# Fileset

[JCS-Jpn 132[6] 257-266 (2024).pdf](https://mdr.nims.go.jp/filesets/f09bd79b-90dc-411e-8611-0c1017552916/download)

## Creator

Kento Ishii, Yuri Taniguchi, Akira Miura, [Shogo Miyoshi](https://orcid.org/0000-0003-0375-1187), [Kazunori Takada](https://orcid.org/0000-0001-7568-1806), Go Kawamura, Hiroyuki Muto, Atsunori Matsuda, Masayoshi Fuji, [Tetsuo Uchikoshi](https://orcid.org/0000-0003-3847-4781)

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Surface modification of Li<sub>3</sub>PO<sub>4</sub> to Li<sub>1.3</sub>Al<sub>0.3</sub>Ti<sub>1.7</sub>(PO<sub>4</sub>)<sub>3</sub> by wet chemical process and its sintering behavior](https://mdr.nims.go.jp/datasets/4c8e3203-f2e1-4f2c-82ef-76de2efbfaa0)

## Fulltext

Surface modification of Li3PO4 to Li1.3Al0.3Ti1.7(PO4)3 (LATP) by wet chemical process and its sintering behaviorJCS-Japan第132巻　第 6号　2024年 6 月 1 日発行（毎月1回1日発行）　ISSN 1348-6535 CODEN: JCSJEWJune2024vol.132Journal of the Ceramic Society of JapanFULL PAPERSurface modification of Li3PO4 to Li1.3Al0.3Ti1.7(PO4)3by wet chemical process and its sintering behaviorKento Ishii1,³, Yuri Taniguchi2, Akira Miura3, Shogo Miyoshi4, Kazunori Takada4, Go Kawamura2,Hiroyuki Muto2, Atsunori Matsuda2, Masayoshi Fuji1 and Tetsuo Uchikoshi4,‡1Nagoya Institute of Technology, Advanced Ceramics Research Center, 3–101–1 Honmachi, Tajimi, Gifu 507–0033, Japan2Toyohashi University of Technology, 1–1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441–8580, Japan3Hokkaido University, Kita 8, Nishi 5, Kita-ku, Sapporo 060–0808, Japan4National Institute for Materials Science, 1–2–1 Sengen, Tsukuba, Ibaraki 305–0047, JapanThe surface modification of lithium phosphate on lithium alumina titanium phosphate (LATP) by a wet chemicalmethod densified the LATP to over 90% at a sintering temperature of 800 °C. This temperature was approx-imately 300 °C lower than that of LATP without additives. Densification of LATP with lithium phosphate signi-ficantly progressed from 700 to 800 °C, and endothermic peaks corresponding to its melting were found in asimilar temperature range, suggesting that the densification mechanism would be due to liquid phase sintering.The liquid phase of Li3PO4 is produced by a multi-step thermal decomposition involving the reaction with LATP.The sintered LATP with lithium phosphate showed a density of about 90% and the highest ionic conductivity of3.5 © 10¹4 S/cm at 25 °C, suggesting an excellent Li ion conducting, solid electrolyte material.Key-words : LATP, Li3PO4, Surface modification, Low-temperature sintering, In-situ SEM[Received December 29, 2023; Accepted March 6, 2024]1. IntroductionAll-solid-state lithium-ion batteries (ASSLiBs) consistof solid materials for all components including the elec-trolyte and electrode active materials.1)–4) ASSLiBs withoxide-based electrolytes are expected to be the next-generation batteries. Compared to conventional batterieswith liquid electrolytes, they are superior in safety anddurability. They are expected to be applied in a wide rangeof fields, such as IoT, mobility, and energy storage sys-tems (microgrids), as the next-generation rechargeable bat-tery.5)–8) Besides oxides, polymer, halide and sulfide mate-rials are used as ASSLiB electrolytes. Among these mate-rials, oxide materials lack plasticity and require a sinteringprocess in addition to the powder pressurization process forthe densification and junction of the cell components. Thisprocess can join the particles and component interfaces,and good Li-ion conduction pathways are formed. Theone-step co-sintering process is one of the efficient meth-ods for sintering battery green cells.9)–12) The sequentialco-sintering method, in which the deposition and sinter-ing of each component are repeated, requires multiple sin-tering cycles. The one-step co-sintering method, in whichsequential deposition followed by sintering of all the com-ponents is performed at once, is a more efficient way. Theone-step co-sintering temperature is limited to the lowestmelting point of the component materials. In general, theactive material often has a lower sintering temperature thanthe electrolyte material, so the co-sintering temperature islimited to the sintering temperature of the electrode.13)–15)The electrolyte material must be densely sintered togetherwith the electrode in a restricted temperature range, and theinterfaces between different phases, such as the electrolyte/electrode layer interface within the composite electrode,must be well-bonded. For this reason, low-temperature sin-tering methods have been developed that can sufficientlydensify the electrolyte at lower temperatures.Various methods have been used for the low-temperature sintering of electrolytes, including elementsubstitution,16)–20) minimization of particle size by ball-milling and liquid-phase synthesis,19),21)–25) the addition ofsintering aids,26)–31) and sintering techniques such as hotpressing, spark plasma sintering, and cold sintering.20),32)–37)For the method of adding a sintering additive, the additivemelts at a lower temperature than