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     

Electrochemical and structural comparison of zinc-cobalt doped spinels with other doped lithium manganese spinels

Doping of lithium manganese spinels by zinc and cobalt of the type (Li_1xZn_x)(Mn_2–2x Co_2x)O₄ may stabilize the delithiated spinels and may offer some indications with respect to the validity of capacity fade models. There are structural chemical arguments for this cation distribution. These and other doped lithium manganese spinels were prepared by heat treatment (in the range between 500 and 800 °C for 12 h) of solution precipitated precursors. Samples were characterised structurally and electrochemically by XRD and galvanostatic cycling. Extended cycling, storage in the charged state and storage in the discharged state were investigated. Pure phases of Zn-Co doped samples were obtained only for quenching the fired samples. Otherwise separation into a tetragonal spinel and a cubic spinel occurred. XRD results prove the occupation of tetrahedral coordinated cation positions by zinc ions, in contradiction to the results of other authors. XRD profiles show an anisotropic line broadening, which is attributed to an anisotropic microstrain, maybe induced by a non-cooperative Jahn-Teller distortion. Capacity retention was better for extended cycling and worse for both kinds of storage, compared to a purely cobalt doped spinel. Therefore, thermodynamic stabilisation of the delithiated spinels could not be confirmed. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998     

Solvothermal synthesis of LiCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript>/C cathode materials for lithium-ion batteries

LiCo_1−x Mn _x PO₄/C cathode materials are selectively synthesized by a solvothermal method in ethylene glycol solvent using glucose, LiCl, H₃PO₄, MnCl₂·4H₂O, and Co(NO₃)₂·6H₂O as precursors. The obtained samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) and the electrochemical performances are also evaluated using a LAND CT2001A battery test system at room temperature. XRD result demonstrates the formation of LiCo_1−x Mn _x PO₄ solid solution and the enlarged channels are benefit for Li⁺ migration. SEM graph indicates that the particle size of LiCo_0.5Mn_0.5PO₄/C is about several hundred nanometers and aggregates to large particles located in the range of 2–3 μm. TEM image illustrates that the core/shell-structured LiCo_0.5Mn_0.5PO₄/C solid solution is indeed obtained by this method. The high specific surface area (35 m²/g) of LiCo_0.5Mn_0.5PO₄/C could make this solid solution contact with the electrolyte more sufficiently and benefit for Li⁺ transportation. The capacity, flat voltage, and cyclical stability of LiCo_1−x Mn _x PO₄/C are improved compared to LiMnPO₄ and LiCoPO₄ due to the improved electronic conductivity and lithium-ion conductivity which resulted from carbon coating and foreign element incorporation.     

Solution synthesis of boron substituted LiMn2O4 spinel oxide for use in lithium rechargeable battery

An attempt has been made to synthesize LiMn₂O₄ spinel and boron substituted LiMn₂O₄ with atomic concentration of boron ranging from 0.01–0.20 and using glutaric acid as a chelating agent. The spinels have been characterized using PXRD, CV and galvanostatic charge-discharge studies. The precursor obtained from the glutaric acid assisted gel was calcined initially at 300 °C for 4 h to obtain the compound and finally at 800 °C for 4 h so as to obtain homogeneity, high degree of purity and crystallinity for better electrochemical performance. This paper suggests that glutaric acid assisted B³⁺ doped (LiB_xMn_2−xO₄) spinel was found to be as an apt candidate with good electrochemical performance for use in lithium battery.     

