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Article

Comparative Study of Stability against Moisture for Solid Garnet Electrolytes with Different Dopants

by
Li Huang
1,
Jian Gao
2,
Zhijie Bi
1,*,
Ning Zhao
1,
Jipeng Wu
3,4,
Qiu Fang
3,4,
Xuefeng Wang
3,4,
Yong Wan
1,* and
Xiangxin Guo
1,*
1
College of Physics, Qingdao University, Qingdao 266071, China
2
Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
3
Laboratory for Advanced Materials & Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
4
College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
*
Authors to whom correspondence should be addressed.
Energies 2022, 15(9), 3206; https://doi.org/10.3390/en15093206
Submission received: 27 March 2022 / Revised: 17 April 2022 / Accepted: 25 April 2022 / Published: 27 April 2022
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Abstract

:
The cubic garnet Li7La3Zr2O12 (c-LLZO) is one of the most promising solid electrolytes due to its high ionic conductivity and large electrochemical window. However, the critical issue of Li2CO3 formation on the c-LLZO surface when exposed to air is problematic, which is detrimental to the ionic conductivity and storage. Herein, comparative studies were carried out on the air stability of Al-doped Li7La3Zr2O12 (Al-LLZO), Al-Ta-doped Li7La3Zr2O12 (Al-LLZTO), and Al-Nb-doped Li7La3Zr2O12 (Al-LLZNO). It was found that Al-LLZTO and Al-LLZNO are less reactive with air than Al-LLZO. The morphology of Li2CO3 on Al-LLZTO micro-sized powders after air exposure was island-like with ~1.5 μm in thickness. The interfacial resistance of Li/Al-LLZTO was also a factor of ~3 smaller than that of Li/Al-LLZO, leading to the improved cycle stability of Li/Al-LLZTO/Li symmetric cells. The first-principles calculations based on density functional theory (DFT) verified that the decomposition energy of Al-LLZTO was larger than that of Al-LLZO, inhibiting the reaction product of Li2O and, thus, the next step product of Li2CO3 following the reactions of Li2O + H2O → LiOH and LiOH + CO2 → Li2CO3.

1. Introduction

With the rapid development of electric vehicles, the requirements for energy density and safety of lithium-ion batteries (LIBs) have become critical [1,2,3]. Currently, commercial LIBs generally use flammable organic liquid electrolytes, which may cause accidents when the thermal runaway loses control [4,5,6]. Under this context, the replacement of liquid electrolytes with solid-state electrolytes (SSEs) is viewed as a powerful means of overcoming safety concerns [7,8]. Among various solid electrolytes, the cubic Li7La3Zr2O12 (LLZO) has attracted extensive attention due to its relatively high ionic conductivity, wide electrochemical window, and good mechanical strength [9,10,11,12]. However, subsequent experimental and computational studies have demonstrated that LLZO tends to react with the humidity in air, resulting in the generation of surface Li2CO3 impurities, which leads to low ionic conductivity, crystal phase decomposition, poor electrochemical stability, poor contact with Li metal, and increased resistance of the garnet surface [13,14,15]. Previous reports indicated that Li+/H+ exchange occurred on the garnet surface when in contact with air [16,17]. The water first reacted with LLZO to form LiOH either as a surface layer or as a precipitate in solution when immersed in water [18]. The formed LiOH then subsequently reacted with CO2 in air to form surface Li2CO3, the amount of which increased with exposure time, seriously deteriorating the ionic conductivity of garnet electrolytes [19]. Furthermore, other studies have also reported a single-step reaction pathway, where the garnet directly reacted with water and CO2 to form Li2CO3 [20,21,22]. Various strategies have been developed to remove the Li2CO3. For example, acid etching is proposed for its high efficiency in removing Li2CO3 from the LLZO surface, but the side reaction of Li+/H+ exchange is hard to avoid [23]. As the garnet electrolytes are used in solid-state batteries, such an inert carbonate contaminant induces a great increase in interfacial resistance at both cathode and anode sides, leading to performance decline and dendrite growth [24,25,26,27,28]. Therefore, the improvement of air stability for garnet electrolytes is important for electrolyte storage and battery performance [29].
Herein, we investigated the air stability of three different element-doped LLZO, i.e., Al-doped LLZO (Li6.25Al0.25La3Zr2O12, Al-LLZO), Al–Ta-doped LLZO (Li5.65Al0.25La3Zr1.4Ta0.6O12, Al-LLZTO), and Al–Nb-doped LLZO (Li5.625Al0.25La3Zr1.375Nb0.625O12, Al-LLZNO). The garnet surfaces were investigated after air exposure for 24 and 72 h. It was found that the growth rate of Li2CO3 on Al-LLZTO and Al-LLZNO was much slower than that on Al-LLZO. The theoretical study indicated that the decomposition product Li2O could evolve into Li2CO3 when in contact with CO2 and H2O. As a result, the decomposition reactions for Al-LLZO, Al-LLZTO, and Al-LLZNO into product Li2O were calculated through density functional theory (DFT) to account for the stability. It was demonstrated that Al–Ta- and Al–Nb-doped LLZOs showed a higher formation energy of Li2O than Al-LLZOs, resulting in a slower Li2CO3 formation rate.

