Sonochemical and soft-chemical intercalation of lithium ions into MnO2 polymorphs

Asynchronous Crystal Cell Expansion during Lithiation of K(+)-Stabilized α-MnO2

α-MnO2 is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li(+). The overall electrochemical performance of α-MnO2 is determined by the tunneled structure stability during its interaction with Li(+), the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal-orthorhombic-tetragonal symmetric transition during lithiation of K(+)-stabilized α-MnO2 is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 × 1 and 2 × 2 tunnels exist along c ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the ⟨100⟩ projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of α-MnO2's tetragonal unit cell along a and b lattice directions, which results in a tetragonal-orthorhombic-tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 × 2 tunnels are the preferred sites for Li(+) occupancy. The sequential Li(+) filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/α-MnO2 batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO2 as an intercalated electrode.

Ultrasound assisted synthesis of nano-sized lithium cobalt oxide

Nano-sized HT-LiCoO(2) powders were prepared by sonochemical synthesis in an aqueous solution of lithium hydroxide containing cobalt hydroxide at approximately 80 degrees C without any further heat treatment at high temperature. The effects of the LiOH concentration, oxidation conditions, ultrasound irradiation time and temperature on the formation of the nano-sized HT-LiCoO(2) phase were investigated. The formation of the HT-LiCoO(2) phase was confirmed by X-ray diffraction and Raman spectroscopy. The TEM images showed the presence of HT-LiCoO(2) aggregates with a mean particle diameter of approximately 20 nm. The reaction mechanism of the ultrasound assisted synthesis of nano-sized LiCoO(2) was proposed on the basis of the XRD, X-ray absorption spectroscopy analysis and TEM observation of the reaction products taken during the course of the synthesis.

Organized and highly dispersed growth of MnO2 nano-rods by sonochemical hydrolysis of Mn3acetate

Highly dispersed and non-agglomerated alpha-MnO(2) nano-needles of dimensions 20-30 nm have been synthesized by the application of ultrasound radiation on the aqueous solution consisting of manganese(3)acetate close to neutral pH followed by mild drying. With a similar reaction system, hot hydrolysis (non-sonochemical process) produced beta-MnO(2) nano-rods of length 100-200 nm but with high degree agglomeration. Sonochemical cavitation phenomenon is suggested to have a pronounced effect for the formation of special phase and morphology. The effect is proved by the difference in the intermediate products which has difference in crystalinity and phase-purity. The intermediate phases are identified to be single-phase gamma-MnOOH for the non-sonochemical reaction and mixture of gamma-MnOOH, alpha-MnO(2) and beta-MnO(2) for the sonochemical products.