Aqueous batteries beyond lithium employing concentrated halide-free electrolytes

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Aqueous batteries beyond lithium employing concentrated halide-free electrolytes
Jin Han1,2, Alessandro Mariani1,2, Maider Zarrabeitia1,2, Alberto Varzi1,2*, Stefano Passerini1,2*
1 Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, D-89081 Ulm, Germany
2 Karlsruhe Institute of Technology (KIT), P.O. Box 3640, D-76021 Karlsruhe, Germany

1 Background
Despite the great success achieved by non-aqueous lithium-ion batteries (LIBs) owing to the high energy density and long cycling life, their cost and safety issues caused by flammable and volatile organic electrolytes hinder their wide application in large-scale stationary energy storage area, [1] in which cost and safety are the determining factors instead of energy density.[2] In this context, rechargeable aqueous batteries, which use non-flammable aqueous electrolytes become an appealing alternative in view of their low cost and limited environmental impact.[3] The first aqueous LIBs based on LiMn2O4//VO2 was proposed by Dahn’s group in 1994.[4] The limited electrochemical window of aqueous electrolytes was extended in 2015 by using a “water-in-salt” (WiSE) electrolyte consisting of 21 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as designed by Wang’s group, demonstrating a LiMn2O4//Mo6S8 full aqueous LIBs with a high output potential of 2.3 V.[5] Afterwards, multiple fluorine-containing salts, such as sodium trifluoromethane sulfonate (NaOTF) and potassium trifluoromethane sulfonate (KCF3SO3),[6] have also been reported to enable high-voltage rechargeable aqueous sodium-ion batteries (ASIBs) and aqueous potassium-ion batteries (APIBs), respectively. However, in spite of the remarkable achievements, these costly and ungreen WiSE based on fluorinated salts spoil the two major advantages of aqueous batteries, which are the low cost and reduced environmental impact.

2 Research contents
2.1 Acetate-based WiSE for Stable Aqueous Sodium-ion Batteries
A WiSE containing 32 m KAc and 8 m NaAc (32K8N) is investigated as double salt system. Interactions between cations, anions and water molecules are studied by MD simulations, which reveals substantially reduced water activity and unique solvation structure. Full aqueous sodium-ion batteries employing NASICON-type Na2VTi(PO4)3/C (NVTP/C) as active material at both the positive and the negative electrode are also demonstrated. Consequently, the full cell using 32K8N electrolyte could achieve a capacity retention of 73% for over 500 cycles at the current density of 60 mA/g.
2.2 Gelified Acetate-based WiSE (20 m KAc+CMC) for Aqueous Potassium-ion Batteries
A gel electrolyte is prepared by adding CMC to the 20 m KAc liquid electrolyte to stabilize the potassium manganese hexacyanoferrate (KMHCF) cathode by tuning the pH value of the electrolyte and reducing Fe and Mn dissolution from cathode. MD simulations suggest that the water in this gel can be further seized by carboxymethyl cellulose (CMC) besides of acetate anions and K cations. K cations even move faster with “jump” mechanism in the gel comparing with the liquid one. A s consequence, KMHCF//AC using the gel electrolyte cells deliver improved cycling performance comparing with the liquid electrolyte.
2.3 Acetate-based WiSE for Stable Zn Anode
A WiSE, consisting of 30m KAc, 3m LiAc and 3m ZnAc2, is developed to improve the reversibility of Zn anode during the Zn plating/stripping. This WiSE enables a relatively high average CE (99.6%) and prolonged stability of Zn plating/stripping. The surface of zinc electrodes cycled in this WiSE appears smooth and stable even after prolonged cycling as demonstrated by SEM and XRD.

3 Conclusion
The results of this poster demonstrate that acetate-based WiSEs are promising electrolytes for aqueous batteries, not only in terms of electrochemical performance but also when taking account cost and environmental aspects.

References
[1] K. Liu, et al., Science advances 2018, 4, eaas9820.
[2] O. Schmidt, et al., Nature Energy 2017, 2, 1-8.
[3] D. Chao, et al., Science Advances 2020, 6, eaba4098.
[4] W. Li, et al., Science 1994, 264, 1115-1118.
[5] L. Suo, et al., Science 2015, 350, 938-943.
[6] a) L. Suo, et al., Advanced Energy Materials 2017, 7, 1701189; b) L. Jiang, et al., Nature Energy 2019.

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