Facile Synthesis of Metastable Mg-Mn Spinel Nanoparticles for Magnesium-Ion Batteries
Authors: Kobayashi Hiroaki , Samukawa Kouta , Honma Itaru 
Published: December 2020

High energy and high density rechargeable batteries are indispensable for widespread portable electronic devices and the spreading of electric vehicles. Rechargeable Mg-ion batteries have gained much attention as promising alternatives to Li-ion batteries due to the high natural abundance, high volumetric energy density, and no dendrite formation of the Mg metal anode. While Mg-ion batteries are nowadays progressing rapidly, their practical use is still hampered by problems related to both cathode materials and electrolytes. One of the crucial problems is the very low rate capability at the cathodes due to the slow diffusion of Mg2+ ions in solids, therefore Mg-ion batteries work only at low current densities or at high temperature. Among various oxide cathode candidates for Mg-ion batteries, spinel oxides have a high redox potential and a relatively high ion-diffusivity. Especially in the common electrochemical window of electrolytes, MgMn2O4 spinel can exhibit a high theoretical capacity of 540 mAh g–1 using both Mn3+/Mn4+ and Mn3+/Mn2+ redox reactions (λ-MnO2 formation and rock-salt Mg2Mn2O4 formation reactions, respectively). However, these reactions should exhibit large polarizations because of the less reversible tetragonal-cubic phase transitions; MgMn2O4 has a tetragonal spinel structure due to the Jahn–Teller effect of Mn3+ ions, while λ-MnO2 and Mg2Mn2O4 are cubic phase. A suppression of the lattice distortion of MgMn2O4 is necessary for improving its cathode performance. Although a cubic MgMn2O4 phase is known as a metastable phase, this phase is only obtained at high-temperature or high-pressure in bulk. In this study, we aimed at synthesizing metastable cubic MgMn2O4 spinel nanoparticles under ambient conditions and applying them as cathodes for Magnesium-ion batteries.

Mg-Mn spinel nanoparticles were synthesized by alcohol-reduction method[1] and hot-injection method. For the alcohol-reduction method, n-Bu4NMnO4 was reduced in anhydrous MgCl2 ethanol solution at room temperature to form fine particles. For the hot-injection method, MgCl2 and Mn(acac)3 were dissolved in anhydrous xylene solution with stearic acid and oleylamine, followed by water injection at 80ºC to form monodisperse nanoparticles. Charge/discharge tests between –1.0~1.0 V vs. Ag+/Ag was carried out using a three-electrode cell with an activated carbon anode and a 0.5 M Mg(ClO4)2/CH3CN electrolyte.

The Rietveld refinements of XRD patterns suggested that the obtained samples were assigned to cubic phase Mg1–x
Mn2O4, and a transformation to tetragonal phase was observed after annealing at 600ºC. Since the both methods utilize rapid nucleation reactions occurred at moderate temperature, once a meta-stable phase formed, the phase transition to more stable tetragonal phase hardly proceed. The cubic phase is obtained probably due to the rapid nucleation of cubic MgMn2O4 spinel nanoparticles at moderate temperature. According to TEM images, the primary particle size of both samples was below 10 nm, suggesting the rapid nucleation without crystal growth. However, the particles were aggregated to form large secondary particles. The electrochemical performances strongly depended on the morphology of secondary particles, therefore suppression of the aggregation is important for improving the performance.

Reference:

[1] H. Kobayashi, K. Yamaguchi, I. Honma, RSC Adv. 9 (2019) 36434–36439.