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[K. Mitsuishi et al.,  preprint.pdf](https://mdr.nims.go.jp/filesets/396f38fd-eb29-4974-a2b7-d138e2992046/download)

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[K. Mitsuishi](https://orcid.org/0000-0002-9361-4057), [T. Ohnishi](https://orcid.org/0000-0002-2333-7752), [K. Niitsu](https://orcid.org/0000-0002-0430-8868), [T. Masuda](https://orcid.org/0000-0001-7462-2177), [S. Miyoshi](https://orcid.org/0000-0003-0375-1187), [K. Takada](https://orcid.org/0000-0001-7568-1806)

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© 2024. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/[Creative Commons BY-NC-ND Attribution-NonCommercial-NoDerivs 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/)

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[Lowering the sintering temperature of LiCoO2 using LiOH aqueous solution](https://mdr.nims.go.jp/datasets/e102b34f-9068-4178-adf5-7625f8698732)

## Fulltext

1  Lowering the sintering temperature of LiCoO2 using LiOH aqueous solution 1  2 K. Mitsuishia, T. Ohnishib, K. Niitsua, T. Masudab, S. Miyoshib, K. Takadab 3  4 a Center for Basic Research on Materials (CBRM), National Institute for Materials Science (NIMS), 5 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047 Japan 6 b Research Center for Energy and Environmental Materials (GREEN), National Institute for Materials 7 Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan 8  9 *Corresponding author at Center for Basic Research on Materials (CBRM), National Institute for 10 Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047 Japan. Tel:+81 29 863 5474. 11 E-mail address: Mitsuishi.Kazutaka@nims.go.jp (K. Mitsuishi) 12  13 Keywords: 14 Lithium cobalt oxide 15 Solid-state reaction 16 Eutectic melting 17 In situ transmission electron microscopy 18 2  ABSTRACT 19 For realizing oxide-based all-solid-state lithium-ion batteries, lowering sintering temperature of 20 LiCoO2 cathode during battery fabrication is important subject to prevent undesired chemical reaction 21 with other constituent substances. Here we report that the addition of LiOH aqueous solution to the 22 LiCoO2 powder upon sintering improves the battery performance. Comparative study for the samples 23 with and without LiOH solution, and with pure water by in situ transmission electron microscopy 24 reveals that the liquid phase appears at much lower temperatures for the sample with the LiOH addition 25 that improves the connectivity of LiCoO2 particles.  26  27 1. Introduction 28 Li ion batteries [1] are the most important energy storage devices that drive a variety of applications 29 from mobile devices to mobility vehicles [2]. With the rise of environmental issues, their importance 30 has never been greater [3]. To improve their energy densities and safety, realization of all-solid-state 31 lithium-ion batteries is strongly desired [4]. However, their performance is still poor for various 32 reasons. The main obstacles are the interfacial resistance of hetero-interfaces, such as electrolyte and 33 active materials, and homo-interfaces i.e., grain boundaries of solid electrolyte and active material[5]. 34 In comparison to sulfide all-solid-state batteries [6, 7], the problem is more prominent for the oxide 35 all-solid-state batteries [8, 9], where the ceramic solid electrolytes need to be combined with active 36 3  materials by sintering because of their very poor plasticity. Increasing the sintering temperature 37 improves the connectivity of homo-interfaces, but it may also promote unfavorable chemical reactions 38 at the hetero-interfaces between electrolytes and active materials, resulting in the poor Li ion 39 conductivity [10-12]. Therefore, various routes to lowering the sintering temperature have been 40 explored. For example, Ohta et al. reported that addition of Li3BO3 and CaO can significantly lower 41 the sintering temperature of Nb-doped Li7La3Zr2O12 to 790 ℃[13] while keeping its lithium ion 42 conductivity comparable to those sintered at high temperature (1000 to 1200 ℃), and Watanabe et al. 43 reported that densification of Li6La3ZrTaO12 was achieved at 775 ℃ by the addition of Bi[14]. 44 To boost the performance of the battery, one also needs to improve the connectivity of homo-45 interfaces in active material in addition to electrolyte particle. In general, LiCoO2, the most widely 46 used cathode material, is often ground to fine powder prior to sintering with solid electrolytes to 47 improve the connectivity, but the performance is still unsatisfactory, and this may partly be due to the 48 insufficient connectivity of homo-interfaces of LiCoO2. 