High Voltage Region Research Articles

Improving the Electrochemical Performance and Structural Stability of the LiNi0.8Co0.15Al0.05O2 Cathode Material at High-Voltage Charging through Ti Substitution

LiNi0.8Co0.15Al0.05O2 (NCA) has been proven to be a good cathode material for lithium-ion batteries (LIBs), especially in electric vehicle applications. However, further elevating energy density of NCA is very challenging. Increasing the charging voltage of NCA is an effective method, but its structural instability remains a problem. In this work, we revealed that titanium substitution could improve cycle stability of NCA under high cutoff voltage significantly. Titanium ions with a relatively larger ion radius could modify the oxygen lattice and change the local coordination environment of NCA, leading to decreased cation migration, better kinetic and thermodynamic properties, and improved structural stability. As a result, the Ti-substituted NCA cathode exhibits impressive reversible capacity (198 mA h g-1 at 0.1C) with considerable cycle stability under a cutoff voltage up to 4.7 V. It is also revealed that Ti could suppress oxygen release in the high-voltage region, benefitting cycle and thermal stabilities. This work provides valuable insight into the design of high-voltage layered cathode materials for high-energy-density LIBs.

Nonlinear resistive switching features of rapid-thermal-annealed aluminum nitride dielectrics with modified charge trapping behaviors

Nonlinear resistive switching (RS) features of aluminum nitride (AlNx)-based resistance random access memories (RRAMs) with rapid thermal annealing (RTA) have been investigated. The operation voltages of AlNx-based RRAMs are improved by RTA because of the reduction in nitride traps within AlNx dielectrics. In addition, the centroids of nitride traps are modified by RTA and a tunneling barrier at the Ir/AlNx interface is formed for the enhancement of nonlinearity to more than 10 during RS operation. The nonlinear behaviors of AlNx-based RRAMs with RTA can be attributed to the combination of conduction mechanisms of direct tunneling (DT) and trap-assisted tunneling (TAT) at low- and high-voltage regions, respectively. Furthermore, superior device reliabilities of AlNx-based RRAMs with RTA are achieved such as an endurance of over 500 cycles and data retention of more than 104 s. The adjustable nonlinear features and superior memory properties render the annealed AlNx-based RRAMs promising for future high-density nonvolatile memory arrays.

Field emission from potassium-doped vertically aligned carbon nanosheets

In this study, field emission from potassium (K-) doped vertically aligned carbon nanosheets is examined. The potassium doping was carried out by dipping samples in potassium hydroxide aqueous solution, which was confirmed by X-ray photoelectron spectroscopy. Ultraviolet photoelectron spectroscopy revealed that the K-doping reduced the work function of vertically aligned carbon nanosheets. Field emission characteristics of the K-doped vertically aligned carbon nanosheets were measured, showing that the reduction of the work function resulted in lower emission barrier and led to lower turn-on voltage and less limitation of the emission current at high voltage region.

Synthesis of a high-quality Al-doped CdS/Si nanoheterojunction array and characterization of its electrical and electroluminescence properties

A high-quality nanoheterojunction composed of p-type Si nanoporous pillar array (Si-NPA) and n-type CdS:Al was successfully constructed by growing Al-doped CdS film on a silicon nanoporous pillar array via a simple chemical bath deposition method. The fabricated CdS:Al/Si-NPA nanoheterojunction exhibits a pronounced rectification behavior at a low turn-on voltage of ~ 0.5 V, a relatively high rectification ratio of 217 at ± 1.5 V, and good ideality factors of 0.85 in the low-voltage region and 2.45 in a high-voltage region. The electroluminescence spectra of a nanolight-emitting diode based on the nanoheterojunction are dominated by an intense band-edge emission located at ~ 520 nm under a small forward voltage and free from defect emissions. The intensity increased rapidly with the forward voltage. The emission mechanism and CIE chromaticity coordinates of the electroluminescence spectra were also discussed. The obtained CdS:Al/Si-NPA nanoheterojunction could be used for important applications in nanoscale optoelectronic devices.

