of space group Ia-3d (230) as shown in Table 1 with following agreement factors Rp:8.81%, Rwp:11.3%, Rexp:8.21 and χ2:1.89. The profile fitted peak positions were used to calculate and refine lattice constant. It was observed that lattice constant a: 12.9282(Å) which shows good agreement with earlier reported value [4]. With addition of LiAlO2 the peak positions are shifting towards higher 2θ values as shown in Fig. 1(b). This may be because as Al content increases, lattice constant decreases as shown in Table 2. This can be attributed to higher amount of Al from LiAlO2 could have occupied the lattice since Al3+ is having smaller ionic radius than Li + which shows the shrinking effect on lattice constant [20].
3.4. Conductivity studies 3.4.1. Nyquist plots Electrical conductivity measurements were carried out in frequency range from 20 Hz to 20 MHz. Complex impedance plots for all LiAlO2 added samples at 25 °C have been shown in Fig. 5. Fig. 6 shows the fitted curves for 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 using R-CPE-W circuit model. The resistance value (R) is obtained from intercept made by a semicircle on z’ axis in the region of high frequency. The Nyquist plots consist of one semicircle which may due to merging of semicircles due to grain and grain boundary at high frequency. In low frequency region advent of tail is because of ion blocking behavior of Ag electrodes used during measurement of ionic conductivity. The formula σtotal = D (SR) −1 was used to calculate the ionic conductivity, where σtotal is ionic conductivity, D is the thickness of the sample, R is the resistance of the sample and S is area of the electrode. It can be observed from plots that 1 wt% LiAlO2 2
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 1. X-ray diffraction patterns of a) Li6.6La3Zr1.6Sb0.4O12 sintered with LiAlO2 addition, b) shift in peak.
Fig. 2. The powder X-ray Refinement profile of Li6.6La3Zr1.6Sb0.4O12. Table 1 Structural parameters of Li6.6La3Zr1.6Sb0.4O12 garnet from Rietveld refinement of X-ray diffraction data. Atom
Wyck. Site
Occupancy
x
y
z
Biso
Li1 Li2 Zr La O Sb
24d 96 h 16a 24c 96h 16a
0.503 0.609 0.167 0.249 1.000 0.007
0.12 0.094 0.000 0.000 −0.0304 0.000
0.000 0.732 0.000 0.250 0.056 0.000
0.250 0.589 0.000 0.125 0.146 0.000
0.237 0.579 0.529 0.584 0.829 0.010
Table 2 Density and Lattice constant of LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics. Wt% LiAlO2
Density (g·cm−3)
Relative density (%)
Lattice constant
0 0.5 1 1.5
4.48 4.57 4.64 4.49
86.15 87.88 89.23 86.34
12.9307 12.9288 12.9282 12.9155
intercepts towards the low frequency region. Although LiAlO2 is having lithium ionic conductivity which is definitely lower than Li6.6La3Zr1.6Sb0.4O12, it acts as a good promoter for lithium ion conduction. The addition of LiAlO2 in composites has shown effect of increasing ionic conductivity [23]. From Fig. 4 it can be observed that lighter region has grown in grain boundaries which is enriched with Al and some intense spot of O also can be seen hence there is strong probability of accumulation of LiAlO2 which is itself a conductive phase
added Li6.6La3Zr1.6Sb0.4O12 ceramic has minimum intercept on z’ axis which shows maximum conductivity of 3.16 × 10−4 Scm−1 among all ceramic samples. This can be ascribed to uniformity in grain size, good connection between the grains as indicated in SEM images of the sample and higher density value. Increase in wt% LiAlO2 shows shift in 3
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 3. SEM morphology of Li6.6La3Zr1.6Sb0.4O12 ceramic with a) 0 wt% LiAlO2 b) 0.5 wt% LiAlO2 c) 1 wt% LiAlO2 and d) 1.5 wt% LiAlO2.
Fig. 4. Elemental mapping of 1 wt% added Li6.6La3Zr1.6Sb0.4O12.
which ultimately helps for migration of Li ions across grain boundary. Similar results have been reported earlier [23].
3.4.2. Arrhenius plots Fig. 7 shows the Arrhenius plots for Li6.6La3Zr1.6Sb0.4O12 added with 0, 0.5, 1 and 1.5 wt% of LiAlO2 heat treated at 1050 °C. 4
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 5. Impedance plots for LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics at 25 °C.
Fig. 6. Fitted curves for LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics at 25 °C.
