LiCoO2 Composite Electrodes Research Articles

Improved electrochemical performance of high voltage LiCoO2 with Al doping and ZrO2 coating

Surface coating and element doping are the key strategies in LIBs development to achieve a high and stable battery performance. In this paper, ZrO2 is directly sputtered on the Al-doped LiCoO2 (Al-LCO) composite electrodes, enabling a controllable oxide coating on the surface of the electrode. With a sputtered coating layer of ∼6 nm, the discharging capacity of the Al doped LCO coated with ZrO2 (Al-LCO-ZrO2) is 141.04 mAh g−1 at the 100th cycle within 3.0–4.7 V at 1 C, while the Al-LCO (without coating layer) is 126.66 mAh g−1. After 100 cycles of charging-discharging at high voltage region, the capacity retention of the Al-LCO-ZrO2-3 (90.7%) is also higher than the Al-LCO (75.57%). It is worth noting that, in this paper, due to the Al doping and the ZrO2 coating, we increase the operation voltage up to 4.7 V, in which the batteries can obtain a higher energy density.

Application of Modified Styrene-Acrylic-Rubber-based Latex Binder to LiCoO<sub>2</sub> Composite Electrodes for Lithium-ion Batteries

A new combination of styrene acrylic rubber (SAR) and sodium carboxymethylcellulose was developed as a water-borne binder for LiCoO2 composite electrodes operating at high voltages. Four novel SAR-based latex binders were synthesized with butyl acrylate or 2-ethylhexyl acrylate monomer and styrene via copolymerization with low and high crosslinking degrees. Composite electrodes prepared using lower-crosslinking-degree SAR binders became more stretchable and flexible. Surface analysis using electron microscopy and X-ray photoelectron spectroscopy revealed that the cycled LiCoO2 electrode was covered with approximately 10-nanometer-thick decomposition products when a conventional poly(vinylidene fluoride) binder was used. The electrodes with SAR-based binders with low cross-linkage formed a stable passivation surface during the initial cycle, and further continuous electrolyte decomposition was successfully suppressed. This passivation improved the cycle stability of LiCoO2 electrode up to 4.5 V, i.e., 87.1 % capacity retention, even after 100 cycles and suppressed self-discharge performance at 45 °C.

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.

What Limits the Rate Capability of Ultrathick Composite Electrodes in Lithium‐Ion Batteries? A Case Study on the Thickness‐Dependent Impedance of LiCoO2 Cathodes

AbstractIncreasing the thickness of composite electrodes in lithium‐ion batteries from about 80–100 μm in state‐of‐the‐art commercial cells to several 100 μm would enhance the energy density, however at the expense of the power density. Despite this common knowledge, quantitative studies on the impedance and rate capability of composite electrode in dependence of the electrode thickness are scarce. Therefore, we have carried out a case study on LiCoO2 composite electrodes with thicknesses up to about 250 μm. We first demonstrate that conventional composite electrode preparation leads to ion transport tortuosities in the electrolyte‐filled pores, which are virtually independent of the thickness. The thickness‐dependent impedance of these electrodes decreases with increasing thickness and follows the predictions of a generalized transmission line (GTLM) model. We use the GTLM model for analyzing in what way cycling of ultrathick electrodes (250–300 μm) with a rate of 1 C should be achievable.

Interface evolution and performance degradation in LiCoO2 composite battery electrodes monitored by advanced EQCM

Unravelling the underlying reasons for degradation mechanism of battery materials is of great fundamental and practical importance. For a classical electrode consisting of an active material, a conductive additive, and a polymeric binder, its capacity fading is commonly related with (i) mechanical degradation of polymeric binder and/or (ii) structural and compositional degradation of active materials. The former is more relevant for electrodes showing volume expansion and represented by the progressive breakage of polymeric binder network during battery operation, leading to the dissolution of the other two components into electrolytes. The latter is generally reflected by an irreversible phase transition in active materials, which may affect the species exchanged at the electrode/electrolyte interface and their interfacial transfer dynamics. By employing a coupled methodology pairing electrochemical techniques with piezoelectric probes derived from quartz crystal microbalance (QCM), this work reports on the evolution of the interfacial processes during electrochemical cycling and correlates to the performance degradation of the electrodes. Shown on a LiCoO2 (LCO) composite electrode as a model system, it was revealed that bare Li+ without a hydration sheath plays a dominant role in charge balance irrespective of the aging degree of the electrode under the experimental conditions of this work. However, Li+ transfer is closely accompanied with free H2O molecules with a Li+:H2O ratio around 10:1 at a polarization state close to LCO redox potential (0.65 V vs. Ag/AgCl). This ratio persists in all cycled electrodes with gradually faded interfacial transfer kinetics of Li+ and H2O along cycling. Such fading in species interfacial transfer kinetics driven by the surficial evolution from LiCoO2 to CoO plays a major role in the electrode performance degradation during cycling.

