Active Material Of Positive Electrode Research Articles

Na2S–NaI solid solution as positive electrode in all-solid-state Na/S batteries

All-solid-state sodium-sulfur (Na/S) batteries are promising next-generation batteries with high safety and high energy density. Sodium sulfide (Na2S) has application as active material in positive electrodes owing to its advantages such as low cost, low toxicity, and a large theoretical capacity. However, the electronic and sodium ion conductivities of Na2S are significantly low, and ascertaining the entire contribution of the active materials to cell capacities is challenging. Therefore, facilitating an electronic and ionic conduction path in the positive electrode is essential. In this study, Na2S was mixed with sodium iodide (NaI) via a mechanochemical process and was used as an active material in an all-solid-state Na/S battery. Consequently, a Na2S–NaI solid solution was formed, and the ionic conductivity of Na2S–NaI increased by five orders of magnitude compared to that of Na2S. Na2S–NaI showed large charge–discharge capacities and cycle capability. In particular, 90Na2S·10NaI showed a large capacity and high cycle efficiency, and 94% of Na2S acted as an active material. The result of this study leads to the development of all-solid-state Na/S batteries with high capacities, high rates, and high cycles.

Applications of different nano-sized conductive materials in high energy density pouch type lithium ion batteries

Lots of lithium ion battery (LIB) products contain lithium metal oxide LiNi5Co2Mn3O2 (LNCM) as positive electrode active material. To increase the conductivity, conductive carbon-based materials including acetylene black and carbon black become necessarily consisted in electrodes. Recently, carbon nano-tube (CNT) appears as a popular choice for conductive carbon in LIB. However, a large quantity of the conductive carbon which cannot provide capacity as the active material will decrease the energy density of batteries. The ultra-high cost of CNT comparing to conventional carbon black is also a problem. In this work, we are going to introduce ‘short length’ and ‘long length’ carbon nano-tube (S-CNT and L-CNT) into electrode in order to design a reduced-amount conductive carbon electrode. The whole experiment will be done in 1 Ah commercial type pouch LIB. By decreasing conductive carbon as well as increasing the active material in positive electrode, the energy density of LNCM-based 1Ah pouch type LIBs with only 0.16% of L-CNT inside LNCM positive electrode could reach 224 Wh/kg and 549 Wh/L, in weight and volume energy density, respectively. Also, this high energy density LIB with L-CNT reveals stable cyclability may become a valuable progress in portable devices and electric vehicle (EV) applications.

The influence of copper and carbon black on electrochemical behavior of nickel positive electrode

Nickel hydroxide is used as an active material in positive electrodes of secondary alkaline batteries. In alkaline batteries, the capacity of the negative electrode is greater than that of the positive electrode, hence the cell capacity is limited by the positive electrode. The practical capacity of the Ni(OH) 2 positive electrode depends on the efficiency of the conductive network connecting the Ni(OH) 2 particles with the current collector. In this study, hot-pressed-type electrodes were prepared using fiber-like β-Ni(OH) 2 powder as the active material on a nickel mesh as a current collector. The effect of carbon black as a conductive network for Ni(OH) 2 active material and the partial substitution of Cu(OH) 2 for β-Ni(OH) 2 material on the electrode performance, are examined. The carbon black powder addition improves the utilization of the active material; however it lead to a decrease in stability of the electrode. The partial substitution of Cu(OH) 2 for β-Ni(OH) 2 significantly improves the coulombic efficiency of the β-Ni(OH) 2 active material and it also increases the specific discharge capacity and enhance the stability of the electrode. These findings showed that the copper modified β-Ni(OH) 2 electrodes possessed an improved specific capacity and stability and thus may be recognized as a promising hybrid material for battery electrode applications. • The β-phase Ni(OH) 2 is formed when hydrothermal treatment is applied. • Carbon black improves specific discharge capacity but not stability. • Cu 2+ substitution improves both capacity and stability.