the base material. It fillsthe gaps between the particles of the base material, therebydensifying the material. When this process is applied toelectrolyte materials, the microstructure must be densified,and good Li-ion conduction pathways must be formedwithout forming highly resistive phases. Therefore, there isa need to select sintering additives that do not significantlyimpair the characteristics of the base material and a meth-od that allows for the uniform addition of a minimum³ Corresponding author: K. Ishii; E-mail: ishii.kento@nitech.ac.jp‡ Corresponding author: T. Uchikoshi; E-mail: UCHIKOSHI.Tetsuo@nims.go.jpJournal of the Ceramic Society of Japan 132 [6] 257-266 2024DOI https://doi.org/10.2109/jcersj2.23208 JCS-Japan©2024 The Ceramic Society of Japan 257This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.23208https://creativecommons.org/licenses/by/4.0/amount of additives. In the field of batteries, surfacemodification methods for improving the electrochemicalstability of electrode active materials have been well stud-ied. These methods are expected to be applied as surfacemodification methods for electrolyte materials.38)–48)Among the oxide-based solid electrolytes, theNASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) has excellentproperties; i.e., high lithium-ion conductivity, low cost,low toxicity, and easy synthesis.19),20),23),49),50) In addition,it has an excellent chemical stability in air and water.51)–54)In the past, cathode active materials compatible with LATPelectrolytes have been investigated.13),14),55)–59) Consider-ing the thermal stability of the active materials and theirreactivity with LATP, the co-sintering process should beperformed at a temperature of 800 °C or below. How-ever, since the sintering temperature required for LATP isaround 950 °C, it is necessary to lower the temperature. Ithas been reported that trace amounts of Co elements inCo-based cathode active materials diffuse into the LATPphase, thereby densifying the LATP phase.56),59)–62) Thesestudies suggested that lithium phosphate salts, which areformed as intermediate products during the reaction of Cowith LATP, improve the sinterability of LATP. Variouslithium salts, such as LiBO2, Li3BO3, Li2CO3, LiCl, LiOHand Li3PO4, have been applied as sintering additives forLATP.27)–29),31),63)–65) Lithium phosphate is a known lithi-um ion conductor and is expected to be a sintering aid thatdoes not block the Li-ion conduction of LATP at its grainboundaries.66)–70) It has been reported that LATP withLi3PO4 is densified at a sintering temperature of 800 °C,which is lower than the melting point of Li3PO4, 837 °C,but the sintering mechanism has not been revealed. Thesintering at temperatures lower than the melting point ofthe sintering aid may be due to the contribution of anintermediate phase produced by the reaction between thematrix phase and the aid. The actual melting occurs at tem-peratures lower than the melting point in many materialsystems has been reported, but the mechanism is still notclarified.71),72) In this study, the effectiveness of the wetchemical method for the uniform addition of the Li3PO4sintering aid to LATP is investigated, and the effects onthe sintering mechanism and electrical conductivity arediscussed.2. Experimental procedure2.1 Sample preparationFigure 1 shows the preparation process of the Li3PO4-modified LATP (LATP-Li3PO4) powder and particles.Commercially-available LATP powder (Toshima Manu-facturing Co., Ltd., Japan) having the average particle size(D50) of 0.6¯m with the NASICON structure and R-3cspace group, lithium acetate dihydrate (CH3COOLi·2H2O,Nacalai Tesque, Inc., Japan), phosphoric acid (H3PO4, 85wt% concentration, Kanto Chemical Co., Inc., Japan), andreagent-grade ethanol (C2H5OH, 99.5% concentration,Nacalai Tesque, Inc., Japan) were used for the experi-ments. The impurity elements and their amounts containedin the LATP powder are shown in Table S1. Lithium ace-tate dihydrate was dissolved in the ethanol solvent so thatthe amount of the Li content in Li3PO4 relative to LATPwas x = 0.0, 0.1, 0.2, 0.3, 0.5, 1.0, 2.0wt%. While thesolution was being ultrasonicated, 15-vol% LATP powderwas dispersed in this solution. H3PO4 was then added tothe slurry while stirring. Lithium acetate dihydrate andphosphoric acid were added in the molar ratio of 3:1.Li3PO4 was precipitated on the LATP particle surface bythe reaction of lithium acetate and phosphoric acid, asexpressed by Eq. (1).73)3CH3COOLi�2H2Oþ H3PO4! Li3PO4 # þ 3CH3COOHþ 2H2O ð1ÞThe prepared slurry was dried at 100 °C for 1 h using a hotplate, and the LATP-Li3PO4 powder was obtained. Thecoarse agglomerates of the dried powder were crushedwith an agate mortar, then the powder was thermally treat-ed at 400 °C for 1 h in air at the heating rate of 200 °C/h toremove the produced acetic acid. The powder was uniaxi-ally pressed into a disk shape with a 10-mm diameter and1-mm thickness at 100MPa. The green bodies were firedat 800 °C for 10 h in air at the heating rate of 200 °C/h.2.2 Sample characterizationThe relative densities of