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).     

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).     

Nano-sized LiMn2O4 spinel cathode materials exhibiting high rate discharge capability for lithium-ion batteries

Nano-sized LiMn₂O₄ spinel with well crystallized homogeneous particles (60 nm) is synthesized by a resorcinol-formaldehyde route. Micro-sized LiMn₂O₄ spinel with micrometric particles (1 μm) is prepared by a conventional solid-state reaction. These two samples are characterized by XRD, SEM, TEM, BET, and electrochemical methods. At current rate of 0.2C (1C = 148 mA g⁻¹), a discharge capacity of 136 mAh g⁻¹ is obtained on the nano-sized LiMn₂O₄, which is higher than that of micro-sized one (103 mAh g⁻¹). Furthermore, compared to the micro-sized sample, nano-sized LiMn₂O₄ shows much better rate capability, i.e. a capacity of 85 mAh g⁻¹, 63% of that at 0.2C, is realized at 60C. The excellent high rate performance of nano-sized LiMn₂O₄ spinel may be attributed to its impurity-free nano-sized particles, higher surface area and well crystalline. The outstanding electrochemical performances demonstrate that the nano-sized LiMn₂O₄ spinel will be the promising cathode materials for high power lithium-ion batteries used in hybrid and electric vehicles.     

A modified Al2O3 coating process to enhance the electrochemical performance of Li(Ni1/3Co1/3Mn1/3)O2 and its comparison with traditional Al2O3 coating process

Al₂O₃-modified Li(Ni_1/3Co_1/3Mn_1/3)O₂ is synthesized by a modified Al₂O₃ coating process. The Al₂O₃ coating is carried out on an intermediate, (Ni_1/3Co_1/3Mn_1/3)(OH)₂, rather than on Li(Ni_1/3Co_1/3Mn_1/3)O₂. As a comparison, Al₂O₃-coated Li(Ni_1/3Co_1/3Mn_1/3)O₂ also is prepared by traditional Al₂O₃ coating process. The effects of Al₂O₃ coating and Al₂O₃ modification on structure and electrochemical performance are investigated and compared. Electrochemical tests indicate that cycle performance and rate capability of Li(Ni_1/3Co_1/3Mn_1/3)O₂ are enhanced by Al₂O₃ modification without capacity loss. Al₂O₃ coating can also enhance the cycle performance but cause evident capacity loss and decline of rate capability. The effect of Al₂O₃ coating and Al₂O₃ modification on kinetics of lithium-ion transfer reaction at the interface of electrode/electrolyte is investigated via electrochemical impedance spectra (EIS). The result support that the Al₂O₃ modification increase Li⁺ diffused coefficient and decrease the activation energy of Li⁺ transfer reaction but the traditional Al₂O₃ coating lead to depression of Li⁺ diffused coefficient and increase of activation energy.     

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).     

Synthesis and electrochemical properties of Na-doped Li3V2(PO4)3 cathode materials for Li-ion batteries

Na-doped Li_3−xNa_xV₂(PO₄)₃/C (x = 0.00, 0.01, 0.03, and 0.05) compounds have been prepared by using sol-gel method. The Rietveld refinement results indicate that single-phase Li_3−xNa_xV₂(PO₄)₃/C with monoclinic structure can be obtained. Among three Na-doped samples and the undoped one, Li_2.97Na_0.03V₂(PO₄)₃/C sample has the highest electronic conductivity of 6.74 × 10⁻³ S cm⁻¹. Although the initial specific capacities for all Na-doped samples have no much enhancement at the current rate of 0.2 C, both cycle performance and rate capability have been improved. At the 2.0 C rate, Li_2.97Na_0.03V₂(PO₄)₃/C presents the highest initial capacity of 118.9 mAh g⁻¹ and 12% capacity loss after 80 cycles. The partial substitution of Li with Na (x = 0.03) is favorable for electrochemical rate and cyclic ability due to the enlargement of Li₃V₂(PO₄)₃ unit cells, optimizing the particle size and morphology, as well as resulting in a higher electronic conductivity.     

Preparation of spherical LiNi0.80Co0.15Mn0.05O2 lithium-ion cathode material by continuous co-precipitation

Micro-spherical Ni_0.80Co_0.15Mn_0.05(OH)₂ precursors with a narrow size-distribution and high tap-density are prepared successfully by continuous co-precipitation of the corresponding metal salt solutions using NaOH and NH₄OH as precipitation and complexing agents. LiNi_0.80Co_0.15Mn_0.05O₂ is then prepared as a lithium battery cathode from this precursor by the introduction of LiOH·H₂O. The pH and NH₃:metal molar ratio show significant effects on the morphology, microstructure and tap-density of the prepared Ni_0.80Co_0.15Mn_0.05(OH)₂ and the R values and I(0 0 3)/I(1 0 4) ratio of lithiated LiNi_0.80Co_0.15Mn_0.05O₂. Spherical LiNi_0.80Co_0.15Mn_0.05O₂ prepared under optimum conditions reveals a hexagonally ordered, layered structure without cation mixing and an initial charging capacity of 176 mAhg⁻¹. More than 91% of the capacity is retained after 40 cycles at the 1 C rate in a cut-off voltage range of 4.3-3.0 V.     

Structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2: Calcination temperature dependence

The structure of the layered LiNi_1/3Co_1/3Mn_1/3O₂ has been investigated by powder X-ray diffraction and electron diffraction, and the relationship of the calcination temperature with the crystal structure, morphology and electrochemical properties has been studied. All the unit cell parameters increase monotonically with increasing the calcination temperature. Some of the [00.1] zone electron diffraction patterns for the sample calcined at higher temperature than 1000 °C show extra spots indicating the 2 × 2 ordering in the basal triangular lattice. These results indicate that the high temperature calcination leads to the formation of vacancies in the transition metal layers with the spinel-like ordering. The calcination at higher temperature lowers the specific capacities and degrades the cycle performances, while the packing density of the powder is increased by the sintering. The optimum calcination temperature is 900 °C in order to obtain the electrochemically active and dense packed oxide particles. The decrease of Li composition leads to coprecipitation of the spinel-like second phase in the range of 0.742 ≤ x ≤ 0.884 for Li_xNi_1/3Co_1/3Mn_1/3O₂, when calcined at 900 °C. The Li-deficient samples show the worse electrochemical properties similarly to the stoichiometric samples calcined at high temperature. For the Li-excess samples, no impurity phase has been detected and their cycle performances are improved.     

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.     

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 properties of spherical spinel Li1.05M0.05Mn1.9O4 (M = Mg and Al) as a cathode material for lithium-ion batteries by co-precipitation method

Cation (Mg and Al)-substituted spinel were synthesized using metal oxide precursor by co-precipitation method. XRD revealed that the prepared substituted spinel has spinel structure with Fd3m space group. In order to compensate the decreased initial capacity of cation-substituted spinel, partial anion (F) substitution was also carried out. The cycling performance of all the substituted spinel was improved, compared to the Li_1.05Mn_1.95O₄ at 55 °C. Li_1.05Al_0.1Mn_1.85O_3.95F_0.05 showed better capacity retention than the other substituted spinels. Both cation and anion substitution appeared to be effective for improving the cycling performance of spinel material at elevated temperature.     

Characterization of LiCoO2 coated Li1.05Ni0.35Co0.25Mn0.4O2 cathode material for lithium secondary cells

In this study, nano-crystalline LiCoO₂ was coated onto the surface of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ powders via sol–gel method. The influence of the coating on the electrochemical behavior of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ is discussed. The surface morphology was characterized by transmission electron microscopy (TEM). Nano-crystallized LiCoO₂ was clearly observed on the surfaces of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂. The phase and structural changes of the cathode materials before and after coating were revealed by X-ray diffraction spectroscopy (XRD). It was found that LiCoO₂ coated Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ cathode material exhibited distinct surface morphology and lattice constants. Cyclic voltammetry (2.8–4.6 V versus Li/Li⁺) shows that the characteristic voltage transitions on cycling exhibited by the uncoated material are suppressed by the 7 wt.% LiCoO₂ coating. This behavior implies that LiCoO₂ inhibits structural change of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ or reaction with the electrolyte on cycling. In addition, the LiCoO₂ coating on Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ significantly improves the rate capability over the range 0.1–4.0C. Comparative data for the coated and uncoated materials are presented and discussed.     

Li4Ti5O12/Sn composite anodes for lithium-ion batteries: Synthesis and electrochemical performance

Li₄Ti₅O₁₂/tin phase composites are successfully prepared by cellulose-assisted combustion synthesis of Li₄Ti₅O₁₂ matrix and precipitation of the tin phase. The effect of firing temperature on the particulate morphologies, particle size, specific surface area and electrochemical performance of Li₄Ti₅O₁₂/tin oxide composites is systematically investigated by SEM, XRD, TG, BET and charge-discharge characterizations. The grain growth of tin phase is suppressed by forming composite with Li₄Ti₅O₁₂ at a calcination of 500 °C, due to the steric effect of Li₄Ti₅O₁₂ and chemical interaction between Li₄Ti₅O₁₂ and tin oxide. The experimental results indicate that Li₄Ti₅O₁₂/tin phase composite fired at 500 °C has the best electrochemical performance. A capacity of 224 mAh g⁻¹ is maintained after 50 cycles at 100 mA g⁻¹ current density, which is still higher than 195 mAh g⁻¹ for the pure Li₄Ti₅O₁₂ after the same charge/discharge cycles. It suggests Li₄Ti₅O₁₂/tin phase composite may be a potential anode of lithium-ion batteries through optimizing the synthesis process.     

