11 research outputs found
New Insights into the Compositional Dependence of Li-Ion Transport in Polymer–Ceramic Composite Electrolytes
Composite electrolytes are widely
studied for their potential in realizing improved ionic conductivity
and electrochemical stability. Understanding the complex mechanisms
of ion transport within composites is critical for effectively designing
high-performance solid electrolytes. This study examines the compositional
dependence of the three determining factors for ionic conductivity,
including ion mobility, ion transport pathways, and active ion concentration.
The results show that with increase in the fraction of ceramic Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (LLZO) phase in
the LLZO–poly(ethylene oxide) composites, ion mobility decreases,
ion transport pathways transit from polymer to ceramic routes, and
the active ion concentration increases. These changes in ion mobility,
transport pathways, and concentration collectively explain the observed
trend of ionic conductivity in composite electrolytes. Liquid additives
alter ion transport pathways and increase ion mobility, thus enhancing
ionic conductivity significantly. It is also found that a higher content
of LLZO leads to improved electrochemical stability of composite electrolytes.
This study provides insight into the recurring observations of compositional
dependence of ionic conductivity in current composite electrolytes
and pinpoints the intrinsic limitations of composite electrolytes
in achieving fast ion conduction
Understanding the Conduction Mechanism of the Protonic Conductor CsH<sub>2</sub>PO<sub>4</sub> by Solid-State NMR Spectroscopy
Local dynamics and hydrogen bonding
in CsH<sub>2</sub>PO<sub>4</sub> have been investigated by <sup>1</sup>H, <sup>2</sup>H, and <sup>31</sup>P solid-state NMR spectroscopy
to help provide a detailed
understanding of proton conduction in the paraelectric phase. Two
distinct environments are observed by <sup>1</sup>H and <sup>2</sup>H NMR, and their chemical shifts (<sup>1</sup>H) and quadrupolar
coupling constants (<sup>2</sup>H) are consistent with one strong
and one slightly weaker H-bonding environment. Two different protonic
motions are detected by variable-temperature <sup>1</sup>H MAS NMR
and <i>T</i><sub>1</sub> spin–lattice relaxation
time measurements in the paraelectric phase, which we assign to librational
and long-range translational motions. An activation energy of 0.70
± 0.07 eV is extracted for the latter motion; that of the librational
motion is much lower. <sup>31</sup>P NMR line shapes are measured
under MAS and static conditions, and spin–lattice relaxation
time measurements have been performed as a function of temperature.
Although the <sup>31</sup>P line shape is sensitive to the protonic
motion, the reorientation of the phosphate ions does not lead to a
significant change in the <sup>31</sup>P CSA tensor. Rapid protonic
motion and rotation of the phosphate ions is seen in the superprotonic
phase, as probed by the <i>T</i><sub>1</sub> measurements
along with considerable line narrowing of both the <sup>1</sup>H and
the <sup>31</sup>P NMR signals
Efficient Co-Nanocrystal-Based Catalyst for Hydrogen Generation from Borohydride
Sodium
borohydride (NaBH<sub>4</sub>) has been proposed as a potential
hydrogen storage material for fuel cells, and the development of highly
active and robust catalysts for hydrolyzing NaBH<sub>4</sub> is the
key for the practical usage of NaBH<sub>4</sub> for fuel cells. Herein
we report Co-nanocrystal assembled hollow nanoparticles (Co-HNP) as
an active and robust catalyst for the hydrolysis of NaBH<sub>4</sub>. A hydrogen generation rate of 10.8 L·min<sup>–1</sup>·g<sup>–1</sup> at 25 °C was achieved by using the
Co-HNP catalyst with a low activation energy of 23.7 kJ·mol<sup>–1</sup>, which is among the best performance of reported
noble and non-noble catalysts for hydrolyzing NaBH<sub>4</sub>. Co-HNP
also showed good stability in the long term cycling tests. The mechanism
of the catalytic hydrolysis of NaBH<sub>4</sub> on Co-HNP was studied
by using <sup>1</sup>H and <sup>11</sup>B solid-state NMR, which provided
unambiguous experimental evidence of the Co–H formation. The
systematically designed NMR experiments unveiled the key role of Co-HNP
in the activation of borohydride and the subsequent transfer of H<sup>–</sup> to water for generating H<sub>2</sub> gas and helped
to distinguish various hypotheses proposed for catalytic H<sub>2</sub> generation reactions. The porous hollow nanostructure of the Co-HNP
catalyst provides large surface area and facilitates mass transfer.
