Abstract
The development of rechargeable Zinc-ion batteries (ZIBs) has been hindered by the lack of efficient cathode materials due to the strong binding of divalent zinc ions with the host lattice. Herein, we report a strategy that eliminates the participation of Zn2+ within the cathode chemistry. The approach involves the use of composite cathode materials that contain Zn halides (ZnCl2, ZnBr2, and ZnI2) and carbon (graphite or activated carbon), where the halide ions act both as charge carriers and redox centers while using a Zn2+-conducting water-in-salt gel electrolyte. The use of graphite in the composite electrode produced batterylike behavior, where the voltage plateau was related to the standard potential of the halogen species. When activated carbon was used in the composite, however, the cell acted as a hybrid Zn-ion capacitor due to the fast, reversible halide ion electrosorption/desorption in the carbon pores. The ZnX2-activated carbon composite delivers a capacity of over 400 mAh g–1 and cell energy density of 140 Wh kg–1 while retaining over 95% of its capacity after 500 cycles. The halogen reaction mechanism has been elucidated using combinations of electrochemical and in situ spectroscopic techniques.
Highlights
Aqueous rechargeable batteries are a promising class of batteries for grid-scale electrochemical energy storage owing to their low cost, ease of fabrication, high ionic conductivity, and high operational safety.[1−3] Research on aqueous batteries in recent years has been gaining momentum from application in low-voltage divalent zinc−ion batteries (ZIB) to highvoltage monovalent lithium-ion batteries (LIBs).[4−6] In particular, Zinc-ion batteries (ZIBs) have attracted substantial interest as one of the most promising next-generation technologies because: (i) they depend on an Earth-abundant metal, which is air-stable unlike Li; (ii) their low cost, safety, and environmental benignancy is attractive for grid-scale energy storage, and (iii) the volumetric energy density is approximately 3 times higher than that of Li.[2,7,8]
It is used as a source of the [TFSI] anion since it is believed that the reduction of [TFSI]− is responsible for the formation of the passivating solid electrolyte interface (SEI), which extends the overall electrochemical window.[5,7]
The most significant findings emerging from this study are that the identity of the Zn halide and carbon structure in the cathode composite produces electrochemical energy storage devices that are fundamentally different from each other
Summary
Aqueous rechargeable batteries are a promising class of batteries for grid-scale electrochemical energy storage owing to their low cost, ease of fabrication, high ionic conductivity, and high operational safety.[1−3] Research on aqueous batteries in recent years has been gaining momentum from application in low-voltage divalent zinc−ion batteries (ZIB) to highvoltage monovalent lithium-ion batteries (LIBs).[4−6] In particular, ZIBs have attracted substantial interest as one of the most promising next-generation technologies because: (i) they depend on an Earth-abundant metal, which is air-stable unlike Li; (ii) their low cost, safety, and environmental benignancy is attractive for grid-scale energy storage, and (iii) the volumetric energy density is approximately 3 times higher than that of Li.[2,7,8] Due to these favorable properties, zinc has been used as an anode material in a series of battery technologies both in conventional static cells (zinc− manganese dioxide batteries, zinc−air batteries, or Zn-graphite dual ion batteries) and in redox flow configurations (Zn− bromine or Zn−iron cells).[9,10]The development of ZIBs is, hindered by a number of factors relating to aqueous electrolytes, the formation of Zn dendrites at the anode, and lack of efficient cathode materials.[7]. Wang et al used water-in-salt electrolytes (WiSEs) to enhance the electrochemical window of water and obtained dendrite-free Zn plating/stripping with near 100% Coulombic efficiency.[7] WiSEs contain a high concentration of the desired salt so that the hydrated ions outnumber free water: as there is no free water to react at the electrode surface, the overall cell voltage can be increased. The less solvated fluorinated anions can be reduced to form a passivating solid electrolyte interface (SEI) on the electrode surface.[5] This SEI formation significantly suppresses the hydrogen evolution reaction and is largely responsible for the overall electrochemical stability window of WiSEs.[5] The highest voltage window (4.9 V) recorded at the hydrophobic graphite of WiSEs contains small metal cations (Li+) and large fluorinated anions such as bis(trifluoromethanesulfonyl)imide ([TFSI]−) and trifluoromethanesulfonate ([TFO]−).[6]
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