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## Creator

[Yasuhiro Domi](https://orcid.org/0000-0003-3983-2202), [Hiroyuki Usui](https://orcid.org/0000-0002-1156-0340), Naoya Wada, [Takayuki Yamamoto](https://orcid.org/0000-0003-3553-3272), [Toshiyuki Nohira](https://orcid.org/0000-0002-4053-554X), [Kei Nishikawa](https://orcid.org/0000-0002-7718-7606), [Hiroki Sakaguchi](https://orcid.org/0000-0002-4125-7182)

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This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Energy Materials, copyright © 2025 American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsaem.5c02478.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Electrochemical Potassiation/Depotassiation Properties of Rare-Earth Antimonide/Antimony Composite Electrodes](https://mdr.nims.go.jp/datasets/1ada6171-5853-41a5-8066-fcd52d5c7d92)

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Template for Electronic Submission to ACS Journals 1 Electrochemical Potassiation/Depotassiation Properties of Rare-Earth Antimonide/Antimony Composite Electrodes Yasuhiro Domi,*,†,§ Hiroyuki Usui,†,§ Naoya Wada,‡,§ Takayuki Yamamoto,⊥ Toshiyuki Nohira,⊥ Kei Nishikawa,|| and Hiroki Sakaguchi*,†,§ †Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, Minami 4–101, Koyama–cho, Tottori 680–8552, Japan ‡Course of Chemistry and Biotechnology, Department of Engineering, Graduate School of Sustainability Science, Tottori University, Minami 4–101, Koyama–cho, Tottori 680–8552, Japan §Center for Research on Green Sustainable Chemistry, Tottori University, Minami 4–101, Koyama–cho, Tottori 680–8552, Japan ⊥Institute of Advanced Energy, Kyoto University, Uji 611–0011, Japan ||International Center of Young Scientists, National Institute for Materials Science, 1–1 Namiki, Tsukuba 305–0044, Japan   2 domi@tottori-u.ac.jp, usui@tottori-u.ac.jp, tottori.naoya@gmail.com, yamamoto.takayuki.2w@kyoto-u.ac.jp, t-nohira@iae.kyoto-u.ac.jp, NISHIKAWA.Kei@nims.go.jp, sakaguch@tottori-u.ac.jp  *Corresponding Authors Yasuhiro Domi Tel/Fax: +81-857-31-5249, E-mail: domi@tottori-u.ac.jp Hiroki Sakaguchi Tel/Fax: +81-857-31-5048, E-mail: sakaguch@tottori-u.ac.jp    3 ABSTRACT (217/300 words)  Potassium-ion batteries have emerged as promising next-generation energy storage systems due to the abundance and low cost of potassium. However, achieving high capacity and long-term cycling stability remains a challenge, especially for metal/alloy anodes such as antimony (Sb), which suffers from significant volume changes during charge–discharge cycling. To address this, we developed composite anodes combining Sb with rare-earth antimonides (RESbx), known for their charge-discharge cycle stability. The electrochemical performances of RESbx/Sb composite electrodes were systematically evaluated in an ionic liquid electrolyte. Charge-discharge testing revealed that the addition of RESbx, even at 10 wt.%, significantly suppressed the rapid capacity fading observed in a pure Sb electrode, leading to excellent cycling stability. Additionally, Sb contributed most of the capacity in the composites, whereas the RESbx component effectively suppressed electrode degradation. Particularly, a CeSb/Sb electrode maintained over three times the cycle life of the pure Sb electrode. Cross-sectional scanning electron microscopic analysis demonstrated that the RESbx phase mitigated electrode expansion and cracking. Further investigation showed that high potassium-ion diffusion coefficient, associated with larger lattice parameters of RESbx, and the mechanical softness (lower breaking strength) of CeSb, played key roles in enhancing cycle life. These properties facilitate uniform K⁺ distribution and reduce mechanical stress in the electrode. The findings contribute to the advancement of high-performance and resource-efficient energy storage technologies.  