High Voltage Region Research Articles

Layered Li-rich Oxide Cathode Microparticle 0.4Li2MnO3∙0.6Li(Mn0.43Ni0.36Co0.21)O2 Decorated with Nano Grains and its Electroochemical Performance

The development of alternative energy sources has become the primary strategy to overcome unstable crude-oil prices and the issues associated with climate change. Seasonal variations in alternative energy sources have prompted the need for an energy storage system. Reduction of carbon dioxide is also a key strategy for solving the problem of climate change. Carbon dioxide produced by transportation constitutes almost one-third of total carbon dioxide production. Replacing gasoline-based transportation with electric vehicles represents a promising solution to reduce carbon dioxide emissions. Among the various types of batteries, lithium secondary batteries are considered the most attractive option because of their light weight and high power density. However, the current technical specifications are not quite satisfactory for full scale commercialization of electric vehicles (EVs) and hybrid electric vehicles (HEVs). Increasing driving range and reducing the cost of batteries remain key issues that need to be addressed. Nano-composite layered compounds of xLi2MnO3·(1-x) LiMO2 (M = Ni, Co, Mn) were recently shown to exhibit high capacities in the high voltage region of –4.6 V and have lower cost and better safety compared to LiCoO2, which is used in commercial lithium-ion batteries. However, the layered xLi2MnO3·(1-x) LiMO2 shows low rate capability due to low electronic conductivity, which is caused by the insulating Li[Li1/3Mn2/3]O2 component. Therefore, the low rate capability of these batteries needs to be addressed for use in EVs and HEVs, which require fast charge/discharge reactions. Various methods have been developed to improve the rate capability of layered xLi2MnO3·(1-x) LiMO2 composite particles, such as doping with another transition metal, surface modification, and coating. However, attempts to control the morphology and particles size has not been reported due to limitations in the preparation methods. The cathode particles are normally prepared by a solid-state reaction or liquid phase methods and these methods require a high calcination temperature and long calcination time to obtain desirable composite phase. These calcination conditions result in aggregation and growth of primary particles, which degrades the diffusion of lithium ions between the electrode and electrolyte. Spray pyrolysis has recently been used for the preparation of multicomponent ceramic particles due to its several advantages, such as being a simple system, ease of controlling particle size and preparing spherical particles of multi-components and formation of a pure well-crystallized phase at low calcination temperature. Thus, in this study, we prepared spherical 0.4Li2MnO3∙0.6Li(Mn0.43Ni0.36Co0.21)O2 microparticles decorated with nano-grains via spray pyolysis and the structural and electrochemical performance of the prepared particles were investigated.

Scalable synthesis of two-dimensional porous sheets of Ni-glycine coordination complexes: A novel high-performance energy storage material

A novel metal-amino acid assembly is successfully prepared using the simplest amino acid structure – glycine – and nickel nitrate hexahydrate. Two-dimensional porous Ni-Glycine coordination complex [Ni(NH2CH2COO)2](H2O) (CQU-Chen-Gly-Ni-2-1) sheets with 3D nanoflake array structures have been prepared by a facile and cost-effective method. As electrode materials for supercapacitors, the Ni-Glycine coordination complexes display a high specific capacitance of 2.34 F·cm−2 at 1.08 mA·cm−2 (2171.71F·g−1 at a current density of 1 A·g−1), outstanding rate performance (80.77% capacity retention from 1.08 to 10.8 mA·cm−2) and superior cycling performance (2.4% loss after 5000 cycles at 10.8 mA·cm−2). In addition, the optimized asymmetric supercapacitor could be cycled reversibly in the high-voltage region of 0–1.8 V. Furthermore, excellent asymmetric supercapacitors of Ni-Glycine//activated carbon were also fabricated, and exhibited high energy densities of 35.05 and 16.75 Wh·kg−1 at power densities of 900 and 9000 W·kg−1, respectively. These fascinating performances can be attributed to the high capacitances and positive synergistic effects of the two electrodes. The impressive results presented here may pave the way for promising applications in high energy density storage systems.

