石墨烯/尖晶石LiMn2O4纳米复合材料制备及电化学性能

采用溶胶凝胶法和还原氧化石墨法制备尖晶石LiMn2O4纳米晶和石墨烯纳米片,并采用冷冻干燥法制备了石墨烯/尖晶石LiMn2O4纳米复合材料,利用XRD、SEM、AFM等对其结构及表面形貌进行表征;利用CV、充放电、EIS研究纳米复合材料的电化学性能和电极过程动力学特征。结果表明:纳米LiMn2O4电极材料及其石墨烯掺杂纳米复合材料的放电比容量分别为107.16 mAh.g-1,124.30 mAh.g-1,循环100周后,对应容量保持率为74.31%和96.66%,石墨烯可显著改善尖晶石LiMn2O4电极材料的电化学性能,归结于其良好的导电性。纳米复合材料EIS上感抗的产生与半导体尖晶石LiMn2O4不均匀地分布在石墨烯膜表面所造成局域浓差有关,并提出了感抗产生的模型。     

Lithium‐Ion‐Transfer Kinetics of Single LiMn2O4 Particles

A stochastic investigation of lithium deinsertion from individual 200‐nm‐sized particles of LiMn₂O₄ reveals the rate‐determining step at high overpotentials to be the transfer of the cation across the particle–electrolyte interface. Measurement of the (electro)chemical behavior of the spinel is undertaken without forming a conductive composite electrode. The kinetics of the interfacial ion transfer defines a theoretical upper limit for the discharge rates of batteries using LiMn₂O₄ in an aqueous environment.     

The effect of nanolayer AlF<Subscript>3</Subscript> coating on LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cycle life in high temperature for lithium secondary batteries

The surface of the spinel LiMn₂O₄ was coated with AlF₃ by a chemical process to improve its electrochemical performance at high temperatures. The morphology and structure of the original and AlF₃-coated LiMn₂O₄ samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM). All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. It was found that the surfaces of the original LiMn₂O₄ samples were covered with a nanolayer AlF₃ after the treatment. The charge/discharge of the materials were carried at 220 mA/g in the range of 3.0 and 4.4 V at 55°C. While the original LiMn₂O₄ showed 17.8% capacity loss in 50 cycles at 55°C, the AlF₃-coated LiMn₂O₄ (118.1 mA h/g) showed only 3.4% loss of the initial capacity (122.3 mA h/g) at 55°C. It is obvious that the improvement in cycling performance of the coated-LiMn₂O₄ electrode at 55°C is attributed to the presence of AlF₃ on the surface of LiMn₂O₄. Published in Russian in Elektrokhimiya, 2009, Vol. 45, No. 7, pp. 817–819. The article is published in the original     

Comparison of different soft chemical routes synthesis of submicro-LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> and their influence on its electrochemical properties

A comparative study of submicro-crystalline spinel LiMn₂O₄ powders prepared by two different soft chemical routes such as hydrothermal and sol–gel methods is made. The dependence of the physicochemical properties of the spinel LiMn₂O₄ powder has been extensively investigated by using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscope, cyclic voltammogram, charge–discharge test, and electrochemical impedance spectroscopy (EIS). The results show that the electrochemical performances of spinel LiMn₂O₄ depend strongly upon the synthesis method. The LiMn₂O₄ powder prepared by hydrothermal route has higher specific capacity and better cycling performance than the one synthesized from sol–gel method. The former has the max discharge capacity of 114.36 and 99.78 mAh g⁻¹ at the 100th cycle, while the latter has the max discharge capacity of 98.67 and 60.25 mAh g⁻¹ at the 100th cycle. The selected equivalent circuit can fit well the EIS results of synthesized LiMn₂O₄. For spinel LiMn₂O₄ from sol–gel method and hydrothermal route in the first charge process R _SEI remain almost invariable, R _e and R _ct first decreasing and then increasing with the increase of polarization potential.     

