48 research outputs found
Structure solution of metal-oxide Li battery cathodes from simulated annealing and lithium NMR spectroscopy
Discerning
the arrangement of transition metal atoms in LiÂ[Ni<sub><i>x</i></sub>Mn<sub><i>y</i></sub>Co<sub><i>z</i></sub>]ÂO<sub>2</sub> cathode materials has remained an
open problem for many years despite the commercial importance of some
stoichiometries and the even more promising characteristics of others.
We present a method for structural determination in this class of
cathode materials. A simple definition of the total energy, based
on the chemical principle of electroneutrality, is used in combination
with a simulated annealing algorithm to generate model structures.
The method reproduces the well-known structure of LiÂ[Li<sub>1/3</sub>Mn<sub>2/3</sub>]ÂO<sub>2</sub> and produces structures of the disordered
LiÂ[Ni<sub><i>x</i></sub>Mn<sub><i>x</i></sub>Co<sub>1–2<i>x</i></sub>]ÂO<sub>2</sub> phases (where <i>x</i> = 0.02, 0.1, 0.33) that are verified by detailed <sup>7</sup>Li NMR spectra. For each LiÂ[Ni<sub><i>x</i></sub>Mn<sub><i>x</i></sub>Co<sub>1–2<i>x</i></sub>]ÂO<sub>2</sub> phase, the solution is found to be heavily disordered,
yet retaining significant ion pairing. Since the underlying notion
of favoring charge-neutral regions is generic, we anticipate its utility
in a much broader family of materials
A Polymer-Rich Quaternary Composite Solid Electrolyte for Lithium Batteries
All-solid-state batteries continue to grow as an alternative to replace the traditional liquid-based ones not only because they provide increased safety but also higher power and energy densities. However, current solid-state electrolytes are either ceramics that are brittle but highly conducting (e.g. Li0.33La0.55TiO3, LLTO) or polymer electrolytes that are poorly conducting but form flexible films with desired mechanical properties (e.g. Poly(ethylene oxide):Lithium bis(trifluoromethanesulfonyl)imide, PEO:LiTFSI). In this work, we have developed quaternary composite solid-state electrolytes (CSEs) to combine the benefits of the two types along with Succinonitrile (SN) as a solid plasticizer. CSEs with different compositions have been fully characterized over the whole compositional range. Guided by neural network simulation results it has been found that a polymer-rich CSE film gives the optimal ionic conductivity (>10−3 S cm−1 at 55 °C) and mechanical properties (Tensile strength of 16.1 MPa; Elongation-at-break of 2360%). Our solid-state coin-type cell which employs our in-house made cathode shows good cycling performance at C/20 and 55 °C maintaining specific discharge capacity at 143.2 mAh g−1 after 30 cycles. This new approach of formulating quaternary CSEs is proven to give the best combination of properties and should be universal and be applied to other CSEs with different chemistry
<i>Ex Situ</i><sup>23</sup>Na Solid-State NMR Reveals the Local Na-Ion Distribution in Carbon-Coated Na<sub>2</sub>FePO<sub>4</sub>F during Electrochemical Cycling
The potential Na-ion
cathode material Na<sub>2</sub>FePO<sub>4</sub>F is investigated here
by <i>ex situ</i> <sup>23</sup>Na
solid-state nuclear magnetic resonance (ssNMR) in order to characterize
the structure and ion mobility as a function of electrochemical cycling.
The use of fast magic angle spinning (MAS) speeds of 65 kHz allows
for the collection of high-resolution <sup>23</sup>Na NMR spectra
that reveal two unique peaks at +450 and −175 ppm, corresponding
to the two crystallographically unique Na sites in the material of
interest. Two-dimensional NMR exchange spectroscopy results reveal
that chemical exchange between the Na ions residing in distinct environments
has a maximum hopping rate of ∼200 Hz. The collection of one-dimensional
NMR spectra as a function of electrochemical cycling reveals the reproducible
formation of a new peak at +320 ppm in the <sup>23</sup>Na NMR spectrum
at all intermediate states of charge. The appearance of this resonance
at +320 ppm is attributed to the fully oxidized (NaFePO<sub>4</sub>F) phase that is present even upon initial electrochemical oxidation.
The simultaneous existence of both the pristine and oxidized phases
suggest formation of two distinct phases upon charging, consistent
with a two-phase desodiation mechanism. This two-phase arrangement
of Na ions persists for multiple charge/discharge cycles and is congruent
with high reversibility of Na (de)Âintercalation in Na<sub>2</sub>FePO<sub>4</sub>F cathodes. These findings imply that the Na<sub>2</sub>FePO<sub>4</sub>F framework is incredibly structurally stable with a robust
intercalation process, despite a lack of ideal sodium-ion kinetics
Differentiating Lithium Ion Hopping Rates in Vanadium Phosphate versus Vanadium Fluorophosphate Structures Using 1D <sup>6</sup>Li Selective Inversion NMR
The electrochemical performance of
lithium ion batteries is strongly
correlated with the ion dynamics within the electrode structures.
