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.     

Synthesis of spinel LiMn2O4 nanoparticles through one-step hydrothermal reaction

Phase pure spinel LiMn₂O₄ nanoparticles can be directly synthesized by one-step hydrothermal reaction of γ-MnO₂ with LiOH in an initial Li/Mn ratio of 1 at 200 °C. The reaction might involve a redox reaction between Mn⁴⁺ and OH⁻, and the formation of LiMn₂O₄ at the same time under the proposed hydrothermal conditions. This hydrothermal process is simple since only γ-MnO₂ powders are used as the Mn source, whereas without use of any oxidants, reductants, or low valence Mn source. The electrochemical performance of the as-synthesized LiMn₂O₄ nanoparticles towards Li⁺ insertion/extraction was examined. Rather good capacity and cycle performance, and an especially excellent high rate capability, were observed for the sample that was hydrothermally reacted for 3 days.     

Morphology and electrochemistry of LiMn2O4 optimized by using different Mn-sources

Spinel-typed LiMn₂O₄ cathode active materials have been prepared for different microstructures by the melt-impregnation method using different forms of manganese. The effect of the starting materials on the microstructure and electrochemical properties of LiMn₂O₄ is investigated by X-ray diffraction, scanning electron microscopy, and electrochemical measurements. The powder prepared from nanostructured γ-MnOOH, with good crystallinity and a regular cubic spinel shape, provided an initial discharge capacity of 114 mAh g⁻¹ with excellent rate and high capacity retention. These advantages render LiMn₂O₄ attractive for practical and large-scale applications in mobile equipment.     

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.     

Effects of supersonic treatment on the electrochemical properties and crystal structure of LiMn1.5Ni0.5O4 as a cathode material for Li ion batteries

The electrochemical properties and crystal structure of LiMn_1.5Ni_0.5O₄ treated with supersonic waves in an aqueous Ni-containing solution were investigated by performing charge-discharge tests, inductively coupled plasma (ICP) analysis, scanning electron microscopy (SEM), iodometry, X-ray diffraction (XRD), powder neutron diffraction and synchrotron powder XRD. The charge-discharge curve of LiMn_1.5Ni_0.5O₄ versus Li/Li⁺ has plateaus at 4.1 and 4.7 V. The 4.1 V versus Li/Li⁺ plateau due to the oxidation of Mn^3+/4+ was reduced by the supersonic treatment. During the charge-discharge cycling test at 25 °C, the supersonic treatment increased the discharge capacity of the 50th cycle. Rietveld analysis of the neutron diffraction patterns revealed that the Ni occupancy of the 4b site in LiMn_1.5Mn_0.5O₄, which is mainly occupied by Ni, was increased by the supersonic treatment. This result suggests that Ni²⁺ is partially substituted for Mn^3+/4+ during the supersonic treatment.     

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.     

A porous spherical aggregation of Li4Mn5O12 nanorods and its electrochemical performance

A porous spherical aggregation of Li₄Mn₅O₁₂ nanorods with the particle size of 3 μm is prepared by oxidizing LiMn₂O₄ powder with (NH₄)₂S₂O₈ under hydrothermal conditions. The result displays that concentration of (NH₄)₂S₂O₈ plays a key role in forming the porous spherical aggregation and the optimal concentration of oxidant is found to be 1.5 mol L⁻¹. The mechanism for the formation of the porous spherical aggregation is proposed. The electrochemical capacitance performance is tested by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge. The porous spherical aggregation exhibits a good electrochemical performance. It could deliver 375 F g⁻¹ within potential range 0-1.4 V at a scan rate of 5 mV s⁻¹ in 1 mol L⁻¹ Li₂SO₄ and the value is cut down to less than 0.024 F g⁻¹ per cycling period in 1000 cycles.     

Synthesis and characterization of Nd doped LiMn2O4 cathode for Li-ion rechargeable batteries

Spinel powders of LiMn_1.99Nd_0.01O₄ have been synthesized by chemical synthesis route to prepare cathodes for Li-ion coin cells. The structural and electrochemical properties of these cathodes were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, cyclic voltammetry, and charge-discharge studies. The cyclic voltammetry of the cathodes revealed the reversible nature of Li-ion intercalation and deintercalation in the electrochemical cell. The charge-discharge characteristics for LiMn_1.99Nd_0.01O₄ cathode materials were obtained in 3.4–4.3 V voltage range and the initial discharge capacity of this material were found to be about 149 mAh g⁻¹. The coin cells were tested for up to 25 charge-discharge cycles. The results show that by doping with small concentration of rare-earth element Nd, the capacity fading is considerably reduced as compared to the pure LiMn₂O₄ cathodes, making it suitable for Li-ion battery applications.     

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.     

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.     

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.     

Nonflammable electrolyte for 3-V lithium-ion battery with spinel materials LiNi0.5Mn1.5O4 and Li4Ti5O12

The compatibility between dimethyl methylphosphonate (DMMP)-based electrolyte of 1 M LiPF₆/EC + DMC + DMMP (1:1:2 wt.) and spinel materials Li₄Ti₅O₁₂ and LiNi_0.5Mn_1.5O₄ was reviewed, respectively. The cell performance and impedance of 3-V LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ lithium-ion cell with the DMMP-based nonflammable electrolyte was compared with the baseline electrolyte of 1 M LiPF₆/EC + DMC (1:1 wt.). The nonflammable DMMP-based electrolyte exhibited good compatibility with spinel Li₄Ti₅O₁₂ anode and high-voltage LiNi_0.5Mn_1.5O₄ cathode, and acceptable cycling performance in the LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ full-cell, except for the higher impedance than that in the baseline electrolyte. All of the results disclosed that the 3 V LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ lithium-ion battery was a promising choice for the nonflammable DMMP-based electrolyte.     