Effect of Substituted Styrene‐Butadiene Rubber Binders on the Stability of 4.5 V‐Charged LiCoO2 Electrode

AbstractThe combination of synthetically modified styrene‐butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) was studied as an aqueous binder for LiCoO2 composite electrodes under high‐voltage operation. Methyl methacrylate (MMA) and 2‐vinylpyridine (VP) substituted SBR copolymers were synthesized and examined as functional polymer binders. The electrolyte‐solvent uptake of the binder‐polymer film was increased by the introduction of the MMA units, but the solubility of the polymer in the solvent remained negligibly low. Hard‐X‐ray photoelectron spectroscopy revealed the formation of a relatively thick passivation layer on the LiCoO2 surface with MMA‐substituted SBR and CMC binder, which suppressed the self‐discharge of 4.5 V‐charged LiCoO2. The capacity retention during battery cycling experiments was similar for the LiCoO2 electrodes with MMA‐modified and unmodified SBR binders. While the effect of VP‐substitution was not significant, incorporation of the MMA unit enhanced the stability of the charged LiCoO2 electrode and provided improved self‐discharge performance.

Enhanced high-voltage performance of LiCoO2 cathode by directly coating of the electrode with Li2CO3 via a wet chemical method

Surface modification is an effective method for improving the high-voltage cycling stability of LiCoO2. In this work, lithium carbonate (Li2CO3), the main component of solid electrolyte interphase (SEI) films, is selected as the coating material to modify LiCoO2 composite electrodes by a wet chemical method, and the effect of the Li2CO3 coating time on the electrochemical performance of the LiCoO2 electrode is investigated. Results show that the Li2CO3 coating significantly improves the cycling performances and initial coulombic efficiencies of the LiCoO2 electrodes in the potential range of 3.0–4.5 V. The electrode with a coating time of 2 min exhibits the best electrochemical performance, in which the capacity retention rate is 90.9% after 100 cycles at 0.2C while the initial coulombic efficiency is 90.04%, whereas the capacity retention rate and initial coulombic efficiency of the uncoated electrode are only 73.11% and 74.66%, respectively. The capacity of the electrode with the 2-min coating reaches 134.3 mA h g−1 after 500 cycles, while that of the uncoated electrode is only 37.7 mA h g−1 under the same conditions. The results of cyclic voltammetry, electrochemical impedance spectroscopy, X-ray diffraction, and scanning electron microscopy show that the Li2CO3 coating stabilizes the electrode surface and structure to effectively inhibit the increase in electrode polarization.

Scrutiny of the LiCoO2 Composite Electrode/Electrolyte Interface by Advanced Electrogravimetry and Implications for Aqueous Li-Ion Batteries

The recent tendency in the battery field toward the use of aqueous electrolytes stimulated several studies searching for compatible electrode materials and charge carriers. Still, another aspect to forge ahead is the fundamental understanding of interfacial processes occurring at electrode/electrolyte interface. To this end, we investigated interfacial properties of a model LiCoO2 composite electrode in Li2SO4 aqueous electrolyte through extensively exploiting operando electrochemical quartz crystal microbalance (EQCM) and its coupling with electrochemical impedance spectroscopy (ac-electrogravimetry). EQCM revealed a global cation-exchange behavior, which is decoded by ac-electrogravimetry into multispecies contribution with different proportions/kinetics, benefiting from the capacity of the latter to study interfacial dynamics. Li+ plays the major role in the charge-compensation mechanism, but a strong interaction with H2O has been revealed with close interfacial transfer kinetics, which provides experimental evidence on the essential character of H2O assisting the Li+ insertion in layered materials due to the weakened charge density by screening the electrostatic interactions between the Li+ ions and the host lattice of cathode materials.

