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Suppression of irreversible capacity for Li1.16Ni0.37Mn0.47O2 due to the chemical treatment with (NH4)2SO4 in lithium ion batteries
Journal Pre-proof Suppression of irreversible capacity for Li1.16Ni0.37Mn0.47O2 due to the chemical treatment with (NH4)2SO4 in lithium ion batteries Hiroaki Konishi, Shohei Terada, Takefumi Okumura PII:
S1572-6657(19)30760-X
DOI:
https://doi.org/10.1016/j.jelechem.2019.113492
Reference:
JEAC 113492
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 7 April 2019 Revised Date:
28 August 2019
Accepted Date: 13 September 2019
Please cite this article as: H. Konishi, S. Terada, T. Okumura, Suppression of irreversible capacity for Li1.16Ni0.37Mn0.47O2 due to the chemical treatment with (NH4)2SO4 in lithium ion batteries, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/j.jelechem.2019.113492. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
Suppression of irreversible capacity for Li1.16Ni0.37Mn0.47O2 due to the chemical treatment with (NH4)2SO4 in lithium ion batteries
Hiroaki Konishi, Shohei Terada, Takefumi Okumura
Research & Development Group, Hitachi Ltd.
7-1-1 Omika-cho, Hitachi, Ibaraki 319-1292, Japan
Corresponding author Hiroaki Konishi
Research & Development Group, Hitachi Ltd.
7-1-1 Omika-cho, Hitachi, Ibaraki 319-1292, Japan
E-mail: [email protected]
Keywords: lithium ion battery; lithium-rich; chemical treatment; rate; potential shift; cathode
1
Abstract A lithium-rich layer-structured cathode material (Li1.2Ni0.2Mn0.6O2) is promising for lithium ion batteries due to its high practical capacity; however, it has several issues such as low rate
performance, potential shift during cycling, and large irreversible capacity. Previously, our group
reported that the change in the metal composition from Li1.2Ni0.2Mn0.6O2 to Li1.16Ni0.37Mn0.47O2 was effective in improving rate performance and suppressing potential shift; however, the large
irreversible capacity could not be addressed by the composition change. In this study, Li2O extraction, which occurs during initial charging and causes irreversible capacity, progressed using
(NH4)2SO4 before charging to reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2. The results obtained in the composition, morphology, specific surface area, and crystal structure analyses
indicate that the (NH4)2SO4 treatment can extract Li2O from the particle surface without changing the crystal structure. The results obtained in the electrochemical measurements indicate that the
irreversible capacity was reduced due to the chemical treatment. Moreover, the high rate and cycling
performance for the Li1.16Ni0.37Mn0.47O2 was maintained after the treatment. The treatment with (NH4)2SO4 can reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2 without harming this material’s advantages such as high rate and cycling performance.
1. Introduction
2
Lithium ion batteries (LIBs) exhibit higher energy density than other batteries such as lead
batteries and nickel hydrogen batteries [1, 2]; therefore, LIBs are used for many electronic devices
and vehicles. LiCoO2 and LiNixMnyCo1−x−yO2 are mainly used as cathode materials for existing LIBs. A variety of materials such as LiMn2O4, LiNi0.5Mn1.5O4, LiFePO4, LiMnPO4, nickel-rich LiNixMnyCo1−x−yO2, and Li1.2NiaMnbCo0.8−a−bO2 have been developed to meet future demand such as high energy and high safety for LIBs [3−17]. Li1.2Ni0.2Mn0.6O2 is one of the promising cathode materials for these batteries because it exhibits high capacity and high thermal stability [18].
However, Li1.2Ni0.2Mn0.6O2 has several issues to be resolved. The discharge capacity drastically decreases as the operating current increases [19]. The potential of charge and discharge reactions
shifts to a low potential as the charge-discharge cycling proceeds [19, 20]. In addition, large
irreversible capacity occurs between charging and discharging in the first cycle [19, 20]. Various
attempts have been made to resolve these issues [21−32]. The elemental substitution was reported to
be useful technique to improve the rate and potential shift during cycling [21−26]. The surface
coating was also the useful method to address these issues [27−32]. Our group has studied another approach and found that the rate performance and potential
shift during cycling were improved by changing the metal composition [33]. The increase in the
Ni/Mn ratio for the conventional lithium-rich cathode (Li1.2Ni0.2Mn0.6O2) improved the rate performance and suppressed the potential shift during cycling. However, it decreased the discharge
3
capacity at a low C-rate [33]. The Li/(Ni+Mn) ratio was controlled to increase the discharge capacity
for a high-Ni/Mn-ratio cathode (Li1.2Ni0.35Mn0.45O2). The decrease in the Li/(Ni+Mn) ratio from 1.2/0.8 (Li1.2Ni0.35Mn0.45O2) to 1.16/0.84 (Li1.16Ni0.37Mn0.47O2) was effective in increasing the discharge capacity without harming the high rate performance and small potential shift [34].
However, the composition change was not effective in reducing the irreversible capacity between
charging and discharging in the first cycle.
