Desirable Energy Density Research Articles

Chitin based heteroatom-doped porous carbon as electrode materials for supercapacitors

Chitin biomass has received much attention as an amino-functional polysaccharide precursor for synthesis of carbon materials. Rich nitrogen and oxygen dual-doped porous carbon derived from cicada slough (CS), a renewable biomass mainly composed of chitin, was synthesized and employed as electrode materials for electrochemical capacitors, for the first time ever. The cicada slough-derived carbon (CSC) was prepared by a facile process via pre-carbonization in air, followed by KOH activation. The weight ratio of KOH and char plays an important role in fabricating the microporous structure and tuning the surface chemistry of CSC. The obtained CSC had a large specific surface area (1243–2217m2g−1), fairly high oxygen content (28.95–33.78 at%) and moderate nitrogen content (1.47–4.35 at%). The electrochemical performance of the CS char and CSC as electrodes for capacitors was evaluated in a three-electrode cell configuration with 6M KOH as the electrolyte. Electrochemical studies showed that the as-prepared CSC activated at the KOH-to-char weight ratio of 2 exhibited the highest specific capacitance (266.5Fg−1 at a current density of 0.5Ag−1) and excellent rate capability (196.2Fg−1 remained at 20Ag−1) and cycle durability. In addition, the CSC-2-based symmetrical device possessed the desirable energy density and power density of about 15.97Whkg−1 and 5000Wkg−1 at 5Ag−1, respectively.

Directing the Lithium-Sulfur Reaction Pathway via Sparingly Solvating Electrolytes for High Energy Density Batteries.

The lithium–sulfur battery has long been seen as a potential next generation battery chemistry for electric vehicles owing to the high theoretical specific energy and low cost of sulfur. However, even state-of-the-art lithium–sulfur batteries suffer from short lifetimes due to the migration of highly soluble polysulfide intermediates and exhibit less than desired energy density due to the required excess electrolyte. The use of sparingly solvating electrolytes in lithium–sulfur batteries is a promising approach to decouple electrolyte quantity from reaction mechanism, thus creating a pathway toward high energy density that deviates from the current catholyte approach. Herein, we demonstrate that sparingly solvating electrolytes based on compact, polar molecules with a 2:1 ratio of a functional group to lithium salt can fundamentally redirect the lithium–sulfur reaction pathway by inhibiting the traditional mechanism that is based on fully solvated intermediates. In contrast to the standard catholyte sulfur electrochemistry, sparingly solvating electrolytes promote intermediate- and short-chain polysulfide formation during the first third of discharge, before disproportionation results in crystalline lithium sulfide and a restricted fraction of soluble polysulfides which are further reduced during the remaining discharge. Moreover, operation at intermediate temperatures ca. 50 °C allows for minimal overpotentials and high utilization of sulfur at practical rates. This discovery opens the door to a new wave of scientific inquiry based on modifying the electrolyte local structure to tune and control the reaction pathway of many precipitation–dissolution chemistries, lithium–sulfur and beyond.

Facile synthesis of graphene nanosheets from humic acid for supercapacitors

A simple, cost-effective, and environmentally friendly route was developed to synthesize graphene nanosheets from humic acid via preliminary carbonization coupled with oxidation-exfoliation-thermal reduction. Such graphene nanosheets have a high specific surface area (495m2·g−1) with large pore volume (2.987cm3·g−1), unique interconnected mesoporous structure and uniform oxygen-containing functional groups in layered graphene framework, which offer a favorable and efficient pathway for the electrolyte propagation and transportation. The electrodes of supercapacitors made from these graphene nanosheets exhibit a maximum specific capacitance of 272F·g−1 at the current density of 50mA·g−1 in aqueous electrolyte, and possess excellent rate capability, low resistance, superior cycling performance with over 96.5% initial capacitance retention after 8000cycles. The corresponding supercapacitors deliver a desirable energy density of 6.47Wh·kg−1 at a powder density of 2250W·kg−1. This study demonstrates a promising synthesis route for large-scale production of graphene nanosheets from renewable and green humic acid for high performance supercapacitors.