Analysis of the Effect of Beam Divergence Angle on Back-Streaming Electron Region in Ion Source for EAST-Neutral Beam Injection

During the process of beam extraction in positive ion source under high voltage region, a large number of electrons are produced in the gaps of grids. After back-streaming acceleration, these electrons go back to arc chamber or impinge grids and then heat back plate or grids, which are harmful for the safety of ion source. Under the situation of poor beam extraction optics, a large part of the primary beam ions bombard the surface of suppressor grid (SG). And this process produces a large number of electrons. Due to the huge extracted voltage, the secondary electron emission coefficient of the SG surface is also high. As a result, the grids' current grows. According to the measurement of the current of SG and the calculation of the perveance of the corresponding shoot, the effect of ion beam divergence angle on back-streaming electrons can be analyzed. When the beam divergence angle increases, the number of back-streaming electrons increases rapidly, and grids' current changes significantly, especially the current of gradient grid and SG. The results can guide the parameters operating on the ion source for Experimental Advanced Superconducting Tokamak-neutral beam injection (EAST-NBI) and find the reasonable operation interval of perveance and to ensure the safety and stable running of the ion source, which has great significance on the development of long pulse, high power ion source.

New Insights into Anionic Redox Reaction and Performance Degradation of Iron-Based Layered Oxides Cathode Materials for Sodium-Ion Batteries

Sodium ion batteries continue to be of immense interest as potential substitution for lithium-ion battery due to its abundance on earth, contrary to limited source of lithium. The beneficial effect on the cost efficiency will increase if other element in the cathode is also earth abundant materials. Many type of iron based layered oxides such as secondary, tertiary, quaternary layered cathode compound for Na ion batteries have been studied. Most of these studies showed severe loss of cycle performance at high voltage. To improve performance of iron based layered oxides, especially at high voltage region, it is necessary to better understand the redox reaction of Fe3+/Fe4+ during sodium extraction. Thus, O3 type NaFeO2 was synthesized and variety of analysis was conducted such as in-situ synchrotron hard X-ray absorption spectroscopy, ex-situ soft X-ray absorption spectroscopy, and transmission electron spectroscopy. In situ hard X-ray absorption spectra showed that Fe K-edge doesn’t shift to higher energy during Na extraction. The shift in Fe K-edge is sensitive with oxidation state of Fe that mean Fe3+/Fe4+ redox reaction does not contribute much during electrochemical test. Instead of Fe, the emerging of new peak on the pre-edge of the oxygen K-edge, indicating charge compensation of oxygen during the whole Na extraction/insertion process. Moreover, we directly reveal the limited reversibility of the oxygen redox reaction and correlation of performance degradation with surface degradation of α-NaFeO2 cathode in sodium-ion cells especially at high voltage. These finding provide more deep understanding for future development of iron-based cathode materials for sodium-ion batteries.

On electrical and interfacial properties of iron and platinum Schottky barrier diodes on (111) n-type Si0.65Ge0.35

Current–voltage (I–V), capacitance–voltage–frequency (C–V–f) and conductance–voltage–frequency (G/[Formula: see text]–V–f) characteristics of Molecular Beam Epitaxy (MBE)-deposited Fe/n-Si[Formula: see text]Ge[Formula: see text] (FM1) and Pt/[Formula: see text]-Si[Formula: see text]Ge[Formula: see text](PM2) (111) orientated Schottky barrier diodes (SBDs) have been investigated at room-temperature. Barrier height ([Formula: see text]), ideality factor (n) and series resistance (R[Formula: see text]) were extracted. Dominant current conduction mechanisms were determined. They revealed that Poole–Frenkel-type conduction mechanism dominated reverse current. Differences in shunt resistance confirmed the difference found in leakage current. Under forward bias, quasi-ohmic conduction is found at low voltage regions and space charge-limited conduction (SCLC) at higher voltage regions for both SBDs. Density of interface states (N[Formula: see text]) indicated a difference in interface reactivity. Distribution profiles of series resistance (R[Formula: see text]) with bias gives a peak in depletion region at low-frequencies that disappears with increasing frequencies. These results show that interface states density and series resistance of Schottky diodes are important parameters that strongly influence electrical properties of FM1 and PM2 structures.

Poly-γ-glutamate Binder To Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-Ion Batteries.

