Electrochemistry of LiMn2O4 epitaxial films deposited on various single crystal substrates

LiMn₂O₄ epitaxial thin films were synthesized on SrTiO₃:Nb(1 1 1) and Al₂O₃(0 0 1) single crystal substrates by pulsed laser deposition (PLD) method and the electrochemical properties were discussed comparing with that of amorphous LiMn₂O₄ film on polycrystalline Au substrate. LiMn₂O₄ epitaxial film showed only a single plateau in charge–discharge curves and a single redox peak at the corresponding voltage of cyclic voltammograms. This phenomenon seems to originate from the effect of the epitaxy: the film is directly connected with the substrate by the chemical bond and this connection would suppress the phase transition of Li_xMn₂O₄ film during lithium (de-)intercalation. The discharge voltage of LiMn₂O₄ epitaxial film on SrTiO₃ was lower than that of LiMn₂O₄ film on Al₂O₃. This lowered discharge voltage may be caused by the electronic interaction between LiMn₂O₄ film and SrTiO₃:Nb n-type semiconductor substrate.     

Fabrication of all solid-state lithium-ion batteries with three-dimensionally ordered composite electrode consisting of Li0.35La0.55TiO3 and LiMn2O4

A composite electrode between three-dimensionally ordered macroporous (3DOM) Li_0.35La_0.55TiO₃ (LLT) and LiMn₂O₄ was fabricated by colloidal crystal templating method and sol–gel process. A close-packed PS beads with the opal structure was prepared by filtration of a suspension containing PS beads. Li–La–Ti–O sol was injected by vacuum impregnation process into the voids between PS beads, and then was heated to form 3DOM-LLT. Three-dimensionally ordered composite material consisting of LiMn₂O₄ and LLT was prepared by sol–gel process. The prepared composite was characterized with SEM and XRD. All solid-state Li-ion battery was fabricated with the LLT–LiMn₂O₄ composite electrode as a cathode, dry polymer electrolyte and Li metal anode. The prepared all solid-state cathode exhibited a volumetric discharge capacity of 220 mAh cm⁻³.     

Surface modification of LiNi0.5Mn1.5O4 by ZrP2O7 and ZrO2 for lithium-ion batteries

The spinel LiNi_0.5Mn_1.5O₄ has been surface modified separately with 1.0 wt.% ZrO₂ and ZrP₂O₇ for the purpose of improving its cycle performance as a cathode in a 5-V lithium-ion cell. Although the modifications did not change the crystallographic structure of the surface-modified samples, they exhibited better cyclability at elevated temperature (55 °C) compared with pristine LiNi_0.5Mn_1.5O₄. The material that was surface modified with ZrO₂ gave the best cycling performance, only 4% loss of capacity after 150 cycles at 55 °C. Electrochemical impedance spectroscopy demonstrated that the improved performance of the ZrO₂-surface-modified LiNi_0.5Mn_1.5O₄ is due to a small decrease in the charge transfer resistance, indicating limited surface reactivity during cycling. Differential scanning calorimetry showed that the ZrO₂-modified LiNi_0.5Mn_1.5O₄ exhibits lower heat generation and higher onset reaction temperature compared to the pristine material. The excellent cycling and safety performance of the ZrO₂-modified LiNi_0.5Mn_1.5O₄ electrode was found to be due to the protective effect of homogeneous ZrO₂ nano-particles that form on the LiNi_0.5Mn_1.5O₄, as shown by transmission electron microscopy.     

Electrochemical performance of nanostructured spinel LiMn2O4 in different aqueous electrolytes

A nanostructured spinel LiMn₂O₄ electrode material was prepared via a room-temperature solid-state grinding reaction route starting with hydrated lithium acetate (LiAc·2H₂O), manganese acetate (MnAc₂·4H₂O) and citric acid (C₆H₈O₇·H₂O) raw materials, followed by calcination of the precursor at 500 °C. The material was characterized by X-ray diffraction (XRD) and transmission electron microscope techniques. The electrochemical performance of the LiMn₂O₄ electrodes in 2 M Li₂SO₄, 1 M LiNO₃, 5 M LiNO₃ and 9 M LiNO₃ aqueous electrolytes was studied using cyclic voltammetry, ac impedance and galvanostatic charge/discharge methods. The LiMn₂O₄ electrode in 5 M LiNO₃ electrolyte exhibited good electrochemical performance in terms of specific capacity, rate dischargeability and charge/discharge cyclability, as evidenced by the charge/discharge results.     

