Low Temperature Synthesis of Nano-Sized Lithium Manganese Oxide Powder by the Sol-Gel Process Using PVA

Lithium manganese oxide (LiMn₂O₄) powder with spinel structure has been synthesized by a sol-gel method using an aqueous solution of metal nitrates containing polyvinyl alcohol (PVA). The role of PVA and the calcination conditions for the formation of LiMn₂O₄ have been studied. Homogeneity and reactivity of the precursor powder are enhanced with an increase in the amount of PVA in the starting solution. When the amount of PVA is low, an impurity phase-Mn₂O₃ is formed at low temperature. On the other hand, when the vinyl alcohol monomer unit of PVA to metal ion ratio is 2 : 1 in the starting solution, only spinel phase is formed at 180°C and organic-free LiMn₂O₄ powder is obtained at as low as 400°C. Nanosized LiMn₂O₄ particles with a narrow size distribution have been successfully prepared by this technique. This method with proper amount of PVA results in much lower calcination temperature and shorter calcination time for producing the single spinel phase in comparison with the conventional solid state reaction and other solution techniques.     

Comparison of different soft chemical routes synthesis of nanocrystalline LiMn2O4 and their influence on its physicochemical properties

A comparative study of nanocrystalline spinel LiMn₂O₄ powders prepared by two different soft chemical routes such as solution and sol-gel methods using lithium and manganese acetates are the precursors under different calcination temperatures. The dependence of the physicochemical properties of the spinel LiMn₂O₄ powder has been extensively investigated by using thermal analysis (TGA/DTA), FTIR, X-ray diffraction studies, SEM, specific surface area (BET) and electrical conductivity measurements. The results show that pure LiMn₂O₄ can be prepared from acetate precursors as starting materials at a low temperature of 600°C from solution route and 500°C from sol-gel method. The charge-discharge characteristics and the cycling behavior of Li/1M LiBF₄-EC/DEC electrolyte / LiMn₂O₄cells revealed that LiMn₂O₄ calcined at higher temperatures showed a high initial capacity, while the LiMn₂O₄calcined at lower temperatures exhibited a good cycling behavior.     

A Facile synthesis of flower-like Co3O4 porous spheres for the lithium-ion battery electrode

The porous hierarchical spherical Co₃O₄ assembled by nanosheets have been successfully fabricated. The porosity and the particle size of the product can be controlled by simply altering calcination temperature. SEM, TEM and SAED were performed to confirm that mesoporous Co₃O₄ nanostructures are built-up by numerous nanoparticles with random attachment. The BET specific surface area and pore size of the product calcined at 280 °C are 72.5 m² g⁻¹ and 4.6 nm, respectively. Our experiments further demonstrated that electrochemical performances of the synthesized products working as an anode material of lithium-ion battery are strongly dependent on the porosity.     

尖晶石型LiMn2-xInxO4(x=0，0.01，0.02，0.05)的合成及电化学性能研究

以醋酸锰、氢氧化锂和三氧化二铟为原料,以柠檬酸为配位剂,采用溶胶-凝胶法制备了掺杂In的尖晶石LiMn2-xInxO4(x=0,0.01,0.02,0.05),采用XRD、SEM对目标材料进行了结构和形貌表征,采用恒流充放电、循环伏安(CV)以及交流阻抗(EIS)谱测试对材料进行了电化学性能表征,考察了不同In掺杂量对材料性能的影响。结果表明,当In掺杂量为1%时,LiMn1.99In0.01O4样品具有纯的尖晶石锰酸锂结构,在0.5C和3.4～4.35 V电压范围条件下,LiMn1.99In0.01O4的初始放电容量为119.9 mAh.g-1,经过1C 30次,2C 30次,再0.5C 5次循环后,其放电容量保持率为84.9%,显示了良好的电化学性能。掺杂1%的In的样品比未掺杂的样品具有更优的高温循环稳定性能。     

溶胶-凝胶法合成Li1-xNaxMn2O4及其作为水系锂离子电池正极材料的电化学性能

用溶胶凝胶法合成了Na⁺离子掺杂的Li_1-xNa_xMn₂O₄(x=0,0.01,0.03,0.05)。X射线衍射图表明Na⁺取代Li⁺进入Li_1-xNa_x Mn₂O₄晶格中,扫描电镜图看出产物是粒径为100~300 nm的颗粒。恒流充放电测试结果表明,Li_0.97Na_0.03Mn₂O₄在2C倍率下循环100圈后放电容量保持率比未掺杂的LiMn₂O₄从51.2%提升到84.1%。循环伏安测试表明Na⁺离子掺杂降低了材料极化且增大了锂离子扩散系数。10C倍率下Li_0.97Na_0.03Mn₂O₄仍有79.0 mAh·g^-1的放电容量,高于未掺杂样品的52.1 mAh·g^-1。Na⁺离子掺杂可以稳定材料结构并提高锂离子扩散系数,从而提高LiMn₂O₄的电化学性能,是一种可行的改性方法。     