Effect of Particle Size of Cathode Active Material on Electrochemical Properties of Lithium-Ion Battery Investigated by In-situ EIS

An electrochemical impedance spectroscopy (EIS) has been applied to the investigation of the electrochemical properties of LIB because the time constant related to the electrode/solution interface can be discriminated in impedance spectrum. In the present study, the effect of particle size of the cathode active material on the electrochemical properties of LIB was investigated by in-situ EIS2). The in-situ EIS2) allows for the impedance measurement of the positive electrode, negative electrode, and cell of LIB simultaneously without stopping the measurement of the charge/discharge curve. A three-electrode cell was used for the impedance measurement. An active material of positive electrode was LiCoO2 (LCO) with particle size of 5 μm, 10 μm and 20 μm. The conductive additive and binder of positive electrode were acetylene black (AB) and polyvinylidene fluoride (PVDF), respectively. The positive electrode was fabricated by hand-screen printing. An Al sheet was used as the current collector. In the present study, the theoretical capacity of the positive electrode was calculated from the capacity density of LCO, whose value was defined as 1C-rate. The C-rate denotes the charge/discharge rate, and the 1 C indicates the charge/discharge rate that can fill/empty the total capacity of a battery in an hour. An active material and a current collector of negative electrode were natural spherical graphite and Cu sheet, respectively. A lithium ring was used as the reference electrode (RE), and a mixture of an ethylene carbonate (EC) and an ethyl methyl carbonate (EMC) (3:7 by volume) containing 1 M LiPF6 were used as the electrolyte. After the charge/discharge curve measurement using potentio-galvanostat (sp-50, Bio-Logic), the impedance measurement of the positive electrode during charge/discharge was measured at 1 C by in-situ EIS. The impedance measurement was carried out in the frequency range of 100 mHz to 100 kHz at 5 frequencies per decade with AC amplitude of 10 mV. An electrochemical measurement system (HZ-7000, Hokuto Denko) was used for the measurement. After the charge/discharge curve measurement at 0.3 C, the charge transfer resistance R ct was estimated from the curve fitting of the impedance spectra of the positive electrode during charge/discharge at 1 C using an equivalent circuit. It was confirmed that the R ct value decreased with increasing SOC at each particle size. Comparing the R ct values at each SOC, the R ct values at discharge were larger than that at the charge. This indicates that R ct is varied depending on the direction of current. Furthermore, the R ct values at each particle size during charge and discharge were decreased with decreasing the particle size, demonstrating the increase of the reaction surface area on the cathode active material. On the basis of these results, the diffusion distance of Li ions in the solid phase during charge/discharge was discussed.

Aging mechanisms under different state-of-charge ranges and the multi-indicators system of state-of-health for lithium-ion battery with Li(NiMnCo)O2 cathode

Understanding the degradation of battery cycled under different state-of-charge ranges is important for the optimal control of operation state-of-charge range. Cycle life tests on 8 A h pouch batteries with Li(NiMnCo)O2 cathode are conducted under 0–20%, 20%–40%, 40%–60%, 60%–80%, 80%–100% and 0–100% state-of-charge ranges with current and temperature at 6C and 25 °C. Results show that among the five ranges with 20% depth-of-discharge, cycling under 0–20% causes more impedance increase and less capacity loss, cycling under 80%–100% cause more capacity loss. The degradation behaviors of batteries under the remaining three ranges are similar. Battery degradation under 20% depth-of-discharge is significantly slower than 100% depth-of-discharge. Battery aging mechanisms are investigated using incremental capacity analysis method. It is indicated that loss of active material in positive electrode and loss of lithium inventory are comparable during battery aging process under 100% depth-of-discharge. However, for the battery under 20% depth-of-discharge, the dominant factor causing aging is loss of lithium inventory. Moreover, six characteristic parameters corresponding to loss of lithium inventory and loss of electrode material respectively are extracted from incremental capacity curve to establish the multi-indicators system describing battery state-of-health, which enhances the convenience of incremental capacity analysis to identify battery aging mechanisms on-board.

