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Atomic-scale origin of the low grain-boundary resistance in perovskite solid electrolytes

Oxide solid electrolytes (OSEs) have the potential to achieve improved safety and energy density for lithium-ion batteries, but their high grain-boundary (GB) resistance is a general bottleneck. In the most well studied perovskite OSE, Li3xLa2/3-xTiO3 (LLTO), the ionic conductivity of GBs is about three orders of magnitude lower than that of the bulk. In contrast, the related Li0.375Sr0.4375Ta0.75Zr0.25O3 (LSTZ0.75) perovskite exhibits low GB resistance for reasons yet unknown. Here, we used aberration-corrected scanning transmission electron microscopy and spectroscopy, along with an active learning moment tensor potential, to reveal the atomic scale structure and composition of LSTZ0.75 GBs. Vibrational electron energy loss spectroscopy is applied for the first time to characterize the otherwise unmeasurable Li distribution in GBs of LSTZ0.75. We found that Li depletion, which is a major reason for the low GB ionic conductivity of LLTO, is absent for the GBs of LSTZ0.75. Instead, the low GB resistivity of LSTZ0.75 is attributed to the formation of a unique defective cubic perovskite interfacial structure that contained abundant vacancies. Our study provides insights into the atomic scale mechanisms of low GB resistivity and sheds light on possible paths for designing OSEs with high total ionic conductivity.


Introduction
As the world transitions toward powering itself solely on sustainable sources of energy, energy storage devices become a critical component in making this process a reality. Among these devices, secondary lithium-ion batteries (LIBs) have been regarded as the most dominant and promising energy storage device because of their high energy density, high power density, long cycle life, and low self-discharge. Liquid electrolytes, as the media for ion transportation for current commercial LIBs, still suffers from a few drawbacks, including the safety concerns of their thermal stability and leakage. [1] On the other hand, solid-state electrolytes are intrinsically safer and leak free.
Among the various solid electrolytes, oxide based ones have drawn significant interest because of their electrochemical, thermal, and structural stability. One bottleneck that limits most oxide solid electrolytes from achieving high total ionic conductivity is the large grain-boundary (GB) resistance. [2][3][4][5][6][7][8][9][10] Many lithium-ion conductors have bulk ionic conductivity that are high enough for building practical all-solid-state battery (ASSB). [3] However, their total ionic conductivity is typically orders of magnitude lower than the bulk ionic conductivity due to large GB resistance. [3] One of the most well studied materials that exhibits this issue is the perovskite type solid electrolyte Li3xLa2/3-xTiO3 (LLTO, 0 < x < 0.16). In its first report by Inaguma and coworkers, LLTO displayed a high bulk ionic conductivity ( ) of 1.0 × 10 -3 S/cm at room temperature. But because its apparent GB ionic conductivity ( ) was 7.5 × 10 -5 S/cm, the total ionic conductivity ( ) was 7 × 10 -5 S/cm at room temperature. [11] Subsequent study revealed that the GBs comprised of severe structural and chemical deviation of about 2-3 unit cells thick, essentially forming a nanoscale TiO2-like insulating interfacial phase. [12,13] Such GBs prohibit the abundance and transport of charge carrier Li + . Nonetheless, few materials are exceptions where the bulk resistance, rather than GB resistance, is the bottleneck that limits the total ionic conductivity. [14][15][16][17] One of them, also a perovskite-type solid electrolyte, is Li0.375Sr0.4375□0.1875Ta0.75Zr0.25O3 (denoted as LSTZ0.75, where 0.75 refers to the value of y in the general formula Li0.5ySr1-0.75y□0.25yTayZr1-yO3 and the □ represents the A-site vacancy). It achieved a of 3.5 × 10 -4 S/cm, a rather high apparent of 1.2 × 10 -3 S/cm (including the grain size effect), and a of 2.7 × 10 -4 S/cm at room temperature. [16] However, it is unknown why LSTZ and LLTO show different trends in grain boundary conduction even though they have the same perovskite (ABO3) crystal structure.
Atomic scale characterization of microstructure and grain boundaries is crucial for understanding the unusual GB ionic conductivity enhancement in LSTZ. However, neither the specific GB ionic conductivity , which represents the intrinsic conduction behavior of the GB, nor the atomic scale microstructures of LSTZ GB and grain bulk have been reported. It is reasonable to expect the specific GB ionic conductivity of LSTZ is higher than those of LLTO and other Li-ion conductors because of the rather high . Thus, it is important to study the GB microstructure of LSTZ0.75, as it will unravel the origin of its low GB resistance and provide insights to overcoming the ubiquitous bottleneck of high GB resistance in other materials. It is also significant to study the bulk phase and microstructure of LSTZ0.75, because this will shed light on how to increase the material's bulk, and thus total, ionic conductivity. Additionally, the atomic scale characterization of Li + ion distribution is important for solid electrolyte materials.
Previous studies have shown distinct Li-K edges were difficult to identify in perovskite-type solid electrolytes, owing to their lower volume densities of Li + ions compared to other types of electrolytes. [18,19] Thus, a technique other than conventional low energy core-loss electron energy loss spectroscopy (EELS) is needed.
Computational studies of complex GB structures have also been constrained by the tradeoff between accurate, but expensive ab initio methods [20][21][22][23][24][25] and cheap, but inaccurate classical interatomic potentials (IAPs). [26] In recent years, IAPs that utilize machine learning (ML) to map the potential energy surface to local environment descriptors have emerged as a promising alternative that combines high accuracy and speed. [26][27][28][29][30][31][32] Nevertheless, studies of GB structures with ML-IAPs remain challenging due to the more complex local environment at GB regions, [29,33] with most studies focusing on low-sigma GB models, [34,35]