the LATP-Li3PO4 fired bodieswere calculated by measuring the bulk density of theLi3PO4-LATP. The thickness at five locations and thediameter at four locations were measured with respect tothe disk-shaped samples, and the volume was calculatedfrom the average of these measurements. The bulk densityof the samples was calculated from the measured weightand volume. The presence of any reaction phases wasevaluated by a powder X-ray diffraction (XRD) measure-ment (RINT-TTRIII, Rigaku Corp., Japan). The state ofthe Li3PO4 modification on the surface of the LATP par-ticles was evaluated by Fourier-transform infrared spec-troscopy and the attenuated total reflection method (ATR-FTIR) (IRSprit with QATR-S, Shimadzu Corp., Japan).The differential scanning calorimetry (DSC) of LATP-Li3PO4 was measured from 25 to 1050 °C in air at theheating rate of 10 °C/min using a thermal analyzer(DSC3300SA, Bruker AXS, Inc., MA, USA). The frac-Fig. 1. The preparation process of the LATP-Li3PO4.Ishii et al.: Surface modification of Li3PO4 to Li1.3Al0.3Ti1.7(PO4)3 by wet chemical process and its sintering behaviorJCS-Japan258tured surface (FS) of the LATP-Li3PO4 fired bodies wasobserved by a scanning electron microscope (SEM) (JSM-6500F, JEOL Ltd., Japan). The FS of the fired specimenswas etched by an argon-ion milling cross-section polisher(IB-09020CP, JEOL Ltd., Japan), and the microstructureobservations and elemental analysis were performed bySEM-EDS (JSM-7800F, JEOL Ltd., Japan). The morphol-ogy change in the LATP-Li3PO4 powder during heatingwas characterized by an in-situ SEM (JSM-IT200 with anattached heating stage, JEOL Ltd., Japan). All the fireddisc-shaped LATP-Li3PO4 pellets were polished and coat-ed with Au electrodes on both sides using a sputteringapparatus (SC-701, Sanyu Electronics Co., Ltd., Japan).The Au-coated samples were sealed in a glass containerfilled with Argon gas to avoid the sub-reaction during theelectrochemical testing. The electrical conductivity wasmeasured using an impedance analyzer (VSP-300, Bio-Logic Science Instruments, Ltd., France) in the frequencyrange from 7MHz to 0.1Hz with the AC amplitude of14mV at temperatures from 25 to 125 °C.3. Results and discussion3.1 Powder characteristicsThe liquid-phase synthesis based on the Eq. (1) reactionlead to the synthesis of the highly-crystalline single-phaseLi3PO4, as shown in Fig. S1. The peaks of the Li3PO4phase could not be detected from the XRD pattern ofLATP modified with Li3PO4 because the amount of Li3PO4modification is minimal. Instead, the peaks of the LATPpowder modified with Li3PO4 were very similar to those ofthe unmodified LATP powder, indicating that no decompo-sition or side reactions of the LATP phase occurred duringthe liquid-phase treatment. EDS mapping of the Li3PO4modified LATP powder showed the presence of a largeamount of the P element, suggesting the presence of phos-phorus compounds (probably Li3PO4) on the surface of thetreated powder, as shown in Fig. S2. We also attempted tocharacterize the surface of the treated powders by trans-mission electron microscopy, but the results did not di-rectly indicate the presence of the Li3PO4 phase, becauseLi3PO4 is susceptible to electron beam damage. The ATR-FTIR spectrum of the LATP-Li3PO4 powders (x = 0.0–2.0)thermally treated at 400 °C and Li3PO4 powder is shown inFig. S3. The peaks around 1000 cm¹1 are attributed to theasymmetric and symmetric stretching vibrations of the P–O bond in the PO43¹ tetrahedral.74)–76) The absorption peakintensity at 1018 cm¹1 increased with the increase in theaddition amount x. The peaks below 750 cm¹1 are due tothe Ti–O stretching vibrations in the TiO6 hexahedron andasymmetric modulation vibrations of the O–P–O bonds inthe PO43¹ tetrahedron. The peak at 1455 cm¹1 is due tovibrations of the Al–O bond. With the increasing Li3PO4addition, the peak intensities originating from the vibra-tions of the Al–O and Ti–O bonds decreased and thespectrum gradually approached that of Li3PO4.77)–79) Theseresults suggest that Li3PO4 is modified on the surface ofLATP.3.2 Relative densityFigure 2 shows the relative densities of LATP-Li3PO4(x = 0.0–2.0) sintered at 600, 700, and 800 °C and its com-pact. The LATP-Li3PO4 sintered at 600 °C was not densi-fied at any addition amount of x = 0.0–2.0, and its densitywas similar to that of the green body. Densification ofLi3PO4-unmodified LATP (x = 0.0) had not significantlyprogressed at any sintering temperature between 600–800 °C. On the other hand, LATP-Li3PO4 (x = 0.1–2.0)densified with the increasing sintering temperature from600 to 800 °C. At the sintering temperature of 800 °C, thesintered density was 63% at x = 0.0 and the sintered den-sity was about 90% at x = 0.1–2.0. Since LATP sinteredat 1100 °C showed a density of about 90%, the sinteringtemperature of LATP could be lowered by 300 °C with theaddition of Li3PO4. Figure 3 shows the relative densitiesof LATP, Li3PO4 and LATP-Li3PO4 (x = 0.3) versus thesintering temperature. The single LATP exhibited a maxi-mum sintering density at 1100 °C. The density of Li3PO4rapidly increased between 600 and 800 °C, with a maxi-mum sintered density of 87% at 800 °C, followed by a de-crease in the sintered density toward 900 °C. The decreasein the density of Li3PO4 from 800 to 900 °C is due to thephase transition from the solid to liquid phase as the melt-ing proceeded. The decrease in density above 900 °C isprobably due to the formation of shrinkage cavities duringFig. 