Quantitative Measurement of Lithium Ion Concentration in an Operating Lib Using Low Energy X-Ray

The lithium ion secondary battery (LIB) is essential device for electric vehicle (EV). However, the fundamental mechanisms of ion transport phenomena in LiB during operation, which determine battery performance, are yet to be elucidated. In this study, inside the operating LIB was visualized by using low energy X-ray microscopy technique. As a result, ion concentration distribution in the negative electrode and the separator were measured quantitatively. Figure 1 shows schematic image of experimental apparatus. Using a low energy X-ray is quite effective to measure the slight difference of ion concentration because the amount of X-ray absorption is greater than that of conventional X-ray. In order to observe the ion concentration distribution in a micro-scale thin layer of the battery, present study achieved high magnification (1 mm/pixel resolution) by irradiating the cone-shaped soft X-ray beam. Cells for visualization, positive electrode, negative electrode, and separator, were cut with the dimension of 6 mm x 20 mm by a cutting machine. Those were sandwiched by aluminum and copper current collector in a plastic frame. After injection of the electrolyte, cell was sealed by covering with the stainless plates that have X-ray transparent windows. All of assembling process was performed in Ar filled glove box. In this study, graphite was employed for negative electrode. Graphite electrode was prepared by mixing graphite (particle size 20 μm), conductive assistant, and binder (polyvinylidene fluoride). Active material for positive electrode was LiMn2O4. Separator was polypropylene microporous membrane. The electrolyte was 1 M LiPF6 / EC:DEC(3:7). Figure 2 shows X-ray transparent image of the graphite negative electrode and the separator. Since Mn significantly absorbs low energy X-rays in the positive electrode, it could not be visualized. The intensity of X-ray transmission indicates density and concentration of the materials that obeys Beer-Lambert law. Thus, by calculating the ratio of X-ray intensity between initial condition and charge/discharge experiments, ion concentration change that is caused by ion transport phenomena can be observed qualitatively. Based on the calibration experiment, quantitative ion concentration distributions were derived from the X-ray intensity ratio distribution. Figure 3 shows quantitative ion concentration distribution in an operating LIB. Constant current discharge was performed (1C, 2C, and 3C). As the discharge rate increases, the ion concentration gradient in the negative electrode becomes steep. Under the discharge condition of 3 C, the maximum ion concentration in the negative electrode reached about 2.5 M. These results suggest that the ion transport resistance increased at higher discharge rate condition because the diffusivity decreases in the higher concentration electrolyte. As mentioned above, using the low energy X-ray microscopy technique, we have succeeded in quantitative measurement of the ion concentration distribution in an operating LIB. In order to improve battery performance, it is important to design an optimum structure (electrode thickness, porosity, etc.) and increases ion transport efficiency in the deep portion of the electrode. Figure 1

Current collectors as reducing agent to dissolve active materials of positive electrodes from Li-ion battery wastes

This study evaluates the electrochemical effect of mineral, organic and metallic reducing agents to promote LiNi1/3Mn1/3Co1/3O2 (NMC) dissolution in acidic media. The main aim is to compare the reactivity of metallic current collectors (Cu and Al) to more conventional reducing agents such as organic acids and hydrogen peroxide. In a more applied perspective, this work points out the economic interest to use current collectors as an alternative chemical reagent for an efficient NMC dissolution as they are inherently present in the fraction treated by an hydrometallurgical approach. It demonstrates that galvanic interactions are suitable to leach the transition metal oxides from spent LIBs without heating and emission unlike carboxylic acid solutions, allowing the development of low ecological footprint processes. In a more fundamental framework, the results obtained clear several parts of the dissolution NMC mechanism. A fast first step is controlled by the solution pH with a low influence of redox reactions on the kinetic. A slower second stage is subjected to the electrochemical processes with a surface-controlled dissolution.

Constructing High Energy and Power Densities Li-Ion Capacitors Using Li Thin Film for Pre-Lithiation

Lithium-ion capacitor (LIC) contains negative electrodes (NEs), positive electrodes (PEs), separator and lithium source for pre-lithiation of NEs. Activated carbon was used as the active materials of PEs, and hard carbon will be chosen as NEs. As we know, the specific capacity of NE is ∼372 mAh/g. The most concerned work for our cells now is the pre-lithiation method. With published data from Cao's group before, pre-lithiation with 99.9% purity Li strips can manufacture cells more efficient and safer than using stabilized lithium metal powder (SLMP). However, even this method may take up to 18 h for electrode to finish the pre-lithiation process. In this paper, five pre-lithiation methods for LICs were evaluated in this work, and strip cells were used as control samples and further provided the base for the whole comparison. This research brought up a new method of using 15∼20 um Li ultra-thin film for pre-lithiation. During tests, both sandwich LIC cells and 200F LICs were fabricated to demonstrate this new method. As supplementary evidence, DC life and cycle life tests were conducted after initial tests. Thus, 200F LICs using 20 μm Li film for pre-lithiation can achieve higher capacitance (246 F) and lower ESR (19 mOhm) than Li strip cells. Higher energy and power densities (26.9 Wh/L and 18.3 kW/L) have also been achieved. Moreover, sandwich cells with 15∼20 μm Li thin film has passed 100,000 charge-discharge cycle tests, and their capacitance and ESR can be maintain stable after 1500 h constantly charging at 3.8 V under 65°C.