Biomimetic Synthesis of Metal Ion‐Doped Hierarchical Crystals Using a Gel Matrix: Formation of Cobalt‐Doped LiMn2O4 with Improved Electrochemical Properties through a Cobalt‐Doped MnCO3 Precursor

We have synthesized spinel type cobalt‐doped LiMn₂O₄ (LiMn_2?yCo_yO₄, 0≤y≤0.367), a cathode material for a lithium‐ion battery, with hierarchical sponge structures via the cobalt‐doped MnCO₃ (Mn_1‐xCo_xCO₃, 0≤x≤0.204) formed in an agar gel matrix. Biomimetic crystal growth in the gel matrix facilitates the generation of both an homogeneous solid solution and the hierarchical structures under ambient condition. The controlled composition and the hierarchical structure of the cobalt‐doped MnCO₃ precursor played an important role in the formation of the cobalt‐doped LiMn₂O₄. The charge–discharge reversible stability of the resultant LiMn_1.947Co_0.053O₄ was improved to ca. 12 % loss of the discharge capacity after 100 cycles, while pure LiMn₂O₄ showed 24 % loss of the discharge capacity after 100 cycles. The parallel control of the hierarchical structure and the composition in the precursor material through a biomimetic approach, promises the development of functional materials under mild conditions.     

Ge4+、Sn4+掺杂尖晶石LiMn2O4的合成与电化学表征

使用Ge⁴⁺、Sn⁴⁺作为掺杂离子, 通过高温固相法制备四价阳离子掺杂改性的尖晶石LiMn₂O₄材料. X射线衍射(XRD)和扫描电子显微镜(SEM)分析表明, Ge⁴⁺离子取代尖晶石中Mn⁴⁺离子形成了LiMn_2-xGe_xO₄ (x=0.02,0.04, 0.06)固溶体; 而Sn⁴⁺离子则以SnO₂的形式存在于尖晶石LiMn₂O₄的颗粒表面. Ge⁴⁺离子掺入到尖晶石LiMn₂O₄材料中, 抑制了锂离子在尖晶石中的有序化排列, 提高了尖晶石LiMn₂O₄的结构稳定性; 而在尖晶石颗粒表面的SnO₂可以减少电解液中酸的含量, 抑制酸对LiMn₂O₄活性材料的侵蚀. 恒电流充放电测试表明, 两种离子改性后材料的容量保持率均有较大幅度的提升, 有利于促进尖晶石型LiMn₂O₄锂离子电池正极材料的商业化生产.     

Synthesis and characterization of submicron size particles of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> by microemulsion route

Among the various positive electrode materials investigated for Li-ion batteries, spinel LiMn₂O₄ is one of the most important materials. Small particles of the active materials facilitate high-rate capability due to large surface to mass ratio and small diffusion path length. The present work involves the synthesis of submicron size particles of LiMn₂O₄ in a quaternary microemulsion medium. The precursor obtained from the reaction is heated at different temperatures in the range from 400 to 900 °C. The samples heated at 800 and 900 °C are found to possess pure spinel phase with particle size <200 nm, as evidenced from XRD, SEM, and TEM studies. The electrochemical characterization studies provide discharge capacity values of about 100 mAh g⁻¹ at C/5 rate, and there is a moderate decrease in capacity by increasing the rate of charge–discharge cycling. Studies also include charge–discharge cycling and ac impedance studies in temperature range from −10 to 40 °C. Impedance data are analyzed with the help of an equivalent circuit and a nonlinear least squares fitting program. From temperature dependence of charge-transfer resistance, a value of 0.62 eV is obtained for the activation energy of Mn³⁺/Mn⁴⁺ redox process, which accompanies the intercalation/deintercalation of the Li⁺ ion in LiMn₂O₄.     

Electrochemical performances of Li–Mn spinel synthesized from nanosized chemical manganese dioxide

Li–Mn spinel (LiMn₂O₄) is prepared from several chemical manganese dioxides (CMDs) and the electrochemical performances are compared. The CMDs are synthesized via a redox process involving the simultaneous reduction and oxidation reaction of manganese ions in starting materials to converge to MnO₂. KMnO₄ is used as an oxidizing agent and various manganese(II) salts as reducing agents in the redox process. The crystalline phase, particle size and shape, and specific surface area (SSA) of the CMDs and the Li–Mn spinels are examined. The influence of the starting manganese salts on the above properties is also investigated. The CMDs form as aggregates of nanosized particles with different crystalline phase according to the starting manganese salts. Pure Li–Mn spinel phase is obtained from the CMDs irrespective of the different crystalline phases. The charge and discharge profiles of the LiMn₂O₄ are measured and compared.     

