Al2O3-coated LiCoO2 Research Articles

Enhanced Interfacial Kinetics and High Rate Performance of LiCoO2 Thin-Film Electrodes by Al Doping and In Situ Al2O3 Coating.

The structure and surface-interface instability of LiCoO2 thin-film electrodes during charge–discharge cycles are one of the main factors leading to the deterioration of electrochemical performance. Element doping and surface coating are effective strategies to tackle this issue. In this work, Al-doped and in situ Al2O3-coated LiCoO2 composite thin-film electrodes are prepared by magnetron sputtering. The results show that the resultant composite thin-film electrodes exhibited excellent cycling stability, with a discharge specific capacity of 40.2 μAh um–1 cm–2 after 240 cycles at 2.5 μA cm–2, with a capacity retention rate of 94.14%, compared to a discharge capacity of the unmodified sample of only 37.7 μAh um–1 cm–2 after 110 cycles, with a capacity retention rate of 80.04%. In addition, the rate performance of the prepared LiCoO2 film is significantly improved, and the discharge specific capacity of the Al-doped sample reaches 43.5 μAh um–1 cm–2 at 100 μA cm–2, which is 38.97% higher than that of the unmodified sample (31.3 μAh um–1 cm–2). The enhancement of electrochemical performance is mainly attributed to the synergistic effect of Al doping and in situ Al2O3 coating. The metal Al forms a conductive network in the film, while part of the Al will enter the LiCoO2 lattice to form a LiAlyCo1–yO2 solid solution, promoting the transport of lithium ions and improving the stability of the electrode structure. The in situ continuous deposition of the coating optimizes the active material coating–electrolyte interface.

Al2O3-surface modification of LiCoO2 cathode with improved cyclic performance

In this article, Al2O3-coated LiCoO2 cathode is synthesized by high-temperature solid-state reaction method, i.e. sintering the mixture of Al2O3 and LiCoO2 powder. X-ray diffraction (XRD) analysis shows that there is no Al2O3 impurity being found in the final product. Scanning electron spectroscopy (SEM) and transmission electron spectroscopy (TEM) measurements show that the micro-morphology of pristine LiCoO2 grains is sustained after coating Al2O3. We analyze the surface chemistry of the pristine and Al2O3-coated LiCoO2 by X-ray photoelectron spectroscopy (XPS) technique. The electrochemical behavior is systematically investigated. The cathodes are cycled in different potential ranges (3.70V–4.30, 4.35, 4.40, 4.45, 4.50, 4.55 and 4.60V). The charge-discharge, cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) measurements show that coating Al2O3 can obviously improve the cyclic stability and decrease the total interfacial resistance. We could conclude that Al2O3-coated LiCoO2 cathode is suitable for practical application in the potential range of 3.70–4.50V vs. Li/Li+. The discharge capacity can reach 113mAh/g after 500cycles. It is very possible that the LiCo1−xAlxO2-like surface phase, formed on the surface of LiCoO2, prevents Co-dissolution. As results, the operation potential range is extended and the cyclic stability is improved.

Understanding the Role of Temperature and Cathode Composition on Interface and Bulk: Optimizing Aluminum Oxide Coatings for Li-Ion Cathodes.

Surface coating of cathode materials with Al2O3 has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition change the chemical composition, morphology, and distribution of coating within the cathode interface and bulk lattice is still missing. In this study, we use a wet-chemical method to synthesize a series of Al2O3-coated LiNi0.5Co0.2Mn0.3O2 and LiCoO2 cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity, and morphology of the coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly dependent on the annealing temperature and cathode composition. For Al2O3-coated LiNi0.5Co0.2Mn0.3O2, higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower Al2O3 coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For Al2O3-coated LiCoO2, the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from "surface coatings" to "dopants", which is not observed for LiNi0.5Co0.2Mn0.3O2. As a result, Al2O3-coated LiCoO2 annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution.

