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.     

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⁺.     

Electrochemical performance of all-solid-state lithium secondary batteries improved by the coating of Li2O-TiO2 films on LiCoO2 electrode

All-solid-state lithium secondary batteries using LiCoO₂ particles coated with amorphous Li₂O-TiO₂ films as an active material and Li₂S-P₂S₅ glass-ceramics as a solid electrolyte were fabricated; the electrochemical performance of the batteries was investigated. The interfacial resistance between LiCoO₂ and solid electrolyte was decreased by the coating of Li₂O-TiO₂ films on LiCoO₂ particles. The rate capability of the batteries using the LiCoO₂ coated with Li₂Ti₂O₅ (Li₂O·2TiO₂) film was improved because of the decrease of the interfacial resistance of the batteries. The cycle performance of the all-solid-state batteries under a high cutoff voltage of 4.6 V vs. Li was highly improved by using LiCoO₂ coated with Li₂Ti₂O₅ film. The oxide coatings are effective in suppressing the resistance increase between LiCoO₂ and the solid electrolyte during cycling. The battery with the LiCoO₂ coated with Li₂Ti₂O₅ film showed a large initial discharge capacity of 130 mAh/g and good capacity retention without resistance increase after 50 cycles at the current density of 0.13 mA/cm².     

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.     

In situ X-ray diffraction studies on the mechanism of capacity retention improvement by coating at the surface of LiCoO2

Synchrotron based in situ X-ray diffraction technique has been used to study the mechanism of capacity fading of LiCoO₂ cycled to a higher voltage above the normal 4.2 V limit and to investigate the mechanism of capacity retention improvement by ZrO₂ surface coating on LiCoO₂. It was found that the capacity fading of LiCoO₂ cycled at higher voltage limit is closely related to the increased polarization rather than the bulk crystal structure damage. The capacity of uncoated LiCoO₂ sample dropped to less than 70 mAh g⁻¹ when charged to 4.8 V after high voltage cycling. However, when the voltage limit was further increased to 8.35 V, the capacity was partially restored and the corresponding structural changes were recovered to the similar level as seen in fresh sample. This indicates that the integrity of the bulk crystal structure of LiCoO₂ was not seriously damaged during cycling to 4.8 V. The increased polarization seems to be responsible for the fading capacity and the uncompleted phase transformation of LiCoO₂. The polarization-induced capacity fading can be significantly improved by ZrO₂ surface coating. It was proposed that the effect of ZrO₂-coating layer on the capacity retention during high voltage cycling is through the formation of protection layer on the surface of LiCoO₂ particles, which can reduce the decomposition of the electrolyte at higher voltages.     

Synthesis and electrochemical performance of doped LiCoO2 materials

Layered intercalation compounds LiM_0.02Co_0.98O₂ (M = Mo⁶⁺, V⁵⁺, Zr⁴⁺) have been prepared using a simple solid-state method. Morphological and structural characterization of the synthesized powders is reported along with their electrochemical performance when used as the active material in a lithium half-cell. Synchrotron X-ray diffraction patterns suggest a single phase HT-LiCoO₂ that is isostructural to α-NaFeO₂ cannot be formed by aliovalent doping with Mo, V, and Zr. Scanning electron images show that particles are well-crystallized with a size distribution in the range of 1–5 μm. Charge–discharge cycling of the cells indicated first cycle irreversible capacity loss in order of increasing magnitude was Zr (15 mAh g⁻¹), Mo (22 mAh g⁻¹), and V (45 mAh g⁻¹). Prolonged cycling the Mo-doped cell produced the best performance of all dopants with a stable reversible capacity of 120 mAh g⁻¹ after 30 cycles, but was inferior to that of pure LiCoO₂.     

Electrochemical studies on LiCoO2 surface coated with Y3Al5O12 for lithium-ion cells

Synthesized yttrium aluminum garnet (YAG) sol was coated on the surface of the LiCoO₂ cathode particles by an in situ sol–gel process, followed by calcination at 923 K for 10 h in air. Based on XRD, TEM, and ESCA data, a compact YAG kernel with an average thickness of ∼20 nm was formed on the surface of the core LiCoO₂ particles, which ranged from ∼90 to 120 nm in size. The charge–discharge cycling studies for the coated materials suggest that 0.3 wt.% YAG-coated LiCoO₂ heated at 923 K for 10 h in air, delivered a discharge capacity of 167 mAh g⁻¹ and a cycle stability of about 164 cycles with a fading rate of 0.2 mAh cycle⁻¹ at a 0.2C-rate between 2.75 and 4.40 V vs. Li/Li⁺. The differential capacity plots revealed that impedance growth was slower for YAG surface treated LiCoO₂, when cells were charged at 4.40 V. DSC results exemplified that the exothermic peak at ∼468 K corresponded to the release of much less oxygen and greater thermal-stability.     

