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.     

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.     

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.     

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.     

尖晶石锰酸锂的柠檬酸辅助溶胶-凝胶法合成及电化学表征

以醋酸锰、氢氧化锂为原料,以柠檬酸为络合剂,n(柠檬酸):n(锂)=1:1,采用柠檬酸辅助溶胶-凝胶法制备了富锂尖晶石Li1+xMn2O4 (x=0,0.02,0.05,0.07),采用TG-DTA、XRD、SEM分别对前驱体和目标材料进行了表征,采用恒流充放电及循环伏安(CV)测试对材料进行了电化学性能表征,考察了不...     

Synthesis and electrochemical characterizations of nano-scaled Zn doped LiMn2O4 cathode materials for rechargeable lithium batteries

The LiZn_xMn_2−xO₄ (x = 0.00-0.15) cathode materials for rechargeable lithium-ion batteries were synthesized by simple sol-gel technique using aqueous solutions of metal nitrates and succinic acid as the chelating agent. The gel precursors of metal succinates were dried in vacuum oven for 10 h at 120 °C. After drying, the gel precursors were ground and heated at 900 °C. The structural characterization was carried out by X-ray powder diffraction and X-ray photoelectron spectroscopy to identify the valance state of Mn in the synthesized materials. The sample exhibited a well-defined spinel structure and the lattice parameter was linearly increased with increasing the Zn contents in LiZn_xMn_2−xO₄. Surface morphology and particle size of the synthesized materials were determined by scanning electron microscopy and transmission electron microscopy, respectively. Electrochemical properties were characterized for the assembled Li/LiZn_xMn_2−xO₄ coin type cells using galvanostatic charge/discharge studies at 0.5 C rate and cyclic voltammetry technique in the potential range between 2.75 and 4.5 V at a scan rate of 0.1 mV s⁻¹. Among them Zn doped spinel LiZn_0.10Mn_1.90O₄ has improved the structural stability, high reversible capacity and excellent electrochemical performance of rechargeable lithium batteries.     

Characterization of Al-doped spinel LiMn2O4 thin film cathode electrodes prepared by Liquid Source Misted Chemical Deposition (LSMCD) technique

A novel process called Liquid Source Misted Chemical Deposition (LSMCD) was used to synthesize Al-doped LiMn₂O₄ cathode films for Lithium microbatteries. The cathode films were characterized by XRD, SEM, cyclic volatmmetry, and charge/discharge test. LiMn_1.8Al_0.2O₄ film crystallized at 800 °C in rapid thermal annealing (RTA) for 5 min under oxygen atmosphere exhibited more improved electrochemical rechargeability than spinel LiMn₂O₄ film because the substitution of Al³⁺ for Mn³⁺ increased Mn---O bonding strength in the spinel framework and suppressed the two-phase behavior of the unsubstituted spinel during the intercalation/deintercalation that is the origin of the failure mechanism in the 4 V region. As a result, LiMn_1.8Al_0.2O₄ film showed an initial discharge capacity of 52 μAh/cm² μm and no capacity fade over 100 cycles.     

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.     

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.     

Effects of surface modification on the cycling stability of LiNi0.8Co0.2O2 electrodes by CeO2 coating

A high-performance LiNi_0.8Co_0.2O₂ cathode was successfully fabricated by a sol-gel coating of CeO₂ to the surface of the LiNi_0.8Co_0.2O₂ powder and subsequent heat treatment at 700 °C for 5 h. The surface-modified and pristine LiNi_0.8Co_0.2O₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), slow rate cyclic voltammogram (CV), and differential scanning calorimetry (DSC). Unlike pristine LiNi_0.8Co_0.2O₂, the CeO₂-coated LiNi_0.8Co_0.2O₂ cathode exhibits no decrease in its original specific capacity of 182 mAh/g (versus lithium metal) and excellent capacity retention (95% of its initial capacity) between 4.5 and 2.8 V after 55 cycles. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion 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.     

