14 research outputs found
Mechanistic Study of Electrolyte Additives to Stabilize High-Voltage CathodeāElectrolyte Interface in Lithium-Ion Batteries
Current developments
of electrolyte additives to stabilize electrodeāelectrolyte
interface in lithium-ion batteries highly rely on a trial-and-error
search, which involves repetitive testing and intensive amount of
resources. The lack of understandings on the fundamental protection
mechanisms of the additives significantly increases the difficulty
for the transformational development of new additives. In this study,
we investigated two types of individual protection routes to build
a robust cathodeāelectrolyte interphase at high potentials:
(i) a direct reduction in the catalytic decomposition of the electrolyte
solvent; and (ii) formation of a ācorrosion inhibitor filmā
that prevents severely attack and passivation from protons that generated
from the solvent oxidation, even the decomposition of solvent cannot
be mitigated. Effect of two exemplary electrolyte additives, lithium
difluoroĀ(oxalato)Āborate (LiDFOB) and 3-hexylthiophene (3HT), on LiNi<sub>0.6</sub>Mn<sub>0.2</sub>Co<sub>0.2</sub>O<sub>2</sub> (NMC 622)
cathode were investigated to validate our hypothesis. It is demonstrated
that understandings of both electrolyte additives and solvent are
essential and careful balance between the cathode protection mechanism
of additives and their side effects is critical to obtain optimum
results. More importantly, this study opens up new directions of rational
design of functional electrolyte additives for the next-generation
high-energy-density lithium-ion chemistries
Cathode Material with Nanorod StructureīøAn Application for Advanced High-Energy and Safe Lithium Batteries
We have developed a novel cathode
material based on lithiumānickelāmanganeseācobalt
oxide, where the manganese concentration remains constant throughout
the particle, while the nickel concentration decreases linearly and
the cobalt concentration increases from the center to the outer surface
of the particle. This full concentration gradient material with a
fixed manganese composition (FCGāMn-F) has an average composition
of LiĀ[Ni<sub>0.60</sub>Co<sub>0.15</sub>Mn<sub>0.25</sub>]ĀO<sub>2</sub> and is composed of rod-shaped primary particles whose length reaches
2.5 Ī¼m, growing in the radial direction. In cell tests, the
FCGāMn-F material delivered a high capacity of 206 mAh g<sup>ā1</sup> with excellent capacity retention of 70.3% after
1000 cycles at 55 Ā°C. This cathode material also exhibited outstanding
rate capability, good low-temperature performance, and excellent safety,
compared to a conventional cathode having the same composition (LiĀ[Ni<sub>0.60</sub>Co<sub>0.15</sub>Mn<sub>0.25</sub>]ĀO<sub>2</sub>), where
the concentration of the metals is constant across the particles
Conductive Polymer Binder-Enabled SiOāSn<sub><i>x</i></sub>Co<sub><i>y</i></sub>C<sub><i>z</i></sub> Anode for High-Energy Lithium-Ion Batteries
A SiOSnCoC composite anode is assembled
using a conductive polymer
binder for the application in next-generation high energy density
lithium-ion batteries. A specific capacity of 700 mAh/g is achieved
at a 1C (900 mA/g) rate. A high active material loading anode with
an areal capacity of 3.5 mAh/cm<sup>2</sup> is demonstrated by mixing
SiOSnCoC with graphite. To compensate for the lithium loss in the
first cycle, stabilized lithium metal powder (SLMP) is used for prelithiation;
when paired with a commercial cathode, a stable full cell cycling
performance with a 86% first cycle efficiency is realized. By achieving
these important metrics toward a practical application, this conductive
polymer binder/SiOSnCoC anode system presents great promise to enable
the next generation of high-energy lithium-ion batteries
Role of Manganese Deposition on Graphite in the Capacity Fading of Lithium Ion Batteries
Lithium
ion batteries utilizing manganese-based cathodes have received
considerable interest in recent years for their lower cost and more
favorable environmental friendliness relative to their cobalt counterparts.
However, Li ion batteries using these cathodes combined with graphite
anodes suffer from severe capacity fading at high operating temperatures.
