Optimization of synthesis condition and electrode fabrication for spinel LiMn2O4 cathode

We used cyclic voltammetry (CV) and galvanostatic cycling test to optimize the synthesis condition and electrode composition for spinel LiMn₂O₄ cathode. Based on a synthetic approach of solid reaction, the most appropriate Li–Mn source for the synthesis of LiMn₂O₄ was found to be LiOH/MnO₂ and the optimum synthesis condition was to react at 750 °C in air for 18 h. The LiMn₂O₄ such obtained has an initial specific capacity of 120–130 mAh/g between 3.5 and 4.2 V. In the electrode films, the carbon that was used as a conducting agent significantly affects performance of the LiMn₂O₄ electrode. Among the carbons examined in this work, we found that carbon black from Alfa Aesar was a better conducting agent and its appropriate content was around 10% in the LiMn₂O₄ electrode.     

The elevated temperature performance of LiMn2O4 coated with LiNi1−XCoXO2 (X = 0.2 and 1)

The surface coating of LiMn₂O₄ using a gel precursor of LiNi_1−XCo_XO₂ (X=0.2 and 1) prepared from a solution-based chemical process was attempted in order to enhance the electrochemical performances of LiMn₂O₄ at elevated temperature. After the surface of LiMn₂O₄ was coated with LiNi_1−XCo_XO₂ (X=0.2 and 1) coating solution and heated at 750 °C, the surface of LiMn₂O₄ was covered with fine LiNi_1−XCo_XO₂ (X=0.2 and 1) particles. LiNi_1−XCo_XO₂ (X=0.2 and 1)-coated LiMn₂O₄ showed an excellent capacity retention at 65 °C compared to pure LiMn₂O₄. While pure LiMn₂O₄ retained 81% of the initial capacity after storage in the discharged state at 65 °C for 300 h, LiCoO₂-coated LiMn₂O₄ showed no capacity loss. The improvement of storage performance at 65 °C is attributed to the suppression of electrolyte decomposition and the reduction of Mn dissolution resulting from encapsulating the surface of LiMn₂O₄ with LiCoO₂. The surface coating with LiNi_0.8Co_0.2O₂ also enhanced the high temperature cycle performance of LiMn₂O₄. Consequently, It is proposed that the surface encapsulation of LiMn₂O₄ with fine LiNi_1−XCo_XO₂ (X=0.2 and 1) particles improve its high temperature performance.     

LiMn1.95M0.05O4 (M: Al,Co, Fe,Ni, Y) cathode materials prepared by combustion synthesis

Spinel LiMn_1.95M_0.05O₄ (M: Al, Co, Fe, Ni, or Y) is prepared by a combustion synthesis, which is a simple, rapid and cost-effective process for obtaining oxide powders. The spinel is obtained by combustion of lithium hydroxide, manganese nitrate, M-nitrates, and urea at about 280°C for 1 min. Well-defined spinel LiMn_1.95M_0.05O₄ was obtained by further heat-treatment at 800°C for 10 h. The synthesized LiMn_1.95M_0.05O₄ shows lower initial capacities, viz. 90–132 mAh/g than undoped LiMn₂O₄. The latter has an initial capacity of 146 mAh/g which fades to 100 mAh/g with cycling. On the other hand, LiMn_1.95Co_0.05O₄, LiMn_1.95Ni_0.05O₄, and LiMn_1.95Y_0.05O₄ display better cycleability than LiMn₂O₄.     

Effect of annealing temperature on electrochemical performance of thin-film LiMn2O4 cathode

Spinel LiMn₂O₄ thin-film cathodes were obtained by spin-coating the chitosan-containing precursor solution on a Pt-coated silicon substrate followed by a two-stage heat-treatment procedure. The LiMn₂O₄ film calcined at 700 °C for 1 h showed the highest Li-ion diffusion coefficient, 1.55 × 10⁻¹² cm² s⁻¹ (PSCA measurement) among all calcined films. It is attributed to the larger interstitial space and better crystal perfection of LiMn₂O₄ film calcined at 700 °C for 1 h. Consequently, the 700 °C-calcined LiMn₂O₄ film exhibited the best rate performance in comparison with the ones calcined at other temperatures.     

