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[Tsuyoshi Ohnishi](https://orcid.org/0000-0002-2333-7752), [Isao Sakaguchi](https://orcid.org/0000-0003-4382-2509), [Kazunori Takada](https://orcid.org/0000-0001-7568-1806)

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[Surface Treatment of Garnet-Type Solid Electrolyte for Suppressing Dendritic Growth](https://mdr.nims.go.jp/datasets/4cbe06a1-601a-4f77-ad22-6206a2f347cc)

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Surface Treatment of Garnet-Type Solid Electrolyte for Suppressing Dendritic GrowthSurface Treatment of Garnet-Type Solid Electrolyte for SuppressingDendritic GrowthTsuyoshi Ohnishi, Isao Sakaguchi, and Kazunori Takada*Cite This: ACS Appl. Energy Mater. 2024, 7, 5321−5325 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Garnet-type solid electrolytes are regarded as promising electrolytes for oxide-basedsolid-state batteries due to their stability against lithium metal. However, lithium metal anodes ongarnet-type solid electrolytes often exhibit dendritic growth, leading to internal short-circuit. Thisstudy reveals that immersion of the solid electrolytes in an aqueous solution of LiOH is a verysimple and effective way to suppress dendritic growth. It removes the contamination layer formedon the electrolyte surface by exposure to ambient air, and it makes the interface to the lithiummetal anode conductive and the current density homogeneous.KEYWORDS: solid electrolyte, garnet, lithium metal, dendrite, critical current density, short-circuit, surface treatmentGarnet-type solid electrolytes are regarded as promisingmaterials for oxide-based solid-state batteries. Among theseveral kinds of solid electrolytes, e.g., NASICON-,1 perov-skite-,2 and garnet-types,3 having ionic conductivities on theorder of 10−3 S cm−1 at room temperature, garnet-type solidelectrolytes exhibit the highest stability against lithium metalanodes,4 although recent studies point out that they undergoslight electrochemical reduction in the vicinity of the lithiummetal interface.5 However, realizing lithium metal anodes insolid-state batteries is still challenging because they often showdendritic growth in garnet-type solid electrolytes, readilyleading to internal short-circuit.6,7The dendritic growth of lithium metal takes place at a highcharging current density, which is called the critical currentdensity. The critical current density will strongly depend onthe surface chemistry of garnet. For example, exposure ofgarnet to ambient air forms a contamination layer of Li2CO3and LiOH with protonation of the garnet.8,9 Since thecontamination layer is a poor ionic conductor, it impedeslithium-ion transfer at the interface to the electrode. Therefore,when a solid-state battery with a lithium metal anode isassembled with such a garnet electrolyte and charged, thecharging current is concentrated to part of the Li/garnetinterface, where the contamination layer is not formed. In thissituation, the solid-state battery is short-circuited easilybecause the local current density at the interface with thecontamination layer is higher than the apparent current densitycalculated from the projection area of the interface and tendsto exceed the critical value for short-circuit. Various methodsto remove the contamination layer have been proposed inorder to suppress the internal short-circuit: surface polish-ing,10,11 heat treatment to react Li2CO3 with protonatedgarnet,12 and reacting Li2CO3 with carbon.13 This paperprovides a very simple method to suppress internal short-circuit effectively.This study reveals effects of the following treatments on theelectrode properties of lithium metal anodes: surface polishingusing 400 grit sandpaper, annealing at 700 °C for 2 h under O2flow with a flow rate of 200 mL min−1, and immersion in asaturated aqueous solution of LiOH. These treatments areapplied to sintered pellets of a garnet-type solid electrolyte oneby one in this order. The garnet-type solid electrolyte used inthis study is Li6.6La3Zr1.6Ta0.4O12. Sintered pellets are 10 mmsquare and 0.5 mm thick and supplied by ToshimaManufacturing Co., Ltd. After these treatments, lithiummetal electrodes with a thickness of 5 μm are formed onboth surfaces of the pellets by thermal evaporation forassembling Li/garnet/Li symmetric cells. Complex impedanceof the cells is recorded in the frequency range from 1 MHz to10 mHz with an ac signal of 20 mV at open-circuit voltage.Effects of the treatments against the internal short-circuit isevaluated by chronopotentiometry with programmed current,in which a ramp-up current of 1 or 5 mA cm−2 min−1 is appliedto Li/garnet/Li symmetric cells until the dendritic growthcauses short-circuit. No stacking pressure is applied to the cellsduring the electrochemical measurements.The symmetric cell assembled with the as-supplied sinteredpellet without any treatments gives a depressed semicircle in itsNyquist plot, as shown in Figure 1a. Since the diameter of theReceived: April 2, 2024Revised: June 12, 2024Accepted: June 16, 2024Published: June 20, 2024Letterwww.acsaem.org© 2024 The Authors. Published byAmerican Chemical Society5321https://doi.org/10.1021/acsaem.4c00805ACS Appl. Energy Mater. 