Limited Rate Capability Research Articles

High-Capacity Layered-Spinel Cathodes for Li-Ion Batteries.

Li and Mn-rich layered oxides with the general structure x Li2 MnO3 ⋅(1-x) LiMO2 (M=Ni, Mn, Co) are promising cathode materials for Li-ion batteries because of their high specific capacity, which may be greater than 250 mA h g(-1) . However, these materials suffer from high first-cycle irreversible capacity, gradual capacity fading, limited rate capability and discharge voltage decay upon cycling, which prevent their commercialization. The decrease in average discharge voltage is a major issue, which is ascribed to a structural layered-to-spinel transformation upon cycling of these oxide cathodes in wide potential ranges with an upper limit higher than 4.5 V and a lower limit below 3 V versus Li. By using four elements systems (Li, Mn, Ni, O) with appropriate stoichiometry, it is possible to prepare high capacity composite cathode materials that contain LiMn1.5 Ni0.5 O4 and Lix Mny Niz O2 components. The Li and Mn-rich layered-spinel cathode materials studied herein exhibit a high specific capacity (≥200 mA h g(-1) ) with good capacity retention upon cycling in a wide potential domain (2.4-4.9 V). The effect of constituent phases on their electrochemical performance, such as specific capacity, cycling stability, average discharge voltage, and rate capability, are explored here. This family of materials can provide high specific capacity, high rate capability, and promising cycle life. Using Co-free cathode materials is also an obvious advantage of these systems.

Spinel cathodes for advanced lithium ion batteries: a review of challenges and recent progress

Spinel-structured LiMn1.5Ni0.5O4 displays a high working voltage plateau of about 4.7 V. It is an attractive cathode material for next-generation lithium ion batteries. However, its pace of commercialisation is hindered by poor cycle performance and inferior rate capability. A lot of researches related to this material have been carried out. This paper presents a review of recent advances, including the structure and electrochemical properties of ordered and disordered spinels. Issues concerning capacity fade and limited rate capability such as surface side reaction, Jahn–Teller distortion and intrinsic poor conductivities, which are challenges for their utilisation in commercial lithium ion batteries will be addressed and recent developments highlighted. Owing to such advances, spinel oxides have a bright future.

Single Wall Carbon Nanotube-Containing PVA Based Carbon Nanofibers for Lithium Ion Battery Anodes

Lithium ion batteries have increasing importance due to the urgent demand for widespread applications in electric/hybrid vehicles, electronic devices, and the grid energy storage. Graphite is one of the most commonly used anode material with low theoretical capacity(372mAh/g) and limited rate capability. In order to improve battery Li-ion storage capacity many researchers have investigated different carbonaceous anode materials with various configurations. Among them, carbon nanofiber is of particular interest for providing high capacity and cycling stability due to its high surface area, porous structure and high electric conductivity which not only shorten the lithium ion path through the electrode but also provide large electrode/electrolyte interface for the charge-transfer reaction properties. Carbon nanofibers can be directly used as free standing and binder free anodes. There are several nanofiber production methods, among which electrospinning is the most promising due to its simplicity and versatility. Carbon nanofibers can be produced with different precursors such as Pitch, polyacrylonitrile(PAN), polyvinypyrrolidone(PVP), polyvinyl alcohol(PVA) etc. Polyvinyl alcohol is a promising material due to its water soluble and environmentally friendly character. In this study we report a method to produce free-standing binder free single wall carbon nanotube containing carbon nanofibers as anode for lithium ion battery. Composite nanofibers prepared via electrospinning of single wall carbon nanotubes (SWCNT) dispersed into polyvinyl alcohol solution, followed by iodination at 100°C and heat treatment in an inert atmosphere at 800°C. Carbon nanotubes were added to the polymer solution in different concentrations 0,1% , 1%, and 5 %. An increase of the carbon nanotube concentration in the fibers led to carbonized nanofiber mats with increased electrical conductivity. TGA and Raman analyses showed that carbon nanotubes presence improves the carbon yield of the mats and their handleability. Galvanostatic cycling tests of half cells revealed that addition of carbon nanotubes improves battery capacity. We attribute the capacity increase of the composite nanofibers to the carbon nanotubes structure. Moreover carbon nanotubes increased the handleabilitiy of the mat which is crucial for freestanding, binder free anode materials.

Insights into the Li Diffusion Dynamics and Nanostructuring of H2Ti12O25 To Enhance Its Li Storage Performance.

