Enhancing the elevated temperature performance of Li/LiMn2O4 cells by reducing LiMn2O4 surface area

Lithium-rich spinels were obtained with the same structure but different surface area by two different synthesis routes, namely the “once-annealed” and the “twice-annealed” methods. The elevated temperature performance of Li/Li_1+xMn₂O₄ cell is significantly improved using a spinel cathode with a small surface area: the cell at 50°C lost 5% of the initial capacity over the first 100 cycles based on a spinel cathode with the small surface area of 1.2 m²/g compared to 8% based on a large one of 6.2 m²/g. Also the mechanism responsible for the reaction of LiMn₂O₄ with LiOH to form lithium-rich spinel has been investigated.     

Influence of synthesis conditions on electrochemical properties of high-voltage Li1.02Ni0.5Mn1.5O4 spinel cathode material

Li_1.02Ni_0.5Mn_1.5O₄ spinel cathode materials were successfully synthesized by a citric acid-assisted sol-gel method. The structure and morphology of the materials have been examined by X-ray diffraction and scanning electron microscopy, respectively. Electrochemical properties of the materials were investigated using cyclic voltammetry and galvanostatic charge/discharge measurements at two different temperatures (25 and 55 °C) using lithium anode. The initial capacity and capacity retention are highly dependent on the particle size, particle size distribution, crystallinity and purity of the materials. The Li_1.02Ni_0.5Mn_1.5O₄ materials synthesized both at 800 and 850 °C have shown best electrochemical performance in terms of capacity and capacity retention between 3.5 and 4.9 V with a LiPF₆ based electrolyte.     

X-ray absorption spectra of the spinel LiCu0.5Mn1.5O4

X-ray absorption spectra of the spinel LiCu_0.5Mn_1.5O₄ were recorded at the Cu K and L₃, Mn K and L₃, and O K edge. The spectra are aligned on a common energy scale with the aim to establish an experimental picture of the conduction band structure. The fine structures observed a few electron volts around the absorption thresholds are discussed in terms of hybridisation of cation and anion orbitals. Emphasis is put on the identification of spectral features correlated with the presence of lithium on the tetrahedral sites of the spinel structure. The consequences of lithium insertion/extraction on the intensities of these spectral structures are discussed. Previous studies by X-ray absorption spectroscopy of lithium insertion/extraction in various spinels are found to agree with our expectations.     

Performance of LiNi0.5Mn1.5O4 prepared by solid-state reaction

LiNi_0.5Mn_1.5O₄ was prepared through a solid-state reaction using various Ni precursors. The effect of precursors on the electrochemical performance of LiNi_0.5Mn_1.5O₄ was investigated. LiNi_0.5Mn_1.5O₄ made from Ni(NO₃)₂·6H₂O shows the best charge–discharge performance. The reversible capacity of LiNi_0.5Mn_1.5O₄ is about 145 mAh g⁻¹ and remained 143 mAh g⁻¹ after 10 cycles at 3.0–5.0 V. The XRD results showed that the precursors and the dispersion methods had significant effect on their phase purity. Pure spinel phase can be obtained with high energy ball-milling method and Ni(NO₃)₂·6H₂O as precursor. Trace amount of NiO and Li₂MnO₃ phase were detected in LiNi_0.5Mn_1.5O₄ with manual-mixture method and using Ni(CH₃COO)₂·6H₂O, NiO and Ni₂O₃ as precursors.     

Improvement of cathode performance of LiMn2O4 as a cathode active material for Li ion battery by step-by-step supersonic-wave treatments

As a novel partial substitution and surface modification process, we focused on a step-by-step (double-step) supersonic-wave treatment in a Zn-containing aqueous solution without any heat-treatments, and performed the treatment on LiMn₂O₄ powder. From XRD measurements, it was demonstrated that the lattice constant of LiMn₂O₄ decreased slightly by the treatments, indicating a partial substitution of Zn for Mn. It was also suggested by SEM–EDX and XPS that Zn was well dispersed in/on the samples and their surfaces were modified by Zn compounds. Such a partial substitution and surface modification was supported by crystal structure analysis based on the Rietveld method using neutron diffraction. Cycle performance of LiMn₂O₄ was significantly improved by the step-by-step supersonic-wave treatments. In the processes, it was especially effective for the improvement to apply lower and higher frequencies at the first and second steps, respectively, keeping the power higher. The cathode property improvement was considered due to the partial substitution and the surface coating caused by the step-by-step supersonic-wave treatments. From the investigation on the cathodes and electrolytes after the cycle tests, it was suggested that the crystal structure of LiMn₂O₄ was stabilized by the treatments.     