Aluminum Secondary Battery Utilizing Graphite-Leaf Powder As an Active Material for Positive Electrode

Secondary battery has driven various technologies forward in past decades. Now we can use a laptop PC on a fright across the Pacific without inserting a plug in a socket. Still development of secondary batteries with improved energy density and reduced cost are of significance. In particular, the volumetric energy density of a battery becomes highly important when apply it in a limited space. To meet the need, several approaches are proposed so far, such as using extremely-high capacity electrodes and multivalent-ion. Some scientists are interested in the Al of which trivalent feature provides a theoretical volumetric capacity as high as 8040 mAh cm–3, nearly four times larger than that of lithium and more than twice of magnesium.1 In fact, electrodeposition of Al has a rich history. Haloaluminate ionic liquids (ILs), allowing reversible deposition/dissolution of Al with a high coulombic efficiency, is suitable for the electrolyte. However, there are only limited researches on positive electrode materials in the haloaluminate ILs. Because of this, most of Al secondary batteries show limited capacity or inferior rate performance.2-4 Herein we report that graphite-leaf powder can be utilized as a new positive electrode material for rechargeable Al batteries, which exhibits a good rate performance and a stable cyclability up to 1,000 cycles. The procedures used for the preparation and purification processes on the AlCl3−[C2mim]Cl IL were identical to those described in our previous articles. In this study, 60.0-40.0 mol% AlCl3−[C2mim]Cl IL was used for the electrolyte. For the positive electrode, after graphite-leaf powder was thoroughly dispersed in N-methyl-2-pyrrolidone solution containing suitable amount of polysulfone binder, the resultant slurry was coated onto a molybdenum current collector and dried in vacuo at 473 K overnight. Graphite-leaf powder used in this study has an averaged thickness of 6-8 nm and thickness of 5 µm. Al coil and Al wire of high purity were used as a negative and reference electrode, respectively. Cell assembly and electrochemical measurements were conducted in an Ar-filled glovebox. Cyclic voltammogram recorded at a graphite-leaf powder electrode in the 60.0 mol% AlCl3−[C2mim]Cl showed several redox waves during the anodic scan from the rest potential. Those waves should be relate to the electrochemical intercalation reaction of the [AlCl4]– as follows:5 n C + [AlCl4]- ⇄ C n +[AlCl4]- + e- Although graphite rod electrode yields carbon powder precipitation after the electrochemical measurements over ca. 2.3 V ((vs. Al(III)/Al)), the graphite-leaf powder electrode was very stable. This favorable behavior would be caused by the flexibility of the graphite-leaf powder. Figure 1 shows the typical charge-discharge curves of graphite-leaf powder electrode. The cut-off voltage range was 0.80 to 2.40 V. Quasi voltage plateaus was observed at approximately 2.30 V, which is higher than previous reported cathodes.2-4 A reversible capacity of 70 mAh g–1 was obtained at current density of 500 mA g–1. These values were estimated from the amount of graphite-leaf powder on the positive electrode. Increasing the current density did not cause a distinct capacity decline, implying that the intercalation/deintercalation of [AlCl4]– anion readily proceeds on the electrode. The electrode performance was certainly enhanced by further improvement of the electrode fabrication method, e.g., the optimization of the type of binder and of the composition in slurry, the use of conductive additive. Acknowledgement Part of this research was supported by the Grant-in-Aid for Scientific Research, Grant Numbers 15H03591, 15K13287, and 15H2202 from the Japanese Ministry of Education, Culture, Sports, Science and Technology and by the ALCA-SPRING program, Japan Science and Technology Agency. Reference s J. Muldoon, C. B. Bucur, and T. Gregory, Chem. Rev., 114, 11683 (2014).L. D. Reed, S. N. Ortiz, M. Xiong, and E. J. Menke, Chem. Commun., 51, 14397 (2015).L. X. Geng, G. C. Lv, X. B. Xing, and J. C. Guo, Chem. Mater., 27, 4926 (2015).T. Tsuda, I. Kokubo, M. Kawabata, M. Yamagata, M. Ishikawa, S. Kusumoto, A. Imanishi, and S. Kuwabata, J. Electrochem. Soc., 161, A908 (2014).M. C. Lin, M. Gong, B. Lu, Y. Wu, D. Y. Wang, M. Guan, M. Angell, C. Chen, J. Yang, B. J. Hwang, and H. Dai, Nature, 520, 324 (2015). Figure 1