KEYWORDS: potassium-ion batteries; antimony; composite; electrode thickness; K+ diffusion coefficient; reference breaking strength; ionic liquid electrolyte 5-8 words   4 1. Introduction  Since their commercialization in 1991, lithium-ion batteries (LIBs) have been widely utilized in various fields, including smartphones and electric vehicles (EVs), due to their high energy density and long cycle life.1–3 Thus, LIBs have thus become indispensable energy storage devices in modern society. However, lithium, the key component of LIBs, is associated with supply risks and price volatility because of its uneven geographical distribution and high extraction costs. Therefore, developing next-generation rechargeable batteries that use inexpensive, abundant resources instead of lithium is critical.  Sodium-ion batteries (NIBs) have attracted considerable attention as one such alternative, and their commercialization for EV applications is currently underway.4–7 Sodium has the significant advantage of being abundant on Earth, with fewer resource constraints. To achieve a decarbonized society, it is essential to expand the use of renewable energy with high-performance storage batteries. It is also extremely important to establish a diverse lineup of storage batteries for various applications. In consideration of this backdrop, potassium-ion batteries (KIBs), which employ potassium ions as charge carriers, are highly attractive from a resource perspective, similar to NIBs.7–10 Furthermore, the low redox potential of K⁺/K in organic electrolytes contributes to the potential for high energy density, while the relatively small solvated radius of K⁺ is expected to enable excellent rate capabilities.   Although these alkali metal secondary batteries share a common "rocking-chair" mechanism, where monovalent cations shuttle between the cathode and anode, the optimal electrode materials for each battery system vary considerably.11,12 For instance, graphite is widely used as an anode for LIBs, whereas hard carbon is predominantly used in NIBs due to the difficulty  5 of Na+ intercalation into graphene layers.4–7 Furthermore, our previous studies have indicated that even when using the same active materials, the specific properties required for superior performance differ between NIBs and KIBs.13 Thus, for the practical realization of KIBs, it is essential to thoroughly investigate the characteristics of diverse electrode active materials and establish a broad portfolio to accommodate diverse applications.  Focusing on anode active materials for KIBs, although graphite demonstrates high Coulombic efficiency and excellent cycle stability10,14–16, achieving higher energy densities requires the development of compound-based or metal-based electrodes with high theoretical capacities.17–23 However, these electrodes suffer from poor cycling performance due to the severe volume expansion and contraction during charge and discharge processes, respectively. Our research group has addressed performance improvement in LIB and NIB anodes through multifaceted approaches, including composite formation with other materials,24–28 alloying or compound formation with different elements,29–32 and impurity doping.33–37   Here, we focus on antimony (Sb) as a promising anode material for KIBs.10,19 Although the gravimetric theoretical capacity of Sb electrodes (K3Sb: 660 mA h g−1) is lower than that of phosphorous electrodes (K4P3: 1154 mA h g−1), Sb offers a significantly higher volumetric theoretical capacity (K3Sb: 4422 mA h cm−3 vs. K4P3: 3104 mA h cm−3), making it an attractive candidate from a practical perspective. Nevertheless, the Sb electrodes undergo a significant volume change (407%) during charge and discharge processes, which generates stress. This leads to strain accumulation over repeated cycles, ultimately causing electrode degradation and capacity fading after a relatively short number of cycles. To overcome this issue, we have synthesized Sb-based alloys and evaluated their anode properties.13 Our findings revealed that FeSb2 electrodes possessed a commendable balance of relatively high reversible capacity and excellent cycling  6 stability. Additionally, we demonstrated that rare-earth antimonide (RESbx) electrodes could maintain stable reversible capacity over 500 cycles despite exhibiting lower initial capacity compared to the FeSb2 electrode.   