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A Water-in-Salt Electrolyte for Potassium-Ion Batteries Daniel P. Leonard, Zhixuan Wei, Gang Chen, Fei Du, and Xiulei Ji ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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ACS Energy Letters

A Water-in-Salt Electrolyte for Potassium-Ion Batteries Daniel P. Leonard†#, Zhixuan Wei†‡#, Gang Chen‡, Fei Du‡*, Xiulei Ji†* †Department of Chemistry, Oregon State University, Corvallis, Oregon, 97331-4003, United States ‡ Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun, 130012, People’s Republic of China AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] #These authors contributed to the article equally.

ABSTRACT Concentrated potassium acetate as a water-in-salt electrolyte provides a wide potential window from -1.7 to 1.5 V vs. Ag/AgClKCl sat.. It facilitates the reversible operation of KTi2(PO4)3, an anode of potassium-ion batteries, that otherwise only functions in non-aqueous electrolytes. KTi2(PO4)3 exhibits high rate capability, long cycle life, and a reversible capacity of 50 mAh/g.

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Recently great interest has been garnered in solvent-in-salt electrolytes, particularly the water-in-salt electrolytes (WiSE).1,2 These non-flammable water-based electrolytes could significantly improve the safety of batteries. Currently most reported WiSE have focused on fluorinated-imide-based salts, usually lithium bis(trifluoromethane)sulfonimide (LiTFSI),3–6 though some recent studies have utilized other metal cations, i.e., Na+ and Zn2+.7,8 However, the use of expensive and possibly toxic fluorinated anions limits the relevance of WiSE for sustainable batteries needed to couple the renewable, though intermittent, energy sources. Thus, it is of high values to identify non-lithium and non-fluorinated WiSE, particularly, based on Kion salts for potassium-ion batteries (KIBs).9,10 Herein we report the K-ion storage performance of KTi2(PO4)3 (KTP) in potassium acetate (KAc) based WiSE, where KTP as a KIB anode would otherwise only exhibit topotactic properties in non-aqueous electrolytes. This study demonstrates the first usage of KAc-based WiSE in batteries that expands the operation potential window of aqueous KIBs to 3.2 V. Indeed, KAc is much more cost competitive than its rivals of fluorinated-imide salts; this highlights the prospect of low-cost aqueous KIBs for grid storage. Note that in the midst of our studies of this


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ACS Energy Letters

WiSE system, Pan et al. first reported the KAc-based WiSE for aqueous supercapacitors, which enables a voltage window of 2 V.11

Figure 1. (A) Linear voltammetry curves recorded at 1 mV/s in 1 m, 10 m, and 30 m KAc electrolytes. Electrochemical stability tests used Al negative electrodes and Ti positive electrodes. We selected the onset potentials, at which a current of 0.01 mA was observed, to define the stability window. (B) Typical CV curves of KTP electrodes at 0.5 mV/s in 1 m (3rd cycle), 10 m (10th cycle), and 30 m (10th cycle) KAc electrolytes. (C) The conductivity of KAc and LiTFSI electrolytes as a function of their molality values. Yamada et al. reported that Al and Ti as the negative and positive current collectors, respectively, allow a wider electrochemical window than other current collectors.5 Impressively, when employing this set of current collectors, we demonstrate that the 30 m KAc displays a wide electrochemical stability window of 3.2 V, from -1.7 to 1.5 V vs. Ag/AgClKCl sat. (Figure 1A). As for 10 m and 1 m KAc electrolytes, the windows are 2.4 and 1.4 V, respectively. With this electrolyte, aqueous KIBs may employ electrode materials previously only viable in non-aqueous applications.12 For example, Xu et al. reported the K-ion storage


