Effects of surface modification by MgO on interfacial reactions of lithium cobalt oxide thin film electrode

Magnesium oxide (MgO)-modified lithium cobalt oxide (LiCoO₂) thin film electrodes were prepared by pulsed laser deposition (PLD) and effects of surface modification by MgO on interfacial reactions of LiCoO₂ were studied. The modification by MgO was carried out by PLD on LiCoO₂ thin film electrode successively after the deposition of LiCoO₂ thin film by PLD. Auger electron spectroscopy suggested that Mg dispersed uniformly in nano-scale on the film electrode. Cyclic voltammetry measurements clearly showed that MgO modification suppresses the increase of resistances caused by repetition of lithium-ion insertion–extraction reaction charged up to 4.2 V versus Li/Li⁺. Moreover, MgO modification decreased the activation energy of lithium-ion transfer reaction at LiCoO₂ thin film electrode–electrolyte interface, indicating that the modification by MgO affects the kinetics of lithium-ion transfer reaction at LiCoO₂–electrolyte interface.     

Performance improvement of LiCoO2 by molten salt surface modification

The surface of commercial LiCoO₂ was modified by molten salt method. The structure and electrochemical and thermal performances of the MgCl₂-treated LiCoO₂ were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy and galvanostatic cycling. It is found that surface modification improves the structural and thermal stability as well as the rate performance of LiCoO₂. These improvements were attributed to the formation of homogeneous solid solution on the surface of the LiCoO₂ particle.     

Coating technique for improvement of the cycling stability of LiCo/NiO2 electrode materials

We provide data on the changes in structure and composition of commercial LiNi_0.8Co_0.2O₂ electrode materials for lithium-ion batteries occurring after surface coating with two types of metal oxides: electrochemically active LiCoO₂ and inactive MgO. XRD analysis, SEM images, IR spectroscopy and EPR of low-spin Ni³⁺ ions were carried out for structural characterisation of coated LiNi_0.8Co_0.2O₂ electrodes. Surface modification with LiCoO₂ was found to be a more effective route for improving the cycling stability of LiNi_0.8Co_0.2O₂. The favourable effect of LiCoO₂-coating was connected with an enhanced stability of the bulk composition and reduction of electrode/electrolyte interaction.     

Effect of FePO4 coating on electrochemical and safety performance of LiCoO2 as cathode material for Li-ion batteries

LiCoO₂ was surface modified by a coprecipitation method followed by a high-temperature treatment in air. FePO₄-coated LiCoO₂ was characterized with various techniques such as X-ray diffraction (XRD), auger electron spectroscopy (AES), field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), electrochemical impedance spectroscopy (EIS), 3 C overcharge and hot-box safety experiments. For the 14500R-type cell, under a high charge cutoff voltage of 4.3 and 4.4 V, 3 wt.% FePO₄-coated LiCoO₂ exhibits good electrochemical properties with initial discharge specific capacities of 146 and 155 mAh g⁻¹ and capacity retention ratios of 88.7 and 82.5% after 400 cycles, respectively. Moreover, the anti-overcharge and thermal safety performance of LiCoO₂ is greatly enhanced. These improvements are attributed to the FePO₄ coating layer that hinders interaction between LiCoO₂ and electrolyte and stabilizes the structure of LiCoO₂. The FePO₄-coated LiCoO₂ could be a high performance cathode material for lithium-ion battery.     

Influence of surface modification of LiCoO2 by organic compounds on electrochemical and thermal properties of Li/LiCoO2 rechargeable cells

