Structural and electrochemical effects of cobalt doping on lithium manganese oxide spinel

Pure LiMn₂O₄ and lithium manganese oxide spinels with partial replacement of manganese by cobalt up to 20 mole%, LiCo_xMn_2−xO₄, were prepared. The effect of extended cycling on the crystal structure was investigated. A capacity decrease with increasing cobalt content was observed in the potential range about 4100 mV vs. Li/Li⁺. Cycling behavior is significantly improved, compared to LiMn₂O₄. LiCo_xMn_2−xO₄ is discharged in a single phase reaction in the upper potential range around 4100 mV vs. Li/Li⁺, whereas pure LiMn₂O₄ shows a two phase behavior. LiMn₂O₄ shows a significant broadening of peaks in plots of differential capacity and change in shape of the voltage profile upon extended cycling. LiCo_xMn_2−xO₄ shows neither broadening nor change. Voltage profiles and plots of the differential capacity differ significantly compared to spinels with lithium substitution, Li_1+xMn_2−xO₄. In contrast to Li_1+xMn_2-xO₄, LiCo_xMn_2-xO₄ is discharged in a two step process in the range of 0 ≤ × ≤ 0,5. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrochemical behavior of Li intercalation processes into a Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 cathode

Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (X=0.17, 0.25, 0.33, 0.5) compounds are prepared by a simple combustion method. The Rietvelt analysis shows that these compounds could be classified as having the α-NaFeO₂ structure. The initial charge-discharge and irreversible capacity increases with the decrease of x in Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂. Indeed, Li[Ni_0.50Mn_0.50]O₂ compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni_0.17Li_0.22Mn_0.61]O₂ and Li[Ni_0.25Li_0.17Mn₀.₅₈]O₂ compounds exhibit high initial discharge capacity over 245 mAh/g and stable cycle performance in the voltage range of 4.8 -2.0 V. On the other hand, XANES analysis shows that the oxidation state of Ni ion reversibly changes between Ni²⁺ and about Ni³⁺, while the oxidation state of Mn ion sustains Mn⁴⁺ during charge-discharge process. This result does not agree with the previously reported ‘electrochemistry model’ of Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂, in which Ni ion changes between Ni²⁺ and NI⁴⁺. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge-discharge process.     

The properties research of ferrum additive on Li [Ni<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>] O<Subscript>2</Subscript> cathode material for lithium ion batteries

Li[Ni_1/3Co_(1-x)/3Mn_1/3Fe _x/3] O₂(x?=?0.0, 0.1, 0.3, 0.5, 0.7, and 0.9) cathode materials have been synthesized via hydroxide co-precipitation method followed by a solid state reaction. Thermogravimetry (TG) and differential thermal analysis (DTA) measurements were utilized to determine the calcination temperature of precursor sample. The crystal structure features were characterized by X-ray diffraction (XRD). The electrochemical properties of Li[Ni_1/3Co_(1-x)/3Mn_1/3Fe _x/3]O₂ were compared by means of cyclic voltammetry (CV), electrochemical impedance spectroscopy(EIS), and galvanostatic charge/discharge test. Electrochemical test results indicate that Li[Ni_1/3Co_0.9/3Mn_1/3Fe_0.1/3] O₂ decrease charge transfer resistance and enhance Li⁺ ion diffusion velocity and thus improve cycling and high-rate capability compared with Li[Ni_1/3Co_1/3Mn_1/3]O₂. The initial discharge specific capacity of Li[Ni_1/3Co_0.9/3Mn_1/3Fe_0.1/3] O₂ was 178.5 mAh/g and capacity retention was 87.11 % after 30 cycles at 0.1C, with the battery showing good cycle performance.     

Structural and electrochemical study of Cr-substituted layered manganese oxide cathode materials

A lithiated layered Mn–Cr compound, Li[Cr_0.29Li_0.24Mn_0.47]O₂ was synthesized by a solution method with subsequent quenching. The crystal structure was investigated by X-ray diffraction (Rietveld refinement) and Electron diffraction showing co-existence of rhombohedral and monoclinic structures. According to the co-indexed electron diffraction patterns and HRTEM images, Li[Cr_0.29Li_0.24Mn_0.47]O₂ electrode was composed of nano-scale domains indexed in monoclinic and hexagonal structures, simultaneously. The nano-composite cathode successfully prevents spinel-like structural transformation during cycling and delivered a good reversible capacity of about 195 mAh/g between 2.4 and 4.7 V.     

