Mirjana Bijelić

Long cycle life of CoMn2O4 lithium ion battery anodes with high crystallinity

CoMn2O4 nanomaterials are prepared by a low temperature precipitation route employing metal acetates and NaOH. Structural changes, induced by different annealing temperatures, are comprehensively analyzed by X-ray powder diffraction and Raman spectroscopy. With rising annealing temperature the crystal lattice of CoMn2O4 undergoes changes ; AO4 tetrahedra expand due to thermally induced substitution of Co2+ by larger Mn2+ metal ions on the A-site of the spinel structure, while in contrast, BO6 octahedra shrink since the B-site becomes partially occupied by smaller Co3+ metal ions on account of the migrated Mn ions. CoMn2O4 particle sizes are easily fine-tuned by applying different annealing temperatures ; the particle size increases with increasing annealing temperature. During the battery operation, pulverization and reduction of particle sizes occurs regardless of the initial size of the particles, but the degree of division of the particles during the operation is dependent on the initial particle properties. Thus, contrary to the common assumption that nanostructuring of the anode material improves the battery performance, samples with the largest particle sizes exhibit excellent performance with a capacity retention of 104% after 1000 cycles (compared to the 2nd cycle).

Graphene-oxide-wrapped ZnMn2O4 as a high performance lithium-ion battery anode

Cation distribution between tetrahedral and octahedral sites within the ZnMn2O4 spinel lattice, along with microstructural features, is affected greatly by the temperature of heat treatment. Inversion parameter can easily be tuned, from 5 to 19%, depending on the annealing temperature. The upper limit of inversion is found for T= 400 °C as confirmed by X-ray powder diffraction and Raman spectroscopy. Excellent battery behavior is found for samples annealed at lower temperatures; after 500 cycles the specific capacities for as-prepared ZnMn2O4 is 909 mAh/g, while ZnMn2O4 heat-treated at 300 °C shows 1179 mAh/g which amounts to 101 % of its initial capacity. Despite excellent performance of sample processed at 300 °C at lower charge/discharge rates (100 mAh/g), a drop in the specific capacity is observed with rate increase. This issue is solved by graphene oxide wrapping; specific capacity obtained after 400th cycle for graphene oxide wrapped ZnMn2O4 heat-treated at 300 °C is 799 mAh/g at charge/discharge rate 0.5 A/g, which is higher by factor 6 compared to sample without graphene oxide wrapping.

Facile route for preparation of nanochristalline ZnMn2O4 ; effect of preparation conditions on structure and microstructure

Traditional synthesis of spinel materials such as solid-state route involving grinding and firing of a mixture of oxides, nitrates or carbonates which requires elevated temperatures and prolonged process times have been abandoned. Indeed, majority of papers on manganese spinels are focused on low cost preparation methods which proceed at moderate temperatures (600°C) with enhanced reaction kinetics [1, 2]. Interestingly, Liu et al. reported a room temperature route for preparation of AMn2O4 (A=Zn, Co, Cd) from metal acetates [3]. Although synthetic route proposed by Liu et al. represents a facile and very efficient route for low temperature synthesis of spinel materials, in order to utilize this route for the targeted design of nanomaterials it is necessary to establish, very precisely, correlations between specific preparation conditions(concentration of NaOH, aging period, and temperature of additional heat treatments), structure, microstructure and properties. Detailed structural investigation using X-ray powder diffraction (XRPD) and Raman spectrocopy have been carried out in order to correlate specific structural and microstructural features with changes in preparation conditions. ZnMn2O4 was prepared by precipitation with NaOH (c=0.25-0.8 M) from solution of Zn and Mn acetates. Also, sample obtained by 0.8 M NaOH was additionaly heat treated at T=300, 400 and 500 °C. Pronounced difference in crystallinity was observed with increase in concentration of NaOH. Based on the results of Raman spectroscopy a model described by the formula: [Zn2+1-xMn2+ x]tetra[Zn2+xMn3+2-2xMn4+x]octaO4 has been proposed and tested upon structural data. It was shown, based on Rietveld structure refinement, that unit-cell constants as well as the inversion parameter of spinel lattice increase with the increase in temperature of thermal treatment. 1. L. Hu et al., Sci. Rep., 2, 217-226, 2012. Y. 2. Li et al., Nanoscale, 5, 2045-2054, 2013. 3. Y. Liu et al. RSC Advances, 4, 4727-4731, 2014.

Crystallographic phase and orientation analysis of GaAs nanowires by ESEM, EDS, TEM, HRTEM and SAED

The novel properties of semiconductor nanowires are interesting [1] for application and could be useful for application in nanoelectronics and photonics. Depending on the band gap and size of particular semiconductor nanowires, the shift in absorption/emission is observed.