Towards High-Safety Potassium-Sulfur Battery Using Potassium

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Towards High-safety Potassium-sulfur Battery Using Potassium Polysulfide Catholyte and Metal-free Anode Jang-Yeon Hwang, Hee-Min Kim, Chong S. Yoon, and Yang-Kook Sun ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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ACS Energy Letters

Towards High-Safety Potassium-Sulfur Battery Using Potassium Polysulfide Catholyte and Metal-Free Anode Jang-Yeon Hwang, † Hee Min Kim, † Chong S. Yoon,‡ Yang-Kook Sun* † †

Department of Energy Engineering, Hanyang University, Seoul, 04763, Republic of Korea

‡

Department of Materials Science and Engineering, Hanyang University, Seoul, 04763, Republic of Korea

* To whom correspondence should be addressed. E-mail: [email protected]

Abstract Based on the reversible conversion reactions: K2Sx (5 ≤ x ≤ 6)

ሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬԦ dıscharge

K2S3

ሬሬሬሬሬሬሬሬሬሬሬሬሬԦ charge

K2S5, the

proposed K-S battery delivered a high discharge capacity of ~ 400 mAh g-1 at 0.1 C-rate with stable cycle retention (94% after 20 cycles) and good rate capability up to 2 C-rate. In addition, instead of an explosive and high reactive potassium metal electrode, a full cell consisting of an electrochemically potassium-impregnated hard carbon and the K2Sx (5 ≤ x ≤ 6) catholyte was constructed to demonstrate the feasibility of a safe K-S battery system free of metallic potassium.

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An integral part of promoting the large-scale use of new renewable energy sources is the development of cost-effective energy storage technologies.1,2 Recently, potassium-ion batteries (KIBs) have attracted widespread interest as a low-cost, alternative energy storage medium, to replace the widely used lithium-ion batteries (LIBs) because potassium has higher natural abundance (the reserve of potassium is close to that of Na in the Earth’s crust) and lower standard redox potential than other metallic elements (E°(Li/ Li+): −3.04 V, E°(K/K+): −2.93 V, E°(Ca/Ca2+): −2.87 V, E°(Na/Na+): −2.71 V, E°(Mg/Mg2+): −2.27 V versus the standard hydrogen redox potential).3 Like (Li,4 Na,5 Mg,6 and Ca7)-sulfur batteries, potassium-sulfur (K-S)8 batteries offer low cost and high specific energy density. Chen et al. in 2014 pioneered a K-S battery system operating at room temperature based on the conversion reaction: S8 K2S3.8 However, no major follow-up work has been reported beyond this, mainly because of the difficulties associated with the high reactivity of the potassium metal anode and the slow reaction kinetics of solid sulfur. Herein we propose a novel K-S cell consisting of a solution-phase potassium-polysulfide catholyte and a 3-dimensional freestanding carbon nanotube (3D-FCN) film electrode (as reservoir for the polysulfide catholyte). First, a series of 0.05 M potassium-polysulfide (K2Sx,1 ≤ x ≤ 6) was prepared benchmarked against a lithium-polysulfide catholyte (Figure S1).9 Unlike lithium-polysulfides, the short-chained potassium-polysulfide, formed by mixing K and S in molar ratios ranging from 2:1 to 2:4, did not dissolve in diethylene glycol dimethyl ether (DEGDME). However, long-chained K2Sx with x ≥ 5, resulting from the reaction of K and S in the ratio 2:5, completely dissolved in DEGDME, forming a dark brown solution. The resulting solution-phase potassium polysulfide was characterized with ultraviolet/visible (UV/Vis) spectroscopy (Figure S2). Since the reference UV absorption data for potassium sulfides was unavailable, the UV spectrum in Figure S2 was compared with that of the lithium sulfides found in a Li-S battery.10 Lithium-polysulfide peaks typically appeared at 232 nm and 264 nm, which closely match the UV absorption peaks at 226 nm and 290 nm for the prepared potassium sulfide solution. In addition, the Raman spectrum of the potassium polysulfide solution exhibited peaks corresponding to those of K2S5 and K2S6 (Figure 1D).11 The solution containing the 3