Structural ordering in LSTZ grain bulk and its effect on bulk ionic conductivity
The LSTZ0.75 ceramics were synthesized via conventional solid state reaction method.
Results from crystal structure, ionic conductivity, and electrochemical stability characterization conducted. Fig. 1d shows SAED pattern of LSTZ0.75 grain bulk along the [100] zone axis.
Surprisingly, the SAED collected from grain bulk exhibits two sets of diffraction spots that had failed to be observed in the corresponding FFT pattern. Twelve representative spots from these two sets of diffraction spots were circled in red. Fig. 1e shows SAED pattern of superlattice structure near (010) faceted GB along the [100] zone axis. The SAED from A-site alternate stacking similarly exhibits an additional set of diffraction spots that was not revealed in the corresponding FFT pattern. Six representative spots from the additional set of diffraction spots were circled in red.
Results from SAED reveal the existence of short coherence length ordering in LSTZ0.75 grain bulk. As for the bulk region near (010) faceted GB, both short and long coherence length ordering were present, though the short coherence length ordering in the (001) plane is masked by the long coherence length ordering in the (010) plane. The short coherence length ordering observed here is similar to the mesoscopic coherence length ordering observed in cubic LLTO. [36] Both features could not be detected by XRD due to the short length scale. However, the mesoscopic ordering in cubic LLTO can be detected by FFT pattern (and no SAED was performed to confirm), whereas the short coherence length ordering in cubic LSTZ was overlooked by FFT pattern and can only be detected using SAED.
The structural ordering and local composition were further elucidated using a moment tensor potential (MTP) that has been fitted to accurately reproduce the DFT potential energy surface of both bulk and GB structures of LSTZ0.75 using an active learning scheme ( Figure  to 140 ℃ reported for a hot-pressed LSTZ0.75 sample, whose A-site alternate stacking was not yet characterized. [37] Similar trends have been observed for the related LLTO material, where higher calcination temperatures lead to a tetragonal to cubic transformation and an increase in from ~7 × 10 -4 S/cm to ~1.5 × 10 -3 S/cm. [38,39] With the SAED results indicating the existence of short coherence length A-site ordering in LSTZ0.75 grain bulk and the trend uncovered in Fig.   2c, we believe the of LSTZ0.75 can be further enhanced by promoting disorder in A-sites.
Meanwhile, the promoted and the decreased Ea of the hot-pressed LSTZ0.75 might be attributed to the elimination of A-site alternate stacking, for which we expect experimental characterization to test out in future.

Microstructure of LSTZ0.75 GBs revealed by STEM-EELS
Next, we investigate the GBs of LSTZ in order to reveal the origin of its low GB resistance. Electron Backscatter Diffraction (EBSD) results ( Figure S4, Supporting Information) indicated that the average grain size is 3.38 ± 1.13 μm, and the GBs mainly consist of randomly orientated grains. In total, we have imaged more than seven hundred GBs, of which ~12% have (010) faceted grain terminal surfaces on one side. As shown in Fig. 1a, an obvious dark band can be observed at the (010) faceted GB. Fig. 1f and 1g  Zr. It is worth noting that despite the compositional change, the cubic perovskite crystal structure is still maintained at the GBs. Figure S5a and S5b of the Supporting Information show high magnification atomic-resolution HAADF-STEM images of the general GB from Fig. 1g and a (010) faceted GB (with respect to the right-side grain), respectively. Since the same crystal lattices from the bulk extend all the way to the GBs, and the only change is the observed decrease in intensity at GBs, crystal structure of the GBs is also cubic perovskite.
To better understand the observed features in HAADF images, high spatial resolution STEM-EELS measurements were performed at (010) faceted GBs and general GBs. Fig. 3a and   3b show the integrated EEL spectra of O-K edge and edges in the high energy loss regime (Sr-L2,3, Zr-L2,3, Ta-M2,3, and Ta-M4,5 edges), respectively. Atomic-resolution HAADF-STEM image of the (010) faceted GB from which the EEL spectra were collected is shown in Fig. 3c. The GB again exhibited a dramatically lower image intensity in comparison with the bulk. For the leftside grain, the alternate stacking between bright and dark A-site atomic columns, which extends ten unit cells from GB into grain interior, is also observed. Fig. 3d, e, f, and g display the atomicresolution elemental maps of Sr, Ta, Zr, and O, respectively. The maps were generated using intensities from Sr-L2,3, Zr-L2,3, Ta-M4,5, and O-K edges. As expected, the elemental maps ( To analyze the correlation between HAADF image and the elemental maps, a plot of vertically integrated intensity profiles across the GB is shown above the HAADF image (Fig.   3c). For the GB, the dramatic decrease in image intensity correlates well with the decrease in intensity of Sr, Ta, and Zr signals, though the decrease in Zr signal is much less than the other two. Specifically, there is a ~26%, 11%, and 7% decrease in Sr, Ta, and Zr signals, respectively, at the GB when compared to those from the abutting grains. This indicates that the GB contains more Sr, Ta, and Zr vacancies than the grain bulk, and its crystal structure resembles that of a defective cubic perovskite. Further analysis reveals that higher Sr and Ta  Obvious elemental segregation of Sr and Zr can be observed at both sides adjacent to the GB.
The differences between (010) faceted GB and general can be attributed to subtle variation in the composition of GBs. Consistent with the lack of ordering observed in the HAADF-STEM images ( Fig. 1f and 1g), the Sr elemental map did not exhibit alternate stacking of A-site.