LiCoO₂ is the most famous positive electrode (cathode) for lithium ion cells. When LiCoO₂ is charged at high charge voltages far from 4.2 V, cycleability of LiCoO₂ becomes worse. Causes for this deterioration are instability of pure LiCoO₂ crystalline structure and an oxidation of electrolyte solutions LiCoO₂ at higher charge voltages. This electrolyte oxidation accompanies with the partial reduction of LiCoO₂. We think more important factor is the oxidation of electrolyte solutions. In this work, influence of 10 organic compounds on electrochemical and thermal properties of LiCoO₂ cells was examined as electrolyte additives. As a base electrolyte solution, 1 M (M: mol L⁻¹) LiPF₆-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) was used. These compounds are o-terphenyl (o-TP), Ph-X (CH₃)_n (n = 1 or 2, X = N, O or S) compounds, adamantyl toluene compounds, furans and thiophenes. These additives had the oxidation potentials (Eox) between 3.4 and 4.7 V vs. Li/Li⁺. These Eox values were lower than that (6.30 V vs. Li/Li⁺) of the base electrolyte. These additives are oxidized on LiCoO₂ during charge of the LiCoO₂ cells. Oxidation products suppress the excess oxidation of electrolyte solutions on LiCoO₂. As a typical example of these organic compounds, o-TP (Eox: 4.52 V) was used to check the fundamental properties of these organic additives. Charge-discharge cycling tests were carried out for the Li/LiCoO₂ cells with and without o-TP. Constant current charge at 4.5 V is mainly used as a charge method. Cells with 0.1 wt.% o-TP exhibited slightly better cycling performance and lower polarization than those without additives. Lower polarization arises from a decrease in a resistance of interface between electrolyte solutions and LiCoO₂ by surface film formation resulted from oxidation of o-TP. Oxidation products were found by mass spectroscopy analysis to be mixture of several polycondensation compounds made from two to four terphenly monomers. Thermal stability of LiCoO₂ with electrolyte solutions did not improve by addition of o-TP. Slightly better charge-discharge cycling properties were obtained by using organic modifiers. However, when industrial applications were considered, drastic improvements have not been obtained yet. One of reasons may be too large influence of the deterioration of stability of pure LiCoO₂ structure at high voltage charging for industrial use. We hope to realize the tremendous improvements of high energy, long cycle life and safe lithium cells by the combination of both LiCoO₂ with more stable structure such as LiCoO₂ treated with MgO and new organic additives with molecular structure more carefully designed.     

Dynamic behavior of surface film on LiCoO2 thin film electrode

Electrochemical oxidation behavior of non-aqueous electrolytes on LiCoO₂ thin film electrodes were investigated by in situ polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy (XPS). LiCoO₂ thin film electrode on gold substrate was prepared by rf-sputtering method. In situ PM-FTIR spectra were obtained at various electrode potentials during cyclic voltammetry measurement between 3.5 V vs. Li/Li⁺ and 4.2 V vs. Li/Li⁺. During anodic polarization, oxidation of non-aqueous electrolyte was observed, and oxidized products remained on the electrode at the potential higher than 3.75 V vs. Li/Li⁺ as a surface film. During cathodic polarization, the stripping of the surface film was observed at the potential lower than 3.9 V vs. Li/Li⁺. Depth profile of XPS also showed that more organic surface film remained on charged LiCoO₂ than that on discharged one. AFM images of charged and discharged electrodes showed that some decomposed products deposited on charged electrode and disappeared from the surface of discharged one. These results indicate that the surface film on LiCoO₂ is not so stable.     

Pt and Ru dispersed on LiCoO2 for hydrogen generation from sodium borohydride solutions

Nano-sized platinum and ruthenium dispersed on the surface LiCoO₂ as catalysts for borohydride hydrolysis are prepared by microwave-assisted polyol process. The catalysts are characterized by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Very uniform Pt and Ru nanoparticles with sizes of <10 nm are dispersed on the surface of LiCoO₂. XRD patterns show that the Pt/LiCoO₂ and Ru/LiCoO₂ catalysts only display the characteristic diffraction peaks of a LiCoO₂ crystal structure. Results obtained from XPS analysis reveal that the Pt/LiCoO₂ and Ru/LiCoO₂ catalysts contain mostly Pt(0) and Ru(0), with traces of Pt(IV) and Ru(IV), respectively. The hydrogen generation rates using low noble metal loading catalysts, 1 wt.% Pt/LiCoO₂ and 1 wt.% Ru/LiCoO₂, are very high. The hydrogen generation rate using Ru/LiCoO₂ as a catalyst is slightly higher compared with that of Pt/LiCoO₂.     