Fig. 8 shows the variation of conductivity at 25 °C and activation energy with wt% of LiAlO2. The ionic conductivity at 25 °C and activation energy for all the ceramic samples has been tabulated in Table 3. The maximum conductivity and minimum activation energy have been found for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. Increase in conductivity by addition of LiAlO2 can be attributed to LiAlO2 phase grown
Measurements were carried in a range of temperature from 180 °C–280 °C. The plots using obtained data show the nature of Ar-
( ) −Ea
rhenius equation i.e. σ (T ) = σOe kB T where Ea is activation energy, σ is the conductivity, kB is the Boltzmann constant, σo is the pre-exponential factor and T is temperature in Kelvin. It is observed that 1 wt% of LiAlO2 addition has the lowest activation energy i.e. 0.33 eV. 5
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 9. Variation of DC conductivity with time for transport number determination. Fig. 7. Ionic conductivity as function of temperature for 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics.
conducting phase which comprises of LiO4 and AlO4 tetrahedra where Li and Al atoms are located in oxygen tetrahedron. In these pairs each corner is shared by two further tetrahedral. Li ion diffusion in this structure may take place via two types of channels formed by octahedral voids in structure, one is along 〈100〉 direction and another is along 〈001〉 [single crystal neutron diffraction]. Ionic conductivity in LiAlO2 is due to the introduction of structural disorder emerging due to point defects. According to density functional theory Li ion can migrate via Li point defect and Frenkel defect between two tetrahedral, and Li conductivity is strongly influenced by distribution of Li vacancy and interstitial lithium in the structure [25]. LiAlO2 coating also affects cycle life and discharge capacity of LiNi1/3Co1/3 Mn1/3O2 due to its characteristic as fast lithium ionic conductor [26]. Secondly Al from LiAlO2 might have entered in the lattice which is supported by decrease in lattice constant as discussed in XRD results. On the basis of charge neutrality, Al3+ should substitute for 3Li+. In this case it replaces one Li+ and creates two vacancies hence increasing Li vacancy concentration eventually increasing the conductivity of the sample [10]. Fig. 9 shows dc polarization plots for pure and 1 wt% LiAlO2 added ceramic samples. Ion transport number was calculated using relation, ti = (σtotal − σe) / σtotal. Ionic transport number has been found to be > 0.999 for 0 wt% and 1 wt% LiAlO2 added samples, which confirms that the conduction is due to ions only.
Fig. 8. Variation of conductivity of Li6.6La3Zr1.6Sb0.4O12 with wt% LiAlO2 at 25 °C and activation energy.
3.4.3. Modulus spectra study The electric modulus study has been done by many researchers to analyze and interpret electrical relaxation data [27]. This formalism highlights on charge transport processes like mechanism of ion dynamics and conductivity relaxation as a function of frequency and temperature. The electric modulus M∗ is defined as
Table 3 Ionic conductivity of LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics. Wt% LiAlO2 0 0.5 1 1.5
σ (Scm−1) 7.08 2.04 3.16 2.75
× × × ×
Ea −05
10 10−04 10−04 10−04
(σ)
(eV)
0.47 0.41 0.33 0.37
M ∗ = 1/ϵ∗ = (ϵ′ − jϵ ″ )/|ϵ∗|2 = M′ + jM″ where M′ and M″ are real and imaginary parts of the complex modulus M∗ respectively and ϵ′ and ϵ″ are real and imaginary parts of complex permittivity ϵ∗ respectively. Fig. 10(a) shows the variation of M″ as a function of frequency at different temperatures for 1 wt% added LiAlO2 in Li6.6La3Zr1.6Sb0.4O12. The existence of a peak in the modulus spectrum represents the conductivity relaxation [19,28]. The frequency region below peak M″ indicates the range where charge carriers are mobile on long distances
over the grain boundaries which serves as a Li conduction medium to facilitate the hopping of Li+ ions in the boundary sites which can be supported by elemental mapping images. Similar results have also been reported by Rong Lan [24] where increase in conductivity is attributed to Li conducting phase available in grain boundary. LiAlO2 itself is a 6
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 10. (a) Imaginary part of electric modulus as a function of frequency and temperature for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. Fig. 10 (b) Modulus scaling behavior of 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. Fig. 10 (c) Imaginary part of electric modulus as a function of frequency for 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 at 25 °C. Fig. 10 (d) Modulus scaling behavior of 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 25 °C. Fig. 10 (e) Variation of relaxation time with temperature for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12.
