# On High-Temperature Thermal Cleaning of Li7La3Zr2O12 Solid-State Electrolytes

Huanyu Zhang, Giulia Paggiaro, Faruk Okur, Janis Huwiler, Claudia

Cancellieri, Lars P. H. Jeurgens, Dmitry Chernyshov, Wouter van Beek,

Maksym V. Kovalenko, Kostiantyn V. Kravchyk

► To cite this version:

Huanyu Zhang, Giulia Paggiaro, Faruk Okur, Janis Huwiler, Claudia Cancellieri, et al. On High-Temperature Thermal Cleaning of Li7La3Zr2O12 Solid-State Electrolytes. ACS Applied Energy Materials, 2023, 6 (13), pp.6972–6980. 10.1021/acsaem.3c00459. http://onlinelibrary.wiley.com/doi/abs/10.1021/acsaem.3c00459

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# On High-Temperature Thermal Cleaning of Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> Solid-State Electrolytes

Huanyu Zhang, Giulia Paggiaro, Faruk Okur, Janis Huwiler, Claudia Cancellieri, Lars P. H. Jeurgens, Dmitry Chernyshov, Wouter van Beek, Maksym V. Kovalenko,* and Kostiantyn V. Kravchyk*

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Cite This: ACS Appl. Energy Mater. 2023, 6, 6972–6980

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**ABSTRACT:** The deployment of Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (LLZO) garnets as solid-state electrolytes for Li-metal solid-state batteries faces challenges due to their reactivity with water and carbon dioxide, resulting in the formation of a Li<sub>2</sub>CO<sub>3</sub>/LiOH contamination layer on the LLZO surface. This contamination layer contributes to high Li/LLZO interfacial resistance, voltage polarization at the Li anode, and the formation of Li dendrites. In this work, we study the feasibility of high temperature annealing (600–900 °C) as a thermal cleaning method for LLZO. By conducting a comprehensive analysis of the LLZO surface using X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, in situ grazing incidence synchrotron X-ray diffraction, and Raman spectroscopy, an inherent issue related to high-temperature thermal cleaning is identified. While heat-treatment effectively reduces the Li<sub>2</sub>CO<sub>3</sub>/LiOH contamination, it also leads to the formation of the La<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> phase on the LLZO surface due to Li losses. The impact of these factors on the voltage polarization, interfacial resistance, and critical current density of Li/LLZO/Li symmetrical cells was assessed by electrochemical impedance spectroscopy and galvanostatic cycling measurements.

**KEYWORDS:** LLZO surface, solid-state electrolytes, solid-state batteries, thermal cleaning, Li anode

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## INTRODUCTION

Cubic Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (LLZO) garnets are emerging Li-ion solid-state electrolyte materials, extensively explored for deployment in next-generation Li-metal solid-state batteries (LMSSBs).<sup>1–6</sup> Surge of research on LLZO has been driven by its favorable properties such as high thermal and mechanical stability,<sup>7</sup> high Li-ion conductivity of up to 1 mS cm<sup>-1</sup> (RT),<sup>8</sup> a low electronic conductivity of ca. 10<sup>-8</sup> S cm<sup>-1</sup> (RT),<sup>9</sup> and a wide electrochemical operation window of 0–6 V versus Li<sup>+</sup>/Li.<sup>10</sup> Despite this promise, the research community has yet to surpass a number of critical issues of LLZO. One of the foremost aspects is its chemical reaction with H<sub>2</sub>O and CO<sub>2</sub> leading to the formation of the Li-ion resistive layer on the LLZO surface, which consists of LiOH and Li<sub>2</sub>CO<sub>3</sub>.<sup>11–18</sup> It was revealed that LLZO reacts first with water forming LiOH, which then further reacts with CO<sub>2</sub> yielding Li<sub>2</sub>CO<sub>3</sub>.<sup>19–22</sup> The electrochemical performance of Li-garnet LMSSBs is greatly impacted by the presence of a Li<sub>2</sub>CO<sub>3</sub>/LiOH layer.<sup>23–27</sup> This layer engenders a rise in the Li/LLZO interfacial resistance, resulting in significant voltage polarization at the Li anode.<sup>16,19,28,29</sup> Furthermore, it causes an uneven distribution of the applied current density at the Li/LLZO interface, leading to the formation of Li dendrites.<sup>13,30–34</sup> The Li<sub>2</sub>CO<sub>3</sub>/LiOH contamination issue can be eliminated by sintering and

handling LLZO ceramics in an inert atmosphere, which, however, adds to the fabrication costs.

