Water-in-Salt Electrolyte for Potassium-Ion Batteries Daniel P. Leonard,†,# Zhixuan Wei,†,‡,# Gang Chen,‡ Fei Du,*,‡ and 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
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S Supporting Information *
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 and 1 m KAc electrolytes, the windows are 2.4
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 nonaqueous electrolytes. Figure 1. (A) Linear voltammetry curves recorded at 1 mV/s in 1, 10, and 30 m KAc electrolytes. 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) Conductivity of KAc and LiTFSI electrolytes as a function of their molality values.
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ecently, great interest has been garnered in solvent-insalt electrolytes, particularly the water-in-salt electrolytes (WiSE).1,2 These nonflammable water-based electrolytes could significantly improve the safety of batteries. Currently, most reported WiSE have focused on fluorinatedimide-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 It is of high value to identify nonlithium and nonfluorinated WiSE, particularly, those based on K-ion 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 nonaqueous electrolytes. This study demonstrates the first use 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 WiSE system Pan et al. first reported the KAcbased WiSE for aqueous supercapacitors, which enables a voltage window of 2 V.11 © 2018 American Chemical Society
and 1.4 V, respectively. With this electrolyte, aqueous KIBs may employ electrode materials previously only viable in nonaqueous applications.12 For example, Xu et al. reported the Kion storage properties of KTP in a nonaqueous electrolyte using ethylene carbonate and dimethyl carbonate as the solvents. We attempted to employ 1 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 a hydrogen evolution reaction (HER) (Figure 1B). Nevertheless, in 30 m KAc electrolyte, KTP anode delivers reversible redox behavior. Received: January 2, 2018 Accepted: January 10, 2018 Published: January 10, 2018 373
DOI: 10.1021/acsenergylett.8b00009 ACS Energy Lett. 2018, 3, 373−374
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Cite This: ACS Energy Lett. 2018, 3, 373−374
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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 afterward 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). In summary, we have demonstrated the utility of an inexpensive and nontoxic potassium-based WiSE electrolyte that provides a potential window of ∼3.2 V. This electrolyte expands the avenues of exploration available for high-energydensity aqueous KIBs.
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 that in the nonaqueous system. The potential hysteresis between the K-ion insertion and extraction decreases from ∼0.5 V in the nonaqueous system to ∼0.3 V in the KAc system (Figure 2A). This means a considerable increase in the
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00009. Experimental details: XRD, SEM, and TEM of KTP (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X.J.). *E-mail:
[email protected] (F.D.). ORCID
Fei Du: 0000-0001-6413-0689 Xiulei Ji: 0000-0002-4649-9594
Figure 2. (A) Typical CV curves of the KTP anode in 30 m KAc and nonaqueous K-ion electrolyte at 0.5 and 0.1 mV/s, respectively. The inset shows typical GCD profiles in 30 m KAc taken at 200 mA/g. (B) First, second, and tenth CV curves in 30 m KAc electrolyte. The inset shows the first 20 GCD cycling with Coulombic efficiency of KTP in 30 m KAc. (C) Rate cycling performance of the KTP anode. (D) Long cycling performance of the KTP anode.
Author Contributions #
D.P.L and Z.W. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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).
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 nonaqueous electrolyte.12 Figure 2B 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 reversible K-ion storage over electrolyte decomposition on 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 2B), where such a phenomenon indicates 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, a 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.
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REFERENCES
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DOI: 10.1021/acsenergylett.8b00009 ACS Energy Lett. 2018, 3, 373−374