indicates that the conduction mechanism is nearly temperature independent. Fig. 10(c) shows the variation of M" as a function of frequency for 0, 0.5, 1 and 1.5 wt% added LiAlO2 in Li6.6La3Zr1.6Sb0.4O12. The nonoverlapping of curves of M″/Mmax″ vs f/fmax in Fig. 10(d)
and frequency region above peak M″ shows charge carriers being mobile on short distances. The frequency associated with Mmax″ gives the relaxation time τ. The Fig. 10(b) depicts plots of M″/Mmax″ vs f/fmax. The figure shows the overlap of curves for all temperatures, which
7
Solid State Ionics 345 (2020) 115185
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suggests the conduction phenomenon is composition dependent which indicates that Al might have entered into the sample leading to change in its composition. Fig. 10(e) shows the variation of relaxation time as a function of temperature for 1 wt% added LiAlO2. Relaxation time plots follow Arrhenius equation τ = τoexp(Ea/kBT), where τo is the pre-exponential factor, Ea is activation energy, kB is the Boltzmann constant and T is temperature in Kelvin. The activation energy Ea(τ) and Ea(σ) are found to be 0.32 eV and 0.33 eV for 1 wt% of LiAlO2 addition respectively. The values of Ea(τ) and Ea(σ) are nearly equal which suggests that both conduction and relaxation processes are due to the same type of charge carriers.
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4. Conclusions Li6.6La3Zr1.6Sb0.4O12 cubic garnet solid electrolyte has been prepared by the solid state reaction method. Phase formation was confirmed by XRD. Density measurements were carried out and microstructural properties were studied by SEM. Study of elemental mapping images showed that Al has been distributed in grain boundaries. Addition of LiAlO2 enhances the ionic conductivity by half an order of magnitude. This has been attributed to LiAlO2 phase grown over the grain boundaries which serves as a Li conductive medium to facilitate the hopping of lithium ions in the boundary region. Modulus analysis carried out over a wide frequency range revealed that both conduction and relaxation processes are due to the same type of charge carriers. 1 wt% addition of LiAlO2 in Li6.6La3Zr1.6Sb0.4O12 has ionic conductivity of 3.16 × 10−4 Scm−1. Addition of LiAlO2 also reduces the sintering temperature by 230 °C to form a cubic phase and thus acts as sintering aid. Declaration of competing interest None. References [1] R. Shin, S. Ick, Y. Soo, Y. Do, H. Kim, S. Ryu, W. Pan, Sintering behavior of garnettype Li7La3Zr2O12-Li3BO3 composite solid electrolytes for all-solid-state lithium batteries, Solid State Ionics 301 (2017) 10–14, https://doi.org/10.1016/j.ssi.2017. 01.005. [2] M.I, T.N.Y. Inaguma, C. Liquan, High ionic conductivity in lithium lanthanum titanate, Solid State Commun. 86 (1993) 689–693. [3] R. Kanno, T. Hata, Y. Kawamoto, M. Irie, Synthesis of a new lithium ionic conductor, thio-LISICON – lithium germanium sulfide system, 130 (2000) 97–104. [4] R. Murugan, V. Thangadurai, W. Weppner, Fast lithium ion conduction in garnettype Li7La3Zr2O12, Angew. Chemie - Int. Ed. 46 (2007) 7778–7781, https://doi.org/ 10.1002/anie.200701144. [5] L. La, O. Zr, L.O.B.O. Sio, E.A. Il, A.A. Raskovalov, N.S. Saetova, B.D. Antonov, O.G. Reznitskikh, Composite electrolyte Li7La3Zr2O12-glassy Li2O-B2O3-SiO2, 296 (2016) 26–30, https://doi.org/10.1016/j.ssi.2016.09.003. [6] L. Zr, Effects of penta- and trivalent dopants on structure and conductivity, 274 (2015) 100–105, https://doi.org/10.1016/j.ssi.2015.03.019. [7] L. Zr, R. Chen, M. Huang, W. Huang, Y. Shen, Y. Lin, C. Nan, Effect of calcining and Al doping on structure and conductivity, Solid State Ionics 265 (2014) 7–12, https://doi.org/10.1016/j.ssi.2014.07.004.
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Enhancement of ionic conductivity by addition of LiAlO2 in Li6.6La3Zr1.6Sb0.4O12 for lithium ion battery Pritee Wakudkar, A.V. Deshpande
T
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Department of Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur, Maharashtra 440010, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Li7La3Zr2O12 Ionic conductivity Solid electrolytes Modulus studies Sintering additives
Li6.6La3Zr1.6Sb0.4O12 has been synthesized by solid state reaction and effect of addition of LiAlO2 on density, structure and ionic conductivity has been studied. XRD pattern is studied for phase confirmation. Density has been calculated and SEM analysis has been done for morphological studies and elemental distribution in the sample. Complex impedance analysis was used to measure ionic conductivity of all samples and transport number measurement has been carried. Highest conductivity of 3.16 × 10−4 Scm−1 at 25°C is observed for 1 wt % LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramic sintered at 1050 °C for 8 h.