Removal of the Li<sub>2</sub>CO<sub>3</sub>/LiOH contamination layer was attempted with various approaches of LLZO surface treatment. For instance, chemical treatments by HCl<sup>35</sup> or LiBF<sub>4</sub><sup>36</sup> have demonstrated effectiveness in the removal of Li-ion insulating surface layers. Moreover, Ar sputtering appeared to be efficient in removing contamination at the LLZO surface,<sup>37</sup> although its applicability is questionable. Last, the removal of Li<sub>2</sub>CO<sub>3</sub>/LiOH contamination by heat-treatment proved to be the most effective. Initial studies of the temperature effect on the chemical composition of the LLZO surface have been done by Sakamoto *et al.*,<sup>38</sup> where it was demonstrated that heat-treatment of LLZO pellets at ca. 500 °C enables partial removal of the Li<sub>2</sub>CO<sub>3</sub>/LiOH contamination layer. Such heat-treatment appeared to be effective in removing LiOH, presumably due to its intensified sublimation in the molten

Received: February 18, 2023

Accepted: May 25, 2023

Published: June 15, 2023

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<figure><img src="image_9.png" /><figcaption>**Figure 1.** (a) PXRD pattern and (b) photograph of as-sintered Al-LLZO pellets after polishing. (c) Top-view and (d, e) cross-sectional SEM images of an Al-LLZO pellet.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 2.** Spectral reconstructions of the charge-corrected La 3d, O 1s, C 1s, Li 1s, and Zr 4s XPS spectra, as recorded from the non-heat-treated and heat-treated [600 °C (1 h), 750 °C (1 h), and 900 °C (1 h)] LLZO pellets using soft Al-α radiation.</figcaption></figure>

state (T = 462 °C), but less so for $Li_2CO_3$. Later, Doeff et al.³⁹ showed that $Li_2CO_3/LiOH$ can be removed at a much lower temperature of 250 °C, which was explained by the reversed chemical reaction of $Li_2CO_3$ with LLZO. In the context of further minimization of $Li_2CO_3/LiOH$ surface contamination, Guo et al.⁴⁰ reported on heat-treatment of the LLZO surface at a significantly higher temperature of 900 °C, which is close to the decomposition temperature of $Li_2CO_3$ (620–1000 °C). However, a chosen heat-treatment time of 24 h appeared to be too long and led to significant Li losses and thus to the formation of a $La_2Zr_2O_7$ (LZO) phase at a depth of more than 0.1 mm from the surface. Considering stringent thickness requirements for the LLZO solid-state electrolyte (20–50 μm) for reaching high gravimetric and volumetric energy densities of Li-garnet LMSSBs,⁴¹⁻⁴³ such a prolonged thermal treatment is clearly not applicable for thin LLZO membranes.

Following the initial reports of Sakamoto et al.,³⁸ Doeff et al.,³⁹ and the recent publication of Guo et al.,⁴⁰ we sought to continue the evaluation of the LLZO surface after its heat-treatment at temperatures close to the melting (T = 723 °C) and decomposition (T = 620–1000 °C) temperatures of $Li_2CO_3$. In this work, we draw the readers' attention to the in-depth characterization of the LLZO surface, which was subject to air exposure and following heat-treatment at various temperatures of 600, 750, and 900 °C. By employing both

X-ray photoelectron spectroscopy (XPS) and hard X-ray photoelectron spectroscopy (HAXPES), our investigation reveals that although an increase in the heat-treatment temperature results in a decrease in $Li_2CO_3/LiOH$ contamination, it is still present at a depth of more than 18 nm from the LLZO surface even after heat-treatment at 900 °C. Another aspect, which was clarified, is the impact of Li losses and the associated formation of the LZO phase. Although the LZO was not identified by powder X-ray diffraction (PXRD) in LLZO heat-treated at temperatures of 600–900 °C for 1 h, in situ grazing incidence synchrotron X-ray diffraction (GISXRD) measurements revealed that the formation of the LZO phase at the LLZO surface occurs even at a relatively low temperature of 600 °C. Both factors, i.e., the presence of $Li_2CO_3/LiOH$ and LZO at the LLZO surface and their effects on the Li/LLZO interfacial resistance and the critical current density of Li/LLZO/Li symmetrical cells, are thoroughly discussed in this work.

## RESULTS AND DISCUSSION

The investigation of the surface of the LLZO solid-state electrolyte has been performed on LLZO pellets, which were fabricated by sintering commercial LLZO powder with a nominal composition of $Li_{6.25}Al_{0.25}La_3Zr_2O_{12}$. After sintering, the surface of the pellets was polished with SiC paper,
<figure><img src="image_11.png" /><figcaption>**Figure 3.** (a) Spectral reconstructions of the charge-corrected O 1s, C 1s, Li 1s, and Zr 4s HAXPS spectra, as recorded from the non-heat-treated and heat-treated [600 °C (1 h), 750 °C (1 h), and 900 °C (1 h)] LLZO pellets using hard Cr-Kα radiation. (b) Comparison of the relative intensities of the resolved O 1s and C 1s peaks from Li₂CO₃ and O₇Lattice for the non-heat-treated and heat-treated LLZO pellets, corresponding to (a). Voltage polarization and CCD of Li/LLZO/Li symmetrical cells assembled with non-heat-treated and heat-treated LLZO pellets.</figcaption></figure>

gradually increasing the grit size from 320 to 4000 (see the **Experimental Section** for details). The pellets were then stored in the air for ca. 24 h. The PXRD (Figure 1a) confirmed the pure cubic phase LLZO structure after sintering (the space group, Ia3d, *a* = 12.9652(4) Å, *V* = 2179.404 Å³, ICSD 430571). The surface and cross-section scanning electron microscope (SEM) image (Figure 1b) shows the homogeneous density within the LLZO pellet (ca. 90% of the theoretical density of 5.1 g cm⁻³).

