Growth of hollow-structured LiMn₂O₄crystals starting from Mn metal in molten KCl through the microscale Kirkendall effect

Abstract

Hollow-structured LiMn 2 O 4 crystals are grown in molten KCl (flux) for the first time via an oxidation reaction of metallic Mn followed by a lithiation reaction. Characterization of the formation process, which is achieved using thermogravimetric differential thermal analysis, X-ray diffraction, and scanning electron microscopy observation, reveals that the hollow structures are formed via a mechanism analogous to the microscale Kirkendall effect, which results from the difference between the solid-state diffusion rates of the core materials and the rate of O 2 diffusion through the shell at elevated temperatures occurring during oxidation. Interestingly, the development of well-defined crystal facets is observed on the surfaces of the LiMn 2 O 4 crystals, implying that the crystal growth is driven by supersaturation in the same manner as the flux growth. We also examine two possible approaches to reduce the volume of the inner space in the crystal: crystal growth under O 2 flow for enhancement of the O 2 diffusion and insertion of a Co and Ni layer between Mn and the substrate to induce the formation of the LiNi 1/3 Co 1/3 Mn 1/3 O 2 phase.

Thin and Dense Solid-solid Heterojunction Formation Promoted by Crystal Growth in Flux on a Substrate

“…S1). Very similar results were observed in our previous reports that describe the formation of hollow-structured LiCoO 2 , LiMn 2 O 4 , and LiNi 0.5 Co 0.2 Mn 0.3 O 2 from the conversion of electrodeposited Co, Mn, and stacked Ni/Co/Mn dots in molten salts at high temperatures, respectively 23,24 . Based on these results, the oxide formation on the Nb substrate could limit the reaction kinetics, since the amount of dissolved oxygen is normally small in the molten salts.…”

Sub-2 nm Thick Fluoroalkylsilane Self-Assembled Monolayer-Coated High Voltage Spinel Crystals as Promising Cathode Materials for Lithium Ion Batteries

“…The LiNi 0.5 Mn 1.5 O 4− δ crystals used were prepared by the flux growth method171819202122. As shown in Fig.…”

Magneto-Revealing and Acceleration of Hidden Kirkendall Effect in Galvanic Replacement Reaction

“…14,23 It has been reported that the KE can happen between Mn and Fe, Fe and O, and Mn and O, respectively, if there exists enough of a chemical potential gradient. 36,40,41 In our experiment, once the precursor Mn 3 O 4 starts to react with Fe 2+ , Mn 3 O 4 converts into Mn x Fe y O 4 , and there will be a gradient difference in element concentration between the inner and surface layers of nanoparticles. Internal Mn is higher than external Mn, while internal Fe is less than external Fe.…”

“…In this secondary reaction, the Mn atoms can diffuse out of the solid without the Mn/Fe exchange, which is the typical characterization of the KE. 36 Another possibility for the secondary reaction is the Ostwald ripening. 37−39 However, according to the crystallization condition of large precursor Mn 3 O 4 crystals (Figure S1) and the relative lower reaction temperature, it is reasonable to exclude the Ostwald ripening, and the KE is dominated for the secondary reaction.…”