Optimizing Design Parameters of Li-Ion Batteries for Phev and EV

A lithium ion battery can be designed as a power battery or an energy battery depending on the power and energy requirements when the battery is applied in plug-in hybrid electric vehicles (PHEV) and electric vehicles(EV). Design parameters of a battery cell usually play an important role in its power density and energy density. For instance, increasing the thickness of the porous electrode offers higher energy density for the battery cell with fixed geometrical dimensions. However, a thicker electrode tends to lengthen the path for lithium ions to diffuse in the electrolyte, which results in poor power capability. Some other parameters such as the particle size and porosity of the active materials in each electrode are also important to the electrochemical performance of a battery cell thus influencing the power density and energy density. The objective of this research is to optimize the battery design parameters to obtain the desired power density and energy density for the batteries used in PHEV and EV. A one-dimensional electrochemical isothermal model is developed by coupling the mass conservation, charge conservation, as well as electrochemical kinetics for a Li-ion cell. The one dimensional cell unit is assumed to be a sandwich structure that is composed of a negative electrode, a separator and a positive electrode. The electrodes are porous solid matrix that consists of active particles with spherical shapes of uniform sizes and additives. The separator is a porous polymer membrane that constitutes a physical barrier between the two electrodes. Both electrodes and separator porous matrix are impregnated with electrolyte ensuring the transfer of lithium ions between two electrodes during charge and discharge. The three coupled governing equations, including mass, charge conservation as well as Butler-Volmer equation, along with other constituent equations and proper boundary conditions, will be solved by COMSOL Multiphysics software. The cell properties, such as geometry parameters and electrochemical properties, are obtained from the cell manufacturer. The experimental data are provided by an industrial partner. The model validation is conducted by comparing simulation results with experimental data of discharging behaviors of different testing samples at various discharge rates. The parameters of interest include the thickness, porosity and particle size of electrode materials at a single cell level. For example, the electrode thickness study was conducted with the same porosity and the same particle size. It has been found that the cell discharge capacity faded more when the thickness of positive electrode increases from 45um to 70um at the same discharge rate (2C), which is consistent with the experimental results. Faster the cell is discharged, and more the discharge capacity fades. More results on the influence of porosity and particle size of electrode material on the cell performance will be reported in the conference. Based on the study of each single parameter, a multi-parameter optimization can be conducted to meet a specific design objective of both power density and energy density properties for cells applied on PHEV or EV. This research reveals the effects of design parameters of a Li-ion battery electrode on the battery energy density and power density, which are very useful for battery modeling and design.

An all-solid-state yarn supercapacitor using cotton yarn electrodes coated with polypyrrole nanotubes

A novel all-solid-state yarn supercapacitor (YSC) has been fabricated by using the cotton yarns coated with polypyrrole (PPy) nanotubes. The interconnected network structure of PPy can increase the surface area as well as the electrode/electrolyte interface area, thus resulting in improved electrochemical performance. For the proposed YSC, a high areal-specific capacitance of 74.0mFcm−2 and a desirable energy density of 7.5μWhcm−2 are achieved. The flexibility of the YSC demonstrates that it is suitable for the integration as flexible power sources in wearable electronic textiles.

Laminar burning characteristics of ethyl propionate, ethyl butyrate, ethyl acetate, gasoline and ethanol fuels

Ethyl esters have been considered as promising second-generation biofuel candidates, due to the available production from low grade biomass waste. Furthermore, with desirable energy densities, emissions performance, low solubility and higher Research Octane Number (RON), ethyl esters have proven attractive as fuel additives or alternatives for gasoline. In this study, high-speed schlieren photography was used to investigate the laminar burning characteristics of three ethyl ester fuels: ethyl acetate, ethyl propionate, and ethyl butyrate, in comparison with gasoline and ethanol at different initial temperatures and a variety of equivalence ratios, with an initial pressure of 0.1MPa in a constant-volume vessel. For the five fuels, the stretched flame speeds, the un-stretched flame speeds, Markstein lengths, Markstein number, laminar burning velocities and laminar burning flux were calculated and analysed using the outwardly spherical flame method. The results show that for all examined initial temperatures (60°C, 90°C and 120°C) and equivalence ratios; ethanol had the highest un-stretched flame propagation speeds, whilst ethyl acetate (EA) had the lowest. At high initial temperatures (120°C), it was observed that the un-stretched flame speed trends of ethyl propionate (EP) and ethyl butyrate (EB) proved faster compared to gasoline, especially for rich conditions. The EB and EA flames demonstrated greater stability when compared to ethanol, EP, and gasoline. Analysis showed that ethanol yielded the fastest flame velocities, whilst EA consistently had the lowest among all the five fuels. The laminar burning velocities of the EP fuel were faster compared to EB and EA, whilst slower than ethanol and gasoline at 60°C. Further increase of the initial temperature, up to 120°C, showed the laminar flame speed of EP and EB to be faster than gasoline, indicating a fast-burning property, and potential of improving engine thermal efficiency.