All-Solid-State Three-Electrode Cells with Reduced Li4Ti5O12 Reference Electrode

Introduction All-solid-state lithium-ion batteries (ASS-LIBs) are expected as next-generation LIBs because of high safety standard and power characteristics. However, ASS-LIBs still have several problems, such as short cycle life and high mass ratio of solid electrolytes. In order to solve these problems, it is necessary to clarify operation mechanisms of ASS-LIBs. In this study, we focused on electrochemical measurements, which have high versatility and high time resolution. Although three-electrode cells have been used in the electrochemical measurements of conventional LIBs to clarify the behavior of single electrodes, there have been few reports on three-electrode cells in ASS systems. [1] Moreover, electrochemical impedance spectroscopy (EIS) with three-electrode cells has never been reported in ASS-LIB systems. Thus, we synthesized chemically reduced Li4Ti5O12 (R-LTO) as an active material of the reference electrode and fabricated ASS three-electrode cells, and investigated electrochemical properties of LiCoO2 (LCO) composite electrode in Li-In|Li10GeP2S12(LGPS)|LCO cells by charge/discharge measurements and EIS measurements. Experimental methods R-LTO powder was synthesized by mixing of white Li4Ti5O12 powder with a tetrahydrofuran solution of lithium-naphthalenide. R-LTO reference electrode was fabricated by coating of the slurry composed of bluish black R-LTO powder, N-methyl-2-pyrrolidone and polyvinylidene fluoride on a nickel mesh current-collector. ASS three-electrode cell consisted of LCO composite electrode (LiNbO3-coated LCO:LGPS=7:3 wt%) as the working electrode, R-LTO reference electrode, LGPS as the solid electrolyte, and Li-In alloy as the counter electrode. Electrochemical measurements were carried out with a multichannel electrochemical system (VSP-300, Biologic). Results and Discussion Charge/discharge curves of the LCO electrodes and Li-In electrodes showed plateaus at around 2.35 V and –0.95 V vs. R-LTO, respectively. Assuming that the redox potential of Li7Ti5O12/ Li4Ti5O12 is 1.55 V vs. Li/Li+, we can calculate the plateau potentials of the LCO and Li-In electrodes to be 3.90 V and 0.60 V vs. Li/Li+, respectively. This clearly shows that the R-LTO reference electrodes successfully worked as an LTO redox couple. In the EIS measurements, the impedances of the LCO and the Li-In were successfully separated with the three-electrode cells since the impedance measured without the reference electrode agreed well with the combined impedance from those of the LCO and Li-In electrodes, which were separately measured with the reference electrode. We also investigated the temperature dependence and the state-of-charge (SOC) dependence of the LCO impedance. Figure 1 (a) shows the Nyquist plots obtained at 273, 288 and 298 K, and Figure 1 (b) gives the Arrhenius plots of two resistance values, R 2 and R 3, calculated with the equivalent circuit shown in the inset of Fig. 1 (a). The activation energy of R 3 was calculated to be 37 kJ mol–1, which is similar to that of the charge transfer resistance of LCO reported in ASS systems [2,3]. In addition, the SOC dependence of R 3 was similar to that of the charge transfer resistance observed for LCO electrode in a liquid electrolyte system [4]. From these results, R 3 can be assign to the charge transfer resistance. The activation energy of R 2 was 57 kJ mol–1, which is much higher than that of ionic conductivity of LGPS and also inconsistent with that of LCO electronic conductivity. Considering the SOC independence of R 2, we can assign R 2 to some interface or interphase resistance in the composite electrodes. The data with other electrode materials will be also reported in the presentation.