溶胶-凝胶法合成Li_(1-x)Na_xMn_2O_4及其作为水系锂离子电池正极材料的电化学性能

用溶胶凝胶法合成了Na+离子掺杂的Li_(1-x)Na_xMn_2O_4(x=0,0.01,0.03,0.05)。X射线衍射图表明Na+取代Li+进入Li_(1-x)Na_xMn_2O_4晶格中,扫描电镜图看出产物是粒径为100~300 nm的颗粒。恒流充放电测试结果表明,Li_(0.97)Na_(0.03)Mn_2O_4在2C倍率下循环100圈后放电容量保持率比未掺杂的LiMn_2O_4从51.2%提升到84.1%。循环伏安测试表明Na+离子掺杂降低了材料极化且增大了锂离子扩散系数。10C倍率下Li0.97Na0.03Mn2O4仍有79.0 m Ah·g-1的放电容量,高于未掺杂样品的52.1 m Ah·g~(-1)。Na+离子掺杂可以稳定材料结构并提高锂离子扩散系数,从而提高LiMn_2O_4的电化学性能,是一种可行的改性方法。     

Solid-solid interactions in pure and Li2O-doped manganese and magnesium mixed oxides system

The solid-solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li₂O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn₂O₃ and MgO were varied between 0.05:1 to 0.5:1. The mixed solids were calcined in air at 400-1000°C. The techniques employed were DTA, XRD and H₂O₂ decomposition at 20-40°C.The results obtained revealed that solid-solid interactions took place between the reacting solids at 600-1000°C yielding magnesium manganates (Mg₂MnO₄, Mg₆MnO₈, MgMnO₄ besides unreacted portions of MgO, Mn₂O₃ and Mn₃O₄). Li₂O-doping (0.75-6 mol%) of the investigated system followed by calcination at 600 and 800°C decreased progressively the intensity of the diffraction lines of Mn₂O₃ (Bixbyite) with subsequent increase in the lattice parameter 'a' of MgO to an extent proportional to the amount of Li₂O added. This finding might suggest that the doping process enhanced the dissolution of Mn₂O₃ in MgO forming solid solution. This treatment led also to the formation of Li₂MnO₃. Furthermore, the doping with 3 and 6 mol% Li₂O conducted at 800°C resulted in the conversion of Mn₂O₃ into Mn₃O₄, a process that took place at 1000°C in absence of Li₂O. The produced Li₂MnO₃ phase remained stable by heating at up to 1000°C. Furthermore, Li₂O doping of the investigated system at 400-1000°C resulted in a progressive measurable increase in the particle size of MgO.The catalytic activity measurements showed that the increase in the molar ratio of Mn₂O₃ in the samples precalcined at 400-800°C was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20°C (k _20°C) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800°C, respectively. Li₂O-doping of the samples having the formula 0.1 Mn₂O₃/MgO conducted at 400-600°C brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k _20°C due to Li₂O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600°C, respectively and opposite effect was found for the doped samples preheated at 800°C.This revised version was published online in November 2005 with corrections to the Cover Date.     

Electron microscope study of alkoxide-derived compositions within the PbZrO3-PbTiO3 phase diagram

Pb(Zr_0.50Ti_0.50)O₃ solid solution was prepared using lead acetate and transition metal n-propoxides in n-propanol or n-butoxides in n-butanol. The complex solutions were hydrolysed with an excess of H₂O. The resultant powders were calcined up to 700°C for 30 minutes in a flowing oxygen atmosphere. Scanning electron microscope analysis showed different morphologies of the resultant powders. The n-propoxide derived powder consisted of gel fragments, while the n-butoxide derived one had agglomerated submicrometre particles. EDS analysis of the powders revealed no chemical heterogeneities in the examined samples upon calcining up to 600°C, notwithstanding the type of precursor used. Both samples, calcined at 700°C, exhibited a slight deficiency of lead in the pyrochlore type phase as compared to the perovskite phase.     