Li-Rich Mn/Ni Layered Oxide as Electrode Material for Lithium Batteries: A 7Li MAS NMR Study Revealing Segregation into (Nanoscale) Domains with Highly Different Electrochemical Behaviors

We present a 7Li MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres) aiming at identifying very precise cationic (Li+, Mn4+/Ni2+) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li2MnO3 cores (volume ∼58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO2-like (volume ∼20%) where M = Mn1/2Ni1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li2MnO3-li...

Structural and electronic studies of metal hexacyanoferrates based cathodes for Li rechargeable batteries

Operando XANES and EXAFS spectra on the newly prepared Fe hexacyanocobaltate active material for positive electrodes in lithium batteries have been recorded at the XAFS beamline of Elettra using a suitable in situ cell. In this way, it was possible to follow in detail the main structural and electronic changes during the charge and discharge processes of the battery. The use of a chemometric approach for data analysis is also underlined.

Structural Evaluation of Amorphous MoS3 Positive Electrode Active Materials in All-Solid-State Lithium Secondary Batteries

Sulfur is an attractive active material for positive electrode because sulfur has a high theoretical capacity. High capacity of the batteries can be achieved by using active materials with a large content of sulfur. Low electronic conductivity is a drawback of sulfur. We thus focus on a transition metal trisulfides such as TiS3 with high electronic conductivity. Moreover, amorphization of active materials is potentially capable of achieving higher capacity and cyclability because of open and random structure in amorphous materials. Recently, we have found that amorphous TiS3 prepared by ball milling had better cyclability than crystalline TiS3 [1, 2]. Very recently, we have reported that amorphous molybdenum trisulfide (a-MoS3) electrode active materials prepared by mechanical milling showed high reversible capacity (about 670 mAh g-1) for 60 cycles in the all-solid-state lithium batteries with Li2S-P2S5 electrolytes at room temperature [3]. However, the electrochemical reaction mechanism of a-MoS3 has not been identified yet. In order to improve the battery performance, it is important to understand the reaction mechanisms of a-MoS3 in the all-solid-sate lithium batteries. In this study, we investigated microstructures and morphologies of a-MoS3 electrodes before and after charge-discharge processes. a-MoS3 was prepared by mechanical milling of the mixture of molybdenum metal and sulfur. The obtained active materials were applied as a positive electrode to all-solid-state cells with sulfide solid electrolytes. Structures and morphologies of a-MoS3 electrodes before and after charge-discharge tests were analyzed by XRD, XPS, SEM and HR-TEM. The HR-TEM observations of a-MoS3 electrodes during the 1st discharge test revealed that the lattice fringes due to MoS2 layer structure disappeared gradually. After the 1st full discharge, the lattice fringes were not observed and the electron diffraction showed halo pattern. However, the lattice fringes like MoS2 were observed again after the 1st charge. These results of HR-TEM observations suggested that the reversible structural changes of a-MoS3 occurred. On the other hand, Li2S nano crystals were observed in HR-TEM image of a-MoS3 after the 10th discharge. The HR-TEM image after the 10th charge showed no lattice fringes of Li2S and MoS2. It is concluded that MoS3 after the 10th charge was completely amorphous. Reference [1] A. Hayashi, et al., Chem. Lett., 41 (2012) 886. [2] T. Matsuyama, et al., J. Solid State Electrochem., 17 (2013) 2697. [3] T. Matsuyama, et al., J. Mater. Chem. A, submitted for publication Acknowledgment This research was financially supported by ALCA-SPRING project.

