Enhanced cycling stability of LiMn2O4 by surface modification with melting impregnation method

The surface of as-prepared LiMn₂O₄ was modified with Al₂O₃ by a melting impregnation method. X-ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) studies indicated that Al₂O₃ nano-particles are distributed around the spinel. X ray absorption fine structure analysis (XAFS) further demonstrated that Al atoms were also doped to the surface of LiMn₂O₄ particles. The nano-Al₂O₃ particle coating improves the capacity retention of spinel LiMn₂O₄ efficiently at both room temperature and 55 °C. The mechanism of improvement for surface modified LiMn₂O₄ can be attributed to the inhibition of a surface Jahn-Teller distortion and the decrease of manganese dissolution, leading to good electric contact among particles.     

Effect of chitosan addition on the electrochemical behavior and crystallization of LiMn2O4 film derived from acetates-containing solution

Spinel LiMn₂O₄ films were obtained by spin-coating the lithium/manganese acetates-containing precursor solution on a Pt-coated silicon substrate. The effect of chitosan addition in the acetates-containing precursor solution on the formation of the LiMn₂O₄ films was investigated by TG/DTA, FT-IR spectroscopy, glancing-angle XRD and cyclic voltammetry. It was demonstrated that the addition of chitosan is very beneficial to the deposition of a single-phase LiMn₂O₄ film due to the fact that chitosan is able to chelate with lithium/manganese ions and form a stable complex compound. Moreover, the electrochemical measurements also showed that the deposited LiMn₂O₄ film from the chitosan-added precursor solution exhibits a higher discharge capacity of 134 mAh/g at 1 C and a better rate performance (86.4% of the discharge capacity at 1 C can be maintained when the discharge rate increases from 1 up to 8 C) in comparison with one from the chitosan-free solution.     

Capacity intermittent titration technique (CITT): A novel technique for determination of Li solid diffusion coefficient of LiMn2O4

The capacity intermittent titration technique (CITT) has been developed on basis of the ratio of the potentio-charge capacity to the galvano-charge capacity (RPG) method to determine continuously the solid diffusion coefficient D of the intercalary species within insertion-host materials. In experiment, CITT is based on the capacity response of galvano–potentio-charge in a small voltage region. In theory, CITT is based on the linear equations of D versus q (value of RPG) in different range of q. By the CITT, the Li⁺ solid diffusion coefficients within LiMn₂O₄ have been determined at different voltages and different galvano-charge currents. Results shows that the order of magnitude of D varies non-linearly with the “W” shape from 10⁻⁹ to 10⁻¹¹ cm² s⁻¹ in the voltage range from 3.3 to 4.3 V. The galvano-charge current also leads to the error due to the semi-conductive character of LiMn₂O₄, and the maximal error may reach as much as one order of magnitude. In addition, the main approximations that lead to errors of CITT are qualitatively analyzed.     

Enhanced high-potential and elevated-temperature cycling stability of LiMn2O4 cathode by TiO2 modification for Li-ion battery

The surface of spinel LiMn₂O₄ was modified with TiO₂ by a simple sol–gel method to improve its electrochemical performance at elevated temperatures and higher working potentials. Compared with pristine LiMn₂O₄, surface-modification improved the cycling stability of the material. The capacity retention of TiO₂-modified LiMn₂O₄ was more than 85% after 60 cycles at high potential cycles between 3.0 and 4.8 V at room temperature and near to 90% after 30 cycles at elevated temperature of 55 °C at 1C charge–discharge rate. SEM studies shows that the surface morphology of TiO₂-modified LiMn₂O₄ was different from that of pristine LiMn₂O₄. Powder X-ray diffraction indicated that spinel was the only detected phase in TiO₂-modified LiMn₂O₄. Introduction of Ti into LiMn₂O₄ changed the electronic structures of the particle surface. Therefore a surface solid compound of LiTi_xMn_2−xO₄ may be formed on LiMn₂O₄. The improved electrochemical performance of surface-modified LiMn₂O₄ was attributed to the improved stability of crystalline structure and the higher Li⁺ conductivity.     