1. Introduction Lithium ion batteries have gained incredible popularity nowadays because they are widely used in most of the portable consumer electronics. Recently much focus has been drawn by solid state lithium ion secondary batteries in which particularly solid state lithium ion conducting electrolytes having some superiority over liquid organic electrolytes in terms of leakage problem, dendrite formation and flammability are used [1]. So, many solid state electrolytes have been extensively studied for solid state lithium ion batteries such as Perovskite Titanate [2], LISICON Sulfides [3] and Garnet Oxides [4–8]. With nominal chemical formula Li7La3Zr2O12 from garnet family has been studied well because of its high conductivity (> 10−04 Scm−1), large electrochemical window and stability against lithium anode [9]. Li7La3Zr2O12 as solid state electrolyte has been reported to have mainly two conductive phases, i.e. tetragonal and cubic, where the cubic phase is having a higher ionic conductivity by two orders of magnitude than tetragonal one hence this phase has been studied broadly. To improve ionic conductivity of cubic Li7La3Zr2O12, doping of various elements was reported by various groups [10,11] also. For forming cubic phase sintering temperature is high around 1230 °C [12], which may result in loss of lithium from sample which can lead to formation of lithium deficient pyrochlore as well as poor density. So there is need to inhibit loss of lithium during annealing. This can be achieved by adding of certain additives as sintering aids. Various groups have studied the effect of sintering additives on the Li7La3Zr2O12 since additives help to reduce the sintering temperature besides formation of cubic phase at low temperature [13]. Janani and
⁎
group [14] have studied the effect of addition of Li3BO3, Li3PO4 and Li4SiO4 on density and conductivity of Al doped Li7La3Zr2O12 where they found that conductivity has increased up to 6.1 × 10−4 Scm−1 by 1 wt% addition of Li4SiO4. Nataly et al. [15] also have observed that addition of 4 wt% of Li2O:B2O3:SiO2:CaO:Al2O3 (LBSCA) glass in Nb doped Li7La3Zr2O12 gives conductivity of 0.8 × 10−4 Scm−1 at room temperature by liquid phase sintering. Li3BO3 added Li7La3Zr2O12 gives 86.4% relative density by sintering at 1100 °C for 8 h [16]. Il'ina E.A. and group [17] has reported that, 3 wt% addition of 65Li2O:27B2O3:8SiO2 in Li7La3Zr2O12 has increased conductivity by two orders of magnitude. Masashi et al. [18] have reported that addition of Al2O3 in Li7La3Zr2O12 lowers the sintering temperature by 230 °C by suppressing the formation of La2Zr2O7 impurity phase with comparable value of ionic conductivity. In the present study we have synthesized highest conducting Sb doped sample Li6.6La3Zr1.6Sb0.4O12 [19] and investigated the effect of addition of LiAlO2 on its density, structure and ionic conductivity where LiAlO2 is used as sintering aid which lowers the sintering temperature as well as acts as conducting medium for Li ion which helps to reduce grain boundary resistance. 2. Experimental 2.1. Material preparation LiAlO2 was synthesized by adding Li2CO3 (Merck) in Al2O3 (Fluka) in an appropriate molar ratio. Mixing was done using agate mortar pestle and then mixed powder was heated at 900 °C for 6 h in alumina
Corresponding author. E-mail address: [email protected] (A.V. Deshpande).
https://doi.org/10.1016/j.ssi.2019.115185 Received 29 August 2019; Received in revised form 3 December 2019; Accepted 9 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
3.2. Density measurement
crucible. A conventional solid state reaction method was used for the preparation of Li6.6La3Zr1.6Sb0.4O12 in which chemicals of high purity Li2CO3 (Merck), La2O3 (Sigma Aldrich), ZrO2 (Merck) and Sb2O3 (Merck) were used. All powders were mixed in Stoichiometric amounts in agate mortar. To suppress the lithium losses during the heating process, 10 wt% of excess Li as Li2CO3 was added. The mixture was heated in an open alumina crucible at 900 °C for 6 h and then cooled to room temperature. Calcined powder was then mixed with 0.5, 1 and 1.5 wt% of LiAlO2 and uniaxially pressed with a hydraulic press into pellets having dimensions of 1.5 mm thickness and 10 mm diameter. Closed alumina crucible containing mother powder was used to sinter pressed pellets at 1050 °C for 8 h.