In-Situ Visualization of Li-Ion Secondary Battery Using Soft X-Ray Microscopy

The lithium ion secondary battery is essential device for electric vehicle (EV) and hybrid electric vehicle (HEV). However, fundamental mechanism of the transport phenomena in an electrode under charge/discharge condition needs to be elucidated in order to improve the battery performance. In this study, in-situ visualization of the Li-ion battery was performed by applying a soft X-ray microscopy technique. As a result, ion concentration distribution in the negative electrode was able to be investigated. Density reduction that was caused by Li intercalation was also observed. Figure 1 shows schematic image of experimental apparatus. Using a low energy X-ray (soft X-ray) is quite effective to visualize the light element like Li because the amount of X-ray absorption is greater than that of conventional X-ray. In order to observe the transport phenomena in a micro-scale thin layer of the battery, present study achieved high magnification (1 mm/pixel resolution) by irradiating the cone-shaped soft X-ray beam. Cells for visualization, positive electrode, negative electrode, and separator, were punched out with the dimension of 6 mm x 20 mm by a cutting machine. Those were sandwiched by aluminum and copper current collector in a plastic frame. After injection of the electrolyte, cell was sealed by covering with the stainless plates that have X-ray transparent windows. All of assembling process was performed in Ar filled glove box. In this study, two active materials, hard carbon or graphite, were employed for negative electrode. Hard carbon negative electrode was prepared by mixing hard carbon (particle size 10 μm), conductive assistant, and binder (polyvinylidene fluoride). Graphite electrode was prepared by mixing graphite (particle size 20 μm), conductive assistant, and binder (polyvinylidene fluoride). Active material for positive electrode was LiMn2O4. Separator was polypropylene microporous membrane. The electrolyte was 1 M LiPF6 / EC:DEC(1:1). Figure 2(a) shows X-ray transparent image of the hard carbon negative electrode. The intensity of X-ray transmission indicates density and concentration of the materials that obeys Beer-Lambert law. Thus, by calculating the ratio of X-ray intensity between initial condition (Ix0 ) and charge/discharge experiments (Ix ), concentration distribution of anion and cation that is caused by transport phenomena can be observed under operating condition. Figure 2(b) and (c) show distribution of Ix /Ix0 in the separator and negative electrode. Constant current charge/discharge was performed (3C and 10C). During the charge process, Ix /Ix0 shows apparent gradient profile that increases toward the negative electrode side. On the other hand, Ix /Ix0 shows an opposite trend during discharge process. Increase of Ix /Ix0 during the charge process means a decrease of concentration of Li+ and PF6 - in the electrolyte. It is considered that the ion concentration in the negative electrode was decreased because Li+ and PF6 - were drawn to the positive electrode by the electric field during the charge process. Thus the change of ion concentration through the charge/discharge process can be observed in Figure 2(b) and (c). In the open circuit condition after charging, Li had inserted in the hard carbon. However, it is difficult to visualize the change of Ix /Ix0 . The amount of Li intercalation was small to cause clear difference of Ix /Ix0 compared with Li+ and PF6 - in the electrolyte. Figure 3(a) shows the time variation (Initial to 7th) of X-ray transparent images of graphite negative electrode. Lithium intercalation changes interlayer distance of the graphite. Thus, the insertion process can be observed by the deformation or density reduction of the negative electrode. As shown in Figure 3(a), swelling of the negative electrode during 0.5 C charge was observed successfully. Ix /Ix0 changes greater than that of hard carbon negative electrode (Figure 3(b)). Thus, Li intercalation can be detected easily. It is considered that Li intercalation occurred not depending on the position because Ix /Ix0 increased uniformly. The expansion rate 1.1 calculated from the experimental result agrees well with the theoretical rate 1.09 that was derived from the ratio between initial interlayer distance (0.34 nm) to inserted distance (0.37nm). As mentioned above, using the soft X-ray microscopy technique, we have succeeded in visualization of the transfer phenomena in the Li-ion secondary battery during charging and discharging. By calculating the ratio of X-ray intensity (Ix /Ix0 ), Li+ and PF6- distributions in the hard carbon negative electrode were obtained clearly. Li intercalation into the graphite negative electrode was also observed. These results suggest that transport phenomena in the electrode can be evaluated quantitatively by the soft X-ray microscopy technique. Figure 1