2. Relative densities of LATP-Li3PO4 fired at temperaturesof 600, 700 and 800 °C, and unfired LATP-Li3PO4.Fig. 3. The relative densities of the fired LATP-Li3PO4 forx = 0.0 and 0.3 of the Li content in Li3PO4 relative to LATP, andthe densities of the fired Li3PO4.Journal of the Ceramic Society of Japan 132 [6] 257-266 2024 JCS-Japan259the dissolution and resolidification of Li3PO4. In addition,the rapid melting or over-sintering may have contributedto the expansion of the residual isolated pores in the sam-ple.80)–83) The sintering behavior of LATP-Li3PO4 (x = 0.3)was more similar to that of Li3PO4 than that of the basematerial, LATP. Despite the very small amount of addedLi3PO4, the sinterability of the LATP-Li3PO4 system wouldbe dominated by Li3PO4 and its intermediate products.3.3 Reactivity during sinteringFigure 4 shows the XRD patterns of LATP-Li3PO4 (x =0.0–2.0) sintered at 800 °C. As shown in the XRD mea-surement results in Fig. 4, no secondary phase appearedin LATP fired at 800 °C. Although not shown here, nosecondary phase appeared even in the single Li3PO4 firedat 800 °C. On the other hand, in LATP-Li3PO4 fired at800 °C, new products were clearly formed as shown inFig. 4, and their intensity became more pronounced as theamount of Li3PO4 modification increased. The above sug-gests that when LATP and Li3PO4 coexist, some reactionand/or decomposition occur during firing at 800 °C. Theformation of Li4P2O7 and LiPO3 by thermal decompositionof Li3PO4 during firing likely follows Eqs. (2)–(4). Be-sides Li3PO4 as an additive, lithium phosphates wouldbe produced as decomposition products from the LATPphase. This is because the XRD of LATP-Li3PO4 beforesintering did not identify the Li3PO4 phase, whereas thepeak intensity of the lithium phosphate phase in the XRDpattern after sintering was high.2Li3PO4 ! Li4P2O7 þ Li2O ð2ÞLi4P2O7 ! 2LiPO3 þ Li2O ð3ÞLi3PO4 ! LiPO3 þ Li2O ð4Þ3.4 Microstructures and elemental mappingFigure 5 shows that the FS and cross-section polishing(CP) microstructures of the sintered LATP-Li3PO4 (x =0.0, 0.3 and 2.0wt%). LATP-Li3PO4 (x = 0.3, 2.0) sin-Fig. 4. The XRD patterns of the LATP-Li3PO4 for x = 0.0–2.0 of the Li content in Li3PO4 relative to LATP,sintered at 800 °C.Fig. 5. The FS and the cross-section polished by Ar-ion milling (CP) of the sintered LATP-Li3PO4 (Li contentin Li3PO4 relative to LATP, x = 0.0, 0.3 and 2.0).Ishii et al.: Surface modification of Li3PO4 to Li1.3Al0.3Ti1.7(PO4)3 by wet chemical process and its sintering behaviorJCS-Japan260tered at 800 °C formed a denser microstructure than LATPsintered at 1000 °C. The LATP sintered at 1100 °C wasdense but very brittle due to intergranular cracking in itsstructure as shown in Fig. S4. The densities of the sinteredLATP-Li3PO4 were calculated by the dimension measure-ment and image analysis as shown in Table S2. The cal-culated density of the LATP single-phase by the imageanalysis was similar to that obtained from the dimensionmeasurements. However, the calculated densities of LATP-Li3PO4 (x = 0.3 and 2.0) by the image analysis werehigher than those obtained from the dimension measure-ments. The SEM image of the fracture surface was taken atthe center of the sample, and the SEM image of the pol-ished surface was taken relatively close to the sample sur-face. Therefore, it is conceivable that the denseness of thesample surface and the sparseness of the interior affectedthe results of the two analytical methods. Furthermore, thereaction products may have caused errors in the calculationof the theoretical density of LATP-Li3PO4. In the case ofLATP-Li3PO4, no intergranular cracks were formed, andthe addition of Li3PO4 was also effective in densifyingLATP without intergranular cracks. The LATP-Li3PO4 sin-tered at 800 °C had a relative density of 93% and the samedensity as the single LATP sintered at 1100 °C which hada relative density of 91%, but its grain growth had notprogressed. This suggested that the melting of the addedLi3PO4 densified the microstructure. In the CP image ofLATP-Li3PO4 at x = 2.0, some segregates with a dark con-trast were present in the microstructure, but no segregatedphases were found in the microstructure at x = 0.3. Atx = 2.0, the amount of the Li3PO4 addition was excessive,and Li3PO4 was segregated to the grain boundaries. Whenthe amount of Li3PO4 added is x = 0.3, Li3PO4 as a sinter-ing aid, does not segregate at the LATP grain boundaries.In addition, an elemental analysis was performed to char-acterize the segregated phases of the LATP-Li3PO4.Figure 6 and Table 1 show the EDX elemental map-ping and point spectrum of LATP-Li3PO4 (x = 2.0) sin-tered at 800 °C, respectively. From the mapping images,three main phases