49  Recently, low-temperature production of LiCoO2 using hydrothermal reactions has been 50 reported[15], in which the appearance of the liquid phase enhances the reactions. The idea of using a 51 hydroxide with low melting temperature should also work for improving the connection between 52 LiCoO2 particles. 53 In this paper, we report that the addition of LiOH aqueous solution to the LiCoO2 powder applied on 54 4  Li6.6La3Zr1.5Ta0.4O12 (LLZT) surface upon sintering improves the charge-discharge characteristics. In 55 situ transmission electron microscopy (TEM) observation of sintering process reveals that with LiOH 56 aqueous solution, appearance of liquid phase improves the connection between particles at much lower 57 temperatures than those with pure water and without any additives. 58  59  2. Experimental 60 Charge-discharge tests were performed for the battery cells consisting of mixed LiCoO2 and 61 Li6.6La3Zr1.5Ta0.4O12 (LLZT) power cathode, LLZT electrolyte, and Li foil anode. The cathode layer 62 was prepared by mixing LiCoO2 powder (Toshima Manufacturing Co. Ltd.) and LLZT electrolyte 63 particles (Toshima Manufacturing Co. Ltd.)  in a weight ratio of 5.4:5.05, which corresponds to 64 a volume ratio of 1:1. The resultant weight percentage of LiCoO2 in the mixture is 48.3%.  The 65 mixture was applied over the entire surface of a 1 cm square substrate (Toshima Manufacturing 66 Co. Ltd.), with a thickness of 0.5 mm. Before the application, a 23-nm-thick metal Ta is coated 67 on the LLZT substrate and heat-treated at 700 ℃ for 2 hours in oxygen atmosphere. Preliminary 68 experimental results indicate that interposing a controlled thickness of Ta reduces resistance and 69 improves cycling performance. 70  Prior to heat treatment, a droplet (10 μL) of saturated LiOH aqueous solution was placed to the one 71 of the cells, and 10 μL of pure water was added to another cell for comparison. The remaining cell was 72 5  heat treated without any liquid. The LiCoO2 contents of those cells were 0.46 mg, 0.51 mg, and 0.51 73 mg, respectively and this corresponds to the loading density of 0.46mg/cm2, 0.51mg/cm2, and 74 0.51mg/cm2, respectively. A saturated LiOH aqueous solution with a concentration of 5.3 75 mol/L was used. Addition of 10 μL of this solution corresponds to 5.3 x 10-6 mol. The LiCoO2 76 contents of the cells were 0.46 mg for LiOH aqueous addition, thus, the molar ratios of LiOH 77 to LiCoO2 was 0.89.  78 After the heat treatment, an indium layer with 5.7 nm in thickness was deposited on the anode side by 79 DC magnetron sputtering, and a Li foil was attached by heating at 220 ℃ for 30 minutes.  80 In situ TEM observations were conducted using a heating holder (Fusion, Protochips Co. Ltd.). The 81 holder is equipped with a micro-electromechanical system (MEMS) heating chip coated with silicon 82 nitride membrane. Two types of chips were used: with and without carbon supporting films over the 83 holes. LiCoO2 particles were ground in a mortar, suspended in a saturated LiOH aqueous solution and 84 dispersed on the heating chip. The chip was dried at 130 ℃ for 10 minutes before observations. 85 Samples dispersed with and without pure water were also observed for comparison. 86 TEM, Scanning TEM (STEM), and electron energy-loss spectroscopy (EELS) observations were 87 performed using a JEOL JEM-ARM200F spherical aberration-corrected microscope equipped with a 88 Gatan Quantum camera. For the observation of deposit, The LiOH-saturated aqueous solution was 89 diluted 10,000 times and drop-cast onto the carbon grid. 90 6   91 3. Results 92 3.1 Comparison of charge-discharge profiles 93 Fig. 1 shows charge-discharge profiles of the cells sintered with (green) and without (blue) LiOH 94 aqueous solution, and with pure water (red). The rate was set to 0.1 C. The LiOH-mediated cell (green)  95 exhibited a much larger capacity in comparison to the cell without the addition of LiOH solution (blue). 96 In contrast, the capacity of the cell with the addition of water significantly decreased (red). This implies 97 that LiOH aqueous solution improves the connectivity of hetero- and homo-interfaces between 98 LiCoO2 particles and/or LiCoO2 and LLZT particles, while the water treatment induces adverse effects. 