The present study aimed to develop high-performance KIB anodes that achieve both high reversible capacity and long-term cycling stability by composite formation with Sb, which offers high capacity but limited cycle stability, and RESbx, known for its superior cycle stability. We systematically investigated the influence of different rare-earth elements on the anode properties and discussed the effects based on reaction behavior analyses. The findings of this work are expected to provide valuable guidelines for the design of high-performance anode active materials, thereby contributing to the practical realization of KIBs and the broader advancement of next-generation energy storage technologies.  2. Experimental 2.1. Synthesis and Characterization of RESbx/Sb Composites.  RESbx/Sb composites were synthesized as active materials using the mechanical alloying (MA) method by 2-step. Rare-earth used in this study was yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), and gadolinium (Gd). Sb powder and Sm chips were purchased from FUJIFILM Wako Pure Chemical Corporation, Ltd. Y powder and Ce ingots were obtained from Nilaco Corporation, Ltd. The shot-like La and Gd supplied by Santoku Corp. were cut into smaller pieces using a nipper and then subjected to a sieve treatment. A mixture of Sb and other elements in a 1:1 molar ratio was enclosed in a stainless-steel container along with stainless-steel balls. The  7 weight ratio of the mixture to the balls was approximately 1:30. The MA was conducted using a planetary ball-milling apparatus (P-6, Fritsch) at a rotational speed of 380 rpm for 10 h at room temperature. The RESbx/Sb composites were obtained by mechanical mixing (MM) of the synthesized RESbx with Sb powder at the same rotational speed for 10 min. The entire process from weighing the raw materials to MM process was carried out in a glovebox (Miwa MFG, DBO-2.5LNKP-TS) with an argon atmosphere, ensuring an oxygen content of 1 ppm or less and a dew point below −90oC.  The synthesized composite powders were characterized by X-ray diffraction (XRD; Ultima IV, Rigaku), field emission scanning electron microscopy (FE-SEM; JSM-IT800, JEOL Ltd.), and laser diffraction particle size analyzer (SALD-2300, Shimadzu Co. Ltd.). The XRD measurement was conducted using Cu-K radiation at a current of 40 mA and a voltage of 40 kV. Cu or Al was used as an internal standard substance. FE-SEM observation was performed with an accelerating voltage between 5 and 10 kV and a working distance of 10 mm. The particle size analysis was conducted through the following procedure: the composite powder and a neutral detergent were mixed in a weight ratio of approximately 100:1. This mixture was then dispersed in deionized water and gently stirred while preventing the formation of bubbles. The neutral detergent was added to suppress the aggregation of composite particles.  The weight ratio of the obtained composite samples was quantitatively analyzed using wavelength-dispersive X-ray fluorescence (WD-XRF; ZSX Primus, Rigaku) and/or inductively coupled plasma atomic emission spectroscopy (ICP-AES; SPECTRO CIROS CCD). In WD-XRF measurements, Rhodium (Rh) was used as the target materials for the X-ray source, and LiF was employed as the spectroscopic crystal. ICP-AES measurements were conducted under the conditions of plasma power at 1400 W and a plasma gas flow rate of 12 L min−1.  8  2.2. Cell Fabrication and Charge–Discharge Testing.  We fabricated various RESbx/Sb electrodes by a slurry coating method.13,18,31,32 The slurry comprised of RESbx/Sb, acetylene black (AB), carboxymethyl cellulose (CMC), and styrene–butadiene–rubber (SBR) in a weight ratio of 70:15:10:5 wt.%. We used deionized water as the dispersing agent. The prepared slurry was coated on a copper foil current collector and dried to develop the active material layer, with a mass loading and thickness of approximately 1.0 mg cm−2 and 8 m, respectively.  