In this paper, we report on how the dissolution of manganese impacts
the capacity fading within the Li ion batteries. Our investigation
reveals that the manganese dissolves from the cathode, transports
to the graphite electrode, and deposits onto the outer surface of
the innermost solid-electrolyte interphase layer, which is known to
be a mixture of inorganic salts (e.g., Li<sub>2</sub>CO<sub>3</sub>, LiF, and Li<sub>2</sub>O). In this location, the manganese facilitates
the reduction of the electrolyte and the subsequent formation of lithium-containing
products on the graphite, which removes lithium ions from the normal
operation of the cell and thereby induces the severe capacity fade
Storage and Effective Migration of Li-Ion for Defected Ī²āLiFePO<sub>4</sub> Phase Nanocrystals
Lithium iron phosphate, a widely
used cathode material, crystallizes typically in olivine-type phase,
Ī±-LiFePO<sub>4</sub> (Ī±LFP). However, the new phase Ī²-LiFePO<sub>4</sub> (Ī²LFP), which can be transformed from Ī±LFP under
high temperature and pressure, is originally almost electrochemically
inactive with no capacity for Li-ion battery, because the Li-ions
are stored in the tetrahedral [LiO<sub>4</sub>] with very high activation
barrier for migration and the one-dimensional (1D) migration channels
for Li-ion diffusion in Ī±LFP disappear, while the Fe ions in
the Ī²-phase are oriented similar to the 1D arrangement instead.
In this work, using experimental studies combined with density functional
theory calculations, we demonstrate that Ī²LFP can be activated
with creation of effective paths of Li-ion migration by optimized
disordering. Thus, the new phase of Ī²LFP cathode achieved a
capacity of 128 mAh g<sup>ā1</sup> at a rate of 0.1 C (1C =
170 mA g<sup>ā1</sup>) with extraordinary cycling performance
that 94.5% of the initial capacity retains after 1000 cycles at 1
C. The activation mechanism can be attributed to that the induced
disorder (such as Fe<sub>Li</sub>Li<sub>Fe</sub> antisite defects,
crystal distortion, and amorphous domains) creates new lithium migration
passages, which free the captive stored lithium atoms and facilitate
their intercalation/deintercalation from the cathode. Such materials
activated by disorder are promising candidate cathodes for lithium
batteries, and the related mechanism of storage and effective migration
of Li-ions also provides new clues for future design of disordered-electrode
materials with high capacity and high energy density
Exploring Highly Reversible 1.5-Electron Reactions (V<sup>3+</sup>/V<sup>4+</sup>/V<sup>5+</sup>) in Na<sub>3</sub>VCr(PO<sub>4</sub>)<sub>3</sub> Cathode for Sodium-Ion Batteries
The development of
highly reversible multielectron reaction per
redox center in sodium super ionic conductor-structured cathode materials
is desired to improve the energy density of sodium-ion batteries.
Here, we investigated more than one-electron storage of Na in Na<sub>3</sub>VCrĀ(PO<sub>4</sub>)<sub>3</sub>. Combining a series of advanced
characterization techniques such as ex situ <sup>51</sup>V solid-state
nuclear magnetic resonance, X-ray absorption near-edge structure,
and in situ X-ray diffraction, we reveal that V<sup>3+</sup>/V<sup>4+</sup> and V<sup>4+</sup>/V<sup>5+</sup> redox couples in the materials
can be accessed, leading to a 1.5-electron reaction. It is also found
that a light change on the local electronic and structural states
or phase change could be observed after the first cycle, resulting
in the fast capacity fade at room temperature. We also showed that
the irreversibility of the phase changes could be largely suppressed
at low temperature, thus leading to a much improved electrochemical
performance
Nanostructured Black Phosphorus/KetjenblackāMultiwalled Carbon Nanotubes Composite as High Performance Anode Material for Sodium-Ion Batteries
Sodium-ion batteries are promising
alternatives to lithium-ion
batteries for large-scale applications. However, the low capacity
and poor rate capability of existing anodes for sodium-ion batteries
are bottlenecks for future developments. Here, we report a high performance
nanostructured anode material for sodium-ion batteries that is fabricated
by high energy ball milling to form black phosphorus/Ketjenblackāmultiwalled
carbon nanotubes (BPC) composite. With this strategy, the BPC composite
with a high phosphorus content (70 wt %) could deliver a very high
initial Coulombic efficiency (>90%) and high specific capacity
with
excellent cyclability at high rate of charge/discharge (ā¼1700