Archimedes' principle was used to measure density of all ceramics where toluene acted as immersion medium. The bulk density and relative density values of ceramics are given in Table 2. From the table it can be observed that with increasing LiAlO2 up to 1 wt%, the bulk density and relative density increases. The density has been found to be maximum for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. This can be understood as the LiAlO2 acts as sintering aid and helps to hold the particles of Li6.6La3Zr1.6Sb0.4O12 together. Density of 1.5 wt% LiAlO2 was observed to be decreasing as lithium aluminate phase has grown enough in the pellet since the density of the LiAlO2 phase is 2.62 g/cc which is lesser than parent phase [21]. 3.3. Morphological studies
2.2. Characterization
Fig. 3(a, b, c and d) shows typical cross sectional SEM images of LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 samples. Fig. 3(a) shows that the grains are having pores in between them. The presence of such pores surely obstructs the diffusion of Li+ ions over the grain boundaries. From Fig. 3(b), (c) and (d) it can be seen that with addition of LiAlO2 the microstructure becomes denser as compared to pure (without addition of LiAlO2), grains are in contact with each other as LiAlO2 phase has grown within the grain boundary area. Fig. 3(c) exhibits the growth of grains with regular size, which are spherical in shape to some extent in 1 wt% LiAlO2 added sample as compared to 0, 0.5 and 1.5 wt% addition of LiAlO2. Fig. 3(d) shows that particles have grown in irregular sizes which might be one of the reasons of decrease in conductivity. Fig. 4 shows elemental mapping of 1 wt% LiAlO2 added fractured pellet where grain boundaries can be seen significantly. La, Zr, Sb and O elements have been observed to be uniformly dispersed over the entire specimen as seen in figure which are main components of Li6.6La3Zr1.6Sb0.4O12. Whereas Al is distributed distinctly in grain boundaries and more intensity has been observed for O in grain boundaries which suggests that formation of Li rich LiAlO2 phase has taken place. Such ceramic additives can enhance the Li ion conductivity by stabilizing the amorphous region within grain boundaries [22]. From the SEM elemental mapping it can be observed that LiAlO2 resides in grain boundary eventually helps to facilitate migration of Li+ ions in between grains. LiAlO2 additive functions as an adhesive to bind the grains within the sample. LiAlO2 also acts as sintering aid which helps to lower down sintering temperature, which eventually helps to control loss of lithium, which generally occurs at high temperature such as 1230 °C. It assists in the formation of pure cubic phase at temperatures about 1050 °C and also helps to inhibit the formation of other unreacted phases within the sample. The average size of particle is found to be 11.13 μm for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramic sample.
To investigate the obtained phase, pellets sintered at 1050 °C were characterized by X-ray Diffraction using Bruker AXS D8 Advance with step size of 0.019 degrees. Diffraction data were collected in the range from 10 to 100 of 2theta with 29.1 s scan step. For the calculation of density of prepared samples Archimedes' principle was employed using K-15 Classic (K-Roy) where toluene is used as immersion medium. Microstructural analysis and composition determination were done by using scanning electron microscope (JSM-7600 F/JEOL) with EDX. To determine conductivity, Novocontrol impedance analyzer was used to measure impedance and modulus in the frequency range of 20 Hz to 20 MHz from 180 °C to 280 °C in an air furnace with 20 °C interval. Before measurement pellets were polished with SiC paper. For plating both surfaces of the sintered pellets, silver paste was used and plated pellets having approximately 1.5 mm thickness and 10 mm diameter were held between silver electrodes (as Li+ blocking electrodes) for AC and DC conductivity measurements. For transport number measurement, the sample holder arrangement was used as that for conductivity measurements having silver electrodes as irreversible electrodes. A DC potential of 1 V was applied and the current through the sample was measured as function of time with digital nanoammeter. AC conductivity which equals the total conductivity (σac = σe + σi) has been considered as 0 time conductivity. 3. Results and discussion 3.1. X-ray diffraction Fig. 1(a) shows the recorded Powder X-ray diffraction spectra of pure and 0.5, 1.0 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics. The XRD peaks show good agreement with the JCPDS file no. 45–109. XRD peaks were indexed to the garnet-type cubic structure and along with main phase, there are no other reflections from LiAlO2 and its derivable phases have been observed which therefore suggests the most of the raw materials were utilized. The Rietveld refinement of sample was performed using Fullprof software and fitted profile has been shown in Fig. 2. The crystal structure and structural parameters of 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramic was refined by Rietveld fitting to cubic symmetry of space group Ia-3d (230) as shown in Table 1 with following agreement factors Rp:8.81%, Rwp:11.3%, Rexp:8.21 and χ2:1.89. The profile fitted peak positions were used to calculate and refine lattice constant. It was observed that lattice constant a: 12.9282(Å) which shows good agreement with earlier reported value [4]. With addition of LiAlO2 the peak positions are shifting towards higher 2θ values as shown in Fig. 1(b). This may be because as Al content increases, lattice constant decreases as shown in Table 2. This can be attributed to higher amount of Al from LiAlO2 could have occupied the lattice since Al3+ is having smaller ionic radius than Li + which shows the shrinking effect on lattice constant [20].