P2-Na2/3Ni1/3Mn2/3O2 (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage by charging up to 4.5 V versus Na+/Na. However, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of ca. 23% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V also brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-γ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the first cycle and capacity retention of 89% after 50 cycles, whereas the PVdF electrode exhibits only 80 and 71%, respectively. The alternating current impedance measurements reveal that the PGluNa electrode shows a much lower resistance during the cycles compared with the PVdF one. From the surface analysis and peeling test of the electrodes, the PGluNa binder was found to cover the surface of the P2-NiMn particles and suppresses the electrolyte decomposition and surface degradation. The PGluNa binder further enhance the mechanical strength of the electrodes and suppresses the electrical isolation of the P2-NiMn particles during sodium extraction/insertion. The efficient binder with noticeable adhesion strength and surface coverage of active materials and carbon has paved a new way to enhance the electrochemical performances of high-voltage positive electrode materials for Na-ion batteries.

Carbon chain-based spintronic devices: Tunable single-spin Seebeck effect, negative differential resistance and giant rectification effects

Abstract Using the density functional theory combined with the Keldysh non-equilibrium Green's function methods, we investigate the thermally (or voltage)-induced spin transport properties of a two-probe device consisting of a carbon atom chain sandwiched between two zigzag graphene nanoribbon (ZGNR) electrodes. When the edge of one ZGNR electrode is partially doped by B atoms, the flowing direction of thermal single-spin current can be reversed in contrast with the undoped case. In addition, when a voltage is applied across the carbon-based device at room temperature, a giant rectification ratio of 10 4 is observed which mainly originates from the band-structure incompatibility between two ZGNR electrodes in the voltage window. Moveover, in the high-voltage region, a single-spin negative differential resistance is also observed in the carbon-based device. Our findings here suggest that the carbon-based systems can be used to design spintronic devices with multiple functions.

Influence of argon and oxygen pressure ratio on bipolar-resistive switching characteristics of CeO2−x thin films deposited at room temperature

Cerium oxide (CeO2−x) film was deposited on Pt/Ti/SiO2/Si substrate by rf magnetron sputtering at room temperature. Resistive switching characteristics of these ceria films have been improved by increasing oxygen content during deposition process. Endurance and statistical analyses indicate that the operating stability of CeO2−x-based memory is highly dependent on the oxygen content. Results indicate that CeO2−x film-based RRAM devices exhibit optimum performance when fabricated at an argon/oxygen ratio of 6:24. An increase in the oxygen content introduced during CeO2−x film deposition not only stabilizes the conventional bipolar RS but also improves excellent switching uniformity such as large ON/OFF ratio (102), excellent switching device-to-device uniformity and good sweep endurance over ~ 500 repeated RS cycles. Conduction in the low-resistance state (LRS) as well as in the low bias field region in the high-resistance state (HRS) is found to be Ohmic and thus supports the conductive filament (CF) theory. In the high voltage region of HRS, space charge limited conduction (SCLC) and Schottky emission are found to be the dominant conduction mechanisms. A feasible filamentary RS mechanism based on the movement of oxygen ions/vacancies under the bias voltage has been discussed.

Enhanced resistive switching and magnetic properties of Gd-doped NiFe2O4 thin films prepared by chemical solution deposition method

Influence of Gd ion doping on resistive switching (RS) and magnetic properties of NiFe2O4 (NiFe2−xGdxO4, NFG-x) thin films prepared by chemical solution deposition method was investigated. The 0.075 Gd-doped NiFe2O4 based devices exhibited much reliable repeatability, endurance (>500 switching cycles), and good data retention (105 s at 25 and 100 °C) without any significant degradation in their RS performance. Dominant conduction mechanisms in devices were Ohmic conduction at low resistance state and at lower voltage region of high resistance state, while Schottky emission dominated at higher voltage region in high resistance state. The physical mechanism of resistive switching was linked with the formation and rupture of conducting filaments, and the improved stability of the switching parameters for Pt/NFG/Pt devices was attributed to appropriate concentration of oxygen vacancies which can easily minimize randomness for formation/rupture of conductive filaments. Due to variation in concentration of oxygen vacancies, saturation magnetization of the RS device at high resistance state was higher than those at virgin state and low resistance state, and changes of saturation magnetization maintained a stable value of 40% between HRS and LRS of Pt/NFG-0.075/Pt device. Our study indicates that doping rare earth element into ferrite thin films is a viable approach for enhancing resistance switching and magnetic properties.