2024, 7, 5321−5325This article is licensed under CC-BY-NC-ND 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on July 8, 2024 at 11:42:09 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tsuyoshi+Ohnishi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Isao+Sakaguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazunori+Takada"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsaem.4c00805&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/13?ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/13?ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/13?ref=pdfhttps://pubs.acs.org/toc/aaemcq/7/13?ref=pdfwww.acsaem.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00805?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsaem.org?ref=pdfhttps://www.acsaem.org?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/semicircle corresponds to the sum of the charge transferresistance at two Li/garnet interfaces, interfacial resistance canbe estimated to be 2.5 kΩ cm2. The ramp-up current applied tothe cell causes an abrupt drop of the cell voltage to 0 V at acurrent density of 1.4 mA cm−2 due to the internal short-circuit, as shown in Figure 1b.If the large interfacial resistance originates from thecontamination layer, a simple way to reduce the resistance isremoving the contamination layer by polishing.10,11 Indeed,polishing the pellet surface dramatically reduces the interfacialresistance to 20 Ω cm2, as shown in Figure 1c; however, ithardly increases the critical current density: the critical currentdensity is increased only to 1.8 mA cm−2, as can be seen inFigures 1b. Another method reported to remove thecontamination layer is heat treatment: it was reported thatannealing at 250 °C reacts contaminated Li2CO3 with theproton-exchanged garnet to recover the garnet surface andstructure and reduce the interfacial resistance.12 However,annealing at 700 °C in this study does not improve theperformance of the lithium metal electrode: interfacialresistance increases again to 650 Ω cm2, and critical currentdensity decreases back to 1.4 mA cm−2, as shown in panels aand b of Figure 1, respectively.Remarkable improvements are brought about by immersionin the LiOH solution, as demonstrated in Figure 1b,c. In thistreatment, the pellet is immersed in a saturated aqueoussolution of LiOH, which is prepared by dissolving an excessamount of LiOH·H2O in deionized water and should be 5.3 Min concentration according to the saturation solubility. Afterimmersion, the pellet is rinsed in a diluted LiOH solution (pH14) and then the residual solution on the rinsed pellet is blownoff by a N2 stream. The pellet is not rinsed in pure waterbecause it may cause proton exchange. It decreases theinterfacial resistance to 10 Ω cm2, which is lower than thatreported for the interface treated with carbon.13 In addition,the cell voltage does not exhibit an abrupt drop up to 5.0 mAcm−2, and instead, it increases and reaches 5 V at this currentdensity. The absence of a voltage drop indicates that internalshort-circuit does not happen up to this current density, andthe increase in the cell voltage may originate from thedepletion of lithium metal in the counter electrode; however,the counter electrode has a sufficient amount of lithium metalfor chronopotentiometry. The areal capacity to the end of themeasurement is 0.21 mAh cm−2, and the thickness of lithiummetal stripped from the counter electrode estimated from theareal capacity is 1.0 μm, as listed in Table S1 in the SupportingInformation, while the thickness of lithium metal formed onthe pellet by thermal evaporation is 5 μm. Therefore, theincrease in the cell voltage should be attributed to loss ofcontact between the garnet pellet and lithium metal; the largeamount of stripped lithium forms many voids at the interface.Increasing the ramp-up rate will enable us to determine thecritical current density in the higher current region because itreduces the consumption of lithium metal in the counterelectrode during the measurement. However, even when thecurrent ramp-up rate is increased to 5 mA cm−2 min−1, internalshort-circuit does not take place up to 10 mA cm−2, at whichthe estimated thickness of lithium metal from the counterelectrode approaches 1 μm and the cell voltage reaches 5 Vagain, as shown in Table S1 and Figure 1b, respectively. Theseresults strongly