Electrochemical performance of spinel LiMn2O4 cathode materials made by flame-assisted spray technology

Spinel lithium manganese oxide LiMn₂O₄ powders were synthesized by a flame-assisted spray technology (FAST) with a precursor solution consisting of stoichiometric amounts of LiNO₃ and Mn(NO₃)₂·4H₂O dissolved in methanol. The as-synthesized LiMn₂O₄ particles were non-agglomerated, and nanocrystalline. A small amount of Mn₃O₄was detected in the as-synthesized powder due to the decomposition of spinel LiMn₂O₄ at the high flame temperature. The impurity phase was removed with a post-annealing heat-treatment wherein the grain size of the annealed powder was 33 nm. The charge/discharge curves of both powders matched the characteristic plateaus of spinel LiMn₂O₄ at 3 V and 4 V vs. Li. However, the annealed powder showed a higher initial discharge capacity of 115 mAh g⁻¹ at 4 V. The test cell with annealed powder showed good rate capability between a voltage of 3.0 and 4.3 V and a first cycle coulombic efficiency of 96%. The low coulombic efficiency from capacity fading may be due to oxygen defects in the annealed powder. The results suggest that FAST holds potential for rapid production of uniform cathode materials with low-cost nitrate precursors and minimal energy input.     

Preparation of manganese oxide with high density by decomposition of MnCO3 and its application to synthesis of LiMn2O4

Manganese oxide with high tap density was prepared by decomposition of spherical manganese carbonate, and then LiMn₂O₄ cathode materials were synthesized by solid-state reaction between the manganese oxide and lithium carbonate. Structure and properties of the samples were determined by X-ray diffraction, Brunauer–Emmer–Teller surface area analysis, scanning electron microscope and electrochemical measurements. With increase of the decomposition temperature from 350 °C to 900 °C, the tap density of the manganese oxide rises from 0.91 g cm⁻³ to 2.06 g cm⁻³. Compared with the LiMn₂O₄ cathode made from chemical manganese dioxide or electrolytic manganese dioxide, the LiMn₂O₄ made from manganese oxide of this work has a larger tap density (2.53 g cm⁻³), and better electrochemical performances with an initial discharge capacity of 117 mAh g⁻¹, a capacity retention of 93.5% at the 15th cycle and an irreversible capacity loss of 2.24% after storage at room temperature for 28 days.     

Hydrogen permeation on Al2O3-based nickel/cobalt composite membranes

Al₂O₃ was synthesized using the sol-gel process with aluminum isopropoxide as the precursor and primary distilled water as the solvent. Nickel and cobalt metal powders were used to increase the strength of the membranes. The Al₂O₃-based membranes were prepared using HPS following a mechanical alloying process. The phase transformation, thermal evolution, surface and cross-section morphology of Al₂O₃ and Al₂O₃-based membranes were characterized by XRD, TG-DTA and FE-SEM. The hydrogen permeation of Al₂O₃-based membranes was examined at 300–473 K under increasing pressure. Hydrogen permeation flux through an Al₂O₃-20wt%Co membrane was obtained to 2.36 mol m⁻² s⁻¹. Reaction enthalpy was calculated to 4.5 kJ/mol using a Van’t Hoff’s plot.     

Enhanced high rate performance of LiMn2O4 spinel nanoparticles synthesized by a hard-template route

A nanosized LiMn₂O₄ (nano-LiMn₂O₄) spinel was prepared by a novel route using a porous silica gel as a sacrificial hard template. This material was found to be made up of 8–20 nm nanoparticles with a mean crystallite size of 15 nm. The electrochemical properties of nano-LiMn₂O₄ were tested in lithium cells at different cycling rates and compared to those of microsized LiMn₂O₄ (micro-LiMn₂O₄) obtained by the classical solid state route. Microsized LiMn₂O₄ is formed by 3–20 μm agglomerates, the size of each individual particle being approximately 0.20 μm. The behaviour of nano-LiMn₂O₄ as a positive electrode improves with increasing current densities (from C/20 to 2C). Moreover, it was found to exhibit a noticeably better performance at high rates (2C), with higher initial capacity values and very good retention (only 2% loss after 30 cycles), with respect to micro-LiMn₂O₄, almost certainly due to enhanced lithium diffusion in the small particles.     