Electrochemical performance of all-solid-state Li batteries based LiMn0.5Ni0.5O2 cathode and NASICON-type electrolyte

LiNi_0.5Mn_0.5O₂ thin films have been deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering followed by annealing. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. All-solid-state Li/PEO₁₈-Li(CF₃SO₂)₂N/LATSP/LiNi_0.5Mn_0.5O₂/Au cells are fabricated using the LiNi_0.5Mn_0.5O₂ thin films and the LATSP electrolyte. The electrochemical performance of the cells is investigated by galvanostatic cycling, cyclic voltammetry (CV), potentiostatic intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS). Interfacial reactions between LiNi_0.5Mn_0.5O₂ and LATSP occur at a temperature as low as 300 °C with the formation of Mn₃O₄, resulting in an increased obstacle for Li-ion diffusion across the LiNi_0.5Mn_0.5O₂/LATSP interface. The electrochemical performance of the cells is limited by the interfacial resistance between LATSP and LiNi_0.5Mn_0.5O₂ as well as the Li-ion diffusion kinetics in LiNi_0.5Mn_0.5O₂ bulk.     

On the LiNi0.2Mn0.2Co0.6O2 positive electrode material

Layered LiNi_0.2Mn_0.2Co_0.6O₂ phase, belonging to a solid solution between LiNi_1/2Mn_1/2O₂ and LiCoO₂ most commercialized cathodes, was prepared via the combustion method at 900 °C for a short time (1 h). Structural, electrochemical and magnetic properties of this material were investigated. Rietveld analysis of the XRD pattern shows this compound as having the α-NaFeO₂ type structure (S.G. R-3m; a = 2.8399(2) ?; c = 14.165(1) ?) with almost none of the well-known Li/Ni cation disorder. SQUID measurements clearly indicate that the studied compound consists of Ni²⁺, Co³⁺ and Mn⁴⁺ ions in the crystal structure. X-ray analysis of the chemically delithiated Li_xNi_0.2Mn_0.2Co_0.6O₂ phases reveals that the rhombohedral symmetry was maintained during Li-extraction, confirmed by the monotonous variation of the potential-composition curve of the Li//Li_xNi_0.2Mn_0.2Co_0.6O₂ cell. LiNi_0.2Mn_0.2Co_0.6O₂ cathode has a discharge capacity of ∼160 mAh g⁻¹ in the voltage range 2.7-4.3 V corresponding to the extraction/insertion of 0.6 lithium ion with very low polarization. It exhibits a stable capacity on cycling and good rate capability in the rate range 0.2-2 C. The almost 2D structure of this cathode material, its good electrochemical performances and its relatively low cost comparing to LiCoO₂, make this material very promising for applications.     

Electrochemical performance of ZnO-coated LiMn1.5Ni0.5O4 cathode material

LiMn_1.5Ni_0.5O₄ cathode material was prepared by sol–gel method and annealed at 850 °C for 15 h. The prepared powder was coated with ZnO by dissolving zinc acetate in methanol and LiMn_1.5Ni_0.5O₄ powder was mixed in this solution followed by the continuous stirring for 4 h. The LiMn_1.5Ni_0.5O₄ and ZnO-coated LiMn_1.5Ni_0.5O₄ powder was structurally characterized using X-ray diffraction and scanning electron microscopy (SEM). The coin cell was fabricated using ZnO-coated LiMn_1.5Ni_0.5O₄ as cathode materials, LiPF₆, dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 wt ratio) as electrolyte, and Li foil as anode. It was found that ZnO-coated LiMn_1.5Ni_0.5O₄ cathode materials had the initial discharge capacity of about 146 mA h g⁻¹. The discharge capacity retention after 50 cycles was found to be nearly 97%.     

PMMA-assisted synthesis of Li1−xNi0.5Mn1.5O4−δ for high-voltage lithium batteries with expanded rate capability at high cycling temperatures

In this work, poly(methyl methacrylate) (PMMA), a non-surfactant polymer was used to synthesize nonstoichiometric Li_0.82Ni_0.52Mn_1.52O_4−δ (0 ≤ δ ≤ 0.25) spinels. The presence of the polymer was found to be beneficial with a view to facilitating the use of the spinel in electrodes for lithium batteries. Thus, PMMA allowed spinel particles of a high crystallinity and uniform size and shape to be obtained, and particle size to be tailored by using an appropriate calcining temperature and time. By controlling these variables, spinels in nanometric, submicrometric and micrometric particle sizes were prepared and characterized by chemical analysis, X-ray diffraction, electron microscopy, thermogravimetry and nitrogen adsorptions measurements. The spinels were obtained as highly crystalline phases with lithium and oxygen deficiency and some cation disorder as revealed by chemical analysis, thermogravimetric and XRD data. Their electrochemical performance in two-electrode cells was tested at room temperature and 50 °C over a wide range of charge/discharge rates (from C/4 to 4C). Cell performance was found to depend on particle size rather than on structural properties. Thus, the spinel best performing at 50 °C was that consisting of submicrometric particles, which delivered a high capacity and exhibited the best capacity retention and rate capability. Particles of submicronic size share the advantages of nanometric particles (viz. the ability to withstand high charge/discharge rates) and micrometric particles (a high capacity and stability at low rates).     

