Columnar-grown porous films of lithium manganese oxide spinel (LiMn2O4) prepared by ultrasonic spray deposition

Using LiNO₃ and Mn(Ac)₂ as raw materials, ultrasonic spray deposition (USD) technique was used to fabricate LiMn₂O₄ films on platinum substrate at different substrate temperatures from 310 to 390 °C. The prepared thick films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical performance of the USD-derived films was also evaluated with LiMn₂O₄/Li cells. It is found that all of the LiMn₂O₄ films are porous and composed of orderly oriented columnar particles. The substrate temperature affects the fine microstructure of the columnar particles. The film prepared at 360 °C substrate temperature give rise to best electrochemical behavior.     

Improvement of electrochemical performance of all-solid-state lithium secondary batteries by surface modification of LiMn2O4 positive electrode

To improve the electrochemical performance of an all-solid-state In/80Li₂S⋅20P₂S₅ (electrolyte)/LiMn₂O₄ cell, a lithium-titanate thin film was used to coat LiMn₂O₄. The interfacial resistance between LiMn₂O₄ and the electrolyte (measured after initial charging) decreased when the LiMn₂O₄ particles were coated with lithium-titanate. A cell with lithium-titanate-coated LiMn₂O₄ had a higher capacity than a cell with noncoated LiMn₂O₄ for current densities in the range 0.064 to 2.6 mA cm^− 2. Additionally, a cell with coated LiMn₂O₄ retained 96% of the 10th-cycle reversible capacity at a current density of 0.064 mA cm^− 2 after 50 cycles.     

Novel approach to preparation of LiMn2O4 core/LiNixMn2−xO4 shell composite

Combining two methods, coating and doping, to modify spinel LiMn₂O₄, is a novel approach we used to synthesize active material. First we coated the LiMn₂O₄ particles with the nickel oxide particles by means of homogenous precipitation, and then the nickel oxide-coated LiMn₂O₄ was calcined at 750 °C to form a LiNi_xMn_2−xO₄ shell on the surface of spinel LiMn₂O₄ particles. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cyclic voltammetry (CV) and charge-discharge test were performed to characterize the spinel LiMn₂O₄ before and after modification. The experimental results indicated that a spinel LiMn₂O₄ core is surrounded by a LiNi_xMn_2−xO₄ shell. The resulting composite showed excellent electrochemical cycling performance with an average fading rate of 0.014% per cycle. This improved cycle stability is greatly attributed to the suppression of Jahn-Teller distortion on the surface of spinel LiMn₂O₄ particles during cycling.     

Spinel LiMn2O4 active material with high capacity retention

Heating the mixture of LiMn₂O₄ and NiO at 650 °C was employed to enhance the cyclability of spinel LiMn₂O₄. The results of scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy analyses implied that a LiNi_xMn_2−xO₄ solid solution was formed on the surface of LiMn₂O₄ particles. And charge-discharge tests showed that the enhancement of the capacity retention of modified LiMn₂O₄ is significant, maintained 97.2% of the maximum capacity after 100 cycles at charge and discharge rate of C/2, while the pure one only 75.2%. The modified LiMn₂O₄ also results in a distinct improvement in rate capability, even at the rate of 12C. The improvement of electrochemical cycling stability is greatly attributed to the suppression of Jahn-Teller distortion at the surface of spinel LiMn₂O₄ particles.     

Surface modification of LiMn2O4 thin films at elevated temperature

In order to improve the cycle stability of spinel LiMn₂O₄ electrode at elevated temperature, the LiCoO₂-coated and Co-doped LiMn₂O₄ film were prepared by an electrostatic spray deposition (ESD) technique. LiCoO₂-coated LiMn₂O₄ film shows excellent cycling stability at 55 °C compared to pristine and Co-doped LiMn₂O₄ films. The samples were studied by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The excellent performance of LiCoO₂-coated LiMn₂O₄ film can be explained by suppression of Mn dissolution. On the other hand, the LiCoO₂-layer on the LiMn₂O₄ surface allows a homogenous Li⁺ insertion/extraction during electrochemical cycles and improves its structure stability.     

Effect of temperature on lithium-ion intercalation kinetics of LiMn1.5Ni0.5O4-positive-electrode material

LiMn_1.5Ni_0.5O₄ is synthesized by a sol–gel method and the intercalation kinetics as positive electrode for lithium-ion batteries is investigated by EIS. LiMn_1.5Ni_0.5O₄ particles prepared via sol–gel process possess spinel phase with Fd-3m space group. The charge-transfer resistance, the exchange-current density and the solid-phase diffusion are found as a function of temperature. The apparent activation energy of the exchange current, the charge transfer, and the lithium diffusion in solid phase are also determined, respectively. This result indicates that the effect of the temperature on the cell capacity and the current dependence of the capacity results mainly from the enhancement of the lithium diffusion at elevated temperatures. It can be concluded that LiMn_1.5Ni_0.5O₄ cell has a bad rate cycling performance at elevated temperatures before any modification due to the high diffusion apparent activation energy. The relevant theoretical elucidations thus provide us some useful insights into the design of novel LiMn_1.5Ni_0.5O₄-based positive-electrode materials.     