Effect of fluorine in Li1.27Cr0.2Mn0.53O2 cathode for secondary lithium batteries

Fluorine substituted Li_1.27Cr_0.2Mn_0.53O₂ electrode, prepared by sol–gel method, was investigated in the present work. Thermal analysis was done on this cathode material and found to be thermally stable with a loss of weight near 300 °C. Influence of fluorine substitution on the structural and electrochemical properties of the Li_1.27Cr_0.2Mn_0.53O₂ electrode was studied by X-ray diffraction (XRD) and field emission scanning electron microscope. XRD pattern of the fluorine-doped Li_1.27Cr_0.2Mn_0.53O₂ cathode material quenched at 900 °C indicates a phase pure material. The charge–discharge profile of the prepared cathode material showed that the fluorine substitution for oxygen in the cathode material resulted in improved capacity retention. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, December 7–9, 2006.     

Enhanced cyclability of triple-metal-doped LiMn2O4 spinel as the cathode material for rechargeable lithium batteries

To improve the cycle performance of spinel LiMn₂O₄ as the cathode of 4-V-class lithium secondary batteries, spinel phases LiM _x Mn_2 − x O₄ (M=Li, Fe, Co; x = 0, 0.05, 0.1, 0.15) and LiFe_0.05M _y Mn_{1.95 − y} O₄ (M=Li, Al, Ni, Co; y = 0.05, 0.1) were successfully prepared using the sol–gel method. The spinel materials were characterized by powder X-ray diffraction (XRD), elemental analysis, and scanning electron microscopy. All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. Electrochemical studies were carried out using the Li|LiM _x Mn_2 − x O₄ (M=Li, Fe, Co; x = 0, 0.05, 0.1, 0.15) and LiFe_0.05M _y Mn_{1.95 − y} O₄ (M=Li, Al, Ni, Co; y = 0.05, 0.1) cells. These cathodes were more tolerant to repeated lithium extraction and insertion than a standard LiMn₂O₄ spinel electrode in spite of a small reduction in the initial capacity. The improvement in cycling performance is attributed to the stabilization in the spinel structure by the doped metal cations.     

Vibrational studies of spinel LixMn2O4 (0.3≤x≤1.0) cathodes

We report on the vibrational properties of spinel LiMn₂O₄ and its electrochemically delithiated forms Li_xMn₂O₄. Raman scattering and infrared absorption spectra have been studied as a function of the delithiation content in the wavenumber range 50–700 cm⁻¹. Results show that lithium ions can be extracted at room temperature to obtain Li_x[Mn₂]O₄ (0.3≤x≤1.0) without disrupting the [Mn₂]O₄ array. The normal modes of the spinel LiMn₂O₄ have been discussed in the O _h ⁷ symmetry and vibrations due to lithium ions with their oxygen neighbors have been identified at ca. 400 cm⁻¹. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Structural and electrochemical study on Li(Li1/3Ti5/3)O4 anode material for lithium ion batteries

Chemical and electrochemical studies have shown that various titanium oxides can incorporate lithium in different ratios. Other compounds with a spinel-type structure and corresponding to the spinel oxides LiTi₂O₄ and Li₄Ti₅O₁₂ have been evaluated in rechargeable lithium cells with promising features. The spinel Li[Li_1/3Ti_5/3]O₄ [1–5] compound is a very appealing electrode material for lithium ion batteries. The lithium insertion-deinsertion process occurs with a minimal variation of the cubic unit cell and this assures high stability which may reflect into long cyclability. In addition, the diffusion coefficient of lithium is of the order of 10⁻⁸ cm²s⁻¹ [5] and this suggests fast kinetics which may reflect in high power capabilities. In this work we report a study on the kinetics and the structural properties of the Li[Li_1/3Ti_5/3]O₄ intercalation electrode carried out by: cyclic voltammetry, galvanostatic cycling and in-situ X-ray diffraction. The electrochemical characterization shows that the Li[Li_1/3Ti_5/3]O₄ electrode cycles around 1.56 V vs. Li with a capacity of the order of 130 mAhg⁻¹ which approaches the maximum value of 175 mAhg⁻¹ corresponding to the insertion of 1 equivalent per formula unit. The delivered capacity remains constant for hundred cycles confirming the stability of the host structure upon the repeated Li insertion-deinsertion process. This high structural stability has been confirmed by in situ Energy Dispersion X-ray analysis. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Studies of the local structure in LixNi0.7Co0.3O2 (0.5≤x≤1.0) cathode materials

