Mila Gorova

Comparing the Behavior of Nano- and Microsized Particles of LiMn1.5Ni0.5O4 Spinel as Cathode Materials for Li-Ion Batteries

We report on a rigorous comparative study of nano- and microparticles of Limn 1.5 Ni 0.5 O 4 spinel as cathode materials for Li-ion batteries. The stability of these materials in LiPF 6 /alkyl carbonate solutions in temperatures up to 70°C was explored. Capacity, cycling, rate capabilities, and impedance behavior were also studied. The methods included X-ray diffraction, Raman, X-ray photelectron, Fourier transform infrared, and electron paramagnetic resonance spectroscopies, and electron microscopy, in conjunction with standard electrochemical techniques: voltammetry, chronopotentiometry, and impedance spectroscopy. These materials show an impressive stability in solutions at elevated temperature. The use of nanomaterials was advantageous for obtaining a better rate capability of LiMn 1.5 Ni 0.5 O 4 electrodes. LiMn 1.5 Ni 0.5 O 4 particles develop a unique surface chemistry in solutions that passivates and protects them from detrimental interactions with solution species at elevated temperatures.

EPR of Mn4+ in spinels Li1+xMn2−xO4 with 0≤x≤0.1

Abstract EPR of Mn4+ has been used to study electron–phonon interactions and cation distribution in Li1+xMn2−xO4 spinels with 0≤x≤0.1. The EPR spectra of Mn4+ in 16d spinel sites has been interpreted in terms of the bottleneck-relaxation mechanism. Two additional weak signals were registered that were attributed to Mn4+ ions located near oxygen vacancies and to Mn4+ ions in a lithium rich environment. These Mn4+ defects are sensitive towards the Li/Mn ratio and the cooling rate. It has been demonstrated that EPR monitoring of Mn4+ allows us to distinguish more precisely the Li1+xMn2−xO4 spinels with respect to the cation distribution.

Studies of nanosized LiNi0.5Mn0.5O2-layered compounds produced by self-combustion reaction as cathodes for lithium-ion batteries

We report here on the synthesis of layered nanosized LiNi 0.5 Mn 0.5 O 2 compound by a modified self-combustion reaction and the studies of its electrochemical behavior as a cathode material for Li cells. Capacities up to 180-200 mAh/g could be reached during cycling of these electrodes even at 60°C in LiPF 6 /alkyl-carbonate solutions. The nano-LiNi 0.5 Mn 0.5 O 2 material exhibits remarkable stability during prolonged aging and cycling at elevated temperatures. Surface films developed on the nano-LiNi 0.5 Mn 0.5 O 2 particles in the course of cycling/aging contain polycarbonates, LiF, nickel and manganese oxides, and fluorides, and lead to efficient passivation and stabilization of the electrodes.

EPR monitoring of Mn4+ distribution in Li4Mn5O12 spinels

Abstract EPR of Mn4+ is used to study the cation-distribution in octahedral sites of Li4Mn5O12 spinel lattice. Analysis of the EPR line width in terms of magnetic dipole–dipole and exchange interactions points to two kinds of Mn4+ in octahedral spinel sites: one of them has as first neighbours five paramagnetic Mn4+ and one diamagnetic Li+, whereas the second type of Mn4+ is in a Li-rich surrounding. On the basis of this, the existing model on the temperature dependence of the magnetic susceptibility of Li4Mn5O12 was refined.