Flame co-synthesis of LiMn2O4 and carbon nanocomposites for high power batteries

A novel method to produce LiMn₂O₄/carbon nanocomposites in a rapid, one-step and industrially scalable process is presented. A flame spray and a diffusion flame are combined to continuously produce LiMn₂O₄ nanoparticles and carbon black, respectively. Powder carbon content is varied by adjusting the diffusion flame conditions. The powders are characterized by X-ray diffraction (XRD), transmission electron microscopy, cyclic voltammetry and galvanostatic cycling for a range of current densities. These LiMn₂O₄/carbon nanocomposites retain over 80% of their initial galvanostatic discharge capacity for current densities ranging from 5 to 50C-rates, significantly better than pure LiMn₂O₄ nanoparticles mixed conventionally with commercial carbon blacks. The improved performance of the LiMn₂O₄/carbon nanocomposites is attributed to the carbon particle contact and/or film coating of the freshly-made LiMn₂O₄ nanoparticles. This additional well-distributed carbon provides an electrically conductive network that induces a more homogeneous charge transfer throughout the electrode. The suitability of these nanocomposites as a hybrid material is discussed by considering the layout of a thin-layer lithium-ion battery containing these flame-made nanocomposites as positive electrode and LiC₆ as negative electrode. The battery’s specific energy is calculated to be 78 Wh kg⁻¹ (50C-rate) based on the results of lithium-ion insertion capacity experiments and reasonable engineering assumptions on the lithium-ion battery design.     

Fabrication and electrochemical properties of lithium-ion batteries for power tools

204056-Type prismatic lithium-ion battery for power tools was developed by using LiMn₂O₄ as cathode and CMS (carbonaceous mesophase spheres) as anode. The performance of batteries and their electrodes were characterized by SEM, ac impedance and electrochemical tests. The bulk density of cathode after pressing was selected as a main factor and it effects on high current rate capability and discharge plateau distinctly, which were investigated in details. Being charged/discharged in the voltage range of 2.5–4.2 V, the normal LiMn₂O₄ battery with cathode bulk density of 2.7 g cm⁻³ shows excellent electrochemical performances. The discharge capacity at 20C rate is 94.1% of that at 1C rate, and the capacity retention ratio charged at 1C and discharged at 5C is 91.7% after 100 cycles at 25 °C. While modified LiMn₂O₄ is used as the cathode material, the cycling performance of batteries is better than that of batteries made from normal LiMn₂O₄. The capacity retention ratios of modified LiMn₂O₄ batteries after 100 cycles at 25 °C and 55 °C are 95.0% and 85.3%, respectively. The discharge capacity at low temperature was tested both at 1C rate and 5C rate, and the capacities discharged at −20 °C were 96.3% and 94.2% of that at 1C at 25 °C. Furthermore, the batteries also show good safety in the test of short circuit, overcharge, and nail penetration.     

Electrochemical performance of LiCoO2 cathodes by surface modification using lanthanum aluminum garnet

LiCoO₂ particles were coated with various wt.% of lanthanum aluminum garnets (3LaAlO₃:Al₂O₃) by an in situ sol–gel process, followed by calcination at 1123 K for 12 h in air. X-ray diffraction (XRD) patterns confirmed the formation of a 3LaAlO₃:Al₂O₃ compound and the in situ sol–gel process synthesized 3LaAlO₃:Al₂O₃-coated LiCoO₂ was a single-phase hexagonal α-NaFeO₂-type structure of the core material without any modification. Scanning electron microscope (SEM) images revealed a modification of the surface of the cathode particles. Transmission electron microscope (TEM) images exposed that the surface of the core material was coated with a uniform compact layer of 3LaAlO₃:Al₂O₃, which had an average thickness of 40 nm. Galvanostatic cycling studies demonstrated that the 1.0 wt.% 3LaAlO₃:Al₂O₃-coated LiCoO₂ cathode showed excellent cycle stability of 182 cycles, which was much higher than the 38 cycles sustained by the pristine LiCoO₂ cathode material when it was charged at 4.4 V.     