Nanocrystalline and long cycling LiMn2O4 cathode material derived by a solution combustion method for lithium ion batteries

A nanostructured LiMn₂O₄ spinel phase is used as a cathode for 4 V lithium batteries and is prepared by solution combustion synthesis using urea as a fuel. Lithium-manganese oxides have received more increasing attention in recent years as high-capacity intercalation cathodes for rechargeable lithium-ion batteries. Nanostructured electrodes have been shown to enhance the cell cyclability. For optimum synthesis, the spinel LiMn₂O₄ showed that the optimal heat treatment protocol was a 10 h calcination at 700 °C, which sustained 229 cycles between 3.0 and 4.3 V at a charge-discharge rate of 0.1 °C before reaching an 80% charge retention cut-off value. X-ray diffraction and electron diffraction pattern investigations demonstrate that all the LiMn₂O₄ products are a spinel phase crystal. TEM micrographs show the prepared products were highly crystalline with an average particle size of 20-50 nm. Cyclic voltammetry shows the absence of phase transitions in the samples ensures negligible strain, resulting in a longer cycle life. This work shows the feasibility of the solution combustion method for obtaining manganese oxides with nano-architecture and high cyclability, and suggests it is a promising method for providing small diffusion pathways that improve lithium-ion intercalation kinetics and minimize surface distortions during cycling.     

Atomic force microscopy study of surface morphology change in spinel LiMn2O4: Possibility of direct observation of Jahn-Teller instability

Surface morphology in 3.5 × 3.5 μm² area of spinel LiMn₂O₄, which is a typical cathode material for Li ion secondary batteries, is studied using an atomic force microscopy (AFM) with a conductive probe. Negative bias voltage is applied to the probe to attract Li⁺ ions toward LiMn₂O₄ surface during the AFM observation. Before applying the voltage (0 V), the whole LiMn₂O₄ surface is covered with scale-shaped grains. Under the negative voltage of 5.5 V, electric current abruptly increases, indicating Li⁺ ionic conduction. Simultaneously, part of the scale-shaped grains expand and flatten. Jahn-Teller phase transition, which is induced by the repulsive interaction between the Mn-e_g and O-2p electrons in Li accumulated layer, is proposed as a possible origin of these results.     

Advanced electrochemical performance of LiMn1.4Cr0.2Ni0.4O4 as 5 V cathode material by citric-acid-assisted method

Spinel LiMn₂O₄ and LiMn_1.4Cr_0.2Ni_0.4O₄ cathode materials were successfully synthesized by the citric-acid-assisted sol-gel method with ultrasonic irradiation stirring. The structure and electrochemical performance of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometer, cyclic voltamogram (CV) and the galvanostatic charge-discharge test in detail. XRD shows that all the samples have high phase purity, and the powders are well crystallized. SEM exhibits that LiMn_1.4Cr_0.2Ni_0.4O₄ has more uniform cubic-structure morphology than that of LiMn₂O₄. EDX reveals that a small amount of Mn³⁺ still exists in LiMn_1.4Cr_0.2Ni_0.4O₄. The galvanostatic charge-discharge test indicates that the initial discharge capacities for the LiMn_1.4Cr_0.2Ni_0.4O₄ and LiMn₂O₄ at 0.15 C discharge rates are 130.8 and 130.2 mAh g⁻¹, respectively. After 50 cycles, their capacity are 94.1% and 85.1%, respectively. The CV curve implies that Ni and Cr dual substitutions are beneficial to the reversible intercalation and deintercalation of Li⁺, and suppress Mn³⁺ generation at high temperatures and provide improved structural stability.     

Characterization of crystallized LiMn2O4 thin films grown by pulsed laser deposition

LiMn₂O₄ thin films were grown on stainless steel substrates at 625°C and 100?mTorr of oxygen by pulsed laser deposition. The deposited film was highly crystallized with an average crystal size of about 260?nm. The initial discharge capacity of the film was about 53.8?µAh?cm^?2?µm^?1 and the capacity decayed at an average rate of about 0.29% per cycle when the film was cycled between 3.0 and 4.5?V vs. Li/Li⁺, with a current density of 20?µA?cm^?2. It was observed that the grains became smaller and the boundaries of grains became obscure after 100 cycles, indicating that manganese dissolution via loss of MnO may be the main factor leading to the capacity fade in pure thin film LiMn₂O₄ electrodes. The apparent diffusion coefficient of Li ions, obtained from cyclic voltammetry scans, was of the order of 10^?12?cm²?s^?1. High charge-transfer resistance was observed at high potentials. Ex-situ X-ray diffraction (XRD) and Raman spectroscopy were used to investigate the structure changes of LiMn₂O₄ thin film with intercalation/de-intercalation of lithium. XRD results revealed a relatively small lattice change with the removal of lithium in crystallized thin film, compared to that of powder LiMn₂O₄ cathode.     