Redox Chemistry in Conventional Layered Lithium Metal Oxide Cathodes

Li-ion battery technology is one of the best performing energy storage devices and holds the best promise to power the upcoming electrical vehicles (EV). Extensive efforts have been devoted to further advancing conventional layered oxide cathodes, such as LiNixMnyCozO2 and LiNixCoyAlzO2 (0 < x, y, z <1), for Li-ion batteries of even better performance and lower cost ($/Wh). Strategies that are largely pursued include increasing Ni content to achieve a higher practical capacity at a given charge cutoff voltage and/or increasing charge cutoff voltage for a given composition. Alongside with these research efforts, questions regarding the stability of electrode, especially at highly delithiated states, electrolyte and electrode/electrolyte interface arise. In this work, of particular interest is the redox chemistry upon electrochemical (de)lithiation of LiNiO2, the parent compound of Ni-rich layered oxides, with a key focus on highly delithiated states in the high voltage region. Electrochemistry of LiNiO2 seems straightforward, in principle, Ni3+/Ni4+ redox can perfectly accommodate 1 Li+ transport during (de)lithiation process. However, the precise state at full oxidation of Ni3+ to Ni4+ and/or full removal of 1 Li+ are difficult to achieve because of the possible side reactions between highly charged electrode and electrolyte, for example, surface Ni reduction and gas evolution upon charge often occur at high voltages. Here, the electronic states of Ni and O are probed at different states of charge through combined soft X-ray absorption (XAS), resonant inelastic X-ray spectroscopy (RIXS) and in situ differential electrochemical mass spectrometry (DEMS) in the high voltage region to elucidate the explicit redox chemistry in LiNiO2. This work is anticipated to provide additional insights into the redox chemistry of conventional Ni-rich layered oxides, especially its utilization in the high voltage region.

Reversible Oxygen Redox Chemistry in Aqueous Zinc-Ion Batteries.

Rechargeable aqueous zinc-ion batteries (ZIBs) are promising energy-storage devices owing to their low cost and high safety. However, their energy-storage mechanisms are complex and not well established. Recent energy-storage mechanisms of ZIBs usually depend on cationic redox processes. Anionic redox processes have not been observed owing to the limitations of cathodes and electrolytes. Herein, we describe highly reversible aqueous ZIBs based on layered VOPO4 cathodes and a water-in-salt electrolyte. Such batteries display reversible oxygen redox chemistry in a high-voltage region. The oxygen redox process not only provides about 27 % additional capacity, but also increases the average operating voltage to around 1.56 V, thus increasing the energy density by approximately 36 %. Furthermore, the oxygen redox process promotes the reversible crystal-structure evolution of VOPO4 during charge/discharge processes, thus resulting in enhanced rate capability and cycling performance.

Stability in water and electrochemical properties of the Na3V2(PO4)2F3 – Na3(VO)2(PO4)2F solid solution

Polyanionic materials have been intensively studied as promising active materials for positive electrodes in Na-ion batteries thanks to their excellent stability upon cycling and the fast ionic mobility in their structural framework. Among them, Na3V2(PO4)2F3 and Na3(VO)2(PO4)2F are two of the most promising ones due to their high voltages for Na+-ion extraction and their high energy densities: 500 mWh g−1 and 495 mWh g−1, respectively. Here, we study the formation mechanism as well as the stability of these phases in aqueous media and the possible use of a washing step in water in order to remove undesirable impurities formed during the synthesis. Furthermore, the origin of the extra capacity observed at the high voltage region for Na3V2(PO4)2F3 and Na3V2(PO4)2F1.5O1.5 was studied by operando X-ray absorption spectroscopy.

Impact of carrier concentration and bandgap on the performance of double gate GNR-FET

Abstract This paper presents the impact of drain carrier density on the drain current saturation of 5 nm channel length double gate graphene nanoribbon field effect transistor (DGGNR-FET) near high drain voltage region. The impact of the substrate and energy band gap on the kink behavior of the device is analyzed by using Nonequilibrium Green's Function with mode space (NEGF_MS) method. We studied the effect of Si, SiC, and the oxide (SiO2) sub'strate on the device performance. The impact of the energy band gap is also examined on the increase in carrier density and current saturation. The necessity of constant carrier density in the drain side for better current saturation is emphasized. The carrier density in the drain side is increased due to the tunnelling from source to drain region (band to band tunnelling) and lateral carrier density modulation. We found that SiC is the better substrate over silicon and SiO2 for better current saturation of DGGNR-FET. Our simulation results provide detailed insight into analog and radio frequency application for the graphene device.

