13 research outputs found
Study on the Electrochemical Reaction Mechanism of NiFe<sub>2</sub>O<sub>4</sub> as a High-Performance Anode for Li-Ion Batteries
Nickel
ferrite (NiFe<sub>2</sub>O<sub>4</sub>) has been previously shown
to have a promising electrochemical performance for lithium-ion batteries
(LIBs) as an anode material. However, associated electrochemical processes,
along with structural changes, during conversion reactions are hardly
studied. Nanocrystalline NiFe<sub>2</sub>O<sub>4</sub> was synthesized
with the aid of a simple citric acid assisted sol–gel method
and tested as a negative electrode for LIBs. After 100 cycles at a
constant current density of 0.5 A g<sup>–1</sup> (about a 0.5
C-rate), the synthesized NiFe<sub>2</sub>O<sub>4</sub> electrode provided
a stable reversible capacity of 786 mAh g<sup>–1</sup> with
a capacity retention greater than 85%. The NiFe<sub>2</sub>O<sub>4</sub> electrode achieved a specific capacity of 365 mAh g<sup>–1</sup> when cycled at a current density of 10 A g<sup>–1</sup> (about
a 10 C-rate). At such a high current density, this is an outstanding
capacity for NiFe<sub>2</sub>O<sub>4</sub> nanoparticles as an anode.
Ex-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy
(XAS) were employed at different potential states during the cell
operation to elucidate the conversion process of a NiFe<sub>2</sub>O<sub>4</sub> anode and the capacity contribution from either Ni
or Fe. Investigation reveals that the lithium extraction reaction
does not fully agree with the previously reported one and is found
to be a hindered oxidation of metallic nickel to nickel oxide in the
applied potential window. Our findings suggest that iron is participating
in an electrochemical reaction with full reversibility and forms iron
oxide in the fully charged state, while nickel is found to be the
cause of partial irreversible capacity where it exists in both metallic
nickel and nickel oxide phases
Structural Evolution of Li<sub><i>x</i></sub>Ni<sub><i>y</i></sub>Mn<sub><i>z</i></sub>Co<sub>1‑y‑z</sub>O<sub>2</sub> Cathode Materials during High-Rate Charge and Discharge
Ni-rich lithium transition
metal oxides have received significant
attention due to their high capacities and rate capabilities determined
via theoretical calculations. Although the structural properties of
these materials are strongly correlated with the electrochemical performance,
their structural stability during the high-rate electrochemical reactions
has not been fully evaluated yet. In this work, transmission electron
microscopy is used to investigate the crystallographic and electronic
structural modifications of Ni-based cathode materials at a high charge/discharge
rate of 10 C. It is found that the high-rate electrochemical reactions
induce structural inhomogeneity near the surface of Ni-rich cathode
materials, which limits Li transport and reduces their capacities.
This study establishes a correlation between the high-rate electrochemical
performance of the Ni-based materials and their structural evolution,
which can provide profound insights for designing novel cathode materials
having both high energy and power densities
Investigation of Thermal Stability of P2–Na<sub><i>x</i></sub>CoO<sub>2</sub> Cathode Materials for Sodium Ion Batteries Using Real-Time Electron Microscopy
Here,
we take advantage of <i>in situ</i> transmission electron
microscopy (TEM) to investigate the thermal stability of P2-type Na<sub><i>x</i></sub>CoO<sub>2</sub> cathode materials for sodium
ion batteries, which are promising candidates for next-generation
lithium ion batteries. A double-tilt TEM heating holder was used to
directly characterize the changes in the morphology and the crystallographic
and electronic structures of the materials with increase in temperature.