Aluminothermal synthesis and characterization of Li3V2−xAlx(PO4)3 cathode materials for lithium ion batteries

Monoclinic Li₃V_2−xAl_x(PO₄)₃ with different Al³⁺ doping contents (x = 0, 0.05, 0.08, 0.10 and 0.12) have been prepared by a facile aluminothermal reaction. Aluminum nanoparticles have been used as source for Al³⁺ and nucleus for Li₃V_2−xAl_x(PO₄)₃ nucleation as well as reducing agent in the aluminothermal strategy. The products were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and electrochemical methods. The XRD results show that the as-obtained Li₃V_2−xAl_x(PO₄)₃ has a phase-pure monoclinic structure, irrespective of the Al³⁺ doping concentration. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) results reveal that the charge-transfer resistance of the Li₃V₂(PO₄)₃ is reduced and the reversibility is enhanced after V³⁺ substituted by Al³⁺. In addition, The Li₃V_2−xAl_x(PO₄)₃ phases exhibit better cycling stability than the pristine Li₃V₂(PO₄)₃.     

Comparison of 4 V and 3 V electrochemical properties of nanocrystalline LiMn2O4 cathode particles in lithium ion batteries prepared by ultrasonic spray pyrolysis

Nanocrystalline LiMn₂O₄ particles were prepared by an ultrasonic spray pyrolysis method using nitrate salts at 800 °C in air atmosphere. Particle properties were characterized by the X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy. In addition, cyclic voltammetry and galvanostatic tests were performed to investigate the effects of structure on electrochemical behavior of both the 4 V and 3 V potential plateaus. Particle characterization studies show that the nanocrystalline particles have spinel structure of submicron size with spherical morphology. Particles, ranging between 75 and 1250 nm, were formed by aggregation of nanoparticles. Discharge capacity of LiMn₂O₄ particles between 3.0 and 4.5 V is 70 mA h g⁻¹ and cumulative capacity between 2.2 and 4.5 V is 111 mA h g⁻¹ at 0.5 C rate. Discharge capacity at the 4 V potential region reduces to 47% of initial capacity, whereas cumulative capacity fade is 62% after 100 cycles at 0.5 C rate. Although nanocrystalline LiMn₂O₄ cathode particles exhibit good rate capability at the 4 V plateau, capacity decreased rapidly by increasing C- rates and cycling between 2.2 and 4.5 V. The loss of capacity can be attributed to phase transformation and dissolution of electrode material. Particle characterization of used cathodes showed that nanocrystalline LiMn₂O₄ electrodes partly dissolve during electrochemical cycling.     

Preparation and electrochemical properties of LiCoO2-LiNi0.5Mn0.5O2-Li2MnO3 solid solutions with high Mn contents

The solid solutions LiCoO₂-LiNi_1/2Mn_1/2O₂-Li₂MnO₃ with higher Mn content have been prepared by a spray drying method between 750 and 950 °C and their electrochemical performances have also been characterized. The effects of the Li content on the structure and electrochemical performance of the samples have been studied. It was found that their lattice parameters a, c and V increase with the increase in Ni content and the decrease in Co content. The solid solutions xLiCoO₂-yLiNi_1/2Mn_1/2O₂-(1−x−y)Li₂MnO₃ with x = 0.18, 0.27 and y = 0.2 have the largest discharge capacity, which is more than 200 mAh/g in the voltages of 3.0-4.6 V. It is believed that the optimum Co content x in xLiCoO₂-yLiNi_1/2Mn_1/2O₂-(1−x−y)Li₂MnO₃ is between 0.2 and 0.3 in the charge-discharge voltage range of 3.0-4.6 V. The solid solutions xLiCoO₂-yLiNi_1/2Mn_1/2O₂-(1−x−y)Li₂MnO₃ with x = 0.18-0.36 and y = 0.2 have the excellent cycling performance and the capacity retention attains to almost 100% after 50 cycles. Moreover, it is found that the discharge capacity gradually increases with the increment of cycle number especially in the initial 10 cycles. XRD showed that the layered structure has been kept all the time in 20 cycles, which is perhaps the reason why the sample has the excellent cycling performance.     