Synthesis of spinel LiMn2O4 with manganese carbonate prepared by micro-emulsion method

Micro-spherical particle of MnCO₃ has been successfully synthesized in CTAB-C₈H₁₈-C₄H₉OH-H₂O micro-emulsion system. Mn₂O₃ decomposed from the MnCO₃ is mixed with Li₂CO₃ and sintered at 800 °C for 12 h, and the pure spinel LiMn₂O₄ in sub-micrometer size is obtained. The LiMn₂O₄ has initial discharge specific capacity of 124 mAh g⁻¹ at discharge current of 120 mA g⁻¹ between 3 and 4.2 V, and retains 118 mAh g⁻¹ after 110 cycles. High-rate capability test shows that even at a current density of 16 C, capacity about 103 mAh g⁻¹ is delivered, whose power is 57 times of that at 0.2 C. The capacity loss rate at 55 °C is 0.27% per cycle.     

Enhanced electrochemical stability of Al-doped LiMn2O4 synthesized by a polymer-pyrolysis method

LiAl_xMn_2−xO₄ samples (x = 0, 0.02, 0.05, 0.08) were synthesized by a polymer-pyrolysis method. The structure and morphology of the LiAl_xMn_2−xO₄ samples calcined at 800 °C for 6 h were investigated by powder X-ray diffraction and scanning electron microscopy. The results show that all samples have high crystallinity, regular octahedral morphology and uniform particle size of 100-300 nm. The electrochemical performances were tested by galvanostatic charge-discharge and cyclic voltammetry. The results demonstrate that the Al-doped LiMn₂O₄ can be very well cycled at an elevated temperature of 55 °C without severe capacity degradation. In particular, the LiAl_0.08Mn_1.92O₄ sample demonstrates excellent capacity retention of 99.3% after 50 cycles at 55 °C, confirming the greatly enhanced electrochemical stability of LiMn₂O₄ by a small quantity of Al-doping.     

Synthesis of spherical spinel LiMn2O4 with commercial manganese carbonate

Spherical spinel LiMn₂O₄ particles were successfully synthesized from a mixture of manganese compounds containing commercial manganese carbonate by sintering of the spray-dried precursor. Different preparation routes were investigated to improve the tap density and to enhance the electrochemical performance of LiMn₂O₄. The structure and morphology of the LiMn₂O₄ particles were confirmed by X-ray diffraction (XRD) and scanning electron microscopy. The results showed that hollow spherical LiMn₂O₄ particles could be obtained when only commercial MnCO₃ was used as the manganese source. These particles had a low tap density (ca.0.8 g/cm³). Perfect micron-sized spherical LiMn₂O₄ particles with good electrochemical performance were obtained by spray-drying a slurry composed of MnCO₃, Mn(CH₃CHOO)₂ and LiOH, followed by a dynamic sintering process and a stationary sintering process. The as-prepared spherical LiMn₂O₄ particles comprised hundreds of nanosize crystal grains and had a high tap density(ca. 1.4 g/cm³). The galvanostatic charge-discharge measurements indicated that the spherical LiMn₂O₄ particles had an initial capacity of 121 mAh/g between 3.0 and 4.2 V at 0.2 C rate and still delivered a reversible capacity of 112 mAh/g at 2 C rate. The retention of capacity after 50 cycles was still 96% of its initial capacity at 0.2 C. All the results showed that the as-prepared spherical LiMn₂O₄ particles had an excellent electrochemical performances. The methods we used for preparing spherical LiMn₂O₄ are energy-saving and suitable for industrial application.     

Enhanced cycling stability of LiMn2O4 cathode by amorphous FePO4 coating

The cycling performance of LiMn₂O₄ at room and elevated temperatures is improved by FePO₄ modification through chemical deposition method. The pristine and FePO₄-coated LiMn₂O₄ materials are characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Their cycling performances are thoroughly investigated and compared. The 3 wt.% FePO₄-coated LiMn₂O₄ exhibits capacity losses of only 32% and 34% at room temperature and 55 °C, respectively, after 80 cycles, much better than those of the pristine material, 55% and 72%. The cyclic voltammograms at 55 °C reveal that the improvement in the cycling performance of FePO₄-coated LiMn₂O₄ electrodes can be attributed to the stabilization of spinel structures. The separation of FePO₄ between active materials and electrolyte and its interaction with SEI (solid electrolyte interphase) film are believed to account for the improved performances.     