LiMn2O4 cathode materials synthesized by the cellulose–citric acid method for lithium ion batteries

A new technique was employed to synthesize spinel LiMn₂O₄ cathode materials by adding cellulose and citric acid to an aqueous solution of lithium and manganese salts. Various synthesis conditions such as the calcination temperature and the citric acid-to-metal ion molar ratio (R) were investigated to determine the ideal conditions for preparing LiMn₂O₄ with the best electrochemical characteristics. The optimal synthesis conditions were found to be R = 1/3 and a calcination temperature of 800 °C. The initial discharge capacity of the material synthesized using the optimal conditions was 134 mAh g⁻¹, and the discharge capacity after 40 cycles was 125 mAh g⁻¹, at a current density of 0.15 mA cm⁻² between 3.0 and 4.35 V. Details of how the initial synthesis conditions affected the capacity and cycling performance of LiMn₂O₄ are discussed.     

Improving the high-temperature performance of LiMn2O4 spinel by micro-emulsion coating of LiCoO2

LiMn₂O₄ spinel, which has known performance deficiency in rechargeable lithium batteries at elevated temperatures, is coated with grains of LiCoO₂ through a novel micro-emulsion process. The resulting composite is characterized by elemental analysis, thermogravimetric analysis, differential scanning calorimetry, powder X-ray diffraction (XRD), and scanning and transmission electron microscopies. For assessment of electrochemical performance, the coated spinel is charged and discharged under typical conditions of battery application, and its cycleability and storage characteristics are determined at 55 °C. The results show substantially improved high-temperature performance, with a small reduction in capacity and specific energy relative to the pristine spinel.     

Improvement of 12 V overcharge behavior of LiCoO2 cathode material by LiNi0.8Co0.1Mn0.1O2 addition in a Li-ion cell

The 12 V overcharge instability of the LiCoO₂ cathode material was improved by the physical blending it with LiNi_0.8Co_0.1Mn_0.1O₂. Even though a Li-ion cell containing a LiCoO₂ cathode did not exhibit thermal runaway at 12 V at the 1 C overcharging rate, it showed thermal runaway at the 2 C overcharging rate, and the cell surface temperature reached more than 400 °C. However, the LiCoO₂ cell containing 40, 50, and 60 wt.% LiNi_0.8Co_0.1Mn_0.1O₂ did not exhibit thermal runaway at the 2 C overcharging rate. In conclusion, 60 wt.% LiNi_0.8Co_0.1Mn_0.1O₂ in the LiCoO₂ cathode showed the lowest cell surface temperature of <90 °C even at a 3 C overcharging rate.     

Characterization of Zn- and Fe-substituted LiMnO2 as cathode materials in Li-ion cells

Layered LiMn_1−xM_xO₂ (M = Zn or Fe) (0 ≤ x ≤ 0.3) samples are synthesized from the corresponding sodium analogues by an ion-exchange method using LiBr in n-hexanol at 160 °C. The samples are subjected to physicochemical and electrochemical characterization. X-ray diffraction data indicate the formation of layered structures for the LiMn_1−xZn_xO₂ samples up to x = 0.3 and for LiMn_1−xFe_xO₂ samples up to x = 0.2. Among these, LiMn_0.95Zn_0.05O₂ and LiMn_0.95Fe_0.05O₂ provide the highest capacity values of 180 and 193 mAh g⁻¹, respectively. Both Zn- and Fe-substituted samples display good capacity retention up to 30 charge–discharge cycles. Electrochemical impedance spectroscopy and galvanostatic intermittent titration data corroborate the results obtained from cyclic volatmmetry and charge–discharge cycling.     