2. Materials and Methods

Cubic-phase Li6.25Al0.25La3Zr2O12 (Al-LLZO), Li5.65Al0.25La3Zr1.4Ta0.6O12 (Al-LLZTO), and Li5.625Al0.25La3Zr1.375Nb0.625O12 (Al-LLZNO) ceramics were prepared via a conventional solid-state reaction in Al2O3 crucibles. The Al dopants were diffused from the crucibles during sinters. The stoichiometric LiOH, La(OH)3 (Alfa Aesar, 99.95%), ZrO2, Nb2O5, and Ta2O5 (Aladdin Reagent, 99.99%) were uniformly mixed, and a 15 wt.% excess of LiOH was adopted to compensate for volatile Li components during synthesis. Then, the uniformly mixed powders were heated in air at 950 °C for 12 h to ensure the formation of cubic-phase garnets. After that, the powders were pressed into pellets with a diameter of 12 mm at 100 MPa, and then sintered at 1140 °C for 9 h in air. The Al-LLZO, Al-LLZTO, and Al-LLZNO pellets and the corresponding powders were exposed to air with a humidity of 70% and temperature of 25 °C for 24 and 72 h for Li2CO3 analysis.
The elemental composition was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) [30]. X-ray diffraction (XRD, Bruker D8 Advance) was used to detect the phase structures of samples with Cu Kα1 radiation (λ = 1.5406 Å) with a 2θ range of 10–80° and a step size of 0.02°. The cross-sectional morphology of the ceramic pellets was analyzed by a focused ion beam scanning electron microscope (FIB-SEM, Thermo Scientific, Scios2 DualBeam). The Scios 2 DualBeam microscope was equipped with an ion beam for sample cutting and an electron beam for morphology observation, allowing the direct observation of sample morphology after ion beam cutting. Raman spectroscopy (WITec Alpha 300) with an excitation source of 532 nm was used to collect spectra of samples with a 50× objective lens. The Fourier-transform infrared (FTIR) spectra were recorded using a NETZSCH X70 FTIR spectrometer. Titration gas chromatography (TGC) [31,32] was carried out as follows: sample powders of 50 mg were weighed in an Ar-filled glovebox (H2O < 0.1 ppm, O2 < 0.1 ppm) and reacted in a closed sample container (42 mL) using an excess of dilute hydrochloric acid for 20 min, until no bubbles were observed in the bottle. CO2 in the sample bottle was measured by gas chromatography (Shimadzu Nexis GC-2030) using He (99.9999%) as the carrier gas. A gastight syringe was used to transfer 50 μL of sample gas from the container into the gas chromatography (GC) system for every measurement. The content of Li2CO3 was determined by converting the corresponding CO2 amount according to a pre-established standard calibration curve.
The electrochemical impedance spectra (EIS) of Li symmetric cells were recorded using a Princeton electrochemical workstation at a frequency range from 7 MHz to 0.1 Hz at 55 °C. The Li symmetric cells were cycled under a current density of 0.2 mA·cm−2 at 55 °C. Considering the stable contact between garnets and Li metals, polyoxyethylene (PEO)-based buffer layers were introduced at the Li/garnet interface. PEO, lithium (bis trifluoromethyl) sulfate (LiTFSI), and succinonitrile (SCN) with a mass ratio of 2:1:0.4 were added to anhydrous acetonitrile (ACN) solution (the mass ratio of ACN to total solution is 80%) and continuously stirred for 12 h. Then, 60 μL of PEO slurry was introduced into the garnet surface, followed by heat treatment at 80 °C for 30 min to evaporate the solvent in the slurry, and the Li foil was tightly pressed on the garnet with PEO electrolyte on the surface. An identical process was applied to both sides of the garnet electrolyte. Finally, the cells were sealed in a Swagelok-type cell mold in an Ar-filled glovebox. All electrochemical measurements were conducted at 55 °C.
First-principles calculations were performed using density functional theory (DFT), implemented in the PWmat package using a GPU [33,34]. For the treatment of the exchange-correlation potential, the generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) functional [35] was adopted, and the NCPP-SG15-PBE pseudopotential [36,37] was used in the geometry optimization, with a force tolerance for the maximal residual force of 0.02 eV·Å−1 and an energy between two successive steps smaller than 2.7 × 10−6 eV as the convergence criteria. A plane wave basis set with a cutoff of 70 Ryd was used. The Brillouin zone was sampled using a 1 × 1 × 1 Monkhorst–Pack k-point grid.