Among the Li_xNi_1−yCo_yO₂ system, LiNi_0.7Co_0.3O₂ is being considered one of the best cathode materials due to its small volume cell expansion upon charge-discharge cycling. In order to study the modifications of structural and physical properties occurring in the cathode materials during charge, different Li_xNi_0.7Co_0.3O₂ samples (0.5 ≥ x ≥ 1.0) were prepared by electrochemical lithium deintercalation in non-aqueous cells. During the first charge of the Li//Li_xNi_0.7Co_0.3O₂ cell, the structural change in the cathode lattice was followed by both x-ray powder diffraction and FTIR spectroscopy at room temperature. A good correlation is found between XRD data and the local environment of the host lattice. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998     

Optical absorption and nuclear magnetic resonance in lithium titanium spinel doped by chromium

The optical absorption and nuclear magnetic resonance spectra of Li_{4 − x} Cr_3x Ti_{5 − 2x} O₁₂ (x = 0, 0.01, 0.02, 0.04) solid solutions have been investigated. It has been found that, in the Li₄Ti₅O₁₂ spinel, lithium ions migrate from tetrahedral to octahedral positions with increasing temperature. Doping of chromium to the spinel favors an increase in the fraction of tetrahedrally coordinated lithium and hinders diffusion.     

Hydrothermal synthesis of spinel Li1+x Mn2O4 as cathode material for rechargeable lithium battery

Abstract

The hydrothermal synthesis of Li-Mn spinel oxide (Li_1+xMn₂O₄) was undertaken in order to develop high quality, low cost cathode material for a rechargeable lithium battery. In our experiments, γ-MnOOH, LiOH · H₂O and H₂O₂ were used as starting materials to synthesize Li-Mn spinel oxide under hydrothermal conditions of 180-230°C and about 1.0-2.8 MPa. The chemical composition and particle size of the Li_1+xMn₂O₄ is easily controlled in the hydrothermal reaction. The Li_1+xMn₂O₄ produced was characterized by X-ray diffraction, with the spinel phase having a Li/Mn ratio of 0.50-0.60. There is convincing evidence, as a result of this work, that our synthesis process is most suitable for producing high quality cathode material that can be used in a rechargeable lithium battery.     

Effect of allied and alien ions on the EPR spectrum of Mn-containing lithium-manganese spinel oxides

Electron paramagnetic resonance spectroscopy was used for studying the effect of allied and alien ions on the EPR spectrum of Mn⁴⁺-containing lithium-manganese spinel oxides. Manganese spinel oxides with paramagnetic Mn⁴⁺ and diamagnetic substituents in the 16d spinel sites were studied: Li[Mg_0.5Mn_1.5]O₄, Li[Mg_0.5−xCo_2xMn_1.5−x]O₄, 0<x≤0.5, and Li[Li_1/3Mn_5/3]O₄. Ni²⁺-ions with integer-spin-ground state (S=1) were selected as alien ions: Li[Mg_0.5−xNi_xMn_1.5]O₄ (0≤x≤0.5), Li[Li_(1−2x)/3Ni_xMn_(5−x)/3]O₄ (0≤x≤0.5), and Li[Ni_0.5Mn_1.5−yTi_y]O₄ (0≤y≤1.0). It was shown that in Ni-substituted oxides the low temperature EPR response comes from magnetically correlated Ni-Mn spins, while at high registration temperature Mn⁴⁺ ions give rise to the EPR profile. Analysis of the EPR line width allows differentiating between the contributions of the density of paramagnetic species and the strength of the exchange interactions in magnetically concentrated systems. The density of allied and alien paramagnetic species has no effect on the EPR line width in cases when the strengths of antiferro- and ferromagnetic interactions on an atomic site are close. On the contrary, when antiferro- or ferromagnetic interactions on an atomic site are dominant, the EPR line width increases with the density of paramagnetic species.     

