In
the near future, the targets for lithium-ion batteries concerning
specific energy and cost can advantageously be met by introducing
layered LiNi
x
Co
y
Mn
z
O2 (NCM) cathode
materials with a high Ni content (x ≥ 0.6).
Increasing the Ni content allows for the utilization of more lithium
at a given cell voltage, thereby improving the specific capacity but
at the expense of cycle life. Here, the capacity-fading mechanisms
of both typical low-Ni NCM (x = 0.33, NCM111) and
high-Ni NCM (x = 0.8, NCM811) cathodes are investigated
and compared from crystallographic and microstructural viewpoints.
In situ X-ray diffraction reveals that the unit cells undergo different
volumetric changes of around 1.2 and 5.1% for NCM111 and NCM811, respectively,
when cycled between 3.0 and 4.3 V vs Li/Li+. Volume changes
for NCM811 are largest for x(Li) < 0.5 because
of the severe decrease in interlayer lattice parameter c from 14.467(1) to 14.030(1) Å. In agreement, in situ light
microscopy reveals that delithiation leads to different volume contractions
of the secondary particles of (3.3 ± 2.4) and (7.8 ± 1.5)%
for NCM111 and NCM811, respectively. And postmortem cross-sectional
scanning electron microscopy analysis indicates more significant microcracking
in the case of NCM811. Overall, the results establish that the accelerated
aging of NCM811 is related to the disintegration of secondary particles
caused by intergranular fracture, which is driven by mechanical stress
at the interfaces between the primary crystallites.
Two major strategies are currently pursued to improve the energy density of lithium-ion batteries using LiNi x Co y Mn z O 2 (NCM) cathode materials. One is to increase the fraction of redox active Ni (≥80%), which allows larger amounts of Li to be extracted at a given cutoff voltage (U max ). The other is to increase U max , in particular for medium-Ni content NCM materials. However, the accompanying lattice changes ultimately lead to capacity fading in both cases. Here the structural changes occurring in Li 1.02 Ni x Co y Mn z O 2 (with x = 1 / 3 , 0.5, 0.6, 0.7, 0.8 and 0.85) during cycling operation in the voltage range between 3.0 and 4.6 V vs Li are quantified by means of operando X-ray diffraction combined with detailed Rietveld analysis. All samples show a large decrease in unit cell volume upon charging, ranging from 2.4% for NCM111 (33% Ni) to 8.0% for NCM851005 (85% Ni). To make a fair comparison of the structural stability of the different NCM materials, energy densities as a function of U max are estimated and correlated with X-ray diffraction results. It is shown that NCMs with a lower Ni content allow for specific energies similar to that of, e.g., Ni-rich NCM811 (80% Ni) when operated at sufficiently high U max , but still undergo less pronounced changes in structure. Nevertheless, as indicated by charge/discharge tests, the capacity retention of low-and medium-Ni content NCMs cycled to high U max is also strongly affected by factors other than stability of the layered crystal lattice (electrolyte decomposition etc.). Overall, it is demonstrated that the complexity of the degradation processes needs to be better understood to identify optimal cycling conditions for specific cathode compositions.
Ni-rich
LiNi
x
Co
y
Mn
z
O2 (NCM) cathode
materials have great potential for application in next-generation
lithium-ion batteries owing to their high specific capacity. However,
they are subjected to severe structural changes upon (de)lithiation,
which adversely affects the cycling stability. Herein, we investigate
changes in crystal and electronic structure of NCM811 (80% Ni) at
high states of charge by a combination of operando X-ray diffraction (XRD), operando hard X-ray absorption
spectroscopy (hXAS), ex situ soft X-ray absorption
spectroscopy (sXAS), and density functional theory (DFT) calculations
and correlate the results with data from galvanostatic cycling in
coin cells. XRD reveals a large decrease in unit cell volume from
101.38(1) to 94.26(2) Å3 due to collapse of the interlayer
spacing when x(Li) < 0.5 (decrease in c-axis from 14.469(1) Å at x(Li) =
0.6 to 13.732(2) Å at x(Li) = 0.25). hXAS shows
that the shrinkage of the transition metal–oxygen layer mainly
originates from nickel oxidation. sXAS, together with DFT-based Bader
charge analysis, indicates that the shrinkage of the interlayer, which
is occupied by lithium, is induced by charge transfer between O 2p
and partially filled Ni eg orbitals (resulting in decrease
of oxygen–oxygen repulsion). Overall, the results demonstrate
that high-voltage operation of NCM811 cathodes is inevitably accompanied
by charge-transfer-induced lattice collapse.