Electrode structure analysis and surface characterization for lithium-ion cells simulated low-Earth-orbit satellite operation: II: Electrode surface characterization

As a sequence work to investigate the performance-degradation mechanism of an aged commercial laminated lithium-ion cell experiencing 4350-cycle charge–discharge in a simulated low-Earth-orbit (LEO) satellite operation, we performed the surface characterization of LiCoO₂ cathode and graphite anode by Fourier transform infrared-Attenuated total reflection (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) analysis in this work. Overall, the graphite anode had a larger change in surface chemistry than that of the LiCoO₂ cathode. Except the common surface components, we detected Co metal at the aged graphite surface in the first time. This Co metal deposition was believed to originate from Co²⁺ dissolution from LiCoO₂ cathode during prolonged cycling, and detrimental to structure stability of LiCoO₂ cathode which was a main cause of cell capacity loss. The amount of surface-film component was also estimated by FTIR analysis. Though the total amount of surface film increased, the organic (inorganic) surface film decreased (increased) with prolonged cycling.     

Inhibition of solid electrolyte interface formation on cathode particles for lithium-ion batteries

Thermal reactions between cathode particles (LiNi_0.8Co_0.2O₂, LiCoO₂, LiMn₂O₄ and LiFePO₄) and ternary electrolyte (1.0 M LiPF₆ in 1:1:1 diethyl carbonate/dimethyl carbonate/ethylene carbonate) with or without the thermal stabilizing additive dimethyl acetamide (DMAc) have been investigated. Ternary electrolyte reacts with the surface of lithiated metal oxides (LiNi_0.8Co_0.2O₂, LiCoO₂ and LiMn₂O₄) upon storage to corrode the surface and generate a complex mixture of organic and inorganic surface species, but the bulk ternary electrolyte does not decompose. There is little evidence for reaction between the surface of carbon coated LiFePO₄ and ternary electrolyte upon storage at elevated temperature (>60 °C), but the bulk ternary electrolyte decomposes. Addition of DMAc to ternary electrolyte reduces the surface corrosion of the lithiated metal oxides and stabilizes the electrolyte in the presence of LiFePO₄.     

Enhancing the thermal stability of LiCoO2 electrode by 4-isopropyl phenyl diphenyl phosphate in lithium ion batteries

To enhance the thermal stability of LiCoO₂ in lithium ion batteries, 4-isopropyl phenyl diphenyl phosphate (IPPP) was investigated as an additive in 1.0 M LiPF₆/EC + DEC (1:1 wt.%) electrolyte. The thermodynamics and kinetics parameters of the single LiCoO₂ and Li_xCoO₂–IPPP-electrolyte are detected and calculated based on the C80 microcalorimeter data. The results indicated that IPPP can enhance the thermal stability of LiCoO₂ electrode in lithium ion battery more or less corresponding to the IPPP content in electrolyte. Furthermore, the electrochemical performances of LiCoO₂/IPPP-electrolyte/Li cells become slightly worse after using IPPP additive in the electrolyte. This alleviated trade-off between thermal stability and cell performance provides a possibility to formulate an electrolyte containing 5–10% of IPPP and enhance the LiCoO₂ electrode thermal stability with minimum sacrifice in performance.     

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.     

Characterization of all-solid-state secondary batteries with LiCoO2 thin films prepared by ECR sputtering as positive electrodes

We fabricated all-solid-state lithium secondary batteries consisting of LiCoO₂ thin films prepared by electron cyclotron resonance (ECR) sputtering LiPON and metallic lithium films, and investigated the influence of the sputtering target composition on the performance of the batteries and LiCoO₂ films. We found that the LiCoO₂ film sputtered with a stoichiometric LiCoO₂ target included many impurities (mainly Co₃O₄) and these impurities were eliminated by adding an excess of Li source to the sputtering target to achieve a Li/Co atomic ratio of 2.0 elsewhere. The LiCoO₂ film sputtered with a Li2.0 target exhibited a larger discharge capacity and a high performance level for large current operation. However, the capacity of a battery employing LiCoO₂ film sputtered with a Li2.0 target decreased more rapidly than that with a Li1.0 or Li1.7 target in a charge–discharge cycle test. We also investigated the cycle performance of LiCoO₂ films in an ordinary liquid electrolyte by using beaker type cells. We found that the decrease in capacity during the cycle tests was caused by the deterioration of the LiCoO₂ film, because the dependence of the target composition on the cycle performance in the beaker type cells was similar to that in the all-solid-state cells. We consider the capacity decrease to be caused by the deterioration in the crystallinity of the LiCoO₂ film when using the Li2.0 target and caused by the formation of a Co₃O₄ layer on the surface of the LiCoO₂ film when using a Li1.7 target on basis of the results of X-ray diffraction analysis and Raman spectroscopy.     

