Effect of ultrasonic irradiation on structure and electrochemical properties of LiMn2O4 thin films cathode material for Li-ion micro-batteries

Two kinds of spinel LiMn₂O₄ thin film for lithium ion micro-batteries were successfully prepared on polycrystal Pt substrates by spin coating methods, which were carried out under ultrasonic irradiation (USG) and magnetic stirring (MSG), respectively. The microstructures and electrochemical performance of LiMn₂O₄ thin films were characterized by thermogravimetry analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge-discharge measurements. It was found that the crystalline structure of USG samples grew better than that of the MSG samples. At the same time, higher discharge capacity and better cycle stability were obtained for the LiMn₂O₄ thin films of USG at the current density of 50 μAh/cm² between 3.0 and 4.3 V. The 1st discharge capacity was 57.8 μAh/cm²-μm for USG thin films and 51.7 μAh/cm²-μm for MSG thin films. After 50 cycles, 91.4% and 69% of discharge capacity could be retained respectively, indicating that ultrasonic irradiation condition during spin coating was more suitable for preparing spinel LiMn₂O₄ thin films with better electrode performance for lithium ion micro-batteries.     

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.     

LiMn2O4 films grown by pulsed-laser deposition

LiMn₂O₄ films have been deposited onto silicon wafer by pulsed-laser deposition (PLD) technique in order to test their reliability as cathode materials in rechargeable lithium microbatteries. The film formation has been studied as a function of the preparation conditions, i.e., composition of the target, substrate temperature, and oxygen partial pressure in the deposition chamber. Depending on the conditions of deposition, Mn₂O₃ was present as an impurity phase. When deposited onto silicon substrate maintained at 300 °C in an oxygen pressure of 100 mTorr from the target LiMn₂O₄+15 % Li₂O, the PLD films are well-textured with crystallite size of 300 nm. It is found that such a film crystallizes in the spinel structure (Fd3m symmetry) as evidenced by x-ray diffraction and Raman scattering measurements. Surface morphologies of layers were investigated by SEM. The cells Li//LiMn₂O₄ have been tested by cyclic voltammetry and galvanostatic charge-discharge techniques in the range 3.0–4.2 V. The voltage profiles show the two expected steps for Li_xMn₂O₄ with a specific capacity as high as 120 mC/cm² μm. The chemical diffusion coefficients for the Li_xMn₂O₄ thin films appear to be in the range of 10⁻¹¹-10⁻¹² cm²/s. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Structural and electrical properties of lithium manganese oxide thin films grown by pulsed laser deposition

LiMn₂O₄ thin films were deposited by reactive pulsed laser deposition technique and studied the microstructural and electrical properties of the films. The LiMn₂O₄ thin films deposited in an oxygen partial pressure of 100 mTorr and at a substrate temperature of 573 K from a lithium rich target were found to be nearly stoichiometric. The films exhibited predominantly (111) orientation representing the cubic spinel structure with Fd3m symmetry. The intensity of (111) peak increased and a slight shift in the peak position was observed with the increase of substrate temperature. The lattice parameter increased from 8.117 to 8.2417 Å with the increase of substrate temperature from 573 to 873 K. The electrical conductivity of the films is observed to be a strong function of temperature. The evaluated activation energy for the films deposited at 873 K is 0.64 eV.     

Glassy carbon - A promising substrate material for pulsed laser deposition of thin Li1+xMn2O4−δ electrodes

The spinel LiMn₂O₄ is a promising candidate for future battery applications. If used as a positive electrode in a battery, the charging capacity of such a battery element is limited by the formation of a solid electrolyte interphase like layer between the electrolyte and the spinel. To study the electrolyte-electrode interaction during electrochemical cycling, spinel thin films are deposited as model electrodes on glassy carbon substrates by pulsed laser ablation. The obtained polycrystalline oxide thin films show a well defined surface morphology and are electrochemical active. Adhesion of these thin films on glassy carbon is in general poor, but can be improved considerably by a surface pretreatment or adding a thin metallic coating to the substrate prior deposition. The best adhesion is obtained for films deposited on argon plasma pretreated as well as Pt coated glassy carbon substrates. During the electrochemical characterization of Li_1.06Mn₂O_3.8 thin film electrodes, no additional reactions of the substrate are observed independent of the used electrolyte. The best cycle stability is achieved for films on Pt coated glassy carbon substrates.     

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.     

Solution synthesis of boron substituted LiMn2O4 spinel oxide for use in lithium rechargeable battery

An attempt has been made to synthesize LiMn₂O₄ spinel and boron substituted LiMn₂O₄ with atomic concentration of boron ranging from 0.01–0.20 and using glutaric acid as a chelating agent. The spinels have been characterized using PXRD, CV and galvanostatic charge-discharge studies. The precursor obtained from the glutaric acid assisted gel was calcined initially at 300 °C for 4 h to obtain the compound and finally at 800 °C for 4 h so as to obtain homogeneity, high degree of purity and crystallinity for better electrochemical performance. This paper suggests that glutaric acid assisted B³⁺ doped (LiB_xMn_2−xO₄) spinel was found to be as an apt candidate with good electrochemical performance for use in lithium battery.     