Effects of Sn doping on electrochemical characterizations of Li[Ni<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>]O<Subscript>2</Subscript> cathode material

Li[Ni_1/3Co_1/3Mn_1/3]O₂ and Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials for lithium battery are synthesized by a solid-state method. The samples are characterized by X-ray diffraction, scanning electron microscope, electrochemical impedance spectroscopy (EIS), and charge–discharge test. The results show that the Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ has a typical hexagonal α-NaFeO₂ structure and strawberry-like shape with uniform particle size. It has also been found that the Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ reveals better electrochemical performances than that without Sn doping. The EIS results suggest that Sn presence decreases the total resistance of Li[Ni_1/3Co_1/3Mn_1/3]O₂, which should be related to the improvement on the electrochemical properties.     

Investigation of the structural and electrochemical performance of Li<Subscript>1.2</Subscript>Ni<Subscript>0.2</Subscript>Mn<Subscript>0.6</Subscript>O<Subscript>2</Subscript> with Cr doping

Cr-doped layered oxides Li[Li_0.2Ni_0.2???x Mn_0.6???x Cr_2x ]O₂ (x?=?0, 0.02, 0.04, 0.06) were synthesized by co-precipitation and high-temperature solid-state reaction. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TRTEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). XRD patterns and HRTEM results indicate that the pristine and Cr-doped Li_1.2Ni_0.2Mn_0.6O₂ show the layered phase. The Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ shows the best electrochemical properties. The first discharge specific capacity of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ is 249.6 mA h g^?1 at 0.1 C, while that of Li_1.2Ni_0.2Mn_0.6O₂ is 230.4 mA h g^?1. The capacity retaining ratio of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ is 97.9% compared with 93.9% for Li_1.2Ni_0.2Mn_0.6O₂ after 80 cycles at 0.2 C. The discharge capacity of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ is 126.2 mA h g^?1 at 5.0 C, while that of the pristine Li_1.2Ni_0.2Mn_0.6O₂ is about 94.5 mA h g^?1. XPS results show that the content of Mn³⁺ in the Li_1.2Ni_0.2Mn_0.6O₂ can be restrained after Cr doping during the cycling, which results in restraining formation of spinel-like structure and better midpoint voltages. The lithium-ion diffusion coefficient and electronic conductivity of Li_1.2Ni_0.2Mn_0.6O₂ are enhanced after Cr doping, which is responsible for the improved rate performance of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂.     

Hydrothermal phase formation of orthorhombic LiMnO2 and its derivatives as lithium intercalation compounds

Different from a conventional solid state reaction, a hydrothermal reaction mechanism is very difficult to illuminate and sometimes it remains undisclosed. Making an attempt to understand the hydrothermal phase formation process of o-LiMnO₂ obtained between the reaction of spinel type Mn₃O₄ precursor and LiOH aqueous solution, the possible reaction route was postulated and experimentally testified. Firstly, the selective dissolution of Mn²⁺ from the tetrahedral site of [Mn^II]^tet_4a[Mn^III₂]^oct_8dO₄, which is considered as to be an ionic exchange reaction with Li⁺, and an additional Li⁺ intercalation into the host structure of precursor would give rise to the formation of meta-stable Li₂Mn₂O₄ ([Li^I]^tet_4a[Li^I]^oct_8c[Mn^III₂]^oct_8dO₄). Secondly, the phase would be simultaneously transformed to thermodynamically stable o-LiMnO₂ phase under hydrothermal state during hydrothermal reaction. Through the above reaction process, the solid solution range of o-LiCo_xMn_1−xO₂ was as large as x ≤ 0.14, and that of o-LiFe_xMn_1−xO₂ was x ≤ 0.05. Co doped o-LiMnO₂ has higher capacity and good cyclability upon cycling, being substantially more stable to cycle than the unsubstituted and Fe doped materials.     

C-rate and temperature dependence of electrochemical performance of Li[Co0.1Ni0.15Li0.2Mn0.55]O2 cathode materials before and after Co3(PO4)2 coating

Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂ was synthesized, as a cathode material with high capacity, by a simple combustion method followed by annealing at 800?°C. Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂ cathode materials were coated with lithium-active Co₃(PO₄)₂ to improve the electrochemical performance of rechargeable lithium batteries. Morphologies and physical properties of Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂ before and after the Co₃(PO₄)₂ coating were analyzed with a scanning electron microscope equipped with an energy dispersive X-ray spectroscope. Transmission electron microscopy, powder X-ray diffraction, and Brunauer?CEmmett?CTeller surface area analyses were also carried out. The electrochemical performances of Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂ cathode material before and after Co₃(PO₄)₂ coating were evaluated by galvanostatic charge?Cdischarge testing at different charge and discharge densities. The temperature dependence of the cathode material before and after Co₃(PO₄)₂ coating was investigated at 0, 10, 20, 30, 40, and 50?°C at a rate of 0.1?C. Co₃(PO₄)₂-Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂ exhibited good electrochemical performance under high C-rate and experimental temperature conditions. The enhanced electrochemical performances were attributed to the formation of a lithium-active Co₃(PO₄)₂-coating layer on Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂.     