3. Results and Discussion

Figure 1 shows the XRD patterns of the Al-LLZO, Al-LLZTO, and Al-LLZNO with different hours of air exposure. Figure 1a shows that Li2CO3 appeared on Al-LLZO surfaces, the intensity of which increased with an increase in time of air exposure, due to the gradually increasing amount of Li2CO3 [38]. On the contrary, XRD patterns of the Al-LLZTO and Al-LLZNO showed a negligible intensity of Li2CO3 even after 72 h exposure (Figure 1b,c). The XRD results indicate that Al-LLZO surfaces had a larger amount of Li2CO3 than Al-LLZTO and Al-LLZNO surfaces.
It is well known that the Li2CO3 formed on the garnet surface is generally not very thick, which leads to a weak peak intensity in XRD patterns. Due to the low thickness of surface Li2CO3 on the garnet matrix, the peak intensity of Li2CO3 is weak and unobvious to distinguish. The observable Li2CO3 peak for Al-LLZO implies that the Al-LLZO surfaces had a large amount of Li2CO3. No observable Li2CO3 peak for Al-LLZTO and Al-LLZNO does not indicated the absence of Li2CO3 on the surface. As revealed by focused ion beam scanning electron microscopy (FIB-SEM), shown in Figure 2a, the formed Li2CO3 is not continuous but island-like on the Al-LLZTO surface after 72 h of exposure, compared with the pristine SEM image of Al-LLZTO (inset of Figure 2a). The maximum island was approximately 1.5 μm in size. Furthermore, as can be seen in Figure 2b, titration gas chromatography (TGC) analysis indicated that the growth of Li2CO3 on the Al-LLZTO surface occurred in two stages. Within the initial 24 h, the growth was fast with a rate of 0.053 mg/h. Then, the growth slowed down with a saturation tendency, the rate of which was approximately 0.007 mg/h. Initially, the pristine surface of garnet was fully exposed to the air, leading to a rapid reaction with H2O/CO2 in the air. Li2CO3 was quickly generated during this initial process, as indicated in Figure 2b. Afterward, the surface sites of garnet were gradually covered by Li2CO3, leading to reduced contact area with air, thereby decreasing the generation rate of Li2CO3. Thus, a decreased slope of Li2CO3 amount vs. time was detected during the middle process. Finally, when the garnet surface was fully covered by Li2CO3, the growth of Li2CO3 reached a saturation state.
Raman spectra were also used to identify Li2CO3, as displayed in Figure 3. The peaks located at 243, 375, 645, and 728 cm−1 denote the characteristic peaks of the cubic garnet phase, which were present for all the three samples [14,39,40,41]. The broad Raman shift around 1100 cm−1 attributed to Li2CO3 was detected in Al-LLZO after 24 and 72 h of air exposure. However, no Li2CO3 could be found in Al-LLZTO and Al-LLZNO after 24 h of exposure. The Li2CO3 peaks were almost absent after 72 h of exposure for both Al-LLZTO and Al-LLZNO, as shown in Figure 3b,c.
Figure 4 shows the FTIR spectra of Al-LLZO, Al-LLZTO, and Al-LLZNO with different exposure time. The Vas (C=O) at 1438 and 863 cm−1 corresponds to the characteristic peaks of Li2CO3 [42,43,44]. The two peaks with large intensity for Al-LLZO confirmed the obvious Li2CO3 formation upon exposure to air for 24 h and 72 h. The peaks of Li2CO3 with weak intensity shown in Figure 4b,c indicate that Al-LLZTO and Al-LLZNO contained a smaller amount of Li2CO3 after exposure to air compared to Al-LLZO.