Effects of Metal Ion Sources on Synthesis and Electrochemical Performance of Spinel LiMn2O4 Using Tartaric Acid Gel Process

The effect of lithium and manganese ions on the synthesis, phase purity, and electrochemical properties of tartaric acid gel processed lithium manganese oxide spinel were investigated. The poor bonding between both lithium and manganese ions with tartaric acid was shown by the FT-IR analysis when lithium nitrate and/or manganese nitrate were used as sources. Li₂MnO₃ and Mn₂O₃ impurities formed in addition to lithium manganese oxides when nitrate salts were used as the sources. When acetate salts were used as sources for the lithium and manganese ions, single-phase LiMn₂O₄ was obtained. These results indicate that homogeneous bonding between acetate salt and tartaric acid was formed. The capacity of single-phase LiMn₂O₄ calcined at 500°C was 117 mAh/g which was much higher than those containing Mn₂O₃ and Li₂MnO₃ impurity compounds. Thus, sources of lithium and manganese ions play an important role in the synthesis and electrochemical behaviors of lithium manganese oxide spinel.     

Preparation of spherical spinel LiMn2O4 cathode material for lithium ion batteries

A novel process is proposed for synthesis of spinel LiMn₂O₄ with spherical particles from the inexpensive materials MnSO₄, NH₄HCO₃, and NH₃H₂O. The successful preparation started with carefully controlled crystallization of MnCO₃, leading to particles of spherical shape and high tap density. Thermal decomposition of MnCO₃ was investigated by both DTA and TG analysis and XRD analysis of products. A precursor of product, spherical Mn₂O₃, was then obtained by heating MnCO₃. A mixture of Mn₂O₃ and Li₂CO₃ was then sintered to produce LiMn₂O₄ with retention of spherical particle shape. It was found that if lithium was in stoichiometric excess of 5% in the calcination of spinel LiMn₂O₄, the product had the largest initial specific capacity. In this way spherical particles of spinel LiMn₂O₄ were of excellent fluidity and dispersivity, and had a tap density as high as 1.9 g cm^–3 and an initial discharge capacity reaching 125 mAh g^–1. When surface-doped with cobalt in a 0.01 Co/Mn mole ratio, although the initial discharge capacity decreased to 118 mAh g^–1, the 100th cycle capacity retention reached 92.4% at 25°C. Even at 55°C the initial discharge capacity reached 113 mAh g^–1 and the 50th cycle capacity retention was in excess of 83.8%.     

The formation and physicochemical characterization of Al2O3-doped manganese ferrites

Mn/Fe mixed oxide solids doped with Al₂O₃ (0.32-1.27 wt.%) were prepared by impregnation of manganese nitrate with finely powdered ferric oxide, then treated with different amounts of aluminum nitrate. The obtained samples were calcined in air at 700-1000 °C for 6 h. The specific surface area (S_BET) and the catalytic activity of pure and doped precalcined at 700-1000 °C have been measured by using N₂ adsorption isotherms and CO oxidation by O₂. The structure and the phase changes were characterized by DTA and XRD techniques. The obtained results revealed that Mn₂O₃ interacted readily with Fe₂O₃ to produce well-crystallized manganese ferrite (MnFe₂O₄) at temperatures of 800 °C and above. The degree of propagation of this reaction increased by Al₂O₃-doping and also by increasing the heating temperature. The treatment with 1.27 wt.% Al₂O₃ followed by heating at 1000 °C resulted in complete conversion of Mn/Fe oxides into the corresponding ferrite phase. The catalytic activity and S_BET of pure and doped solids were found to decrease, by increasing both the calcination temperature and the amount of Al₂O₃ added, due to the enhanced formation of MnFe₂O₄ phase which is less reactive than the free oxides (Mn₂O₃ and Fe₂O₃). The activation energy of formation (ΔE) of MnFe₂O₄ was determined for pure and doped solids. The promotion effect of aluminum in formation of MnFe₂O₄ was attributed to an effective increase in the mobility of reacting cations.     

A new approach to the preparation of microcrystalline ZnNb2O6 photocatalysts via a water-soluble niobium-citrate-peroxo compound

A water-soluble niobium-citrate-peroxo compound was synthesized by using Nb₂O₅ as a precursor. This niobium compound solution was successfully applied to the preparation of microcrystalline ZnNb₂O₆ photocatalysts via a water-based sol-gel method. The results indicated that pure ZnNb₂O₆ could be obtained in a temperature range from 750 to 950 °C. The absorption edge of ZnNb₂O₆ located at about 305 nm, corresponding to a band gap of ca. 4.06 eV. The photocatalytic activities of the as-prepared samples for the methyl orange degradation were evaluated under UV light (λ = 254 nm). It was found that the sample obtained at 850 °C showed the highest photocatalytic activity due to its suitable surface area and crystallinity.     