demonstrate that immersion in a LiOHsolution is effective in increasing critical current density.The differences in the electrode properties will originatefrom changes in the pellet surface that form interfaces withlithium metal electrodes, and the changes can be followed bygrazing incidence X-ray diffraction (GIXRD) at an incidentangle of 0.25° and secondary ion mass spectrometry (SIMS).The diffraction patterns are taken using Cu-Kα radiation, andthe depth profiles are recorded on a mass spectrometer using aprimary ion beam of Cs+. Li2CO3 and LiOH, which areFigure 1. Electrochemical behaviors of Li/garnet/Li symmetric cells. (a) Nyquist plots before the i−V measurement and (b) i−V characteristicsobserved under the ramp-up current. Parts of the Nyquist plots in (a) are enlarged in (c), and some data points are labeled with the measuredfrequencies. The ramp-up current rate in (b) is 1.0 mA cm−2 min−1 unless otherwise indicated, and the cells are operated at 25 °C.ACS Applied Energy Materials www.acsaem.org Letterhttps://doi.org/10.1021/acsaem.4c00805ACS Appl. Energy Mater. 2024, 7, 5321−53255322https://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00805/suppl_file/ae4c00805_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00805/suppl_file/ae4c00805_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00805/suppl_file/ae4c00805_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00805/suppl_file/ae4c00805_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig1&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00805?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aspossible contaminants, are detectable in the SIMS measure-ment as the signals of 12C and 1H, respectively. On the otherhand, immersion in the aqueous solution of LiOH may induceLi+/H+ exchange. Therefore, the samples for the SIMSmeasurement are immersed in a heavy water solution ofLiOH in order to distinguish 1H in contaminant LiOH from2D introduced during the immersion.Since the as-supplied pellet shows large interfacial resistanceto the lithium metal electrode, a contamination layer thatimpedes charge transfer at the Li/garnet interface will beformed on the surface of the pellet. However, it is not observedeven in the surface-sensitive GIXRD pattern displayed inFigure 2a, and all of the diffractions in the GIXRD pattern areattributable to the cubic garnet structure. On the other hand,the depth profile obtained by SIMS clearly reveals thecontamination. Signal intensity for Zr and La decreases andthat for C and H increases at the pellet surface, as shown inFigure 3, which suggests that the contaminants are LiOH andLi2CO3, and they are present over a range of 2 μm from thesurface. Because the contaminants are generated by exposureto ambient air at room temperature, their crystallinity will below, which is the reason for the absence of their diffractions inthe GIXRD pattern.Since the thickness of the contamination layer is 2 μm, it willbe readily removed by polishing. In fact, it significantly reducesthe resistance at the Li/garnet interface from 2.5 kΩ cm−2 to20 Ω cm−2; however, it hardly increases the critical currentdensity, which may be due to the polishing damage. Figure 2bcompares the diffraction patterns for the pellets before andafter annealing recorded by normal ω-2θ X-ray diffraction(XRD) and GIXRD. The annealing increases the diffractionintensity observed in the XRD pattern to some extent, whereasthe increase in the diffraction intensity is significant in theGIXRD pattern. Because GIXRD collects diffraction from thesurface of the sample, the remarkable increase in the diffractionintensity observed in the GIXRD pattern indicates thatcrystallinity near the surface is low after polishing; that is,the surface of the garnet pellet forming the interface to themetal lithium electrode suffers polishing damage.Annealing eliminates the polishing damage, which can berecognized as the increasing diffraction intensity, as shown inFigure 2b. However, it increases the interfacial resistance anddecreases the critical current density, which is due toreformation of a contamination layer. Strong signals for Cand H observed in the SIMS depth profiles at the pelletsurface, where signals for Zr and La are weakened, suggest thatLiOH and Li2CO3 exist as the contaminants in the surfaceregion with 0.1 μm in depth, and a diffraction at 2θ = 20.5°appearing in the GIXRD pattern is attributable to the 001reflection from LiOH. That is, the annealing segregates theresidual contaminants inside the pellet after the polishing toform a contamination layer on the surface again, whichincreases the interfacial resistance and decreases the criticalcurrent density.Immersion in a LiOH solution successfully removes thecontaminant layer without damaging the solid electrolyte. Thedepth