Electrochemical performance and thermal stability of LiCoO2 cathodes surface-modified with a sputtered thin film of lithium phosphorus oxynitride

A lithium phosphorus oxynitride (LiPON) glass-electrolyte thin film is coated on a lithium cobalt oxide (LiCoO₂) composite cathode by means of a radio frequency (RF) magnetron sputtering method. The effect of the LiPON coating layer on the electrochemical performance and thermal stability of the LiCoO₂ cathode is investigated. The thermal stability of the delithiated LiCoO₂ cathode in the presence of liquid electrolyte is examined by differential scanning calorimetry (DSC). It is found that the LiPON coating, improves the rate capability and the thermal stability of the charged LiCoO₂ cathode. The LiPON film appears to suppress impedance growth during cycling and inhibits side-reactions between delithiated LiCoO₂ and the electrolyte.     

Electrochemical and impedance investigation of the effect of lithium malonate on the performance of natural graphite electrodes in lithium-ion batteries

Lithium malonate (LM) was coated on the surface of a natural graphite (NG) electrode, which was then tested as the negative electrode in the electrolytes of 0.9 M LiPF₆/EC-PC-DMC (1/1/3, w/w/w) and 1.0 M LiBF₄/EC-PC-DMC (1/1/3, w/w/w) under a current density of 0.075 mA cm⁻². LM was also used as an additive to the electrolyte of 1.0 M LiPF₆/EC-DMC-DEC (1/1/1, v/v/v) and tested on a bare graphite electrode. It was found that both the surface coating and the additive approach were effective in improving first charge-discharge capacity and coulomb efficiency. Electrochemical impedance spectra showed that the decreased interfacial impedance was coupled with improved coulomb efficiency of the cells using coated graphite electrodes. Cyclic voltammograms (CVs) on fresh bare and coated natural graphite electrodes confirmed that all the improvement in the half-cell performance was due to the suppression of the solvent decomposition through the surface modification with LM. The CV data also showed that the carbonate electrolyte with LM as the additive was not stable against oxidation, which resulted in lower capacity of the full cell with commercial graphite and LiCoO₂ electrodes.     

Mechanism study of enhanced electrochemical performance of ZrO2-coated LiCoO2 in high voltage region

In this study, the mechanism of enhanced performance of ZrO₂-coated LiCoO₂ especially at high potential range is systematically investigated. Firstly, when overcharging to 4.5 V (higher than 4.2 V, the normal cutoff charging potential), phase transformation from H1 to H2 takes place with less volume expansion for ZrO₂-coated LiCoO₂ (1.2% and 2.2% for as-received one). EIS analysis indicates the growth of interfacial impedance during charging/discharging can be effectively suppressed with ZrO₂ coating on the LiCoO₂ surface. It is demonstrated as well that cation mixing of the cycled LiCoO₂ caused by re-intercalation of dissolved Co²⁺ is inhibited with the ZrO₂ coating on the LiCoO₂. Therefore the ZrO₂-coated LiCoO₂ shows great enhancement in the electrochemical properties with 85% capacity retention after 30 cycles from 3 to 4.5 V at a rate of 0.5C. Nevertheless, under the same evaluation process, the as-received LiCoO₂ possesses only 21% capacity retention, which is resulted from the formation of polymeric layers by the electrolyte decomposition on its surface, the higher volumetric changes during charging/discharging and possible cation mixing by re-intercalation of the dissolved Co²⁺.     