First, the impact of temperature on the chemical composition of the LLZO surface has been systematically investigated by XPS, examining the LLZO surface before and after heat-treatment at 600, 750, and 900 °C for 1 h in an Ar-filled glovebox. It should be noted that the XPS/HAXPES instrument was directly connected to an Ar-filled glovebox, and therefore, the fabricated samples could be transferred for XPS analysis without air exposure. To account for the variation in charging across the insulating LLZO surfaces during XPS measurements, the recorded photoelectron spectra were corrected by adjusting them based on the difference between the observed adventitious C 1s peak (located on the lower binding energy side of the C 1s peak envelope, representing the C-C bond) and the reference value for adventitious carbon at 284.8 eV (Figure S1). Figure 2 shows the La 3d<sub>5/2</sub>, O 1s, C 1s, Li 1s, and Zr 4s XPS spectra of non-heat-treated and 1 h-heat-treated (600, 750, and 900 °C) LLZO pellets. The recorded probing depths of the La 3d, O 1s, C 1s, and Li 1s photoelectron lines obtained from LLZO using Al Kα X-ray radiation (*h*ν = 1486.7 eV) are 3.6, 4.8, 5.5, and 6.6 nm, respectively.³⁷ Notably, the adventitious C 1s main peak at the lower binding energy (BE) side of the C 1s peak envelope (as employed for charge correction) is very pronounced, even though the LLZO surfaces were heat-treated and transferred under Ar shielding gas in a glovebox environment; i.e., the adventitious C 1s main peak is much stronger than any typical adventitious C 1s peak recorded from an air-exposed material surface by XPS. However, as verified by independent XPS analyses of the same (oxide) material surface after short exposure to either a typical Ar glovebox environment used for chemical synthesis or to ambient conditions, sample exposure to a synthesis glovebox environment generally gives rise to a much stronger “adventitious” C 1s peak. This suggests that material synthesis using organic solvents in a glovebox environment inevitably releases volatile organic molecular

species, which are adsorbed on the synthesized material surfaces with time.

Accordingly, the lower-BE-side C 1s main peaks at 284.8, 285.6, and 288.8 eV are attributed to C-C, C-O, and O=C-O bonds, respectively, in the adventitious carbon species adsorbed on the outer surface originating from the synthesis glovebox environment.³⁸ In addition, the smaller C 1s main peak at the higher BE side of the C 1s peak envelope can be assigned to a Li₂CO₃ surface reaction layer (Figure 2). The Li₂CO₃ surface reaction layer gives rise to Li 1s, C 1s, and O 1s main peaks at BE values of 55.3, 290.1, and 531.8 eV. Importantly, no Zr signal from the LLZO lattice (in the range of 178-186 eV) was detected before the heat-treatment of the LLZO surface. This indicates that the thickness of the Li₂CO₃-based surface layer for the non-heat-treated pellet is (much) larger than the XPS probing depth of about 6 nm (see above).

A different chemical constitution of the LLZO surface region is observed after heat-treatment of the LLZO pellets at 600 °C. The C 1s peaks at 284.8, 285.6, and 288.8 eV, as well as the O 1s peak at 532.8 eV, can again be assigned to adventitious carbon specious adsorbed on the heat-treated LLZO sample surfaces due to short exposure to a synthesis glovebox environment (see above). Noticeably, the spectral contributions from adventitious C species are largely reduced with respect to those from the Li₂CO₃ reaction layer. More importantly, additional spectral contributions from O 1s at 529.0 eV, La 3d at 838-833 eV (with the peak at 830.8 eV originating from the La MNN transition), and Zr 4s at 51.6 eV peaks were detected, attributed to the LLZO lattice. The detection of surface-sensitive photoelectron intensities from the LLZO lattice indicates that the heat-treatment removes a significant part of the Li₂CO₃ surface contamination layer. However, there are still detectable signal intensities of O 1s and C 1s originating from Li₂CO₃, indicating the presence of a remaining Li₂CO₃ layer with an effective thickness well below 6 nm. Moreover, comparison of the relative peak intensities of C 1s (290.1 eV) and O 1s (531.8 eV) from Li₂CO₃ with those of O 1s (529.0 eV), La 3d (838-833 eV, 830.8 eV), and Zr 4s (51.6 eV) from the LLZO lattice shows that the Li₂CO₃ contamination layer is further reduced with increasing temperature of the heat-treatment.