profiles in Figure 3 indicate that contents of C and H atthe surface decrease significantly by the immersion. Moreover,immersion does not decrease the diffraction intensity from thegarnet, as shown in Figure 2a. It should be noted that the finaltreatment should be done in a LiOH solution. Immersion inwater also removes LiOH and Li2CO3 from the surface todecrease the interfacial resistance to 10 Ω cm2, asdemonstrated in Figure 1c. However, the immersion exchangesthe lithium ions in the solid electrolyte with protons todeteriorate Li transfer kinetics in the Li/garnet interface.14 Infact, the critical current density after immersion in water is 5.7mA cm−2 under the ramp-up current rate of 1 mA cm−2 min−1,as shown in Figure 1b. Under the higher ramp-up rate of 5 mAcm−2 min−1, a voltage drop does not appear at the currentFigure 2. XRD patterns for the garnet pellets. (a) GIXRD patterns forthe pellets before and after the treatments. Reflections in (a) areindexed on the basis of the cubic garnet structure. (b) Comparison ofGIXRD and XRD patterns for the pellets after polishing andannealing.Figure 3. SIMS depth profiles of the pellets. Signal intensities areplotted against the depth from the pellet surface, d, in the left panel,and the profiles in d = 0−0.55 μm are enlarged on the right.ACS Applied Energy Materials www.acsaem.org Letterhttps://doi.org/10.1021/acsaem.4c00805ACS Appl. Energy Mater. 2024, 7, 5321−53255323https://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig3&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00805?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asdensity of 5.7 mA cm−2, which is because dendrites do notgrow enough to bridge between the working and counterelectrodes. Increasing the ramp-up current rate decreases theamount of deposited lithium metal when the ramp-up currentreaches the same current density. Even so, internal short-circuittakes place at 10 mA cm−2. Such proton exchange can beavoided in the LiOH solution.15This surface treatment enabled us to assemble a solid-statebattery with a garnet electrolyte. The cathode of this battery isa mixture of LiCoO2 and the solid electrolyte applied on asintered pellet of garnet and heated at 700 °C, and the anode isa Li foil attached to the other side of the pellet. A 23 nm thickTa layer and a 5.7 nm thick In layer are deposited as theinterlayers on the cathode and anode sides, respectively.Details of the battery assembly will be reported elsewhere.16Figure 4a shows charge−discharge curves of the battery atvarious charge−discharge rates under constant-current con-ditions. The charge−discharge rates increase from 0.1C to 2C,which correspond to the current densities from 7.7 to 150 μAcm−2, and the battery is cycled four times in each C-rate. Thebattery does not reach an internal short-circuit during thecycling.The surface treatment is also effective in suppressingdendritic growth in Li-free batteries. The Li-free battery doesnot have lithium metal in its anode when assembled, and alithium metal anode is formed on the anode current collectorin situ during the first charging.17 The current collector isgenerally made of Cu, and thus, effects of the surface treatmenton the dendritic growth at the Li-free anode can be evaluatedin a Li/garnet/Cu asymmetric cell. However, the as-suppliedsintered pellet shows interfacial resistance exceeding 50 kΩcm2 to the in situ plated Li anode on the Cu current collector.Therefore, a Au interlayer with 94 nm in thickness is formedon the garnet pellet to reduce the resistance18 and perform thechronopotentiometry. Figure 4b compares the chronopotentio-grams for the Li/garnet/Au asymmetric cells with the as-supplied pellet and immersed in the LiOH solution under acurrent ramp-up rate of 0.1 mA cm−2 min−1. The cell with theas-supplied pellet reaches an internal short-circuit at 0.48 mAcm−2. This value is lower than that observed for the Li/garnet/Li cell in Figure 1b. The reason is that the internal short-circuittends to take place at a higher current density under a highercurrent ramp-up rate, as discussed above, and vice versa. Whenthe current ramp-up rate is set to 1 mA cm−2 min−1, the Li/garnet/Au cell reached an internal short-circuit at the highercurrent density of 1.5 mA cm−2. On the other hand, internalshort-circuit is not observed at least until 7 mA cm−2 for thecell assembled with the pellet after immersion in the LiOHsolution.This study demonstrates that immersion in an aqueoussolution of LiOH is a simple and effective way to suppressinternal short-circuit occurring in a solid-state battery with alithium metal electrode and a garnet-type solid electrolyte byremoving the contamination layer that forms on the surface ofthe solid electrolyte. Because many studies have revealed thatlithium