Effects of dopant on the electrochemical properties of Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> anode materials

The effects of dopant on the electrochemical properties of spinel-type Li_3.97M_0.1Ti_4.94O₁₂ (M = Mn, Ni, Co) and Li_(4-x/3)Cr_xTi_(5-2x/3)O₁₂(x = 0.1, 0.3, 0.6, 0.9, 1.5) were systematically investigated. Charge-discharge cycling were performed at a constant current density of 0.5 mA/cm² between the cut-off voltages of 3.0 and 1.0 V, the experimental results showed that Cr³⁺ dopant improved the reversible capacity and cycling stability over the pristine Li₄Ti₅O₁₂. The substitution of the Mn³⁺ and Ni³⁺ slightly decreased the capacity of the Li₄Ti₅O₁₂. Dopants such as Co³⁺ to some extent worsened the electrochemical performance of the Li₄Ti₅O₁₂.     

Effect of V5+ in the complex Li3xLa2/3−xTiO3-LaPO4 system

Lithium fast ion conductors of the composition Li_0.3La_2/3Ti_0.7P_0.3−xV_xO_3.3 (LTV) based on mixtures of Li_3xLa_2/3−xTiO₃ and LaPO₄ were prepared by solid state reaction at high temperature (≈ 1300 °C). AC impedance measurements indicate total conductivities of about 1 × 10⁻⁴ Scm⁻¹ for compositions of x=0∼0.3 at room temperature with an activation energy of ≈18 kJ·mol⁻¹ in the temperature range from 30 to 400 °C. X-ray powder diffraction patterns showed that the LTV system is composed of Li_3xLa_2/3−xTiO₃ perovskite solid solution and LaP_1−xV_xO₄ solid solution.     

Evidence of Jahn–Teller distortion in Li<Subscript>x</Subscript>Mn<Subscript>2</Subscript>O<Subscript>4</Subscript> by thermal diffusivity measurements

Thermal diffusivities of Li_xMn₂O₄ (x=0.9, 1.0 and 1.1) polycrystalline pellets are determined by photoacoustic technique in the reflection configuration, at two temperatures, 298 K and 280 K, above and below their Jahn–Teller phase transition temperature (290 K). The diffusivities of Li_xMn₂O₄ at 280 K show a drastic reduction from their corresponding room temperature values (298 K) and the percentage of reduction in the thermal diffusivity for Li_xMn₂O₄ increases with their Li content. These effects are associated with the reduction in crystal symmetry due to structural deformation by Jahn–Teller distortion observed in Li_xMn₂O₄ below its transition temperature. PACS 78.20.Nv; 82.47.Aa; 63.20.Mt     

Li4Ti2.5Cr2.5O12 as anode material for lithium battery

Chromium-substituted Li₄Ti₅O₁₂ has been investigated as a negative electrode for future lithium batteries. It has been synthesized by a solid-state method followed by quenching leading to a micron-sized material. The minimum formation temperature of Li₄Ti_2.5Cr_2.5O₁₂ was found to be around 600 °C using thermogravimetric and differential thermal analysis. X-ray diffraction, scanning electron microscopy, cyclic voltammetry (CV), impedance spectroscopy, and charge–discharge cycling were used to evaluate the synthesized Li₄Ti_2.5Cr_2.5O₁₂. The particle size of the powder was around 2–4 μm. CV studies reveal a shift in the deintercalation potential by about 40 mV, i.e., from 1.54 V for Li₄Ti₅O₁₂ to 1.5 V for Li₄Ti_2.5Cr_2.5O₁₂. High-rate cyclability was exhibited by Li₄Ti_2.5Cr_2.5O₁₂ (up to 5  C) compared to the parent compound. The conduction mechanism of the compound was examined in terms of the dielectric constant and dissipation factor. The relaxation time has been evaluated and was found to be 0.07 ms. The mobility was found to be 5.133 × 10⁻⁶ cm² V⁻¹ s⁻¹.