Synthesis and electrochemical characterization of nano-CeO2-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

LiMn₂O₄ spinel cathode materials were coated with 0.5, 1.0, and 1.5 wt.% CeO₂ by a polymeric process, followed by calcination at 850 °C for 6 h in air. The surface-coated LiMn₂O₄ cathode materials were physically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron microscopy (XPS). XRD patterns of CeO₂-coated LiMn₂O₄ revealed that the coating did not affect the crystal structure or the Fd3m space group of the cathode materials compared to uncoated LiMn₂O₄. The surface morphology and particle agglomeration were investigated using SEM, TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 20 nm. The XPS data illustrated that the CeO₂ completely coated the surface of the LiMn₂O₄ core cathode materials. The galvanostatic charge and discharge of the uncoated and CeO₂-coated LiMn₂O₄ cathode materials were measured in the potential range of 3.0-4.5 V (0.5 C rate) at 30 °C and 60 °C. Among them, the 1.0 wt.% of CeO₂-coated spinel LiMn₂O₄ cathode satisfies the structural stability, high reversible capacity and excellent electrochemical performances of rechargeable lithium batteries.     

Enhanced electrochemical and thermal stability of surface-modified LiCoO2 cathode by CeO2 coating

CeO₂-coated LiCoO₂ particles were successfully synthesized by a sol-gel coating of CeO₂ on the surface of the LiCoO₂ powder and subsequent heat treatment at 700 °C for 5 h. The surface-modified and pristine LiCoO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), slow rate cyclic voltammogram (CV), and differential scanning calorimetry (DSC). Cyclic voltammetry curves suggested that the CeO₂ coating suppressed the phase transitions. Unlike pristine LiCoO₂, the CeO₂-coated LiCoO₂ cathode exhibited better capacity retention than the pristine LiCoO₂ electrode in the higher cutoff voltage. Differential scanning calorimetric data revealed the higher thermal stability of the CeO₂-coated LiCoO₂ electrode.     

Improvement of structural and electrochemical properties of commercial LiCoO2 by coating with LaF3

Commercial LiCoO₂ has been modified with LaF₃ as a new coating material. The surface modified materials were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), field emission scanning electron microscopy (FE-SEM), auger electron spectroscopy (AES) and galvanostatic charge–discharge cycling. The LaF₃-coated LiCoO₂ had an initial discharge specific capacity of 177.4 mAh g⁻¹ within the potential ranges 2.75–4.5 V (vs. Li/Li⁺), and showed a good capacity retention of 90.9% after 50 cycles. It was found that the overcharge tolerance of the coated cathode was significantly better than that of the pristine LiCoO₂ under the same conditions – the capacity retention of the pristine LiCoO₂ was 62.3% after 50 cycles. The improvement could be attributed to the LaF₃ coating layer that hinders interaction between LiCoO₂ and electrolyte and stabilizes the structure of LiCoO₂. Moreover, DSC showed that the coated LiCoO₂ had a higher thermal stability than the pristine LiCoO₂.     

Effects of surface modification on the cycling stability of LiNi0.8Co0.2O2 electrodes by CeO2 coating

A high-performance LiNi_0.8Co_0.2O₂ cathode was successfully fabricated by a sol-gel coating of CeO₂ to the surface of the LiNi_0.8Co_0.2O₂ powder and subsequent heat treatment at 700 °C for 5 h. The surface-modified and pristine LiNi_0.8Co_0.2O₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), slow rate cyclic voltammogram (CV), and differential scanning calorimetry (DSC). Unlike pristine LiNi_0.8Co_0.2O₂, the CeO₂-coated LiNi_0.8Co_0.2O₂ cathode exhibits no decrease in its original specific capacity of 182 mAh/g (versus lithium metal) and excellent capacity retention (95% of its initial capacity) between 4.5 and 2.8 V after 55 cycles. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.     