Improvement of electrochemical stability of LiMn2O4 by CeO2 coating for lithium-ion batteries

CeO₂-coated LiMn₂O₄ spinel cathode was synthesized using two-step synthesis method. All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. The results of the electrochemical performances on CeO₂-coated electrode are compared to those of electrodes based on LiMn₂O₄ spinel without CeO₂ coating. CeO₂-coated LiMn₂O₄ cathode improved the cycling stability of the electrode. The capacity retention of 2 wt% CeO₂-coated LiMn₂O₄ was more than 82% after 150 cycles between 3.0 and 4.4 V at room temperature and 82% after 40 cycles at elevated temperature of 60 °C. The amounts of dissolved manganese-ions in CeO₂-coated LiMn₂O₄ significantly are smaller than pristine LiMn₂O₄ systems especially at elevated temperatures. Surface-modified LiMn₂O₄ can suppress the dissolution reaction of manganese-ions at elevated temperature and clearly improve the cyclability of the spinel LiMn₂O₄ cathode materials.     

Li-ion diffusion kinetics in LiMn2O4 thin films prepared by pulsed laser deposition

LiMn₂O₄ thin films were deposited on Au substrates by pulsed laser deposition (PLD). The Li-ion chemical diffusion coefficients of the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), potentiostatic intermittent titration technique (PITT), and electrochemical impedance spectroscopy (EIS). It was found that the values by CV and PITT were in the order of 10⁻¹³ cm² s⁻¹, and those by EIS and GITT were in the range of 10⁻¹³ to 10⁻¹¹ and 10⁻¹⁴ to 10⁻¹¹ cm² s⁻¹, respectively. These data were compared with the previously reported values.     

Synthesis of spherical LiMn2O4 cathode material by dynamic sintering of spray-dried precursors

Spherical LiMn₂O₄ particles were successfully synthesized by dynamically sintering spherical precursor powders, which were prepared by a slurry spray-drying method. The effect of the sintering process on the morphology of LiMn₂O₄ was studied. It was found that a one-step static sintering process combined with a spray-drying method could not be adopted to prepare spherical products. A two-step sintering procedure consisting of completely decomposing sprayed precursors at low temperature and further sintering at elevated temperature facilitated spherical particle formation. The dynamic sintering program enhanced the effect of the two-step sintering process in the formation of spherical LiMn₂O₄ powders. The LiMn₂O₄ powders prepared by the dynamic sintering process, after initially decomposing the spherical spray-dried precursor at 180 °C for 5 h and then sintering it at 700 °C for 8 h, were spherical and pure spinel. The as-prepared spherical material had a high tap density (ca. 1.6 g/cm³). Its specific capacity was about 117 mAh/g between 3.0 and 4.2 V at a rate of 0.2 C. The retention of capacity for this product was about 95% over 50 cycles. The rate capability test indicated that the retention of the discharge capacity at 4C rate was still 95.5% of its 0.2 rate capacity. All the results showed that the spherical LiMn₂O₄ product made by the dynamic sintering process had a good performance for lithium ion batteries. This novel method combining a dynamic sintering system and a spray-drying process is an effective synthesis method for the spherical cathode material in lithium ion batteries.     

A comparative electrochemical study of LiMn2O4 spinel thin-film and porous laminate

Novel Electrostatic Spray Deposition (ESD) technique was used to fabricate LiMn₂O₄ spinel thin-films. Cyclic voltammograms of both the ESD and porous laminate films show the double peaks in the 4.0 V range characteristic of the LiMn₂O₄ spinel materials. The porous laminates exhibit two semicircles in the impedance spectra while the ESD films show only one single semicircle. The diffusion time constant in the laminate films was typically one order of magnitude larger than that in the ESD thin-films. The apparent lithium-ion chemical diffusion coefficient in LiMn₂O₄ was found to be of the order of 10⁻⁹ cm²/s for both the porous laminate film and the ESD films despite the difference in the diffusion time constants.     