A 2032-type coin-cell was fabricated with the slurry-coated working electrode, a K metal sheet counter electrode, and a glass fiber separator (Whatman GF/A). We utilized an ionic-liquid electrolyte composed of potassium bis(fluorosulfonyl)amide (KFSA) dissolved in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13–FSA or [C3C1pyrr][FSA]). The molar ratio of KFSA to Py13–FSA was 20:80 mol%, amounting to a molar concentration of 0.9744 mol dm−3 (~1M) at 303 K. Detailed electrochemical and physicochemical properties of the electrolyte were previously reported.38 The use of FSA-based electrolytes has been reported to provide remarkable superior anode properties in LIBs, NIBs, and KIBs.29,35,39–43 Charge–discharge testing was galvanostatically performed between 0.005 and 2.000 V vs. K+/K at 303 K with a current density of 50 or 200 mA g(AM)⁻1 using an electrochemical measurement system (HJ-1001SD8, Hokuto Denko Co., Ltd.). Here, AM denotes the active materials including RESbx/Sb, RESbx, or Sb. Unless otherwise specified, the specific capacity and current density in charge-discharge curves and cycling performance were expressed as a value per AM weight. Cyclic voltammetry (CV)  9 measurement was performed between 0.005 and 2.000 V vs. K+/K at 303 K at scan rate of 0.02 mV s−1 using Compactstat.h (Ivium Technologies).   2.3. Determination of Potassium Diffusion Coefficient.  The K+ diffusion coefficient (DK+) in the RESbx was determined by a galvanostatic intermittent titration technique (GITT).44 After the first charge-discharge cycle at a constant current density of 50 mA g(RESbx)−1, the GITT measurement was carried out under the following conditions; the electrode was charged at a constant current density of 25 mA g(RESbx)−1 for 20 min and relaxed at open circuit voltage (OCV) for 4 h.   2.4. Measurement of Reference Breaking Strength.  A reference breaking strength (Cx) of the RESbx particles was measured by a uniaxial compression using a dynamic ultra-microhardness tester (DUH-211S, Shimadzu Co. Ltd.). Herein, the Cx is defined as the force required to the applied pressure at which a compressed RESbx particle exhibits 10% deformation compared to its original size. The RESbx powder was added to ethanol and dispersed by sonication. The suspension was dropped onto a glass slide and dried. The particle (diameter: d) was compressed with a plane indenter (size: 50 m) under a test force (P) of 49 mN and a loading velocity of 2.22 mN s–1. The Cx value was estimated by equation (1), where a coefficient a is 2.48 with the 50 m plane indenter. 𝐶𝑥 =𝑎𝑃𝜋𝑑2                                               (1)  10  3. Results and discussion 3.1. Characterization of each RESbx/Sb Composite.  Figure 1a shows an XRD pattern of synthesized powder from La and Sb in a 1:1 molar ratio (blue line) and LaSb/Sb with weight ratio of 10:90 wt.% (red line). All the XRD peaks were assigned to the standard data of LaSb (ICSD No. 01-073-6810) and Sb (ICSD No. 00-005-0562) and no peaks of elemental La and Sb appeared; and thus, it is confirmed that the successful synthesis of the desired sample (LaSb/Sb composite). Figures 1b and 1c depict the SEM image and particle size distribution of LaSb/Sb powder, respectively. The LaSb/Sb composite (10/90 wt.%) was an aggregate (secondary particles) and its average particle size was 7.77 m. Other RESbx/Sb were similarly characterized (Figures S1–S4), demonstrating successful synthesis by the MA and MM treatments, with all the D50 values of approximately 10 m. WD-XRF and ICP-AES analysis also demonstrated that the weight ratio of the synthesized samples closely matched the initial composition. TEM observation of LaSb/Sb composite (50/50 wt.%) revealed that the Sb and LaSb phases were homogeneously mixed at the nanosized level (Figure S5).    