The facile preparation and outstanding performance of Co-HNP enables
it as a very competitive catalyst for hydrogen production
Composite Polymer Electrolytes with Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> Garnet-Type Nanowires as Ceramic Fillers: Mechanism of Conductivity Enhancement and Role of Doping and Morphology
Composite polymer
solid electrolytes (CPEs) containing ceramic
fillers embedded inside a polymer-salt matrix show great improvements
in Li<sup>+</sup> ionic conductivity compared to the polymer electrolyte
alone. Lithium lanthanum zirconate (Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub>, LLZO) with a garnet-type crystal structure
is a promising solid Li<sup>+</sup> conductor. We show that by incorporating
only 5 wt % of the ceramic filler comprising undoped, cubic-phase
LLZO nanowires prepared by electrospinning, the room temperature ionic
conductivity of a polyacrylonitrile-LiClO<sub>4</sub>-based composite
is increased 3 orders of magnitude to 1.31 × 10<sup>–4</sup> S/cm. Al-doped and Ta-doped LLZO nanowires are also synthesized
and utilized as fillers, but the conductivity enhancement is similar
as for the undoped LLZO nanowires. Solid-state nuclear magnetic resonance
(NMR) studies show that LLZO NWs partially modify the PAN polymer
matrix and create preferential pathways for Li<sup>+</sup> conduction
through the modified polymer regions. CPEs with LLZO nanoparticles
and Al<sub>2</sub>O<sub>3</sub> nanowire fillers are also studied
to elucidate the role of filler type (active vs passive), LLZO composition
(undoped vs doped), and morphology (nanowire vs nanoparticle) on the
CPE conductivity. It is demonstrated that both intrinsic Li<sup>+</sup> conductivity and nanowire morphology are needed for optimal performance
when using 5 wt % of the ceramic filler in the CPE
Operando EPR for Simultaneous Monitoring of Anionic and Cationic Redox Processes in Li-Rich Metal Oxide Cathodes
Anionic
redox chemistry offers a transformative approach for significantly
increasing specific energy capacities of cathodes for rechargeable
Li-ion batteries. This study employs operando electron paramagnetic
resonance (EPR) to simultaneously monitor the evolution of both transition
metal and oxygen redox reactions, as well as their intertwined couplings
in Li<sub>2</sub>MnO<sub>3</sub>, Li<sub>1.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>O<sub>2</sub>, and Li<sub>1.2</sub>Ni<sub>0.13</sub>Mn<sub>0.54</sub>Co<sub>0.13</sub>O<sub>2</sub> cathodes. Reversible
O<sup>2–</sup>/O<sub>2</sub><sup><i>n</i>–</sup> redox takes place above 3.0 V, which is clearly distinguished from
transition metal redox in the operando EPR on Li<sub>2</sub>MnO<sub>3</sub> cathodes. O<sup>2–</sup>/O<sub>2</sub><sup><i>n</i>–</sup> redox is also observed in Li<sub>1.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>O<sub>2</sub>, and Li<sub>1.2</sub>Ni<sub>0.13</sub>Mn<sub>0.54</sub>Co<sub>0.13</sub>O<sub>2</sub> cathodes,
albeit its overlapping potential ranges with Ni redox. This study
further reveals the stabilization of the reversible O redox by Mn
and e<sup>–</sup> hole delocalization within the Mn–O
complex. The interactions within the cation–anion pairs are
essential for preventing O<sub>2</sub><sup><i>n</i>–</sup> from recombination into gaseous O<sub>2</sub> and prove to activate
Mn for its increasing participation in redox reactions. Operando EPR
helps to establish a fundamental understanding of reversible anionic
redox chemistry. The gained insights will support the search for structural
factors that promote desirable O redox reactions
Lithiation and Delithiation Dynamics of Different Li Sites in Li-Rich Battery Cathodes Studied by <i>Operando</i> Nuclear Magnetic Resonance
Li
in Li-rich cathodes mostly resides at octahedral sites in both