Solvothermal synthesis of NiWO4 nanostructure and its application as a cathode material for asymmetric supercapacitors.

This study proposes a facile solvothermal synthesis of nickel tungstate (NiWO4) nanowires for application as a novel cathode material for supercapacitors. The structure, morphology, surface area and pore distribution were characterized and their capacitive performances were investigated. The results showed that the NiWO4 nanowires synthesized in ethylene glycol solvent could offer a high specific capacitance of 1190 F g−1 at a current density of 0.5 A g−1 and a capacitance retaining ratio of 61.5% within 0.5–10 A g−1. When used as a cathodic electrode of an asymmetric supercapacitor (ASC), the NiWO4 nanowire based device can be cycled reversibly in a high-voltage region of 0–1.7 V with a high specific capacitance of 160 F g−1 at 0.5 A g−1, which therefore contributed to an energy density of 64.2 W h kg−1 at a power density of 425 W kg−1. Moreover, 92.8% of its initial specific capacitance can be maintained after 5000 consecutive cycles (5 A g−1). These excellent capacitive properties make NiWO4 a credible electrode material for high-performance supercapacitors.

High-performance asymmetric supercapacitors based on reduced graphene oxide/polyaniline composite electrodes with sandwich-like structure

The sandwich-like structure of reduced graphene oxide/polyaniline (RGO/PANI) hybrid electrode was prepared by electrochemical deposition. Both the voltage windows and electrolytes for electrochemical deposition of PANI and RGO were optimized. In the composites, PANI nanofibers were anchored on the surface of the RGO sheets, which avoids the re-stacking of neighboring sheets. The RGO/PANI composite electrode shows a high specific capacitance of 466 F/g at 2 mA/cm2 than that of previously reported RGO/PANI composites. Asymmetric flexible supercapacitors applying RGO/PANI as positive electrode and carbon fiber cloth as negative electrode can be cycled reversibly in the high-voltage region of 0–1.6 V and displays intriguing performance with a maximum specific capacitance of 35.5 mF cm−2. Also, it delivers a high energy density of 45.5 mW h cm−2 at power density of 1250 mW cm−2. Furthermore, the asymmetric device exhibits an excellent long cycle life with 97.6% initial capacitance retention after 5000 cycles. Such composite electrode has a great potential for applications in flexible electronics, roll-up display, and wearable devices.

Hysteresis-free DC conduction in zinc oxide films with a conducting polymer counter electrode

A conducting polymer, namely poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been employed as an alternative metal counter electrode to study steady state charge conduction through a spin-coated 5.2 µm thick ZnO film in a sandwich structure on a fluorine doped tin oxide coated glass substrate. The room temperature current–voltage characteristics exhibit rectifying behaviour without hysteresis as the bias voltage is cycled between ± 10V for varying voltage sweep rate from 100 to 1000 mV/s. Thermionic emission is believed to be the dominant conduction mechanism at forward bias voltage V ≤ 2.5 V whereas the space charge limited current conduction becomes effective in a higher voltage region V > 2.5 V. The barrier height (φb), ideality factor (n) and series resistance (Rs) are found to be strongly temperature dependent parameters showing the increase of {phi _b} and simultaneous decreases of n and Rs within the range of 218–298 K. The behaviour of the temperature dependent charge carrier mobility in the higher voltage region has also been discussed. The use of PEDOT:PSS is quite promising as an alternative to metal electrodes in semiconductor devices.

Synthesis of Cu2O by oxidation-assisted dealloying method for flexible all-solid-state asymmetric supercapacitors

Supercapacitors are promising energy storage devices for flexible electronics because of their integration of lightweight, tiny volume and high flexibility. Here, a facile yet effective method was developed to synthesize Cu2O, by oxidation-assisted dealloying and subsequent electrochemical strategies. The assembled asymmetric supercapacitor device, using Cu2O as the positive electrode and active carbon as the negative electrode could be operated with high voltage region of 1.65 V and exhibited a high energy density of 20.04 Wh kg−1. Additionally, our device had an excellent long-term cycling stability with more than 93.3% retention after 5000 cycles. To demonstrate potential application of our prepared flexible all-solid-state ASC device, four charged devices were constructed in series, they were able to illuminate 52 red colored LEDs.