Dodecatitanate H2Ti12O25 crystal has a condensed layered structure and exhibits noteworthy Li storage performance that makes it an anode material with great potential for use in Li-ion batteries. However, an unknown Li diffusion mechanism and a sluggish level of Li dynamics through elongated diffusion paths inside this crystal has impeded any forward development in resolving its limited rate capability and cyclic stability. In this study, we investigated the Li diffusion dynamics inside the H2Ti12O25 crystal that play an essential role in Li storage performance. A study of density functional theory combined with experimental evaluation confirmed a strong dependence of Li storage performance on its diffusion. In addition, a nanostructured H2Ti12O25 containing a bundle of nanorods is developed via the introduction of a kinetic gap during the structural transformation, which conferred a significantly shortened diffusion time/length for Li in H2Ti12O25. The nanostructured H2Ti12O25 has high specific capacity (∼230 mAh g(-1)) and exhibits enhanced cyclic stability and rate capability compared with conventional bulky H2Ti12O25. The H2Ti12O25 proposed in this study has high potential for use as an anode material with excellent safety and stability.

Flexible Electrode Design: Fabrication of Freestanding Polyaniline-Based Composite Films for High-Performance Supercapacitors.

Polyaniline (PANI) is a promising pseudocapacitance electrode material. However, its structural instability leads to low cyclic stability and limited rate capability which hinders its practical applications. In view of the limitations, flexible PANI-based composite films are developed to improve the electrochemical performance of electrode materials. We report in the research a facile and cost-effective approach for fabrication of a high-performance supercapacitor (SC) with excellent cyclic stability and tunable energy and power densities. SC electrode containing a very high mass loading of active materials is a flexible film of PANI, tissue wiper-based cellulose, graphite-based exfoliated graphite (ExG), and silver nanoparticles with potential applications in wearable electronics. The optimum preparation weight ratios of silver nitrate/aniline and ExG/aniline used in the research are estimated to be 0.18 and 0.65 (or higher), respectively. Our results show that an ultrahigh capacitance of 3.84 F/cm(2) (240.10 F/g) at a discharge rate of 5 mA can be achieved. In addition, our study shows that the power density can be increased from 1531.3 to 3000 W/kg by selecting the weight ratio of ExG/aniline to be more than 0.65, with a sacrifice in the energy density. The obtained promising electrochemical properties are found to be mainly attributed to an effective combination of PANI, ExG, cushiony cellulose scaffold, and silver as well as the porosity of the composite.

Dynamic study of sub-micro sized LiFePO4 cathodes by in-situ tender X-ray absorption near edge structure

Olivine-type phosphates (LiMPO4, M = Fe, Mn, Co) are promising cathode materials for lithium-ion batteries that are generally accepted to follow first order equilibrium phase transformations. Herein, the phase transformation dynamics of sub-micro sized LiFePO4 particles with limited rate capability at a low current density of 0.14 C was investigated. An in-situ X-ray Absorption Near Edge Structure (XANES) measurement was conducted at the Fe and P K-edge for the dynamic studies upon lithiation and delithiation. Fe K-edge XANES spectra demonstrate that not only lithium-rich intermediate phase LixFePO4 (x = 0.6–0.75), but also lithium-poor intermediate phase LiyFePO4 (y = 0.1–0.25) exist during the charge and discharge, respectively. Furthermore, during charge and discharge, a fluctuation of the FePO4 and LiFePO4 fractions obtained by liner combination fitting around the imaginary phase fractions followed Faraday's law and the equilibrium first-order two-phase transformation versus reaction time is present, respectively. The charging and discharging process has a reversible phase transformation dynamics with symmetric structural evolution routes. P K-edge XANES spectra reveal an enrichment of PF6−1 anions at the surface of the electrode during charging.

Systematic Effect for an Ultralong Cycle Lithium-Sulfur Battery.

Rechargeable lithium-sulfur (Li-S) batteries are attractive candidates for energy storage devices because they have five times the theoretical energy storage of state-of-the-art Li-ion batteries. The main problems plaguing Li-S batteries are poor cycle life and limited rate capability, caused by the insulating nature of S and the shuttle effect associated with the dissolution of intermediate lithium polysulfides. Here, we report the use of biocell-inspired polydopamine (PD) as a coating agent on both the cathode and separator to address these problems (the "systematic effects"). The PD-modified cathode and separator play key roles in facilitating ion diffusion and keeping the cathode structure stable, leading to uniform lithium deposition and a solid electrolyte interphase. As a result, an ultralong cycle performance of more than 3000 cycles, with a capacity fade of only 0.018% per cycle, was achieved at 2 C. It is believed that the systematic modification of the cathode and separator for Li-S batteries is a new strategy for practical applications.

LiNi0.5Mn1.5O4 Cathodes for Lithium Ion Batteries: A Review.

