title: In operando X-ray absorption fine structure studies of polyoxometalate molecular cluster batteries: polyoxometalates as electron sponges.
abstract: We carried out in operando Mo K-edge X-ray absorption fine structure measurements on the rechargeable molecular cluster batteries (MCBs) of polyoxometalates (POMs), in which a Keggin-type POM, [PMo(12)O(40)](3-), is utilized as a cathode active material with a lithium metal anode. The POM-MCBs exhibit a large capacity of ca. 270 (A h)/kg in a voltage range between V = 4.0 V and V = 1.5 V. X-ray absorption near-edge structure analyses demonstrate that all 12 Mo(6+) ions in [PMo(12)O(40)](3-) are reduced to Mo(4+) in the discharging process. This means the formation of a super-reduced state of the POM, namely, [PMo(12)O(40)](27-), which stores 24 electrons, and this electron number can explain the large capacity of the POM-MCBs. Furthermore, extended X-ray absorption fine structure analyses reveal the molecular structure of [PMo(12)O(40)](27-), which is slightly reduced in size compared to the original [PMo(12)O(40)](3-) and involves Mo(4+) metal-metal-bonded triangles. Density functional theory calculations suggest that these triangles are formed because of the large number of additional electrons in the super-reduced state.
authors.affiliation: Department of Chemistry and Research Center for Materials Science (RCMS), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
authors.forename: Heng
authors.initials: H
authors.lastname: Wang
authors.forename: Shun
authors.initials: S
authors.lastname: Hamanaka
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Nishimoto
authors.forename: Stephan
authors.initials: S
authors.lastname: Irle
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Yokoyama
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Yoshikawa
authors.forename: Kunio
authors.initials: K
authors.lastname: Awaga