Nanosized Li4Ti5O12/graphene hybrid materials with low polarization for high rate lithium ion batteries

We report a simple strategy to prepare a hybrid of lithium titanate (Li₄Ti₅O₁₂, LTO) nanoparticles well-dispersed on electrical conductive graphene nanosheets as an anode material for high rate lithium ion batteries. Lithium ion transport is facilitated by making pure phase Li₄Ti₅O₁₂ particles in a nanosize to shorten the ion transport path. Electron transport is improved by forming a conductive graphene network throughout the insulating Li₄Ti₅O₁₂ nanoparticles. The charge transfer resistance at the particle/electrolyte interface is reduced from 53.9 Ω to 36.2 Ω and the peak currents measured by a cyclic voltammogram are increased at each scan rate. The difference between charge and discharge plateau potentials becomes much smaller at all discharge rates because of lowered polarization. With 5 wt.% graphene, the hybrid materials deliver a specific capacity of 122 mAh g⁻¹ even at a very high charge/discharge rate of 30 C and exhibit an excellent cycling performance, with the first discharge capacity of 132.2 mAh g⁻¹ and less than 6% discharge capacity loss over 300 cycles at 20 C. The outstanding electrochemical performance and acceptable initial columbic efficiency of the nano-Li₄Ti₅O₁₂/graphene hybrid with 5 wt.% graphene make it a promising anode material for high rate lithium ion batteries.     

Thermal stability of Li1−yNixMn(1−x)/2Co(1−x)/2O2 layer-structured cathode materials used in Li-Ion batteries

In order to develop safe lithium-ion batteries using Ni-based cathode active materials, such as LiNi_xMn_(1−x)/2Co_(1−x)/2O₂, thermal stability is one of the most important requirements. We used XRD and TDS-MS in the first step of our study to elucidate the thermal stability and to improve it under anomalous high temperature conditions. We investigated the relationship between the thermal stability and cathode composition, especially for that of the nickel and lithium content. The XRD indicated that the crystal structure of electrochemically delithiated materials changed from a layered into a spinel structure followed by a rock-salt structure as the temperature rose. The TDS-MS indicated that these changes coincided with the release of oxygen from the cathode materials. We found that decreasing the lithium content and increasing the nickel content made the temperature of the crystal structure change and oxygen release lower, and thus, influenced the cathode composition.     

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.     

High rate capabilities of all-solid-state lithium secondary batteries using Li4Ti5O12-coated LiNi0.8Co0.15Al0.05O2 and a sulfide-based solid electrolyte

Rate capability of LiNi_0.8Co_0.15Al_0.05O₂ in solid-state cells was investigated with 70Li₂S-30P₂S₅ glass-ceramics as a sulfide solid electrolyte. It showed higher rate capability than LiCoO₂; discharge capacity observed at a current density of 10 mA cm⁻² was ca. 70 mAh g⁻¹. Surface coating with Li₄Ti₅O₁₂ onto the LiNi_0.8Co_0.15Al_0.05O₂ particles further improved the high-rate performance to give ca. 110 mAh g⁻¹. The rate capability promises to realize all-solid-state lithium secondary batteries with very high performance.     

Relationship between surface chemistry and electrochemical behavior of LiNi1/2Mn1/2O2 positive electrode in a lithium-ion battery

The formation and the evolution of lithium-containing species on the surface of grains of a layered 4 V material such as LiNi_1/2Mn_1/2O₂ along the electrochemical cycling have been followed using ⁷Li MAS NMR, electrochemical impedance spectroscopy (EIS) and XPS. Materials displaying different specific surface areas and stored in different atmospheres have been investigated in order to study the influence of the surface/volume ratio and the influence of the initial surface state, respectively. It is shown that the presence of an initial interphase of Li₂CO₃ influences the electrochemical behavior of the electrode, emphasizing the importance of the history of the electrode prior cycling. ⁷Li MAS NMR experiments performed upon cycling indicate the formation of interphase species in reduction and their partial removal in oxidation, indicating the dynamic character of the interphase upon cycling. Combined NMR, EIS and XPS experiments show the strong influence of the electrode/electrolyte interphase evolution on the electrochemical performance. Such results lead us to draw conclusions on the optimal storage conditions of layered 4 V materials for Li-ion batteries such as LiNi_1/2Mn_1/2O₂.     

Effect of carbon coating on LiNi1/3Mn1/3Co1/3O2 cathode material for lithium secondary batteries

Amorphous carbon is coated on LiNi_1/3Mn_1/3Co_1/3O₂ cathode material for lithium batteries. The carbon-coated material shows improved thermal stability and electrochemical performance compared with bare material.