To investigate the effect of Li2CO3 on interfacial resistance, Li symmetric cells with 24 h air-exposed pellets were assembled. The electrochemical impedance spectra (EIS) of Li symmetric cells are plotted in Figure 5. As can be seen in Figure 5a,b, all the fitted EIS lines consisted of two half-circles in the measuring ranges. The high-frequency half-circle corresponds to the resistance of bulk impedance, while the medium-frequency half-circle corresponds to the interfacial resistance. Considering the charge transfer across two Li/LLZO interfaces in each symmetric cell, the interfacial resistance determined by the half-circle should be divided by 2 to obtain the value for each Li/LLZO interface. The fitted results of interfacial resistance are listed in Figure 5c. After air exposure for 24 h, the interfacial area resistance of Li/Al-LLZO greatly increased from 160 Ω·cm2 to 836 Ω·cm2, which was attributed to the blocking effect of Li2CO3 coverage [45]. For Al-LLZTO and Al-LLZNO, the interfacial area resistance also increased after 24 h of air exposure. The specific values for the three samples are given in Figure 5c. The interfacial resistance for Al-LLZO increased by a factor of approximately 5 after 24 h exposure, while that for Al-LLZTO and Al-LLZNO increased by a factor of 2–3. The galvanostatic cycles of Li symmetric cells for the three types of samples are shown in Figure 5d–f. It can be seen that Al-LLZO after 24 h exposure showed a larger potential increase during cycles than Al-LLZTO and Al-LLZNO. This is consistent with the above conclusions drawn from XRD, Raman, and FTIR analyses, i.e., the amount of formed Li2CO3 for Al-LLZO was larger than that for Al-LLZTO and Al-LLZNO.
In order to deeply understand the mechanism of Li2CO3 formation, density functional theory (DFT) was further used to calculate the decomposition energy of Al-LLZO, Al-LLZTO, and Al-LLZNO, referring to the decomposition product of Li2O [46]. Here, Li5.625Al0.25La3Zr1.375Ta0.625O12 was adopted instead of experimental Li5.65Al0.25La3Zr1.4Ta0.6O12 in consideration of the eight times supercell and the integer number of atoms. The decomposed Li2O reacts with H2O and CO2 in air to form surface Li2CO3. The energies of reactants and products are shown in Table 1, and the three decomposition reaction energies are presented in Table 2. Considering the partial substitution of Li+ by introducing 0.25/f.u. (formula units) Al3+ in LLZO, the decomposition energy of Li6.25Al0.25La3Zr2O12 is −19.89 meV/atom, indicating the preferred decomposition trend and the intensive Li2CO3 formation [47]. The decomposition is significantly promoted in the existence of H2O and CO2 with the final formation of Li2CO3. In comparison, when theoretically substituting 0.625/f.u. Ta5+ and Nb5+ in LLZO for Zr4+, the cell parameters were close to the cubic phase, with the parameters shown in Table 3. In addition, an increment of 11.08 and 11.07 meV/atom were required for the decomposition reaction, indicating the better thermodynamic stability of Al-LLZTO and Al-LLZNO than Al-LLZO. In fact, the former samples were thermodynamically stable in the absence of CO2 and H2O. This is highly in agreement with the experimental results.

