13 research outputs found
Aramid Nanofiber Composites for Energy Storage Applications
Lithium ion batteries and non-aqueous redox flow batteries represent two of the most important energy storage technologies to efficient electric vehicles and power grid, which are essential to decreasing U.S. dependence on fossil fuels and sustainable economic growth. Many of the developmental roadblocks for these batteries are related to the separator, an electrically insulating layer between the cathode and anode. Lithium dendrite growth has limited the performance and threatened the safety of lithium ion batteries by piercing the separator and causing internal shorts. In non-aqueous redox flow batteries, active material crossover through microporous separators and the general lack of a suitable ion conducting membrane has led to low operating efficiencies and rapid capacity fade. Developing new separators for these batteries involve the combination of different and sometimes seemingly contradictory properties, such as high ionic conductivity, mechanical stability, thermal stability, chemical stability, and selective permeability. In this dissertation, I present work on composites made from Kevlar-drived aramid nanofibers (ANF) through rational design and fabrication techniques. For lithium ion batteries, a dendrite suppressing layer-by-layer composite of ANF and polyethylene oxide is present with goals of high ionic conductivity, improved safety and thermal stability. For non-aqueous redox flow batteries, a nanoporous ANF separator with surface polyelectrolyte modification is used to achieve high coulombic efficiencies and cycle life in practical flow cells. Finally, manufacturability of ANF based separators is addressed through a prototype machine for continuous ANF separator production and a novel separator coated on anode assembly. In combination, these studies serve as a foundation for addressing the challenges in separator engineering for lithium ion batteries and redox flow batteries.PHDMacromolecular Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/138543/1/situng_1.pd
Annulated Dialkoxybenzenes as Catholyte Materials for Nonâaqueous Redox Flow Batteries: Achieving High Chemical Stability through Bicyclic Substitution
1,4âDimethoxybenzene derivatives are materials of choice for use as catholytes in nonâaqueous redox flow batteries, as they exhibit high openâcircuit potentials and excellent electrochemical reversibility. However, chemical stability of these materials in their oxidized form needs to be improved. Disubstitution in the arene ring is used to suppress parasitic reactions of their radical cations, but this does not fully prevent ringâaddition reactions. By incorporating bicyclic substitutions and ether chains into the dialkoxybenzenes, a novel catholyte molecule, 9,10âbis(2âmethoxyethoxy)â1,2,3,4,5,6,7,8âoctahydroâ1,4:5,8âdimethanenoanthracene (BODMA), is obtained and exhibits greater solubility and superior chemical stability in the charged state. A hybrid flow cell containing BODMA is operated for 150 chargeâdischarge cycles with a minimal loss of capacity.A novel bicyclical substituted dialkoxyâbenzene molecule, 9,10âbis(2âmethoxyâethoxy)â1,2,3,4,5,6,7,8âoctahydroâ1,4:5,8âdimethanenoanthracene (BODMA), is developed for use as catholyte materials in nonâaqueous redox flow batteries with greater solubility (in their neutral state) and improved chemical stability (in their charged state). A hybrid flow cell using BODMA demonstrates stable efficiencies and capacity over 150 cycles. The molecular design approach of BODMA can be inspirational for future development of redox active molecules.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139992/1/aenm201701272.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139992/2/aenm201701272-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139992/3/aenm201701272_am.pd
Pushing the Limits: 3D Layer-by-Layer-Assembled Composites for Cathodes with 160 C Discharge Rates
Deficiencies of cathode materials severely limit cycling performance and discharge rates of Li batteries. The key problem is that cathode materials must combine multiple properties: high lithium ion intercalation capacity, electrical/ionic conductivity, porosity, and mechanical toughness. Some materials revealed promising characteristics in a subset of these properties, but attaining the entire set of often contrarian characteristics requires new methods of materials engineering. In this paper, we report high surface area 3D composite from reduced graphene oxide loaded with LiFePO<sub>4</sub> (LFP) nanoparticles made by layer-by-layer assembly (LBL). High electrical conductivity of the LBL composite is combined with high ionic conductivity, toughness, and low impedance. As a result of such materials properties, reversible lithium storage capacity and Coulombic efficiency were as high as 148 mA h g<sup>â1</sup> and 99%, respectively, after 100 cycles at 1 C. Moreover, these composites enabled unusually high reversible chargeâdischarge rates up to 160 C with a storage capacity of 56 mA h g<sup>â1</sup>, exceeding those of known LFP-based cathodes, some of them by several times while retaining high content of active cathode material. The study demonstrates that LBL-assembled composites enable resolution of difficult materials engineering tasks
Materials Engineering of High-Performance Anodes as Layered Composites with Self-Assembled Conductive Networks
The
practical implementation of nanomaterials in high capacity
batteries has been hindered by the large mechanical stresses during
ion insertion/extraction processes that lead to the loss of physical
integrity of the active layers. The challenge of combining the high ion storage capacity
with resilience to deformations and efficient charge transport is
common for nearly all battery technologies. Layer-by-layer (LBL/LbL)
engineered nanocomposites are able to mitigate structural design challenges
for materials requiring the combination of contrarian properties.