3.4. Conductivity studies 3.4.1. Nyquist plots Electrical conductivity measurements were carried out in frequency range from 20 Hz to 20 MHz. Complex impedance plots for all LiAlO2 added samples at 25 °C have been shown in Fig. 5. Fig. 6 shows the fitted curves for 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 using R-CPE-W circuit model. The resistance value (R) is obtained from intercept made by a semicircle on z’ axis in the region of high frequency. The Nyquist plots consist of one semicircle which may due to merging of semicircles due to grain and grain boundary at high frequency. In low frequency region advent of tail is because of ion blocking behavior of Ag electrodes used during measurement of ionic conductivity. The formula σtotal = D (SR) −1 was used to calculate the ionic conductivity, where σtotal is ionic conductivity, D is the thickness of the sample, R is the resistance of the sample and S is area of the electrode. It can be observed from plots that 1 wt% LiAlO2 2
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 1. X-ray diffraction patterns of a) Li6.6La3Zr1.6Sb0.4O12 sintered with LiAlO2 addition, b) shift in peak.
Fig. 2. The powder X-ray Refinement profile of Li6.6La3Zr1.6Sb0.4O12. Table 1 Structural parameters of Li6.6La3Zr1.6Sb0.4O12 garnet from Rietveld refinement of X-ray diffraction data. Atom
Wyck. Site
Occupancy
x
y
z
Biso
Li1 Li2 Zr La O Sb
24d 96 h 16a 24c 96h 16a
0.503 0.609 0.167 0.249 1.000 0.007
0.12 0.094 0.000 0.000 −0.0304 0.000
0.000 0.732 0.000 0.250 0.056 0.000
0.250 0.589 0.000 0.125 0.146 0.000
0.237 0.579 0.529 0.584 0.829 0.010
Table 2 Density and Lattice constant of LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics. Wt% LiAlO2
Density (g·cm−3)
Relative density (%)
Lattice constant
0 0.5 1 1.5
4.48 4.57 4.64 4.49
86.15 87.88 89.23 86.34
12.9307 12.9288 12.9282 12.9155
intercepts towards the low frequency region. Although LiAlO2 is having lithium ionic conductivity which is definitely lower than Li6.6La3Zr1.6Sb0.4O12, it acts as a good promoter for lithium ion conduction. The addition of LiAlO2 in composites has shown effect of increasing ionic conductivity [23]. From Fig. 4 it can be observed that lighter region has grown in grain boundaries which is enriched with Al and some intense spot of O also can be seen hence there is strong probability of accumulation of LiAlO2 which is itself a conductive phase
added Li6.6La3Zr1.6Sb0.4O12 ceramic has minimum intercept on z’ axis which shows maximum conductivity of 3.16 × 10−4 Scm−1 among all ceramic samples. This can be ascribed to uniformity in grain size, good connection between the grains as indicated in SEM images of the sample and higher density value. Increase in wt% LiAlO2 shows shift in 3
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 3. SEM morphology of Li6.6La3Zr1.6Sb0.4O12 ceramic with a) 0 wt% LiAlO2 b) 0.5 wt% LiAlO2 c) 1 wt% LiAlO2 and d) 1.5 wt% LiAlO2.
Fig. 4. Elemental mapping of 1 wt% added Li6.6La3Zr1.6Sb0.4O12.
which ultimately helps for migration of Li ions across grain boundary. Similar results have been reported earlier [23].
3.4.2. Arrhenius plots Fig. 7 shows the Arrhenius plots for Li6.6La3Zr1.6Sb0.4O12 added with 0, 0.5, 1 and 1.5 wt% of LiAlO2 heat treated at 1050 °C. 4
Solid State Ionics 345 (2020) 115185
P. Wakudkar and A.V. Deshpande
Fig. 5. Impedance plots for LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics at 25 °C.