Estimation of energy density of Li-S batteries with liquid and solid electrolytes

Abstract With the exponential growth of technology in mobile devices and the rapid expansion of electric vehicles into the market, it appears that the energy density of the state-of-the-art Li-ion batteries (LIBs) cannot satisfy the practical requirements. Sulfur has been one of the best cathode material choices due to its high charge storage (1675 mAh g−1), natural abundance and easy accessibility. In this paper, calculations are performed for different cell design parameters such as the active material loading, the amount/thickness of electrolyte, the sulfur utilization, etc. to predict the energy density of Li-S cells based on liquid, polymeric and ceramic electrolytes. It demonstrates that Li-S battery is most likely to be competitive in gravimetric energy density, but not volumetric energy density, with current technology, when comparing with LIBs. Furthermore, the cells with polymer and thin ceramic electrolytes show promising potential in terms of high gravimetric energy density, especially the cells with the polymer electrolyte. This estimation study of Li-S energy density can be used as a good guidance for controlling the key design parameters in order to get desirable energy density at cell-level.

Lignite-derived high surface area mesoporous activated carbons for electrochemical capacitors

Mesoporous activated carbons (ACs) were successfully prepared from lignite using KOH as activation agent at the temperature above 700°C. The pore structure and surface chemistry of the as-prepared ACs were characterized by means of nitrogen adsorption–desorption, X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy. The results show that such prepared mesoporous ACs have a high specific surface area (~3036m2·g−1) with a hierarchical macro–meso–micro-pore structure and oxygen-enriched surface. The electrochemical performances of the ACs as electrode materials for electrochemical capacitors (ECs) were assessed by galvanostatic charge–discharge, cyclic voltammetry and cycling durability tests. It was demonstrated that the mesoporous ACs produced in this study possessed a maximum specific capacitance of 355F·g−1 and 196F·g−1 in 3M KOH aqueous and 1M (C2H5)4NBF4/PC organic electrolytes, respectively, at a current density of 50mA·g−1, and exhibited a desirable energy and power density with a superior cycling performance. The excellent capacitive behavior of the prepared mesoporous ACs in aqueous system is attributed to their unique macro–meso–micro-hierarchical pore structure with high surface area and oxygen-containing surface. Their superb electrochemical performance in the organic electrolyte is attributed to their well-developed mesoporous structure.

Visualizing Longitudinal and Transversal Strain Effects in High-Energy NMC Cathodes Induced By Intense Calendering

One of several challenges in improving the performance of lithium-ion batteries is the enhancement of energy density. Apart from the development of ever better active materials, considerable potential for improvement can be found in the complex manufacturing process. The manufacturing of lithium-ion battery cells exhibits a long, mostly sequential process chain with numerous interdependent parameters and quality-determining characteristics. Among slurry mixing and coating, calendering is the most challenging process step and considered of superior importance to the quality of the electrode. Calendering refers to compressing the porous coating layers of the electrode to a predetermined thickness. To obtain the desired energy density and to prepare a homogeneous electrode thickness it is necessary to densify the coating of the electrodes. In this, more electrochemically active material can be accommodated in every unit of electrode volume. In essence, this defines the potential energy density of the cell. As for high-energy battery cells energy density is one of the most important optimization criteria to achieve ambitious requirements for automotive applications calendering is of crucial importance.However, compression of the electrode is not merely beneficial to the overall performance of the cell. It is a trade-off between achieving high energy density and, on the contrary, damaging the electrode’s structure by applying high mechanical forces. An effect rarely taken into account is mechanical strain induced to the electrode’s metal foil underlying the coating, which can lead to serious issues in manufacturing and malfunction of the battery cell.This paper addresses macroscopic changes in electrode’s metal foils resulting from intense calendering procedures of high-energy NMC cathodes. Figure 1 exemplarily depicts an area-resolved longitudinal strain map showing irreversible deformation from calendering. The left green section reflects an uncoated edge of the electrode which is not deformed during calendering. The coated section of the specimen is deformed significantly. The data is derived from before/after comparison of the electrode’s surface, conducted with a non-contact optical 3D deformation measuring system. To minimize undesired strain inducement, multi-calendering strategies are assessed which gently apply loads and at the same time still provide for highly compressed electrodes.

Fluoro-Polymer@BaTiO3 Hybrid Nanoparticles Prepared via RAFT Polymerization: Toward Ferroelectric Polymer Nanocomposites with High Dielectric Constant and Low Dielectric Loss for Energy Storage Application

Polymer nanocomposites with high energy density and low dielectric loss are highly desirable in electronic and electric industry. Achieving the ability to tailor the interface between polymer and nanoparticle is the key issue to realize desirable dielectric properties and high energy density in the nanocomposites. However, the understanding of the role of interface on the dielectric properties and energy density of polymer nanocomposites is still very poor. In this work, we report a novel strategy to improve the interface between the high dielectric constant nanoparticles (i.e., BaTiO3) and ferroelectric polymer [i.e., poly(vinylidene fluoride-co-hexafluoro propylene)]. Core–shell structured BaTiO3 nanoparticles either with different shell thickness or with different molecular structure of the shell were prepared by grafting two types of fluoroalkyl acrylate monomers via surface-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization. The dielectric properties and energy storage c...