potassium sulfide, K2Sx (5 ≤ x ≤ 6) was used as a catholyte for simultaneously acting as a source for reactive sulfur species and also as K+ ion-conducting medium. For electrochemical testing, a 3D-FCN electrode was employed because of its high electrical conductivity and porous structure. This provides three-dimensional interlinked pathways for electrons and K+ ions, thereby improving the electrochemical response of the K2Sx (5 ≤ x ≤ 6) catholyte. The preparation of the 3D-FCN film electrode was described in detail in our previous paper.12 The crosssectional scanning electron microscopy (SEM) image (Figure 1A) confirms that the 3D-FCN film has indeed a highly porous structure with an average thickness of ~220 µm. The fundamental electrochemical performance was tested in the voltage range of 1.0–2.4 V (vs. K+/K metal) in a half cell. The upper cut-off potential was kept below 2.4 V because the over-activity between the metallic potassium and the potassium-polysulfide above this value leads to detrimental shuttling reactions (Figure S3).8 The initial charge-discharge curves of the K | K2Sx (5 ≤ x ≤ 6) catholyte | 3D-FCN cell in Figure 1B shows that the cell had a high initial discharge capacity of ~400 mAh g-1 at 0.1 C-rate. The dQ dV-1 curves (Figure 1C) provide evidence of multiple oxidation/reduction reactions. During the discharge process, a reduction peak appeared at ~2.1 V, which corresponds to the formation of highorder potassium polysulfides (K2Sx, 5 ≤ x ≤6), whereas the second reduction peak at ~1.8 V corresponds to the formation of low-order lithium polysulfide (K2S3). In the following charge process, the dQ/dV curve shows two matching oxidation peaks at 2.05 V and 2.3 V, confirming the reversibility of the two redox reactions.8 To better understand the reaction mechanism, ex-situ Raman analysis and X-ray diffraction (XRD) measurements were performed. After the first discharge, the Raman spectrum showed peaks at 235 cm-1 and 465 cm-1, originating from the formation of K2S3 (Figure 1D).11 The ex-situ XRD pattern after the first discharge (Figure 1E) also clearly indicates the formation of pure K2S3 compounds.8 The transmission electron microscopy (TEM) image and the corresponding elemental mapping data after the first discharge (Figure 1F), further corroborates that the major discharge product is K2S3. After the subsequent first charge, the peak corresponding to K2S5 was clearly observed at 435 cm-1 in the Raman spectrum shown in Figure 1D. The K2S5 formation 4


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ACS Energy Letters

cannot be observed in the XRD patterns (Figure 1E) because it was completely dissolved in DEGDME. The Raman and the XRD results for the recharge process indicates that K2S3 (major discharge product) could be reversibly charged form a solution phase K2S5. The solution-phase polysulfide catholyte provided better reversibility (see the dQ dV-1 curves at the 5th cycle in Figure S4) and faster reaction kinetics than the solid phase elemental sulfur,8 which enhanced the cycling stability (94% after 20 cycles while maintaining Coulombic efficiencies above 95%) and the rate capability (up to 2C-rate) (Figure 2A). Based on the aforementioned results (Raman spectra, XRD, TEM analysis, and electrochemical data), the K | K2Sx (5 ≤ x ≤ 6) polysulfide catholyte | 3D-FCN cell can be operated based on the reversible conversion reactions: K2Sx (5 ≤ x ≤ 6)

ሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬሬԦ dıscharge

K2S3

ሬሬሬሬሬሬሬሬሬሬሬሬሬԦ charge

K2S5.

Finally, we also constructed a K-S full cell using hard carbon (HC) as the anode to avoid the safety concern arising from the use of a potassium metal as an anode. Firstly, a K | K2Sx (5 ≤ x ≤ 6) catholyte | HC cell was used to impregnate the HC anode with potassium (250 mAh g-1 at 0.1 C-rate in Figure S5). After the electrochemical potassium impregnation of the HC anode, we assembled a HC | K2Sx (5 ≤ x ≤ 6) catholyte |3D-FCN full cell. This cell exhibited excellent reversibility and possessed a high initial discharge capacity of 235 mAh g-1 at 0.1 C-rate (1C-rate: 558 mA g-1) in the voltage range of 0.7-1.85 V (Figure 2B). This clearly demonstrates the feasibility of a safe practical K-S battery. In summary, this paper describes a high-performance and high-safety K-S battery, which operates at room temperature and based on the K2Sx (5 ≤ x ≤ 6) catholyte. By following a similar procedure to that used for preparing a lithium-polysulfide catholyte, we successfully synthesized a solution-phase potassium catholyte, 0.05 M K2Sx (5 ≤ x ≤ 6) dissolved in DEGDME. The DEGDME solvent provided an additional advantage of being non-flammable, such that the K2Sx (5 ≤ x ≤ 6) catholyte did not ignite under flame as demonstrated in Figure S6. Based on the reversible conversion reaction between K2S3↔K2S5, the K | K2Sx (5 ≤ x ≤ 6) catholyte | 3D-FCN produces a high specific capacity of ~ 400 mAh g-1 and had a superior rate capability up to 2 C-rate in a half-cell configuration. In addition, a working HC | K2Sx (5 ≤ x ≤ 6) catholyte | 3D-FCN full cell was constructed to highlight the alleviation of the safety issue that plagued the K-S batteries, by using the K2Sx (5 ≤ x ≤ 6) catholyte. 5



Although the proposed K-S battery will require further work to make it commercially viable, the results presented here provides a new opportunity for developing safe practical K-S batteries.

ASSOCIATED CONTENT Supporting Information The experimental section includes the preparation of used materials and the characterization method.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGEMENTS This work was mainly supported by the Global Frontier R&D Programme (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM), by the Ministry of Science, ICT & Future Planning and supported by a Human Resources Development programme (No. 20154010200840) of a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant, funded by the Ministry of Trade, Industry and Energy of the Korean government.

