R. Stoyanova

Stabilization of the layered crystal structure of LiNiO2 by Co-substitution

Abstract XRD analysis, IR spectroscopy, magnetic susceptibility measurements and thermal analysis are used to investigate the effect of Co substitution on the Ni 3+ content and long-range order in Li x Ni 2− x O 2 (0.7 x x (Co 1− y Ni y ) 2− x O 2 (0.8≤ y ≤1) and are associated with the competition between the solid state reaction and the volatility of the unreacted lithium salt: Co-dopants in NiO enhance the reactivity of the oxide towards lithium salts (LiNO 3 ), as a result of which the lithium amount (the M 3+ content, correspondingly) in the reaction product Li x (Co 1− y Ni y ) 2− x O 2 increases from x ≈0.65 to x ≈0.92. It has been proved that the nonstoichiometry of Li x (Co 1− y Ni y ) 2− x O 2 with respect to lithium is associated with the stabilization of Ni 2+ ions but not of Co 2+ ions. Further enhancement of the cobalt amount (0≤ y ≤0.8) results in the formation of “ideal” Li x (Co 1− y Ni y ) 2− x O 2 solid solutions with nearly stoichiometric composition (0.96 x ≤1). Moreover, Co-substitution in lithium-nickel oxide over the whole concentration range (0≤ y ≤1) stabilizes the layered crystal structure, which is manifested by the increase of the trigonal distortion of the cubic lattice. The shift of the IR bands with the contraction of the unit cell during cobalt substitution confirms the formation of mixed nickel-cobalt layers.

Effect of Mg doping and MgO-surface modification on the cycling stability of LiCoO2 electrodes

Two synthetic routes including Mg doping and MgO-surface modification were applied to the preparation of LiCoO2 showing enhanced reversible cycling behaviour as cathode material in lithium ion batteries. Mg-doped LiCoO2 was obtained by the citrate precursor method in the temperature range 750–900°C. The surface of LiCoO2 was modified by coating with Mg(CH3COO)2 and subsequent heating at 600°C. XRD, chemical oxidative analysis and electron paramagnetic resonance (EPR) of Ni3+ spin probes were used to characterize the Mg distribution in LiCoO2. Substitution of Co by Mg in the CoO2-layers was found to have a positive effect on the cycling stability, while Mg dopants in LiO2-layers did not influence the capacity fade. The accumulation of MgO on the surface of LiCoO2 improves the cycling stability without loss of initial capacity.

Effect of Mn-substitution for Co on the crystal structure and acid delithiation of LiMnyCo1−yO2 solid solutions

Lithium-manganese-cobalt oxides, LiMnyCo1−yO2, where Li/(Mn+Co)≈1and 0<y<1, have been obtained by a solid state reaction between lithium hydroxide and manganese-cobalt spinels in air (0<y<0.2) or under nitrogen (0.2< y<1). The crystal structure of the ternary oxides depends on the Mn/(Mn+Co) ratio: up to y<0.2, oxides with a trigonal structure (s.g. R3m) are formed; at 0.2<y<0.7, oxides with a rock-salt type structure (s.g. Fm3m) appear; finally, at y<0.7, the oxides obtained have a tetragonal spinel structure (I41/amd). Acid delithiation of the lithium-manganese-cobalt oxides proceeds differently depending on their composition and crystal structure. With cobalt-rich oxides, the acid extraction of lithium proceeds within the initial trigonal structure, while with rock-salt and tetragonal oxides acid delithiation yields Lix(MnyCo1−y)O2 oxides with a cubic spinel structure (s.g. Fd3m). Among LiMnyCo1−yO2 oxides, lithium is removed more easily and to a higher extent from the tetragonal oxides. Due to the high oxidation degree of the manganese and cobalt ions in delithiated samples, the electrochemical cell consisting of Li/Lix(MnyCo1−y)O2 displays a high discharge voltage (4.1–4.2 V), which allows its direct discharge. The highest reversibility and capacity per formula unit (about 0.75 Li) are observed on manganese-rich spinel oxides with y<0.7.

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.

Characterisation of mesocarbon microbeads (MCMB) as active electrode material in lithium and sodium cells

Mesocarbon microbeads (MCMB) derived from petroleum residua and heat treated under different experimental conditions were characterised by X-ray and electron diffraction, proton magnetic resonance (PMR), Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR). The presence of two different forms of hydrogen is retained after heating to 750°C under vacuum. Graphitisation to 3000°C leads to graphite ribbon-like particles surrounding microbeads of a few microns in size. The crystalline graphite monodomains are with a small band gap or are semimetallic as observed by EPR. Heat treatment even at 3000°C does not eliminate completely the localised paramagnetic defects in the microbeads. These properties condition the aptitude of these materials toward their use in lithium and sodium electrochemical cells. The samples prepared at 750°C have a reversible intercalation behaviour, while samples prepared at 3000°C evidence solvent decomposition resulting in a non-reversible extended discharge plateau when using sodium perchlorate electrolyte dissolved in pure propylene carbonate (PC).