Influence of Sm<Superscript>3+</Superscript> ion in structural,morphological, and electrochemical properties of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> synthesized by microwave calcination

LiSm_xMn_2–xO₄ samples were synthesized via co-precipitation technique. The structural properties of the synthesized materials were studied using X-ray diffraction analysis and it confirmed the cubic spinel structure for all the compounds. The lattice parameter of LiMn₂O₄ was observed to be 8.2347 Ǻ and it decreased with Sm³⁺ concentration, due to the shrinkage in cell volume aided by higher binding energy between Sm-O bond. The SEM micrographs were analyzed using Image processing software (Image-J) to ascertain the pore and grain properties. The microwave synthesis had been observed to control the bulk grain formation and had yielded lesser porous and nanoparticles. The particle size distributions obtained through photocross correlation laser diffraction analysis had shown that LiMn₂O₄ with 60 nm and Sm-doped compounds with ∼30 nm, respectively. The cyclic voltammetry studies had revealed the decrease in electrocatalytic behavior in the initial cycle for compounds doped with Sm³⁺ ion. The initial capacities of LiMn₂O₄, LiSm_0.05Mn_1.95O₄ and LiSm_0.10Mn_1.90O₄ substituted compounds were observed to be 134.87 mAhg⁻¹, 132.22 mAhg⁻¹ and 126.41 mAhg⁻¹, respectively. The cells were simulated using 1D model namely Dualfoil5.1 program. The simulated results coincide well with the measured results. The cycle life studies reveal 93% capacity retention of samarium-0.05-doped samples when compared with 78.4% of the LiMn₂O₄.     

Effects of structural disorder in lithium manganite and titanate oxides

The structures and magnetic states of stoichiometric lithium manganite LiMn₂O₄ and manganites and titanates Li_1.33Mn_1.67O₄ and Li_1.33Ti_1.67O₄ with excess lithium in both the initial (as-synthesized) state and after irradiation by fast (E _eff ≥ 1 MeV) neutrons with a fluence of 2 × 10²⁰ cm⁻² have been studied using neutron diffraction, X-ray diffraction, and magnetic methods. It has been established that the irradiation brings about a noticeable redistribution of manganese, titanium, and lithium cations over nonequivalent tetrahedral (8a) and octahedral (16d) positions of a spinel lattice. This structural disorder causes a radical change in the physical properties of the materials under study. The charge order existing in the initial LiMn₂O₄ sample is destroyed. There arises a strong intersublattice indirect exchange interaction Mn(8a)-O-Mn(16d). The disorder is accompanied by the antiferromagnet-ferrimagnet (LiMn₂O₄) and paramagnet-ferrimagnet (Li_1.33Mn_1.67O₄) magnetic transitions.     

Significance of Mg doped LiMn2O4 spinels as attractive 4 V cathode materials for use in lithium batteries

LiMn₂O₄ spinel is one of the most promising cathode materials for lithium-ion batteries because of its cheapness and eco-friendliness. Due to Jahn-Teller distortion, the capacity fades, however, upon repeated cycling. Attempts are being made to improve the cycle life of the spinel by substitution of manganese with other cations. In this paper we report the effect of partial substitution of manganese by Mg²⁺ ions in the LiMn₂O₄ phase. LiMg_yMn_2−yO₄ (y=0 – 0.3) has been synthesized by a thermal method and characterized using XRD, TG/DTA and FTIR. The electrochemical performance is correlated with the dopant concentration.     

Preparation, characterization and electrochemical performance of γ-MnO2 and LiMn2O4 as cathodes for lithium batteries

The spinel LiMn₂O₄ is a very promising cathode material with economical and environmental advantages. LiMn₂O₄ materials have been synthesized by solid state method using γ-MnO₂ as manganese source, and Li₂CO₃ or LiNO₃ as Li sources. γ-MnO₂ is a commercial battery grade electrolytic manganese dioxide (TOSOH-Hellas GH-S) and LiMn₂O₄ samples were synthesized at a calcinations temperature up to 800 °C. γ-MnO₂ and LiMn₂O₄ samples were characterized by X-ray diffraction, thermal and electrochemical measurements. X-ray powder diffraction of as prepared LiMn₂O₄ showed a well-defined highly pure spinel single phase. The electrochemical performance of LiMn₂O₄ and its starting material γ-MnO₂ was evaluated through cyclic voltammetry, galvanostatic (constant current charge-discharge cycling) The electrochemical properties in terms of cycle performance were also discussed. γ-MnO₂ showed fairly high initial capacity of about 200 mAhg⁻¹ but poor cycle performance. LiMn₂O₄ samples showed fairly low initial capacity but good cycle performance.     