Structural and electrical performance of epitaxial InP based heterojunctions prepared by liquid phase epitaxy

Thin films of InP was grown on single crystalline substrates of Si to form InP/Si heterojunctions by liquid phase epitaxy (LPE) and its morphology and crystalline characteristics were achieved. The essential electrical properties and its main parameters were extracted using the current density-voltage. The analysis was done to obtain the rectification characteristics which has its maximum value at a certain voltage of 0.7 V. Moreover, the heterojunction obeys ohmic behavior followed by quadratic space charge limited conduction at lower and higher voltage regions, respectively. The conductivity under AC bias as well as the dielectric behaviors of the heterojunction was explored in the frequency range 100 kHz–5 MHz and in the temperature range 298–623 K. The AC conductivity is interpreted by the correlated barrier hopping model via single polaron with activation energy dependent on the applied frequency. The response of the dielectric constants confirms its remarkable dependence on both frequency and temperature.

Demonstration and analysis of channel mobility, trapped electron density and Hall effect at SiO2/SiC (03) interfaces

Low interface state density (Dit) and high field-effect mobility (μfe) at SiO2/4H-SiC (03) Metal-oxide-semiconductor (MOS) interfaces are known. In order to understand the behavior and the scattering mechanisms induced electrons in more detail, we fabricated the Hall-bar lateral MOSFETs on the 4H-SiC (03) substrate with various channel doping concentrations and evaluated the trapped electron densities and the Hall mobilities by split capacitance–voltage and Hall-effect measurements. Our results demonstrated that more than 80% of the induced electrons at the SiO2/4H-SiC MOS (03) interfaces contribute to the current conduction as the free electrons. The majority of the electron traps seemed to be located mainly at the edge of the conduction band because the trapped electron density increased around the threshold voltage and was saturated in the high gate voltage region.

Vacuum‐free fabrication of a low‐voltage multi‐bit memory device based on a ferroelectric polymer and photosensitive film

A multi-bit memory device was fabricated using a ferroelectric polymer. For multi-bit storage, a capacitive-type memory was configured to have two different thicknesses: a thinner sub-capacitor to represent the polarization reversal at a lower voltage and a thicker sub-capacitor that allows the two sub-capacitors to operate in different voltage ranges. The thin film was composed of a single ferroelectric polymer film and the thick film was composed of a hybrid film of a ferroelectric polymer and a photosensitive polymer. The photosensitive polymer reacted with ultraviolet light, so it was patterned by a conventional lithography process. The external voltage was divided into two layers, the ferroelectric layer and the photosensitive layer, which resulted in polarisation reversal at a higher voltage region. The most important feature of this work is that no vacuum equipment was used in the manufacturing process of the memory device. In the manufacturing process of the conventional multi-bit memory device, vacuum equipment was required to remove the polymer film. Therefore, the size of the substrate is limited and the productivity is inevitably lowered. The proposed multi-bit memory device and its fabrication process will be useful because it has many advantages in terms of productivity compared to existing processes.

Electronic Transport Mechanisms in PVDF/ZnO Nanocomposite Films

In this report, we present fundamental DC conduction mechanisms of pol-yvinyl difluoride/nano-zinc oxide (PVDF/ZnO) composite films. Nano-composite films of zinc oxide in the PVDF matrix have been fabricated via a solution casting method. Space charge limited conduction (SCLC) was found to be a prominent conduction mechanism in higher voltage region and Schottky- Richardson mechanism was a possible conduction mechanism in a higher temperature region. DC activation energy for composites was found to decrease with increasing concentration of ZnO nanoparticles and reduced graphene oxide (RGO) with 1.41 eV (9.9 wt%), 1.04 eV (33.33 wt%), and 0.95 eV (33.33 wt% RGO), respectively.