Lithium cobalt oxide cathode film prepared by rf sputtering

Thin films of LiCoO₂ prepared by radio frequency magnetron sputtering on Pt-coated silicon are investigated under various deposited parameters such as working pressure, gas flow rate of Ar to O₂, and heat-treatment temperature. The as-deposited film was a nanocrystalline structure with (1 0 4) preferred orientation. After annealing at 500–700 °C, single-phase LiCoO₂ is obtained when the film is originally deposited under an oxygen partial pressure (P_O2) from 5 to 10 mTorr. When the sputtering process is performed outside these P_O2 values, a second phase of Co₃O₄ is formed in addition to the HT-LiCoO₂ phase. The degree of crystallization of the LiCoO₂ films is strongly affected by the annealing temperature; a higher temperature enhances the crystallization of the deposited LiCoO₂ film. The grain sizes of LiCoO₂ films annealed at 500, 600 and 700 °C are about 60, 95, and 125 nm, respectively. Cyclic voltammograms display well-defined redox peaks. LiCoO₂ films deposited by rf sputtering are electrochemically active. The first discharge capacity of thin LiCoO₂ films annealed at 500, 600 and 700 °C is about 41.77, 50.62 and 61.16 μAh/(cm² μm), respectively. The corresponding 50th discharge capacities are 58.1, 72.2 and 74.9% of the first discharge capacity.     

Electrochemical and solid-state NMR studies on LiCoO2 coated with Al2O3 derived from carboxylate-alumoxane

The surface of LiCoO₂ cathodes was coated with various wt.% of Al₂O₃ derived from methoxyethoxy acetate-alumoxane (MEA-alumoxane) by a mechano-thermal coating procedure, followed by calcination at 723 K in air for 10 h. The structure and morphology of the surface modified LiCoO₂ samples have been characterized with XRD, SEM, EDS, TEM, BET, XPS/ESCA and solid-state ²⁷Al magic angle spinning (MAS) NMR techniques. The Al₂O₃ coating forms a thin layer on the surface of the core material with an average thickness of 20 nm. The corresponding ²⁷Al MAS NMR spectrum basically exhibited the same characteristics as the spectrum for pristine Al₂O₃ derived from MEA-alumoxane, indicating that the local environment of aluminum atoms was not significantly changed at coating levels below 1 wt.%. This provides direct evidence that Al₂O₃ was on the surface of the core materials. The LiCoO₂ coated with 1 wt.% Al₂O₃ sustained continuous cycle stability 13 times longer than pristine LiCoO₂. A comparison of the electrochemical impedance behavior of the pristine and coated materials revealed that the failure of pristine cathode performance is associated with an increase in the particle–particle resistance upon continuous cycling. Coating improved the cathode performance by suppressing the characteristic structural phase transitions (hexagonal to monoclinic to hexagonal) that occur in pristine LiCoO₂ during the charge–discharge processes.     

Li+ ion diffusion in LiMn2O4 thin film prepared by PVP sol–gel method

LiMn₂O₄ thin film (1 μm thick) was prepared on a gold substrate by the PVP sol–gel method. The electrochemical properties of the thin-film electrode were studied in an electrolyte 1 mol dm⁻³ LiClO₄/(ethylene carbonate + diethyl carbonate). The prepared LiMn₂O₄ showed a good charge–discharge performance, and the capacity fade was ca. 20% during 200 cycles. The Li⁺ ion diffusion in the LiMn₂O₄ thin film was investigated by means of potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. The chemical diffusion coefficients were estimated to be 10⁻⁸ to 10⁻¹⁰ cm² s⁻¹.     

A stabilized NiO cathode prepared by sol-impregnation of LiCoO2 precursors for molten carbonate fuel cells

Layers of LiCoO₂ were formed on the internal surface of a porous NiO cathode to reduce the rate of NiO dissolution into the molten carbonate. A sol-impregnation technique assisted by acrylic acid (AA) was used to deposit gel precursors of LiCoO₂ on the pore surface of the Ni plate. Thermal treatment of the gel-coated cathode above 400 °C produced LiCoO₂ layers on the porous cathode. A number of bench-scale single cells were fabricated with LiCoO₂-coated cathodes and the cell performance was examined at atmospheric pressure for 1000 h. With the increase in the LiCoO₂ content in the cathode, the initial cell voltage decreased, but the cell performance gradually improved during the cell test. It was found from symmetric cathode cell test that the cathode was initially flooded with electrolyte, but redistribution of the electrolyte took place during the test and cell performance became comparable to that of a conventional NiO cathode. The amount of Ni precipitated in the matrix during the cell operation for 1000 h was significantly reduced by the LiCoO₂ coating. For instance, coating 5 mol% of LiCoO₂ in the cathode led to a 56% reduction of Ni precipitation in the matrix. The results obtained in this study strongly suggest that LiCoO₂ layers formed on the internal surface of the porous NiO cathode effectively suppress the rate of NiO dissolution for 1000 h.     