11  Figure 1. (a) XRD patterns of synthesized powder from La and Sb with molar ratio of 1 : 1 by MA treatment for 10 hours and from LaSb and Sb with weight ratio of 10 : 90 by MM treatment for 10 minutes. (b) SEM image and (c) particle size distribution of LaSb/Sb.  3.2. Influence of LaSb/Sb Mass Ratio on the Potassiation–Depotassiation Properties  Figure 2 shows the initial charge-discharge curve of LaSb/Sb composite electrodes with different weight ratios in 1 M KFSA/Py13-FSA at 50 mA g−1. The charge (potassiation) and discharge (depotassiation) capacities increased with an increase in the amount of elemental Sb in the composite electrodes. Regardless of the weight ratio of LaSb and elemental Sb, the two potential plateaus at approximately 0.62 and 0.20 V were confirmed on the charge curves. Additionally, a potential plateau and potential shoulder appeared at around 0.61 and 1.10 V, respectively, on the discharge curve. The potential plateaus and the shoulder were confirmed on a pure Sb electrode at around above-mentioned potentials, whereas a pure LaSb electrode showed plateaus and shoulders at other potential regions. The LaSb electrode did not exhibit a distinct potential plateau but displayed potential slopes ranging from 0.49 and 0.02 V during charging and  12 from 0.36 and 2.00 V during discharging. Hence, it is considered that only pure Sb in the composite electrode contributes to most of the capacities. 00.511.520 200 400 600Potential / V vs. K+/KCapacity / mA h g-1SbLaSb10/90 wt.%30/70 wt.%50/50 wt.%70/30 wt.%LaSb/Sb2nd cycle Figure 2. Charge–discharge profiles of Sb, LaSb, and LaSb/Sb electrodes of various weight ratios in 1 M KFSA/Py13-FSA at current density of 50 mA g–1.   Table 1 summarized the Coulombic efficiencies (CEs) of Sb, LaSb, and LaSb/Sb composite electrodes during the initial five cycles. Comparing the CEs of the Sb and LaSb electrodes, the former exhibited higher CE over the initial five cycles. As the proportion of Sb increased, the LaSb/Sb composite electrodes with different weight ratios tended to show higher CE over the initial five cycles.    13  Table 1. CEs of Sb, LaSb, and LaSb/Sb electrodes (unit : %). Cycle Sb LaSb LaSb/Sb 10/90 wt.% 30/70 wt.% 50/50 wt.% 70/30 wt.% 1st 75.6 47.8 72.9 70.1 70.4 56.1 2nd 99.3 92.1 98.3 98.3 97.8 95.4 3rd 99.3 94.3 98.9 98.9 98.7 97.5 4th 99.1 95.3 99.1 99.3 99.1 98.4 5th 99.3 96.1 99.3 99.3 99.3 98.9   Figure 3 displays the cycling performance of LaSb/Sb composite electrodes with different weight ratios. As previously reported,22 the pure Sb electrode had a high initial reversible capacity, whereas the reversible capacity decayed early. In contrast, we have reported that the pure LaSb electrode had only approximately a quarter of the discharge capacity of the Sb electrode but maintained its capacity stably over 400 cycles.13 The LaSb/Sb composite electrodes suppressed the rapid capacity fading of the Sb electrode at the initial cycle regardless of the amount of LaSb added. It was demonstrated that excellent cycling stability and a high discharge capacity can be achieved even with as little as 10 wt.% of LaSb. For example, in the case of FeSi2/Si composite electrodes for LIBs, FeSi2 compensates for the disadvantage of Si, which has a high capacity but undergoes significant volume changes during lithiation and delithiation.45 When the amount of FeSi2 added is small, capacity decay occurs in the early stages of the cycle, similar to the pure Si electrode. However, surprisingly, the suppression of capacity decay even with as little as 10 wt.% of LaSb in this electrode system was almost identical to that confirmed at other weight ratios. Subsequent experiments used RESbx/Sb composite electrodes with a 10/90 wt.% ratio for high-capacity applications.   14 01002003004005006000 100 200 300 400Discharge capacity / mA h g-1Charge-discharge cycle numberSb10/90 wt.%30/70 wt.%50/50 wt.%70/30 wt.%LaSb/SbLaSb Figure 3. Cycle dependency of discharge capacities of LaSb/Sb electrodes with various weight ratios and Sb alone electrode in 1 M KFSA/Py13-FSA at a current density of 200 mA g–1.   To investigate the reaction behavior of the LaSb/Sb composite electrode in detail, CV measurements of Sb, LaSb, and LaSb/Sb electrodes were performed (Figure 4). The pure Sb electrode exhibited two cathodic peaks at approximately 0.59 and 0.12 V and two anodic peaks at approximately 0.69 and 1.12 V. Each peak is attributed to the reaction shown in Figure 4a.46,47 Additionally, each peak potential was nearly identical to the potential plateaus and shoulder of the Sb electrode shown in Figure 2. The peaks resulting from the potassiation and depotassiation of the Sb electrode appeared on the LaSb/Sb electrode. In contrast, the