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ACS Energy Letters

References (1) Scrosati, B.; Hassoun, J.; Sun, Y.-K. Lithium-Ion Batteries. A Look into The Future.

Energy Environ. Sci. 2011, 4, 3287-3295. (2) Hwang, J.-Y.; Myung, S.-T.; Sun, Y.-K. Sodium-Ion Batteries: Present and Future. Chem.

Soc. Rev. 2017, 46, 3529-3856. (3) Wu, X.; Daniell, P.; Ji, X. Emerging Non-Aqueous Potassium-Ion Batteries: Challenges and Opportunities. Chem. Mater. 2017, 29, 5031-5042. (4) Hwang, J.-Y.; Kim, H. M.; Lee, S.-K.; Lee, J.-H.; Abouimrane, A.; Khaleel, M. A.; Belharouak, I.; Manthiram, A.; Sun, Y.-K. High-Energy, High-Rate, Lithium-Sulfur batteries: Synergetic Effect of Hollow TiO2-Webbed Carbon Nanotubes and a Dual Functional Carbon-Paper Interlayer. Adv. Energy Mater. 2016, 6, 1501480. (5) Wei, S.; Xu, S.; Agrawral, A.; Choudhry, S.; Lu, Y.; Tu, Z.; Ma, L.; Archer, L. A. A Stable Room-Temperature Sodium-Sulfur Battery. Nat. Commun. 2016, 7, 11722. (6) Robba, A.; Vizintin, A.; Bitenc, J.; Mali, G.; Arčon I.; Kavčič, M.; Žitnik, M.; Bučar, K.; Aquilanti, G.; Macrtineau-Corcos, C. et al. A. Mechanistic Study of Magnesium-Sulfur Batteries. Chem. Mater. 2017, 29, 955-9564. (7) See, K. A.; Gerbec, J. A.; Jun, Y. A.; Wudl, F.; Stucky, G. D.; Seshadri, R. A high Capacity Calcium Primary Cell Based on the Ca-S System. Adv. Energy Mater. 2013, 3, 1056-1061.

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(8) Zhao, Q.; Hu, Y.; Zhang, K.; Chen, J. Potassium-Sulfur Batteries: A New Member of Room-Temperature Rechargeable Metal-Sulfur Batteries. Inorg. Chem. 2014, 53, 90009005. (9) Lee, D.-J.; Agostini, M; Park, J.-W.; Sun, Y.-K.; Hassoun, J.; Scrosati, B. Progress in Lithium-Sulfur Batteries: The Effective Role of a Polysulfide-Added Electrolyte as Buffer to Prevent Cathode Dissolution. ChemSusChem. 2013, 6, 2245-2248. (10) Dong, K.; Wang, S.; Yu, J. Electrochemical reactions of lithium-sulfur batteries: An analytical study using the organic conversion technique. Phys. Chem. Chem. Phys. 2014,

16, 9344-9350. (11) Janz, G. J.; Coutts, J. W.; Downey, J. R.; Roduner, E. Raman Studies of SulfurContaining Anions in Inorgainc Polysulfides. Potassium Polysulfides. Inorg. Chem. 1976,

15, 1755-1759. (12) Hwang, J.-Y.; Kim H. M.; Sun, Y.-K. Controlling the Wettability between Freestanding Electrode and Electrolyte for High Energy Density Lithium-Sulfur Batteries. J.

Electrochem. Soc. 2018, 165, A5006-A5013.

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ACS Energy Letters

Figure 1. (A) Cross sectional SEM image of the 3D-FCN film and digital photograph of liquid phase potassium-polysulfide (inset). (B) Initial charge-discharge curves of K | K2Sx (5 ≤ x ≤ 6) polysulfide catholyte |3D-FCN half-cell. (C) dQ dV-1 curves at initial chargedischarge. (D) Ex-situ Raman spectra of the electrodes: as prepared (black line), 1st discharge (red line) and 1st charge (blue line). (E) Ex-situ XRD patterns after the first discharge (brown line) and first charge (black line). (F) TEM image (a) and the corresponding EDX elemental mapping (b-d) after the first discharge. 9


ACS Energy Letters

A

0.05M K2Sx (5≤x≤6) Catholyte

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0.05M K2Sx (5≤x≤6) in DEGDME 2.5

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Figure 2. (A) K | K2Sx (5 ≤ x ≤ 6) polysulfide catholyte |3D-FCN half-cell: Cycle life test at 0.1 C-rate and rate capability test up to 2 C-rate in a voltage range of 1.2-2.4 V. (B) Initial charge-discharge curves of potassium impregnated Hard Carbon | K2Sx (5 ≤ x ≤ 6) polysulfide catholyte | 3D-FCN full cell in a voltage range of 0.7 V - 1.85 V.

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Towards High-Safety Potassium-Sulfur Battery Using Potassium

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