Malonic acid assisted sol-gel synthesis and characterization of chromium doped LiMn2O4 spinel

Pristine LiMn₂O₄ and LiCr_xMn_2-xO₄ (x=0.01−0.20) have been synthesized by sol-gel method using malonic acid as chelating agent. This technique involves less impurities, shorter heat treatment time, sub-micron sized particles, good surface morphology, better homogeneity, good agglomeration and better crystallinity. The synthesized spinel materials have been characterized by XRD, SEM, TEM, EDAX and electrochemical studies like charge-discharge studies, cyclic voltammogram, cycleability studies have also been carried out. All the results exhibit that chromium substitution improves the structural stability of LiMn₂O₄ spinel upon repeated cycling.     

Synchrotron X-ray diffraction studies on the phase transitions in the spinel LixMn3−xO4 intercalation compounds

Detailed investigations have been undertaken of the lithium for manganese substitution effect on the LiMn₂O₄, in the system Li_xMn_3−xO₄, for 0.95≤x≤1.05, that is for the nearly stoichiometric lithium content. Synchrotron X-ray measurements have been performed in the temperature range 10–300 K. The diffraction experiments were carried out at the DESY-HASYLAB high-resolution powder diffractometer (beamline B2), equipped with a closed-cycle He-cryostat. Very small changes in the lithium content influence clearly the low-temperature crystal structure of Li_xMn_3−xO₄, spinels and the nature of phase transitions. It was found that for x=0.95 the sample remains tetragonal in the whole 10–300 K temperature range. The stoichiometric LiMn₂O₄ transforms from cubic to orthorhombic at about 280 K. For x=1.0125 the temperature of phase transition from cubic to orthorhombic decreases down to about 260 K, whereas for x=1.025 the transformation goes from cubic to tetragonal phase, at the temperature 220 K. No phase transition has been observed for the cubic sample with x=1.0375. These results partly explain the divergences in recent reports on the low-temperature structure and phase transformations of lithium manganese oxides.     

Structure, morphology and electrochemistry of doped lithium cobalt oxides

The structure, morphology and electrochemical performance of LiCoO₂ powders doped with various trivalent cations such as M³⁺=Ni³⁺, Al³⁺, and B³⁺ have been investigated. X-ray diffraction patterns and FTIR data show that LiCo_1−yNi_yO₂ oxides display a solid solution in the whole range 0≤y≤1, while the solubility limit of LiCo_1−yAl_yO₂ and LiCo_1−yB_yO₂ materials is about y=0.35. Electrochemical features of LiCo_1−yNi_yO₂ cathode materials show remarkable stability in their charge-discharge profiles. Aluminum substitution also stabilizes the two-dimensional framework and induces an increasing average cell potential than for LiCoO₂. The overall capacity of the LiCo_1−yB_yO₂ oxides has been reduced due to the sp metal substitution, however, a more stable charge-discharge cycling characteristic has been observed when electrodes are charged up to 4.4 V as compared to the performances of the native oxides. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electronic properties of LiMn2?xTixO4

Ti-substituted LiMn₂O₄ (LiMn_2−x Ti _x O₄, x=0, 0.15, 0.30, 0.45, 0.60, and 0.75) has been synthesized using solid-state reactions. Their crystal and electronic structures were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XRD data suggested that the lattice parameters of LiMn_2−x Ti _x O₄ increase due to the replacement of Mn by Ti ions. XPS results indicated that the substituted Ti ions were in +4 oxidation state; consequently, the normal oxidation state of Mn ions has been detected by measuring the binding energy splitting of Mn 3s states, which decreases with the content of substituted Ti. The valence band spectra suggested that the intensity of e _g level of Mn 3d orbitals increased due to the increase of the Mn³⁺/Mn⁴⁺ ratio.     