Spinel structured LiNi0.5Mn1.5O4 based compounds are promising cathode materials for high power lithium ion batteries, owing to its superior 4.7 V discharge plateau and large reversible capacity. However, its pace of commercialization is hindered by limited rate capability, poor cyclic performance at high current densities and instability with electrolyte. Extensive works have been devoted to tackle these difficulties. This paper presents a review of challenges and recent developments of LiNi0.5Mn1.5O4 based compounds.

Structural and Chemical Evolution of Li and Mn Rich Layered Oxide Cathode and Correlation with Capacity and Voltage Fading

Lithium-rich, magnesium-rich (LMR) cathode materials have been regarded as one of the very promising cathodes for Li-ion battery applications. However, their practical application is still limited by several challenges, especially by their limited electrochemical stability and rate capability. In this work, we present recent progresses on the understanding of the structural and composition evolution of LMR cathode materials with emphasis being placed on the correlation between structural/chemical evolution and electrochemical properties. In particular, using Li[Li0.2Ni0.2Mn0.6]O2as a typical example, we clearly illustrate the structural characteristics of the pristine materials and their dependence on the materials processing history, cycling induced structural degradation/chemical partition and their correlation with degradation of electrochemical performance. The fundamental understanding obtained in this work may also guide the design and preparation of new cathode materials based on ternary system of

Synthesis of three-dimensionally interconnected sulfur-rich polymers for cathode materials of high-rate lithium-sulfur batteries.

Elemental sulfur is one of the most attractive cathode active materials in lithium batteries because of its high theoretical specific capacity. Despite the positive aspect, lithium–sulfur batteries have suffered from severe capacity fading and limited rate capability. Here we report facile large-scale synthesis of a class of organosulfur compounds that could open a new chapter in designing cathode materials to advance lithium–sulfur battery technologies. Porous trithiocyanuric acid crystals are synthesized for use as a soft template, where the ring-opening polymerization of elemental sulfur takes place along the thiol surfaces to create three-dimensionally interconnected sulfur-rich phases. Our lithium–sulfur cells display discharge capacity of 945 mAh g−1 after 100 cycles at 0.2 C with high-capacity retention of 92%, as well as lifetimes of 450 cycles. Particularly, the organized amine groups in the crystals increase Li+-ion transfer rate, affording a rate performance of 1210, mAh g−1 at 0.1 C and 730 mAh g−1 at 5 C.

Recent Advances on the Understanding of Structural and Composition Evolution of LMR Cathodes for Li-ion Batteries

Lithium-rich, magnesium-rich (LMR) cathode materials have been regarded as very promising for lithium (Li-ion battery applications. However, their practical application is still limited by several barriers such as their limited electrochemical stability and rate capability. In this work, we present recent progress on the understanding of structural and compositional evolution of LMR cathode materials, with an emphasis being placed on the correlation between structural/chemical evolution and electrochemical properties. In particular, using Li[Li0.2Ni0.2Mn0.6]O2 as a typical example, we clearly illustrate the structural characteristics of pristine materials and their dependence on the material-processing history, cycling-induced structural degradation/chemical partition, and their correlation with electrochemical performance degradation. The fundamental understanding that resulted from this work may also guide the design and preparation of new cathode materials based on the ternary system of transitional metal oxides.

Review—Li-Rich Layered Oxide Cathodes for Next-Generation Li-Ion Batteries: Chances and Challenges

Since their commercialization Li-ion batteries have relied on the use of layered oxides (LiMO2) as positive electrodes. Over the years, via skilful chemical substitution their performances have drastically improved in terms of safety and capacity, which has nearly doubled (280 mAh/g) with the recent arrival of Li-rich NMC, i.e. layered LiCoO2 in which Co has been simultaneously replaced by Mn, Ni and Li. This review will aim to describe the chemical rationale which has led to this material evolution prior to focus on Li-rich NMC phases which are sources of excitement but challenges as well. The benefits of going back to fundamentals to rationalize and understand the new science at work with these Li-rich NMC phases will be stressed and illustrated by the discovery of a new reversible anionic redox process. Issues regarding voltage fade and limited rate capability which are plaguing their present utilization in commercial Li-ion cells will be addressed as well and solutions proposed. Owing to such advances, layered oxides which are over performing spinel or polyanionic-based compounds have a bright future.

ChemInform Abstract: Recent Advancements in the Conversion‐Type Pnictide‐Based Electrodes for Li‐Ion Batteries

AbstractReview: 99 refs.

