Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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High Cycle Capability of All-Solid-State Lithium−Sulfur Batteries Using Composite Electrodes by Liquid-Phase and Mechanical Mixing Kota Suzuki,†,‡,§ Naohiro Mashimo,§ Yuki Ikeda,§ Toshiyuki Yokoi,∥ Masaaki Hirayama,†,‡,§ and Ryoji Kanno*,†,‡,§ †
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ‡ All-Solid-State Battery Unit, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohma 226-8503, Japan § Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ∥ Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *
ABSTRACT: All-solid-state lithium−sulfur batteries were fabricated using composite electrodes incorporating sulfur, carbon replica, and a solid electrolyte. Novel liquid-phase mixing contributed to improving electrochemical properties through solidelectrolyte penetration into the mesopores of the carbon replica. Combined mechanical and liquid-phase mixing realized a solidstate battery with high electrochemical performance comparable to a liquid battery system. A discharge capacity in excess of 1500 mAh g−1 and a high Coulombic efficiency of about 100% were demonstrated at an applied pressure of 213 MPa. The novel sulfur-electrode fabrication method enhances battery performance, since the ionic conduction pathway in the composite electrode is increased. KEYWORDS: all-solid-state battery, Li−S battery, composite electrode, electrode fabrication, mechanical and liquid-phase mixing
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conductivity but also suitable space for the volume change seen in sulfur during battery reactions. We focus on a carbon replica structure as a framework; the matrix of this structure has a three-dimensional arrangement of pores in the 10−100 nm range.10 However, although optimization of mesopore size (∼14 nm) and mixing conditions for the preparation of the sulfur−carbon replica composites realized high battery performance, cycle and rate capabilities were still lower than those of batteries using liquid electrolyte systems.5,11 Sulfur−carbon replica composites are prepared by gas-phase mixing, which introduces sulfur into the mesopores, while mechanical milling
ll-solid-state lithium−sulfur batteries (>1600 mAh gsulfur−1) are among the most promising candidates for new-generation high-energy-density power sources.1 In this system, the serious problem of the dissolution of lithium polysulfides into the liquid electrolytes during battery reaction is prevented by the all-solid-state configuration.2−5 Other problems associated with sulfur electrodes include the low electrical conductivity of sulfur (5 × 10−30 S cm−1) that contributes to a low sulfur utilization ratio and deteriorating rates and cycle capabilities of these batteries. To address these problems, a wide variety of carbon materials have been used as conductive frameworks in sulfur−carbon composite electrodes.6−9 Mesoporous carbons that contain mesosized (2−50 nm) pores in their matrices are attractive since they incorporate sulfur into their pores and provide not only electrical © XXXX American Chemical Society
Received: February 20, 2018 Accepted: May 18, 2018
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DOI: 10.1021/acsaem.8b00227 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
gradually decreased during the following 10 cycles. Subsequent stable discharge capacities of around 1500 mAh g−1, with high Coulombic efficiencies (≈100%), were maintained up to the 50th cycle (Figure 1b). These excellent performance properties are among the best reported for all-solid-state lithium−sulfur batteries.16−18 The combination of mixing processes provides ionic/electronic conduction pathways within the composite electrode, while the applied pressure suppresses the volume change experienced by sulfur during charge−discharge reactions.15 Sulfur electrodes show large volume changes (∼180%) during charge−discharge reactions. Consequently, the capacity fading observed for these batteries in the absence of applied pressure is possibly due to the isolation of sulfur within the composite as a result of volume changes (see the Supporting Information). Since sulfur readily undergoes changes in volume within the composite void space, inside the mesopores, or in low-density regions of the composite, a higher density composite lacking the void structure could suppress the isolation phenomenon. Because the walls of the carbon replica are part of a strong framework, the pressure inside the composite matrix increases as the sulfur volume expands. Consequently the development of novel techniques to fill sulfur and the solid electrolyte into the framework pores will contribute to further enhancement of the electrochemical properties of all-solid-state batteries because, under high pressure, these batteries exhibit electrochemical performance comparable to that of liquid-electrolyte batteries.1,9,19 We next examined the stability of the solid electrolyte when immersed in THF. Li10.05Ge1.05P1.95S12 did not show complete dissolution in THF; rather, a suspension of the solid electrolyte was observed (Figure S3). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements revealed no noteworthy changes in morphology or crystal structure (Figure S4). However, the ionic conductivity of the coldpressed solid-electrolyte powder decreased from 5.3 × 10−3 to 1.1 × 10−3 S cm−1 as a consequence of the mixing process (Figure S5). Although residual THF, after drying, could result in conductivity deterioration, no additional weight loss due to residual THF was observed by thermogravimetric (TG) analysis. Sharper XRD peaks (Table S1) indicate partial dissolution of the solid electrolyte in THF, since a similar phenomenon was reported in an argyrodite solid electrolyte system (i.e., crystallinity is enhanced during reprecipitation).13 Therefore, this change in conductivity might be due to small amounts of an impurity phase, formed at the grain boundary and/or particle surface during the dissolution/reprecipitation process.13 Even though conductivity decreased as a result of liquid-phase mixing, its absolute value was still sufficiently high (≈10−3 S cm−1) for use as the solid electrolyte in the battery. SEM was used to analyze the composites at the macroscale. Before mixing, a large variety of particle sizes (1−30 μm) was observed for the carbon replica (Figure S6a). In the image of the S/CR, no aggregation of deposited sulfur was observed, since the sulfur introduced by gas-phase mixing was highly distributed over the surface of the carbon replica (Figure S6b).5,11 Slight decreases in particle size were observed for the carbon replica (1−20 μm), due to hand grinding using an agate mortar, after the introduction of sulfur. SEM images of the sulfur−carbon replica−solid-electrolyte (S/CR/SE) composites are shown in Figures 2 and S7. After mechanical mixing (process i), the particle sizes decreased to 0.5−5 μm and the particles were well dispersed in the composite (Figure S7). The
is used to mix the thiolithium superionic conductor solid electrolyte (Li3.25Ge0.25P0.75S4, ≈2.0 × 10−3 S cm−1) and the sulfur−carbon replica composite to obtain the electrode material. The poor battery properties that result from this process are attributed to the small contact area between sulfur and the solid electrolyte, which cannot penetrate deeply enough into the carbon replica (average pore size, 14 nm) by mechanical milling. Therefore, in the present study we examined a liquid-phase method12,13 for introducing the solid electrolyte into the mesopores. In order to fabricate the composite electrode, the prepared sulfur−carbon replica composites and the solid electrolyte Li10.05Ge1.05P1.95S12,14 a material with high ionic conductivity (≈1.0 × 10 −2 S cm−1), were immersed in tetrahydrofuran (THF), followed by mixing and drying. The structures and electrochemical properties of the composites were investigated, and the effects of the liquidphase method compared to the mechanical-milling method were elucidated. Since the performance of the battery, which includes an active material that undergoes large volume changes, can be improved by applying pressure,15 the effect of applied pressure during charge−discharge cycling was investigated in order to improve the cycle stability of the allsolid-state lithium−sulfur battery. The pore size of the carbon replica was evaluated by BJH (Barrett−Joyner−Halenda) plot (Supporting Information Figure S1) using BET (Brunauer−Emmett−Teller) data. The aperture size of the carbon replica corresponds to the observed pore size of 14 nm. Sulfur−carbon replica composites (S/CRs) were mixed with the solid electrolyte using three different mixing methods (Figure S2): (i) mechanical mixing, (ii) liquidphase mixing, and (iii) combination mixing using both liquidphase and mechanical mixing. The battery fabricated using the electrode prepared by combination mixing exhibited a large initial discharge capacity of >2000 mAh g−1 under an applied pressure of 213 MPa (Figure 1a). The discharge capacity
Figure 1. (a) Charge−discharge curves for all-solid-state battery using composite electrode prepared by combination mixing. (b) Discharge capacity and Coulombic efficiency of battery as functions of cycle number. The battery was operated at a 0.5 C rate, and the battery cell was compressed at 213 MPa during charge−discharge measurements. B
DOI: 10.1021/acsaem.8b00227 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