were identified, i.e., the LATP matrixphase (spots 1, 2, and 3), the Al-poor phase with a lightercontrast (spots 4, 5, and 6), and the Ti-poor phase with adarker contrast (spots 7, 8, 9, and 10). The ratio of theelements present in the entire image area was similar to thetheoretical value of LATP as shown in Table 1. The phasesin spots 1, 2, and 3 can be regarded as the LATP phases,although they are somewhat enriched in Al elements. Thephases in spots 4, 5 and 6 are presumed to be the LiTiPO5Fig. 6. The elemental mapping images of the LATP-Li3PO4 (Li content in Li3PO4 relative to LATP, x = 2.0)fired at 800 °C.Table 1. The elemental ratios at each point in the EDS analysisSpectrum/At% Al Ti P O O/P Condition Predicted PhaseSpot 1 2.7 8.6 17.0 71.7 4.2 Al-rich LATPSpot 2 2.6 7.8 15.7 73.6 4.7 Al-rich LATPSpot 3 2.3 9.1 17.0 71.6 4.2 Al-rich LATPSpot 4 — 12.8 14.3 72.8 5.1 No Al content LiTiPO5Spot 5 — 12.2 14.7 73.1 5.0 No Al content LiTiPO5Spot 6 — 12.8 13.5 73.7 5.5 No Al content LiTiPO5Spot 7 0.8 4.8 20.0 74.3 3.7 Al, Ti-poor Li3PO4, Li4P2O7Spot 8 1.3 5.2 19.1 74.5 3.9 Ti-poor LATP, Li3PO4Spot 9 0.6 2.1 27.8 69.4 2.5 Al- and Ti-poor, P-rich LiPO3, P2O5Spot 10 1.7 5.4 22.6 70.3 3.1 Ti-poor, P-rich Li4P2O7, LiPO3Whole region 1.9 8.6 18.2 71.4 3.9 — Li3PO4-LATP (x = 2.0)LATP (Theoretical) 1.8 10.2 18 72 4.0 Amount of 6 molar LATP —Journal of the Ceramic Society of Japan 132 [6] 257-266 2024 JCS-Japan261phases since they contain no Al and have an O/P ratio =5. Spots 7, 8, 9, and 10 are Ti-poor phases, and their O/Pratios suggested that they are ortho-, pyro-, and meta-lithium phosphate salts.3.5 Thermal analysisFigure 7 shows the DSC curves of LATP-Li3PO4 atx = 0.0 and 2.0 and Li3PO4. No noticeable change wasobserved between 600–800 °C for the single LATP. ForLATP-Li3PO4 at x = 0.2, an endothermic reaction occur-red between 771 and 900 °C, along with a large endother-mic peak at 816 °C. In addition to this large peak, twosmall peaks were seen between 771–816 °C. These peaksindicated the decomposition and melting of Li3PO4, reac-tion of LATP with Li3PO4, and the decomposition reactionof Li3PO4 as shown by Eqs. (2)–(4). The reactions ofEqs. (2) and (3) sequentially proceeded at about 816 °C,so that these reactions are summarized by Eq. (4).Li3PO4 becomes the intermediate product, Li4P2O7, andthe final product, LiPO3. The melting point of Li3PO4 is837 °C, and its sintering proceeded in the LATP-Li3PO4system with the formation and melting of LiPO3. This isbecause the melting point of LiPO3 is 650 °C29),70),84) thusthe produced LiPO3 immediately melts at 800 °C. Themelting of Li3PO4 is accompanied by thermal decompo-sition and melting of the other lithium phosphate com-pounds. Therefore, the melting point of the Li3PO4 singlephase is 837 °C, but in the LATP-Li3PO4 system, the liquidphase is formed at a temperature lower than the meltingpoint of Li3PO4, and sintering occurs. The sintering mech-anism of LATP-Li3PO4 was suggested to be liquid phasesintering as already described.3.6 In-situ SEM observationFigures 8(a) and 8(b) show the morphology of theLATP and LATP-Li3PO4 (x = 2.0) secondary particlesduring heating as observed by in-situ SEM, respectively.Morphological changes in the secondary particles duringthe sintering and shrinking process were analyzed byImageJ, an image analysis software program.85)–87) TheFig. 7. The DSC curves of the LATP-Li3PO4 powder (Li con-tent in Li3PO4 relative to LATP, x = 0.0 and 2.0) at the heatingrate of 10 °C/min in air.Fig. 8. (a) The morphology of the LATP particles during heating observed by in-situ SEM, (b) the morphologyof the LATP-Li3PO4 (Li content in Li3PO4 relative to LATP, x = 2.0) particles during heating observed by in-situSEM.Ishii et al.: Surface modification of Li3PO4 to Li1.3Al0.3Ti1.7(PO4)3 by wet chemical process and its sintering behaviorJCS-Japan262SEM images at each temperature were classified intoparticle and spatial areas by a binarization process, and thevalues of the secondary particle areas were calculated.Their area shrinkage rates were calculated as the differencebetween the secondary particle area at room temperatureand at each temperature. The actual sample dimensionswere measured before and after sintering, and the volu-metric shrinkage was determined. Figures 9(a) and 9(b)show the sintering shrinkage rates of the LATP and LATP-Li3PO4 (x = 2.0) calculated by a sample dimensionalmeasurement and image analysis. Although the secondaryparticles observed in situ were only a few microns indiameter and not composed of many primary particles, themeasured and calculated shrinkage rates at each temper-ature were similar. Therefore, the morphological change inthe particles during heating observed by in-situ SEM isconsidered to represent the sintering behavior in the entirebulk body. For LATP-Li3PO4 (x = 2.0), the morphology ofthe secondary particle changed and significantly shrankfrom 600 to 800 °C. During the initial stage of sintering upto 700 °C, the shrinkage progressed by ¹25% of abouthalf the total, and the primary particles joined each otherinto a