A thin layer electrode of birnessite-type manganese oxide was prepared by brushing a mixed solution of KOCOCH3 and Mn(OCOCH3)2 on a platinum substrate, followed by heating at 1073 K. The chemical composition of the electrode was K x MnO y (x = 0.33 and y ∼ 2) with an interlayer spacing of c 0 = 0.697 nm. The positive-potential going sweep on the electrode in an aqueous phase caused the deintercalation of K+ with an increase in c 0. The quasi-reversible intercalation of K+ occurred with a subsequent negative-potential going sweep in a 0.2 mol/dm3 KCl solution. The electrochemical measurements suggested that K+ is not electrochemically active in the deintercalation/intercalation reaction but H+ is. The reaction proceeds based on a mechanism consisting of an electrochemical reaction (the redox reaction between Mn3+ and Mn4+) and an ion-exchange reaction between K+ and H+. The intercalation experiments in various alkali-metal chloride solutions showed the intercalation capacity to be in the order of Na ∼ K > Li > Rb > Cs.

Solid electrolytes potentially provide safety, Li dendrites blocking, and electrochemical stability in Li-metal batteries. Large efforts have been devoted to disperse ceramic nanoparticles in a poly­(ethylene oxide) (PEO) matrix to improve the ions transport. However, it is challengeable to create efficient framework for ions transport with nanoparticles. Here we report for the first time garnet nanosheets to provide interconnected Li-ions transport pathway in a PEO matrix. The garnet nanosheet fillers would not only facilitate ions transport but also enhance ionic conductivity in comparison with their nanoparticle counterparts. A composite solid polymer electrolyte containing 15 wt % garnet nanosheets exhibits a practically useful conductivity of 3.6 × 10–4 S cm–1 at room temperature. Besides, the composite electrolyte can robustly isolate Li dendrites in a symmetric lithium metal-composite electrolyte battery during reversible Li dissolution/deposition at a relatively low temperature of 40 °C. The symmetric cell with composite electrolyte shows flat voltage and low interfacial resistance over a galvanostatic cycling of 200 h at a current density of 0.1 mA cm–2. A solid-state Li/LiFePO4 battery with the composite polymer electrolyte exhibits a capacity of 98.1 mAh g–1 and a capacity retention of 97.5% after 30 cycles at a temperature of 40 °C. This finding provides a strategy to explore superionic conductors.

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.

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Composites consisting of b-MnO 2 nanocrystals and acetylene black were synthesized by thermal decomposition of Mn(NO 3 ) 2 with acetylene black. The effects of heating temperature and specific surface area (S BET ) of the starting acetylene black on the formation and properties of the composite were investigated. The decomposition of Mn(NO 3 ) 2 mixed with acetylene black progressed at a lower temperature than that of Mn(NO 3 ) 2 alone. The formation and the size of b-MnO 2 nanocrystals depended strongly on the heating temperature (T) and S BET of the starting acetylene black. The b-MnO 2 /acetylene black composite could be produced for acetylene black of S BET ~60 m 2 g 21 at 160 uC ¡ T ¡ 320 uC and S BET ~133 m 2 g 21 at 160 uC ¡ T ¡ 300 uC. However, the composite could not be obtained for acetylene black with larger specific surface area (S BET ~300 m 2 g 21 ). The size of b-MnO 2 nanocrystals decreases and their dispersion in the composites increases with an increase in the S BET or the heating temperature. The lithium insertion behavior and voltage feature in the first discharge process depend strongly on the size of the b-MnO 2 nanocrystals as well as their dispersion. The electrochemical lithium insertion progressed topotactically for well-dispersed b-MnO 2 nanocrystals, retaining the framework of their rutile structure to permit a large amount of lithium insertion (Li/Mn ~1.15 in the solid). The charge/ discharge curves showed a flat voltage plateau around 2.8 V and a stable cycling feature up to the 20th cycle. It is likely that the easy lattice expansion of b-MnO 2 nanocrystals along the a crystal axis plays an important role in the topotactic lithium insertion/extraction reactions.

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