On electrochemistry of Al2O3-coated LiCoO2 composite cathode with improved cycle stability

LiCoO2-based cathode does still have a powerful competition in high-end mobile electronics due to its relatively high true density (about 5.2 g/cm3). When the operation potential range is extended, the improvement in its cycle stability has attracted more attention. The extension of its operation potential can be realized by partial replacement of Co by Ni and Mn or by surface modification. However, Ni and Mn replacing partial Co results in decreased true density; for example, the true density of LiNi0.5Mn0.3Co0.2O2 is about 4.6 g/cm3. In this case, the increase in its practical energy density is impossible. As a result, the surface modification technology becomes very important to extend its operation potential range. In this article, an Al2O3-coated LiCoO2 cathode was synthesized. X-ray diffraction test did not show any impurity. Scanning electron spectroscopy measurements showed that the basic microstructure of pristine LiCoO2 grain is sustained after coating Al2O3. The surface characteristic of pure and Al2O3-coated LiCoO2 was also analyzed using an X-ray photoelectron spectroscopy (XPS) technique. Unusual XPS peaks of O 1s, Al 2p, and Co 2p binding energy were found and may be caused by the possible H existence in crystal structure. The electrochemical behavior was systematically investigated, and the cathode was cycled at different charge cutoff voltages (4.30∼4.60 V). The charge-discharge and cyclic voltammetry measurements showed an obviously improved cyclic performance after coating Al2O3. The electrocatalytic activity is not clearly changed before and after coating Al2O3. From our systematical investigation, it could be concluded that the Al2O3-coated LiCoO2 cathode is suitable for practical application in the potential range of 3.70∼4.50 V vs. Li/Li+.

Comparative study of Al2O3-coated LiCoO2 electrode derived from different Al precursors: uniformity, microstructure and electrochemical properties

In this work, two different Al precursors are used to modify the surface of LiCoO2 particles with Al2O3 layer, and the corresponding structural and electrochemical properties of as-prepared Al2O3-coated LiCoO2 are examined. The SEM, TEM and charge-discharge cycling results suggest that the method using aluminum isopropoxide (AIP) as Al precursor can obtain more uniform Al2O3 coating layer and more excellent electrochemical performance. The ICP results reveal that the cobalt dissolution ability is different for coated LiCoO2 prepared by two Al precursors when the charge cutoff voltage is different, and when charge cutoff voltage further increasing to 4.6V, the dominant factor of cycling decay transform from cobalt dissolution to structural collapse caused by the excessive of Li+ delithiation. Besides, EIS analysis indicates that two kinds of Al2O3 coating layer lead to different polarization impedance, and the growth of polarization impedance during charging/discharging can be effectively suppressed by Al2O3 coating, and thus the electrochemical performance of the coated samples is greatly enhanced.

Vapor-assisted synthesis of Al2O3-coated LiCoO2 for high-voltage lithium ion batteries

A simple and facile vapor-assisted hydrolysis route has been used to synthesize Al2O3-coated LiCoO2. The effects of Al2O3 coating on the structure and electrochemical performance have been systematically studied. After deposition of the Al2O3 coating, the crystal structure and morphology of LiCoO2 are maintained. Galvanostatic charge-discharge tests show that Al2O3-coated LiCoO2 exhibits markedly improved capacity retention and rate capability at high charge voltage (4.5V). The Al2O3-coated LiCoO2 exhibits an initial discharge capacity of 165.5mAh/g at 360mA/g, and the capacity retention is 98.6% after 180 cycles. The electrochemical impedance spectroscopy results demonstrate that the Al2O3 coating can slow the increase of charge-transfer resistance during cycling, which is in accord with the excellent electrochemical cycling performance of Al2O3-coated LiCoO2.

Suppression of interface reaction of LiCoO2 thin films by Al2O3-coating

Al2O3-coated LiCoO2 thin films were prepared to characterize the effects of coating in detail. The LiCoO2 film electrodes served as effective pristine cathodes to investigate the interface reaction between a cathode and electrolyte because they have wide and smooth surface and do not contain conducting agents (such as carbon) or binders. The electrochemical properties, surface morphology, and concentration profile (vs. depth) were characterized to identify the effects of coating. The Al2O3-coated film showed a much improved rate capability and cyclic performance compared with those of the pristine film. Specifically, based on the results of SIMS analyses and SEM images, dissolution of transition-metal ions and surface damage during storage at high temperature were suppressed by Al2O3 coating. This result directly shows the successful protective effect of surface coating, which is associated with enhanced electrochemical properties of the coated film.