Comparison of AlPO4- and Co3(PO4)2-coated LiNi0.8Co0.2O2 cathode materials for Li-ion battery

Electrochemical and thermal properties of Co₃(PO₄)₂- and AlPO₄-coated LiNi_0.8Co_0.2O₂ cathode materials were compared. AlPO₄-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 170.8 mAh g⁻¹ and had a capacity retention (89.1% of its initial capacity) between 4.35 and 3.0 V after 60 cycles at 150 mA g⁻¹. Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 177.6 mAh g⁻¹ and excellent capacity retention (91.8% of its initial capacity), which was attributed to a lithium-reactive Co₃(PO₄)₂ coating. The Co₃(PO₄)₂ coating material could react with LiOH and Li₂CO₃ impurities during annealing to form an olivine Li_xCoPO₄ phase on the bulk surface, which minimized any side reactions with electrolytes and the dissolution of Ni⁴⁺ ions compared to the AlPO₄-coated cathode. Differential scanning calorimetry results showed Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathode material had a much improved onset temperature of the oxygen evolution of about 218 °C, and a much lower amount of exothermic-heat release compared to the AlPO₄-coated sample.     

Effects of the nanostructured SiO2 coating on the performance of LiNi0.5Mn1.5O4 cathode materials for high-voltage Li-ion batteries

LiNi_0.5Mn_1.5O₄ spinels coated with various amounts of fumed silica have been synthesized and investigated as cathode materials for high-voltage lithium-ion batteries at the elevated temperature (55 °C). The morphology and structure of the coated LiNi_0.5Mn_1.5O₄ samples were characterized by XRD, TEM and EDX. It was found that the surfaces of the coated LiNi_0.5Mn_1.5O₄ samples are covered with a porous, amorphous, nanostructured SiO₂ layer. The results of electrochemical experiments showed that the SiO₂-coated LiNi_0.5Mn_1.5O₄ samples display obviously improved capacity retention rate, and the improvement effect enhances with the increase of SiO₂ content. The XPS results revealed that the surfaces of the SiO₂-coated LiNi_0.5Mn_1.5O₄ cathode materials have relatively low content of LiF, and this is mainly responsible for their improved electrochemical cycling stability.     

Low-temperature atomic layer deposited Al2O3 thin film on layer structure cathode for enhanced cycleability in lithium-ion batteries

The deposition of Al₂O₃ on LiCoO₂ electrodes using a low-temperature atomic layer deposition has been investigated. Scanning electron microscopy confirms that Al₂O₃ films can be homogeneously deposited on LiCoO₂ particles of porous electrodes at 120 °C. The results of X-ray photoelectron spectroscopy show that the Al₂O₃ preferentially deposits on the LiCoO₂. Furthermore, the results of cycling stability tests show that the cells with Al₂O₃-coated LiCoO₂ electrodes have enhanced performance.     

Synthesis and electrochemical characteristics of Al2O3-coated LiNi1/3Co1/3Mn1/3O2 cathode materials for lithium ion batteries

LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials have been coated with Al₂O₃ nano-particles using sol-gel processing to improve its electrochemical properties. The X-ray diffraction (XRD) pattern of the as-prepared Al₂O₃ nano-particles was indexed to the cubic structure of the γ-Al₂O₃ phase and had an average size of ∼4 nm. The XRD showed that the structure of LiNi_1/3Co_1/3Mn_1/3O₂ was not affected by the Al₂O₃ coating. However, the Al₂O₃ coatings on LiNi_1/3Co_1/3Mn_1/3O₂ improved the cyclic life performance and rate capability without decreasing its initial discharge capacity. These electrochemical properties were also compared with those of LiAlO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ cathode material. The electrochemical impedance spectroscopy (EIS) was studied to understand the enhanced electrochemical properties of the Al₂O₃-coated LiNi_1/3Co_1/3Mn_1/3O₂ compared to uncoated LiNi_1/3Co_1/3Mn_1/3O₂.     

Control of AlPO4-nanoparticle coating on LiCoO2 by using water or ethanol

The electrochemical properties of AlPO₄-coated LiCoO₂ cathodes prepared in a water or ethanol solvent were characterized with the view of stabilizing LiCoO₂ at charge-cutoff voltages of 4.6 and 4.8 V. Under the influence of the AlPO₄ crystallinity, the coated LiCoO₂ prepared in ethanol had better capacity retention than those prepared in water. This enhancement also correlated with the improved suppression of Li-diffusivity decay in the coated cathode from the ethanol compared to that from water. In addition, the differential scanning calorimetry (DSC) results of the AlPO₄ nanoparticle-coated LiCoO₂ with ethanol showed an enhanced thermal stability.     