mAh g<sup>ā1</sup> after 100 cycles at 1.3 A g<sup>ā1</sup> based on the mass of P). In situ electrochemical impedance spectroscopy,
synchrotron high energy X-ray diffraction, ex situ small/wide-angle
X-ray scattering, high resolution transmission electronic microscopy,
and nuclear magnetic resonance were further used to unravel its superior
sodium storage performance. The scientific findings gained in this
work are expected to serve as a guide for future design on high performance
anode material for sodium-ion batteries
Modifying the Surface of a High-Voltage Lithium-Ion Cathode
Ni-rich
lithium nickel manganese cobalt oxides (LiNi<sub><i>x</i></sub>Mn<sub><i>y</i></sub>Co<sub>1ā<i>x</i>ā<i>y</i></sub>O<sub>2</sub>, NMCs) suffer from
poor cycling stability at potentials above 4.2 V vs Li/Li<sup>+</sup>. This degraded cyclability at high potentials has been largely ascribed
to the parasitic reactions between the delithiated cathode and the
nonaqueous electrolyte. In this study, we mitigated the performance
degradation of high-voltage NMC 622 by designing a functional interfacial
layer that consists of a surface doping by Ti<sup>4+</sup> and a TiO<sub>2</sub> coating at the same time. The doping of Ti<sup>4+</sup> near
the surface of NMC can suppress the irreversible phase transformation,
while the TiO<sub>2</sub> coating can kinetically reduce the rate
of the electron-transfer reaction between the delithiated cathode
and the solvent. It is revealed that this interfacial engineering
approach significantly enhanced both the cycling stability and the
rate performance of NMC 622
Revisiting the Corrosion of the Aluminum Current Collector in Lithium-Ion Batteries
The
corrosion of aluminum current collectors and the oxidation
of solvents at a relatively high potential have been widely investigated
with an aim to stabilize the electrochemical performance of lithium-ion
batteries using such components. The corrosion behavior of aluminum
current collectors was revisited using a home-build high-precision
electrochemical measurement system, and the impact of electrolyte
components and the surface protection layer on aluminum foil was systematically
studied. The electrochemical results showed that the corrosion of
aluminum foil was triggered by the electrochemical oxidation of solvent
molecules, like ethylene carbonate, at a relative high potential.
The organic radical cations generated from the electrochemical oxidation
are energetically unstable and readily undergo a deprotonation reaction
that generates protons and promotes the dissolution of Al<sup>3+</sup> from the aluminum foil. This new reaction mechanism can also shed
light on the dissolution of transitional metal at high potentials
Tuning of Thermal Stability in Layered Li(Ni<sub><i>x</i></sub>Mn<sub><i>y</i></sub>Co<sub><i>z</i></sub>)O<sub>2</sub>
Understanding and further designing
new layered LiĀ(Ni<sub><i>x</i></sub>Mn<sub><i>y</i></sub>Co<sub><i>z</i></sub>)ĀO<sub>2</sub> (NMC) (<i>x</i> + <i>y</i> + <i>z</i> = 1) materials
with optimized thermal stability
is important to rechargeable Li batteries (LIBs) for electrical vehicles
(EV). Using ab initio calculations combined with experiments, we clarified
how the thermal stability of NMC materials can be tuned by the most
unstable oxygen, which is determined by the local coordination structure
unit (LCSU) of oxygen (TMĀ(Ni, Mn, Co)<sub>3</sub>-O-Li<sub>3ā<i>x</i>ā²</sub>): each O atom bonds with three transition
metals (TM) from the TM-layer and three to zero Li from fully discharged
to charged states from the Li-layer. Under this model, how the lithium
content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li
disorder to tune the thermal stability of NMC materials by affecting
the sites, content, and the release temperature of the most unstable
oxygen is proposed. The synergistic effect between Li vacancies and
raised valence state of Ni during delithiation process can aggravate
instability of oxygen, and oxygen coordinated with more nickel (especially
with high valence state) in LSCU becomes more unstable at a fixed
delithiation state. The Ni/Li mixing would decrease the thermal stability
of the āNiī»Mnā group NMC materials but benefit
the thermal stability of āNi-richā group, because the
Ni in the Li layer would form 180Ā° NiāOāNi super
exchange chains in āNi-richā NMC materials. Mn and Co
doping can tune the initial valence state of Ni, local coordination
environment of oxygen, and the Ni/Li disorder, thus to tune the thermal
stability directly