No abstract

Rechargeable batteries based on an abundant metal such as aluminum with a three-electron transfer per atom are promising for large-scale electrochemical energy storage. Aluminum can be handled in air, thus offering superior safety, easy fabrication, and low cost. However, the development of Al-ion batteries has been challenging due to the difficulties in identifying suitable cathode materials. This study presents the use of a highly open framework Mo VO as a cathode for Al-ion batteries. The open-tunnel oxide allows a facile diffusion of the guest species and provides sufficient redox centers to help redistribute the charge within the local host lattice during the multivalent-ion insertion, thus leading to good rate capability with a specific capacity among the highest reported in the literature for Al-based batteries. This study also presents the use of Mo VO as a model host to develop a novel ultrafast technique for chemical insertion of Al ions into host structures. The microwave-assisted method employing diethylene glycol and aluminum diacetate (Al(OH)(C H O ) ) can be performed in air in as little as 30 min, which is far superior to the traditional chemical insertion techniques involving moisture-sensitive organometallic reagents. The Al-inserted Al Mo VO obtained by the microwave-assisted chemical insertion can be used in Al-based rechargeable batteries.

In search of a suitable cathode host for aqueous Zn-ion batteries (ZIBs), polyanionic materials have been explored due to their open-framework structure that is believed to improve Zn-ion diffusion. Among them, Na3V2(PO4)2F3 was recently shown in the aqueous ZIB to exhibit attractive electrochemical performance, and the charge storage mechanism was attributed to reversible Zn2+ insertion into the cathode. Here, however, we investigate the puzzling differences in the electrochemical behavior of Na3V2(PO4)2F3 as a cathode material between nonaqueous and aqueous ZIBs. Ex situ analyses of the cathode after cycling in both systems unveil that the observed disparity in the electrochemical behavior stems from the difference in the charge storage mechanism. In the nonaqueous ZIB, guest ions are determined to be both Zn2+ and Na+ initially that gradually shift to be pure Zn2+. In the aqueous ZIB, however, H+ is found to be the guest ion species rather than Zn2+. This explains the attractive electrochemical performance in the aqueous ZIB as H+ insertion and diffusion would be extremely facile unlike Zn2+. Moreover, even with Zn­(CF3SO3)2 electrolyte, the formation of zinc salt byproduct on the cathode after discharge further supports that H+ ions are inserted into the cathode, as observed with the aqueous ZnSO4 electrolyte. This byproduct formation on the cathode in the aqueous ZIB system calls for careful analyses to be performed to categorically elucidate that Zn ions are indeed the guest ions at the cathode when investigating cathodes for “Zn-ion” batteries.

Detailed structural analysis reveals that Zn-insertion into V2(PO4)3 induces heavy distortion due to strong host–guest interactions.

Electrochemical insertion of Zn ions into 2H-NbS2 has been investigated, for the first time, to delineate whether the limited insertion of Zn ions into 2H phases of MoS2 and WS2 is related to the geometrical features of the 2H phase. Electrochemical and ex situ structural and elemental characterization analyses reveal that significant and reversible Zn-ion insertion into 2H-NbS2 can be achieved by reducing the particle size to facilitate Zn-ion diffusion, unlike that in 2H-MoS2 or 2H-WS2. By qualitatively comparing the electronic structures of the 2H phases of NbS2, MoS2, and WS2, it is determined that Zn-ion insertion can occur with metallic phases such as 2H-NbS2 as electrochemical reduction by Zn is thermodynamically favored, while in the semiconducting phases such as 2H-MoS2 and 2H-WS2, Zn-ion insertion would not occur as electrochemical reduction by Zn is thermodynamically unfavored.