High-Capacity P2-Type NaxLi0.25Mn0.75O2 Cathode Enabled by Anionic Oxygen Redox

Sodium-ion battery technology has attracted significant attention due to its substantial cost advantage and similar operating mechanism to Li-ion batteries. P2-type sodium manganese oxide cathode is one of the most promising candidates, demonstrating both high capacity and good cycling stability. Here, we explore the lattice oxygen activity in layered sodium transition metal oxides. We synthesize a series of sodium lithium manganese oxides, NaxLi0.25Mn0.75O2 (x = 0.75 – 0.833), to optimize Na content. We further investigate the charge compensation mechanism for the best performing Na0.75Li0.25Mn0.75O2 over an extensive electrochemical cycling window. The large charge and discharge capacity is enabled by reversible lattice oxygen redox in the high voltage region (≥2.5 V), along with Mn redox at the voltages below 2.5 V. Additionally, we reveal a small amount of oxygen gas evolution, 0.04% of the total oxygen in Na0.25Li0.25Mn0.75O2. This initial study will trigger an interest in the lattice oxygen activity in layered sodium metal oxide cathode, therefore, leading to better understanding of its correlation with crystal structure and electrochemical performance.

Unveiling the Effect of Voltage Regulation System on the Structure and Electrochemical Properties of Lithium-Rich Cathode Materials

Lithium-rich layered oxides exhibit a superior specific capacity more than 280 mAh g−1 in the range of 2.0–4.8 V. Meanwhile, capacity attenuation and voltage decay are also caused by such a wide voltage range. The structure evolution and electrochemical properties of materials at different voltage regulation systems have been studied. Complex phase transition and interplanar spacing increase can be blocked under the lower voltage cycle and the diffusion ability of Li+ has been significantly improved after pre-cycling treatment, especially at the condition of 4.5 V. Li1.2Ni0.13Co0.13Mn0.54O2 exhibits excellent capacity retention of 92.77% with slight voltage attenuation of 0.3731 V compared to 55.74% and 0.7883 V at high voltage after 200 cycles under the synergistic effect of pre-cycling and lower voltage operation. Such superior electrochemical performance is attributed to that: (1) pre-cycling can gentle the activation process and rationalize the intercalation/deintercalation interval of Li+; (2) the lower voltage operation will restrain the release of oxygen in high voltage region to stabilize the phase structure and inhibit the decomposition of electrolyte. The rational voltage regulation system can improve the reversibility of anion redox and maintain excellent cycle performance.

Belt-like MnO2 cathode to enable high energy density and ultra-stable aqueous asymmetric supercapacitor

A belt-like MnO2 nanomaterials is designed as cathode material to enable neutral aqueous, high energy density and ultra-long life asymmetric supercapacitor. The energy density and power density can reach as high as 33.8 Wh kg−1 and 5000 W kg−1 respectively even in the high-voltage region of 0–2 V. The asymmetric supercapacitor exhibits 91% capacity retention after 10,000 cycles indicating outstanding cycling property. The ultra-stability of the belt-like MnO2//activated carbon asymmetric supercapacitor makes it a promising device for large-scale applications.

Evaluation of Electrical Carrier Transport Mechanism in Plasma Polymerized n-Butyl Methacrylate Thin Films

Aluminium/poly(n-butyl methacrylate)/aluminium pattern thin films of different thickness were fabricated through plasma polymerization technique and their DC electrical characterization was conducted with a view of better understanding of the electrical charge transport mechanism. The current density—voltage and current density—film thickness characteristics of fabricated thin films of different thickness indicated that the dominant conduction mechanism in the higher voltage region is space charge limited conduction. The estimated carrier mobility, trap density and free carrier density decreased with the increase of thickness of the prepared thin films.

Charge transport mechanism under applied DC electric field in plasma polymerized diethanolamine thin films prepared by a capacitively coupled reactor

The structural and electrical characterizations of plasma polymerized (PPDEOA) thin films formed using a parallel plate capacitively coupled reactor were carried out. In Fourier transform infrared spectroscopy, it is observed that the absorption peaks appeared for different bonds in PPDEOA thin films are shifted to higher/lower wavenumbers in contrast to the corresponding peaks for the monomer, which indicate changed structure of PPDEOA. Al/PPDEOA/Al sandwiched samples were used to study the electrical properties of the thin films. The values of the power index, n, determined from the relation, J ∝ Vn are 0.53 ˂ n ˂ 1.1, that represents Ohmic behavior at the lower voltage region and in the higher voltage region, n values lie between 1.3 ˂ n ˂ 4.5 that indicate non-Ohmic behavior. In the non-Ohmic region, comparison of the theoretical values of Schottky and Poole-Frenkel coefficients with the values of experimental coefficients, the Schottky type conduction mechanism has been observed. In the low voltage region, the activation energies of PPDOA thin films are 0.06–0.14 eV and 0.23–0.69 eV in the lower and higher temperatures, respectively. The increased activation energies in the higher voltage region are 0.12 to 0.19 eV and 0.52 to 0.74 eV in the low and high temperatures, respectively.