Lithium difluoro(oxalato)borate/ethylene carbonate + propylene carbonate + ethyl(methyl) carbonate electrolyte for LiMn2O4 cathode

Lithium difluoro(oxalato)borate (LiODFB) was investigated as a lithium salt for non-aqueous electrolytes for LiMn₂O₄ cathode in lithium-ion batteries. Linear sweep voltammetry (LSV) tests were used to examine the electrochemical stability and the compatibility between the electrolytes and LiMn₂O₄ cathode. Through inductively coupled plasma (ICP) analysis, we compared the amount of Mn²⁺ dissolved from the spinel cathode in 1 mol L⁻¹ LiPF₆/EC + PC + EMC (1:1:3 wt.%) and 1 mol L⁻¹ LiODFB/EC + PC + EMC (1:1:3 wt.%). AC impedance measurements and scanning electron microscopy (SEM) analysis were used to analyze the formation of the surface film on the LiMn₂O₄ cathode. These results demonstrate that ODFB anion can capture the dissolution manganese ions and form a denser and more compact surface film on the cathode surface to prevent the continued Mn²⁺ dissolution, especially at high temperature. It is found that LiODFB, instead of LiPF₆, can improve the capacity retention significantly after 100 cycles at 25 °C and 60 °C, respectively. LiODFB is a very promising lithium salt for LiMn₂O₄ cathode in lithium-ion batteries.     

Comparison of the electrochemical properties of LiBOB and LiPF6 in electrolytes for LiMn2O4/Li cells

The electrochemical stability and conductivity of LiPF₆ and lithium bis(oxalato)borate (LiBOB) in a ternary mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were compared. The discharge capacities of LiMn₂O₄/Li cells with the two electrolytes were measured at various current densities. At room temperature, LiMn₂O₄/Li cells with the electrolyte containing LiBOB cycled equally well with those using the electrolyte containing LiPF₆ when the discharge current rate was under 1 C. At 60 °C, the LiBOB-based electrolyte cycled better than the LiPF₆-based electrolyte even when the discharge current rate was above 1 C. Compared with the electrolyte containing LiPF₆, in LiMn₂O₄/Li cells the electrolyte containing LiBOB exhibited better capacity utilization and capacity retention at both room temperature and 60 °C. The scanning electron microscopy (SEM) images and the a.c. impedance measurements demonstrated that the electrode in the electrolyte containing LiBOB was more stable. In summary, LiBOB offered obvious advantages in LiMn₂O₄/Li cells.     

Performance improvement of LiMn2O4 as cathode material for lithium ion battery with bismuth modification

Spinel lithium manganese oxides (LiMn₂O₄) modified with and without bismuth by sol–gel method were investigated by theoretical calculation and experimental techniques, including galvanostatic charge/discharge test (GC), cyclic voltammetry (CV), chronopotentiometry (CP), electrochemical impedance spectroscopy (EIS), inductively coupled plasma (ICP), powder X-ray diffraction (XRD), BET measurement, and infrared spectroscopy (IR). It is found that the performance of LiMn₂O₄ can be improved by the bismuth modification. The modified and the unmodified samples have almost the same initial discharge capacity, 118 and 120 mAh g⁻¹, respectively. However, the modified sample has better cyclic stability than the unmodified sample. After 100 cycles, the capacity remains 100 and 89 mAh g⁻¹ for the modified and the unmodified samples, respectively. Moreover, the results from EIS show that the modified sample has a quicker kinetic process for Li ion intercalation/de-intercalation than the unmodified one; the charge-transfer resistance of the former is less than one-sixth of that of the latter. After immersion in electrolyte (DMC:EC:EMC = 1:1:1, 1 mol L⁻¹ LiPF₆) for 10 h at room temperature, the modified sample has less change in open circuit potential, crystal volume, and vibration absorption of Mn–O bond, and has less dissolution of manganese into solution than the unmodified sample.     