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。     

Surface modifications of Li-ion battery electrodes with various ultrathin amphoteric oxide coatings for enhanced cycleability

Ultrathin ZnO, ZrO₂, and Al₂O₃ surface coatings are deposited via atomic layer deposition (ALD) with high conformality and atomic scale thickness control to enhance the electrochemical performance of LiMn₂O₄ for applications in lithium ion batteries. Two types of ALD-modified LiMn₂O₄ electrodes are fabricated: one is ALD-coated LiMn₂O₄ composite electrode and the other is electrode composed of ALD-coated LiMn₂O₄ particles and uncoated carbon/polyvinylidenefluoride network. Cycling performance and cyclic voltammetric patterns reveal that ZnO ALD coating is the most effective protective film for improving the electrochemical performance of LiMn₂O₄ at either 25 or 55 °C, followed by ZrO₂ and Al₂O₃. After 100 electrochemical cycles in 1 C at 55 °C, the electrode consisting of LiMn₂O₄ particles coated with six ZnO ALD layers (as thin as ~1 nm) delivers the highest final capacity, more than twice that of the bare electrode. It is also found that amphoteric oxide coating on LiMn₂O₄ particles can enhance the cycleability of LiMn₂O₄ more effectively than coating on the composite electrode. Furthermore, for ALD coating either on the composite electrode or on LiMn₂O₄ particles, the effect of oxide ALD modification for improving capacity retention and increasing specific capacity of LiMn₂O₄ is more phenomenal at elevated temperature than at room temperature.     

Improving the high-temperature performance of LiMn2O4 spinel electrodes by coating the active mass with MgO via a sonochemical method

LiMn₂O₄ spinel coated sonochemically with MgO was studied as an active mass in composite cathodes in standard Li electrolyte solutions for Li-ion batteries at 60 °C. Solutions comprising 1 M LiPF₆ in a mixture of ethylene, dimethyl, and diethyl carbonates (EC–DMC–DEC; 2:2:1) were used. It was possible to obtain LiMn₂O₄ particles fully covered by porous magnesia films that allow a free transport of Li-ions. Electrodes comprising LiMn₂O₄, modified by MgO showed a higher capacity retention compared to the electrodes comprising an uncoated active mass, specially at elevated temperatures. We suggest that the presence of an MgO film on the surface of the LiMn₂O₄ particles, reduces the detrimental effect of HF contamination present in LiPF₆ solutions, reduces the electrodes’ impedance and improves their kinetics.     

Facile synthesis of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> microsheets with porous micro-nanostructure as high-rate cathode materials for Li-ion batteries

Porous LiMn₂O₄ microsheets with micro-nanostructure have been successfully prepared through a simple carbon gel-combustion process with a microporous membrane as hard template. The crystal structure, morphology, chemical composition, and surface analysis of the as-obtained LiMn₂O₄ microsheets are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscope (XPS). It can be found that the as-prepared LiMn₂O₄ sample presents the two-dimensional (2-D) sheet structure with porous structure comprised with nano-scaled particles. As cathode materials for lithium-ion batteries, the obtained LiMn₂O₄ microsheets show superior rate capacities and cycling performance at various charge/discharge rates. The LiMn₂O₄ microsheets exhibit a higher charge and discharge capacity of 137.0 and 134.7 mAh g^?1 in the first cycle at 0.5 C, and it remains 127.6 mAh g^?1 after 50 cycles, which accounts for 94.7% discharge capacity retention. Even at 10 C rate, the electrode also delivers the discharge capacity of 91.0 mAh g^?1 after 300 cycles (93.5% capacity retention). The superior electrochemical properties of the LiMn₂O₄ microsheets could be attributed to the unique microsheets with porous micro-nanostructure, more active sites of the Li-ions insertion/deinsertion for the higher contact area between the LiMn₂O₄ nano-scaled particles and the electrolyte, and better kinetic properties, suggesting the applications of the sample in high-power lithium-ion batteries.     