Preparation of Li2S-GeSe2-P2S5 electrolytes by a single step ball milling for all-solid-state lithium secondary batteries

Glass-ceramic and glass Li₂S-GeSe₂-P₂S₅ electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) with/without heat treatment (HT) from x = 0.55 to x = 1.00 were systematically investigated. Structural analysis by X-ray diffraction (XRD) showed gradual increase of the lattice constant followed by significant phase change with increasing GeSe₂. HT also affected the crystallinity. Incorporation of GeSe₂ in Li₂S-P₂S₅ kept high conductivity with a maximum value of 1.4 × 10⁻³ S cm⁻¹ at room temperature for x = 0.95 in Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) without HT. All-solid-state LiCoO₂/Li cells using Li₂S-GeSe₂-P₂S₅ as solid-state electrolytes (SE) were tested by constant-current constant-voltage (CCCV) charge-discharge cycling at a current density of 50 μA cm⁻² between 2.5 and 4.3 V (vs. Li/Li⁺). In spite of the extremely high conductivity of the SE, LiCoO₂/Li cells showed a large irreversible reaction especially during the first charging cycle. LiCoO₂ with SEs heat-treated at elevated temperature exhibited a capacity over 100 mAh g⁻¹ at the second cycle and consistently improved cycle retention, which is believed to be due to the better interfacial stability.     

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.     

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.     

Thickness estimation of interface films formed on Li1−xCoO2 electrodes by hard X-ray photoelectron spectroscopy

Solid electrolyte interface (SEI) films formed on Li_1−xCoO₂ electrodes were observed with hard X-ray photoelectron spectroscopy (HX-PES). This paper particularly focuses on film thickness estimation using HX-PES with theoretical calculation. The validity of the calculation was proven by experiments using model SEI films. The native film formed on a LiCoO₂ composite electrode was estimated to be LiF with its thickness of 5 nm. Formation of Co (II) species on top of LiCoO₂ was also indicated. Storage of the electrode at 60 °C brought about considerable film growth (30-40 nm) with carbonate compounds formation. SEI film changes during charging of the LiCoO₂ electrode were also examined. The main component in the film was deduced to be LiF or a kind of fluorite, with its thickness decreased during charging. The SEI formation mechanisms are also elucidated.     

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.     

AlF3-coated LiCoO2 and Li[Ni1/3Co1/3Mn1/3]O2 blend composite cathode for lithium ion batteries

Surface modifications of electrode materials can improve the electrochemical and thermal properties of cathodes for use in lithium batteries. In this study, AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are blended, as both have the same crystal structure and exhibit similar electrochemical properties. The composite electrodes exhibit high discharge capacities of 180-188 mAh g⁻¹ in a voltage range of 3.0-4.5 V at room temperature. The capacity retention of the composite electrode is greater than 95% of the initial capacity after 50 cycles. The thermal stability of these composite electrodes is greatly improved because of the superior thermal stability of AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂. The blended AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode shows two exothermic peaks, one at 227 °C from AlF₃-coated LiCoO₂ and another at 277 °C from AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂, accompanied by significantly reduced exothermic heat generation.     

Fabrication of all-solid-state lithium battery with lithium metal anode using Al2O3-added Li7La3Zr2O12 solid electrolyte

Li₇La₃Zr₂O₁₂ (LLZ) solid electrolyte is one of the promising electrolytes for all-solid-state battery due to its high Li ion conductivity and stability against Li metal anode. However, high calcination temperature for LLZ preparation promotes formation of La₂Zr₂O₇ impurity phase. In this paper, an effect of Al₂O₃ addition as sintering additive on LLZ solid electrolyte preparation and electrochemical properties of Al₂O₃-added LLZ were examined. By the Al₂O₃ addition, sintered LLZ pellet could be obtained after 1000 °C calcination, which is 230 °C lower than that without Al₂O₃ addition. Chemical and electrochemical properties of the Al₂O₃-added LLZ, such as stability against Li metal and ion conductivity, were comparable with the LLZ without Al₂O₃ addition, i.e. σ_bulk and σ_total were 2.4 × 10⁻⁴ and 1.4 × 10⁻⁴ S cm⁻¹ at 30 °C, respectively. All-solid-state battery with Li/Al₂O₃-added LLZ/LiCoO₂ configuration was fabricated and its electrochemical properties were tested. In cyclic voltammogram, clear redox peaks were observed, indicating that the all-solid-state battery with Li metal anode was successfully operated. The redox peaks were still observed even after one year storage of the all-solid-state battery in the Ar-filled globe-box. It can be inferred that the Al₂O₃-added LLZ electrolyte would be a promising candidate for all-solid-state battery because of facile preparation by the Al₂O₃ addition, relatively high Li ion conductivity, and good stability against Li metal and LiCoO₂ cathode.     