Influence of surface modification with Sn6O4(OH)4 on electrochemical performance of ZnO in Zn/Ni secondary cells

The surface-modified ZnO by Sn₆O₄(OH)₄ was prepared by a simple hydrolyzation process and the influence of Sn₆O₄(OH)₄ on electrochemical performance of ZnO was investigated by charge/discharge cycling test, slow rate cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared with the unmodified ZnO, the Sn₆O₄(OH)₄-modified ZnO showed improved electrochemical properties, such as superior electrochemical cycle stability, higher discharge capacity and utilization ratio. The surface modification could suppress the dissolution of ZnO in the alkaline electrolyte and maintain the electrochemical activity of ZnO. When the Sn₆O₄(OH)₄ content reached 27 wt.%, the discharge capacity of the modified ZnO hardly declined over 80 cycling test, the average utilization ratio could reach 98.5%, and the modified ZnO electrodes had no obvious weight loss after the cycling tests. However, the charge/discharge plateau voltage with the Sn₆O₄(OH)₄-modified ZnO slightly decreased. For the modified ZnO electrodes, two anodic peaks occurred in the CV curves, and the charge transfer resistance increased from the EIS results, both of which were ascribed to the suppressive effect of surface modification on the electrochemical reactions.     

In situ X-ray diffraction studies of mixed LiMn2O4-LiNi1/3Co1/3Mn1/3O2 composite cathode in Li-ion cells during charge-discharge cycling

The structural changes of the composite cathode made by mixing spinel LiMn₂O₄ and layered LiNi_1/3Co_1/3Mn_1/3O₂ in 1:1 wt% in both Li-half and Li-ion cells during charge/discharge are studied by in situ XRD. During the first charge up to ∼5.2 V vs. Li/Li⁺, the in situ XRD spectra for the composite cathode in the Li-half cell track the structural changes of each component. At the early stage of charge, the lithium extraction takes place in the LiNi_1/3Co_1/3Mn_1/3O₂ component only. When the cell voltage reaches at ∼4.0 V vs. Li/Li⁺, lithium extraction from the spinel LiMn₂O₄ component starts and becomes the major contributor for the cell capacity due to the higher rate capability of LiMn₂O₄. When the voltage passed 4.3 V, the major structural changes are from the LiNi_1/3Co_1/3Mn_1/3O₂ component, while the LiMn₂O₄ component is almost unchanged. In the Li-ion cell using a MCMB anode and a composite cathode cycled between 2.5 V and 4.2 V, the structural changes are dominated by the spinel LiMn₂O₄ component, with much less changes in the layered LiNi_1/3Co_1/3Mn_1/3O₂ component, comparing with the Li-half cell results. These results give us valuable information about the structural changes relating to the contributions of each individual component to the cell capacity at certain charge/discharge state, which are helpful in designing and optimizing the composite cathode using spinel- and layered-type materials for Li-ion battery research.     

Low-temperature synthesis of highly crystallized LiMn2O4 from alpha manganese dioxide nanorods

One-dimensional alpha manganese dioxide (α-MnO₂) nanorods synthesized by a hydrothermal route were explored as the starting material for preparing lithium manganese spinel LiMn₂O₄. Pure and highly crystalline spinel LiMn₂O₄ was easily obtained from α-MnO₂ nanorods through a low-temperature solid-state reaction route, while Mn₂O₃ impurity was present along with the spinel phase when commercial MnO₂ was used as starting material. The particle size of LiMn₂O₄ prepared from α-MnO₂ nanorods was about 100 nm with a homogenous distribution. Electrochemical tests demonstrated that the LiMn₂O₄ thus prepared exhibited a higher capacity than that prepared from commercial MnO₂. Therefore, α-MnO₂ nanorods are proved to be a promising starting material for the preparation of high quality LiMn₂O₄.     