Excellent cycling stability of spherical spinel LiMn2O4 by Y2O3 coating for lithium-ion batteries

The Y₂O₃ nano-film is coated on the surface of the spherical spinel LiMn₂O₄ by precipitation method and subsequent heat treatment at 550 °C for 5 h in air. The structure and performance of the bare LiMn₂O₄ and Y₂O₃-coated LiMn₂O₄ are characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive analysis X-ray spectroscopy, galvanostatic charge–discharge, cyclic voltammetry, and impedance spectroscopy. It has been found that the addition of Y₂O₃ does not change the bulk structure of LiMn₂O₄, and the thickness of the Y₂O₃ coating layer is approximate to 3.0 nm. The 1 wt% Y₂O₃-coated LiMn₂O₄ electrode reveals excellent cycling performance with 80.3 % capacity retention after 500 cycles at 1 C at 25 °C. When cycling at elevated temperature 55 °C, the as-prepared sample still shows 76.7 % capacity retention after 500 cycles. These remarkable improvements indicate that thin Y₂O₃ coating on the surface of LiMn₂O₄ is an effective way to improve the electrochemistry performance. Besides, the suppression of Mn dissolution into the electrolyte via the Y₂O₃ coating layer can be accounted for the improved performances.     

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%.     

Improvement in stability of LiMn2O4 thin-film electrodes by oxygen-plasma irradiation to precursor gel

LiMn₂O₄ thin-film electrodes were prepared by the sol?Cgel method combined with oxygen-plasma irradiation. Oxygen plasma with a power of 10 or 90?W was irradiated to the precursor thin film prepared from lithium acetate, manganese acetate tetrahydrate and polyvinylpyrrolidone on a Pt plate, and then it was fired at 723 or 973?K. X-ray diffraction and Raman measurements indicated that oxygen-plasma irradiation was effective to increase the crystallinity of the resulting LiMn₂O₄. Atomic force microscope observation showed that the particle size of LiMn₂O₄ in the resulting thin-film electrode was decreased and homogeneous distribution of LiMn₂O₄ particles was achieved. Oxidation of the electrolyte at higher potentials was suppressed and capacity retention at 328?K was dramatically improved for the LiMn₂O₄ thin-film electrode obtained at 973?K. The improved electrochemical stability is ascribed to the elimination of organic materials from precursor by oxygen-plasma irradiation.     

Electrochemical properties of tetravalent Ti-doped spinel LiMn2O4

Ti-doped spinel LiMn₂O₄ is synthesized by solid-state reaction. The X-ray photoelectron spectroscopy and X-ray diffraction analysis indicate that the structure of the doped sample is Li( Mn^{3 +} Mn_{1 - x} ^{4 +} Ti_x^{4 +} )O₄ {\hbox{Li}}\left( {{\hbox{M}}{{\hbox{n}}^{3 + }}{\hbox{Mn}}_{1 - x\,}^{4 + }{\hbox{Ti}}_x^{4 + }} \right){\hbox{O}}{}_4 . The first principle-based calculation shows that the lattice energy increases as Ti doping content increases, which indicates that Ti doping reinforces the stability of the spinel structure. The galvanostatic charge–discharge results show that the doped sample LiMn_1.97Ti_0.03O₄ exhibits maximum discharge capacity of 135.7 mAh g⁻¹ (C/2 rate). Moreover, after 70 cycles, the capacity retention of LiMn_1.97Ti_0.03O₄ is 95.0% while the undoped sample LiMn₂O₄ shows only 84.6% retention under the same condition. Additionally, as charge–discharge rate increases to 12C, the doped sample delivers the capacity of 107 mAh g⁻¹, which is much higher than that of the undoped sample of only 82 mAh g⁻¹. The significantly enhanced capacity retention and rate capability are attributed to the more stable spinel structure, higher ion diffusion coefficient, and lower charge transfer resistance of the Ti-doped spinel.     