Li layers (Li<sub>Li</sub>) and transition metal layers (Li<sub>TM</sub>). Extraction and insertion of Li<sub>Li</sub> and Li<sub>TM</sub> are strongly influenced by surrounding transition metals. pjMATPASS
and <i>operando</i> Li nuclear magnetic resonance are combined
to achieve both high spectral and temporal resolution for quantitative
real time monitoring of lithiation and delithiation at Li<sub>Li</sub> and Li<sub>TM</sub> sites in Li<sub>2</sub>MnO<sub>3</sub>, Li<sub>1.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>O<sub>2</sub>, and Li<sub>1.2</sub>Ni<sub>0.13</sub>Mn<sub>0.54</sub>Co<sub>0.13</sub>O<sub>2</sub> cathodes. The results have revealed that Li<sub>TM</sub> are
preferentially extracted for the first 20% of charge and then Li<sub>Li</sub> and Li<sub>TM</sub> are removed at the same rate. No preferential
insertion or extraction of Li<sub>Li</sub> and Li<sub>TM</sub> is
observed beyond the first charge. Ni and Co promote faster and more
complete removal of Li<sub>TM</sub>. The recovery of the removed Li
is <60% for Li<sub>TM</sub> and >80% for Li<sub>Li</sub> upon
first discharge. The study sheds light on the activity of Li<sub>Li</sub> and Li<sub>TM</sub> during electrochemical processes as well as
their respective contributions to cathode capacity
Li Distribution Heterogeneity in Solid Electrolyte Li<sub>10</sub>GeP<sub>2</sub>S<sub>12</sub> upon Electrochemical Cycling Probed by <sup>7</sup>Li MRI
All-solid-state
rechargeable batteries embody the promise for high
energy density, increased stability, and improved safety. However,
their success is impeded by high resistance for mass and charge transfer
at electrode–electrolyte interfaces. Li deficiency has been
proposed as a major culprit for interfacial resistance, yet experimental
evidence is elusive due to the challenges associated with noninvasively
probing the Li distribution in solid electrolytes. In this Letter,
three-dimensional <sup>7</sup>Li magnetic resonance imaging (MRI)
is employed to examine Li distribution homogeneity in solid electrolyte
Li<sub>10</sub>GeP<sub>2</sub>S<sub>12</sub> within symmetric Li/Li<sub>10</sub>GeP<sub>2</sub>S<sub>12</sub>/Li batteries. <sup>7</sup>Li
MRI and the derived histograms reveal Li depletion from the electrode–electrolyte
interfaces and increased heterogeneity of Li distribution upon electrochemical
cycling. Significant Li loss at interfaces is mitigated via facile
modification with a poly(ethylene oxide)/bis(trifluoromethane)sulfonimide
Li salt thin film. This study demonstrates a powerful tool for noninvasively
monitoring the Li distribution at the interfaces and in the bulk of
all-solid-state batteries as well as a convenient strategy for improving
interfacial stability
Li Distribution Heterogeneity in Solid Electrolyte Li<sub>10</sub>GeP<sub>2</sub>S<sub>12</sub> upon Electrochemical Cycling Probed by <sup>7</sup>Li MRI
All-solid-state
rechargeable batteries embody the promise for high
energy density, increased stability, and improved safety. However,
their success is impeded by high resistance for mass and charge transfer
at electrode–electrolyte interfaces. Li deficiency has been
proposed as a major culprit for interfacial resistance, yet experimental
evidence is elusive due to the challenges associated with noninvasively
probing the Li distribution in solid electrolytes. In this Letter,
three-dimensional <sup>7</sup>Li magnetic resonance imaging (MRI)
is employed to examine Li distribution homogeneity in solid electrolyte
Li<sub>10</sub>GeP<sub>2</sub>S<sub>12</sub> within symmetric Li/Li<sub>10</sub>GeP<sub>2</sub>S<sub>12</sub>/Li batteries. <sup>7</sup>Li
MRI and the derived histograms reveal Li depletion from the electrode–electrolyte