Printed thermoelectrics (TE) could significantly reduce the production cost of energy harvesting devices by large-scale manufacturing. However, developing a high-performance printable TE material is a substantial challenge. In this work,...
LiNi0.5Mn1.5O4 (LNMO) based spinel cathode materials for lithium-ion batteries are promising alternatives to the widely used mixed transition-metal layered Li(Ni,Co,Mn)O2 (NCM) oxides. LNMO is cobalt free and thus cost efficient,...
Commercially used
LiNi1/3Mn1/3Co1/3O2 (NMC111)
in lithium-ion batteries mainly consists of
a large-grained nonporous active material powder prepared by coprecipitation.
However, nanomaterials are known to have extreme influence on gravimetric
energy density and rate performance but are not used at the industrial
scale because of their reactivity, low tap density, and diminished
volumetric energy density. To overcome these problems, the build-up
of hierarchically structured active materials and electrodes consisting
of microsized secondary particles with a primary particle scale in
the nanometer range is preferable. In this paper, the preparation
and detailed characterization of porous hierarchically structured
active materials with two different median secondary particle sizes,
namely, 9 and 37 μm, and primary particle sizes in the range
300–1200 nm are presented. Electrochemical investigations by
means of rate performance tests show that hierarchically structured
electrodes provide higher specific capacities than conventional NMC111,
and the cell performance can be tuned by adjustment of processing
parameters. In particular, electrodes of coarse granules sintered
at 850 °C demonstrate more favorable transport parameters because
of electrode build-up, that is, the morphology of the system of active
material particles in the electrode, and demonstrate superior discharge
capacity. Moreover, electrodes of fine granules show an optimal electrochemical
performance using NMC powders sintered at 900 °C. For a better
understanding of these results, that is, of process-structure–property
relationships at both granule and electrode levels, 3D imaging is
performed with a subsequent statistical image analysis. Doing so,
geometrical microstructure characteristics such as constrictivity
quantifying the strength of bottleneck effects and descriptors for
the lengths of shortest transportation paths are computed, such as
the mean number of particles, which have to be passed, when going
from a particle through the active material to the aluminum foil.
The latter one is at lowest for coarse-grained electrodes and seems
to be a crucial quantity.
High-performance Ag−Se-based n-type printed thermoelectric (TE) materials suitable for room-temperature applications have been developed through a new and facile synthesis approach. A high magnitude of the Seebeck coefficient up to 220 μV K −1 and a TE power factor larger than 500 μW m −1 K −2 for an n-type printed film are achieved. A high figure-of-merit ZT ∼0.6 for a printed material has been found in the film with a low in-plane thermal conductivity κ F of ∼0.30 W m −1 K −1 . Using this material for n-type legs, a flexible folded TE generator (flexTEG) of 13 thermocouples has been fabricated. The open-circuit voltage of the flexTEG for temperature differences of ΔT = 30 and 110 K is found to be 71.1 and 181.4 mV, respectively. Consequently, very high maximum output power densities p max of 6.6 and 321 μW cm −2 are estimated for the temperature difference of ΔT = 30 K and ΔT = 110 K, respectively. The flexTEG has been demonstrated by wearing it on the lower wrist, which resulted in an output voltage of ∼72.2 mV for ΔT ≈ 30 K. Our results pave the way for widespread use in wearable devices.
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