Hierarchical Hollow Prussian Blue Rods Synthesized via Self‐Sacrifice Template as Cathode for High Performance Sodium Ion Battery

AbstractIn this work, a facile strategy is proposed to synthesize hierarchical hollow rod‐like Mn‐doped Prussian blue (R‐PB) using MnO2 nanosheets as a self‐sacrifice template. The as prepared R‐PB with low contents of vacancies and coordinated water, and high content of sodium, exhibits appealing electrochemical performance as a cathode for sodium ion batteries (SIBs) achieving a high discharge capacity of 117.3 mAh g−1 and a capacity retention of 98.5% after 200 cycles at 1 C. Moreover, the specific capacity reaches 111.5 mAh g−1 at 5 C with a Coulombic efficiency (CE) of 93.8%. The high capacity, superior cycling stability, and enhanced kinetics of Na+ migration of R‐PB are attributed to the positive effect of doped Mn ions and the hierarchical hollow rod‐like structure. The doped Mn ions in R‐PB activate the redox of FeLS(C) in the high voltage region to reach a high capacity. The unique hierarchical hollow rod‐like structure provides a buffer space for the volume change during the insertion/extraction of sodium ions, enlarges the contact area between the electrode materials and electrolyte, and enhances the apparent diffusion coefficient of sodium ions in electrode materials.

Measurement Methodology for Accurate Modeling of SiC MOSFET Switching Behavior Over Wide Voltage and Current Ranges

This paper presents two novel measurement methods to characterize silicon carbide (SiC) MOSFET devices. The resulting data are utilized to significantly improve the extraction of a custom device model that can now accurately reproduce device switching behavior. First, we consider the $I_{\text{d}}- V_{\text{ds}}$ output characteristics of power devices such as SiC transistors. These are typically measured using traditional curve tracers, but the characterization of the high-voltage and high-current (HVHC) region is very challenging because of device power compliance and self-heating. In this paper, we introduce a measurement technique that overcomes self-heating and derives the HVHC region from switching waveforms. The switching transient characteristics of devices are used to determine drain current $(I_{\text{d}})$ as a function of drain–source voltage $(V_{\text{ds}})$ in the HVHC range. Second, we consider another challenging characterization area: measurement of nonlinear capacitances when device is turned on. These capacitance characteristics of on-state devices are important for correcting disagreements between simulations and measurements in turn-off switching transient waveforms and cannot be measured using a conventional capacitance-voltage meter. We introduce S-parameter measurements as an effective method to obtain the capacitance characteristics of both off-state devices and on-state devices. These novel measurement techniques have been applied to the modeling of a SiC device. The extracted device model, a modified version of the popular Angelov−GaN high-electron-mobility transistor model, shows significant improvement in terms of the accuracy of switching waveforms of devices over a wide range of operating conditions.

Effect of thickness and crystalline morphology on electrical properties of rf-magnetron sputtering deposited Bi4Ti3O12 thin films

Bi4Ti3O12 (BiT) thin films with thicknesses ranging from 535 to 870 nm were deposited on Pt(111)/Ti/SiO2/Si substrates by rf-magnetron sputtering method. The films were crystallized by direct thermal annealing in air at 650 °C. The effects of film thicknesses and crystalline morphology on ferroelectric, leakage current conduction and dielectric properties were investigated. XRD and SEM analysises reveal that BiT thin film deposited at 400 °C is amorphous and exhibits a non-crystalline morphology. After annealing treatment, the amorphous film transformed into Aurivillius Bi4Ti3O12 crystalline phase with rodlike grains. The electrical properties show that the as-deposited thin film is a lossy dielectric with absence of ferroelectricity. The remnant polarization of crystalline BiT thin films increases slightly with decreasing thickness. The leakage currents of annealed BiT films exhibit ohmic behavior in low voltage region and Schottky emission conduction characteristic in high voltage region, respectively. For crystalline BiT films, the dielectric constant decreases significantly with increasing film thicknesses.