title: Critical Role of pH Evolution of Electrolyte in the Reaction Mechanism for Rechargeable Zinc Batteries.
abstract: The reaction mechanism of α-MnO2having 2×2 tunnel structure with zinc ions in a zinc rechargeable battery, employing an aqueous zinc sulfate electrolyte, was investigated by in situ monitoring structural changes and water chemistry alterations during the reaction. Contrary to the conventional belief that zinc ions intercalate into the tunnels of α-MnO2, we reveal that they actually precipitate in the form of layered zinc hydroxide sulfate (Zn4(OH)6(SO4)⋅5 H2O) on the α-MnO2surface. This precipitation occurs because unstable trivalent manganese disproportionates and is dissolved in the electrolyte during the discharge process, resulting in a gradual increase in the pH value of the electrolyte. This causes zinc hydroxide sulfate to crystallize from the electrolyte on the electrode surface. During the charge process, the pH value of the electrolyte decreases due to recombination of manganese on the cathode, leading to dissolution of zinc hydroxide sulfate back into the electrolyte. An analogous phenomenon is also observed in todorokite, a manganese dioxide polymorph with 3×3 tunnel structure that is an indication for the critical role of pH changes of the electrolyte in the reaction mechanism of this battery system.
authors.affiliation: Centre for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792, Korea.|Department of Chemical and Biomolecular Engineering, Yonsei University, Yonseiro 50, Seodaemoon-gu, Seoul, 03722, Korea.
authors.forename: Boeun
authors.initials: B
authors.lastname: Lee
authors.affiliation: Centre for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792, Korea.
authors.forename: Hyo Ree
authors.initials: HR
authors.lastname: Seo
authors.affiliation: Centre for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792, Korea.
authors.forename: Hae Ri
authors.initials: HR
authors.lastname: Lee
authors.affiliation: Department of Materials Science and Engineering, Hanyang University, 222 Wangsimni, Seongdong-gu, Seoul, 04763, Korea.
authors.forename: Chong Seung
authors.initials: CS
authors.lastname: Yoon
authors.affiliation: Department of Chemical and Biomolecular Engineering, Yonsei University, Yonseiro 50, Seodaemoon-gu, Seoul, 03722, Korea.
authors.forename: Jong Hak
authors.initials: JH
authors.lastname: Kim
authors.affiliation: Centre for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792, Korea.
authors.forename: Kyung Yoon
authors.initials: KY
authors.lastname: Chung
authors.affiliation: Centre for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792, Korea.
authors.forename: Byung Won
authors.initials: BW
authors.lastname: Cho
authors.affiliation: Centre for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14 gil 5, Seongbuk-gu, Seoul, 02792, Korea. sho74@kist.re.kr.
authors.forename: Si Hyoung
authors.identifier: http://orcid.org/0000-0002-7063-9235
authors.initials: SH
authors.lastname: Oh
mesh_terms: D018360:Crystallography, X-Ray; D011211:Electric Power Supplies; D004566:Electrodes; D004573:Electrolytes / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D017895:Manganese Compounds / Q000737:chemistry; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D010087:Oxides / Q000737:chemistry; D013052:Spectrometry, X-Ray Emission; D014961:X-Ray Diffraction; D015032:Zinc / Q000737:chemistry*
chemical_list: D004573:Electrolytes; D017895:Manganese Compounds; D010087:Oxides; D015032:Zinc; C016552:manganese dioxide
keywords: batteries; conversion; intercalation; manganese dioxide; zinc