pure LaSb electrode showed only a cathodic peak at approximately 0.11 V, an anodic shoulder at approximately 0.38 V and three anodic peaks at approximately 0.65, 1.06, and 1.60 V. These results indicated that only Sb of the LaSb/Sb electrode alloyed and dealloyed with K, whereas the potassiation and depotassiation reactions occurred on the pure Sb and LaSb electrode. The same phenomena were confirmed on the Si-based composite electrodes for LIBs.24,26 Due to its low formation energy,  15 LaSb does not undergo phase separation into La and Sb during potassiation, enabling it to reversibly store and release K+.13,48  -0.05-0.02500.0250 0.5 1 1.5 2Potential / V vs. K+/KcCurrent density / mA cm-2b-0.100.1Sweep rate 0.02 mV s-12nd cyclea-0.100.1Sb + K+ + e- → KSbKSb + 2K+ + 2e- → K3SbK3Sb → KSb + 2K+ + 2e-KSb → Sb + K+ + e- Figure 4. Cyclic voltammograms of (a) Sb, (b) LaSb/Sb (10/90 wt.%), and (c) LaSb electrodes.  3.3. Charge–Discharge Cycling Performance of Various RESbx/Sb Electrodes  Figure 5 shows the charge–discharge cycle dependence of the gravimetric and volumetric discharge capacities of different RESbx/Sb (10/90 wt.%) and pure Sb electrodes. The results of pure RESbx electrodes are also shown. The high density of Sb and RESbx (5.8–7.3 g cm−3) resulted in a high capacity per volume. All pure RESbx electrodes exhibited long cycle stability. Among all RESbx electrodes, the CeSb and LaSb electrodes exhibited the highest and the lowest discharge capacities, respectively. The discharge capacity of the GdSb, SmSb, and YSb electrodes was comparable and the second highest. All RESbx/Sb composite electrodes exhibited high initial  16 capacity of approximately 530 mA h g−1 and suppressed the rapid capacity decay of the Sb electrode at the initial cycle. In particular, the CeSb/Sb and LaSb/Sb electrodes showed superior cycle stability. It appeats that the cycling performance of RESbx/Sb composite electrodes is not affected by the discharge capacity of pure RESbx electrodes.  0100200300400500600SbLaSbSmSbYSbYSb/SbLaSb/SbSmSb/SbCeSb/SbCeSbDischarge capacity / mA h g-1GdSba GdSb/Sb010002000300040000 100 200 300 400Discharge capacity / mA h cm-3Charge-discharge cycle numberSbLaSb SmSbYSbYSb/SbLaSb/SbSmSb/SbCeSb/SbCeSbGdSbGdSb/Sbb Figure 5. Cycle dependency of (a) gravimetric and (b) volumetric discharge capacities of RESbx/Sb (10/90 wt.%) and Sb alone electrodes in 1 M KFSA/Py13-FSA at a current density of 50 mA g–1.   The theoretical capacity achievement rate and the capacity retention of the RESbx/Sb (10/90 wt.%) composite electrodes are provided in Figure S6. As mentioned above, only Sb in RESbx/Sb (10/90 wt.%) alloyed and dealloyed with K, and hence, the theoretical capacity of the composite electrode is 594 mA h g−1 regardless of the RESbx. The capacity retention also indicates  17 how much capacity is maintained over subsequent cycles, with the initial discharge capacity being 100%. These showed a similar trend to their cycling performances. Figure S7 also shows the capacity retention of the pure RESbx electrodes. As can be easily seen from Figure 5, the pure RESbx exhibited high capacity retention over 400 cycles. Figures S8 and S9 display the initial charge-discharge curve of RESbx/Sb (10/90 wt.%) composite and pure RESbx electrodes, respectively. For comparison, the result of the pure Sb electrode is also shown in Figure S8. Regardless of the type of RESbx, the composite electrodes exhibited charge-discharge curves similar to those of the pure Sb electrode. Conversely, the pure RESbx electrodes exhibited different charge-discharge behavior from that of the Sb-alone electrode. This trend was confirmed by CV measurements as shown in Figures S10-S13.   To clarify the differences in cycling performance between the different RESbx, the cycle life tests were conducted with a charge capacity limitation of 400 mA h g−1 (Figure 6). To shorten the measurement time, the current density was set to 200 mA g−1 which is four times higher than that in the previous test. The results clearly confirmed that the cycle life depended on the RESbx. All composite electrodes showed a longer cycle life than the pure Sb electrode. Notably, the CeSb/Sb composite electrode exhibited a cycle life more than three times longer than the pure Sb electrode. To determine the extent to which the addition of RESbx suppresses electrode disintegration, the cause of capacity fading, SEM observations of electrode cross-sections before and after charge and discharge were performed (see the next section).    