The electron diffraction patterns and the electron energy loss spectra
demonstrated the presence of cobalt oxides (Co<sub>3</sub>O<sub>4</sub>, CoO) and even metallic cobalt (Co) at higher temperatures as a
result of reduction of Co ions and loss of oxygen. The bright-field
TEM images revealed that the surface of Na<sub><i>x</i></sub>CoO<sub>2</sub> becomes porous at high temperatures. Higher cutoff
voltages result in degrading thermal stability of Na<sub><i>x</i></sub>CoO<sub>2</sub>. The observations herein provide a valuable
insight that thermal stability is one of the important factors to
be considered in addition to the electrochemical properties when developing
new electrode materials for novel battery systems
Nanoscale Zirconium-Abundant Surface Layers on Lithium- and Manganese-Rich Layered Oxides for High-Rate Lithium-Ion Batteries
Battery
performance, such as the rate capability and cycle stability
of lithium transition metal oxides, is strongly correlated with the
surface properties of active particles. For lithium-rich layered oxides,
transition metal segregation in the initial state and migration upon
cycling leads to a significant structural rearrangement, which eventually
degrades the electrode performance. Here, we show that a fine-tuning
of surface chemistry on the particular crystal facet can facilitate
ionic diffusion and thus improve the rate capability dramatically,
delivering a specific capacity of ∼110 mAh g<sup>–1</sup> at 30C. This high rate performance is realized by creating a nanoscale
zirconium-abundant rock-salt-like surface phase epitaxially grown
on the layered bulk. This surface layer is spontaneously formed on
the Li<sup>+</sup>-diffusive crystallographic facets during the synthesis
and is also durable upon electrochemical cycling. As a result, Li-ions
can move rapidly through this nanoscale surface layer over hundreds
of cycles. This study provides a promising new strategy for designing
and preparing a high-performance lithium-rich layered oxide cathode
material
P2 Orthorhombic Na<sub>0.7</sub>[Mn<sub>1–<i>x</i></sub>Li<sub><i>x</i></sub>]O<sub>2+<i>y</i></sub> as Cathode Materials for Na-Ion Batteries
P2-type
manganese-based oxide materials have received attention as promising
cathode materials for sodium ion batteries because of their low cost
and high capacity, but their reaction and failure mechanisms are not
yet fully understood. In this study, the reaction and failure mechanisms
of β-Na<sub>0.7</sub>[Mn<sub>1–<i>x</i></sub>Li<sub><i>x</i></sub>]O<sub>2+<i>y</i></sub> (<i>x</i> = 0.02, 0.04, 0.07, and 0.25), α-Na<sub>0.7</sub>MnO<sub>2+<i>y</i></sub>, and β-Na<sub>0.7</sub>MnO<sub>2+<i>z</i></sub> are compared to clarify the dominant factors
influencing their electrochemical performances. Using a quenching
process with various amounts of a Li dopant, the Mn oxidation state
in β-Na<sub>0.7</sub>[Mn<sub>1–<i>x</i></sub>Li<sub><i>x</i></sub>]O<sub>2+<i>y</i></sub> is
carefully controlled without the inclusion of impurities. Through
various in situ and ex situ analyses including X-ray diffraction,
X-ray absorption near-edge structure spectroscopy, and inductively
coupled plasma mass spectrometry, we clarify the dependence of (i)
reaction mechanisms on disordered Li distribution in the Mn layer,
(ii) reversible capacities on the initial Mn oxidation state, (iii)
redox potentials on the Jahn–Teller distortion, (iv) capacity
fading on phase transitions during charging and discharging, and (v)
electrochemical performance on Li dopant vs Mn vacancy. Finally, we
demonstrate that the optimized β-Na<sub>0.7</sub>[Mn<sub>1–<i>x</i></sub>Li<sub><i>x</i></sub>]O<sub>2+<i>y</i></sub> (<i>x</i> = 0.07) exhibits excellent electrochemical
performance including a high reversible capacity of ∼183 mA
h g<sup>–1</sup> and stable cycle performance over 120 cycles
Anatase Titania Nanorods as an Intercalation Anode Material for Rechargeable Sodium Batteries
For
the first time, we report the electrochemical activity of anatase
TiO<sub>2</sub> nanorods in a Na cell. The anatase TiO<sub>2</sub> nanorods were synthesized by a hydrothermal method, and their surfaces
were coated by carbon to improve the electric conductivity through
carbonization of pitch at 700 °C for 2 h in Ar flow. The resulting
structure does not change before and after the carbon coating, as
confirmed by X-ray diffraction (XRD). Transmission electron microscopic
images confirm the presence of a carbon coating on the anatase TiO<sub>2</sub> nanorods. In cell tests, anodes of bare and carbon-coated
anatase TiO<sub>2</sub> nanorods exhibit stable cycling performance
and attain a capacity of about 172 and 193 mAh g<sup>–1</sup> on the first charge, respectively, in the voltage range of 3–0
V. With the help of the conductive carbon layers, the carbon-coated
anatase TiO<sub>2</sub> delivers more capacity at high rates, 104
mAh g<sup>–1</sup> at the 10 C-rate (3.3 A g<sup>–1</sup>), 82 mAh g<sup>–1</sup> at the 30 C-rate (10 A g<sup>–1</sup>), and 53 mAh g<sup>–1</sup> at the 100 C-rate (33 A g<sup>–1</sup>). By contrast, the anode of bare anatase TiO<sub>2</sub> nanorods delivers only about 38 mAh g<sup>–1</sup> at the 10 C-rate (3.3 A g<sup>–1</sup>). The excellent cyclability
and high-rate capability are the result of a Na<sup>+</sup> insertion
and extraction reaction into the host structure coupled with Ti<sup>4+/3+</sup> redox reaction, as revealed by X-ray absorption spectroscopy
Lattice Water for the Enhanced Performance of Amorphous Iron Phosphate in Sodium-Ion Batteries
We report amorphous iron phosphate
with lattice water, namely FePO<sub>4</sub>·<i>x</i>H<sub>2</sub>O (<i>x</i> ∼ 2.39), as a promising
sodium-ion battery (SIB) cathode.