Sodium Titanate Nanotubes as Negative Electrode Materials for Sodium-Ion Capacitors

The lithium-based energy storage technology is currently being considered for electric automotive industry and even electric grid storage. However, the hungry demand for vast energy sources in the modern society will conflict with the shortage of lithium resources on the earth. The first alternative choice may be sodium-related materials. Herein, we propose an electric energy storage system (sodium-ion capacitor) based on porous carbon and sodium titanate nanotubes (Na-TNT, Na(+)-insertion compounds) as positive and negative electrode materials, respectively, in conjunction with Na(+)-containing non-aqueous electrolytes. As a low-voltage (0.1-2 V) sodium insertion nanomaterial, Na-TNT was synthesized via a simple hydrothermal reaction. Compared with bulk sodium titanate, the predominance of Na-TNT is the excellent rate performance, which exactly caters to the need for electrochemical capacitors. The sodium-ion capacitors exhibited desirable energy density and power density (34 Wh kg(-1), 889 W kg(-1)). Furthermore, the sodium-ion capacitors had long cycling life (1000 cycles) and high coulombic efficiency (≈ 98 % after the second cycle). More importantly, the conception of sodium-ion capacitor has been put forward.

Electrolytes for high-energy lithium batteries

From aqueous liquid electrolytes for lithium–air cells to ionic liquid electrolytes that permit continuous, high-rate cycling of secondary batteries comprising metallic lithium anodes, we show that many of the key impediments to progress in developing next-generation batteries with high specific energies can be overcome with cleaver designs of the electrolyte. When these designs are coupled with as cleverly engineered electrode configurations that control chemical interactions between the electrolyte and electrode or by simple additives-based schemes for manipulating physical contact between the electrolyte and electrode, we further show that rechargeable battery configurations can be facilely designed to achieve desirable safety, energy density and cycling performance.

Thermal properties characterization of sodium alanates

High energy density hydrogen storage is a critical technology requirement in a hydrogen-based energy infrastructure. Although there are no current storage methods that meet desired energy density goals for vehicular hydrogen storage, complex metal hydride based systems are among the most promising. These materials form compounds with hydrogen under appropriate conditions and release hydrogen by thermal decomposition. The complex hydride, sodium alanate, is particularly useful due to its favorable reversibility. Thermal properties characterization of sodium alanate has been performed at Sandia National Laboratories to gain a detailed understanding of how complex hydrides will behave in a storage system. Thermal properties were investigated using the thermal probe method (ASTM D5334). Custom test hardware was designed and built to accommodate the complex decomposition and recombination of sodium alanate. Thermal conductivity and thermal wall resistance were determined by utilizing analytical and numerical data analysis methods. The thermal conductivity of sodium alanate was found to vary by more than 90% with changes in phase composition and hydrogen gas pressures between 1 and 100 atm. The quality of thermal contact between the alanate and the vessel wall was characterized numerically for various pressures and phase compositions. The contact resistance is high for all states, indicating poor contact between the material and the vessel wall.

Chemical double-layer capacitor power source for electromechanical thrust vector control actuator

Power sources for electrical actuation systems need to be made as compact as possible for many potential applications. Size and weight constraints require that the energy and power densities of the source be as large as possible. While, in general, it is not possible to optimize both these parameters simultaneously, chemical double-layer (CDL) capacitor technology offers an excellent combination of the desirable energy and power density characteristics required in a source for these applications. The work presented here focuses on the design, testing, and evaluation of the CDL capacitor technology in combination with batteries as a unique power source for electrical actuation systems. In this research effort CDL capacitor technology was applied in the design and development of a power source for thrust vector control electromechanical actuators currently being developed for use on the Space Shuttle's solid rocket boosters. CDL capacitors have many properties that make them well suited for actuator applications. Among them are that they have the highest demonstrated energy density for capacitive storage (about a factor of 5-10 less than NiCd batteries), have power densities up to 50 times greater than NiCd batteries, are capable of 500,000 charge-discharge cycles, can be charged at extremely high rates, and have nonexplosive failure modes. Thus, CDL capacitors exhibit a combination of desirable battery and capacitor characteristics.