One-step synthesis LiMn2O4 cathode by a hydrothermal method

Spinel LiMn₂O₄ powders have been successfully synthesized by a hydrothermal method directly, which is no any pretreatment and following treatment in the process. The structure and morphology of the powders were studied in detail by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and thermogravimetric analysis (TGA). The data reveal that the products have well-defined stable spinel structure, and the particles show distinctive crystal faces with 50–300 nm in particle sizes. The electrochemical characteristics of the spinel materials are measured in the coin-type cells in a potential range of 3.2–4.35 V versus Li/Li⁺. The as-synthesized LiMn₂O₄ delivers reversible capacity of about 121 mAh g⁻¹ at a current density of 1/10 C. Cycled the cell to 40 cycles, the capacity remains at about 111 mAh g⁻¹ at 1/2 C.     

Nanostructured electrodes for next generation rechargeable electrochemical devices

Nanostructured intercalating electrodes offer immense potential for significantly enhancing the performance of rechargeable rocking chair (e.g. Li⁺ and Mg²⁺) and asymmetric hybrid batteries. The objective of this work has been to develop a variety of cathode (e.g. V₂O₅, LiMnO₂ and LiFePO₄) and anode (e.g. Li₄Ti₅O₁₂) materials with unique particle characteristics and controlled composition to reap the maximum benefits of nanophase electrodes for rechargeable Li-based batteries. Different processing routes, which were chosen on the basis of the final composition and the desired particle characteristics of electrode materials, were developed to synthesize a variety of electrode materials. Vapor phase processes were used to synthesize nanopowders of V₂O₅ and TiO₂. TiO₂ was the precursor used for producing ultrafine particles of Li₄Ti₅O₁₂. Liquid phase processes were used to synthesize nanostructured LiMn_xM_1−xO₂ and LiFePO₄ powders. It was found that (i) nanostructured V₂O₅ powders with a metastable structure have 30% higher retention capacity than their coarse-grained counterparts, for the same number of cycles; (ii) the specific capacity of nanostructured LiFePO₄ cathodes can be significantly improved by intimately mixing nanoparticles with carbon particles and that cathodes made of LiFePO₄/C composite powder exhibited a specific capacity of ∼145 mAh/g (85% of the theoretical capacity); (iii) nanostructured, layered LiMn_xM_1−xO₂ cathodes demonstrated a discharge capacity of ∼245 mAh/g (86% of the theoretical capacity) at a slow discharge rate; however, the composition and structure of nanoparticles need to be optimized to improve their rate capabilities and (iv) unlike micron-sized (1–10 μm) powders, ultrafine Li₄Ti₅O₁₂ showed exceptional retention capacity at a discharge rate as high as 10 C in Li-test cells.     

Characterization of a LiCoO2 thick film by screen-printing for a lithium ion micro-battery

A thick film cathode has been fabricated by a screen-printing technique using LiCoO₂ paste to improve the discharge capacity in lithium ion micro-batteries. The LiCoO₂ thick film (about 6 μm) was obtained by screen-printing, but high discharge capacity and a suitable surface roughness of printed LiCoO₂ film cathodes could not be obtained by adding carbon black only to the LiCoO₂ paste. On the other hand, the printed cathode which was prepared using the mixture of carbon-coated LiCoO₂ powders and carbon black showed a typical discharge curve of a LiCoO₂ cathode with a high discharge capacity (179 μAh cm⁻²).     

