Article
DOI: 10.1002/bkcs.10679
BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
J.-h. Park et al.
Stability of LiNi0.6Mn0.2Co0.2O2 as a Cathode Material for Lithium-Ion
Batteries against Air and Moisture
Jae-hoon Park, Joon-ki Park, and Jae-won Lee*
Department of Energy Engineering, Dankook University, Cheonan 330-714, Korea.
*E-mail: [email protected]
Received September 8, 2015, Accepted November 18, 2015, Published online February 14, 2016
LiNi0.6Co0.2Mn0.2O2 (C622), a Ni-rich cathode material, was prepared and its durability tested in air and moisture. For the test, C622 was stored in a glovebox and in a humidity chamber at 50% RH. For each sample, the
cell performance was tested in terms of the specific capacity, rate capability, and cycling stability; the changes
in the crystal structure were also investigated. The results indicate that when C622 is stored in a humidity chamber, severe degradation occurs in all measurements relating to the cell performance, whereas when it is stored in
a glovebox, no degradation is seen. This indicates that C622 is very sensitive to air and moisture. A larger quantity of residual lithium compounds, which are by-products of the reaction between the cathode material, CO2,
and moisture in the air, could be seen on the surface of the C622 that was stored in a humidity chamber when
compared to the one stored in a glovebox. The formation of residual lithium compounds and the corresponding
changes in the crystal structure of C622 kept in the humidity chamber may be the reason for the degradation in
the cell performance.
Keywords: Ni-rich cathode, Storage, Stability, Moisture, Air
Introduction
Today, most mobile IT devices, including smartphones and
laptop PCs, have adopted the use of lithium-ion batteries as
their power source because of their advantages, including high
energy density, long cycle life, and lack of a memory effect.
Much attention has been paid to next-generation batteries,
including lithium-air, lithium-sulfur, and sodium-ion batteries, but these still face numerous technical problems. Therefore, lithium-ion batteries remain the most advanced batteries
that are commercially available.
The need for cathode materials that have a higher energy
density has motivated extensive studies on Ni-rich cathode
materials.1–3 NMC materials (LiNixMnyCo1–x–yO2) with various molar ratios of Ni:Mn:Co are now being used as cathode
materials for lithium-ion batteries, and the Ni content has been
increased to meet the increasing requirements for a higher specific capacity.4
However, one of the greatest problems of using Ni-rich
NMC is that a high content of residual lithium compounds
(Li2CO3 and LiOH) can be seen on the surface of the particles,
and this may cause gelation in the electrode slurry, gas evolution, and swelling of batteries during cycling. The residual lithium compounds may be a result of the excessive LiOH or
Li2CO3 that has been added as a reactant, or they can be spontaneously generated from the reactions between Li+ in the
cathode material and carbonate or hydroxide ions from air
and moisture.4 LiNiO2-based materials have been reported
to have rapid CO2 and moisture uptake, leading to the formation of both Li2CO3 and LiOH impurities on the surface of the
oxide particles.5–8 Some researchers have tried to prevent the
Bull. Korean Chem. Soc. 2016, Vol. 37, 344–348
degradation in LiNiO2 by substituting cobalt for Ni.9 As discussed above, several studies have investigated the effects of
moisture and CO2 on LiNiO2, LiNi1/3Mn1/3Co1/3O2, and
LiNi0.8Co0.15Al0.05O2, but those on Ni-rich NMC materials
can be seldom found.
In this study, we stored Ni-rich NMC (LiNi0.6Mn0.2Co0.2O2, C622) in an atmosphere where the moisture
content and temperature were controlled for a certain
period, and we examined the variation in the electrochemical
performance and physical/chemical properties over that
time. C622 is a Ni-rich NMC material that is recently
being used in high-energy power sources. We believe that
the results of this study can offer guidance for storing Nirich NMCs.
Experimental
Ni-rich NMC (LiNi0.6Mn0.2Co0.2O2) was supplied by ENF
Technology Co., Ltd. (Asan, Korea) C622 was dried under
vacuum for 8 h before placing it in a humidity chamber controlled at 25 C and 50% RH, which is similar to that of the
actual atmosphere. C622 was stored in the chamber for
7, 15 and 30 days and was also stored in a glovebox for
same period to determine the differences. The C622 samples
that were stored in the humidity chamber and in the glovebox
for x number of days are denoted as C622-C-x and C622-G-x,
respectively.
X-ray diffraction (XRD) analyses were carried out on the
samples using an X-ray diffractometer (Rigaku, ULTIMA
VI, Tokyo, Japan) over the 2θ range of 10 –80 with monochromatized Cu Kα radiation. The relative intensities of
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
344
Article
ISSN (Print) 0253-2964
| (Online) 1229-5949
(003)/(104) and [(006) + (102)]/(101) reflections were calculated to estimate the degree of cation disorder and the hexagonal ordering of the sample.