4. Conclusions

Differently doped garnets Al-LLZO, Al-LLZTO, and Al-LLZNO were prepared to investigate their air stabilities. The results demonstrated that there was less Li2CO3 on the surface of Al-LLZTO and Al-LLZNO than on Al-LLZO after air exposure. Titration gas chromatography (TGC) analysis showed that the growth of Li2CO3 on the Al-LLZTO surface occurred in two stages: a fast growth stage and a slow growth stage with a tendency to saturation. FIB-SEM indicated Li2CO3 grew in an island-like manner on the garnet surfaces. In theory, the decomposition energies of Al-, Ta-, and Nb-doped LLZOs were calculated, with the product of Li2O as the cause of Li2CO3 formation. The negative decomposition energy of Al-LLZO indicated that it is unstable and tends to decompose compared with the positive decomposition energy of Al-LLZTO and Al-LLZO. Therefore, the development of such doped electrolytes is one step closer to the practical application of lithium metal batteries and offers prospects and directions for solid-state batteries.

Author Contributions

Conceptualization, X.G. and Y.W.; methodology, Z.B.; software, J.G.; resources, X.W., N.Z., J.W. and Q.F.; data curation, L.H.; writing—original draft preparation, L.H.; writing—review and editing, X.G. and Z.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (U1932205, 22005163, 52002197, 22109005), the Key R&D Program of Shandong Province (Grant No. 2021CXGC010401), the Taishan Scholars Program (ts201712035), and the Project of Qingdao Talents in Entrepreneurship and Innovation (193210zhc).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 15 03206 g001 550
Figure 1. XRD patterns of (a) Al-LLZO, (b) Al-LLZTO, and (c) Al-LLZNO before and after 24 and 72 h of air exposure.
Figure 1. XRD patterns of (a) Al-LLZO, (b) Al-LLZTO, and (c) Al-LLZNO before and after 24 and 72 h of air exposure.
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Figure 2. (a) The cross-sectional SEM image in combination with FIB for Al-LLZTO. The inset is the polished pristine SEM image of Al-LLZTO. (b) Quantitative analysis of Li2CO3 based on TGC technique for micro-sized Al-LLZTO.
Figure 2. (a) The cross-sectional SEM image in combination with FIB for Al-LLZTO. The inset is the polished pristine SEM image of Al-LLZTO. (b) Quantitative analysis of Li2CO3 based on TGC technique for micro-sized Al-LLZTO.
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Figure 3. Raman spectra of as-prepared, 24 h air-exposed, and 72 h air-exposed (a) Al-LLZO, (b) Al-LLZTO, and (c) Al-LLZNO.
Figure 3. Raman spectra of as-prepared, 24 h air-exposed, and 72 h air-exposed (a) Al-LLZO, (b) Al-LLZTO, and (c) Al-LLZNO.
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Figure 4. FTIR spectra of as-prepared, 24 h air-exposed, and 72 h air-exposed (a) Al-LLZO, (b) Al-LLZTO, and (c) Al-LLZNO.
Figure 4. FTIR spectra of as-prepared, 24 h air-exposed, and 72 h air-exposed (a) Al-LLZO, (b) Al-LLZTO, and (c) Al-LLZNO.