TPR investigations on the reducibility of Cu supported on Al2O3, zeolite Y and SAPO-5

Reducibility of Cu supported on Al₂O₃, zeolite Y and silicoaluminophosphate SAPO-5 has been investigated in dependence on the Cu content using a method combining conventional temperature programmed reduction (TPR) by hydrogen with reoxidation in N₂O followed by a second the so-called surface-TPR (s-TPR). The method enables discrimination and a quantitative estimation of the Cu oxidation states +2, +1 and 0. The quantitative results show that the initial oxidation state of Cu after calcination in air at 400 °C, independent on the nature of the support, is predominantly +2. Cu²⁺ supported on Al₂O₃ is quantitatively reduced by hydrogen to metallic Cu⁰. Comparing the TPR of the samples calcined in air and that of samples additionally pre-treated in argon reveals that in zeolite Y and SAPO-5 Cu²⁺ cations are stabilized as weakly and strongly forms. In both systems, strongly stabilized Cu²⁺ ions are not auto-reduced by pre-treatment in argon at 650 °C, but are reduced in hydrogen to form Cu⁺. The weakly stabilized Cu²⁺ ions, in contrast, may be auto-reduced by pre-treatment in argon at 650 °C forming Cu⁺ but are reduced in hydrogen to metallic Cu⁰.     

NMR and FT-IR Investigation of Spinel LiMn2O4 Cathode Prepared by the Tartaric Acid Gel Process

The structure of the lithium manganese tartrate precursor and the synthesis mechanism of LiMn₂O₄ were investigated by FT-IR, NMR, TG/DSC, and XRD in this study. The results of FT-IR and ⁷Li and ¹³C NMR measurements revealed that lithium ions bond with carboxylic acid ligands and the O–H stretching modes of tartaric acid. Manganese ion bonds only with carboxylic acid. Lithium and manganese ions were trapped homogeneously on an atomic scale throughout the precursor. Such a structure eliminates the need for long-range diffusion during the formation of lithium manganese oxides. Therefore, spinel LiMn₂O₄ was synthesized at temperatures as low as 300°C. In this work, the electrochemical properties of Li/Li_xMn₂O₄ were studied. It is clear that the discharge curves exhibit two pseudo plateaus as the LiMn₂O₄ is fired to higher temperatures. The discharge capacity of LiMn₂O₄ increases from 84 to 117 mAh/g as the calcination temperature increases from 300 to 500°C. The LiMn₂O₄ powders calcined at low temperatures with a high specific surface area and an average valence of manganese exhibit a better cycle life.     

Influence of the crystallinity on the Li+ intercalation process in Nb2O5 films

Nb₂O₅ thin films were prepared by the Pechini method. The effect of the film crystallinity on the electrochemical and electrochromic properties was investigated. A relationship between the crystalline structure and the Li⁺ intercalation/extraction process, stability and kinetics was observed. A significant decrease in the electrochemical response was observed as a function of the number of cycles for films treated at 400 and 450 °C. However, as the calcination temperature increases this effect disappears. XRD studies shown that at 400 °C, the material is amorphous, evolving to orthorhombic phase. The transmittance variation as well as the coloration efficiency increases as the temperature is increased. In the initial cycles the intercalation charge is higher for the amorphous oxide than for the orthorhombic phase. However, the variation in the optical density is small. On the other hand, the charge of the orthorhombic phase oxide does not change. These results suggest that there are two different processes associated with Li⁺ intercalation, but only one of them leads to the coloration process.     

Preparation and performances of nanosized Ta2O5 powder photocatalyst

Nanosized-Ta₂O₅ powder photocatalyst was successfully synthesized by using sol-gel method via TaCl₅ butanol solution as a precursor. Ta₂O₅ species can be formed under 500 °C via the decomposition of the precursor. The crystalline phase of Ta₂O₅ powder photocatalyst can be obtained after being calcined above 600 °C for 4 h. The crystal size and particle size of Ta₂O₅ powder photocatalyst was about 50 nm. A good photocatalytic performance for the degradation of gaseous formaldehyde was obtained for the nanosized-Ta₂O₅ powder. The Ta₂O₅ powder formed at 700 °C for 4 h and at 650 °C for 12 h showed the best performance. The calcination temperature and time play an important role in the crystallization and photocatalytical performance of nanosized-Ta₂O₅ powder.     