The amount of residual lithium compounds that was present
on the surface of the cathode particles was measured using
Warder’s method. The cathode material powder 10 g was
placed in 100 mL of DI (deionized) water and was stirred vigorously. After stirring for 10 min, the cathode material powder
was separated by vacuum filtration, and the filtrate was titrated
with 1.0 N HCl solution. Phenophthalein and methyl orange
were used as indicators for the titration.
To measure the electrochemical performance, 2016 cointype half-cells were assembled in an Ar-filled glovebox with
a moisture content that was controlled below 1.0 ppm. The
lithium metal was used as a counter electrode, and the cathode
was fabricated by casting a slurry with a formulation of 90 wt
% active material, 5 wt % acetylene black, and 5 wt %
poly(vinylidene difluoride) (PVdF) binder on an aluminum
foil. The electrolyte consisted of a 1.0 M LiPF6 solution in
3:7 (vol %) ethylene carbonate (EC)/diethyl carbonate
(DEC). The specific capacity was measured at 0.1 C-rate,
and a rate capability test was conducted at various current densities (0.1, 1.0, 3.0, 5.0 and 7.0 C-rate). The cells were cycled
for 100 cycles at 1.0 C-rate to examine the cycling stability.
Results and Discussion
Figure 1 shows the XRD patterns obtained for the cathode
materials stored in the glovebox and in the humidity chamber
Figure 1. XRD pattern for fresh C622 stored (a) in a humidity chamber and (b) in a glovebox.
Bull. Korean Chem. Soc. 2016, Vol. 37, 344–348
BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
for 7 and 15 days. The (003) and (104) peak positions shifted to
a higher angle after storage in the humidity chamber while no
changes in the peak position were observed for the sample
stored in a glovebox. This means that the crystal structure
changed after storage in the humidity chamber. CO2 and
H2O in air have been reported to be likely to be absorbed on
the LiNiO2-based cathode material as a result of the spontaneous reduction of Ni3+ to Ni2+ ion.10 Subsequently, Li+ forms
Li2CO3 and LiOH on the surface of the cathode particles,
reacting with the CO2 and H2O molecules and leaving the
Li site vacant. Subsequently, the vacant site may accommodate nickel ions from the transition-metal layer, resulting in
cation mixing. According to Choi et al., the lowering of c/a
is an indicator of cation mixing,11 and the results of our
XRD analysis showed a slight decrease in the c/a value as
the storage time in the humidity chamber increased.
In order to investigate the variation in the degree of cation
mixing after storage, the relative intensities I(003)/I(104) (=
Rw) and I(006) + I(102)/I(101) (= Rf) were plotted against
the storage time. Rw and Rf have been reported to represent
the degree of cation mixing and hexagonal ordering, respectively.12,13 The correlation between these factors and the storage time in the glovebox and in the humidity chamber is shown
in Figure 2. The Rw value of C622-C-x decreases as the storage
time increases, while that of C622-G-x does not show any discernable variation in the storage time, which indicates that the
degree of cation mixing for C622 increases upon exposure to
moisture and air. In the meantime, the storage of the cathode
material in a humidity chamber appears to degrade the hexagonal ordering, considering that the Rf of C622-C-x increased
from 0.44 to 0.46 after 15 days while that of C622-G-x
remained almost constant. These observations indicate that
the cell performance of the cathode stored in the humidity
chamber may be degraded as the storage time increases
because the cell performance is highly related to the crystal
structure.
The amounts of the residual lithium compounds (Li2CO3,
LiOH) on the surface of the cathode materials were estimated
using Warder’s method, and the results are shown in Figure 3.
Figure 2. Relative intensities I(003)/I(104) (= Rw) and I(006) + I
(102)/I(101) (= Rf) of C622-C-x and C622-G-x.
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Stability of LiNi0.6Mn0.2Co0.2O2 against Air and Moisture
BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
Figure 3. Variations of the amount of residual lithium compounds in
C622-C-x and C622-G-x with the storage time.
The LiOH and Li2CO3 concentrations of the fresh C622 were
measured to be approximately 2000 and 3000 ppm, respectively. After storage in the humidity chamber for 15 days,
the concentration of Li2CO3 increased to 15 000 ppm while
that of LiOH decreased to almost 0 ppm. Li2CO3 and LiOH
are the products of the reaction of lithium ions in C622 with
CO2 and water molecules, respectively. The data suggests that
the formation reaction for Li2CO3 seems to be much more
likely to occur than that for LiOH when C622 is exposed to
air. The reason for this could be that Li2CO3 forms more easily
from LiOH and CO2 in air at room temperature, as expected
from the Gibbs free energy of the following reaction:
2 LiOH + CO2 ! Li2 CO3 + H2 O,
ΔG0 = − 97 kJ=mol Li2 CO3
ð1Þ
LiOH is widely used as a carbon dioxide absorbent.