single particle form. At this intermediate stage asshown in Fig. 10, it is considered that several pores re-mained in the integrated particle group.80)–83) Furthermore,during the final stage of sintering from 700 to 800 °C, theshrinkage progressed by ¹20%, and the joined particlesbecame smaller while maintaining their shape. The meltingtemperature range was understood to be 771–816 °C basedon the DSC measurement results; the integration of theparticle group seemed to occur before the melting of thelithium phosphate.3.7 Electrical conductivityThe Nyquist plots of LATP-Li3PO4 (x = 0.0–2.0) mea-sured by the AC impedance method exhibited a clearsemicircle as shown in Fig. S5. From the origin to the x-axis intercept of the arc is the bulk resistance (Rb), thesemi-circular arc is the grain boundary resistance (Rgb) andpseudo-capacitance (CPE1), and in the low-frequencyregion, the electric double layer corresponds to the pseudo-capacitance (CPE2), similar to the equivalent circuit inFig. S5. The relaxation frequencies of LATP-Li3PO4 ob-tained from the semicircles corresponding to the grainboundary impedance are shown in Table S3. The relaxa-tion frequencies are similar between x = 0.1 and 1.0, ex-cept x = 0.0 (pure LATP) and x = 2.0 (excess Li3PO4-added LATP), indicating that there was no significantchange in the grain boundary composition.Figure 11(a) shows the electrical conductivity of LATP-Li3PO4 (x = 0.0–2.0) calculated from the Nyquist plot.The electrical conductivity at 25 °C of LATP-Li3PO4 (x =0.0–2.0) is shown in Fig. 11(b). The electrical conductiv-ity of LATP-Li3PO4 increased from x = 0.0 to 0.2, thenremained constant until x = 0.5. The electrical conductiv-ity decreased above x = 0.5. From x = 0.0 to 0.2, the con-ductivity increased due to the increased sintering densityFig. 9. The thermal shrinkages of (a) LATP and (b) LATP-Li3PO4 (Li content in Li3PO4 relative to LATP, x = 2.0) calcu-lated by size measurement and in-situ SEM image analysis.Fig. 10. The sintering mechanism of the LATP-Li3PO4.Journal of the Ceramic Society of Japan 132 [6] 257-266 2024 JCS-Japan263and decreased length of the lithium conduction pathways.For x = 0.2–0.5, the sintered density did not change andthe added Li3PO4 was uniformly present without segrega-tion, so there was no significant change in the conductivity.The decrease in conductivity at x = 0.5–2.0 is due to theexcess amount of segregated Li3PO4 acting as a resistivephase, and the significantly increased amount of the reac-tion phases. No significant changes were observed in anyof the activation energies. The grain boundary componentaccounts for a large fraction of the total conductivity, andthe activation energy also depends on factors related to thegrain boundary. Since the amount of Li3PO4 added by thismodification method was minimal and the amount of thereaction phase formed between the LATP particles wasvery small, it is postulated that there was no change in theactivation energy. LATP sintered at 800 and 1000 °C ex-hibited an electrical conductivity of 6.7 © 10¹5 and 2.7 ©10¹4 S/cm at 25 °C, respectively. LATP-Li3PO4 (x = 0.5wt%) sintered at 800 °C exhibited the highest electricalconductivity of 3.5 © 10¹4 S/cm at 25 °C. The obtainedconductivities of LATP-Li3PO4 were comparable to or ex-ceeded those of previous reports.29),88) The difference inthese values is due to the addition method of the sinter-ing aid. Sintering aids were added by the powder mixingmethod in previous reports and by surface modificationutilizing the wet chemical reaction method in this study.The powder mixing method is simple, but its structuretends to be heterogeneous, resulting in a low conductivity.The surface modification method can uniformly add thesintering aid and form a more homogeneous microstruc-ture without segregation resulting in a higher conductivity.By surface-modifying the electrolyte particles using a wetchemical reaction method, a minimum amount of sinteringaid can be added uniformly, and as a result, a sintered bodywith a uniform structure can be obtained. Li3PO4 effec-tively acts as a sintering aid for LATP, and LATP with asintered density equivalent to 1100 °C can be obtained at asintering temperature of 800 °C. As a result, LATP-Li3PO4sintered at 800 °C exhibits high electrical conductivity.4. ConclusionsLATP-Li3PO4 was densified to more than 90% relativedensity by sintering up to 800 °C, and its sinterability wasrather higher than that of single Li3PO4, suggesting theinterface reaction between Li3PO4 and LATP contributedto the improvement of sinterability of LATP-Li3PO4. Thesintering of LATP-Li3PO4 was found to be not only due tothe sintering and melting of Li3PO4, but also to a liquidphase sintering mechanism related to the melting of somelithium phosphates formed by the reaction of LATP andLi3PO4. Although several reaction phases were formedduring the sintering of LATP-Li3PO4, it was possible toachieve both a high sintering density and high electricalconductivity by adding the minimum amount of Li3PO4 bythis surface-modification method.Acknowledgments We thank