Interestingly, an additional O chemical state at 530.1 eV has appeared in the recorded O 1s spectra for the heat-treated LZZO samples. Considering the reported O 1s BE difference between LiOH and Li₂CO₃ (ΔBE<sub>Li₂CO₃ - LiOH</sub> = 0.9 eV) with
<figure><img src="image_12.png" /><figcaption>**Figure 4.** Critical current density measurements of Li/LLZO/Li symmetrical cells based on (a) non-heat-treated and heat-treated [(b) 600 °C, (c) 750 °C, and (d) 900 °C] LLZO pellets. The measurements were conducted at room temperature without external pressure, with a capacity limitation of 0.1 mAh cm⁻² per half-cycle.</figcaption></figure>

the O 1s BE difference between Li₂O and Li₂CO₃ (ΔBE<sub>Li₂CO₃ - Li₂O</sub> = 3.4 eV),<sup>44,45</sup> the additional O 1s peak at 530.1 eV can be attributed to Li-O(H) species. Accordingly, the Li 1s peak at 55.3 eV is assigned to a mixture of Li₂CO₃ and Li-O(H); however, the chemical shift of the Li 1s line between Li₂CO₃ and Li-O(H) cannot be resolved unambiguously due to a lack of energy resolution. However, as the temperature of the heat-treatment increases, the Li 1s peak shifts from 55.2 to 54.9 eV, in accordance with an increased spectral contribution from the LLZO lattice as compared to other possible Li chemical states (predominantly Li₂CO₃/Li-O(H)).

As the next step, HAXPES analysis was performed on the same set of samples to investigate the chemical composition of the LLZO samples over a much larger probing depth, far beyond the outer surface (i.e., extending into the interior of the LLZO lattice), as shown in Figure 3a. The probing depths for the O 1s, C 1s, and Li 1s photoelectron lines, as recorded from LLZO using Cr-Kα X-ray radiation (hν = 5414.7 eV), are equal to 17.6, 18.3, and 19.2 nm, respectively (i.e., about 3 times that for the conventional XPS spectra in Figure 2).<sup>37</sup> Such non-destructive HAXPES analysis with hard Cr-Kα X-rays (i.e., at large probing depths) is superior to destructive XPS sputter-depth profiling analysis, as conducted by performing alternating steps of XPS analysis with soft Al-Kα X-rays (i.e., at small probing depths) and Ar⁺ sputtering. Namely, surface bombardment with Ar⁺ ions (even at low beam voltages of, say, 1 kV) will induce changes in the chemistry and/or crystal structure of the LLZO lattice, which can be excluded from the HAXPES analysis. As for the XPS spectra in Figure 2, the measured HAXPES spectra were charge-corrected using the reference value for adventitious carbon at 284.8 eV (Figure S1). As exhibited from the absence of O 1s (529.0 eV) and Zr 4s (51.6 eV) spectral contributions from the LLZO lattice in the HAXPES analysis of the non-heat-treated sample, the thickness of the initial Li₂CO₃ reaction layer is larger than 18 nm. Notably, for the HAXPES analysis of the heat-treated samples, the signal intensities from the LLZO

lattice are increased relative to those from the adventitious carbon species and the Li₂CO₃ contamination layer (compare Figures 2 and 3a), which confirms the chemical assignment of the La 3d⁵/₂, O 1s, C 1s, Li 1s, and Zr 4s main peaks for the LLZO lattice in Figure 2. The observed trends with increasing temperature of the heat-treatment, as observed by conventional XPS, in particular the reduction of the Li₂CO₃ contamination layer and the appearance of signals from the LLZO lattice, as well as from Li₂O/Li-O(H), are confirmed by the HAXPES analysis. Importantly, although the contribution from the LLZO lattice is very pronounced after the heat-treatment at 900 °C, some surface contamination by Li₂CO₃ is still detected. As discussed in ref 37, these Li₂CO₃ contaminations not necessarily originate from the outer LLZO surface but can be present on the inner surfaces of the remaining porosity and/or grain boundaries of the pellet.

Importantly, the obtained XPS and HAXPES results correlate well with the Li/LLZO interfacial resistance measured on Li/LLZO/Li symmetrical cells by electrochemical impedance spectroscopy (EIS). Comparison of the impedance spectra of symmetrical cells assembled with non-heat-treated and heat-treated LLZO pellets showed that the heat-treatment leads to a decrease in the Li/LLZO interfacial resistance from 9.1 kΩ cm² (RT) to 164 Ω cm² (600 °C), 66 Ω cm² (750 °C), and 55 Ω cm² (900 °C) (Figure S2). Such difference is clearly associated with the decrease in the Li₂CO₃/LiOH content on the LLZO surface, as evident from XPS/HAXPES data, when the heat-treatment temperature is increased. Notably, as revealed from Raman spectroscopy measurements (Figure S3), the Li₂CO₃ contamination layer was not present on the LLZO pellets that were not exposed to air after sintering.