dendrites grow and penetrate through garnet pelletsalong the grain boundaries,6,19 influence of the LiOH solutionon grain boundaries should be noted. It is not possible toconsider that the immersion in this study affects thepropagation of lithium metal through the grain boundariesbecause the sintered pellets used in this study are dense. Therelative density is around 97%, and there are few voids in thepellets, as shown in the cross-sectional secondary electronmicrograph in Figure S1 in the Supporting Information.Therefore, the LiOH solution will not infiltrate into the pelletduring the immersion to affect the grain boundaries.In addition, this study suggests that only reduction of theresistance at the Li/garnet interface is not enough to avoid theshort-circuit, as indicated by the critical current densitiesobserved for the surface-polished solid electrolyte and thatimmersed in water. Although the reason for this has not beenfully understood, it may be because these treatments induceinhomogeneity in the current density. The protonation causedby immersion in water is reported to lower the transfer kineticsat the Li/garnet interface, as mentioned above. In addition,since lowering crystallinity decreases the ionic conductivity,20polishing damage will also block the ionic diffusion. Thesedefects scattered at the interface make current densityinhomogeneous, and the local current density can exceed thecritical value for the internal short-circuit.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsaem.4c00805.Cross-sectional scanning electron micrograph of asintered garnet pellet and estimated thickness of platedLi at the internal short-circuit or at the end of themeasurements (PDF)■ AUTHOR INFORMATIONCorresponding AuthorKazunori Takada − Research Center for Energy andEnvironmental Science, National Institute for MaterialsScience, Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0001-7568-1806; Phone: +81 29 8604317;Email: takada.kazunori@nims.go.jpFigure 4. (a) Charge−discharge curves of a Li/garnet/LiCoO2 solid-state battery with various charge−discharge rates and (b) i−Vcharacteristics observed for Li/garnet/Au cells. The horizontal axis in(a) indicates the specific capacity (Q) based on the weight of LiCoO2.The battery is cycled four times in each C-rate in constant-currentcharge−discharge mode, and typical charge−discharge curves arepresented in the figure.ACS Applied Energy Materials www.acsaem.org Letterhttps://doi.org/10.1021/acsaem.4c00805ACS Appl. Energy Mater. 2024, 7, 5321−53255324https://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00805/suppl_file/ae4c00805_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00805/suppl_file/ae4c00805_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsaem.4c00805/suppl_file/ae4c00805_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazunori+Takada"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-7568-1806https://orcid.org/0000-0001-7568-1806mailto:takada.kazunori@nims.go.jphttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.4c00805?fig=fig4&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.4c00805?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asAuthorsTsuyoshi Ohnishi − Research Center for Energy andEnvironmental Science, National Institute for MaterialsScience, Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0002-2333-7752Isao Sakaguchi − Research Center for Electronic and OpticalMaterials, National Institute for Materials Science, Tsukuba,Ibaraki 305-0044, Japan; orcid.org/0000-0003-4382-2509Complete contact information is available at:https://pubs.acs.org/10.1021/acsaem.4c00805NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThe authors would like to thank Ms. Mai Uchida and Ms.Naomi Tsurumi for their assistance in scanning electronmicroscopy. This work was partly supported by the AdvancedLow Carbon Technology Research and Development Program,Specially Promoted Research for Innovative Next GenerationBatteries (ALCA-SPRING) of the Japan Science andTechnology Agency (JST), Japan (grant numberJPMJAL1301); GteX Program of JST, Japan (grant numberJPMJGX23S2); Materials Processing Science project (“Matere-alize”) of Ministry of Education, Culture, Sports, Science andT e c h n o l o g y (MEXT) , J a p a n ( g r a n t n umb e rJPMXP0219207397); and the Program on Open InnovationPlatforms for Industry-academia Co-creation of JST (grantnumber JPMJPF2016).■ REFERENCES(1) Aono, H.; Sugimoto, E.; Sadaoka, Y.; Imanaka, N.; Adachi, G.Ionic Conductivity of Solid Electrolytes Based on Lithium TitaniumPhosphate. J. Electrochem. Soc. 1990, 137, 1023−1027.(2) Inaguma, Y.; Liquan, C.; Itoh, M.; Nakamura, T.; Uchida, T.;Ikuta, H.; Wakihara, M. High Ionic Conductivity in LithiumLanthanum Titanate. 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