Optical and electro-optic properties of polymer-stabilized blue phase liquid crystal cells with photoalignment layers

A comparative study of the optical and electro-optic properties of polymer-stabilized blue phase liquid crystal (PS-BPLC) cells with photoalignment layers and rubbing alignment layers is performed. The optical microscope (OM) images of the photoaligned and rubbing-aligned cells depict that they have uniform PS-BPLC textures, while the post-processed OM images of the two cells reveal that the former has a more uniform PS-BPLC texture than the latter. The relatively uniform PS-BPLC texture in the photoaligned cell is verified by the reflective spectra of the two cells. The voltage-dependent transmittance curves and time-dependent transmittance curves of another two photoaligned and rubbing-aligned cells with in-plane-switching (IPS) electrodes indicate that the contrast ratio of the former is 41% larger than that of the latter, and the operation voltage (response time) of the former is 11 (10) % smaller than that of the latter. This result arises from the fact that the photoaligned cell with the IPS electrodes exhibits a relatively uniform PS-BPLC texture and relatively weak voltage-shielding effect. Therefore, the photoalignment can improve the electro-optic properties of PS-BPLC cells with alignment layers. The photoaligned and rubbing-aligned cells have no thermal hysteresis in low voltage regions due to the uniform surface alignment layers, and exhibit very low voltage-induced hysteresis in high voltage regions owing to no thermal hysteresis. Briefly, uniform surface alignment layers can eliminate the thermal hysteresis of PS-BPLC cells, suppressing their voltage-induced hysteresis. In other words, the hysteresis of PS-BPLC cells with alignment layers is independent of alignment methods as long as the alignment layers have strong surface anchoring energy.

Redox-Active Organic Positive Electrodes for Li- and Na-Ion Capacitors

Li-ion capacitors employ capacitive positive electrodes from electrochemical capacitors (ECs) and intercalation negative electrodes from Li-ion batteries (LIBs), bridging the performance gap between ECs and LIBs. However, the low capacity of capacitive positive electrodes limits the energy density of the Li-ion capacitors. Incorporating pseudocapacitance (surface redox reaction) into the carbon materials is a promising strategy to enhance the capacity of the carbon-based positive electrodes. We have been investigating the redox reactions of various organic materials with Li- and Na-ions for hybrid capacitor applications. We have revealed that the organic electrodes, which are prepared from pyrene derivatives,1 hydrothermally synthesized carbon from glucose,2 and polymerized dopamine,3 have redox reactions with Li- and (or) Na- ions. In this study, we further reveal that the organic electrodes can store high capacities in both Li- and Na-cells in the high voltage region by utilizing the redox reactions between oxygen functional groups and metal ions. Finally, we demonstrate the high-performance Li-ion capacitors by combining the organic positive electrode and high-energy battery negative electrode.