Ms. Makiko Oshida at theNational Institute for Materials Science Battery Research Plat-form for her help with the argon-ion milling and SEM-EDSmeasurements. The authors wish to thank Dr. Hiroyo Segawain the Electroceramics Group, Research Center for FunctionalMaterials, NIMS, for her help with the DSC measurements.We thank Ms. Masae Sawamoto at Hokkaido University forher help with the in-situ SEM measurements. This study wassupported in part by the Materials Processing Science Proj-ect (“Materealize”), grant number JPMXP0219207397 fromMEXT, and supported by JST, PRESTO Grant NumberJPMJPR21Q8, Japan.Supporting Information Figures and photographs thatcould not be included in the main part of this paper can bereferred to as “Supplemental materials”.References1) K. Takada, Acta Mater., 61, 759–770 (2013).2) S. Sen, E. Trevisanello, E. Niemöller, B. X. Shi, F. J.Simon and F. H. Richter, J. Mater. Chem. A, 9, 18701–18732 (2021).3) R. Chen, W. Qu, X. Guo, L. Li and F. Wu, Mater.Horizons, 3, 487–516 (2016).4) L. Liang, X. Sun, J. Zhang, J. Sun, L. Hou, Y. Liu andC. Yuan, Mater. Horizons, 6, 871–910 (2019).5) J. B. Goodenough and Y. Kim, Chem. Mater., 22, 587–603 (2010).Fig. 11. (a) The electrical conductivities of the LATP-Li3PO4(Li content in Li3PO4 relative to LATP, x = 0.0–2.0) body firedat 800 °C, (b) the electrical conductivity at 25 °C of the LATP-Li3PO4 at x = 0.0–2.0.Ishii et al.: Surface modification of Li3PO4 to Li1.3Al0.3Ti1.7(PO4)3 by wet chemical process and its sintering behaviorJCS-Japan2646) Y. Zhu, J. C. Gonzalez-Rosillo, M. Balaish, Z. D. Hood,K. J. Kim and J. L. M. Rupp, Nat. Rev. Mater., 6, 313–331 (2021).7) R. Chen, Q. Li, X. Yu, L. Chen and H. Li, Chem. Rev.,120, 6820–6877 (2020).8) X. Chen, Z. Guan, F. Chu, Z. Xue, F. Wu and Y. Yu,InfoMat, 4, 1–20 (2022).9) G. Ye, F. Ju, C. Lin, S. Gopalan, U. Pal and D. A.Seccombe, Proc. - Electrochem. Soc., PV 2005-07,451–459 (2005).10) P. Xu, W. Rheinheimer, S. N. Shuvo, Z. Qi, O. Levit, H.Wang, Y. Ein-Eli and L. A. Stanciu, ChemElectroChem,6, 4576–4585 (2019).11) B. Wang, L. Bi and X. S. Zhao, J. Eur. Ceram. Soc., 38,5620–5624 (2018).12) C. Seok, J. Moon, M. Park, J. Hong, H. Kim, J. W. Son,J. H. Lee, B. K. Kim, H. W. Lee and K. J. Yoon, J. Eur.Ceram. Soc., 36, 1417–1425 (2016).13) Y. Wu, S. Wang, H. Li, L. Chen and F. Wu, InfoMat, 3,827–853 (2021).14) L. Miara, A. Windmüller, C. L. Tsai, W. D. Richards, Q.Ma, S. Uhlenbruck, O. Guillon and G. Ceder, ACS Appl.Mater. Inter., 8, 26842–26850 (2016).15) K. J. Kim and J. L. M. Rupp, Energ. Environ. Sci., 13,4930–4945 (2020).16) S. Ramakumar, C. Deviannapoorani, L. Dhivya, L. S.Shankar and R. Murugan, Prog. Mater. Sci., 88, 325–411 (2017).17) M. M. Raju, F. Altayran, M. Johnson, D. Wang and Q.Zhang, Electrochem, 2, 390–414 (2021).18) A. Rossbach, F. Tietz and S. Grieshammer, J. PowerSources, 391, 1–9 (2018).19) J. A. Dias, S. H. Santagneli and Y. Messaddeq, J. Phys.Chem. C, 124, 26518–26539 (2020).20) K. Yang, L. Chen, J. Ma, Y. He and F. Kang, InfoMat, 3,1195–1217 (2021).21) L. Xingang, T. Jiang, J. Fu, R. Yuan, H. Wen and C.Zhang, ACS Appl. Mater. Inter., 9, 11696–11703 (2017).22) Y. Ren, H. Deng, H. Zhao, Z. Zhou and Z. Wei, Ionics,7, 6049–6056 (2020).23) R. Dewees and H. Wang, ChemSusChem, 12, 3713–3725 (2019).24) K. Waetzig, A. Rost, C. Heubner, M. Coeler, K.Nikolowski, M. Wolter and J. Schilm, J. Alloy. Compd.,818, 153237 (2020).25) L. Vijayan and G. Govindaraj, J. Phys. Chem. Solids,72, 613–619 (2011).26) X. Zhang, T. S. Oh and J. W. Fergus, J. Electrochem.Soc., 166, A3753–A3759 (2019).27) H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka andG. ya Adachi, Chem. Lett., 19, 331–334 (1990).28) H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka andG. ya Adachi, Solid State Ionics, 47, 257–264 (1991).29) K. Waetzig, C. Heubner and M. Kusnezoff, Crystals,10, 408 (2020).30) S. Chen, X. Hu, W. Bao, Z. Wang, Q. Yang, L. Nie, X.Zhang, J. Zhang, Y. Jiang, Y. Han, C. Wan, J. Xie, Y. Yuand W. Liu, Cell Reports Phys. Sci., 2, 0–16 (2021).31) H. Bai, J. Hu, X. Li, Y. Duan, F. Shao, T. Kozawa, M.Naito and J. Zhang, Ceram. Int., 44, 6558–6563 (2018).32) P. M. G. Puente, S. Song, S. Cao, L. Ziwen, X. Xiang,Q. Shen and F. Chen, J. Adv. Ceram., 10, 933–972(2021).33) N. Hamao, Y. Yamaguchi and K. Hamamoto, Materials,14, 4737 (2021).34) Y. Liu, Q. Sun, D. Wang, K. Adair, J. Liang and X. Sun,J. Power Sources, 393, 193–203 (2018).35) H. Zhu, A. Prasad, S. Doja, L. Bichler and J. Liu,Nanomaterials-Basel, 9, 6–8 (2019).36) Y. Liu, J. Liu, Q. Sun, D. Wang, K. R. Adair, J. Liang,C. Zhang, L. Zhang, S. Lu, H. Huang, X. Song and X.Sun, ACS Appl. Mater. Inter., 11, 27890–27896 (2019).37) A. Paolella, W. Zhu, G. Bertoni, A. Perea, H. Demers, S.Savoie, G. Girard, N. Delaporte, A. Guerfi, M. Rumpel,H. Lorrmann, G. P. Demopoulos and K. Zaghib, Adv.Mater. Interfaces, 7, 2000164 (2020).38) J. M. Kim, X. Zhang, J. G. Zhang, A. Manthiram, Y. S.Meng and W. Xu, Mater. Today, 46, 155–182 (2021).39) H. Zhang, J. Xu and J. Zhang, Front. Mater., 6, 1–10(2019).40) Z. Chen, Y. Qin, K. Amine and Y. K. Sun, J. Mater.Chem., 20, 7606–7612 (2010).41) X. Bian, Q. Fu, X. Bie, P. Yang, H. Qiu, Q. Pang, G.Chen, F. Du and Y. Wei, Electrochim. Acta, 174, 875–884 (2015).42) H. Chen, L. Xiao, H. Chen, Y. Zhu, K. Xiang and H.Liao, Electrochim. Acta, 344, 1–11 (2020).43) Z. Chen, D. Chao, J. Lin and Z. Shen, Mater. Res. Bull.,96, 491–502 (2017).44) V. Mereacre, N. Bohn, P. Stüble, L. Pfaffmann and J. R.Binder, ACS Appl. Energ. Mater., 4, 4271–4276 (2021).45) W. Guo, Y. Meng, Y. Hu, X. Wu, Z. Ju and Q. Zhuang,Front. Energy Res., 8, 1–20 (2020).46) C. Li, H. P. Zhang, L. J. Fu, H. Liu, Y. P. Wu, E. Rahm,R. Holze and H. Q. Wu, Electrochim. Acta, 51, 3872–3883 (2006).47) C. Gong, Z. Xue, S. Wen, Y. Ye and X. Xie, J. PowerSources, 318, 93–112 (2016).48) A. Banerjee, X. Wang, C. Fang, E. A. Wu and Y. S.Meng, Chem. Rev., 120, 6878–6933 (2020).49) X. Lu, R. Wang, F. Zhang and J. Li, Solid State Ionics,354, 115417 (2020).50) H. Aono, E. Sugimoto, N. Imanaka and G. Adachi,J. Electrochem. Soc., 137, 1023–1027 (1990).51) K. He, C. Zu, Y. Wang, B. Han, X. Yin, H. Zhao, Y. Liuand J. Chen, Solid State Ionics, 254, 78–81 (2014).52) S. Hasegawa, N. Imanishi, T. Zhang, J. Xie, A. Hirano,Y. Takeda and O. Yamamoto, J. Power Sources, 189,371–377 (2009).53) M. A. Pogosova, I. V. Krasnikova, A. O. Sanin, S. A.Lipovskikh, A. A. Eliseev, A. V. Sergeev and K. J.Stevenson, Chem. Mater., 32, 3723–3732 (2020).54) E. Dashjav, Q. Ma, Q. Xu, C. L. Tsai, M. Giarola, G.Mariotto and F. Tietz, Solid State Ionics, 321, 83–90(2018).55) M. Rumpel, F. Nagler, L. Appold, W. Stracke, A.Flegler, O. Clemens and G. Sextl, Mater. Adv., 3, 4015–4025 (2022).56) P. Hofmann, F. Walther, M. Rohnke, J. Sann, W. G.Zeier and J. Janek, Solid State Ionics, 342, 1–10 (2019).57) M. Gellert, E. Dashjav, D. Grüner, Q. Ma and F. Tietz,Ionics, 24, 1001–1006 (2018).58) C. Y. Yu, J. Choi, V. Anandan and J. H. Kim, J. Phys.Chem. C, 124, 14963–14971 (2020).59) K. Nagata and T. Nanno, J. Power Sources, 174, 832–837 (2007).Journal of the Ceramic Society of Japan 132 [6] 257-266 2024 JCS-Japan26560) K. Ishii, M. Ode, K. Mitsuishi, S. Miyoshi, T. Ohno, K.Takada and T. Uchikoshi, J. Power Sources, 546,231954 (2022).61) K. Ishii, T. Uchikoshi, S. Miyoshi, M. Ode, T. Ohno andK. Takada, Mater. Lett., 324, 132736 (2022).62) K. Ishii, T. Uchikoshi and K. Takada, J. Ceram. Soc.Jpn., 131, 298–305 (2023).63) K. Kwatek, W. Ślubowska, C. Ruiz, I. Sobrados, J.Sanz, J. E. Garbarczyk and J. L. Nowiński, J. Alloy.Compd., 838, 1–13 (2020).64) B. Davaasuren and F. Tietz, Solid State Ionics, 338,144–152 (2019).65) Z. Zou, Y. Li, Z. Lu, D. Wang, Y. Cui, B. Guo, Y. Li, X.Liang, J. Feng, H. Li, C. W. Nan, M. Armand, L. Chen,K. Xu and S. Shi, Chem. Rev., 120, 4169–4221 (2020).66) E. Kartini, V. Yapriadi, H. Jodi, M. Manawan, C.Panghegar and Wahyudianingsih, Prog. Nat. Sci.-Mater., 30, 168–173 (2020).67) N. I. P. Ayu, E. Kartini, L. D. Prayogi, M. Faisal andSupardi, Ionics, 22, 1051–1057 (2016).68) K. Gaur, A. J. Pathak and H. B. Lal, J. Mater. Sci., 23,4257–4262 (1988).69) J. Li and W. Lai, Solid State Ionics, 351, 115329 (2020).70) A. Robertson, J. G. Fletcher, J. M. S. Skakle and A. R.West, J. Solid State Chem., 109, 53–59 (1994).71) A. Wustrow, G. Huang, M. J. McDermott, D. O’Nolan,C. H. Liu, G. T. Tran, B. C. McBride, S. S. Dwaraknath,K. W. Chapman, S. J. L. Billinge, K. A. Persson, K.Thornton and J. R. Neilson, Chem. Mater., 33, 3692–3701 (2021).72) A. Miura, C. J. Bartel, Y. Goto, Y. Mizuguchi, C.Moriyoshi, Y. Kuroiwa, Y. Wang, T. Yaguchi, M. Shirai,M. Nagao, N. C. Rosero-Navarro, K. Tadanaga, G.Ceder and W. Sun, Adv. Mater., 33, 2100312 (2021).73) M. Hofmann, F. Nagler, M. Kapuschinski, U. Guntowand G. A. Giffin, ChemSusChem, 13, 5962–5971 (2020).74) J. Li, C. Liu, C. Miao, Z. Kou and W. Xiao, Ionics, 28,63–72 (2022).75) B. Yang, X. Li, H. Guo, Z. Wang and W. Xiao, J. Alloy.Compd., 643, 181–185 (2015).76) S. He, Y. Xu, B. Zhang, X. Sun, Y. Chen and Y. Jin,Chem. Eng. J., 345, 483–491 (2018).77) A. H. Ahmad, M. Z. A. Yahya, R. Puteh and A. K. Arof,Indones. J. Phys., 15, 65–69 (2016).78) S. M. Benoy, S. Singh, M. Pandey and B. Manoj,Mater.Res. Express, 6, 125624 (2019).79) A. H. Ahmad and A. K. Arof, Ionics, 10, 200–205(2004).80) G. Wang, H. Zhang, C. Liu, H. Su, L. Jia, J. Li, X.Huang and G. Gan, J. Electron. Mater., 47, 4672–4677(2018).81) B. Wang, J. Wang, A. Chang and J. Yao, RSC Adv., 9,25488–25495 (2019).82) R. M. German, P. Suri and S. J. Park, J. Mater. Sci., 44,1–39 (2009).83) X. Wang, Y. Li, Z. Chen, H. Zhang, H. Su, G. Wang, Y.Liao and Z. Zhong, J. Alloy. Compd., 797, 566–572(2019).84) R. K. Osterheld, J. Inorg. Nucl. Chem., 30, 3173–3175(1968).85) W. S. Rasband, https://imagej.nih.gov/ij/ (1997–2018).86) C. A. Schneider, W. S. Rasband and K. W. Eliceiri, Nat.Methods, 9, 671–675 (2012).87) M. D. Abràmoff, P. J. Magalhães and S. J. Ram,Biophoton. Int., 11, 36–41 (2004).88) M. Rumpel, L. Appold, J. Baber, W. Stracke, A. Fleglerand G. Sextl, Mater. Adv., 3, 8157–8167 (2022).Ishii et al.: Surface modification of Li3PO4 to Li1.3Al0.3Ti1.7(PO4)3 by wet chemical process and its sintering behaviorJCS-Japan266https://imagej.nih.gov/ij/