Recent advances in the study of layered lithium transition metal oxides and their application as intercalation electrodes

Abstract A review is presented on the extensive work carried out during the last 30 years on layered oxides structurally related to LiCoO2 and LiNiO2. The studies considered here range from the structural and chemical characterization of the layered solids to the detailed evaluation of their aptitude towards lithium deintercalation-intercalation reactions, which form the basis of their successful application in rechargeable battery technology. The different challenges remaining in this area, such as the development of advanced preparation procedures and the optimization of the electrochemical performance by controlled changes in composition, structure, and particle morphology, are discussed.

Lithium Storage Mechanisms and Effect of Partial Cobalt Substitution in Manganese Carbonate Electrodes

A promising group of inorganic salts recently emerged for the negative electrode of advanced lithium-ion batteries. Manganese carbonate combines low weight and significant lithium storage properties. Electron paramagnetic resonance (EPR) and magnetic measurements are used to study the environment of manganese ions during cycling in lithium test cells. To observe reversible lithium storage into manganese carbonate, preparation by a reverse micelles method is used. The resulting nanostructuration favors a capacitive lithium storage mechanism in manganese carbonate with good rate performance. Partial substitution of cobalt by manganese improves cycling efficiency at high rates.

Changes in Structure and Cathode Performance with Composition and Preparation Temperature of Lithium Cobalt Nickel Oxide

The electrochemical behavior of Li 1-x (Ni y Co 1-y ) 1+x O 2 (y = 0.88 and 0.13) solid solutions obtained in the 450-800 °C temperature range was studied by step potential electrochemical spectroscopy. The intensity vs voltage curves for cobalt-rich compositions displayed two well-defined signals during cell charge, which were located at ca. 3.6 and 3.9 V The low-voltage signal was due to nickel oxidation. For samples prepared below 750 °C, the first step of the deintercalation from the pseudospinel phase contributed to this effect, whereas the second was responsible for the 3.9 V peak. For samples prepared above 650 C, the single-phase extraction preserved the trigonal lattice up to 4.2 V with a marked increase in the c/a ratio. For Ni-rich compositions, cell performance was poor for samples prepared below 750 °C. However, a sample prepared at 750 °C showed reversible extraction by a single-phase mechanism and a simple voltammogram, which differed markedly from LiNiO 2 electrodes. Ex situ electron paramagnetic resonance studies of Ni 3+ ions in the electrodes after the first cycle revealed a better recovery of the oxidation state of metal ions for the nickel-rich composition at 750 °C.

On the Performance of LiNi1 / 3Mn1 / 3Co1 / 3O2 Nanoparticles as a Cathode Material for Lithium-Ion Batteries

We report on the behavior of nanometric LiMn 1/3 Ni 1/3 CO 1/3 O 2 (LiMNC) as a cathode material for Li-ion batteries in comparison with the same material with submicrometric particles. The LiMNC material was produced by a self-combustion reaction, and the particle size was controlled by the temperature and duration of the follow-up calcination step. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, Raman spectroscopy, electron paramagnetic resonance, inductively coupled plasma, and atomic force microscopy were used in conjunction with standard electrochemical techniques (cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy) for characterizing the electrode materials. The effect of cycling and aging at 60°C was also explored. Nanomaterials are much more reactive in standard electrolyte solutions than LiMNC with a submicrometric particle. They develop surface films that impede their electrochemical response, while their bulk structure remains stable during aging and cycling at elevated temperatures. The use of nanomaterials in Li-ion batteries is discussed.

Structure and Electrochemical Properties of Li1 − x ( Ni y Co1 − y ) 1 + x O 2 Effect of Chemical Delithiation at 0°C

Acid treatment at 0°C of Li 1-x (Ni y Co 1-y ) 1+x O 2 solid solutions yields delithiated well crystalline oxides Li 0.5 (Ni y Co 1-y ) 1+x O 2 which do not contain protons. For the nearly stoichiometric nickel-rich oxides, removal of more than 20% of the lithium leads to a monoclinic distortion of the rhombohedral unit cell. Electron paramagnetic resonance (EPR) of low-spin Ni 3+ ions shows that for up to 0.2 extracted lithium, the generated Ni 4+ ions are statistically distributed in the NiO 2 layers while above 0.2 extracted lithium the Ni 4+ ions trend to order. During acid delithiation the oxidation of Ni 3+ to Ni 4+ ions proceeds prior to the oxidation of Co 3+ to Co 4+ as for a nonaqueous electrolyte. Discharge-charge cycles of lithium cells using acid-delithiated oxides as cathode materials show a significant improvement of reversibility compared with the charge-discharge cycles of the corresponding pristine samples