The effect of Al2O3-coating coverage on the electrochemical properties in LiCoO2 thin films

The electrochemical properties of nanoscale Al2O3-coated LiCoO2 thin films were examined as a function of the coating coverage. Al2O3-coated LiCoO2 films showed enhanced cycle-life performance with increasing degree of coating coverage, which was attributed to the suppression of Co dissolution and F− concentration in the electrolyte. Moreover, an Al2O3-coating layer with partial coverage clearly improved the electrochemical properties, even at 60 °C or with a water-contaminated electrolyte. Even though metal-oxide coating on LiCoO2 has been actively investigated, the mechanisms of nanoscale coating have yet to be clearly identified. In this article, surface analysis suggested that the Al2O3-coating layer had transformed to an AlF3 ∙3H2O layer during cycling, which inhibited the generation of HF by scavenging H2O molecules present in the electrolyte.

Fabrication of All-Solid-State Lithium Polymer Secondary Batteries Using Al2O3-Coated LiCoO2

Al2O3-coated LiCoO2 cathode particles were prepared by a spray coating method, and a uniform nanoscaled Al2O3 layer was formed on the surface of LiCoO2. The capacity retention in a LiCoO2 cathode material for the all-solid-state lithium polymer secondary batteries was improved by Al2O3 coating on LiCoO2 as well as liquid-type lithium batteries.

Effect of Al2O3 coating on electrochemical performance of LiCoO2 as cathode materials for secondary lithium batteries

Al2O3-coated LiCoO2 particles are prepared by a gas-suspension spray method with coatings in the range from 0.1 to 2.0 wt.%, and are examined for their electrochemical performance with special attention to the surface coverage of cathode materials for lithium-ion batteries. The Al2O3 coating increases both the surface area and the electrical conductivity of LiCoO2, improves the cycle performance even at a higher cut-off charge voltage, and induces higher thermal stability. In the experimental range studied, different coating amounts (i.e., surface coverage of Al2O3) causes different capacity retention after 50 cycles. The optimum Al2O3 coating amount for the highest capacity retention is 0.2 wt.%. This corresponds to 13.7% of surface coverage.

Impacts of Electrolyte Solvent Soakage on Structure and Electrochemical Performance of LiCoO[sub 2] for Lithium-Ion Batteries

Commercial LiCoO2 for lithium-ion batteries is high-energy ball-milled to nanoscale sizes. Bare and Al2O3-coated nano- LiCoO2 particles are soaked in a mixture of ethylene carbonate and dimethyl carbonate. The influences of solvent soakage on the structure and electrochemical performance of the material are characterized with X-ray diffraction, Raman spectroscopy, scanning electron microscopy, inductively coupled plasma technique, and galvonostatic electrochemical cycling. It is shown that solvent soakage has significant impact on the structure and electrochemical properties of bare and Al2O3-coated LiCoO2 positive electrode materials. (C) 2004 The Electrochemical Society.

Electrochemical Stability of Thin-Film LiCoO2 Cathodes by Aluminum-Oxide Coating

High-performance thin-film LiCoO2 cathodes were successfully fabricated by aluminum-oxide coating. Both the galvanostatic charge−discharge experiments and the cyclic voltammograms (CVs) showed enhanced electrochemical properties in the Al2O3-coated LiCoO2 films compared to those in the uncoated ones. The improved cycling behaviors in the coated samples are caused by the suppression of cobalt dissolution from the LiCoO2 thin films, with the formation of an aluminum-oxide solid electrolyte residing between the LiCoO2 cathode and liquid electrolyte. Galvanostatic intermittent titration technique (GITT) results clearly showed that the Al2O3-coated samples had higher Li diffusivities than the uncoated ones after 80 cycles. The effect of Al2O3 thickness on the electrochemical properties up to 300 nm was also studied.