This study characterizes Li ion hopping rates and energy barriers
in the layered phase, Li<sub>5</sub>VÂ(PO<sub>4</sub>)<sub>2</sub>F<sub>2</sub>, using <sup>6</sup>Li selective inversion (SI) NMR measurements.
Li<sub>5</sub>VÂ(PO<sub>4</sub>)<sub>2</sub>F<sub>2</sub> has six crystallographically
distinct lithium sites giving the possibility of fifteen exchange
partners between nonequivalent lithium environments. Here, <sup>6</sup>Li 1D SI measurements over a variable temperature range were used
to quantify the time scales and energy barriers of ion mobility for
several ion pairs observed to participate in ion hopping. The rates
determined in this material are similar in range to the previously
determined rates found in tavorite Li<sub>2</sub>VPO<sub>4</sub>F
yet considerably slower than results from both α-Li<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> and α-Li<sub>3</sub>Fe<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>. A detailed analysis of
the structural features that enhance or inhibit fast ion mobility
is discussed. This includes a consideration of the bond valence density
maps of the diffusion pathway. Comparison of the ion mobilities in
the phosphates and fluorophosphates shows how the gains in redox potential
come at the expense of fast ion mobility, meaning that any improvements
to the energy output of the lithium ion battery through higher voltage
may be compromised due to slow charge/discharge rates
Structure and Dynamics in Functionalized Graphene Oxides through Solid-State NMR
Graphene
oxide (GO), a derivative of the supermaterial graphene,
has intrinsic proton conductivity, which is similar to Nafion, the
most popular proton exchange membrane material currently used in fuel
cells. Research into acid-functionalized GOs and determining the role
of acidic groups in increasing proton conductivity will help to improve
polymer electrolyte membrane performance in fuel cell systems. Multinuclear
solid-state NMR (ssNMR) spectroscopy was used to analyze the structure
and dynamics of GO and a number of sulfonic acid derivatives of GO,
both novel and previously reported. <sup>13</sup>C CP-MAS spectra
showed the disappearance of surface-based oxygen groups upon GO functionalization
and can be used to identify linker group carbon sites in previously
synthesized and novel functionalized GO samples with high specificity.
Dehydration of these samples allows the collection of <sup>1</sup>H spectra with resolved acidic proton and water peaks. The effect
of dehydration on the proton spectrum is partially reversible through
rehydration. Deuteration of the acidic groups in high temperature
and acidic conditions was virtually unsuccessful, indicating that
only the surface and not the intercalated functional groups play a
role in the enhanced proton conductivity of ionomer/functionalized
GO composites. Increased surface area and increased delamination of
functionalized GO are suggested to be important to improved proton
exchange membrane fuel cell performance. This synthesis and method
of analysis prove the utility of ssNMR in the study of structure and
dynamics in industrially relevant amorphous carbon materials despite
the obvious difficulties caused by naturally broad signals and low
sensitivity
Structure and ionic mobility of anti-perovskite compounds as solid-state electrolytes
International audienc
Synthèse et caractérisation de conducteurs ioniques Li3-xHxOCl de structure anti-pérovskite
International audienceLe remplacement de l’électrolyte liquide utilisé dans les accumulateurs Li-ion actuels .par un électrolyte solide ininflammable et plus stable (électro)chimiquement constitue une voie privilégiée pour l’amélioration de la sécurité des batteries..
Synthèse et caractérisation de conducteurs ioniques Li3-xHxOCl de structure anti-pérovskite
International audienceLe remplacement de l’électrolyte liquide utilisé dans les accumulateurs Li-ion actuels .par un électrolyte solide ininflammable et plus stable (électro)chimiquement constitue une voie privilégiée pour l’amélioration de la sécurité des batteries..
<sup>19</sup>F Double Quantum NMR Spectroscopy: A Tool for Probing Dynamics in Proton-Conducting Fluorinated Polymer Materials
Solid-state NMR spectroscopy is an
important technique for probing
the structure and local dynamics of materials at the molecular level.
For example, <sup>1</sup>H double quantum (DQ) NMR is a well-established
probe of local dynamics. Here, this concept has been extended to characterize
fluorinated ionomer materials for the first time. <sup>19</sup>F DQ
recoupling NMR experiments are applied to investigate the site-specific
local dynamics of the polymer electrolyte material, Nafion 117, under
various conditions with respect to temperature and hydration level.
The initial rise of the normalized double quantum (nDQ) build-up curves
generated from NMR dipolar recoupling experiments is compared as a
measure of the motionally averaged <sup>19</sup>F–<sup>19</sup>F dipolar couplings for spectroscopically resolved domains of the
polymers. Since the side-chain and backbone fluorines can be distinguished
by their chemical shifts, it was possible to demonstrate a difference
between the side-chain and backbone local dynamics profiles. The side
chain is shown to be more sensitive toward the temperature and relative
humidity (%RH) changes, and generally the side chain exhibits greater
local dynamics as compared to the hydrophobic backbone, which is consistent
with subsegmental motion known as β-relaxation. Elevated temperature
and increased relative humidity give rise to increased local dynamics,
which is reflected by the slower initial increase of the nDQ build-up
curves. This NMR technique has been validated as a comparative analysis
tool, suitable for a range of perfluorinated ionomers