A study on electrochemical characteristics of LiCoO2/LiNi1/3Mn1/3Co1/3O2 mixed cathode for Li secondary battery

In this study, the LiCoO₂/LiNi_1/3Mn_1/3Co_1/3O₂ mixed cathode electrodes were prepared and their electrochemical performances were measured in a high cut-off voltage. As the contents of LiNi_1/3Mn_1/3Co_1/3O₂ in the mixed cathode increases, the reversible specific capacity and cycleability of the electrode enhanced, but the rate capability deteriorated. On the contrary, the rate capability of the cathode enhanced but the reversible specific capacity and cycleability deteriorated, according to increasing the contents of LiCoO₂ in the mixed cathode. The cell of LiCoO₂/LiNi_1/3Mn_1/3Co_1/3O₂ (50:50, wt.%) mixed cathode delivers a discharge capacity of ca. 168 mAh/g at a 0.2 C rate. The capacity of the cell decreased with the current rate and a useful capacity of ca. 152 mAh/g was obtained at a 2.0 C rate. However, the cell shows very stable cycleability: the discharge capacity of the cell after 20th charge/discharge cycling maintains ca. 163 mAh/g.     

Synthesis of LiNi1/3Co1/3Mn1/3O2 cathode materials by using a supercritical water method in a batch reactor

LiNi_1/3Co_1/3Mn_1/3O₂ and LiCoO₂ cathode materials were synthesized by using a supercritical water (SCW) method with a metal salt solution in a batch reactor. Stoichiometric LiNi_1/3Co_1/3Mn_1/3O₂ was successfully synthesized in a 10-min reaction without calcination, while overlithiated LiCoO₂ (Li_1.15CoO₂) was synthesized using the batch SCW method. The physical properties and electrochemical performances of LiNi_1/3Co_1/3Mn_1/3O₂ were compared to those of Li_1.15CoO₂ by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and charge/discharge cycling tests. The XRD pattern of LiNi_1/3Co_1/3Mn_1/3O₂ was found to be similar to that of Li_1.15CoO₂, showing clear splitting of the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) peak pairs as particular characteristics of the layered structure. In addition, both cathode powders showed good crystallinity and phase purity, even though a short reaction time without calcination was applied to the SCW method. The initial specific discharge capacities of the Li_1.15CoO₂ and LiNi_1/3Co_1/3Mn_1/3O₂ powders at a current density of 0.24 mA/cm² in 2.5-4.5 V were 149 and 180 mAh/g, and their irreversible capacity loss was 20 and 17 mAh/g, respectively. The discharge capacities of the Li_1.15CoO₂ and LiNi_1/3Co_1/3Mn_1/3O₂ powders decreased with cycling and remained at 108 and 154 mAh/g after 30 cycles, which are 79% and 89% of the initial capacities. Compared to the overlithiated LiCoO₂ cathode powders, the LiNi_1/3Co_1/3Mn_1/3O₂ cathode powders synthesized by SCW method had better electrochemical performances.     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

Preparation of micro-dot electrodes of LiCoO2 and Li4Ti5O12 for lithium micro-batteries

Fabrications of micro-dot electrodes of LiCoO₂ and Li₄Ti₅O₁₂ on Au substrates were demonstrated using a sol-gel process combined with a micro-injection technology. A typical size of prepared dots was about 100 μm in diameter, and the dot population on the substrate was 2400 dots cm⁻². The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were characterized with scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy, and cyclic voltammetry. The prepared LiCoO₂ and Li₄Ti₅O₁₂ micro-dot electrodes were evaluated in an organic electrolyte as cathode and anode for lithium micro-battery, respectively. The LiCoO₂ micro-dot electrode exhibited reversible electrochemical behavior in a potential range from 3.8 to 4.2 V versus Li/Li⁺, and the Li₄Ti₅O₁₂ micro-dot electrode showed sharp redox peaks at 1.5 V.     