Application of modified styrene-butadiene-rubber-based latex binder to high-voltage operating LiCoO2 composite electrodes for lithium-ion batteries

Styrene-butadiene rubber (SBR) is modified either by changing the degree of cross linkage, or by the partial substitution of acrylonitrile for styrene. Four types of SBR-based latex binders (SBRx), which are used as a mixture with sodium carboxymethyl cellulose, are compared with conventional poly(vinylidene fluoride) (PVdF) binder by applying them to LiCoO2 composite electrodes in high-voltage operations up to 4.5 V vs. Li. The results demonstrate not only better electrode performance of SBRx compared with PVdF binder, but also some differences among SBRx binders. Further, they demonstrate good correlation between electrode performance and electrode analysis data by electrochemical impedance spectroscopy and hard X-ray photoelectron spectroscopy, suggesting better coverage with uniformity and stability on LiCoO2 particles composed of SBRx-based binder and deposition products of decomposed electrolyte than that by PVdF binder. The results reveal a clear dependency on the degree of cross linkage, and SBRlow modified by the low degree of cross linkage shows the best cyclability and rate capability. SBRCN, which is modified by acrylonitrile, shows a high solvent uptake and remarkable electrode performance in terms of rate capability and self-discharge suppression.

Performance of Li4Ti5O12-based reference electrode for the electrochemical analysis of all-solid-state lithium-ion batteries

The establishment of three-electrode cells for all-solid-state lithium-ion batteries is an important issue to clarify the electrochemical behavior of the battery components and solve problems for the practical applications. Here we prepared an all-solid-state three-electrode cell with a chemically-reduced Li4Ti5O12 reference electrode and investigated electrochemical properties of a LiCoO2 composite electrode in a Li-In|Li10GeP2S12|LiCoO2 cell. Plateau potentials in charge-discharge curves were coincident with reported values. Cell impedance measured with two-electrode configuration agreed well with the sum of the impedances of the positive and the negative electrodes measured with the reference electrode. These results show that chemically-reduced Li4Ti5O12 reference electrode was worked as a Li7Ti5O12|Li4Ti5O12 reference electrode and electrochemical properties of the LiCoO2 electrode and the Li-In electrode were separated. From the temperature and the state of charge dependences of LiCoO2 impedance, resistances related to interface or interphase and charge-transfer reaction were identified. These results show that all-solid-state cells with chemically-reduced Li4Ti5O12 reference electrodes are promising candidates for three-electrode cells for all-solid-state lithium-ion battery systems.

Reaction uniformity visualized by Raman imaging in the composite electrode layers of all-solid-state lithium batteries.

The reaction uniformity of LiCoO2 composite positive electrodes in all-solid-state cells was compared quantitatively by investigating the Raman band shifts corresponding to the state-of-charge (SOC) of LiCoO2. The quantitative SOC analysis was conducted using the Raman imaging data of composite electrodes with smaller or larger solid electrolytes. The electrodes exhibited different reaction uniformity although the cells showed similar initial charge capacities and average SOC. In the case of larger solid electrolytes, most LiCoO2 particles showed higher or lower SOC than the average SOC, and lower battery performance. The quantitative analysis of SOC in each LiCoO2 electrode demonstrated that a variable SOC outside the average SOC resulted in larger irreversible capacity and lower rate performance. The quantitative SOC analysis newly developed in the present study is a useful technique for designing composite electrodes showing higher battery performance.

Visualization of Inhomogeneuous Reactivity on Battery Material Using Scanning Electrochemical Cell Microscopy