Electrochemical activities of yttrium doped spinel LiMn2O4

It was found for the first time that the catalysis of yttrium doping of spinel LiMn₂O₄ can enhance the electrochemical activities of manganese, leading to both improvement of electrochemical capacity and reactivity with the electrolyte of manganese. A proper amount of doping was 0.5%, and such yttrium-doped sample, Li(Y_0.005Mn_0.995)₂O₄, had an initial capacity of 130 mAh g⁻¹ over that of the undoped one with the capacity retention to reach 92.3% exceeding that of the undoped one at 100th cycle.     

Magnetism of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> manganite in structurally ordered and disordered states

The specific features of the crystal structure and the magnetic state of stoichiometric lithium manganite in the structurally ordered Li[Mn₂]O₄ and disordered Li_{1 − δ}Mn_δ[Mn_{2 − δ}Li_δ]O₄ (δ = 1/6) states have been investigated using neutron diffraction, X-ray diffraction, and magnetic methods. The structurally disordered state of the manganite was achieved under irradiation by fast neutrons (E _eff ≥ 1 MeV) with a fluence of 2 × 10²⁰ cm⁻² at a temperature of 340 K. It has been demonstrated that, in the initial sample, the charge ordering of manganese ions of different valences arises at room temperature, which is accompanied by orthorhombic distortions of the cubic spinel structure, and the long-range antiferromagnetic order with the wave vector k = 2π/c(0, 0, 0.44) is observed at low temperatures. It has been established that the structural disordering leads to radical changes in the structural and magnetic states of the LiMn₂O₄ manganite. The charge ordering is destroyed, and the structure retains the cubic symmetry even at a temperature of 5 K. The antiferromagnetic type of ordering transforms into ferrimagnetic ordering with local spin deviations in the octahedral sublattice due to the appearance of intersublattice exchange interactions.     

A novel method for preparing lithium manganese oxide nanorods from nanorod precursor

Lithium manganese oxide nanorods were prepared from manganese dioxide nanorods precursor. The structure and morphology were confirmed by X-ray diffraction (XRD) and transmission electron microscope (TEM). The data of the Rietveld refinement indicate that the nanorods preferentially grow along the [111] direction. After charge–discharge test at 1.0 mA cm⁻² in 3.0–4.4 V, the nanorods LiMn₂O₄ showed the 134.5 mAh g⁻¹ initial discharge capacity and only lost 1.1% of initial capacity after 30 cycles, which is better than that of bulk particles LiMn₂O₄ prepared by traditional solid-state reaction method. This effective and simple route to synthesis nanorods LiMn₂O₄ from one-dimensional (1D) precursor could also be extended to prepare 1D other nanomaterials with special electrochemical properties.     

Co-precipitation synthesis and performance of multi-doped LiCrxNixMn2-2xO4-zFz cathode materials for lithium ion batteries

Multiple ion-doped lithium manganese oxides LiCr_xNi_xMn_2-2xO_4-zF_z (0 < x ≤ 0.25, z =  0.05, 0.1) with a spinel structure and space group Fd m were prepared by using the co-precipitation procedure carried out in water–alcohol solvent using adipic acid as the chelating agent. The electrochemical measurements indicated that the charge/discharge capacities of the samples prepared at 600 °C are higher than that of the treatment at 800 °C or microwave heating. The capacitance-voltage (CV) curves of LiCr_xNi_xMn_2-2xO_4-zF_z (0 < x ≤ 0.25, z = 0.05, 0.1) showed that when x ≤ 0.1, the samples had two reduction–oxidation peaks at 4.0 to 4.2-V region, whereas when x > 0.1, the samples had only one reduction–oxidation peak at 4.0- to 4.2-V region in CV measurements and could offer more stable voltage plateau in a 4-V region and also had stable electrical conductivity after 20 cycles. Another reduction–oxidation peak appeared in 4.6-4.8-V region (Ni²⁺–Ni⁴⁺ reduction–oxidation peaks); this suggests that the LiCr_xNi_xMn_2-2xO_4-zF_z (0.1 < x≤ 0.25, z = 0.05, 0.1) cathode material could offer 4.6 to 4.8-V charge/discharge plateaus, and its specific capacity increases with increasing Ni²⁺. The impedance measurements of the cell proved that the F⁻ anion doped can not only prevent Mn³⁺ from disproportion but also can prevent the passivation film from forming and can help keep stable the cell’s electrical properties. The LiCr_0.05Ni_0.05Mn_1.9O_3.9F_0.1 sintered at 600 °C shows the best cycle performance and the largest capacity in all prepared samples; its first discharge capacity is 120 mAh/g, and the discharge capacity loses only 1.78% after 20 cycles. After 100 cycles, it still remains in the spinel structure.     