Electrochemical supercapacitor based on multiferroic BiMn2O5

Submicrometre particles of multiferroic BiMn2O5 are prepared by a hydrothermal method. We demonstrate for the first time that BiMn2O5-multiwalled carbon nanotube (MWCNT) composite can be used as a new active material for positive electrodes of electrochemical supercapacitors (ES). The possibility of fabrication of BiMn2O5–MWCNT composites from colloidal suspensions using Celestine blue dye as a co-dispersant for BiMn2O5 and MWCNT is demonstrated. The composite BiMn2O5–MWCNT electrodes with high mass loading and high active material to current collector mass ratio show a capacitance of 6.0 F cm−2 (540 F cm−3) at a scan rate of 2 mV s−1 and capacitance retention of 75 and 58% at scan rates of 100 and 200 mV s−1, respectively. The new findings pave the way to the fabrication of efficient asymmetric devices, containing BiMn2O5–MWCNT positive electrodes and activated carbon–carbon black (AC–CB) negative electrodes. The asymmetric device shows good capacitive behavior and good cyclic stability in a voltage window of 1.8 V. The analysis of power-energy characteristics indicates that maximum energy density of 13.0 W h L−1 (9.0 W h kg−1) and maximum power density of 3.6 kW L−1 (2.5 kW kg−1) can be achieved.

Study of multi-walled carbon nanotubes for lithium-ion battery electrodes

The potential use of multi-walled carbon nanotubes (MWCNTs) produced by Chemical Vapor Deposition (CVD) as conductive agent for electrodes in Li-ion batteries has been investigated. LiNi0.33Co0.33Mn0.33O2 (NCM) has been chosen as the active material for positive electrodes, and a nano-sized TiO2-rutile for the negative electrodes. Also the MWCNTs ability of reversibly inserting Li has been characterized. The electrochemical performances of the electrodes are studied by galvanostatic techniques and cyclic voltammetry. In particular the influence of the nanotubes on the rate capability is evaluated. The addition of MWCNTs significantly enhances the rate performances of NCM-based cathodes at all investigated C-rates. The 1wt.% MWCNTs in TiO2 rutile-based anodes accounts for an increase in the rate capability when the electrodes are cycled in the potential range 1.0–3.0V. The range extension to more negative potentials (i.e. 0.1–3.0V), however, causes a capacity fading especially at higher current rates. The obtained results demonstrate that the addition of MWCNTs to the electrode composition, even in low amounts, enables an increase in both energy and power densities of a Li-ion battery.

Effects of addition of different carbon materials on the electrochemical performance of nickel hydroxide electrode

Nickel hydroxide is used as an active material in positive electrodes of rechargeable alkaline batteries. The capacity of nickel–metal hydride (Ni–MH) batteries depends on the specific capacity of the positive electrode and utilization of the active material because of the Ni(OH) 2/NiOOH electrode capacity limitation. The practical capacity of the positive nickel electrode depends on the efficiency of the conductive network connecting the Ni(OH) 2 particle with the current collector. As β-Ni(OH) 2 is a kind of semiconductor, the additives are necessary to improve the conductivity between the active material and the current collector. In this study the effect of adding different carbon materials (flake graphite, multi-walled carbon nanotubes (MWNT)) on the electrochemical performance of pasted nickel–foam electrode was established. A method of production of MWNT special type of catalysts had an influence on the performance of the nickel electrodes. The electrochemical tests showed that the electrode with added MWNT (110–170 nm diameter) exhibited better electrochemical properties in the chargeability, specific discharge capacity, active material utilization, discharge voltage and cycling stability. The nickel electrodes with MWNT addition (110–170 nm diameter) have exhibited a specific capacity close to 280 mAh g −1 of Ni(OH) 2, and the degree of active material utilization was ∼96%.