To understand the metal oxide coating effect on battery performance, the following two techniques are required: 1) constructing a flat thin-film electrode surface to realize a well-defined interface and 2) analyzing the electrode/electrolyte interface reaction with nanoscale resolution. We previously studied flat LiCoO2 thin-film electrodes using in situ surface-sensitive X-ray absorption spectroscopy (XAS) and reported that Co reduction at the LiCoO2 surface resulting from electrolyte contact caused the initial degradation. We also showed that the ZrO2 layer successfully prevented physical contact between LiCoO2 and the electrolyte. And it confirmed that a thicker ZrO2 layer (above 2 nm) increased the diffusion resistance of the lithium ions in the ZrO2 layer. However, since XAS lacks in-plane resolution and provides only averaged information, it is impossible to analyze the ZrO2 morphology in detail. Recently, Taguchi et al. investigated a thin Li-Zr-layer (ca. 2 nm) on a LiCoO2 composite electrode by transmission electron microscopy (TEM). They suggested that this thin layer could improve the durability. However, it is difficult to analyze the electrochemical properties using TEM. To evaluate the intrinsic mechanism of the metal oxide coating effect, it is necessary to develop a novel in-situ method that can analyze the surface morphology with high spatial resolution and simultaneously determine the local electrochemical properties. Scanning electrochemical microscopy (SECM) is a powerful technique for linking the surface morphology of a sample to its electrochemical properties. For the battery materials, the SECM feedback mode is effective in monitoring solid electrolyte interphase formation. To directly and quantitatively investigate spatially resolved ionic processes, mercury-capped platinum ultramicroelectrodes were developed and employed for Li+ imaging based on Li stripping. However, it is difficult to visualize the Li+ flux in battery materials at the sub-micrometer scale by SECM. Scanning electrochemical cell microscopy (SECCM), which uses a nanopipette as a probe and forms a local electrochemical cell, is effective in characterizing surface reactivity. We recently applied SECCM for visualization of electrochemical activities on a lithium-ion battery cathode material at sub-micrometer resolution. The SECCM was applied to collect or provide Li in specified area confined by the nanopipette. Further, it collection visualized the electrochemical properties by scanning the nanopipette as an image. There are some strong advantages in SECCM for battery material research such as its high spatial resolution, small capacitive current, and isolated electrochemical cell. In this report, we applied SECCM to characterize a ZrO2-coated LiCoO2 thin-film electrode prepared by pulsed laser deposition. Local cyclic voltammetry (CV) and galvanostatic charge/discharge were performed to characterize the cycle durability and rate performance of ZrO2-coated LiCoO2 thin-film electrodes and to reveal the relationship between the ZrO2 morphology and thickness.

Operando Observation of Formation and Annihilation of Inhomogeneous Reaction Distribution in a Composite Electrode for Lithium‐Ion Batteries

AbstractInhomogeneous reaction distribution in battery electrodes can be a cause of serious capacity loss and degradation. Therefore, it is important to understand the mechanism of the reaction distribution formation. In this study, formation and annihilation of inhomogeneous charge/discharge reaction in a model LiCoO2 composite electrode were directly observed by operando two‐dimensional X‐ray absorption spectroscopy. The reaction distribution was clearly observed, and it was demonstrated that the reacted area became smaller with increasing the current density. The inhomogeneous charged situation remained even after 20 h of the open‐circuit rest and the discharge treatments. These results suggested that it is difficult to remove the inhomogeneity once it is formed in a composite electrode. High‐rate charge/discharge cycles may accumulate the inhomogeneity of the state of charge and result in significant capacity losses. The formation of the reaction distribution was strongly affected by the salt concentration in the liquid electrolyte. The expansion and annihilation rates of the charge/discharge reaction did not simply depend on the apparent ionic conductivity of the liquid electrolyte. This suggested that the salt concentration in a composite electrode varies locally by the lithiation/delithiation reactions especially under high‐rate operation. Such a local variation of the salt concentration can suppress or facilitate the charge/discharge reactions in a composite electrode.

Effects of C60 film coating on the electrochemical performance of composite LiCoO2 cathodes for lithium secondary batteries

In this work, LiCoO2(LCO) composite electrodes were coated by fullerene C60 thin film with different thickness of 60, 100 and 200 nm using a plasma thermal evaporation technique. The surface morphology of bare and coated samples was observed by scanning electron microscope (SEM). The electrochemical characteristics of the coated electrodes as cathode materials in Lithium-Ion Batteries (LIB)were investigated by a galvanostatic charge-discharge test at various C-rates between 3.0 and 4.5 V and compared with those of uncoated samples. An improvement of the performances of the coated electrodes in terms of higher initial coulombic efficiency, higher capacity retention and better rate capability was shown by the 60 nm thick C60 coated LiCoO2 electrodes. It can be said that the thin C60 coating layer can minimize the dissolution of Co from the electrode to the electrolyte. As the thickness of coating layer was increased, the coated electrodes show a more severe capacity fade due to longer Li-ion diffusion path.

Surface heterogeneity in Li0.5CoO2 within a porous composite electrode.