Fig. 6. Fitted curves for LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics at 25 °C.
Fig. 8 shows the variation of conductivity at 25 °C and activation energy with wt% of LiAlO2. The ionic conductivity at 25 °C and activation energy for all the ceramic samples has been tabulated in Table 3. The maximum conductivity and minimum activation energy have been found for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. Increase in conductivity by addition of LiAlO2 can be attributed to LiAlO2 phase grown
Measurements were carried in a range of temperature from 180 °C–280 °C. The plots using obtained data show the nature of Ar-
( ) −Ea
rhenius equation i.e. σ (T ) = σOe kB T where Ea is activation energy, σ is the conductivity, kB is the Boltzmann constant, σo is the pre-exponential factor and T is temperature in Kelvin. It is observed that 1 wt% of LiAlO2 addition has the lowest activation energy i.e. 0.33 eV. 5
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P. Wakudkar and A.V. Deshpande
Fig. 9. Variation of DC conductivity with time for transport number determination. Fig. 7. Ionic conductivity as function of temperature for 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics.
conducting phase which comprises of LiO4 and AlO4 tetrahedra where Li and Al atoms are located in oxygen tetrahedron. In these pairs each corner is shared by two further tetrahedral. Li ion diffusion in this structure may take place via two types of channels formed by octahedral voids in structure, one is along 〈100〉 direction and another is along 〈001〉 [single crystal neutron diffraction]. Ionic conductivity in LiAlO2 is due to the introduction of structural disorder emerging due to point defects. According to density functional theory Li ion can migrate via Li point defect and Frenkel defect between two tetrahedral, and Li conductivity is strongly influenced by distribution of Li vacancy and interstitial lithium in the structure [25]. LiAlO2 coating also affects cycle life and discharge capacity of LiNi1/3Co1/3 Mn1/3O2 due to its characteristic as fast lithium ionic conductor [26]. Secondly Al from LiAlO2 might have entered in the lattice which is supported by decrease in lattice constant as discussed in XRD results. On the basis of charge neutrality, Al3+ should substitute for 3Li+. In this case it replaces one Li+ and creates two vacancies hence increasing Li vacancy concentration eventually increasing the conductivity of the sample [10]. Fig. 9 shows dc polarization plots for pure and 1 wt% LiAlO2 added ceramic samples. Ion transport number was calculated using relation, ti = (σtotal − σe) / σtotal. Ionic transport number has been found to be > 0.999 for 0 wt% and 1 wt% LiAlO2 added samples, which confirms that the conduction is due to ions only.
Fig. 8. Variation of conductivity of Li6.6La3Zr1.6Sb0.4O12 with wt% LiAlO2 at 25 °C and activation energy.
3.4.3. Modulus spectra study The electric modulus study has been done by many researchers to analyze and interpret electrical relaxation data [27]. This formalism highlights on charge transport processes like mechanism of ion dynamics and conductivity relaxation as a function of frequency and temperature. The electric modulus M∗ is defined as
Table 3 Ionic conductivity of LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 ceramics. Wt% LiAlO2 0 0.5 1 1.5
σ (Scm−1) 7.08 2.04 3.16 2.75
× × × ×
Ea −05
10 10−04 10−04 10−04
(σ)
(eV)
0.47 0.41 0.33 0.37
M ∗ = 1/ϵ∗ = (ϵ′ − jϵ ″ )/|ϵ∗|2 = M′ + jM″ where M′ and M″ are real and imaginary parts of the complex modulus M∗ respectively and ϵ′ and ϵ″ are real and imaginary parts of complex permittivity ϵ∗ respectively. Fig. 10(a) shows the variation of M″ as a function of frequency at different temperatures for 1 wt% added LiAlO2 in Li6.6La3Zr1.6Sb0.4O12. The existence of a peak in the modulus spectrum represents the conductivity relaxation [19,28]. The frequency region below peak M″ indicates the range where charge carriers are mobile on long distances
over the grain boundaries which serves as a Li conduction medium to facilitate the hopping of Li+ ions in the boundary sites which can be supported by elemental mapping images. Similar results have also been reported by Rong Lan [24] where increase in conductivity is attributed to Li conducting phase available in grain boundary. LiAlO2 itself is a 6
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Fig. 10. (a) Imaginary part of electric modulus as a function of frequency and temperature for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. Fig. 10 (b) Modulus scaling behavior of 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12. Fig. 10 (c) Imaginary part of electric modulus as a function of frequency for 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 at 25 °C. Fig. 10 (d) Modulus scaling behavior of 0, 0.5, 1 and 1.5 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12 25 °C. Fig. 10 (e) Variation of relaxation time with temperature for 1 wt% LiAlO2 added Li6.6La3Zr1.6Sb0.4O12.