99 Hence, LiOH is a key additive that improves the capacity of the cell. 100  101 3.2 TEM observations of LiOH deposits  102 The role of LiOH aqueous solution in the sintering process was elucidated by TEM. First, the deposits 103 of LiOH were observed by dispersing only a LiOH solution onto a TEM grid. To make deposits with 104 appropriate thickness for EELS observations, diluted solution was used, and the grid was dried under 105 air at 130 ℃ for 10 minutes. From the diluted LiOH solutions, deposits with various sizes were formed 106 as shown in Fig. 2(a). The edge of the deposits appears as aggregation of small platelets that are in 107 crystalline state assignable to Li2CO3 as evident by diffraction pattern shown in Fig. 2(b). The EELS 108 7  feature near the Li-K edge (Supporting information) resembles the Li2CO3 spectrum. [16] Those 109 results indicates that the deposits of LiOH were completely changed to Li2CO3. Because a small 110 amount of diluted LiOH solution was used, the resulting LiOH might be completely reacted with CO2 111 in air during the drying process to form the Li2CO3 crystalline deposits.  112  113 3.3 In situ observation of sintering process of LiCoO2 with LiOH additives 114 Sintering with LiOH additives was also investigated by in situ TEM. Fig. 3 shows in-focus (a) and 115 defocused (b) images of LiCoO2 particles that were suspended in a saturated LiOH aqueous solution 116 and dispersed on the heating chip.  Light-element compounds such as LiOH and Li2CO3 generate a 117 small phase change to the incident electrons; hence they are almost invisible in the in-focus (Fig. 3(a)) 118 but certainly appear in the defocused image (Fig. 3(b)). In contrast to the deposits from the diluted 119 aqueous solution (Fig. 2), deposits are in amorphous phase and filling the space between particles. 120 These deposits were generally thicker than those observed in the previous section and may be a mixture 121 of LiOH and Li2CO3. It is known that the formation of LiOH-Li2CO3 eutectics lowers the melting 122 temperature [17], and melted liquid phase should enhance the inter-diffusion of LiCoO2. Thus, the 123 formation of Li2CO3 should enhance sintering. 124 Fig. 4 illustrates in situ TEM images measured at different temperatures. The corresponding video is 125 available in the supporting information (1. LCO with LOH_RT to 600 deg.mp4). At around 200 ℃, a 126 8  “sea-island” structure appeared in the LiOH deposit region (the arrow in Fig 4 (b)), and the pattern 127 grew and coarsened as temperature increased (the arrow in Fig. 4(c)). This is probably due to the 128 melting of LiOH.  LiCoO2 particles started to melt at around 340 ℃, and almost completely melted 129 by 400 ℃. Since the liquid phase of LiOH appears first, the melting of LiCoO2 should rather be 130 considered as "dissolving.". At around 575 ℃, the boundaries between LiOH and LiCoO2 disappeared, 131 and a single molten phase was formed at 600 ℃. After keeping the sample at 600 ℃, some areas 132 turned to show diffraction contrast indicating those areas changed into crystalline state as indicated by 133 an arrow in Fig. 4(f). EDS measurement performed later from those areas at room temperature (Fig. 134 5) revealed that those crystalline areas are rich in Co and likely represent LiCoO2.   135 These in situ observations of sintering process were performed using a heating chip with carbon 136 supporting film. Similar results were obtained without the film and are presented in the supporting 137 information (7. LCO with LiOH with no carbon film RT to 600 deg .mp4.) confirming that the above-138 mentioned phenomena such as a single molten phase formed below 600 ℃, was not affected by the 139 carbon support.  140  141 3.4 In situ observation sintering process of LiCoO2 with pure water 142 In situ observations were also conducted for particles dispersed using pure water. The results are 143 summarized in Fig. 6, and the corresponding video is available in supporting information (2. LCO 144 9  with water _RT to 600 deg.mp4, 3. LCO with water 600 deg to 750 deg.mp4). In contrast to heating 145 with LiOH additives, no noticeable change occurred until 750 ℃. Around 750℃, however, the 146 particles suddenly became round, the connections between them became thicker, and some small 147 particles were merged to form larger particles within about 10 seconds (Fig. 6 (c)). After that, the 148 image has stabilized. Although some small particles were absorbed, the particles were, as a whole, not 149 melted, and preserved their original configurations meaning the connection between particles becomes 150 thicker by surface and volume diffusion.  Therefore, the observed process was likely a solid phase 151 rection. The temperature of 750 ℃ (~1020 K) is 300 ℃ lower than the melting point of pure LiCoO2 152 (1130 ℃ or ~1400 K), which is a rather common difference for sintering.  