After carbon coating, micrometer-sized FePO<sub>4</sub>·<i>x</i>H<sub>2</sub>O exhibits a reversible capacity that is higher
than that of its counterpart without lattice water (130.0 vs 50.6
mAh g<sup>–1</sup> at 0.15<i>C</i> rate) along
with clearly enhanced rate capability and cyclability. The superior
electrochemical performance of FePO<sub>4</sub>·<i>x</i>H<sub>2</sub>O is attributed to the lattice water that facilitates
sodium-ion diffusion via enlarged channel dimensions and the screening
of the electrostatic interactions between sodium ions and host anions.
The amorphous phase is also advantageous in accommodating the stress
created in the host framework during sodium-ion (de)intercalation.
The presence of lattice water also protects the oxidation state of
Fe from reductive surface carbon coating and slightly lowers the operation
voltage via reduced inductive effect. The current study provides a
useful insight into how to design SIB electrode materials particularly
focusing on facile sodium-ion diffusion
Investigation of Changes in the Surface Structure of Li<sub><i>x</i></sub>Ni<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Cathode Materials Induced by the Initial Charge
We
use transmission electron microscopy (TEM) to investigate the
evolution of the surface structure of Li<sub><i>x</i></sub>Ni<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> cathode
materials (NCA) as a function of the extent of first charge at room
temperature using a combination of high-resolution electron microscopy
(HREM) imaging, selected area electron diffraction (SAED), and electron
energy loss spectroscopy (EELS). It was found that the surface changes
from the layered structure (space group <i>R</i>3̅<i>m</i>) to the disordered spinel structure (<i>Fd</i>3̅<i>m</i>), and eventually to the rock-salt structure
(<i>Fm</i>3̅<i>m</i>), and that these changes
are more substantial as the extent of charge increases. EELS indicates
that these crystal structure changes are also accompanied by significant
changes in the electronic structure, which are consistent with delithiation
leading to both a reduction of the Ni and an increase in the effective
electron density of oxygen. This leads to a charge imbalance, which
results in the formation of oxygen vacancies and the development of
surface porosity. The degree of local surface structure change differs
among particles, likely due to kinetic factors that are manifested
with changes in particle size. These results demonstrate that TEM,
when coupled with EELS, can provide detailed information about the
crystallographic and electronic structure changes that occur at the
surface of these materials during delithiation. This information is
of critical importance for obtaining a complete understanding of the
mechanisms by which both degradation and thermal runaway initiate
in these electrode materials
Correlating Structural Changes and Gas Evolution during the Thermal Decomposition of Charged Li<sub><i>x</i></sub>Ni<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> Cathode Materials
In this work, we present results from the application
of a new
in situ technique that combines time-resolved synchrotron X-ray diffraction
and mass spectroscopy. We exploit this approach to provide direct
correlation between structural changes and the evolution of gas that
occurs during the thermal decomposition of (over)charged cathode materials
used in lithium-ion batteries. Results from charged Li<sub><i>x</i></sub>Ni<sub>0.8</sub>Co<sub>0.15</sub>Al<sub>0.05</sub>O<sub>2</sub> cathode materials indicate that the evolution of both
O<sub>2</sub> and CO<sub>2</sub> gases are strongly related to phase
transitions that occur during thermal decomposition, specifically
from the layered structure (space group <i>R</i>3̅<i>m</i>) to the disordered spinel structure (<i>Fd</i>3̅<i>m</i>), and finally to the rock-salt structure
(<i>Fm</i>3̅<i>m</i>). The state of charge
also significantly affects both the structural changes and the evolution
of oxygen as the temperature increases: the more extensive the charge,
the lower the temperature of the phase transitions and the larger
the oxygen release. Ex situ X-ray absorption spectroscopy (XAS) and
in situ transmission electron microscopy (TEM) are also utilized to
investigate the local structural and valence state changes in Ni and
Co ions, and to characterize microscopic morphology changes. The combination
of these advanced tools provides a unique approach to study fundamental
aspects of the dynamic physical and chemical changes that occur during
thermal decomposition of charged cathode materials in a systematic
way
Controlling the Intercalation Chemistry to Design High-Performance Dual-Salt Hybrid Rechargeable Batteries
We
have conducted extensive theoretical and experimental investigations
to unravel the origin of the electrochemical properties of hybrid
Mg<sup>2+</sup>/Li<sup>+</sup> rechargeable batteries at the atomistic
and macroscopic levels. By revealing the thermodynamics of Mg<sup>2+</sup> and Li<sup>+</sup> co-insertion into the Mo<sub>6</sub>S<sub>8</sub> cathode host using density functional theory calculations,
we show that there is a threshold Li<sup>+</sup> activity for the
pristine Mo<sub>6</sub>S<sub>8</sub> cathode to prefer lithiation
instead of magnesiation. By precisely controlling the insertion chemistry
using a dual-salt electrolyte, we have enabled ultrafast discharge
of our battery by achieving 93.6% capacity retention at 20 C and 87.5%
at 30 C, respectively, at room temperature