indicates that the conduction mechanism is nearly temperature independent. Fig. 10(c) shows the variation of M" as a function of frequency for 0, 0.5, 1 and 1.5 wt% added LiAlO2 in Li6.6La3Zr1.6Sb0.4O12. The nonoverlapping of curves of M″/Mmax″ vs f/fmax in Fig. 10(d)
and frequency region above peak M″ shows charge carriers being mobile on short distances. The frequency associated with Mmax″ gives the relaxation time τ. The Fig. 10(b) depicts plots of M″/Mmax″ vs f/fmax. The figure shows the overlap of curves for all temperatures, which
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suggests the conduction phenomenon is composition dependent which indicates that Al might have entered into the sample leading to change in its composition. Fig. 10(e) shows the variation of relaxation time as a function of temperature for 1 wt% added LiAlO2. Relaxation time plots follow Arrhenius equation τ = τoexp(Ea/kBT), where τo is the pre-exponential factor, Ea is activation energy, kB is the Boltzmann constant and T is temperature in Kelvin. The activation energy Ea(τ) and Ea(σ) are found to be 0.32 eV and 0.33 eV for 1 wt% of LiAlO2 addition respectively. The values of Ea(τ) and Ea(σ) are nearly equal which suggests that both conduction and relaxation processes are due to the same type of charge carriers.
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4. Conclusions Li6.6La3Zr1.6Sb0.4O12 cubic garnet solid electrolyte has been prepared by the solid state reaction method. Phase formation was confirmed by XRD. Density measurements were carried out and microstructural properties were studied by SEM. Study of elemental mapping images showed that Al has been distributed in grain boundaries. Addition of LiAlO2 enhances the ionic conductivity by half an order of magnitude. This has been attributed to LiAlO2 phase grown over the grain boundaries which serves as a Li conductive medium to facilitate the hopping of lithium ions in the boundary region. Modulus analysis carried out over a wide frequency range revealed that both conduction and relaxation processes are due to the same type of charge carriers. 1 wt% addition of LiAlO2 in Li6.6La3Zr1.6Sb0.4O12 has ionic conductivity of 3.16 × 10−4 Scm−1. Addition of LiAlO2 also reduces the sintering temperature by 230 °C to form a cubic phase and thus acts as sintering aid. Declaration of competing interest None. References [1] R. Shin, S. Ick, Y. Soo, Y. Do, H. Kim, S. Ryu, W. Pan, Sintering behavior of garnettype Li7La3Zr2O12-Li3BO3 composite solid electrolytes for all-solid-state lithium batteries, Solid State Ionics 301 (2017) 10–14, https://doi.org/10.1016/j.ssi.2017. 01.005. [2] M.I, T.N.Y. Inaguma, C. Liquan, High ionic conductivity in lithium lanthanum titanate, Solid State Commun. 86 (1993) 689–693. [3] R. Kanno, T. Hata, Y. Kawamoto, M. Irie, Synthesis of a new lithium ionic conductor, thio-LISICON – lithium germanium sulfide system, 130 (2000) 97–104. [4] R. Murugan, V. Thangadurai, W. Weppner, Fast lithium ion conduction in garnettype Li7La3Zr2O12, Angew. Chemie - Int. Ed. 46 (2007) 7778–7781, https://doi.org/ 10.1002/anie.200701144. [5] L. La, O. Zr, L.O.B.O. Sio, E.A. Il, A.A. Raskovalov, N.S. Saetova, B.D. Antonov, O.G. Reznitskikh, Composite electrolyte Li7La3Zr2O12-glassy Li2O-B2O3-SiO2, 296 (2016) 26–30, https://doi.org/10.1016/j.ssi.2016.09.003. [6] L. Zr, Effects of penta- and trivalent dopants on structure and conductivity, 274 (2015) 100–105, https://doi.org/10.1016/j.ssi.2015.03.019. [7] L. Zr, R. Chen, M. Huang, W. Huang, Y. Shen, Y. Lin, C. Nan, Effect of calcining and Al doping on structure and conductivity, Solid State Ionics 265 (2014) 7–12, https://doi.org/10.1016/j.ssi.2014.07.004.
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