The amount of residual lithium compounds on the surface
should be minimized, since it may cause swelling of the battery during charge–discharge cycling and gelation of the slurry
during the manufacturing process of the electrode.14
The specific discharge capacities of the samples were measured, which are displayed in Figure 4. Fresh C622 exhibited a
discharge capacity of 189 mAh/g, which decreased to 184 and
179 mAh/g after 7 and 15 days, respectively. In contrast, a
much smaller drop in capacity could be observed with
C622-G-x. The irreversible formation of the residual lithium
compounds would consume the lithium ions in the lattice of
C622, thus reducing the amount of lithium ions available to
participate in the intercalation and de-intercalation, which is
associated with the capacity fading in the C622-C-x samples
along with an alteration in the crystal structure of the cathode
material.
The rate capability of the samples was measured under various current densities (0.1, 1, 3, 5, 7 C-rate) in order to examine
the effect of the storage atmosphere on the kinetics of the intercalation/de-intercalation processes of the lithium ions
(Figure 5). As can be seen in the figure, C622-C-7 and -15
show drastic capacity fading with increase in the current
Bull. Korean Chem. Soc. 2016, Vol. 37, 344–348
Figure 4. Initial specific discharge capacities of (a) C622-C-x and
(b) C622-G-x.
Figure 5. Capacity retention rates of (a) C622-C-x and (b) C622-G-x
at various current densities.
density, while C622-G-x did not show any significant decrease
in capacity even at high C-rates. At the 7 C-rate, fresh C622
retained approximately 82% of the capacity at 0.1 C, but only
8–13% of the capacity could be obtained at the same current
density after storage in the humidity chamber. Moreover,
C622-C-7 and -15 showed an almost similar degradation in
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Stability of LiNi0.6Mn0.2Co0.2O2 against Air and Moisture
the rate capability, which indicates that C622 could deteriorate
in the early stages of exposure to air and moisture. The severe
degradation in the rate capability of C622-C-x may be interpreted as due to two reasons: a high resistance due to the formation of a layer of residual lithium compounds, and a high
degree of cation mixing of C622-C-x. The layer of residual
lithium compounds may act as an insulating layer for electron
and lithium-ion conduction, and Ni atoms located at the Li site
would also retard the motion of the lithium ions.15
The cycling stability of the cathode materials is shown in
Figure 6. The cells were charged and discharged at a 1 C-rate,
and the initial capacity for C622-C-7 and -15 was approximately 150 mAh/g, which is much lower than that of the fresh
sample due to the poor rate capability of these samples. C622C-7 and -15 showed a capacity retention of only 58% and 43%
after 100 cycles, respectively, while the fresh sample retained
92% of the initial capacity. However, C622-G-7 and -15
BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
retained 92% and 93% of the initial capacity, which is almost
the same retention rate as the fresh one. Moreover, C622-G-x
showed initial capacities with a much smaller difference from
the fresh sample at the 1 C-rate. The change in the crystal structure and the formation of residual lithium compounds during
storage in a humidity chamber are believed to be the cause of
the poor cycling stability of the C622-C-x samples.
The results given above indicate that storage of C622 in air
should be avoided and that the storage conditions should be
strictly controlled. In particular, the concentration of CO2
should be controlled at a low level in order to prevent the formation of Li2CO3 on the surface of the cathode material.
Conclusion
C622 was stored in a humidity chamber at constant temperature and humidity for certain periods. The electrochemical performance of the samples was measured in order to examine the
stability of the material against moisture and air. For the control group, C622 was stored in a glovebox filled with argon
with strictly controlled moisture content. Storage in the
humidity chamber resulted in changes in the crystal structure,
including cation mixing and hexagonal ordering. This also
caused the formation of more residual lithium compounds
on the surface of the cathode particles. In particular, a large
amount of Li2CO3 was formed, while the quantity of LiOH
decreased. On the contrary, C622 stored in the glovebox did
not show any significant changes in the crystal structure and
in the amount of residual lithium compounds. As a result of
these changes, a severe degradation in the cell performance
in terms of the specific capacity, rate capability, and cycling
stability could be observed with C622 stored in the humidity
chamber compared to that stored in the glovebox, and this degradation was closely associated with a change in the crystal
structure. All of these results indicate that the Ni-rich cathode
materials for the lithium-ion batteries should be handled in a
controlled atmosphere of moisture and CO2.
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Figure 6. Cycling stability of (a) C622-C-x and (b) C622-G-x.
Bull. Korean Chem. Soc. 2016, Vol. 37, 344–348
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