8 research outputs found
Factors Affecting the Volumetric Energy Density of Lithium-Ion Battery Materials: Particle Density Measurements and Cross-Sectional Observations of Layered LiCo<sub>1–<i>x</i></sub>Ni<sub><i>x</i></sub>O<sub>2</sub> with 0 ≤ <i>x</i> ≤ 1
Volumetric
capacity <i>Q</i><sub>vol</sub> (mAh cm<sup>–3</sup>), more correctly, volumetric energy density <i>W</i><sub>vol</sub> (mWh cm<sup>–3</sup>), is a crucial
property of lithium-ion battery (LIB) materials, because LIBs are
devices that operate in a limited space. The actual value of <i>W</i><sub>vol</sub> (<i>W</i><sub>vol</sub><sup>act</sup>) is currently limited to
40–60% of the maximum (theoretical) value of <i>W</i><sub>vol</sub> (<i>W</i><sub>vol</sub><sup>max</sup>), for reasons that have not yet been fully
clarified. Thus, to gain information that will enable an increase
in <i>W</i><sub>vol</sub><sup>act</sup> such that it is closer to <i>W</i><sub>vol</sub><sup>max</sup>, systematic
studies of the values for <i>Q</i><sub>vol</sub>, <i>W</i><sub>vol</sub>, true density (<i>d</i><sub>XRD</sub>), and particle density (<i>d</i><sub>p</sub>) obtained
using gas pycnometry were undertaken for LiCo<sub>1–<i>x</i></sub>Ni<sub><i>x</i></sub>O<sub>2</sub> samples
with 0 ≤ <i>x</i> ≤ 1. Here, <i>d</i><sub>p</sub> is the density that includes the volume of the closed
pores in the particles, and consequently is less than <i>d</i><sub>XRD</sub>, which is determined by X-ray diffraction (XRD) measurement. <i>D</i><sub>XRD</sub> monotonically decreased from 5.062(1) g
cm<sup>–3</sup> for <i>x</i> = 0 to 4.779(1) g cm<sup>–3</sup> for <i>x</i> = 1, as expected. On the contrary, <i>d</i><sub>p</sub> decreased almost linearly from 4.98(2) g cm<sup>–3</sup> for <i>x</i> = 0 to 4.80(2) g cm<sup>–3</sup> for <i>x</i> = 0.5, then suddenly dropped to 4.63(2) g
cm<sup>–3</sup> for <i>x</i> = 0.667, and finally
leveled off to a constant value (∼4.6 g cm<sup>–3</sup>) at larger values of <i>x</i>. The cross-sectional observations
using a Focused Ion Beam system revealed that the significantly smaller
values for <i>d</i><sub>p</sub> compared with those for <i>d</i><sub>XRD</sub>, particularly when <i>x</i> >
0.5, is due to the presence of closed pores in agglomerated secondary
particles. This indicates that the closed pores in the secondary particles
play an important role in determining the value of <i>W</i><sub>vol</sub><sup>act</sup> for
LIBs. The formation of well-developed primary particles as a mean
for increasing the value of <i>d</i><sub>p</sub> was also
investigated
Are All-Solid-State Lithium-Ion Batteries Really Safe?–Verification by Differential Scanning Calorimetry with an All-Inclusive Microcell
Although
all-solid-state lithium-ion batteries (ALIBs) have been believed as
the ultimate safe battery, their true character has been an enigma
so far. In this paper, we developed an all-inclusive-microcell (AIM)
for differential scanning calorimetry (DSC) analysis to clarify the
degree of safety (DOS) of ALIBs. Here AIM possesses all the battery
components to work as a battery by itself, and DOS is determined by
the total heat generation ratio (Δ<i>H</i>) of ALIB
compared with the conventional LIB. When DOS = 100%, the safety of
ALIB is exactly the same as that of LIB; when DOS = 0%, ALIB reaches
the ultimate safety. We investigated two types of LIB-AIM and three
types of ALIB-AIM. Surprisingly, all the ALIBs exhibit one or two
exothermic peaks above 250 °C with 20–30% of DOS. The
exothermic peak is attributed to the reaction between the released
oxygen from the positive electrode and the Li metal in the negative
electrode. Hence, ALIBs are found to be flammable as in the case of
LIBs. We also attempted to improve the safety of ALIBs and succeeded
in decreasing the DOS down to ∼16% by incorporating Ketjenblack
into the positive electrode as an oxygen scavenger. Based on Δ<i>H</i> as a function of voltage window, a safety map for LIBs
and ALIBs is proposed
Role of Oxide Ions in Thermally Activated Lithium Diffusion of Li[Li<sub>1/3</sub>Ti<sub>5/3</sub>]O<sub>4</sub>: X‑ray Diffraction Measurements and Raman Spectroscopy
Although Li[Li<sub>1/3</sub>Ti<sub>5/3</sub>]O<sub>4</sub> (LTO)