In-situ XRD investigations on structure changes of ZrO2-coated LiMn0.5Ni0.5O2 cathode materials during charge

The structural changes of pristine and ZrO₂-coated LiMn_0.5Ni_0.5O₂ cathode materials were investigated by using in situ X-ray diffraction (XRD) during charging process. An obviously solid solution phase transition from a hexagonal structure (H1) to another hexagonal structure (H2) was observed during the charging process at a constant current of 0.3 mA in the potential range of 2.5–5.7 V. The second hexagonal structure has a shorter a-axis and a longer c-axis before the crystal collapse. Before the structure collapses the c-axis length increases to maximum and then significantly decreases to 14.1 Å. The c-axis length of the pristine and ZrO₂-coated LiMn_0.5Ni_0.5O₂ increases to the maximum at the charge capacity of 119.2 and 180.9 mAh g⁻¹, respectively. It can be concluded that the ZrO₂ coating can strongly stabilize the crystal structure of the LiMn_0.5Ni_0.5O₂ compound from the comparison of the lattice parameter variations between the pristine and the ZrO₂-coated LiMn_0.5Ni_0.5O₂ compounds upon charge. The potential fluctuation resulting from the decomposition of electrolytes starts at the charge capacity of around 200 and 260 mAh g⁻¹ for the pristine and ZrO₂-coated LiMn_0.5Ni_0.5O₂, respectively. It suggests that the ZrO₂ coating layer can impede the reaction between the cathode material and electrolyte.     

A high rate,high capacity and long life (LiMn2O4 + AC)/Li4Ti5O12 hybrid battery–supercapacitor

A hybrid battery–supercapacitor (LiMn₂O₄ + AC)/Li₄Ti₅O₁₂ using a Li₄Ti₅O₁₂ anode and a LiMn₂O₄/activated carbon (AC) composite cathode was built. The electrochemical performances of the hybrid battery–supercapacitor (LiMn₂O₄ + AC)/Li₄Ti₅O₁₂ were characterized by cyclic voltammograms, electrochemical impedance spectra, rate charge–discharge and cycle performance testing. It is demonstrated that the hybrid battery–supercapacitor has advantages of both the high rate capability from hybrid capacitor AC/Li₄Ti₅O₁₂ and the high capacity from secondary battery LiMn₂O₄/Li₄Ti₅O₁₂. Moreover, the electrochemical measurements also show that the hybrid battery–supercapacitor has good cycle life performance. At 4C rate, the capacity loss in constant current mode is no more than 7.95% after 5000 cycles, and the capacity loss in constant current–constant voltage mode is no more than 4.75% after 2500 cycles.     

Influence of S and Ni co-doping on structure, band gap and electrochemical properties of lithium manganese oxide synthesized by soft chemical method

Ni-doped lithium manganese oxysulfides with a nominal composition of LiNi_xMn_2−xO_4−δS_δ (0 ≤ x ≤ 0.5 and 0 ≤ δ ≤ 0.1) have been synthesized by an alanine-assisted, low-temperature combustion process, followed by calcination at 700 °C in air. Quantitative X-ray phase analyses show that the spinel structure of LiMn₂O₄ is retained for all compositions. However, analysis of the vibrational peaks observed in Fourier transformed infrared (FTIR) spectroscopy suggests that the Fd3m crystal symmetry is retained only up to x ≤ 0.4 and changes to P4₃32 symmetry for x = 0.5. A systematic change in microstructure is observed with increasing Ni content in presence of S. The shape of the particles changes from spherical (LiMn₂O₄) to icosahedron (LiNi_0.2Mn_1.8O_4−δS_δ) to octahedron (LiNi_0.5Mn_1.5O_4−δS_δ). UV-vis spectroscopy shows that the band structure of pristine LiMn₂O₄ is strongly influenced by hybridization among Mn 3d and O 2p orbitals near the Fermi level and the band gap (1.45 eV) gradually decreases with increasing nickel content and reaches the minimum (1.35 eV) for LiNi_0.4Mn_1.6O_4−δS_δ. Electrochemical results on 2032 coin-type cells, fabricated with the synthesized powders as the positive electrode (cathode) and Li metal as the negative electrode (anode), reveal that the substitution of S for O and Ni for Mn in LiMn₂O₄ enhances the structural integrity of the spinel host, which in turn increases the electrochemical cycleability. A high initial discharge capacity of 155 mAh g⁻¹ is obtained for a LiNi_0.4Mn_1.6O_4−δS_δ/Li cell with about 87% capacity retention (135 mAh g⁻¹) after 25 cycles at a current density of 0.2 mA cm⁻². All LiNi_xMn_2−xO_4−δS_δ/Li cells (x = 0.2-0.5) show excellent reversibility with nominal capacity fading (0.04-0.2 mAh per cycle) at a current density of 0.2 mA cm⁻².