Zn-Al共掺和形貌调控制备LiMn2O4正极材料及电化学性能

采用固相燃烧法和不同的焙烧温度制备了600、650、700和750 ℃的LiZn_0.05Al_0.03Mn_1.92O₄材料。实验结果表明,Zn-Al复合掺杂和焙烧未改变LiMn₂O₄的晶体结构,样品结晶性随焙烧温度的升高而增加,650 ℃及以上时形成了较多包含高暴露{111}、小面积{110}和{100}晶面的截断八面体形貌晶粒,但750 ℃时部分样品发生分解。优化焙烧温度650 ℃的样品具有优良的倍率容量和容量保持率,在5 C和10 C下,初始放电比容量和1000次循环后容量保持率分别为101.3 mAh/g、81.5%和99.9 mAh/g、74.3%。CV和EIS表明,其具有较好的循环可逆性和较大的Li₊^{扩散系数。}Zn-Al共掺和形貌调控改性LiMn₂O₄正极材料有效抑制了LiMn₂O₄材料的Jahn-Teller效应,形成的截断八面体颗粒形貌降低了Mn的溶解,同时提供了更多的Li₊^{迁移三维通道},改善了材料的倍率容量及循环寿命。     

Investigations into capacity fading as a result of a Jahn-Teller distortion in 4 V LiMn2O4 thin film electrodes

This study reports the onset of the Jahn-Teller distortion in 4 V LiMn₂O₄ thin film electrodes that was investigated using an in situ bending beam method (BBM). The phase transformation during lithium insertion/extraction could be detected using the BBM technique. The phase transformation between the cubic and tetragonal phases was depicted by the larger value of the compressive or tensile differential strain, which is consistent with a well-known phase transformation between those phases in 3 V LiMn₂O₄. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously. The potential ranges responsible for the Jahn-Teller distortion in 4 V range, which takes place at the electrode surface, was determined by the charge versus. differential strain (dε/dQ) curve. The onset of the Jahn-Teller distortion was observed at the end of the cathodic scan, and the relaxation of the Jahn-Teller distortion was observed at the beginning of anodic scan. Furthermore, the onset of the Jahn-Teller distortion was found to be dependent on the lithium ion insertion rate, which was controlled by the scan rate.     

Synthesis of highly crystalline spinel LiMn2O4 by a soft chemical route and its electrochemical performance

Highly crystalline spinel LiMn₂O₄ was successfully synthesized by annealing lithiated MnO₂ at a relative low temperature of 600 °C, in which the lithiated MnO₂ was prepared by chemical lithiation of the electrolytic manganese dioxide (EMD) and LiI. The LiI/MnO₂ ratio and the annealing temperature were optimized to obtain the pure phase LiMn₂O₄. With the LiI/MnO₂ molar ratio of 0.75, and annealing temperature of 600 °C, the resulting compounds showed a high initial discharge capacity of 127 mAh g⁻¹ at a current rate of 40 mAh g⁻¹. Moreover, it exhibited excellent cycling and high rate capability, maintaining 90% of its initial capacity after 100 charge-discharge cycles, at a discharge rate of 5 C, it kept more than 85% of the reversible capacity compared with that of 0.1 C.     

Fractal study of LiMn2O4 film electrode surface for lithium batteries application

Fractal structure of a LiMn₂O₄ film electrode has been investigated and its fractal dimensions was determined using different electrochemical techniques, viz. cyclic voltammetry and chronoamperometry. The results obtained from both these methods are in good agreement indicating the reliability of the estimated D_f. The fractal study of the LiMn₂O₄ film electrode suggests a complex surface with high fractal dimension. In addition, length scales of the electrode surface were also calculated.     