has been considered as an ideal electrode material for lithium-ion
batteries (LIBs) because of its “zero-strain” character,
initial LTO exhibits high Li conductivity (σ<sub>Li</sub>) at
high temperatures (<i>T</i>). In this paper, to clarify
the inter-relation between LTO’s Li-diffusive nature and structural
environment, we performed a systematic structural study on LTO using
X-ray diffraction (XRD) measurements and Raman spectroscopy. The average
and static information obtained by XRD measurements suggested that
the bottleneck radius for Li conduction is limited to ∼0.41
even at 873 K, which is too small to explain the high σ<sub>Li</sub> values in LTO. However, Raman spectroscopy demonstrated
the dynamic structural changes of the LiO<sub>6</sub> octahedron with <i>T</i>; the bond interaction between Li and O atoms decreases
with <i>T</i> because of its anharmonic potential energy.
Because the Raman-active modes in LTO correspond to changes in oxide
ion position, oxide ions are determined to play a crucial role in
obtaining high σ<sub>Li</sub> values
Toward Positive Electrode Materials with High-Energy Density: Electrochemical and Structural Studies on LiCo<sub><i>x</i></sub>Mn<sub>2–<i>x</i></sub>O<sub>4</sub> with 0 ≤ <i>x</i> ≤ 1
To obtain positive electrode materials
with higher energy densities
(<i>W</i>s), we performed systematic structural and electrochemical
analyses for LiCo<sub><i>x</i></sub>Mn<sub>2–<i>x</i></sub>O<sub>4</sub> (LCMO) with 0 ≤ <i>x</i> ≤ 1. X-ray diffraction measurements and Raman spectroscopy
clarified that the samples with <i>x</i> ≤ 0.5 are
in the single-phase of a spinel structure with the <i>Fd</i>3̅<i>m</i> space group, whereas the samples with <i>x</i> ≥ 0.75 are in a mixture of the spinel-phase and
Li<sub>2</sub>MnO<sub>3</sub> phase with the <i>C</i>2/<i>m</i> space group. The <i>x</i>-dependence of the
discharge capacity (<i>Q</i><sub>dis</sub>) indicated a
broad maximum at <i>x</i> = 0.5, although the average operating
voltage (<i>E</i><sub>ave</sub>) monotonically increased
with <i>x</i>. Thus, the <i>W</i> value obtained
by <i>Q</i><sub>dis</sub> × <i>E</i><sub>ave</sub> reached the maximum (=627 mW h·g<sup>–1</sup>) at <i>x</i> = 0.5, which is greater than that for Li[Ni<sub>1/2</sub>Mn<sub>3/2</sub>]O<sub>4</sub>. Furthermore, the change
in the lattice volume (Δ<i>V</i>) during charge and
discharge reactions approached 0%, that is, zero-strain, at <i>x</i> = 1. Because Δ<i>V</i> for <i>x</i> = 0.5 was smaller than that for Li[Ni<sub>1/2</sub>Mn<sub>3/2</sub>]O<sub>4</sub>, the <i>x</i> = 0.5 sample is found to be
an alternative positive electrode material for Li[Ni<sub>1/2</sub>Mn<sub>3/2</sub>]O<sub>4</sub> with a high <i>W</i>
Toward Improving the Thermal Stability of Negative Electrode Materials: Differential Scanning Calorimetry and <i>In Situ</i> High-Temperature X‑ray Diffraction/X-ray Absorption Spectroscopy Studies of Li<sub>2</sub>ZnTi<sub>3</sub>O<sub>8</sub> and Related Compounds
Negative electrode materials with high thermal stability
are a
key strategy for improving the safety of lithium-ion batteries for
electric vehicles without requiring built-in safety devices. To search
for crucial clues into increasing the thermal stability of these materials,
we performed differential scanning calorimetry (DSC) and in
situ high-temperature (HT)-X-ray diffraction (XRD)/X-ray
absorption (XAS) up to 450 °C with respect to a solid-solution
compound of Li4/3–2x/3ZnxTi5/3–x/3O4 with 0 ≤ x ≤ 0.5. The DSC
profile of fully discharged x = 0.5 (Li2ZnTi3O8) with a LiPF6-based electrolyte
could be divided into three temperature (T) regions:
(i) T ≤ 250 °C for ΔHaccumi, (ii)
250 °C T ≤ 350 °C for ΔHaccumii, and (iii) T > 350 °C for ΔHaccumiii, where
ΔHaccumn is the accumulated change
in enthalpy in region n. The HT-XRD/XAS analyses
clarified that ΔHaccumi and ΔHaccumii originated
from the decomposition of solid electrolyte interphase (SEI) films
and the formation of a LiF phase, respectively. Comparison of the
DSC profiles with x = 0 (Li[Li1/3Ti5/3]O4) and graphite revealed the operating voltage,
i.e., amount of SEI films, and stability of the crystal lattice play
significant roles in the thermal stability of negative electrode materials.