The influences of some additives on electrochemical behaviour of nickel electrodes

Nickel hydroxide is used as an active material in positive electrodes of rechargeable alkaline batteries. Since the nickel hydroxide electrode exhibits a poor performance which results not only from the competitive reactions of the oxidation of the active material but also from the evolution of oxygen. Its reduced charge acceptance is suspected to be related to a relatively long distance between nickel hydroxide particles and the nearest portion of the substrate. The practical capacity of the positive nickel electrode depends on the efficiency of the conductive network connecting the Ni(OH) 2 particle with the current collector. In this study, a pasted-type electrode is prepared using spherical nickel hydroxide powder as the main active material on a foamed nickel grid as a current collector. The effects of additives such as metallic cobalt powder, cobalt hydroxide, calcium carbonate and cobalt powder with calcium carbonate on the electrode performance are examined. The calcium carbonate addition increases the oxygen evolution potential while the metallic cobalt powder and its compounds enhance the conductivity of the active material. This combined effect in nickel hydroxide electrode in turn increases the capacity of the Ni–MH battery due to the augmentation in the utilization of the active material of the positive electrode.

加水分解による PbO2 ナノ粒子の調製と鉛蓄電池正極活物質への応用

加水分解による PbO2 ナノ粒子の調製と鉛蓄電池正極活物質への応用

An investigation of silicon-doped LiCoO 2 as cathode in lithium-ion secondary batteries

Powders of layered–structured LiCo (1− x) Si x O 2 ( x = 0, 0.01, 0.05, 0.10, 0.35) were synthesized by a co-precipitation method followed by a 950 °C sintering. Their structures were analyzed by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). Iodine titration method was also employed to measure the average valence of cobalt ions. With these powders as the active materials of positive electrodes versus lithium, the electrochemical behaviors of the cells were investigated using charge–discharge cycling, AC impedance spectroscopy and DC resistance measurement. It is found that the Si-doping results in the decrease of cobalt valence and the crystallite size. When the Si-content is less than 10%, pure LiCo 1− x Si x O 2 phases are obtained. A second phase Li 2CoSiO 4 is also obtained when the Si-content is 35%. Among the five compositions, the non-doped LiCoO 2 exhibits a high initial specific capacity (about 150 and 185 mA h/g at a current density of 0.4 mA/cm 2 from 2.8 to 4.2 and from 2.8 to 4.5 V, respectively), but degrades in the following cycles; while the Si-doped LiCo 1− x Si x O 2 electrodes especially LiCo 0.99Si 0.01O 2 show the best performance of long-life cycling. And all of the doped powders have better stability of the 3.6 V-plateau efficiency due to the improved stability effect on the cell impedance.

放電／充電サイクルの初期における鉛蓄電池正極活物質の質量変化

In order to investigate the early stages of discharge-charge of the active materials for positive electrode in the lead-acid battery, a PbO2 thin film was prepared by reactive sputtering, and then cyclic voltammetry on the PbO2 film electrode was repeated 5 times in various H2SO4 solutions ranging from 0.1 to 2.0 kmol m-3 in concentration at 303 K. The mass change of the PbO2 film electrode was in situ observed using the electrochemical quartz crystal microbalance (EQCM) technique. The mass of the active material of positive electrode, PbO2 increases during the discharge reaction of PbO2→PbSO4 and it decreases during charge reaction of PbSO4→PbO2. A part of the discharge product, PbSO4 was not restored to its original state, PbO2 on 1∼3 cycles of discharge-charge, although the mass change curves had the loop shapes on 4∼5 cycles. The discharge-charge became steady-state and the current efficiency of discharge was nearly equal to that of charge on and after 4 cycle. On the other hand, the average grain size of the discharge product, PbSO4 in 0.1 kmol m-3 H2SO4 solution was about 0.35 μm and it was considerably smaller than those in 0.5∼2.0 kmol m-3 H2SO4 solutions. Moreover, the utilization rate of PbO2 in the former solution is approximately 6∼8% higher than those in the latter solutions. It may be concluded from the results that the utilization rate of PbO2 depends on the shape of the PbSO4 crystal produced by the first discharge. Therefore, it seems that refining the grain size of the first discharge product is of great significance for the improvement of the discharge-charge property of the active materials for positive electrode in the lead-acid battery.