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Figure 5. Comparison of EIS profiles of Li symmetric cells with (a) Al-LLZO and (b) Al-LLZTO at 55 °C. (c) The interfacial resistances of the Li/LLZO/Li symmetric cells with Al-LLZO, Al-LLZTO, and Al-LLZNO. The galvanostatic cycles of Li symmetric cells at a current density of 0.2 mA·cm−2 and a capacity of 0.1 mA·h·cm−2 for as-prepared and 24 h air-exposed (d) Al-LLZO, (e) Al-LLZTO, and (f) Al-LLZNO.
Figure 5. Comparison of EIS profiles of Li symmetric cells with (a) Al-LLZO and (b) Al-LLZTO at 55 °C. (c) The interfacial resistances of the Li/LLZO/Li symmetric cells with Al-LLZO, Al-LLZTO, and Al-LLZNO. The galvanostatic cycles of Li symmetric cells at a current density of 0.2 mA·cm−2 and a capacity of 0.1 mA·h·cm−2 for as-prepared and 24 h air-exposed (d) Al-LLZO, (e) Al-LLZTO, and (f) Al-LLZNO.
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Table
Table 1. The energies of reactants and products.
Table 1. The energies of reactants and products.
SamplesEnergy keV/f.u.
Li2O−0.825603
Li6Zr2O7−6.811825
La2O3−3.053699
LiTaO3−3.101137
LiNbO3−3.057770
Al2O3−5.079534
Li6.25Al0.25La3Zr2O12−12.13005
Li5.625Al0.25La3Zr1.375Ta0.625O12−12.19829
Li5.625Al0.25La3Zr1.375Nb0.625O12−12.17118
Table
Table 2. The simulated thermodynamic stability of Al-LLZO, Al-LLZTO, and Al-LLZNO.
Table 2. The simulated thermodynamic stability of Al-LLZO, Al-LLZTO, and Al-LLZNO.
SamplesDecomposition EquationsΔ Energy meV/atom
Li6.25Al0.25La3Zr2O121/8Li2O + Li6Zr2O7 + 3/2La2O3 + 1/8Al2O3−19.89
Li5.625Al0.25La3Zr1.375Ta0.625O127/16Li2O + 11/16Li6Zr2O7 + 5/8LiTaO3 + 3/2La2O3 + 1/8Al2O3+11.08
Li5.625Al0.25La3Zr1.375Nb0.625O127/16Li2O + 11/16Li6Zr2O7 + 5/8LiNbO3 + 3/2La2O3 + 1/8Al2O3+11.07
Table
Table 3. Lattice parameters for calculated crystals and XRD refinement.
Table 3. Lattice parameters for calculated crystals and XRD refinement.
SamplesCalculated Lattice ParameterRefinement Lattice Parameter
a (Å)b (Å)c (Å)α (°)β (°)γ (°)a = b = c (Å)α = β = γ (°)
Al-LLZO12.944112.970713.072489.990.189.912.938790
Al-LLZTO13.003212.905512.943190.090.089.912.924390
Al-LLZNO13.016712.917612.929190.190.189.912.928890
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Huang, L.; Gao, J.; Bi, Z.; Zhao, N.; Wu, J.; Fang, Q.; Wang, X.; Wan, Y.; Guo, X. Comparative Study of Stability against Moisture for Solid Garnet Electrolytes with Different Dopants. Energies 2022, 15, 3206. https://doi.org/10.3390/en15093206

AMA Style

Huang L, Gao J, Bi Z, Zhao N, Wu J, Fang Q, Wang X, Wan Y, Guo X. Comparative Study of Stability against Moisture for Solid Garnet Electrolytes with Different Dopants. Energies. 2022; 15(9):3206. https://doi.org/10.3390/en15093206

Chicago/Turabian Style

Huang, Li, Jian Gao, Zhijie Bi, Ning Zhao, Jipeng Wu, Qiu Fang, Xuefeng Wang, Yong Wan, and Xiangxin Guo. 2022. "Comparative Study of Stability against Moisture for Solid Garnet Electrolytes with Different Dopants" Energies 15, no. 9: 3206. https://doi.org/10.3390/en15093206

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