Stable spinel type cobalt and copper oxide electrodes for O2 and H2 evolutions in alkaline solution

The stability of one material, Ti/Cu_xCo_3−xO₄, as anode and also cathode was investigated for electrolysis of alkaline aqueous solution. The electrodes were prepared by thermal decomposition method with x varied from 0 to 1.5. The accelerated life test illustrated that the electrodes with x = 0.3 nominally showed the best performance, with a total service life of 1080 h recorded in 1 M NaOH solution under alternating current direction at 1 A cm⁻² and 35 °C. The effects of copper content in electrode coating were examined in terms of electrode stability, surface morphology, coating resistivity and coating compositions. The presence of Cu in the spinel structure of Co₃O₄ could significantly enhance the electrochemical and physicochemical properties. The trends of crystallographic properties and surface morphology have been analyzed systemically before, during and after the electrodes were employed in alkaline electrolysis. The oxygen evolution would lead to the consumption of the coating material and the progressive cracking of the coating. Along with hydrogen evolution, cobalt oxide could be reduced to metal Co and Co(OH)₂ with particle sizes changed to smaller values in crystal and/or amorphous form at the cathode. The formation of Co is the key process for this electrode to serve as both anode and cathode. It is also the main reason leading to the eventual failure of the electrodes.     

Lanthanum-doped LiCoO2 cathode with high rate capability

Lanthanum-doped LiCoO₂ composite cathode materials, containing 0.1-10 mol% of La were synthesized by citric acid aided combustion technique. Thermal analyses showed that the sharp decomposition reaction for pristine LiCoO₂ became sluggish upon addition of lanthanum. X-ray diffraction analyses of the composites revealed existence of minute quantities of lanthanum-rich perovskite phases—rhombohedral LaCoO₃ and tetragonal La₂Li_0.5Co_0.5O₄ (14/mmm), along with rhombohedral LiCoO₂. Electron microscopy showed a distinct grain growth with increasing La content. An increase of about two orders of magnitude in the electrical conductivity (1.09 × 10⁻³ S cm⁻¹) was observed for 1.0 mol% La-doped LiCoO₂. An excellent cycling performance with capacity retention by a factor of ∼10 in comparison to the pristine LiCoO₂ was observed for the composite cathode containing 5.0 mol% La, when 2032 type coin cells were cycled at 5C rate. This has been ascribed to the structural stability induced by La doping and presence of the ion-conducting phase La₂Li_0.5Co_0.5O₄ which acts as a solid electrolyte for Li⁺ ions. A negligible growth of impedance upon repeated cycling has been observed. Cyclic voltammetry showed a remarkable improvement in reversibility and stability of the La-doped electrodes. These composite cathodes might be very useful for high rate power applications.     

High voltage stability of LiCoO2 particles with a nano-scale Lipon coating

For high-voltage cycling of rechargeable Li batteries, a nano-scale amorphous Li-ion conductor, lithium phosphorus oxynitride (Lipon), has been coated on surfaces of LiCoO₂ particles by combining a RF-magnetron sputtering technique and mechanical agitation of LiCoO₂ powders. LiCoO₂ particles coated with 0.36 wt% (∼1 nm thick) of the amorphous Lipon, retain 90% of their original capacity compared to non-coated cathode materials that retain only 65% of their original capacity after more than 40 cycles in the 3.0–4.4 V range with a standard carbonate electrolyte. The reason for the better high-voltage cycling behavior is attributed to reduction in the side reactions that cause increase of the cell resistance during cycling. Further, Lipon coated particles are not damaged, whereas uncoated particles are badly cracked after cycling. Extending the charge of Lipon-coated LiCoO₂ to higher voltage enhances the specific capacity, but more importantly the Lipon-coated material is also more stable and tolerant of high voltage excursions. A drawback of Lipon coating, particularly as thicker films are applied to cathode powders, is the increased electronic resistance that reduces the power performance.     