and large pore volumes of >1.6 m3 gCR−1. Following process i, a relatively large change in V (−48%) was observed. Although mechanical mixing decreases particle size, the feasibility of the solid electrolyte penetrating the mesopores (≈14 nm) was still low. Therefore, the observed decrease was possibly due to the coating of the carbon replica surface by the solid electrolyte, thereby preventing nitrogen gas flow into the mesopores (poreblocking) for some of the carbon replica particles. In the case of process ii, a relatively small change in V (−23%) was observed after mixing. The introduction of the solid electrolyte into the mesopores contributes to these changes, since liquid-phase mixing is expected to have a capillary effect. There are two expected routes for introducing the solid electrolyte into the pores; (i) nanosized solid electrolyte particles, which cannot be observed in SEM images, could penetrate the mesopores; and (ii) the solid electrolyte partially dissolved in THF could be absorbed in the mesopores. The effect of surface coating is likely to be negligible, since the aggregation of small particles of the carbon replica was confirmed by SEM. Process iii resulted in the largest change in V (−71%) after mixing. Both penetration of the solid electrolyte by the capillary effect and pore-blocking resulting from liquid-phase and mechanical mixing, respectively, contribute to this large change in pore volume. Therefore, mesoscale characterization revealed that liquid-phase mixing enhances the filling ratio of the solid electrolyte in the mesopores. From macro- and mesoscale analysis of the structures, we conclude that the composite prepared by each method has the following characteristics: (i) homogeneous particle distribution (0.5−5 μm) with a poor filling ratio of the solid electrolyte in mesopores; (ii) micrometer-scale particle aggregation (small (0.5−1 μm) particles on large (>5 μm) S/CR) with a high filling ratio of the solid electrolyte in mesopores; and (iii) homogeneous particle distribution (0.5−5 μm) with a high filling ratio of the solid electrolyte in mesopores. We next examined the effect of the fabrication process on battery performance. The composite electrodes, prepared under different conditions, were subjected to charge−discharge testing without applying pressure. All of the batteries showed larger discharge capacities than the theoretical capacity of sulfur (Figure S9a−c), indicating that carbon is reacting with lithium8,11 and/or the decomposition of the solid electrolyte proceeds.20,21 These reactions may contribute to the observed capacity in excess of the theoretical value. The composite electrode prepared by liquid-phase mixing (process ii) exhibited severe capacity fading over several initial cycles (Figure 3). This is ascribable to inadequate macroscale ion-conduction pathways in the composite due to the aggregation of the solid electrolyte on the S/CR surface, as observed by SEM (Figure 2a). In contrast, the composite electrodes prepared by mechanical mixing (process i) and combination mixing (process iii) provided larger discharge capacities (>1000 mAh g−1) even at the 50th cycle. The homogeneous macroscale particle distribution obtained by mechanical mixing contributes to the high usability of the sulfur in the composite and to better cycling capability. However, the electrode prepared through combination mixing showed slightly higher capacity than that prepared by mechanical mixing. In order clarify the differences between these composites, the rate capabilities of the electrodes prepared by mechanical mixing and combination mixing were examined at various currents rates between 0.1 and 5.0 C. The battery with the
Figure 2. SEM images of sulfur−carbon replica−solid-electrolyte (S/ CR/SE) composites prepared by (a) liquid-phase mixing and (b) combination mixing.
mechanical mixing pulverized the carbon replica and solid electrolyte, contributing to the homogeneous distributions of the S/CR and the solid electrolyte at the macroscale. Conversely, large S/CR particles remained in the composites prepared by liquid-phase mixing (process ii). In addition, partial aggregates of small S/CR and/or solid electrolyte particles (0.5−1 μm) were observed on large-sized S/CR surfaces (>5 μm). Therefore, liquid-phase mixing has a small effect on reducing particle size, while more strongly affecting the macroscale particle distribution within the composite. The SEM image of the composite prepared by combination mixing (process iii) reveals no significant difference in particle size and distribution between the materials produced by processes i and iii. This result indicates that the mechanical mixing process determines composite attributes such as particle size and distribution on the macroscale. BET surface-area measurements were used to analyze the composites at the mesoscale. The mesoporous structure of the carbon replica is maintained after the various mixing processes, as confirmed by type IV isotherms (Figure S8). The adsorption gas volume (Va) decreased as a result of the mixing process, indicating decreases in surface area and pore volume. The calculated pore volume (V) and changes in pore volume (ΔV) of each sample are summarized in Table S2. Before mixing, all S/CR samples exhibited large surface areas of >500 m2 gCR−1 C
DOI: 10.1021/acsaem.8b00227 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00227. Full experimental details, Figures S1−S9, Tables S1 and S2, and supplementary references (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kota Suzuki: 0000-0002-2473-0724 Ryoji Kanno: 0000-0002-0593-2515 Notes
The authors declare no competing financial interest.
