title: Indigo carmine: an organic crystal as a positive-electrode material for rechargeable sodium batteries.
abstract: Using sodium, instead of lithium, in rechargeable batteries is a way to circumvent the lithium's resource problem. The challenge is to find an electrode material that can reversibly undergo redox reactions in a sodium-electrolyte at the desired electrochemical potential. We proved that indigo carmine (IC, 5,5'-indigodisulfonic acid sodium salt) can work as a positive-electrode material in not only a lithium-, but also a sodium-electrolyte. The discharge capacity of the IC-electrode was ~100 mAh g(-1) with a good cycle stability in either the Na or Li electrolyte, in which the average voltage was 1.8 V vs. Na(+)/Na and 2.2 V vs. Li(+)/Li, respectively. Two Na ions per IC are stored in the electrode during the discharge, testifying to the two-electron redox reaction. An X-ray diffraction analysis revealed a layer structure for the IC powder and the DFT calculation suggested the formation of a band-like structure in the crystal.
authors.affiliation: Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
authors.forename: Masaru
authors.initials: M
authors.lastname: Yao
authors.affiliation: Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
authors.forename: Kentaro
authors.initials: K
authors.lastname: Kuratani
authors.affiliation: Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
authors.forename: Toshikatsu
authors.initials: T
authors.lastname: Kojima
authors.affiliation: Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
authors.forename: Nobuhiko
authors.initials: N
authors.lastname: Takeichi
authors.affiliation: Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Senoh
authors.affiliation: Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
authors.forename: Tetsu
authors.initials: T
authors.lastname: Kiyobayashi