Surface of LiCo1/3Ni1/3Mn1/3O2 modified by CeO2-coating

A novel method to improve the cycling performance of LiCo_1/3Ni_1/3Mn_1/3O₂ in lithium-ion batteries by 1.0 wt.% CeO₂-coating is presented in this work. The crystalline structure and morphology of the synthesized powder have been characterized by XRD, SEM, TEM and their electrochemical performances were evaluated by CV, EIS and galvonostatic charge/discharge tests. It is found that CeO₂ forms a layer on the surface of LiCo_1/3Ni_1/3Mn_1/3O₂ without destroying the crystal structure of the core material. Electrochemical test indicates that CeO₂-coating could improve the cycling performance of LiCo_1/3Ni_1/3Mn_1/3O₂. At room temperature, the capacity retention of 1.0 wt.% CeO₂-coated material is 93.2% after 12 cycles at 3.0 C while that of the bare sample is only 86.6%. ICP-OES proves the coating layer could protect the dissolution of the transition metal ions from LiCo_1/3Ni_1/3Mn_1/3O₂. From the analysis of EIS, the improvement of cycle ability could be attributed to the suppression of the reaction between cathode and electrolyte.     

Improvement of electrochemical stability of LiMn2O4 by CeO2 coating for lithium-ion batteries

CeO₂-coated LiMn₂O₄ spinel cathode was synthesized using two-step synthesis method. All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. The results of the electrochemical performances on CeO₂-coated electrode are compared to those of electrodes based on LiMn₂O₄ spinel without CeO₂ coating. CeO₂-coated LiMn₂O₄ cathode improved the cycling stability of the electrode. The capacity retention of 2 wt% CeO₂-coated LiMn₂O₄ was more than 82% after 150 cycles between 3.0 and 4.4 V at room temperature and 82% after 40 cycles at elevated temperature of 60 °C. The amounts of dissolved manganese-ions in CeO₂-coated LiMn₂O₄ significantly are smaller than pristine LiMn₂O₄ systems especially at elevated temperatures. Surface-modified LiMn₂O₄ can suppress the dissolution reaction of manganese-ions at elevated temperature and clearly improve the cyclability of the spinel LiMn₂O₄ cathode materials.     

Crystal structure studies of thermally aged LiCoO2 and LiMn2O4 cathodes

LiCoO₂ and LiMn₂O₄ cathodes were studied by X-ray diffractometry (XRD) and electron diffraction after ageing in the charged state at elevated temperature. Some cathodes were stopped at different times during ageing and XRD measurements were taken to monitor changes in the crystal structure over ageing time. The results indicate that Li-ions intercalate into the cathodes lattice during ageing thus decreasing the available discharge capacity. Analysis of electron diffraction patterns of LiCoO₂ and LiMn₂O₄ retrieved from the cathodes after ageing shows that irreversible crystallographic transformations have taken place in both electrodes. Dark field imaging illustrates that LiCoO₂ forms a layer of spinel phase on its surface. In LiMn₂O₄ a tetragonal distorted spinel is observed when the cathode has been in the 3 V regime for considerable length of time.     

Effect of ZnO modification on the performance of LiNi0.5Co0.25Mn0.25O2 cathode material

ZnO was coated on LiNi_0.5Co_0.25Mn_0.25O₂ cathode (positive electrode) material for lithium ion battery via sol–gel method to improve the performance of LiNi_0.5Co_0.25Mn_0.25O₂. The X-ray diffraction (XRD) results indicated that the lattice structure of LiNi_0.5Co_0.25Mn_0.25O₂ was not changed distinctly after surface coating and part of Zn²⁺ might dope into the lattice of the material. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) proved that ZnO existed on the surface of LiNi_0.5Co_0.25Mn_0.25O₂. Charge and discharge tests showed that the cycle performance and rate capability were improved by ZnO coating, however, the initial capacity decreased dramatically with increasing the amount of ZnO. Differential scanning calorimetry (DSC) results showed that thermal stability of the materials was improved. The XPS spectra after charge–discharge cycles showed that ZnO coated on LiNi_0.5Co_0.25Mn_0.25O₂ promoted the decomposition of the electrolyte at the early stage of charge–discharge cycle to form more stable SEI layer, and it also can scavenge the free acidic HF species from the electrolyte. The electrochemical impedance spectroscopy (EIS) results showed ZnO coating could suppress the augment of charge transfer resistance upon cycling.