153  154  155 3.5 In situ observation of sintering process of mechanically dispersed LiCoO2 156 Lastly, to investigate the effect of pure water and LiOH solution on sintering process, additive-free 157 LiCoO2 particles were mechanically dispersed on a heating chip and observed in situ TEM. When a 158 heating chip with carbon supporting film was used, LiCoO2 reacts with the carbon film to produce 159 metallic Co. This may be because the heating was performed under vacuum, so that LiCoO2 was 160 reduced by the carbon film upon heating and CO2 was formed.  The corresponding video is available 161 in supporting information (8. LCO only on carbon film RT to 600 deg.mp4).  162 10  Thus, a heating chip without carbon supporting film was used in the present experiment as shown 163 in Fig. 7. The corresponding video can be found in supporting information (4. LCO only with no 164 carbon film RT to 600 deg.mp4, 5. LCO only with no carbon film 600 to700 deg.mp4, 6. LCO only 165 with no carbon film 700 to 750 deg.mp4). Unlike the previous two experiments where the rounding 166 started at 340 ℃ and 750 ℃, the rounding of the particles started at around 600 ℃ and gradually 167 progressed up to 700 ℃. At around 740 ℃, the coarsening started, and a single large grain was formed 168 at around 750 ℃. As compared with the water addition case, the connection is more continuous, and 169 there is no trace of original particles in the merged grain. This result agrees with the fact that the 170 charge-discharge capacities in Figure 1 are higher for LiCoO2 with LiOH, without LiOH, and with 171 water in that order. 172  173 4. Discussion 174 As demonstrated by Liang et al. [15], the eutectic effect of LiOH and Li2CO3 lithium precursors can 175 lower the solid-state reaction temperature.  In the present study, although the samples for the charge-176 discharge test were heat-treated in oxygen, since the samples were handled in the air, some part of 177 LiOH might be reacted with CO2 to form Li2CO3, forming a eutectic binary phase with even lower 178 melting temperature. 179 In the in situ observations of sintering process, the observations were conducted for relatively small 180 11  LiCoO2 particles that were being heated in vacuum. Although the heating rate in the TEM heating 181 experiment is set faster compared to the heating rate in a tube furnace for the bulk, we believe that the 182 particles being observed reach a sufficiently uniform temperature at each temperature range because 183 the particle size is very small relative to the sample holder acting as a thermal bath. As for the 184 atmosphere, we have confirmed that a functioning battery can be fabricated by heat treatment under 185 an Ar atmosphere (i.e., oxygen-free). Since the temperature at which the liquid phase appears during 186 in-situ TEM observation matches the temperature at which battery performance improves in bulk cells, 187 we believe that the mechanism by which LiOH generates a liquid phase, thereby promoting the 188 sintering of LiCoO2, occurs similarly in bulk batteries, and the in-situ experiment accurately replicates 189 the behavior of bulk batteries.  190 The melting temperature of the LiOH-Li2CO3 depends on the ratio of these compounds [17] . In our 191 experiment, sintering was performed in pure oxygen, and hence precise control of the ratio was not 192 possible. However, if we could sinter the mixture in a controlled atmosphere and fine-tune the ratio 193 upon sintering, we might be able to further lower the sintering temperature. 194 In this work, we focused on the effect of LiOH on the sintering between LiCoO2 particles. 195 However, it is quite possible that LiOH also improve the connection between LiCoO2/LLZT 196 particles and LLZT/LLZT particles. We intend to further investigate the effect of LiOH on 197 the sinterability between LLZT particles in future 198 12   199 5. Conclusions 200 The addition of LiOH aqueous solution can improve the connection of LiCoO2 particles at much lower 201 sintering temperatures, resulting in greatly improved charge-discharge performance in comparison 202 with the samples without LiOH solution and with pure water. In situ observations of sintering process 203 reveal that liquid phase appears at much lower temperatures, which improves the connection of 204 LiCoO2 particles. The lowering of the sintering temperature widens the process condition when 205 fabricating batteries by suppressing side reactions between cathode and electrolyte materials. Hence 206 these results may be useful for the development of all-solid base oxide batteries. 207  208 Acknowledgement 209 The authors acknowledge the financial support provided by the Materials Processing Science project 210 “Materealize” of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT; 211 Grant Number JPMXP0219207397).  