18 01002003004005006000 50 100 150 200 250SbYSb/SbLaSb/SbSmSb/SbCharge-discharge cycle numberDischarge capacity / mA h g-1 CeSb/SbGdSb/Sb Figure 6. Cycle life of RESbx/Sb (10/90 wt.%) and Sb alone electrodes in 1 M KFSA/Py13-FSA at 200 mA g–1 with a charge capacity limitation of 400 mA h g –1.  3.4. Reaction Behaviors of RESbx/Sb Electrodes  Figures 7a–7f depict the cross-sectional SEM images of the pure Sb and the RESbx/Sb composite electrodes after the 50th cycle and Figure 7g shows change in the relative thickness of each electrode. Large cracks were observed at the pure Sb electrode (Figure 7a), whereas small cracks were confirmed at the composite electrodes (Figures 7b–7f) and the electrode disintegration was found to be suppressed. Figures S14–S19 display the cross-sectional SEM images of these electrodes at other cycles. It can be seen that these small cracks increased in size with an increase in the cycle number (Figures S17–S19). Additionally, the CeSb/Sb electrode with the longest cycle life continued to maintain the lowest relative thickness over all cycles. We speculated that the difference in the relative thickness was due to differences in K+ migration ease in RESbx. Thus, the DK+ was investigated using GITT measurements.    19  Figure 7. Cross-sectional SEM image of (a) pure Sb, (b) YSb/Sb, (c) GdSb/Sb, (d) SmSb/Sb, (e) LaSb/Sb, and (f) CeSb/Sb electrodes after 50th cycle. (g) Changes in relative thickness (t/t0) of each electrode over the cycle. t0 and t represent the thickness measured before and after cycling, respectively. The value of t was investigated in the depotassiation state.   Figures S20 and S21 show the relationship between the change in potential and time obtained by repeated charging and relaxation and its enlarged view of the pure Sb electrode, respectively. The DK+ was calculated using equation (2) and the obtained parameters of E and Es in Figure S21, where , mB, Vm, MB, and S means the charging time, the mass of active material, molar volume, formula weight of active material, and the area of the active material layer, respectively. 𝐷𝐾+ =4𝜋𝜏(𝑚𝐵𝑉𝑚𝑀𝐵𝑆)2(∆𝐸𝑆∆𝐸𝜏)2                                               (2) Figure 8a shows the relationship between potential and DK+, whereas Figure 8b provides the correlation between the lattice parameter and the average DK+ at the potassiation potential of each RESbx electrode. The crystal system of all RESbx is cubic, whereas that of elemental Sb is trigonal. Table S1 summarizes the lattice parameter and potassiation potential of RESbx. The DK+ of RESbx increased with an increase in lattice parameter. It is considered that the reason for lower DK+ of  20 pure Sb is due to a difference in the crystal structure and/or a smaller lattice constant. LaSb and CeSb particularly possess high DK+, and the composite electrodes with these RESbx exhibited a superior cycle life. RESbx with high DK+ can function as an excellent K+ conduction path. Thus, the disintegration of the RESbx/Sb composite electrode would be suppressed by uniform K+ storage and release throughout the electrode layer. 10-1610-1510-1410-1310-120 0.2 0.4 0.6 0.8DK+ / cm2 s-1Potential / V vs. K+/KSbLaSbSmSbYSbCeSb●●●●●GdSb●a10-1510-1410-130 0.4 0.5 0.6DK+ / cm2 s-1Lattice parameter / nmSbCeSbSmSbYSbLaSb10-12GdSbb Figure 8. Relationship between (a) DK+ and potential and (b) DK+ and lattice parameter.   