Recent Advancements in the Conversion-Type Pnictide-Based Electrodes for Li-Ion Batteries

Nowadays conversion-type electrode materials definitively lie as the core of any research programs related to Li-ion batteries. Requirements are high capacity, good rate capability, and a long cycle life. Indeed, the goal of much lithium battery research is to achieve the highest energy density battery as possible. In the case of pnictide materials, such performances are the results of the following conversion reaction: MxXy + 2yLi ↔ xM0 + yLi3X (X = P, Sb; M = Fe, Ni, Co, ...). However, these materials are still suffering from serious issues such as (i) low Coulombic efficiency, (ii) high polarization, (iii) poor cycle life (volume expansion), and (iv) limited rate capability that unfortunately still prevent them for any close commercial viability. In this article, the most recent research developments of our group and through collaborations in this specific field will be reported. In the interest of overcoming the limitations listed above, a cautious and rigorous scrutinizing of the electrochemical beha...

Revisiting the capacitance of polyaniline by using graphene hydrogel films as a substrate: the importance of nano-architecturing

Polyaniline has long been explored as a potential candidate for supercapacitors. However, its limited rate capability and cyclability, along with large variation of reported capacitance, cast doubt on its potential for real-world applications. We use a recently developed graphene hydrogel film as a substrate to revisit the capacitance of polyaniline, and reveal that if its nanostructure is properly engineered, polyaniline can provide a combination of high capacitance, excellent rate performance and long cycling life and is promising for real applications.

A Monte Carlo simulation study to evaluate septal spacing using triple-head hybrid PET imaging

The low coincidence fraction (ratio of true coincidences to single events detected) and a limited count rate capability reduce performance of hybrid PET systems as compared with dedicated PET. Axial collimation (i.e., using septa oriented perpendicular to the axis of rotation) has been used to reduce acceptance of photons from outside the field of view, reduce scattered events, and allow the use of 2-D image reconstruction algorithms. The major drawback of axial collimation is a reduced coincidence detection efficiency. The goal of this study was to evaluate the effect of axial collimator septal spacing, as well as to evaluate a new hybrid parallel-fan beam axial collimator design which may better shield out of field-of-view (FOV) photons, reduce the number of scattered events, and increase the sensitivity to the central FOV. The GEANT 3.21 Monte Carlo (MC) simulation code was modified to model a Philips IRIX triple-head hybrid PET system in the 0/spl deg/-90/spl deg/-270/spl deg/ geometry and used to investigate different axial collimator designs. Both lead and tungsten septa were investigated. Basic PET system parameters such as scatter fraction, global true coincidences to single events (T/S) ratio, axial profiles of the T/S ratio, and energy spectra were examined. Noise equivalent count (NEC) rate curves were plotted. Noise and contrast were evaluated for different collimator designs and reconstruction strategies. Comparisons to an experimentally acquired line source phantom showed that the GEANT 3.21 MC simulation code accurately modeled the Philips IRIX camera. The T/S ratio was observed to increase with increasing septal spacing from 0.77 to 2.27 cm and was approximately constant for larger septal spacing. Widening the septal spacing modified the T/S axial profiles making them more nonuniform with a strong peak toward the axial center of the FOV. Using the hybrid parallel-fan beam axial collimator provided a more uniform axial T/S profile for some septal spacings and improved contrast and decreased the noise in the reconstructed slices. In addition, for a given single events rate, a larger proportion of single events were measured within the photopeak energy window as septal spacing decreased. This observation can have implications on the recorded random rate for some PET systems.

Counting Rate Performance of Pulsed - Tube Systems

Energy - dispersive x-ray fluorescence systems are characterized by limited count rate capability, which is an important practical limitation on their application. This arises from two factors: 1) The amplifier time constant is many times greater than needed in wavelength-dispersive systems, in order to permit accurate measurement of pulse height, and reducing the time constant degrades system resolution; 2) x-ray pulses from ail the major and minor elements, as welt as scattered background and/or peaks from the incident radiation, must all be processed, reducing further the effective count rate for a single element of interest. Hence there has been a strong desire to find ways to increase the count rate capability of energy-dispersive systems.

Event Priority Instrumentation System for Nuclear Experiments on Scientific Satellites

The number of information bits per unit time, or data rate, which can be telemetered back to earth from a scientific satellite is limited by restrictions on weight, volume and power. Thus, each experimenter (generally one of many) is allotted a very limited data rate capability. In nuclear experiments, where a wide variety of events may occur per unit time, use of this rate capability may be optimized by selection of events on a priority basis. Described here is an event priority instrumentation system developed for use in scientific satellites. The system allows for the selection of the highest priority event occurring during a telemetry sequence. Also, provision is made for periodically changing the order of priority. All circuitry was designed and developed to minimize weight, volume and power drain and to maintain stable characteristics while unattended for periods as long as one year. The operating temperature range is -20°C to 60°C.