Effect of sulfolane and lithium bis(oxalato)borate-based electrolytes on the performance of spinel LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathodes at 55 °C

To seek a promising candidate electrolyte at elevated temperature for lithium manganese oxide (LiMn₂O₄)/Li cells, the electrochemical performance of 0.7 mol L^?1 LiBOB (lithium bis(oxalate)borate)-SL (sulfolane)/DEC (diethyl carbonate) (1:1, in volume) electrolyte was studied at 55 °C. The Mn dissolution in electrolyte was analyzed by inductively coupled plasma (ICP) analysis. AC impedance measurement and scanning electron microscopy (SEM) analysis were used to analyze the formation of the surface film on the LiMn₂O₄ electrode. The results demonstrate that the LiBOB-SL/DEC electrolyte can slow down the dissolution and erosion of Mn ions, and decrease the interface impedance. Moreover, the LiBOB-SL/DEC electrolyte could obviously improve the capacity retention, the operating voltage (4.05 V), and the rate performance of LiMn₂O₄/Li cells.     

Improvement of storage performance of LiMn2O4/graphite battery with AlF3-coated LiMn2O4

LiMn₂O₄/graphite batteries using AlF₃-coated LiMn₂O₄ have been fabricated and their electrochemical performance including discharge capacity and cyclic and storage performances have been tested and compared with pristine LiMn₂O₄/graphite batteries. The LiMn₂O₄/graphite battery with AlF₃-coated LiMn₂O₄ shows better capacity (108.5 mAhg^?1), cyclic performance (capacity retention of 92.7 % after 70 cycles), and capacity recovery ratio (98.6 %) than the pristine LiMn₂O₄ battery. X-ray diffraction patterns shows that the spinel structure of AlF₃-coated LiMn₂O₄ can be controlled better than that of pristine LiMn₂O₄ after storage. The improvement in electrochemical performance of the AlF₃-coated LiMn₂O₄/graphite battery is due to the fact that AlF₃ acts as a stabilizer and can protect the oxide structure from damaging during storage, leading to a smaller resistance and polarization after storage.     

Structure and electrochemistry of thin-film oxides grown by laser-pulsed deposition

Thin films of V₂O₅, LiCoO₂ and LiMn₂O₄ were grown by pulsed-laser deposition in the view of their use in lithium microbatteries. Lithiated polycrystalline crystalline thin dense films grown without post-deposition annealing were formed onto substrates maintained at low temperature (300 °C) from a sintered composite target including Li₂O as additive. The structural characterizations of these films have been carried out by X-ray diffraction and Raman scattering spectroscopy. The electrochemical features of thin films are investigated by cyclic voltammetry and their charge-discharge profiles in lithium microbatteries are shown. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

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.     

Preparation, characterization and electrochemical performance of γ-MnO2 and LiMn2O4 as cathodes for lithium batteries

The spinel LiMn₂O₄ is a very promising cathode material with economical and environmental advantages. LiMn₂O₄ materials have been synthesized by solid state method using γ-MnO₂ as manganese source, and Li₂CO₃ or LiNO₃ as Li sources. γ-MnO₂ is a commercial battery grade electrolytic manganese dioxide (TOSOH-Hellas GH-S) and LiMn₂O₄ samples were synthesized at a calcinations temperature up to 800 °C. γ-MnO₂ and LiMn₂O₄ samples were characterized by X-ray diffraction, thermal and electrochemical measurements. X-ray powder diffraction of as prepared LiMn₂O₄ showed a well-defined highly pure spinel single phase. The electrochemical performance of LiMn₂O₄ and its starting material γ-MnO₂ was evaluated through cyclic voltammetry, galvanostatic (constant current charge-discharge cycling) The electrochemical properties in terms of cycle performance were also discussed. γ-MnO₂ showed fairly high initial capacity of about 200 mAhg⁻¹ but poor cycle performance. LiMn₂O₄ samples showed fairly low initial capacity but good cycle performance.     

Preparation and characterization of Li2CoMn3O8 thin film cathodes for high energy lithium batteries

An ion layer gas reaction (ILGAR) dip-coating process for the deposition of homogeneous spinel structured Li₂CoMn₃O₈ thin layers has been developed. Thin film cathodes for use in high-energy density lithium batteries with thicknesses of about 200 nm have been prepared. The films were found to be X-ray amorphous after preparation. After annealing at 700°C in air for 2 h, the spinel structure of Li₂CoMn₃O₈ was observed by X-ray diffraction analysis. The composition of the surface was studied by XPS, which indicated enhanced Li and Mn concentrations as a result of the rinsing process and different solubilities of the precursor salts. The electrochemical behavior was investigated by separating the annealed electrode sample from a conventional organic lithium ion-conducting electrolyte by a layer of LiPON solid electrolyte and using elemental lithium as counter electrode. A capacity of 110.8 mAh/g was observed which is related to the valence changes of Mn and Co in the spinel structure.     