Preparation and characterization of layered LiMn1/3Ni1/3Co1/3O2 as a cathode material by an oxalate co-precipitation method

The layered LiMn_1/3Ni_1/3Co_1/3O₂ cathode materials were synthesized by an oxalate co-precipitation method using different starting materials of LiOH, LiNO₃, [Mn_1/3Ni_1/3Co_1/3]C₂O₄·2H₂O and [Mn_1/3Ni_1/3Co_1/3]₃O₄. The morphology, structural and electrochemical behavior were characterized by means of SEM, X-ray diffraction analysis and electrochemical charge–discharge test. The cathode material synthesized by using LiNO₃ and [Mn_1/3Ni_1/3Co_1/3]C₂O₄·2H₂O showed higher structural integrity and higher reversible capacity of 178.6 mAh g⁻¹ in the voltage range 3.0–4.5 V versus Li with constant current density of 40 mA g⁻¹ as well as lower irreversible capacity loss of 12.9% at initial cycle. The rate capability of the cathode was strongly influenced by particle size and specific surface area.     

Electrochemical properties of iodine-containing lithium manganese oxide spinel

Iodine-containing, cation-deficient, lithium manganese oxides (ICCD-LMO) are prepared by reaction of MnO₂ with LiI. The MnO₂ is completely transformed into spinel-structured compounds with a nominal composition of Li_1−δMn_2−2δO₄I_x. A sample prepared at 800 °C, viz. Li_0.99Mn_1.98O₄I_0.02, exhibits an initial discharge capacity of 113 mA h g⁻¹ with good cycleability and rate capability in the 4-V region. Iodine-containing, lithium-rich lithium manganese oxides (ICLR-LMO) are also prepared by reaction of LiMn₂O₄ with LiI, which results in a nominal composition of Li_1+xMn_2−xO₄I_x. Li_1.01Mn_1.99O₄I_0.02 shows a discharge capacity of 124 mA h g⁻¹ on the first cycle and 119 mA h g⁻¹ a on the 20th cycle. Both results indicate that a small amount of iodine species helps to maintain cycle performance.     

Preparation of LiMn2O4 at the low temperature of 250 °C using eutectic self-mixing method

Spinel LiMn₂O₄ as a cathode material for lithium rechargeable batteries is prepared at the low temperature of 250 °C without any artificial mixing procedures of reactants. The phase transitions of lithium manganese oxide are found three times on heating at 250 °C. The prepared material exhibits the initial discharge capacity of 85.5 mAh g⁻¹ and the discharge capacity retention of 91.5% after 50 cycles.     

Synthesis of LiMn2O4 powder by auto-ignited combustion of poly(acrylic acid)-metal nitrate precursor

Spinel LiMn₂O₄ powder has been prepared by an auto-ignited combustion of poly(acrylic acid) (PAA)-metal nitrate precursor. The formation route to spinel LiMn₂O₄ phase from the precursor strongly depends on the mole ratio (MR) of PAA to metal nitrate. The precursors with MR=1.0 and 6.0 directly produced crystalline LiMn₂O₄ powders by the auto-ignited combustion at very low temperature (200°C), whereas the precursor with MR=3.0 produced Mn₃O₄, MnO, and carbonate at the auto-ignited combustion stage and formed spinel LiMn₂O₄ phase after further calcinations at higher temperatures. The obtained LiMn₂O₄ powders are composed of very fine primary particles (<100 nm) but highly agglomerates and have surface area of 14–29 m² g⁻¹.     

In situ XRD studies of the structural changes of ZrO2-coated LiCoO2 during cycling and their effects on capacity retention in lithium batteries

Synchrotron based in situ X-ray diffraction was used to study the structural changes of a ZrO₂-coated LiCoO₂ cathode in comparison with the uncoated sample during multi-cycling in a wider voltage window from 2.5 to 4.8 V. It was found that the improved cycling performance of ZrO₂-coated LiCoO₂ is closely related to the larger lattice parameter “c” variation range, which is an indicator of how far the structural change has proceeded towards the two end members of the phase transition stream during charge–discharge cycling. At fifth charge, the lattice parameter variation ranges for both uncoated and ZrO₂-coated LiCoO₂ were reduced compared with those for the first charge, reflecting the capacity fading caused by the high voltage cycling. However, this variation range reduction is smaller in ZrO₂-coated LiCoO₂ than that in the uncoated sample, and so is the capacity fading. These results point out an important direction for studying the fading mechanism and coating effects: the key issues are the surface protection, the interaction between the cathode surface and the electrolyte and the electrolyte decomposition. In order to improve the capacity retention during cycling, the variation range of lattice parameter “c” of LiCoO₂ should be preserved, not reduced.