Preparation and electrochemical properties of LiMn2O4 by the microwave-assisted rheological phase method

Pure-phase and well-crystallized spinel LiMn₂O₄ powders as cathode materials for lithium-ion batteries were successfully synthesized by a new simple microwave-assisted rheological phase method, which was a timesaving and efficient method. The physical properties of the as-synthesized samples compared with the pristine LiMn₂O₄ obtained from the rheological phase method were investigated by thermogravimetry analysis (TGA), X-ray diffraction (XRD) and scanning electronic microscope (SEM). The as-prepared powders were used as positive materials for lithium-ion battery, whose charge/discharge properties and cycle performance were examined in detail. The powders resulting from the microwave-assisted rheological phase method were pure, spinel structure LiMn₂O₄ particles of regular shapes with distribution uniformly, and exhibited promising electrochemical properties for battery. Furthermore, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the reactions of Li-ion insertion into and extraction from LiMn₂O₄ electrode.     

Electrochemical insertion and extraction of lithium-ion at nano-sized LiMn2O4 particles prepared by a spray pyrolysis method

Nano-sized LiMn₂O₄ particles were prepared at 1023 K by electrospray pyrolysis in which they were directly deposited on a Pt substrate in gas phase. Cyclic voltammetry gave very sharp and symmetrical redox peaks at ca. 4.0 and 4.1 V vs. Li/Li⁺ owing to the insertion and extraction of lithium-ion at LiMn₂O₄. However, the redox peaks broadened and their peak separation in an electrode potential increased when aggregated nano-sized LiMn₂O₄ particles were used. In Nyquist plots, a semi-circle due to lithium-ion transfer resistance appeared at potentials above 3.90 V. The values of the lithium-ion transfer resistances were small for dispersed nano-sized LiMn₂O₄ particles. On the other hand, the lithium-ion transfer resistances increased and the Warburg impedance became obvious as the nano-sized LiMn₂O₄ particles aggregated. These results clearly indicate that the apparent rapid diffusion of lithium-ion can be attained using well-dispersed nano-sized particles of electroactive materials.     

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.     

Improved electrochemical properties of Li1.2Ni0.18Mn0.59Co0.03O2 by surface modification with LiCoPO4

Lithium-rich nickel–manganese–cobalt oxide, Li_1.2Ni_0.18Mn_0.59Co_0.03O₂, prepared by spray-dry process, exhibits rapid capacity fade and poor rate capability. The surface of Li_1.2Ni_0.18Mn_0.59Co_0.03O₂ can be modified with LiCoPO₄ through co-precipitation method in order to improve its electrochemical properties. The resultant LiCoPO₄ particles are in nano-scale and accumulate on the surface of the Li_1.2Ni_0.18Mn_0.59Co_0.03O₂ particles. The surface modification by LiCoPO₄ is shown to significantly improve both the cyclic performance and the rate capability of Li_1.2Ni_0.18Mn_0.59Co_0.03O₂.     

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.     

Electrochemical performance and local cationic distribution in layered LiNi1/2Mn1/2O2 electrodes for lithium ion batteries

LiNi_1/2Mn_1/2O₂ electrodes with layered structure were synthesized by solid-state reaction between lithium hydroxide and mixed Ni,Mn oxides obtained from co-precipitated Ni,Mn carbonates and hydroxides and freeze-dried Ni,Mn citrates. The temperature of the solid-state reaction was varied between 800 and 950 °C. This method of synthesis allows obtaining oxides characterized with well-crystallized nanometric primary particles bounded in micrometric aggregates. The extent of particle agglomeration is lower for oxides obtained from freeze-dried Ni,Mn citrates. The local Mn⁴⁺ surrounding in the transition metal layers was determined by X-band electron paramagnetic resonance (EPR) spectroscopy. It has been found that local cationic distribution is consistent with α,β-type cationic order with some extent of disordering that depends mainly on the precursors used. The electrochemical extraction and insertion of lithium was tested in lithium cells using Step Potential Electrochemical Spectroscopy. The electrochemical performance of LiNi_1/2Mn_1/2O₂ oxides depends on the precursors used, the synthesis temperature and the potential range. The best electrochemical response was established for LiNi_1/2Mn_1/2O₂ prepared from the carbonate precursor at 900 °C. The changes in local environment of Mn⁴⁺ ions during electrochemical reaction in both limited and extended potential ranges were discussed on the basis of ex situ EPR experiments.