Next, to examine the impact of the heat-treatment of the LLZO surface on the effectiveness of the Li plating/stripping at the Li/LLZO interface, galvanostatic cycling experiments of Li/LLZO/Li symmetrical cells have been performed. Importantly, the absence of any significant morphological changes of the LLZO surface before and after heat-treatment (Figure
S4) excluded its possible impact on the electrochemical performance of the Li/LLZO/Li symmetrical cells. Tests were conducted by sequentially applying positive and negative current density to the symmetrical cells in steps of 0.02 mA cm⁻² with a capacity limitation of 0.1 mAh cm⁻² for each half cycle, as shown in Figure 4. This type of measurement allows assessing the achievable critical current density (CCD) of the studied systems, representing the threshold current density at which the growth of Li dendrites/filaments begins. As exhibited from Figure 4, non-heat-treated LLZO pellets demonstrate a very low CCD of 0.04 cm⁻² and high voltage polarization of 712 mV at 0.02 mA cm⁻². On the contrary, much higher CCDs of 0.2, 0.24, and 0.4 mA cm⁻² and lower overpotentials of 14.5, 12.3, and 11.3 mV at 0.02 mA cm⁻² were reproducibly reached for a symmetrical cell assembled with 600, 750, and 900 °C-heat-treated LLZO pellets, respectively. Notably, a significant decrease in the over-potentials and an increase in CCD for heat-treated LLZO pellets has a direct correlation with the data obtained by HAXPES (Figure 3b). This correlation, as in the case of Li/LLZO interfacial resistance, is related to the decrease in Li₂CO₃/LiOH content on the LLZO surface as the heat-treatment temperature increases. As explained by Zhang et al.,³² the presence of a Li₂CO₃/LiOH contamination layer between Li and LLZO leads to an inhomogeneous current density distribution during Li plating/stripping, causing the formation of Li dendrites.

Having established that heat-treatment of LLZO allows to reduce the content of the Li₂CO₃/LiOH contamination layer on the LLZO surface, we characterized the surface of LLZO for the presence of LZO phase by in situ GISXRD as a function of heat-treatment duration. These experiments were impelled by the well-known fact of Li losses from LLZO at high temperatures of >850 °C.⁴⁶ The latter leads to the formation of the LZO phase on the LLZO surface and hence to the increase in the Li/LLZO interfacial resistance. The in situ GISXRD measurements were performed during heat-treatment of LLZO pellets under a N₂ atmosphere at 600 °C (Figure 5a, b). The analysis of GISXRD data demonstrates that the surface of LLZO pellets contains minor quantities of LZO phase at the initial stage of the measurements. As exhibited from the comparison of GISXRD patterns measured at RT and 600 °C, its presence is related to minor Li losses, which occurred during the heating step (20 min) and the time required to determine the grazing incidence angle (ca. 10 min at 600 °C). However, the intensity of LZO peaks gradually increased during the heat-treatment of LLZO at 600 °C for 1 h. Notably, the appearance of LZO phase was not detected from PXRD patterns of heat-treated LLZO pellets (Figure S5), indicating that the LZO formation occurs only at the surface.

Importantly, similar data on the appearance of LZO phase upon LLZO heat-treatment has been obtained by Raman spectroscopy measurements. As exhibited from Figure 5c, comparing the Raman spectra of LLZO pellets after the heat-treatment at 900 °C for 10 min and 1 h, the longer heat-treatment time leads to an increase in the LZO amount at the surface. This is evident from the significant increase in the LZO peak intensity at 300 cm⁻¹ for the 1 h-heat-treated sample. Other Raman peaks, in the range of 100–600 cm⁻¹, which correspond to the cubic structure of LLZO, did not change.

Next, to identify the impact of the LZO formation on the Li/LLZO interfacial resistance and CCD, we have performed

<figure><img src="image_13.png" /><figcaption>**Figure 5.** (a) Measured GISXRD map of LLZO pellets during heat-treatment at 600 °C for 1 h under a N₂ atmosphere. The acquired time of each pattern is 10 s. (b) Selected SGIXRD patterns of LLZO pellets at different stages of the heat-treatment (10 s, 10 min, and 1 h). The SGIXRD pattern of the non-heat-treated LLZO surface is given for comparison (see Figure S6 for the detailed comparison of SGIXRD patterns for non-heat-treated and 1 h-heat-treated (600 °C, N₂) LLZO surfaces). (c) Raman spectra of LLZO pellets after heat-treatment in Ar at 900 °C for 10 and 60 min. (d) Impedance spectra of Li/LLZO/Li symmetrical cells assembled with LLZO pellets heat-treated in Ar at 900 °C for 10 and 60 min.</figcaption></figure>