Fabrication of LiCoO2 cathode powder for thin film battery by aerosol flame deposition

Crystalline LiCoO₂ nano-particles for thin film battery were synthesized and deposited by aerosol flame deposition (AFD). The aqueous precursor solution of the lithium nitrate and cobalt acetate was atomized with an ultrasonic vibrator and subsequently carried into the central tube of the torch by flowing dry Ar gas. LiCoO₂ were formed by oxy-hydrogen flame and deposited on a substrate placed in a heating stage. The deposited soot film composed of nano-sized particles was subsequently consolidated into a dense film by high temperature heat treatment at 500–800 °C for 5 h and characterized by SEM, XRD, and Raman spectroscopy. The crystalline carbonates and oxide were first formed by the deposition and the subsequent heat treatment converted those to LiCoO₂. The FWHMs of the XRD peaks were reduced and their intensity increased as the heat treatment temperature increased, which is due to improved crystallinity. When judged from the low enough cation mixing and well-developed layered structure, it is believed that the LiCoO₂ film satisfied the quality standard for the real application. SEM measurements showed that LiCoO₂ were nano-crystalline structure with the average particle size <70 nm and the particle size increased with the increase of heat treatment temperature. The thickness of thin film LiCoO₂ before the consolidation process was about 15 μm and reduced to about 4 μm after sintering.     

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.     

Fe and Ni impurity distribution and electrochemical performance of LiCoO2 electrode materials for lithium ion batteries

The distribution of Fe³⁺ and Ni³⁺ impurities and the electrochemical performance of LiCoO₂ electrodes were examined. Commercial LiCoO₂ powders supplied by Aldrich were used. The electrochemical performance of LiCoO₂ was modified by rotor blade grinding of LiCoO₂ followed by thermal treatment. Structural information on Fe³⁺ and Ni³⁺ impurities was obtained using both conventional X-band and high-frequency electron paramagnetic resonance spectroscopy (EPR). It was found that Fe³⁺ occupies a Co-site having a higher extent of rhombic distortion, while Ni³⁺ is in a trigonally distorted site. After rotor blade grinding of LiCoO₂, isolated Fe³⁺ ions display a tendency to form clusters, while isolated Ni³⁺ ions remain intact. Re-annealing of ground LiCoO₂ at 850 °C leads to disappearance of iron clusters; isolated Fe³⁺ ions are recovered. The electrochemical performance of LiCoO₂ was discussed on the basis of isolated and clustered ions.     

Formation of the high lithium ion conducting phase from mechanically milled amorphous Li2S-P2S5 system

The fast ionic conducting structure similar to thio-Lithium Super Ionic Conductor (LISICON) phase is synthesized in the Li₂S-P₂S₅ system. The Li₂S-P₂S₅ glass-ceramics with the composition of xLi₂S·(100−x)P₂S₅ (75 ≤ x ≤ 80) are prepared by the heat-treatment of mechanically milled amorphous sulfide powders. In the binary Li₂S-P₂S₅ system, 78.3Li₂S·21.7P₂S₅ glass ceramic prepared by mechanical milling and subsequent heat-treatment at 260 °C for 3 h shows the highest conductivity of 6.3 × 10⁻⁴ S cm⁻¹ at room temperature and the lowest activation energy for conduction of 30.5 kJ mol⁻¹. The enhancement of conductivity with increasing x up to 78.3 is probably caused by the introduction of interstitial lithium ions at the Li sites which affects the Li ion distribution. The prepared electrolyte exhibits the lithium ion transport number of almost unity and voltage stability of 5 V vs. Li at room temperature.     

Amorphous Si film anode coupled with LiCoO2 cathode in Li-ion cell

An amorphous silicon film with an average thickness of up to 2 μm was deposited on copper foil by direct-circuit (dc) magnetron sputtering and coupled with commercial LiCoO₂ cathode to fabricate cells. Their cycle performance and high rate capability at room temperature have been investigated. In the voltage range 2.5–3.9 V at the current density of 0.2C (0.11 mA cm⁻²), the lithiation and delithiation capacity of this cell was first increased to 0.55 mAh cm⁻² within 80 cycles and maintained stable during the following cycles. After 300 cycles its capacity still retained 0.54 mAh cm⁻². High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) image indicated that the sputtered film could keep an amorphous structure although the volume expansion ratio during the lithiation and delithiation was still up to 300% after 300 cycles observed from scanning electron microscopy (SEM) image. This recovered amorphous structure was believed to be beneficial for the improvement of the cycle life of the cell. Rate performance showed that the cells had a promising high rate capability. At 30C, its lithiation/delithiation capacity remained 25% of that at 0.2C.