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properties of KTP in a non-aqueous electrolyte using ethylene carbonate and dimethyl carbonate as the solvents. We attempted to employ 1 m and 10 m KAc as the electrolyte for KTP electrodes. However, the cyclic voltammetry (CV) studies reveal pronounced cathodic current from the KTP electrode in these electrolytes, which is due to hydrogen evolution reaction (HER) (Figure 1B). Nevertheless, in 30 m KAc electrolyte, KTP anode delivers reversible redox behavior. In the 30 m KAc, we are able to probe the redox couple, Ti4+/Ti3+, of the KTP electrode. As shown in Figure 1C, the conductivity of the KAc electrolytes exceeds that of the LiTFSI, and other reported fluorinated imide salts, at the same concentrations.7 The higher conductivity may be attributed to either the weak Lewis acidity of potassium ions or the advantage of an acetate salt, which requires future studies. Furthermore, KTP exhibits better kinetics in this WiSE electrolyte than in the non-aqueous system. The potential hysteresis between the K-ion insertion and extraction decreases from ~0.5 V in the non-aqueous system to ~0.3 V in the KAc system (Figure 2A). This means a considerable increase in the battery energy efficiency. The typical galvanostatic charge/discharge (GCD) profiles show plateaus at similar potentials as the CV results (-1.3/-1.0 V vs. Ag/AgClKCl sat. or 1.8/2.1 V vs. K+/K) (inset of Figure 2A), delivering a specific capacity of 53 mAh/g, where one K-ion insertion per Ti4+/Ti3+ corresponds to the theoretical capacity of 64 mAh/g. Figure 2B shows the rate performance of the KTP electrode, conducted at current densities, ranging from 100 mA/g to 5 A/g, which appears superior compared to the non-aqueous electrolyte.12 Figure 2C depicts the initial CV curves of the KTP anode, where the first cathodic scan shows considerable current due to electrolyte reduction, but in the subsequent cycling the redox behavior becomes favorable for the reversible K-ion storage over electrolyte decomposition on


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ACS Energy Letters

the anode surface. In GCD cycling, the Coulombic efficiency increases from 62% in the first cycle to 98.7% by the 10th cycle (inset of Figure 2C), where such a phenomenon indicates the passivation on the surface of the KTP electrode that prohibits the HER reaction. Similar evolution of Coulombic efficiency was observed for other WiSE battery systems that use fluorinated-imide salts.13,14 In those WiSE systems, solid electrolyte interphase (SEI) was observed. However, a SEI layer from an aqueous KAc system, particularly from the reduction of acetate groups could be highly intriguing, and warrants future studies. We also investigated the cycling stability of KTP in 30 m KAc at 1 A/g. The cell underwent 20 conditioning cycles at a current density of 200 mA/g before the long cycling tests were carried out at 1 A/g. KTP’s capacity fades from 58 to 35 mAh/g over the initial 500 cycles, and the capacity is stabilized at 30 mAh/g afterwards over the following 5500 cycles. Interestingly, the capacity started to increase during the next 5000 cycles, which reached 40 mAh/g at the end of 11000 cycles with a total capacity retention of 69% (Figure 2D).


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Figure 2. (A) Typical CV curves of the KTP anode in 30 m KAc and non-aqueous K-ion electrolyte showing the difference in redox polarization at 0.5 mV/s and 0.1 mV/s, respectively. Inset shows typical GCD profiles in 30 m KAc taken at 200 mA/g. (B) Rate cycling performance and the corresponding Coulombic efficiency of the KTP anode. (C) The 1st, 2nd, and 10th CV curves in 30 m KAc electrolyte showing the progressively more reversible redox peaks. Inset shows the first 20 GCD cycling with Coulombic efficiency of KTP in 30 m KAc. (D) Long cycling performance and Coulombic efficiency of the KTP anode. In summary, we have demonstrated the utility of an inexpensive and non-toxic potassium-based WiSE electrolyte that provides a potential window of ~3.2 V. This electrolyte


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ACS Energy Letters

expands the avenues of exploration available for high-energy-density aqueous K-ion batteries. The 30 m KAc electrolyte allows the KTP anode to deliver a reversible specific capacity of 53 mAh/g together with promising rate performance and long cycle stability. It is our hope that the results presented here can promote the development of cathode materials that are compatible with this electrolyte and potential range.

Associated content Supporting Information: Experimental details. XRD, SEM and TEM of KTP.