Improved electrochemical performance of AlPO4-coated LiMn1.5Ni0.5O4 electrode for lithium-ion batteries

LiMn_1.5Ni_0.5O₄ materials coated with AlPO₄ are prepared by a sol-gel method with citric acid to improve their electrochemical performance; the physical and electrochemical properties are characterized by various analytical techniques. The coated AlPO₄ layer completely covers the surfaces of the LiMn_1.5Ni_0.5O₄ particles and the thickness of the coated layer is ∼15 nm. 1 wt.% AlPO₄-coated LiMn_1.5Ni_0.5O₄ has much lower surface and charge-transfer resistances and shows a higher lithium diffusion rate in comparison with the pristine sample. The modified material demonstrates dramatically enhanced electrochemical reversibility and stability under elevated temperature conditions. This is because the coated AlPO₄ layer reduces the contact area between the electrode and electrolyte and suppresses the formation of undesirable solid electrolyte interface films.     

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.     

Mg-doped LiNi0.5Mn1.5O4 spinel for cathode materials

Lithium-ion batteries are becoming more and more important not only for portable electronic devices, but also in prevision of high power electric vehicles. In such an optic, deep studies regarding all the components of a secondary battery are in development. In this study, high voltage cathode materials have been selected. Crystals with spinel structure have a 3D vacancy pathway suitable for Li-ions transport. The material under study was LiNi_0.5Mn_1.5O₄ doped with magnesium replacing the nickel. Various samples were synthesized via three different routes: a solid-state method, a modified sol–gel method and a xerogel method. The structure and morphology of the powders were analyzed with HRTEM and XRD. Electrochemical tests were also performed. A wide range of particle sizes (from micro to nanosize) was the result of the different synthesis routes. Unfortunately pure materials were not always obtained. The electrochemical tests showed improvement of the material's cyclability, by reducing the particle size. The electrochemical tests further confirmed the existence of a Li_1+dMn_2−dO₄ impurity. The results are quite promising, however, further improvement of the purity of the electrode composition are needed.     

Synthesis and electrochemical properties of LiNi0.8Co0.2O2 nanopowders for lithium ion battery applications

Nitrates of lithium, cobalt and nickel are utilized to synthesize LiNi_0.8Co_0.2O₂ cathode material through sol-gel technique. Various synthesis parameters such as calcination time and temperature as well as chelating agent are studied to determine the optimized condition for material processing. Using TG/DTA techniques, the optimized calcination temperatures are selected. Different characterization techniques such as ICP, XRD and TEM are employed to characterize the chemical composition, crystal structure, size and morphology of the powders. Micron and nano-sized powders are produced using citric/oxalic and TEA as chelating agent, respectively. Selected powders are used as cathode material to assemble batteries. Charge-discharge testing of these batteries show that the highest discharge capacity is 173 mAh g⁻¹ at a constant current of 0.1 mA cm⁻², between 3.0 and 4.2 V. This is obtained in a battery assembled with the nanopowder produced by TEA as chelating agent.     

Synthesis and electrochemical performances of core-shell structured Li[(Ni1/3Co1/3Mn1/3)0.8(Ni1/2Mn1/2)0.2]O2 cathode material for lithium ion batteries

Micro-scale core-shell structured Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ powders for use as cathode material are synthesized by a co-precipitation method. To protect the core material Li[Ni_1/3Co_1/3Mn_1/3]O₂ from structural instability at high voltage, a Li[Ni_1/2Mn_1/2]O₂ shell, which provides structural and thermal stability, is used to encapsulate the core. A mixture of the prepared core-shell precursor and lithium hydroxide is calcined at 770 °C for 12 h in air. X-ray diffraction studies reveal that the prepared material has a typical layered structure with an space group. Spherical morphologies with mono-dispersed powders are observed in the cross-sectional images obtained by scanning electron microscopy. The core-shell Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ electrode has an excellent capacity retention at 30 °C, maintaining 99% of its initial discharge capacity after 100 cycles in the voltage range of 3-4.5 V. Furthermore, the thermal stability of the core-shell material in the highly delithiated state is improved compared to that of the core material. The resulting exothermic onset temperature appear at approximately 272  °C, which is higher than that of the highly delithiated Li[Ni_1/3Co_1/3Mn_1/3]O₂ (261 °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.     

