title: Sodium-Ion Intercalation Mechanism in MXene Nanosheets.
abstract: MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state (23)Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na(+) electrolyte exhibits reversible Na(+) intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na(+) is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na(+) and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na(+) intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability.
authors.affiliation: Department of Chemical System Engineering, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Kajiyama
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science , 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Lucie
authors.initials: L
authors.lastname: Szabova
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science , 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.|Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University , Nishikyo-ku, Kyoto 615-8510, Japan.
authors.forename: Keitaro
authors.initials: K
authors.lastname: Sodeyama
authors.affiliation: Department of Chemical System Engineering, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Iinuma
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University , 3-1-1 Tsushima-naka, Okayama 700-8530, Japan.
authors.forename: Ryohei
authors.initials: R
authors.lastname: Morita
authors.affiliation: Graduate School of Natural Science and Technology, Okayama University , 3-1-1 Tsushima-naka, Okayama 700-8530, Japan.|Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University , Nishikyo-ku, Kyoto 615-8510, Japan.
authors.forename: Kazuma
authors.initials: K
authors.lastname: Gotoh
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science , 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.|Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University , Nishikyo-ku, Kyoto 615-8510, Japan.
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Tateyama
authors.affiliation: Department of Chemical System Engineering, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.|Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University , Nishikyo-ku, Kyoto 615-8510, Japan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Okubo
authors.affiliation: Department of Chemical System Engineering, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.|Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University , Nishikyo-ku, Kyoto 615-8510, Japan.
authors.forename: Atsuo
authors.initials: A
authors.lastname: Yamada
keywords: 23Na NMR; MXene; intercalation; negative electrode; sodium-ion battery