All-solid-state lithium secondary batteries using LiCoO2 particles with pulsed laser deposition coatings of Li2S-P2S5 solid electrolytes

Electrode-electrolyte composite materials were prepared by coating a highly conductive Li₂S-P₂S₅ solid electrolyte onto LiCoO₂ electrode particles using pulsed laser deposition (PLD). Cross-sections of the composite electrode layers of the all-solid-state cells were observed using a transmission electron microscope to investigate the packing morphology of the LiCoO₂ particles and the distribution of solid electrolyte in the composite electrode. All-solid-state cells based on a composite electrode composed entirely of solid-electrolyte-coated LiCoO₂ were fabricated, and their performance was investigated. The coating amounts of Li₂S-P₂S₅ solid electrolytes on LiCoO₂ particles and the conductivity of the coating material were controlled to increase the capacity of the resulting all-solid-state cells. All-solid-state cells using LiCoO₂ with thick solid electrolyte coatings, grown over 120 min, had a capacity of 65 mAh g⁻¹, without any addition of Li₂S-P₂S₅ solid electrolyte particles to the composite electrode. The capacity of the all-solid-state cell increased further after increasing the conductivity of the Li₂S-P₂S₅ solid electrolyte coating by heat treatment at 200 °C. Furthermore, an all-solid-state cell based on a composite electrode using both a solid electrolyte coating and added solid electrolyte particles was fabricated, and the capacity of the resulting all-solid-state cell increased to 95 mAh g⁻¹.     

Electrochemical properties and performance of poly(acrylonitrile)-based polymer electrolyte for Li/LiCoO2 cells

It has been demonstrated that LiCoO₂ is a very attractive cathode active material for lithium rechargeable cells. Poly(acrylonitrile) (PAN)-based polymer electrolyte is used for Li/LiCoO₂ cells. PAN-based polymer electrolyte shows ionic conductivity of the order 1 mS cm⁻¹ at room temperature, irrespective of time evolution, and wide electrochemical stability up to 4.3 V (versus Li⁺/Li). An LiCoO₂ composite cathode containing 4 wt.% conductive material displays a good cycling performance. From a.c. impedance results, the interfacial resistance of Li/LiCoO₂ cells is dominated by the passive layer formed at the lithium/polymer electrolyte interface.     

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.     

Characterization of the interface between LiCoO2 and Li7La3Zr2O12 in an all-solid-state rechargeable lithium battery

The interfacial layer formed between a lithium-ion conducting solid electrolyte, Li₇La₃Zr₂O₁₂ (LLZ), and LiCoO₂ during thin film deposition was characterized using a combination of microscopy and electrochemical measurement techniques. Cyclic voltammetry confirmed that lithium extraction occurs across the interface on the first cycle, although the nonsymmetrical redox peaks indicate poor electrochemical performance. Using analytical transmission electron microscopy, the reaction layer (∼50 nm) was analyzed. Energy dispersive X-ray spectroscopy revealed that the concentrations of some of the elements (Co, La, and Zr) varied gradually across the layer. Nano-beam electron diffraction of this layer revealed that the layer contained neither LiCoO₂ nor LLZ, but some spots corresponded to the crystal structure of La₂CoO₄. It was also demonstrated that reaction phases due to mutual diffusion are easily formed between LLZ and LiCoO₂ at the interface. The reaction layer formed during high temperature processing is likely one of the major reasons for the poor lithium insertion/extraction at LLZ/LiCoO₂ interfaces.     

Lithium-ion transfer on a LixCoO2 thin film electrode prepared by pulsed laser deposition—Effect of orientation-

LiCoO₂ thin films with different orientations were fabricated by pulsed laser deposition, and Li-ion transfer at the interface between the electrolyte and a LiCoO₂ thin film electrode was investigated. This study particularly focused on the effect of orientation on Li-ion transfer. The thin films were shown to be highly crystallized by X-ray diffraction. Charge transfer resistance ascribed to Li-ion transfer at the interface was observed by ac impedance spectroscopy. While charge transfer resistance was strongly influenced by the preferred orientation of LiCoO₂ thin film, the activation energy evaluated from the temperature-dependence of Li-ion transfer resistance appeared to be independent of the orientation.