Facile synthesis and electrochemical performance of ordered LiNi0.5Mn1.5O4 nanorods as a high power positive electrode for rechargeable Li-ion batteries

One-dimensional ordered LiNi_0.5Mn_1.5O₄ nanorods have been fabricated and investigated for use as a high power cathode in rechargeable Li-ion batteries. These highly crystalline nanorods, with an ordered spinel structure and diameters and lengths around 130 nm and 1.2 μm, respectively, were synthesized in two steps by using a hydrothermal reaction to produce β-MnO₂ nanorods followed by solid-state lithiation. Electrochemical analysis showed the superior performance of nanorods as a cathode in Li-ion half cells. The specific charge and discharge capacities were found to be 120 and 116 mAh g⁻¹ at a 0.5 C rate, and 114 and 111 mAh g⁻¹ at a 1 C rate between 3.5 and 5.0 V vs. Li⁺/Li. Moreover, the nanorods exhibit high power capability, maintaining capacities of 103 and 95 mAh g⁻¹ at specific currents of 732.5 and 1465 mA g⁻¹ (5 and 10 C rates), respectively.     

Research progress in high voltage spinel LiNi0.5Mn1.5O4 material

Lithium-ion batteries are now considered to be the technology of choice for future hybrid electric and full electric vehicles to address global warming. LiCoO₂ has been the most widely used cathode material in commercial lithium-ion batteries. Since LiCoO₂ has economic and environmental issues, intensive research has been directed towards the development of alternative low cost, environmentally friendly cathode materials as possible replacement of LiCoO₂. Among them, spinel LiNi_0.5Mn_1.5O₄ material is one of the promising and attractive cathode materials for next generation lithium-ion batteries because of its high voltage (4.7 V), acceptable stability, and good cycling performance. Research advances in high voltage spinel LiNi_0.5Mn_1.5O₄ are reviewed in this paper. Developments in synthesis, structural characterization, effect of doping, and effect of coating are presented. In addition to conventional synthesis methods, several alternative synthesis methods are also summarized. Apart from battery performance, the application of spinel LiNi_0.5Mn_1.5O₄ material in asymmetric supercapacitors is also discussed.     

A high energy Li-ion battery based on nanosized LiNi0.5Mn1.5O4 cathode material

The electrochemical performance of a Li-ion battery made from nanometric, highly crystalline LiNi_0.5Mn_1.5O₄ as positive electrode and mesoporous carbon microbeads (MCMBs) as negative electrode was assessed. The best performance was obtained by using a slight excess of spinel (a cathode/anode mole ratio of 1.3) and lithium bis-oxalate borate (LiBOB) instead of LiPF₆ as an electrolyte salt. Higher spinel contents caused the formation of metallic Li in the carbon and the rapid degradation of battery performance as a result. The calculated output energy was 322 Wh kg⁻¹ which is higher than the value reported for the LiMn₂O₄/C cell (250 Wh kg⁻¹).     

Influence of lithium content on high rate cycleability of layered Li1+xNi0.30Co0.30Mn0.40O2 cathodes for high power lithium-ion batteries

Layered Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) materials have been synthesized using citric acid assisted sol-gel method. The materials with excess lithium showed distinct differences in the structure and the charge and discharge characteristics. The rate capability tests were performed and compared on Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) cathode materials. Among these materials, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode demonstrated higher discharge capacity than that of the other cathodes. Upon extended cycling at 1C and 8C, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ showed better capacity retention when compared to other materials with different lithium content. Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ exhibited 93 and 90% capacity retention where as Li_1.05Ni_0.30Co_0.30Mn_0.40O₂, Li_1.15Ni_0.30Co_0.30Mn_0.40O₂, and Li_1.00Ni_0.30Co_0.30Mn_0.40O₂ exhibited only 84, 71, and 63% (at 1C), and 79, 66 and 40% (at 10C) capacity retention, respectively, after 40 cycles. The enhanced high rate cycleability of Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode is attributed to the improved structural stability due to the formation of appropriate amount of Li₂MnO₃-like domains in the transition metal layer and decreased Li/Ni disorder (i.e., Ni content in the Li layer).     

Effect of capacity matchup in the LiNi0.5Mn1.5O4/Li4Ti5O12 cells

The effect of the capacity matchup between cathode and anode in the LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ cell system on cycling property, choice of electrolyte, high voltage and overcharge tolerances was investigated by comparing the cells with Li₄Ti₅O₁₂ limiting capacity with the cells with LiNi_0.5Mn_1.5O₄ limiting capacity. The former exhibits better cycling performance and less limitation of electrolyte choice than the latter. Furthermore, the Li₄Ti₅O₁₂-limited cell exhibits better tolerance to high voltage and overcharge than the LiNi_0.5Mn_1.5O₄-limited cell, owing to taking advantage of the extra capacity of Li₄Ti₅O₁₂ below 1 V. It is thus recommended that the LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ cell whose capacity is limited by Li₄Ti₅O₁₂ anode should be used to extend the application of the state-of-the-art lithium-ion batteries.