CeSb exhibited the longest cycle life, whereas LaSb demonstrated the highest K+. We considered the possibility that other parameters may contribute to cycle stability and focused on the mechanical properties of the RESbx, particularly the reference breaking strength, Cx. The strength is the ability to resist deformation or breakage when external forces are applied. Figure 9 shows the compressive deformation–indentation force curve and the Cx obtained for RESbx and Sb powders. Sb with the highest Cx is difficult to deform, meaning it is hard. In contrast, the RESbx with a low Cx are easily deformed, indicating that they are soft. Consequently, the CeSb was found to be the softest. It is presumed that the presence of CeSb between the hard Sb phases suppressed the pulverization of Sb and stabilized the electrode structure. Although the CeSb has slightly lower  21 DK+ than LaSb, its mechanical properties are excellent. Thus, the CeSb/Sb composite electrode is believed to exhibit excellent cycle life.  Figure 9. (a) Compressive deformation–indentation force curve and (b) reference breaking strength of RESbx and Sb powders. The inset in part (a) shows the schematic diagram of the experiment.  4. Conclusions  We investigated the electrochemical potassiation and depotassiation properties of various RESbx/Sb composite electrodes in an ionic liquid electrolyte. Based on charge–discharge curves and cyclic voltammograms, only pure Sb in the composite electrode would contribute to most of the capacities. Due to the significant volumetric changes in most metal/alloy electrodes during charging and discharging, adding 10 wt.% of a substance to compensate for this drawback had little impact on the electrochemical performance. However, surprisingly, the suppression of capacity decay at 90 wt.% Sb in this electrode system was nearly identical to that at other weight ratios. The CeSb/Sb and LaSb/Sb electrodes exhibited the longest cycle life among various  22 RESbx/Sb electrodes. The cross-sectional SEM revealed that an increase in the relative thickness of both electrodes was suppressed during charge–discharge cycling. Additionally, small cracks were observed at the RESbx/Sb electrodes and large cracks were seen at the pure Sb electrode. The pure LaSb and CeSb particularly possess high DK+ due to their large lattice parameter. Composite electrodes with these antimonides exhibited a superior cycle life. These antimonides can function as excellent K+ conduction paths; therefore, the disintegration of the composite electrodes was suppressed by uniform K+ storage and release throughout the electrode layer. The CeSb/Sb electrode exhibited the longest cycle life, but the LaSb had the highest K+. The reference breaking strength of CeSb was lower than that of LaSb, indicating that CeSb is soft and easily deformed. The presence of CeSb between the hard Sb phases stabilized the electrode structure and suppressed the pulverization of the Sb. Therefore, the CeSb/Sb composite electrode exhibited the longest cycle life.     23 ASSOCIATED CONTENT Supporting Information.  XRD patterns, crystal structures, SEM images, particle size distribution, TEM image, theoretical capacity achievement rates, capacity retentions, charge–discharge curves, cyclic voltammograms, cross-sectional SEM images, GITT curves, and lattice parameters are available free of charge via the Internet at http://pubs.acs.org.  AUTHOR INFORMATION Corresponding Authors *Yasuhiro Domi and Hiroki Sakaguchi E-mail: domi@tottori-u.ac.jp and sakaguch@tottori-u.ac.jp Notes The authors have declared no competing financial interests to report.  ACKNOWLEDGMENT This work was partially supported by the Joint Usage/Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE2021A-28, ZE2022A-07, and ZE2023A-21) and the NIMS Joint Research Hub Program. We thank Mr. R. Matsui of the Technical Department, Tottori University for his technical support.    24 Author Contributions The manuscript was written with contributions from all authors.    25 REFERENCES (1) Armand, M.; Tarascon, J.M. Building Better Batteries. Nature 2008, 451 (7179), 652–657. DOI: 10.1038/451652a (2) Whittingham, M.S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104 (10), 4271–4301. DOI: 10.1021/cr020731c (3) Goodenough, J.B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22 (3), 587–603. DOI: 10.1021/cm901452z (4) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium–Ion Batteries. Chem. Rev. 2014, 114 (23), 11636–11682. 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