Indeed, the highest thermal stability was attained at x = 0.25 using this approach
Understanding the Zero-Strain Lithium Insertion Scheme of Li[Li<sub>1/3</sub>Ti<sub>5/3</sub>]O<sub>4</sub>: Structural Changes at Atomic Scale Clarified by Raman Spectroscopy
Lithium titanium oxide Li[Li<sub>1/3</sub>Ti<sub>5/3</sub>]O<sub>4</sub> (LTO) is regarded as an ideal
electrode material for lithium-ion
batteries because of its “zero-strain” characteristic,
high thermal stability, and structural stability. Here, the zero-strain
means that the change in cubic lattice parameter is negligibly small
during charge and discharge reactions. We performed <i>ex situ</i> Raman spectroscopy on Li<sub>1+<i>x</i></sub>[Li<sub>1/3</sub>Ti<sub>5/3</sub>]O<sub>4</sub> samples with 0 ≤ <i>x</i> ≤ 0.94 to gain information about the relationship between
a zero-strain reaction scheme and structural change at the atomic
scale. The <i>x</i> = 0 (initial) sample exhibits three
major Raman bands at 671, 426, and 231 cm<sup>–1</sup> and
six minor Raman bands at 751, 510, 400, 344, 264, and 146 cm<sup>–1</sup>. According to Raman spectroscopy results on other lithium titanium
oxides such as Li<sub>2</sub>TiO<sub>3</sub> and TiO<sub>2</sub>,
the Raman bands at 510, 400, and 146 cm<sup>–1</sup> are attributed
to TiO<sub>2</sub> anatase, which is used as a starting material.
As <i>x</i> increases from 0 to 0.94, the two major Raman
bands at 426 and 231 cm<sup>–1</sup> show a blue shift, while
the major Raman band at 671 cm<sup>–1</sup> maintains frequency.
The three major Raman bands at 671, 423, and 231 cm<sup>–1</sup> are assigned to the <i>A</i><sub>1<i>g</i></sub> mode of symmetric stretching vibration ν<sub>sym</sub>(Ti–O),
the <i>E</i><sub><i>g</i></sub> mode of asymmetric
stretching vibration ν<sub>asym</sub>(Li–O), and the <i>F</i><sub>2<i>g</i></sub> mode of bending vibration
δ(Ti–O), respectively. Thus, the change in the Raman
spectrum with <i>x</i> indicates that the bond length between
the Ti and O atoms in the TiO<sub>6</sub> octahedron is independent
of <i>x</i>, while that between the Li and O atoms in the
LiO<sub>6</sub> octahedron and the bond angle between the Ti and O
atoms in the TiO<sub>6</sub> octahedron change with <i>x</i>. Raman studies with decreasing <i>x</i> from 0.94 to 0.10
clarified that such local structural changes are reversible, as in
the case for the electrochemical reaction. The zero-strain insertion
scheme is discussed from the perspective of Raman spectroscopy
Structural Phase Transition from Rhombohedral (<i>R</i>3̅<i>m</i>) to Monoclinic (<i>C</i>2/<i>m</i>) Symmetry in Lithium Overstoichiometric Li<sub>1+δ</sub>Co<sub>1−δ</sub>O<sub>2−δ</sub>
Stoichiometric lithium cobalt oxide
LiCoO<sub>2</sub> is known
to exhibit several structural phase transitions with <i>x</i> in Li<sub><i>x</i></sub>CoO<sub>2</sub> at ambient temperature
(<i>T</i>); e.g., an initial rhombohedral (<i>R</i>3̅<i>m</i>) phase transforms into a monoclinic (<i>C</i>2/<i>m</i>) phase at <i>x</i> ∼
0.5. In contrast, lithium overstoichiometric (Li)<sub>3<i>b</i></sub>[Li<sub>δ</sub>Co<sub>1−δ</sub>]<sub>3<i>a</i></sub>O<sub>2−δ</sub> with δ ≥