尖晶石LiMn2O4高温电化学容量衰减及改进

综述了高温下尖晶石LiMn2O4容量衰减的原因、机理研究和改进它的高温性能的方法以及目前的进展,且指出了可能的提高它的高温性能的途径。     

Synthesis of Single‐Crystalline Spinel LiMn2O4 Nanorods for Lithium‐Ion Batteries with High Rate Capability and Long Cycle Life

The long‐standing challenge associated with capacity fading of spinel LiMn₂O₄ cathode material for lithium‐ion batteries is investigated. Single‐crystalline spinel LiMn₂O₄ nanorods were successfully synthesized by a template‐engaged method. Porous Mn₃O₄ nanorods were used as self‐sacrificial templates, into which LiOH was infiltrated by a vacuum‐assisted impregnation route. When used as cathode materials for lithium‐ion batteries, the spinel LiMn₂O₄ nanorods exhibited superior long cycle life owing to the one‐dimensional nanorod structure, single‐crystallinity, and Li‐rich effect. LiMn₂O₄ nanorods retained 95.6 % of the initial capacity after 1000 cycles at 3C rate. In particular, the nanorod morphology of the spinel LiMn₂O₄ was well‐preserved after a long‐term cycling, suggesting the ultrahigh structural stability of the single crystalline spinel LiMn₂O₄ nanorods. This result shows the promising applications of single‐crystalline spinel LiMn₂O₄ nanorods as cathode materials for lithium‐ion batteries with high rate capability and long cycle life.     

One-step hydrothermal synthesis of LiMn2O4 cathode materials for rechargeable lithium batteries

LiMn₂O₄ cathode materials with high discharge capacity and good cyclic stability were prepared by a simple one-step hydrothermal treatment of KMnO₄, aniline and LiOH solutions at 120–180 °C for 24 h. The aniline/KMnO₄ molar ratio (R) and hydrothermal temperature exhibited an obvious influence on the component and phase structures of the resulting product. The precursor KMnO₄ was firstly reduced to birnessite when R was less than 0.2:1 at 120–150 °C. Pure-phased LiMn₂O₄ was formed when R was 0.2:1, and the LiMn₂O₄ was further reduced to Mn₃O₄ when R was kept in the range of 0.2–0.3 at 120–150 °C. Moreover, LiMn₂O₄ was fabricated when R was 0.15:1 at 180 °C. Octahedron-like LiMn₂O₄ about 300 nm was prepared at 120 °C, and particle size decreased with an increase in hydrothermal temperature. Especially, LiMn₂O₄ synthesized at 150 °C exhibited the best electrochemical performance with the highest initial discharge capacity of 127.4 mAh g⁻¹ and cycling capacity of 106.1 mAh g⁻¹ after 100 cycles. The high discharge capacity and cycling stability of the as-prepared LiMn₂O₄ cathode for rechargeable lithium batteries were ascribed to the appropriate particle size and larger cell volume.     

Synthesis and electrochemical characterization of duo doped spinels (zinc and praseodymium) for use in lithium rechargeable batteries

LiMn₂O₄ and LiZn_xPr_yMn_2?x?yO₄ (x = 0.10–0.24; y = 0.01–0.10) powders have been synthesized by sol–gel method using palmitic acid as chelating agent. The synthesized samples have been subjected to thermo gravimetric and differential thermal analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX). The sol–gel route bestows low calcination temperature, shorter heating time, high purity, good control over stoichiometry, small particle size, high surface area, good surface morphology and better homogeneity, The XRD patterns reveal high degree of crystallinity and better phase purity. SEM and TEM images exhibit nano-sized nature particles with good agglomeration. EDAX peaks of Zn, Pr, Mn and O have been confirmed in actual compositions of LiMn₂O₄ and LiZn_xPr_yMn_2?x?yO₄. Charge–discharge studies of pristine spinel LiMn₂O₄ heated at 850 °C delivers discharge capacity of 132 mA h g^?1 corresponding to columbic efficiency of 73 % during the first cycle. At the end of 10th cycles, it delivers maximum discharge capacity of 112 mA h g^?1 with columbic efficiency of 70 % and capacity fade of 0.15 mA h g^?1 cycle^?1 over the investigated 10 cycles. Inter alia, all dopants concentrations, LiZn_0.10Pr_0.10Mn_1.80O₄ exhibits the better cycling performance (1st cycle discharge capacity: 130 mA h g^?1 comparing to undoped spinel 132 mA h g^?1) corresponding to columbic efficiency of 73 % with capacity fade of 0.12 mA h g^?1 cycle^?1.