Electrochemical stability of thin film LiMn2O4 cathode in organic electrolyte solutions with different compositions at 55 °C

A survey of the electrochemical stability of electrostatic spray deposited thin film of LiMn₂O₄ was performed in LiClO₄-EC-PC, LiBF₄-EC-PC, and LiPF₆-EC-PC solutions at 55 °C. The solution resistance, the surface film resistance, and the charge-transfer resistance were all found to depend on the electrolyte composition. Among the LiX-salts studied, the lowest charge transfer-resistance, and surface layer resistance were obtained in LiBF₄-EC-PC solution. There is no major influence of the electrolyte solution compositions upon lithium ion transport in the LiMn₂O₄ bulk at 55 °C. The diffusion coefficient of lithium in the solid phase varied within 10⁻¹⁰-10⁻⁸ cm² s⁻¹ in the three solutions. In general, it seems that in LiBF₄ solutions, the surface chemistry is the most stable in the three solutions examined, and hence the electrode impedance in LiBF₄ solutions was the lowest. In LiPF₆ solutions, HF seems to play an important role, and thus, the electrode impedance is relatively high due to the precipitation of surface LiF.     

Synthesis and electrochemical characterization of nano-CeO2-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

LiMn₂O₄ spinel cathode materials were coated with 0.5, 1.0, and 1.5 wt.% CeO₂ by a polymeric process, followed by calcination at 850 °C for 6 h in air. The surface-coated LiMn₂O₄ cathode materials were physically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron microscopy (XPS). XRD patterns of CeO₂-coated LiMn₂O₄ revealed that the coating did not affect the crystal structure or the Fd3m space group of the cathode materials compared to uncoated LiMn₂O₄. The surface morphology and particle agglomeration were investigated using SEM, TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 20 nm. The XPS data illustrated that the CeO₂ completely coated the surface of the LiMn₂O₄ core cathode materials. The galvanostatic charge and discharge of the uncoated and CeO₂-coated LiMn₂O₄ cathode materials were measured in the potential range of 3.0-4.5 V (0.5 C rate) at 30 °C and 60 °C. Among them, the 1.0 wt.% of CeO₂-coated spinel LiMn₂O₄ cathode satisfies the structural stability, high reversible capacity and excellent electrochemical performances of rechargeable lithium batteries.     

Synthesis of spherical LiMn2O4 microparticles by a combination of spray pyrolysis and drying method

Lithium manganese oxide (LiMn₂O₄) has been synthesized by a spray pyrolysis method from the precursor solution; LiNO₃ and Mn(NO₃)₂·6H₂O were stoichiometrically dissolved into distilled water. The synthesized LiMn₂O₄ particles exhibited a pure cubic spinel structure in the X-ray diffraction (XRD) patterns, however they were spherical hollow spheres for various concentrations of precursor solution. Thus, the as-prepared LiMn₂O₄ particles were then ground in a mortar and dispersed into distilled water. To make a well dispersed slurry solution, a dispersion agent was also added into the slurry solution. The LiMn₂O₄ microparticles with a spherical nanostructure were finally prepared by a spray drying method from the slurry solution. The tap density of the LiMn₂O₄ microparticle prepared by a combination of spray pyrolysis and drying method was larger than that by a conventional spray pyrolysis method.The as-prepared samples were sintered at 750 °C for 1 h in air and used as cathode active materials for lithium batteries. Test experiments in the electrochemical cell Li|1 M LiClO₄ in EC:DEC = 1:1|LiMn₂O₄ demonstrate that the sample prepared by the present method is a promising cathode active material for 4 V lithium-ion batteries at high-charge-discharge and elevated temperature operation. The LiMn₂O₄ compounds by the combination of spray pyrolysis and drying method are superior to that by the conventional spray pyrolysis method in terms of electrochemical characteristics and tap density.     

Co-precipitation synthesis of spherical Li1.05M0.05Mn1.9O4 (M = Ni, Mg, Al) spinel and its application for lithium secondary battery cathode

Metal-doped spinel lithium manganese oxides were successfully synthesized via co-precipitation. The synthesized Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) had highly crystalline cubic spinel phase with the space group Fd3m, as confirmed by X-ray diffraction study. From scanning electron microscopic observation, it was found that the prepared materials had spherical morphology with high tap-density. The Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) electrode exhibited improved cycling performance at elevated temperature. Especially, the Li_1.05Al_0.05Mn_1.9O₄ showed capacity retention of 91.5% during 100 cycles at elevated temperature (55 °C). Also the thermal stability of the Li_1.05M_0.05Mn_1.9O₄ (M = Ni, Mg, Al) was also significantly improved.