Direct micron-sized LiMn2O4 particle coating on LiCoO2 cathode material using surfactant

Micron-sized LiMn₂O₄ particles were easily coated on LiCoO₂ cathodes using an amphoteric gelatin surfactant at pH4–5. The coated LiCoO₂ material showed a significantly higher thermal stability during charging and capacity retention on cycling at 4.6 V, compared to the bare LiCoO₂.     

Effect of reaction temperature on morphology and electrochemical behavior

LiCoO₂ and LiMn₂O₄ compounds were synthesized using two different methods, viz., low-temperature-aided hydrothermal and high-temperature-assisted co-precipitation method. Keeping the reaction parameters such as type of precursors chosen and the medium of reaction as same for both the hydrothermal and co-precipitation methods, the effect of temperature in producing LiCoO₂ and LiMn₂O₄ with varying physical as well as electrochemical properties has been studied. As expected, the effect of low-temperature-involved hydrothermal method rendered finer particles of nanocrystalline nature with minimum strain, and the high-temperature synthesis of co-precipitation method produced slightly enhanced particle size with an increased strain value. The effect of size-grown particles resulting from co-precipitation method exhibited inferior electrochemical properties such as increasing resistance of the cell upon cycling and a significant decline in capacity behavior, irrespective of LiCoO₂ or LiMn₂O₄ cathodes. On the other hand, hydrothermal synthesis of LiCoO₂ and LiMn₂O₄ has exhibited acceptable specific capacity with an admissible capacity fade behavior and negligible internal resistance of the cell, thus qualifying the same as better-performing cathodes. Hence, the effect of low temperature in producing LiCoO₂ and LiMn₂O₄ cathodes with facile intercalation and de-intercalation of lithium is demonstrated.     

The effect of cation doping on spinel LiMn2O4: a first-principles investigation

The effect of the cation doping on the electronic structure of spinel LiM_yMn_2−yO₄ (M=Cr, Mn, Fe, Co and Ni) has been calculated by first-principles. Our calculation shows that new M-3d bands emerge in the density of states compared with that in LiMn₂O₄. Simultaneously, the new O-2p bands appear accordingly in almost the same energy range around the Fermi energy owing to the M-3d/O-2p interaction. It is found that the appearance of new O-2p bands in the lower energy position results in a higher intercalation voltage. Consequently, the origin of higher intercalation voltage in LiM_yMn_2−yO₄ can be ascribed to the lower O-2p level introduced by the doping cation M.     

Al掺杂的尖晶石型LiMn2O4的结构和电子性质

采用基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)和GGA+U方法下对尖晶石型LiMn₂O₄及其Al掺杂 的尖晶石型LiAl_0.125Mn_1.875O₄晶体的结构和电子性质进行了计算. 结果表明: 采用GGA方法得到尖晶石型LiMn₂O₄是立方晶系结构, 其中的Mn离子为+3.5价, 无法解释它的Jahn-Teller 畸变. 给出的LiMn₂O₄能带结构特征也与实验结果不符. 而采用GGA+U方法得到在低温下的LiMn₂O₄和其掺杂 体系LiAl_0.125Mn_1.875O₄的晶体都是正交结构, 与实验一致. 也能明确地确定Mn的两种价态Mn³⁺/Mn⁴⁺的分布并且能够说明Mn³⁺O₆的z方向有明显的Jahn-Teller 畸变, 而Mn⁴⁺O₆则没有畸变. LiMn₂O₄的能带结构与实验比较也能够符合. 采用GGA+U方法对Al掺杂体系的LiAl_0.125Mn_1.875O₄的研究表明, 用Al替换一个Mn不会明显地改变晶体的电子性质, 但可以有效地消除Al³⁺O₆ 八面体的Jahn-Teller畸变, 从而改善正极材料LiMn₂O₄的性能, 这与电化学实验的观察结果相一致.     

Precursor derived LiMn2O4 thin films as ionic conductor

Thin films of spinel LiMn₂O₄ have been fabricated using a metallorganic precursor. Crystalline films have been deposited on Au substrates to exhibit as the cathode in rechargeable thin film lithium batteries. The nucleation and growth of spinel LiMn₂O₄ crystallites were investigated with heat treatment of the deposited thin films. Film capacity density as high as 22 μAh/cm² was measured for LiMn₂O₄. The film heat treated at 700 °C were cycled electrochemically up to 30 cycles against Li metal without any degradation of the capacity. There were neither open area nor amorphous layers which prevent the Li⁺ions transfer at the boundaries in the LiMn₂O₄ thin film. The microscopic study revealed that (111) planes in the two grains directly bonded at the grain boundary which could proceed the lithium ion intercalation or deintercalation smoothly.     