Synthesis and electrical characterisation of different doped magnesium titanates

The synthesis and characterization are reported for the cubic spinel titanate Mg_(2−x)Ni_xTiO₄ (x≤0.25) and Mg_(2−x)Mn_xTiO₄ (x≤1). Single phase samples were observed for Mg_(2−x)Ni_xTiO₄ and with x≤0.4 for Mg_(2−x)Mn_xTiO₄. AC measurements were carried out on four different compositions (x=0.01, 0.03, 0.04 and 0.15) in the Mg_(2−x)Ni_xTiO₄ series and for Mg_1.9Mn_0.1TiO₄. For all these compounds, increasing conductivity with temperature and Arrhenius conductivity dependence are observed, the activation energy is around 0.28 eV for the Ni compounds and is 0.184 eV for Mg_1.9Mn_0.1TiO₄. The DC conductivity was recorded over a range of oxygen partial pressures (10⁻¹⁹ to 1 atm) at 930 °C. The Mg_(2−x)Ni_xTiO₄ compounds show a n-type behaviour whereas the Mg_(2−x)Mn_xTiO₄ show a p-type behaviour at high p(O₂) and n-type at low p(O₂). The stability under reduced conditions was checked and discussed for the different synthesized compounds. Paper presented at the 5th Euroconference onSolid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Magnetic phases in Pr0.5Sr0.5Mn1 ? xCoxO3 (x ≤ 0.5) solid solutions

The magnetic and magnetotransport properties of Pr_0.5Sr_0.5Mn_{1 − x} Co _x O₃ (x ≤ 0.5) solid solutions have been investigated using neutron diffraction methods. The magnetization and electrical conductivity have been measured in magnetic fields up to 140 kOe. It has been established that, during cooling in the temperature range from 160 to 110 K, the compounds of compositions with a cobalt content x ≤ 0.07 undergo a structural phase transition from the high-temperature ferromagnetic phase to the antiferromagnetic phase. A further substitution of cobalt for manganese leads to a stabilization of the inhomogeneous dielectric ferromagnetic state, whereas a state of the cluster spin-glass type has been revealed in compositions with x = 0.15 and 0.20. At x ≥ 0.25, a new magnetic phase with a Curie temperature up to 210 K is formed as a result of the magnetic interaction between manganese and cobalt ions. A magnetic phase diagram of the system under investigation has been constructed.     

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.     

Synthesis of Co-coated lithium manganese oxide and its characterization as cathode for lithium ion battery

Co-coated LiMn₂O₄ was synthesized by electroless plating. The phase identification, surface morphology, and electrochemical properties of the synthesized powders were studied by X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy, and galvanostatic charge–discharge experiments, respectively. The result shows that Co-coated LiMn₂O₄ particle has a coarse surface with a lot of holes. The specific capacity of Co-coated LiMn₂O₄ is 118 mAh g⁻¹, which is a bit less than 123 mAh g⁻¹ for the uncoated LiMn₂O₄. The capacity retention of Co-coated LiMn₂O₄ is 11% higher than the uncoated LiMn₂O₄ when the electrode is cycled at room temperature for 20 times. When cycled at the temperature of 55 °C, the capacity retention of Co-coated LiMn₂O₄ becomes 15% higher than the uncoated one.