Crystallization of LiMn2O4 observed with high temperature X-ray diffraction

We investigated the formation of LiMn₂O₄ phases by calcinating a stoichiometric mixture of Li₂CO₃ and various manganese compounds with high temperature X-ray diffraction (HT-XRD) technique to understand the influence of starting materials on the electrochemical performance. XRD measurements were carried out during heating processes from room temperature to 700 °C. In case of Li₂CO₃/electrolytic manganese dioxide and Li₂CO₃/MnCO₃ mixtures used as starting materials, Li_0.33MnO₂ phase and low crystalline phase, respectively, appeared as intermediate products during heating process followed by the crystallization into the spinel. HT-XRD observation confirmed that the LiMn₂O₄ phase was directly formed from starting Li₂CO₃/Mn₂O₃ and Li₂CO₃/Mn₃O₄ mixtures. The reactivity of the mixture, meant by the lower reaction temperature between Li and Mn compounds and the faster evolution of Li–Mn–O phase, depended on manganese compounds. The purity and stoichiometry of spinel type LiMn₂O₄ was not achieved only by the higher reactivity. From these results, the dependence of reversible capacities and cycleability of synthesized LiMn₂O₄s on the formation process which varied with the starting materials was discussed.     

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.     

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.     

Physical and photoelectrochemical studies for hydrogen photo-evolution over the spinel ZnCr2O4

The present work deals with the photoelectrochemical hydrogen production over the spinel ZnCr₂O₄. The photoactivity is dependent on the synthesis conditions and the oxide has been prepared by nitrate way in order to produce homogeneous powder with large active surface. The transport properties indicate p-type conductivity with activation energy of 0.21 eV. A corrosion potential of 0.404 V_SCE and an exchange current density of 50 μA cm⁻² have been determined from the semi logarithm plot. The photocurrent onset potential, assimilated to the flat band potential, was found to be −0.39 V_SCE. ZnCr₂O₄/S₂O₃²⁻ is a self driven system where absorption of light promotes electrons into the conduction band with a potential (−1 V) sufficient to reduce water into hydrogen. The activity shows a tendency toward saturation whose deceleration is the result of the competitive reductions of end products namely S₂O₆²⁻ and S₂O₄²⁻ with water. A comparative study with CuCr₂O₄ is reported.     

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.     

Synthesis and electrochemical performance of the high voltage cathode material Li[Li0.2Mn0.56Ni0.16Co0.08]O2 with improved rate capability

The high voltage layered Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode material, which is a solid solution between Li₂MnO₃ and LiMn_0.4Ni_0.4Co_0.2O₂, has been synthesized by co-precipitation method followed by high temperature annealing at 900 °C. XRD and SEM characterizations proved that the as prepared powder is constituted of small and homogenous particles (100-300 nm), which are seen to enhance the material rate capability. After the initial decay, no obvious capacity fading was observed when cycling the material at different rates. Steady-state reversible capacities of 220 mAh g⁻¹ at 0.2C, 190 mAh g⁻¹ at 1C, 155 mAh g⁻¹ at 5C and 110 mAh g⁻¹ at 20C were achieved in long-term cycle tests within the voltage cutoff limits of 2.5 and 4.8 V at 20 °C.     

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.     

Preparation and characterization of sub-micro LiNi0.5−xMn1.5+xO4 for 5 V cathode materials synthesized by an ultrasonic-assisted co-precipitation method

Sub-micro spinel LiNi_0.5−xMn_1.5+xO₄ (x < 0.1) cathode materials powder was successfully synthesized by the ultrasonic-assisted co-precipitation (UACP) method. The structure and electrochemical performance of this as-prepared powder were characterized by powder XRD, SEM, XPS, CV and the galvanostatic charge–discharge test in detail. XRD shows that there is a small Li_yNi_1−yO impurity peak placed close to the (4 0 0) line of the spinel LiNi_0.5−xMn_1.5+xO₄, and the powders are well crystallized. XPS exhibits that the Mn oxidation state is between +3 and +4, and Ni oxidation state is +2 in LiNi_0.5−xMn_1.5+xO₄. SEM shows that the prepared powders (UACP) have the uniform and narrow size distribution which is less than 200 nm. Galvanostatic charge–discharge test indicates that the initial discharge capacities for the LiNi_0.5−xMn_1.5+xO₄ (UACP) at C/3, 1C and 2C, are 130.2, 119.0 and 110.0 mAh g⁻¹, respectively. After 100 cycles, their capacity retentions are 99.8%, 88.2%, and 73.5%, respectively. LiNi_0.5−xMn_1.5+xO₄ (UACP) at C/3 discharge rate exhibits superior capacity retention upon cycling, and it also shows well high current discharge performance. CV curve implies that LiNi_0.5−xMn_1.5+xO₄ (x < 0.1) spinel synthesized by ultrasonic-assisted co-precipitation method has both reversibility and cycle capability because of the ultrasonic-catalysis.