LSTZ0.75 GB composition and its effect on ionic conductivity revealed by MTP
The relationship between GB composition and ionic conductivity was probed using the  [11,42] However, the GB Li + diffusivities DLi, 300K of LSTZ0.75 are only 2-3 times lower than those of bulk LSTZ0.75 (Table S3, Supporting Information). This is in sharp contrast to LLTO, where it has been reported that the GB ionic conductivity is 1-2 orders of magnitude lower than bulk ionic conductivity. [11,41] The GB activation energies in LSTZ0.75 are also comparable to the bulk value, while in LLTO, the GB activation energies are generally higher than bulk. [41] Furthermore, we note that the non-equilibrated GBs have a higher Ea and lower DLi, 300K (Table S3, Figure S8, Supporting Information), which indicates that Li enrichment and Sr vacancies play a role in increasing the GB Li + diffusivity. Most importantly, these trends were consistently observed in the low-sigma GBs and the high-sigma GB, indicating fast Li + diffusion at low-sigma GBs as well as general GBs of LSTZ0.75. We note that these general observations remain valid with and without equilibration.

Li distribution in LSTZ0.75 microstructure revealed by vibration EELS
Although attempts were made to investigate the Li distribution at the GBs by identifying To resolve this issue, we map the Li distributions by probing Li-O vibrations. To this end, we employ a state-of-the-art vibrational EELS technique enabled by high energy resolution monochromation which has facilitated the spectroscopy of vibrational excitations at the nanometer [43] and even atomic scales. [44,45] We employ a dark field vibrational EELS (DF VibEELS) beam-detector geometry ( Figure S10, Supporting Information) to ensure only high spatial resolution signals are acquired. [44,46] Representative background-subtracted spectra of LSTZ in the grain interior, grain boundary, and SL region spanning 10-150 meV (Fig. 5e)  Here the dip at the GB is less drastic owing to slight vacancies in Ta and Zr which can be more clearly seen in Fig. 5h. A similar contrast for Ta, Zr-O vibrations in the SL region is seen in Fig. 5f and, to a much lesser degree, in the intensity profiles in Fig. 3, which suggests that DF VibEELS is extremely sensitive to local elemental modulation and can be extended to map minute variations in composition. This is not unlike VibEELS ability to measure structural changes that manifest as defect modes. [47] The vacancies, the relative atomic % of Li is higher at the GB, which is consistent with Fig. 4b and With Li + ion charge carriers preserved, cubic perovskite structure maintained, and vacancies added at the GBs, Li + ion transport at the GBs is enhanced in LSTZ0.75. In comparison, Li + ions were depleted, crystal structure was dramatically altered, and no vacancies were created at the GBs for LLTO 12 . As a result, Li + ion transport at the GBs is severely impeded and LLTO has a large GB resistance.

Conclusion
Based on the results presented above, vacancy engineering of the GBs should be a viable approach to increase the GB ionic conductivity of perovskite oxide solid electrolytes. Previous studies have established that ionic conduction in LLTO occurs via the migration of Li + ions to nearby A-site vacancies. [48][49][50] Our simulation results indicate the ionic conduction mechanism of LSTZ0.75 is similar to that of LLTO. Since LSTZ0.75 maintained the perovskite framework at GBs, the additional Sr vacancies at its GBs naturally increases the number of percolation pathways and facilitates Li-ion transport. Meanwhile, preservation of the structural framework is important, for it prevents the GBs from forming structures that generate new barriers for Li-ion transport. One method to realize such goals might be incorporation of appropriate dopants into crystal lattice. It has been reported that appropriate crystal doping can improve overall ionic conductivity by introducing additional vacancies while preserving the desired crystal structure. [15,[51][52][53][54][55] Beyond this particular LSTZ system, our proposed approach should also benefit other Li-ion conductors, as diffusion to nearby structural defects is a common Li-ion conduction mechanism for solid electrolytes. [55,56] In conclusion, we report the first atomic-scale study of LSTZ to reveal its microstructures