Electron energy-loss spectroscopy of LiMn2O4, LiMn1.6Ti0.4O4 and LiMn1.5Ni0.5O4

The dielectric properties of LiMn₂O₄, LiMn_1.6Ti_0.4O₄ and LiMn_1.5Ni_0.5O₄ powders, synthesized by sol-gel method, were determined by analyzing the low-loss region of the electron energy-loss spectroscopy (EELS) spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) was obtained by Kramers-Kronig analysis. Since maxima observed in the OJDS spectra are assigned to interband transitions above the Fermi level, these spectra can be interpreted on the basis of calculated density of states (DOS), carried out with the CASTEP code. Experimental and theoretical results are in good agreement.     

Characterization and electrochemical performance of the spinel LiMn2O4 prepared from -MnO2

The preparation and characterization of the spinel LiMn₂O₄ obtained by solid state reaction from quasi-amorphous -MnO₂ is reported. A well-defined highly pure spinel was characterized from X-ray diffractograms. The average manganese valence of -MnO₂ and spinel samples was found to be 3.89±0.01 and 3.59±0.01, respectively. The electrochemical performance of the spinel was evaluated through cyclic voltammetry and chronopotentiometry. The voltammetric profiles obtained at 1 mV/s for the LiMn₂O₄ electrode in 1 M LiClO₄ dissolved in a 2:1 mixture of ethylene carbonate and dimethyl carbonate showed typical peaks for the lithium insertion/extraction reactions. The charge capacity of this electrode was found to be 110 mA h g⁻¹ for the first charge/discharge cycles.     

A galvanostatic study of lithium intercalation in RuO2

The diffusion process in a host structure has been studied at constant current with the cells: RuO₂ composite/ LiClO₄PEO/Li and RuO₂ powder/LiClO₄PC/Li. The effect of grain size distribution and temperature has been investigated and the diffusion coefficient for Li in RuO₂ calculated along with the diffusion activation energy (0.52 eV). Results show no intercalation of PC at 25°C.     

Multi-faceted highly crystalline LiMn2O4 and LiNi0.5Mn1.5O4 cathodes synthesized by a novel carbon exo-templating method

LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ powders have been synthesized by a novel cost-effective carbon exo-templating process. It has been observed that controlled nucleation in the pores of highly surface active carbon produces a distinct effect on the powder morphology and crystallinity. Quantitative X-ray phase analyses show single phase spinel structure having Fd3m symmetry for both samples. Field emission electron microscopy reveals particles of size 0.5–1.0 µm with well defined multi-faceted crystals. Cyclic voltammetry results show well separated distinct redox peaks at 4.05/3.92 and 4.17/4.08 V for LiMn₂O₄/Li and 4.91/4.61 V for LiNi_0.5Mn_1.5O₄/Li coin cells indicating good crystallinity and reversibility of the cathodes compared to that of pristine LiMn₂O₄ synthesized by conventional combustion process. The LiMn₂O₄/Li and LiNi_0.5Mn_1.5O₄/Li cells deliver an initial discharge capacity of 110 mA h/g and 122 mA h/g respectively at a current density of 0.05 mA/cm² and when cycled at 0.2 mA/cm², the cells maintain 81% and 96% of their initial discharge capacity respectively even after 20 cycles. On the other hand, at the same current density, LiMn₂O₄ synthesized by conventional combustion process suffers from severe capacity fading (only 37.5% capacity retention after the 25th cycle). The capacity fading rate is found to be very less even at further higher current densities (0.4–0.8 mA/cm²) for both LiMn₂O₄/Li and LiNi_0.5Mn_1.5O₄/Li cells synthesized by the templating process. The present study reveals that high crystallinity along with multi-faceted morphology shows a remarkable enhancement in capacity as well as rate performance of pristine LiMn₂O₄ and its Ni derivative.     

Ab initio studies on n-type and p-type Li4Ti5O12

This paper studies the structure and electronic properties of Li4Ti5O12, as anode material for lithium ion batteries, from first principles calculations. The results suggest that there are two kinds of unit cell of Li4Ti5O12: n-type and p-type. The two unit cells have different structures and electronic properties:the n-type with two 16d site Li ions is metallic by electron, while the p-type with three 16d Li ions is metallic by hole. However, the Li4Ti5O12 is an insulator. It is very interesting that one n-type cell and two p-type cells constitute one Li4Ti5O12 supercell which is insulating. The results show that the intercalation potential obtained with a p-type unit cell with one additional electron is quite close to the experimental value of 1.5 V.