EIS and galvanostatic cycling measurements of Li/LLZO/Li symmetrical cells assembled with LLZO pellets heat-treated at 900 °C for 10 min and 1 h. As exhibited from Figure 5d, comparing the EIS spectra of both symmetrical cells, lowering the heat-treatment time from 1 h to 10 min leads to a significant decrease in the Li/LLZO interfacial resistance from 55 to 21 Ω cm², respectively. Importantly, by lowering the heat-treatment time and therefore lowering the content of LZO phase at the LLZO surface and Li/LLZO interfacial resistance, we enabled to decrease the voltage polarization down to 10.5 mV at 0.02 mA cm⁻² and increase the CCDs up to 0.64 mA cm⁻² (Figure 6a,b). Additional galvanostatic measurements of Li/LLZO/Li symmetrical cells assembled with LLZO pellets heat-treated at 900 °C for 10 min have demonstrated high cycling stability over 550 cycles, corresponding to a cumulative capacity of 110 mAh cm⁻² (Figure 6c).

## ■ CONCLUSIONS

In this work, we have examined the impact of heat-treatment on the chemical composition of the LLZO surface, Li/LLZO interfacial resistance, and critical current density of Li/LLZO/Li symmetrical cells. A side-by-side characterization of non-heat-treated and heat-treated LLZO pellets by XPS/HAXPES followed by EIS and CCD measurements of Li/LLZO/Li symmetrical cells confirmed that heat-treatment of LLZO pellets is a compelling approach for cleaning the LLZO surface. Thermal cleaning decreases the Li₂CO₃ content at the LLZO surface, resulting in a significant decrease in Li/LLZO interfacial resistance and the increase in CCD of Li/LLZO/Li symmetrical cells. Specifically, we determined that LLZO pellets annealed at 900 °C (1 h) have both the lowest content of Li₂CO₃ at the LLZO surface and the lowest Li/LLZO
<figure><img src="image_14.png" /><figcaption>**Figure 6.** Critical current density and cycling stability measurements of Li/LLZO/Li symmetrical cells comprising heat-treated (900 °C, 10 min) LLZO pellets. (a, b) Voltage profiles of the Li/LLZO/Li symmetrical cell measured at different current densities ranging from 0.02 mA cm⁻² to 1.0 mA cm⁻² with a capacity limitation of 0.1 mAh cm⁻² per half-cycle. (c) Voltage profile of the Li/LLZO/Li symmetrical cell, which was measured at a current density of 0.1 mA cm⁻², while maintaining a capacity limitation of 0.1 mAh cm⁻² per half-cycle. Both critical current density and cycling stability measurements were performed at room temperature without external pressure.</figcaption></figure>

interfacial resistance as compared to pellets heat-treated at lower temperatures of 600 °C (1 h) and 750 °C (1 h) or non-heat-treated LLZO pellets. Additionally, Li/LLZO/Li symmetrical cells assembled with 900 °C (10 min)-heat-treated LLZO pellets exhibited the highest critical current density of up to 0.64 mA cm⁻² and cycling stability over 550 cycles at a current density of 0.1 mA cm⁻² while maintaining an areal capacity limitation of 0.1 mAh cm⁻².

Albeit these findings, additional characterization of the LLZO surface by *in situ* GISXRD and Raman spectroscopy showed that heat-treatment as a surface cleaning methodology has limitations. Although on one side, it helps to decrease the Li₂CO₃ surface contamination, on the other side, it leads to the formation of LZO phase, resulting in an increase in Li/LLZO interfacial resistance. In other words, these results point to the intrinsic trade-off issue of thermal cleaning—the formation of LZO phase due to Li losses at high temperatures of >600 °C. Most likely, a successful solution to this problem could be the employment of fast heating rates to the targeted temperature of 900 °C, enabling the removal of Li₂CO₃ contamination with minimized Li losses. Another promising avenue could be an adjustment of the amount of added additional Li source (for instance, Li₂O) to LLZO, not just to compensate for Li losses occurring during LLZO sintering, but also during its following heat-treatment.

## EXPERIMENTAL SECTION

**Preparation of Al-Doped LLZO Pellets.** Commercial Al-LLZO powder (AmpceraTM, Al-doped LLZO, 500 nm nanopowder) was placed into a pressing die with a diameter of 1 cm and uniaxially pressed at a pressure of approximately 10 kN. The resulting pellets were then subjected to isostatic pressing at a pressure of 1000 kN (approximately 350 MPa). Subsequently, the pellets were carefully polished on all sides using polishing paper to eliminate any contamination from the pressing die present on the surface. The

polished green pellets were then completely covered with pristine LLZO powder and sandwiched between two Al₂O₃ crucible lids. The assembly was inserted into a tube furnace (Gero Carbolite) for sintering. The sintering process was carried out under a nitrogen atmosphere with a constant gas flow, at a temperature of 1235 °C for 15 min. The heating rates were set at 600 °C per hour up to 700 °C followed by 400 °C per hour up to 1235 °C. For cooling, the rate was set at 450 °C per hour until room temperature was reached. After sintering, the pellets were further polished using a grinding machine (Grinding Machine Buhler MetaServ 250) in ambient air. The grit size of the polishing paper was gradually increased (320, 800, 1200, and 4000) to achieve a final thickness of approximately 1 mm. Subsequently, the pellets were transferred to an argon-filled glovebox, where they underwent additional polishing using polishing paper with a grit size of 800. Following this, the pellets were subjected to heat-treatment at various temperatures: 600 °C for 1 h, 750 °C for 1 h, and 900 °C for 1 h or 10 min. The heat-treatment of LLZO pellets was performed in a muffle furnace using LLZO pillars with a thickness of approximately 5 mm as substrates.