Acknowledgements: X. J. is grateful to the United State National Science Foundation, Award Number 1551693 for the support. F. D. thanks the Joint Foundation between Jilin Province and Jilin University (SXGJQY2017-10).

References (1) Suo, L.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. “Water-in-Salt” Electrolyte Enables High-Voltage Aqueous Lithium-Ion Chemistries. Science 2015, 350, 938–943. (2) Suo, L.; Hu, Y.-S.; Li, H.; Armand, M.; Chen, L. A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481. (3) Suo, L.; Han, F.; Fan, X.; Liu, H.; Xu, K.; Wang, C. “Water-in-Salt” Electrolytes Enable Green and Safe Li-Ion Batteries for Large Scale Electric Energy Storage Applications. J. Mater. Chem. A 2016, 4, 6639–6644. (4) Suo, L.; Borodin, O.; Sun, W.; Fan, X.; Yang, C.; Wang, F.; Gao, T.; Ma, Z.; Schroeder, M.; von Cresce, A.; et al. Advanced High-Voltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chem. Int. Ed. 2016, 55, 7136–7141. (5) Yamada, Y.; Usui, K.; Sodeyama, K.; Ko, S.; Tateyama, Y.; Yamada, A. Hydrate-Melt Electrolytes for High-Energy-Density Aqueous Batteries. Nat. Energy 2016, 1, 16129.


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(6) Sun, W.; Suo, L.; Wang, F.; Eidson, N.; Yang, C.; Han, F.; Ma, Z.; Gao, T.; Zhu, M.; Wang, C. “Water-in-Salt” Electrolyte Enabled LiMn2O4 /TiS2 Lithium-Ion Batteries. Electrochem. Commun. 2017, 82, 71–74. (7) Kühnel, R.-S.; Reber, D.; Battaglia, C. A High-Voltage Aqueous Electrolyte for SodiumIon Batteries. ACS Energy Lett. 2017, 2, 2005–2006. (8) Hu, P.; Yan, M.; Zhu, T.; Wang, X.; Wei, X.; Li, J.; Zhou, L.; Li, Z.; Chen, L.; Mai, L. Zn/V2O5 Aqueous Hybrid-Ion Battery with High Voltage Platform and Long Cycle Life. ACS Appl. Mater. Interfaces 2017 DOI: 10.1021/acsami.7b13110. (9) Fan, L.; Liu, Q.; Xu, Z.; Lu, B., An Organic Cathode for Potassium Dual-Ion Full Battery. ACS Energy Lett. 2017, 2, 1614-1620. (10) Tian, B.; Tang, W.; Leng, K.; Chen, Z.; Tan, S. J. R.; Peng, C.; Ning, G.-H.; Fu, W.; Su, C.; Zheng, G. W.; Loh, K. P., Phase Transformations in TiS2 during K Intercalation. ACS Energy Lett. 2017, 2, 1835-1840. (11) Tian, Z.; Deng, W.; Wang, X.; Liu, C.; Li, C.; Chen, J.; Xue, M.; Li, R.; Pan, F. Superconcentrated Aqueous Electrolyte to Enhance Energy Density for Advanced Supercapacitors. Funct. Mater. Lett. 2017, 1750081. (12) Han, J.; Niu, Y.; Bao, S.; Yu, Y.-N.; Lu, S.-Y.; Xu, M. Nanocubic KTi2(PO4)3 Electrodes for Potassium-Ion Batteries. Chem. Commun. 2016, 52, 11661–11664. (13) Suo, L.; Oh, D.; Lin, Y.; Zhuo, Z.; Borodin, O.; Gao, T.; Wang, F.; Kushima, A.; Wang, Z.; Kim, H.-C.; et al. How Solid-Electrolyte Interphase Forms in Aqueous Electrolytes. J. Am. Chem. Soc. 2017 DOI: 10.1021/jacs.7b10688. (14) Yang, C.; Chen, J.; Qing, T.; Fan, X.; Sun, W.; von Cresce, A.; Ding, M. S.; Borodin, O.; Vatamanu, J.; Schroeder, M. A.; et al. 4.0 V Aqueous Li-Ion Batteries. Joule 2017, 1, 122– 132.


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