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Figure 3. Battery discharge capacities as a function of cycle number.
ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (S) (No. 17H06145) and an Early-Career Scientists grant (No. 18K14304) of the Japan Society for the Promotion of Science.
electrode prepared by combination mixing (process iii) showed higher capacities than that prepared by mechanical mixing (process i) across all C rates (Figure 4). In addition, when the
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Figure 4. Discharge capacities during rate capability testing of all-solidstate batteries containing sulfur−carbon replica−solid-electrolyte (S/ CR/SE) composites prepared by mechanical and combination mixing, as functions of charge−discharge rate.
C rate was returned to 0.1 C at the end of the testing period (15 cycles), enhanced recovery of capacity, to more than 1500 mAh g−1, was observed for the battery containing the electrode prepared by combination mixing. These results indicate that the solid electrolyte inside the matrix pores contributes to both rate and cycle capabilities of the battery. In summary, the high-capacity retention and excellent rate capability of the electrode prepared by combination mixing (process iii) are attributed to (a) high ionic conductivities inside the mesopores, resulting from liquid-phase mixing and (b) size reduction and dispersion of particles by mechanical milling at the macroscale. The use of liquid-phase mixing, for fabricating composite sulfur electrodes that incorporate mesoporous carbon, contributes to realizing a high-powerand -energy-density all-solid-state lithium−sulfur battery.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications Web site. The Supporting Information is D
DOI: 10.1021/acsaem.8b00227 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials Li6PS5Cl Solid Electrolyte from Ethanol Solution for All-Solid-State Lithium Batteries. J. Power Sources 2015, 293, 941−945. (14) Kwon, O.; Hirayama, M.; Suzuki, K.; Kato, Y.; Saito, T.; Yonemura, M.; Kamiyama, T.; Kanno, R. Synthesis, Structure, and Conduction Mechanism of the Lithium Superionic Conductor Li10+δGe1+δP2−δS12. J. Mater. Chem. A 2015, 3, 438−446. (15) Li, W. J.; Hirayama, M.; Suzuki, K.; Kanno, R. Fabrication and All Solid-State Battery Performance of TiS2/Li10GeP2S12 Composite Electrodes. Mater. Trans. 2016, 57, 549−552. (16) Nagata, H.; Chikusa, Y. An All-Solid-State Lithium−Sulfur Battery Using Two Solid Electrolytes Having Different Functions. J. Power Sources 2016, 329, 268−272. (17) Nagata, H.; Chikusa, Y. All-Solid-State Lithium−Sulfur Battery with High Energy and Power Densities at the Cell Level. Energy Technol. 2016, 4, 484−489. (18) Hakari, T.; Hayashi, A.; Tatsumisago, M. Li2S-Based Solid Solutions as Positive Electrodes with Full Utilization and Superlong Cycle Life in All-Solid-State Li/S Batteries. Adv. Sustainable Syst. 2017, 1, 1700017. (19) Manthiram, A.; Chung, S.-H.; Zu, C. Lithium-Sulfur Batteries: Progress and Prospects. Adv. Mater. 2015, 27, 1980−2006. (20) Han, F.; Zhu, Y.; He, X.; Mo, Y.; Wang, C. Electrochemical Stability of Li10GeP2S12 and Li7La3Zr2O12 Solid Electrolytes. Adv. Energy Mater. 2016, 6 (8), 1501590. (21) Han, F.; Gao, T.; Zhu, Y.; Gaskell, K. J.; Wang, C. A Battery Made from a Single Material. Adv. Mater. 2015, 27 (23), 3473−3483.
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DOI: 10.1021/acsaem.8b00227 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