**Material Characterization.** PXRD measurements were conducted using an STOE STADIP powder X-ray diffractometer in either transmission mode (using Cu-Kα irradiation with a wavelength of 1.5406 Å) or reflection mode (using Mo-Kα1 irradiation with a wavelength of 0.7093 Å). SEM (scanning electron microscopy) analysis was carried out using a Hitachi TM3030Plus Tabletop microscope, operating at an acceleration voltage of 10 kV.

XPS/HAXPES analysis was carried out using a PHI Quantes spectrometer (ULVAC-PHI) equipped with two X-ray sources: a low-energy Al-Kα source (1486.6 eV) and a high-energy Cr-Kα source (5414.7 eV). Both sources employ high-flux-focused monochromatic X-ray beams. The XPS spectrometer was directly linked to an Ar-filled glovebox, allowing for the transfer of LLZO pellets without exposing them to air. Notably, most laboratory XPS spectrometers do not have a direct connection to the glovebox. Therefore, the majority of the reported XPS experiments on LLZO are performed after their short exposure to air. The energy scale of the hemispherical analyzer was calibrated following the ISO 15 472 standard by referencing the Au 4f7/2 and Cu 2p3/2 main peaks. These peaks were measured *in situ*
using sputter-cleaned, high-purity metal references and aligned to the recommended binding energy (BE) positions of 83.96 and 932.62 eV, respectively. To maintain charge neutrality during each measurement cycle, a dual beam charge neutralization system was employed. This system utilized low-energy electron and argon-ion beams with a bias of 1 V and a current of 20 µA.

XPS and HAXPES survey spectra were obtained within specific binding energy (BE) ranges. For XPS, the range covered 0 to 1200 eV, while for HAXPES, it covered 0 to 5200 eV. The spectra were recorded with a step size of 0.5 eV, and a constant pass energy of 280 eV was maintained. The Al-Kα source with a power of 51 W and a beam diameter of approximately 200 µm was used for XPS measurements. On the other hand, the Cr-Kα source with a power of 100 W and a beam diameter of around 100 µm was utilized for HAXPES measurements. To capture specific regions of interest, such as Li 1s, La 3d, C 1s, and O 1s, the spectra were recorded with a finer step size of 0.05 eV. The same constant pass energy of 280 eV was maintained, and both the Al-Kα and the Cr-Kα sources were sequentially employed for these measurements. For more comprehensive survey spectra and detailed Zr 3d spectra, please refer to the Supporting Information (Figures S1 and S7).

To quantify the intensity of C 1s and O 1s peaks, a constrained peak fitting method was employed on the linear background-corrected spectra. This analysis involved using one or more symmetrical, mixed Gaussian-Lorentzian line shape functions, and it was performed using the CasaXPS software. The Gaussian fraction, which represents instrumental broadening, was constrained to a value of 0.5 for each peak component. Initially, the full width at half maximum (FWHM) and the binding energy (BE) position of each peak component were determined individually for all four samples by analyzing every region of interest. Subsequently, specific regions from all four samples were simultaneously fitted using a predefined number of individual components. This approach allowed for obtaining more accurate values of the FWHM and BE for each individual component within a specific sample.

In-situ SGIXRD measurements were carried out at the European Synchrotron Radiation Facility (Beamline BM01). A PILATUS@ SNBL diffractometer was utilized, with a wavelength (λ) of 0.68966 Å, operating in low-intensity beam mode at approximately 20 mA. The experiments were conducted under a nitrogen (N₂) atmosphere, employing a custom-made furnace. The furnace was gradually heated at a ramp rate of 30 °C per minute until reaching a temperature of 600 °C. Each pattern was acquired for 10 s. During the XRD measurements while rotating the LLZO pellet, it was determined that the grazing incidence angle was set to 2.2°. The 2D diffraction data obtained from a Pilatus 2 M detector were processed using the SNBL Toolbox and BUBBLE software. To calculate the size of crystallites, the SGIXRD peaks were fitted using the Modulation-Enhanced Diffraction Viewer and Editor (Medved) software, developed at the Swiss Norwegian Beamlines (SNBL).⁴⁷

Raman spectroscopy measurements were performed using a confocal Raman microscope, specifically the Horiba LabRAM HR Evolution model. The microscope was equipped with a Nd:Yag laser operating at a wavelength of 532 nm, specifically the Cobalt SambaTM laser.

