Preparation of Plate-Form Manganese Oxide by Selective Lithium Extraction from Monoclinic Li₂MnO₃ under Hydrothermal Conditions

Abstract

Hydrothermal lithium extraction from well-crystallized Li 2 MnO 3 crystals was studied by scanning electron microscopy (SEM) observation, chemical, and X-ray diffraction analyses. Monoclinic Li 2 MnO 3 polyhedral crystals were prepared in an LiCl flux at 650 °C. The lithium extraction was carried out in a variety of H 2 SO 4 solutions with mole ratios of H + in the solution to Li + in the solid ([H + ] l /[Li + ] s ) of 0.1, 0.15, 0.25, 0.50, 0.75, and 1.0 under a hydrothermal condition at 140 °C. The lithium extraction from Li 2 MnO 3 proceeds consuming equimolar H + in the solution by the mechanism of a Li + /H + ion exchange reaction when [H + ] l /[Li + ] s e 0.15 and by the mechanism of a mixture of the Li + /H + ion exchange reaction and lithium dissolution when 0.25 e [H + ] l /[Li + ] s e 0.75. The lithium-extracted phase preserves the crystal structure of Li 2 MnO 3 with a partial contraction of the lattice when 0.25 e [H + ] l /[Li + ] s e 0.75. SEM observation showed that the lithium extraction brings about a cleavage of the Li 2 MnO 3 particle along the (001) plane to result in a unique form in which platelike particles are stacked, preserving the polyhedral outline. High-magnification SEM observation showed that the platelike particles consist of an aggregate of subplates with nanometer thickness in parallel arrangement. Slit-shaped mesopores were formed among the subplates. γ-MnO 2 was formed as a final product when [H + ] l /[Li + ] s ) 1. A structural model is proposed to explain the mechanism of lithium extraction as well as the cleavage of the particle.

Investigation of the possible incorporation of protons into oxide cathodes during chemical delithiation

“…Since the oxidation of Mn 4+ to Mn 5+ in an octahedral oxygen environment is not common or accessible, one would expect Li 2 MnO 3 to be electrochemically inactive. However, this compound has been reported to show some electrochemical activity [18,19] due to the ion-exchange of Li + by H + as well as by the loss of oxygen from the lattice, similar to that observed on the acid leached Li 2 MnO 3 samples [20][21][22][23][24].…”

“…The authors reported that both the ionexchange of Li + by H + as well as loss of oxygen from the lattice takes place on deep charging. Also, chemical delithiation studies on Li 2 MnO 3 with aqueous acids have indicated that the delithiation proceeds both by an ion-exchange of Li + by H + as well as by an oxidation of oxide ions, involving an effective removal of Li 2 O from the lattice [20][21][22][23][24].…”

Controlled synthesis of Mn3O4 nanocrystallites and MnOOH nanorods by a solvothermal method

“…Due to the considerable importance in many applications, such as batteries, electrochromic and magnetic materials, much attention has been paid to manganese oxides and oxyhydroxides [1][2][3][4][5][6].…”

Effects of deposition variables on spray-deposited MnO2 thin films prepared from Mn(C2H3O2)2·4H2O

“…Although there have been a number of investigation on the electrical, optical and electrochromic properties of the films [14][15][16], no systematic study appears to have been done on electrical and optical properties at varying deposition conditions. Moreover, there is a considerable lack of understanding [7,9,17], concerning the surface properties of the oxides in different applications oriented measuring techniques. Different microstructures caused by different deposition conditions could be the probable reasons for the lacks in understanding.…”