Surface phase heterogeneity within a fully charged LiCoO2 (LCO) composite electrode before and after thermal decomposition has been shown to strongly depend on the distribution of the binder and carbon additives, but be independent of the crystalline facet and size of LCO, as revealed by X-ray photoemission electron microscopy (X-PEEM). The obtained new insights deepen the understanding of thermal stability in a real composite electrode, which would be valuable for further development of long-life and safe cathodes for lithium-ion batteries.

In situ Raman spectroscopic studies on concentration change of electrolyte salt in a lithium ion model battery with closely faced graphite composite and LiCoO2 composite electrodes by using an ultrafine microprobe

The concentration of ions in the electrolyte solution in lithium ion batteries changes during operation, reflecting the resistance to ion migration and the positions of diffusion barriers. The change causes various negative effects on the performance of batteries. Thus, it is important to elucidate how the concentration changes during operation. In this work, the concentration change of ions in the electrolyte solution in deep narrow spaces in a realistic battery was studied by in situ ultrafine microprobe Raman spectroscopy. Graphite composite and LiCoO2 composite electrodes, which are the most commonly used electrodes in practical batteries, were placed facing each other and their distance was set to 80μm, which is close to the distance between electrodes in practical batteries. After repeated charge/discharge cycles, the concentration of ions increased and decreased greatly during charging and discharging, respectively. The maximum concentration was more than three-times higher than the minimum concentration. The rate of changes in concentration increased almost linearly with increase in current density. The results have important implications about concentration changes of ions occurring in practical batteries.

Sputtering TiO2 on LiCoO2 composite electrodes as a simple and effective coating to enhance high-voltage cathode performance

Abstract Surface coating is a key strategy in lithium-ion battery technologies to achieve a high and stable battery performance. Increasing the operation voltage is a direct way to increase the energy density of the battery. In this work, TiO 2 is directly sputtered on as-fabricated LiCoO 2 composite electrodes, enabling a controllable oxide coating on the topmost of the electrode. With an optimum coating, the discharge capacity is able to reach 160 mAh g −1 (86.5% retention) after 100 cycles within 3.0–4.5 V at 1 C, which is increased by 40% compared to that of the bare electrode. The high-voltage rate capability of LiCoO 2 is also remarkably enhanced after TiO 2 -coating as reflected by the much larger capacity at 10 C (109 vs. 74 mAh g −1 ). The artificially introduced oxide coating is believed to make the LiCoO 2 electrode more resistant to interfacial side reactions at high voltage and thus minimizes the irreversible loss of the active material upon long cycling. The TiO 2 coating layer is also possible to partially react with the decomposition product of electrolyte (e.g. HF) and form a more stable and conductive interphase containing TiF x , which is responsible for the improvement of the rate capability.

Visualization of Inhomogeneous Reaction Distribution in the Model LiCoO2 Composite Electrode of Lithium Ion Batteries

Two-dimensional X-ray absorption spectroscopy was carried out to observe the reaction distribution in a LiCoO2 composite electrode from the shift of the peak top energy in Co K-edge X-ray absorption spectra. The influence of ionic transportation to the inhomogeneous reaction was evaluated by using the model electrode, which sandwiched the LiCoO2 composite electrode between an aluminum foil and a polyimide ion blocking layer. When the model electrode was charged with the currents of 6, 9, and 12 mA cm–2, the observed capacities were 51, 20, and 12 mAh g–1 and the charged areas visualized from the shift of the peak top energy in Co K-edge X-ray absorption spectra were formed within ca. 700, 500, and 200 μm from the edge of the electrode, respectively. The observed reaction distribution indicated that the electrochemically active region decreases with increasing the current density because of the large potential loss of the electrochemical processes.

Nanoscale chemical imaging of the additive effects on the interfaces of high-voltage LiCoO2 composite electrodes

X-ray photoemission electron microscopy (X-PEEM) of cycled LiCoO2 composite electrodes has revealed the interfaces of various components within the composite electrodes and their dependence on additives in the electrolyte and the interplay of multiple components in the electrodes. This study visualizes CoF2 distribution and Co-O bonding variation along with local component agglomeration and degradation. The obtained new insights will assist further development of long-life high-voltage LiCoO2/C batteries.