**Electrochemical Characterization.** Symmetrical cells were prepared by isostatic pressing of Li metal discs (thickness ≈ 30 µm, d ≈ 6 mm) onto the two opposite sides of an Al-LLZO pellet at 1000 kN for about 4–5 min under inert conditions. The drop of the resistance (Figure S8) and the overpotential of Li/LLZO/Li symmetrical cells were used as evidence of cell short-circuit.

Electrochemical measurements were carried in the argon-filled glovebox, utilizing a multichannel workstation (MPG-2, Bio-Logic SAS). For EIS (electrochemical impedance spectroscopy) measurements, a frequency range from 1 MHz to 0.1 Hz was employed. It was verified that the electrochemical system remains linear up to an amplitude of approximately 300 mV (Figure S9); therefore, the applied amplitude was set at 70 mV. The obtained EIS data were fitted using ZView4 software.

■ ASSOCIATED CONTENT

S1 Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsaem.3c00459.

Further experimental details and characterization (Figures S1–S9): charge-corrected XPS and HAXPES surveys, impedance spectra, Raman spectra, top-view SEM images, PXRD patterns, SGIXRD patterns, spectral reconstructions of the charge-corrected Zr 3d XPS and HAXPES spectra, and typical impedance spectra (PDF)

■ AUTHOR INFORMATION

Corresponding Authors

Maksym V. Kovalenko — Laboratory of Inorganic Chemistry,
Department of Chemistry and Applied Biosciences, ETH
Zürich, CH-8093 Zürich, Switzerland; Laboratory for Thin
Films and Photovoltaics, Empa - Swiss Federal Laboratories
for Materials Science & Technology, CH-8600 Dübendorf,
Switzerland; ● orcid.org/0000-0002-6396-8938;
Email: mvkovalenko@ethz.ch

Kostiantyn V. Kravchyk — Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland; Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Materials Science & Technology, CH-8600 Dübendorf, Switzerland; ● orcid.org/0000-0001-6149-193X; Email: Kostiantyn.Kravchyk@empa.ch

Authors

Huanyu Zhang — Laboratory of Inorganic Chemistry,
Department of Chemistry and Applied Biosciences, ETH
Zürich, CH-8093 Zürich, Switzerland; Laboratory for Thin
Films and Photovoltaics, Empa - Swiss Federal Laboratories
for Materials Science & Technology, CH-8600 Dübendorf,
Switzerland

Giulia Paggiaro — Laboratory of Inorganic Chemistry,
Department of Chemistry and Applied Biosciences, ETH
Zürich, CH-8093 Zürich, Switzerland; Laboratory for Thin
Films and Photovoltaics, Empa - Swiss Federal Laboratories
for Materials Science & Technology, CH-8600 Dübendorf,
Switzerland

Faruk Okur — Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland; Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Materials Science & Technology, CH-8600 Dübendorf, Switzerland

Janis Huwiler — Laboratory of Inorganic Chemistry,
Department of Chemistry and Applied Biosciences, ETH
Zürich, CH-8093 Zürich, Switzerland; Laboratory for Thin
Films and Photovoltaics, Empa - Swiss Federal Laboratories
for Materials Science & Technology, CH-8600 Dübendorf,
Switzerland

Claudia Cancellieri — Laboratory for Joining Technologies & Corrosion, Empa - Swiss Federal Laboratories for Materials Science & Technology, CH-8600 Dübendorf, Switzerland;
● orcid.org/0000-0003-4124-4362

Lars P. H. Jeurgens — Laboratory for Joining Technologies & Corrosion, Empa - Swiss Federal Laboratories for Materials Science & Technology, CH-8600 Dübendorf, Switzerland;
● orcid.org/0000-0002-0264-9220
Dmitry Chernyshov – Swiss-Norwegian Beamlines, European Synchrotron Radiation Facility, 38000 Grenoble, France;
● orcid.org/0000-0001-7738-9358

Wouter van Beek – Swiss-Norwegian Beamlines, European Synchrotron Radiation Facility, 38000 Grenoble, France

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsamc.3c00459

Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

M.V.K. and K.V.K. express sincere appreciation to Belenos Clean Power Holding Ltd. for their generous funding, which has greatly contributed to this research. This work is conducted as part of the joint Empa-Fraunhofer ICS project “IE4B” and is financially supported through the ICON funding scheme. The authors extend their gratitude to Neal Fairley for his invaluable guidance in utilizing the CasaXPS software. The authors would like to acknowledge the support received from ETH Zurich research facilities, specifically the Scientific Center for Optical and Electron Microscopy and Small Molecule Crystallography Center (Department of Chemistry and Applied Biosciences). The authors are also thankful for the assistance and access to instruments provided by Laboratory for Mechanics of Materials & Nanostructures (Empa) and ESFR Beamline BM01.

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