Herein, we show that materials engineering capabilities of LBL augmented
by self-organization of nanoparticles (NPs) can be exploited for constructing
multiscale composites for high capacity lithium ion anodes that mitigate
the contrarian nature of three central parameters most relevant for
advanced batteries: large intercalation capacity, high conductance,
and robust mechanics. The LBL multilayers were made from three function-determining
components, namely polyurethane (PU), copper nanoscale particles,
and silicon mesoscale particles responsible for the high nanoscale
toughness, efficient electron transport, and high lithium storage
capacity, respectively. The nanocomposite anodes optimized in respect
to the layer sequence and composition exhibited capacities as high
as 1284 and 687 mAh/g at the first and 300th cycle, respectively,
with a fading rate of 0.15% per cycle. Average Coulombic efficiencies
were as high as 99.0îž99.4% for 300 cycles at 1.0 C rate (4000
mA/g). Self-organization of copper NPs into three-dimensional (3D)
networks with lattice-to-lattice connectivity taking place during
LBL assembly enabled high electron transport efficiency responsible
for high battery performance of these Si-based anodes. This study
paves the way to finding a method for resolution of the general property
conflict for materials utilized in for energy technologies
Toward Improved Catholyte Materials for Redox Flow Batteries: What Controls Chemical Stability of Persistent Radical Cations?
Catholyte materials are used to store
positive charge in energized fluids circulating through redox flow
batteries (RFBs) for electric grid and vehicle applications. Energy-rich
radical cations (RCs) are being considered for use as catholyte materials,
but to be practically relevant, these RCs (that are typically unstable,
reactive species) need to have long lifetimes in liquid electrolytes
under the ambient conditions. Only few families of such energetic
RCs possess stabilities that are suitable for their use in RFBs; currently,
the derivatives of 1,4-dialkoxybenzene look the most promising. In
this study, we examine factors that define the chemical and electrochemical
stabilities for RCs in this family. To this end, we used rigid bis-annulated
molecules that by design avoid the two main degradation pathways for
such RCs, viz., their deprotonation and radical addition. The decay
of the resulting RCs are due to the single remaining reaction: O-dealkylation.
We establish the mechanism for this reaction and examine factors controlling
its rate. In particular, we demonstrate that this reaction is initiated
by the nucleophile attack of the counteranion on the RC partner. The
reaction proceeds through the formation of the aroxyl radicals whose
secondary reactions yield the corresponding quinones. The O-dealkylation
accelerates considerably when the corresponding quinone has poor solubility
in the electrolyte, and the rate depends strongly on the solvent polarity.
Our mechanistic insights suggest new ways of improving the RC catholytes
through molecular engineering and electrolyte optimization
Redox Flow Batteries: Annulated Dialkoxybenzenes as Catholyte Materials for Nonâaqueous Redox Flow Batteries: Achieving High Chemical Stability through Bicyclic Substitution (Adv. Energy Mater. 21/2017)
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139916/1/aenm201770121.pd
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Development and validation of risk prediction models for COVID-19 positivity in a hospital setting
ObjectivesTo develop: (1) two validated risk prediction models for coronavirus disease-2019 (COVID-19) positivity using readily available parameters in a general hospital setting; (2) nomograms and probabilities to allow clinical utilisation.MethodsPatients with and without COVID-19 were included from 4 Hong Kong hospitals. The database was randomly split into 2:1: for model development database (n = 895) and validation database (n = 435). Multivariable logistic regression was utilised for model creation and validated with the Hosmer-Lemeshow (H-L) test and calibration plot. Nomograms and probabilities set at 0.1, 0.2, 0.4 and 0.6 were calculated to determine sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV).ResultsA total of 1330 patients (mean age 58.2 ± 24.5 years; 50.7% males; 296 COVID-19 positive) were recruited. The first prediction model developed had age, total white blood cell count, chest x-ray appearances and contact history as significant predictors (AUC = 0.911 [CI = 0.880-0.941]). The second model developed has the same variables except contact history (AUC = 0.880 [CI = 0.844-0.916]). Both were externally validated on the H-L test (p = 0.781 and 0.155, respectively) and calibration plot. Models were converted to nomograms. Lower probabilities give higher sensitivity and NPV; higher probabilities give higher specificity and PPV.ConclusionTwo simple-to-use validated nomograms were developed with excellent AUCs based on readily available parameters and can be considered for clinical utilisation