Thin aluminum film improving the cycle performance of positive electrode of lithium ion battery

A novel method was investigated to improve the cycle performance of the spinel LiMn₂O₄. It is widely different from the traditional way of modifying LiMn₂O₄ particle with compounds or metals. In our study, instead of coating LiMn₂O₄ particle itself with compounds or metals, first we covered the current collector with the mixture of LiMn₂O₄ particle, conductive agents and binders, and then deposited an aluminum film onto it by vacuum evaporation technique. Both of the pristine electrode and the modified one were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and charge-discharge tests. The results of SEM and XRD demonstrate that the aluminum film was formed successfully onto the positive electrode. And the charge-discharge tests show that the capacity retention of pristine electrode and modified one are 63.7% and 93.5% at C/2 rate in the voltage range of 3.5-4.3 V after 200 cycles, respectively. The modified electrode also shows better rate capability in comparison with the pristine one. The improved cycling stability is attributed to the minimizing of Mn dissolution into electrolyte solution and the good electronic conductivity of deposited aluminum film.     

Vibrational studies of spinel LixMn2O4 (0.3≤x≤1.0) cathodes

We report on the vibrational properties of spinel LiMn₂O₄ and its electrochemically delithiated forms Li_xMn₂O₄. Raman scattering and infrared absorption spectra have been studied as a function of the delithiation content in the wavenumber range 50–700 cm⁻¹. Results show that lithium ions can be extracted at room temperature to obtain Li_x[Mn₂]O₄ (0.3≤x≤1.0) without disrupting the [Mn₂]O₄ array. The normal modes of the spinel LiMn₂O₄ have been discussed in the O _h ⁷ symmetry and vibrations due to lithium ions with their oxygen neighbors have been identified at ca. 400 cm⁻¹. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrochemical properties of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> prepared with tartaric acid chelating agent

Lithium manganese oxide (LiMn₂O₄) has been prepared using sol-gel technique under acidic (pH = 5.8) and alkaline (pH = 9) conditions with tartaric acid as chelating agent. X-ray studies show that under acidic condition, an Mn₂O₃ peak was observed indicating the presence of impurities. No impurity was observed for LiMn₂O₄ under alkaline conditions. The particle size is mostly in the range of 124 to 185 nm from HR-TEM. The lithium diffusion coefficient, D _Li+ in LiMn₂O₄ is of the order 10^?9 cm² s^?1. By using density functional theory (DFT) calculations, structural properties have been obtained. The specific discharge capacity of the cells with LiMn₂O₄ prepared under alkaline condition and with LiMn₂O₄ prepared under acidic condition discharged at 0.5 C is in the ranges of 132 to 142 and 128 to 139 mAh g^?1, respectively.     

Synthesis of lithium-manganese oxide spinel in hydrothermal conditions

Abstract

Lithium-manganese oxide spinel LiMn₂O₄ was synthesized in hydrothermal conditions (400°C, 20 MPa) in the course of thermovaporous treatment mixtures of MnO₂ and LiOH/or Li₂CO₃. The conditions of synthesis of the spinel as monophase product were determined. The obtained product has been characterized by means of various physical and chemical methods. The spinel has been used for manufacturing cathodes of rechargeable lithium cells. The cells discharged in the potential range 2.8-3.5 V. The specific capacity was 100-140mAh/g.     

Physical properties of LiMn2O4 spinel prepared at moderate temperature

A moderate-temperature method of preparation of the spinel LiMn₂O₄ was developed around 500 °C. Physical features of the products were identified by X-ray photoelectron spectroscopy, X-ray diffractometry, Raman scattering and FTIR spectroscopy. The electronic conductivity of LiMn₂O₄ has been studied as a function of annealing temperature. The product LiMn₂O₄ is identified as a micron-sized powder and analysis of the local environment is in good accordance with the classical structural model of Fd3m space group. LiMn₂O₄ exhibits an electrical conductivity of 1.9×10⁻⁵ S/cm at room temperature with an activation energy of 0.16 eV which corresponds to an electron hopping mechanism between the two charge states of Mn³⁺ and Mn⁴⁺ ions. A first-order phase transition is observed at 292 K.     

Structural and magnetic properties of bulk and thin films of Mg0.95Mn0.05Fe2O4

We present here a comparative study on structural and magnetic properties of bulk and thin films of Mg_0.95Mn_0.05Fe₂O₄ ferrite deposited on two different substrates using X-ray diffraction (XRD) and dc magnetization measurements. XRD pattern indicates that the bulk sample and their thin films exhibit a polycrystalline single phase cubic spinel structure. It is found that the film deposited on indium tin oxide coated glass (ITO) substrate has smaller grain size than the film deposited on platinum coated silicon (Pt–Si) substrate. Study of magnetization hysteresis loop measurements infer that the bulk sample of Mg_0.95Mn_0.05Fe₂O₄ and its thin film deposited on Pt–Si substrate shows a well-defined hysteresis loop at room temperature, which reflects its ferrimagnetic behavior. However, the film deposited on ITO does not show any hysteresis, which reflects its superparamagnetic behavior at room temperature.     

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.