∼0.02, where δ is the Li<sup>+</sup> ions at the 3<i>a</i> (Co) site, maintains the <i>R</i>3̅<i>m</i> symmetry until <i>x</i> ∼ 0.5 in Li<sub><i>x</i></sub>(Li<sub>δ</sub>Co<sub>1−δ</sub>)O<sub>2−δ</sub> at ambient <i>T</i>, and
this is the reason why such material has been widely used in commercial
lithium ion batteries. We performed X-ray diffraction measurements
in the <i>T</i> range between 100 and 300 K for the lithium
overstoichiometric Li<sub><i>x</i></sub>(Li<sub>0.02</sub>Co<sub>0.98</sub>)O<sub>1.98</sub> samples with <i>x</i> = 1, 0.56, and 0.51 to understand the factors that govern the structural
changes in Li<sub><i>x</i></sub>(Li<sub>δ</sub>Co<sub>1−δ</sub>)O<sub>2−δ</sub> with δ
≥ 0. Both <i>x</i> = 0.56 and 0.51 samples exhibit
a structural phase transition from the high-<i>T R</i>3̅<i>m</i> phase to the low-<i>T C</i>2/<i>m</i> phase at 250 K (=<i>T</i><sub>s1</sub>). Furthermore,
these samples indicate another structural phase transition at 170
K (=<i>T</i><sub>s2</sub>); although their crystal structures
still have the <i>C</i>2/<i>m</i> symmetry, the
degree of monoclinic distortion starts to decrease below <i>T</i><sub>s2</sub>, associated with a magnetic anomaly and a freezing
of the Li<sup>+</sup> ions at the 3<i>b</i> site. Because
the two structural phase transitions of <i>T</i><sub>s1</sub> (=330 K) and <i>T</i><sub>s2</sub> (=150 K) are also observed
for the stoichiometric Li<sub><i>x</i></sub>CoO<sub>2</sub> compound with <i>x</i> ∼ 0.5, the <i>C</i>2/<i>m</i> phase in Li<sub><i>x</i></sub>(Li<sub>δ</sub>Co<sub>1−δ</sub>)O<sub>2−δ</sub> is found to appear in the limited <i>x</i> and <i>T</i> ranges. The characteristics and possible origin of <i>T</i><sub>s1</sub> and <i>T</i><sub>s2</sub> for both
stoichiometric Li<sub><i>x</i></sub>CoO<sub>2</sub> and
lithium overstoichiometric Li<sub><i>x</i></sub>(Li<sub>0.02</sub>Co<sub>0.98</sub>)O<sub>1.98</sub> samples are discussed
Superior Low-Temperature Power and Cycle Performances of Na-Ion Battery over Li-Ion Battery
The most simple and clear advantage
of Na-ion batteries (NIBs)
over Li-ion batteries (LIBs) is the natural abundance of Na, which
allows inexpensive production of NIBs for large-scale applications.
However, although strenuous research efforts have been devoted to
NIBs particularly since 2010, certain other advantages of NIBs have
been largely overlooked, for example, their low-temperature power
and cycle performances. Herein, we present
a comparative study of spirally wound full-cells consisting of Li<sub>0.1</sub>Na<sub>0.7</sub>Co<sub>0.5</sub>Mn<sub>0.5</sub>O<sub>2</sub> (or Li<sub>0.8</sub>Co<sub>0.5</sub>Mn<sub>0.5</sub>O<sub>2</sub>) and hard carbon and report that the power of NIB at −30
°C is ∼21% higher than that of LIB. Moreover, the capacity
retention in cycle testing at 0 °C is ∼53% for NIB but
only ∼29% for LIB. Raman spectroscopy and density functional
theory calculations revealed that the superior performance of NIB
is due to the relatively weak interaction between Na<sup>+</sup> ions
and aprotic polar solvents