Comparison of structure and electrochemical properties for 5 V LiNi0.5Mn1.5O4 and LiNi0.4Cr0.2Mn1.4O4 cathode materials

Spinel LiNi_0.5Mn_1.5O₄ and LiMn_1.4Cr_0.2Ni_0.4O₄ cathode materials have been successfully synthesized by the sol–gel method using citric acid as a chelating agent. The structure and electrochemical performance of these as-prepared powders have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the galvanostatic charge–discharge test in detail. XRD results show that there is a small Li _y Ni_1-y O impurity peak placed close to the (4 0 0) line of the spinel LiNi_0.5Mn_1.5O₄, and LiMn_1.4Cr_0.2Ni_0.4O₄ has high phase purity, and the powders are well crystallized. SEM indicates that LiMn_1.4Cr_0.2Ni_0.4O₄ has a slightly smaller particle size and a more regular morphological structure with narrow size distribution than those of LiNi_0.5Mn_1.5O₄. Galvanostatic charge–discharge testing indicates that the initial discharge capacities of LiMn_1.4Cr_0.2Ni_0.4O₄ and LiNi_0.5Mn_1.5O₄ cycled at 0.15 C are 129.6 and 130.2 mAh g⁻¹, respectively, and the capacity losses compared to the initial value, after 50 cycles, are 2.09% and 5.68%, respectively. LiMn_1.4Cr_0.2Ni_0.4O₄ cathode has a higher electrode coulombic efficiency than that of the LiNi_0.5Mn_1.5O₄ cathode, implying that Ni and Cr dual substitution is beneficial to the reversible intercalation and de-intercalation of Li⁺.     

锂离子电池正极材料LiMn2O4的低热固相合成与性能表征

锂离子电池具有比能量高、环境污染小等优点,广泛应用于手提电话、便携式电脑、摄像机等设备中。其正极材料的研究是锂离子电池的研究重点。层状结构的LiCoO2、LiNiO2和尖晶石结构的LiMn2O4是仅有的三种能在3.5V以上电位可嵌入Li的正极材料[1~3]。目前市售的锂离子电池主要采用LiCoO2作正极材料,但由于Co资源缺乏和价格相对昂贵,而锰资源丰富,价格低廉且无毒,对环境友好,因此世界各国都在大力进行以LiMn2O4为正极材料的锂离子电池的实用化研究。LiMn2O4传统的制备方法是高温固相反应合成法[4~7],但由于Mn的变价多,与Li形成贫Li或…     

异质双碳源对LiMn0.8Fe0.2PO4复合材料电化学性能的影响

采用水基流变相辅助的固相法,以异质碳蔗糖和石墨为碳源,合成了LiMn_0.8Fe_0.2PO₄/C复合材料,研究了不同石墨加入方式对所制复合材料电化学性能的影响,并对所制备的LiMn_0.8Fe_0.2PO₄/C复合材料进行了X射线衍射(XRD)、N₂吸附-脱附测试、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等表征。结果表明,不同石墨包覆工艺对材料结构和电化学性能具有显著影响。前驱体煅烧后再加入石墨获得的样品纯度高,形貌呈均一的椭圆形,在0.1C下的放电比容量为149 mAh·g^-1,达到其理论比容量的87%;在5C下最大的放电比容量为133 mAh·g^-1;在2C倍率下经过300次循环后比容量维持在127 mAh·g^-1,衰减率仅为1.9%,表现出了优良的循环稳定性。     

异质双碳源对LiMn0.8Fe0.2PO4复合材料电化学性能的影响

采用水基流变相辅助的固相法,以异质碳蔗糖和石墨为碳源,合成了LiMn_0.8Fe_0.2PO₄/C复合材料,研究了不同石墨加入方式对所制复合材料电化学性能的影响,并对所制备的LiMn_0.8Fe_0.2PO₄/C复合材料进行了X射线衍射(XRD)、比表面积测试、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等表征。结果表明,不同石墨包覆工艺对材料结构和电化学性能具有显著影响。前驱体煅烧后再加入石墨获得的样品纯度高,形貌呈均一的椭圆形,在0.1C下的放电比容量为149 mAh·g^-1,达到其理论比容量的 87%;在 5C 下最大的放电比容量为 133 mAh·g^-1;在 2C 倍率下经过 300 次循环后比容量维持在 127 mAh·g^-1,衰减率仅为1.9%,表现出了优良的循环稳定性。