interfaces and increased heterogeneity of Li distribution upon electrochemical
cycling. Significant Li loss at interfaces is mitigated via facile
modification with a poly(ethylene oxide)/bis(trifluoromethane)sulfonimide
Li salt thin film. This study demonstrates a powerful tool for noninvasively
monitoring the Li distribution at the interfaces and in the bulk of
all-solid-state batteries as well as a convenient strategy for improving
interfacial stability
Local Structure and Dynamics in the Na Ion Battery Positive Electrode Material Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub>
Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub> is a novel electrode material that
can be used in both Li ion and
Na ion batteries (LIBs and NIBs). The long- and short-range structural
changes and ionic and electronic mobility of Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub> as a positive electrode
in a NIB have been investigated with electrochemical analysis, X-ray
diffraction (XRD), and high-resolution <sup>23</sup>Na and <sup>31</sup>P solid-state nuclear magnetic resonance (NMR). The <sup>23</sup>Na NMR spectra and XRD refinements show that the Na ions are removed
nonselectively from the two distinct Na sites, the fully occupied
Na1 site and the partially occupied Na2 site, at least at the beginning
of charge. Anisotropic changes in lattice parameters of the cycled
Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub> electrode upon charge have been observed, where <i>a</i> (= <i>b</i>) continues to increase and <i>c</i> decreases, indicative of solid-solution processes. A noticeable
decrease in the cell volume between 0.6 Na and 1 Na is observed along
with a discontinuity in the <sup>23</sup>Na hyperfine shift between
0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement
of unpaired electrons within the vanadium t<sub>2g</sub> orbitals.
The Na ion mobility increases steadily on charging as more Na vacancies
are formed, and coalescence of the resonances from the two Na sites
is observed when 0.9 Na is removed, indicating a Na1–Na2 hopping
(two-site exchange) rate of ≥4.6 kHz. This rapid Na motion
must in part be responsible for the good rate performance of this
electrode material. The <sup>31</sup>P NMR spectra are complex, the
shifts of the two crystallograpically distinct sites being sensitive
to both local Na cation ordering on the Na2 site in the as-synthesized
material, the presence of oxidized (V<sup>4+</sup>) defects in the
structure, and the changes of cation and electronic mobility on Na
extraction. This study shows how NMR spectroscopy complemented by
XRD can be used to provide insight into the mechanism of Na extraction
from Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub> when used in a NIB
Sidorenkite (Na<sub>3</sub>MnPO<sub>4</sub>CO<sub>3</sub>): A New Intercalation Cathode Material for Na-Ion Batteries
Na-ion
batteries represent an effective energy storage technology
with slightly lower energy and power densities but potentially lower
material costs than Li-ion batteries. Here, we report a new polyanionic
intercalation cathode material of an unusual chemical class: sidorenkite
(Na<sub>3</sub>MnPO<sub>4</sub>CO<sub>3</sub>). This carbonophosphate
compound shows a high discharge capacity (∼125 mAh/g) and specific
energy (374 Wh/kg). <i>In situ</i> X-ray diffraction measurement
suggests that sidorenkite undergoes a solid solution type reversible
topotactic structural evolution upon electrochemical cycling. <i>Ex situ</i> solid state NMR investigation reveals that more
than one Na per formula unit can be deintercalated from the structure,
indicating a rarely observed two-electron intercalation reaction in
which both Mn<sup>2+</sup>/Mn<sup>3+</sup> and Mn<sup>3+</sup>/Mn<sup>4+</sup> redox couples are electrochemically active