Unusual Exothermal Behavior of Degraded Lithium-Ion Batteries during Charging

When the lithium-ion batteries (LIBs) are used in the spacecraft such as low-orbit satellite, LIBs in earlier generation are selected because reliability is extremely important and long-term durability test are required to guarantee the life of the batteries. On the other hand, LIBs of high energy density beyond 200 Wh kg-1is also desired to weight saving the spacecraft, which is recently achieved in several LIBs for consumer use. Our goal is development of accelerated life estimation method applicable to such next generation LIBs with high energy density, and in order to understand the degradation mechanism, we are investigating durability test of LIBs. In this work, extraordinary heat generation behaviors appeared in some degraded LIBs is reported. For the durability test, commercially available 18650 type lithium-ion batteries were selected as the test samples of which nominal capacity was 2900 mAh at 25°C and the energy density was ca. 220 Wh kg-1. The test cells were set in a thermostatic chamber and the temperature was controlled at 25°C. In the test, 0.5C (1.35 A) CC – 4.2 V CV charging for max. 4 h, and 1CCC discharging with 2.5 V cutoff were repeated. The rest times after charge and discharge were 5 min and 10 min, respectively. The test was finish after 870 cycles where the capacity retention of the cells was less than 68%. In the final CV charging process, remarkable rising behaviors were found in both the current and the surface temperature of the same cells. Nonetheless, discharging behavior after the extraordinary charging was general. After the test, isothermal calorimetry was carried out for the degraded cells in order to analyze the heat generation mechanism. Figure 1 shows the heat flow from the cells during 0.5C CC – 4.2 V CV charging at 25°C. In the fresh cell (a), heat flow was not so large, and the values are same order as that predicted from entropic heat and resistive (including polarization) heat. On the while, extremely large heat generation was observed during CV charging process of the cycled cell (b). The peak value was not steady and it reduced in the next charging. However, after long time rest, it became large again. Note that this unusual exothermal behavior was not remarkable during 4.2 V CV process if the preceding CC process had been low rate such as 0.1C. After storage at SOC 0% under ambient temperature beyond half year including several weeks storage at 10°C, heat generation behavior of the cell finally changed to curve (c) in the figure, where CC charging continued 4 hours without CV process because the voltage did not reach to 4.2 V. Finally, evolved gases during charging of the cell were analyzed by gas chromatography after making a cut the case of the cell and sealed in a laminate bag. As the result, evolution of some kinds of gasses such as H2, CO, CO2, and CH4 was confirmed in the highly exothermic region, and consequently, it can be concluded that the electrolyte decomposition with evolution of the gasses proceeded as the side-reaction during charging the degraded cell in the high voltage region, and that the side-reaction was origin of the large heat generation. Essentially, the electrolyte decomposition becomes remarkable when the cell is overcharged. However, in the test sample, this reaction began in the voltage range under normal charging condition. In the cycled cell, enlargement of overvoltage was also observed. This suggested that the interface structure between the electrodes and the electrolyte changed during cycling in the negative way to disturb the charging reaction. From the viewpoint of safety, such kind of degradation is unfavorable, and improvement methods should be considered. Figure 1

Influence of the Charge/Discharge Voltage Range on the Capacity Reversibility and Electrode/Electrolyte Interface Stability of LiCo1/3Ni1/3Mn1/3O2

Introduction Many attempts have been made to increase the energy density of Li-ion batteries using positive electrode materials such as LiMO2 (LiCoO2, LiNiO2, LiNi1/3Mn1/3Co1/3O2, etc.), which have high charging voltages (typically ≥4.4 V). Various studies to understand the associated degradation mechanism and improve the capacity reversibility during high-voltage charge/discharge have been reported. Generally, the inter-terminal voltage of Li-ion batteries can be practically controlled by changing the electric potential of the positive electrode. Thus, the positive electrodes used for high-voltage charge/discharge are exposed to a high electric potential as well as a large change in the electric potential. In this study, we examined the influence of the operating voltage range on the charge/discharge characteristics, and found that the discharge cutoff voltage greatly influences the capacity reversibility during high-voltage charge/discharge. Additionally, we discuss the mechanisms that determine the capacity reversibility based on the results of electrochemical impedance analysis and soft X-ray absorption spectroscopy (soft XAS). Experimental LiNi1/3Co1/3Mn1/3O2 powder, with a secondary particle diameter of 10 μm (Toda Kogyo Corp.), was used as the active material. Positive electrodes were fabricated from a mixture of 90 wt% LiNi1/3Co1/3Mn1/3O2, 5 wt% acetylene black, and 5 wt% polyvinylidene fluoride. The electrochemical characteristics of the samples were examined in coin cells with a Li-metal counter electrode. A 1.0 mol dm-3 solution of LiPF6 in ethylene carbonate + diethyl carbonate was used as the electrolyte. The cells were cycled at discharge-charge cutoff voltages of 2.5–4.6, 3.0–4.6, 3.8–4.6, and 4.2–4.6 V, at a current rate of 1 C. The Li-ion transfer characteristics were measured by alternating current impedance spectroscopy. The electronic structure of the LiNi1/3Co1/3Mn1/3O2was investigated using soft XAS at the beam line BL11 of Ritsumeikan University SR Center (Shiga, JAPAN). Results and Discussion Figures 1a and b show the discharge capacity and discharge capacity retention versus cycle number for LiNi1/3Co1/3Mn1/3O2 cycled with different voltage ranges. As the discharge cutoff voltage was increased, the initial discharge capacity decreased, accompanied by an improvement in the discharge capacity retention. The retentions at the 143rd cycle for LiNi1/3Co1/3Mn1/3O2cycled with 2.5–4.6, 3.0–4.6, 3.8–4.6, and 4.2–4.6 V, were 8, 37, 56, and 81%, respectively. Figure 2a shows the Nyquist plots for LiNi1/3Co1/3Mn1/3O2 cycled with different voltage ranges at an open circuit voltage of ~4.2 V after 3 cycles. The charge transfer resistances (R ct) calculated from the semicircles in the lower frequency region were almost equal (5–8 Ω) regardless of the discharge cutoff voltage. From the Nyquist plots after 143 cycles (Figure 2b), the R ct for LiNi1/3Co1/3Mn1/3O2 cycled with 2.5–4.6, 3.0–4.6, 3.8–4.6, and 4.2–4.6 V, were obtained as 3800, 300, 65, and 17 Ω, respectively. The increase of R ct with the number of cycles was significantly suppressed as the discharge cutoff voltage was increased, resulting in the higher capacity retention observed in Fig. 1b. These results suggest that a stable interface is retained between the electrode and electrolyte when the charge/discharge voltage is limited to the high-voltage region only. The interface structure and the capacity reversibility mechanism will be discussed along with the electronic state of LiNi1/3Co1/3Mn1/3O2analyzed by soft XAS. Acknowledgements This work is financially supported by the RISINGII project of the NEDO and METI, Japan. Figure 1