212  213 References 214 [1]K. Ozawa, Solid State Ionics 69 (1994) (3-4) 212. 215 [2]M. Armand, J.M. Tarascon, Nature 451 (2008) (7179) 652. 216 [3]D. Lindley, Nature 463 (2010) (7277) 18. 217 [4]K. Takada, Acta Materialia 61 (2013) (3) 759. 218 13  [5]S.A. Pervez, M.A. Cambaz, V. Thangadurai, M. Fichtner, ACS Appl Mater Interfaces 11 (2019) 219 (25) 22029. 220 [6]A. Hayashi, S. Hama, T. Minami, M. Tatsumisago, Electrochemistry Communications 5 (2003) 221 (2) 111. 222 [7]R. Kanno, M. Murayama, Journal of The Electrochemical Society (2001). 223 [8]H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, Journal of the Electrochemical 224 Society 137 (1990) (4) 1023. 225 [9]S. Stramare, V. Thangadurai, W. Weppner, Chemistry of Materials 15 (2003) (21) 3974. 226 [10]Y. Kobayashi, T. Takeuchi, M. Tabuchi, K. Ado, H. Kageyama, Journal of Power Sources 81 227 (1999)  853. 228 [11]J. Wakasugi, H. Munakata, K. Kanamura, Electrochemistry 85 (2017) (2) 77. 229 [12]K. Park, B.-C. Yu, J.-W. Jung, Y. Li, W. Zhou, H. Gao, S. Son, J.B. Goodenough, Chemistry 230 of Materials 28 (2016) (21) 8051. 231 [13]S. Ohta, J. Seki, Y. Yagi, Y. Kihira, T. Tani, T. Asaoka, Journal of Power Sources 265 (2014)  232 40. 233 [14]K. Watanabe, A. Tashiro, Y. Ichinose, S. Takeno, K. Suematsu, K. Mitsuishi, K. Shimanoe, 234 Journal of the Ceramic Society of Japan 130 (2022) (7) 416. 235 [15]R. Liang, S. Yonezawa, J.-H. Kim, T. Inoue, Journal of Asian Ceramic Societies 6 (2018) (4) 236 332. 237 [16]F. Wang, J. Graetz, M.S. Moreno, C. Ma, L. Wu, V. Volkov, Y. Zhu, ACS Nano 5 (2011) (2) 238 1190. 239 [17]A.D. Pelton, C.W. Bale, P.L. Lin, Canadian Journal of Chemistry 62 (1984) (3) 457. 240  241   242 14  Figure captions 243  244 Fig.1. Charge-discharge profiles of the cells sintered with (green) and without (blue) LiOH aqueous 245 solution, and with pure water (red). The performance improves for the cell with LiOH aqueous 246 (green), while it degrades with water (red). 247  248 Fig. 2. (a) Deposits formed from diluted LiOH aqueous solution dispersed onto a carbon grid. (b) 249 Diffraction pattern from a deposit that can be indexed as Li2CO3. 250  251 Fig. 3. TEM images of LiCoO2 particles that were suspended in a saturated LiOH aqueous solution 252 and dispersed on the heating chip: (a) in focus, and (b) defocused. 253  254 Fig. 4. In situ heating result of LiCoO2 with LiOH deposits. The arrows in (b) and (c) indicate 255 the LiOH deposit region where “sea-island” structure appears. The arrow in (f) indicates a 256 crystalline area appeared in melted region. 257  258 Fig. 5. EDS map of LiCoO2 with LiOH deposits heated up to 600 ℃. Green, red, and blue correspond 259 to cobalt, oxygen, and carbon, respectively. 260 15   261 Fig. 6. In situ observation result of LiCoO2 particles dispersed with pure water. The sintering 262 suddenly progresses at around 750℃ at which connection between particles becomes thicker while 263 keeping their positions. 264  265 Fig. 7. In situ observation result of LiCoO2 particles dispersed mechanically without using LiOH 266 solution nor pure water. The rounding of particle shapes started around 600℃  and gradually 267 progressed up to 700 ℃. 268   269 16   270 271 Fig. 1. Charge-discharge profiles of the cells sintered with (green) and without (blue) LiOH aqueous solution, and with pure water (red). The performance improves for the cell with LiOH aqueous (green), while it degrades with water (red). 17    272 Fig. 2. (a) Deposits formed from diluted LiOH aqueous solution dispersed onto a carbon grid. (b) Diffraction pattern from a deposit that can be indexed as Li2CO3. 18    273 Fig. 3. TEM images of LiCoO2 particles that were suspended in a saturated LiOH aqueous solution and dispersed on the heating chip: (a) in focus, and (b) defocused. 19    274 Fig. 4. In situ heating result of LiCoO2 with LiOH deposits. The arrows in (b) and (c) indicate the LiOH deposit region where “sea-island” structure appears. The arrow in (f) indicates a crystalline area appeared in melted region. 20    275 Fig. 5. STEM (b) and EDS map (b) of LiCoO2 with LiOH deposits heated up to 600 ℃. Green, red, and blue correspond to cobalt, oxygen, and carbon, respectively. 21    276 Fig. 6. In situ observation result of LiCoO2 particles dispersed with pure water. The sintering suddenly progresses at around 750℃ at which connection between particles becomes thicker while keeping their positions. 22   277 Fig. 7. In situ observation result of LiCoO2 particles dispersed mechanically without using LiOH solution nor pure water. The rounding of particle shapes started around 600℃ and gradually progressed up to 700 ℃.