Microstructure of Li1+xMn2–xO4 spinels obtained from metal-organic precursors

A metal-organic precursor method was adopted for the preparation of lithium-manganese spinel oxides. EPR spectra of Mn 2+ in solutions and in freeze-dried compositions, as well as IR and DSC of freeze-dried compositions, show that α-hydroxyorganic acids (such as lactic, malic and citric acids) chelate Mn 2+ via hydroxy and carboxylate groups. At 450°C, thermal decomposition of Li-Mn-organic acid precursors with Li/Mn=1.05/1.95 leads to the formation of non-stoichiometric Li[Li y Mn 2–y– Δ □ Δ ]O 4 spinels (0.03<y<0.05; 0.02<δ<0.05). EPR of Mn 4+ was used to throw light on the microstructure of the spinels obtained. For the annealed spinels at 750°C there is a transition from uniform to non-uniform distribution of the excess Mn 4+ ions and the corresponding metal vacancies. As a result, the bulk of the spinel becomes close to the stoichiometric composition Li[Li 0.05 Mn 1.95 ]O 4 , whereas the spinel surface accommodates the excess Mn 4+ ions. The formation of Mn 4+ -rich surface regions depends both on the cooling rate and on the organic acid used in the precursors. Lactate precursors are most appropriate for the preparation of lithium-manganese spinels with a homogeneous cation distribution.

EPR evidence on short-range Co/Mn order in LiCoMnO4 spinels

EPR of Mn4+ has been used to specify the electronic structure and cation distribution in LiCoMnO4 compositions that belong to high-potential electrode materials. To prepare single phase LiCoMnO4, two synthesis techniques have been adopted: solid state reactions and a lactate precursor method. EPR of Mn4+ shows that diamagnetic Co3+ (low-spin configuration, S = 0) and paramagnetic Mn4+ (S = 3/2) account for the electronic structure of LiCoMnO4. Analysis of the EPR line width in terms of dipole–dipole and exchange interactions allows one to estimate the mean number of paramagnetic neighbours of Mn4+. It has been demonstrated that the Co3+/Mn4+ distribution in 16d spinel sites is sensitive towards the cooling rates. For the high-temperature LiCoMnO4 phase obtained by air quenching from 750 °C, three paramagnetic Mn4+ and three diamagnetic Co3+ give rise to the local coordination of Mn4+, indicating a random Co/Mn distribution. For the low-temperature LiCoMnO4 phases obtained by air quenching from 600 °C and by slow-cooling from 750 to 25 °C, the mean paramagnetic number of Mn4+ decreases from 3 to 2, which is interpreted in the framework of short-range Co3+/Mn4+ ordering. The Co3+/Mn4+ octahedral ordering process is developed within a small-scale range (approximately up to a distance of the third metal shell) and does not affect the cubic spinel symmetry of LiCoMnO4.

Co/Mn distribution and electrochemical intercalation of Li into Li[Mn2−yCoy]O4 spinels, 0<y≤1

Abstract Single-phase oxides with Li[Mn 2− y Co y ]O 4 (0≤ y ≤1) stoichiometry and cubic spinel structure have been prepared by using the lactate precursor method. The oxides were alternatively quenched or slow cooled from 750°C. X-ray diffraction and EPR spectroscopy show that Co and Mn are disordered over 16d spinel sites for samples obtained by air quenching. Local environment of Mn 4+ was assessed by comparison of the observed EPR spectra for Li[Mn 2− y Co y ]O 4 compositions with the EPR signal from Mn 4+ in LiMnCoO 4 and the EPR spectrum of LiMn 2 O 4 . Thus, a local Co/Mn ordering is found for spinels obtained by slow-cooling. The use of these materials as the cathodic active compound in lithium cells reveals a higher capacity but lower capacity retention in the 3–4 V regions for metal-quenched samples as compared with slow-cooled products.

Microstructure of Li1+xMn2-xO4 Cathode Materials Monitored by EPR of Mn4+

Lithium-ion batteries based on lithium manganese spinel electrodes, Li1+xMn2-xO4, are the state-of-the-art power sources because they appear to be safer, are cheaper to manufacture and less toxic and polluting [1,2]. Extensive research has been devoted to the synthesis, the composition and the electrochemical properties of Li1+xMn2-xO4 spinels, but little work has been done on their microstructure. The stoichiometric LiMnO4 is a normal spinel where Li and Mn occupy tetrahedral and octahedral spinel sites, respectively. The valence distribution corresponds to Li[Mn3+Mn4+]O4. With an increase of Li/Mn ratio, the tetrahedral sites remain unchanged, while the excess of lithium and the manganese reside in octahedral sites. The compensation of the lithium charge is achieved by appearance of Mn4+.