Bipolar resistive switching in PVDF and Graphene Oxide hetero-structure thin films

Resistive switching behavior in Graphene oxide (GO) is well studied, however, the various mechanisms responsible for this phenomenon are still under extensive debate. We present repeatable bipolar resistive switching in GO thin films sandwiched between two insulating polymer PVDF (Polyvinylidene Fluoride) grown on conducting indium tin oxide (ITO) covered glass substrate. The device heterostructure (Al/PVDF/GO/PVDF/ITO) showed bipolar resistance states switching between low resistance state (LRS) to high resistance state (HRS) with a large ON/OFF ratio of 103 and resistance retention potential up to 104 s. In LRS, in the low applied voltage region, ohmic conduction was the main reason for current conduction in devices; however, traps filled/assisted conduction mechanism dominates in the higher voltage region. The PVDF/GO/PVDF heterostructure shows that oxygen vacancies are responsible for the formation of current conducting filaments. The low operating voltage (<3 V) and long-term stability of resistance states make it a promising candidate for possible applications as Resistive Random Access Memory (ReRAM) elements.

Ce-doping induced enhancement of resistive switching performance of Pt / NiFe 2 O 4 / Pt memory devices

Abstract Ce-doping NiFe2O4 spinel ferrite thin films were prepared on Pt / Ti / SiO2 / Si substrates using a chemical solution deposition method. It has been observed that Ce-doping induced enhancement of unipolar resistive switching properties, such as uniform switching voltages, enlarged ON / OFF ratio, and long retention in Pt / NiFe2O4 / Pt memory devices. The dominant conduction mechanisms in the thin film devices were Ohmic conduction at low resistance state and lower voltage region of high resistance state, while Schottky emission dominated at higher voltage region in high resistance state. The physical mechanism of resistive switching is related to the formation and rupture of conducting filament. The improved stability of the switching parameters for the devices can be attributed to Ce-doping minimizing random formation and rupture of conductive filament. This study indicates that doping rare earth ions in ferrite thin films would be an effective approach for obtaining stable resistance switching memory devices.

Electrical, photovoltaic and photosensitivity characteristics of p-ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for photodiode applications

In this work, heterojunctions of p-ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for photovoltaic and photodiode application were grown epitaxially by using molecular beam system. Current-voltage profile of the heterojunction device was studied in dark and under various illumination intensities. The obtained photocurrent is found to depend on the light intensity. The main heterojunction parameters, such as shunt and series resistances, the barrier height and the ideality factor were extracted from the current-voltage profile using diverse methods at ambient temperature. Values of the barrier height and the series resistance were obtained from Cheung's functions. A large value of the series resistance causes the non-ideal characteristics of current–voltage measurements. The study of the current-voltage characteristics of high voltage region suggests a predominant space charge limited mechanism. Moderate values of short circuit current and open circuit voltage were obtained through a light intensity of 140mW/cm2, a current and voltages of 0.336mA and 370mV, respectively. The high photosensitivity and responsivity for the current under illumination condition suggests that the prepared heterojunction device could be employed as a photodiode sensor.