The cause of the large irreversible capacity for Li1.2NiaMnbCo0.8−a−bO2 was reportedly attributed to the irreversible Li2O extraction that occurred above 4.4 V during initial charging [35−37]. Subsequently, chemical treatments, which can extract Li2O before charging, have been found to be a useful technique to reduce the irreversible extraction of Li2O during charging [38−40]. Yu et al. reported that, due to the chemical treatment with (NH4)2SO4, Li2O can be extracted from the cathode according to the formula ((NH4)2SO4 + 2Li + 1/2O2 → Li2SO4 + 2NH3 + H2O (i)). The Li2O extraction before charging was effective in reducing irreversible capacity [38]. Zheng et al. reported that Li[Li0.2Mn0.54Ni0.13Co0.13]O2 treated with Na2S2O8 exhibited low irreversible capacity [39]. Xu et al. found that the irreversible capacity of a cathode was reduced after a mild acid
treatment [40]. Previously, our group prepared Li1.2Ni0.2Mn0.6O2 using metal acetates as raw material and found that the irreversible capacity of Li1.2Ni0.2Mn0.6O2 was reduced after a chemical treatment with (NH4)2SO4 [41]. Our group has changed raw material from metal acetates to metal carbonates
4
to reduce the particle size of the cathode, and Li1.2Ni0.2Mn0.6O2 and Li1.16Ni0.37Mn0.47O2, which both exhibit high capacity, were obtained [33, 34].
In
this
study,
a
chemical
treatment
with
(NH4)2SO4
was
conducted
with
Li1.16Ni0.37Mn0.47O2 to obtain cathode which exhibits high rate performance, small potential shift, and low irreversible capacity. Li1.2Ni0.2Mn0.6O2 treated with (NH4)2SO4 was also prepared as a reference, and the effects of the chemical treatment on the electrochemical properties for both
materials were investigated.
2.
Experimental
2.1. Preparation of active materials
Li2CO3, NiCO3･2Ni(OH)2･4H2O, MnCO3 were mixed in proportion of 1.2: 0.2: 0.6 or 1.16: 0.37: 0.47 (Li: Ni: Mn (mol)). The resulting mixture was pre-annealed at 500°C for 12 hr in air
and then re-annealed at 1000°C (Li1.2Ni0.2Mn0.6O2) or 950°C (Li1.16Ni0.37Mn0.47O2) for 12 hr in air, and Li1.2Ni0.2Mn0.6O2 and Li1.16Ni0.37Mn0.47O2 were obtained. Then, 2.5, 5, or 10 wt% (NH4)2SO4 was mixed with Li1.2Ni0.2Mn0.6O2 or Li1.16Ni0.37Mn0.47O2 in distilled water at 120°C until the water was evaporated. After that, the residual materials were annealed at 300°C for 6 hr in air and washed
with distilled water. Next, the residue materials were dried at 120°C for 6 hr in vacuum to remove
residual water. The samples prepared using 0, 2.5, 5, and 10 wt% (NH4)2SO4 for conventional
5
Li1.2Ni0.2Mn0.6O2 and high-Ni/Mn-ratio Li1.16Ni0.37Mn0.47O2 cathodes were classified as C-N0, C-N2.5, C-N5, C-N10, H-N0, H-N2.5, H-N5, and H-N10, respectively.
2.2. Evaluation of metal composition, morphology, specific surface area, crystal structure, and
electrochemical properties for prepared materials
Inductively coupled plasma atomic emission spectroscopy (ICP-AES; PerkinElmer,
OPTIMA8300) was used to measure the metal composition of the prepared materials. Scanning
electron microscope (SEM; Hitachi 4300) was used to observe morphology of the prepared materials.
Gas adsorption method based on the Brunauer–Emmett–Teller theory (BET theory; MircotracBEL
BELSORP-6) was used to evaluate the specific surface area (SSA) of the prepared materials. X-ray
diffraction (XRD; Rigaku Rint-2200) with Cu Kα radiation was used to evaluate their crystal
structure. Three-electrode electrochemical cells were used to evaluate the electrochemical properties
for the prepared materials. The cathode was comprised of active material (85 wt%), acetylene black
(10 wt%), and binder (5 wt%). Lithium metal was used as the anode and reference electrode. A
mixture of ethylene carbonate (20 vol%), ethylmethyl carbonate (40 vol%), and dimethyl carbonate (40 vol%) with dissolved LiPF6 (1 mol dm−3) was used as an electrolyte. The charge and discharge measurements were conducted under two potential ranges (2.5−4.6 and 2.5−4.4 V (vs. Li/Li+)). The charge was conducted using constant current (CC) and constant voltage mode (operating current
6
condition: 0.05 C (1 C: 260 A kg−1) and cut-off current condition: 0.005 C). The discharge was conducted using CC mode (operating current condition: 0.05, 0.2, and 1 C). The open circuit
potential (OCP), which was defined as the potential 5 hr after charging and discharging were stopped,
during the first charging was measured for each cathode. The cycling test was conducted as follows.
The electrochemical cells were cycled at 0.05 C two times, at 1 C hundred times, and then at 0.05 C
one time.
3. Results and discussions 3.1. Effects of treatment with (NH4)2SO4 on the physical properties Effects of chemical treatment on the physical properties such as composition, morphology,
SSA, and crystal structure were evaluated. The metal composition of non-treated and treated
materials was measured using ICP-AES to determine the progress of Li2O extraction (shown in formula (i)) with (NH4)2SO4 treatment for Li1.16Ni0.37Mn0.47O2 and Li1.2Ni0.2Mn0.6O2. Fig. 1 summarizes the Li/(Ni+Mn) ratio for each material. The Li/(Ni+Mn) ratio decreased as the used
(NH4)2SO4
content
increased
for
Li1.16Ni0.37Mn0.47O2.
The
same
change
as
that
of
Li1.16Ni0.37Mn0.47O2 was observed for Li1.2Ni0.2Mn0.6O2. These results indicate that the progress of the Li2O extraction was affected by the used (NH4)2SO4 content. The effects of treatment with (NH4)2SO4 on the morphology were evaluated using SEM.
7
Fig. 2 shows the particles of H-N0, H-N10, C-N0, and C-N10. The surface of H-N10 (Fig. 2(b))
particle was more coarse than that of H-N0 particle (Fig. 2(a)). The surface of C-N0 particle was
roughened due to treatment with (NH4)2SO4. (Fig. 2(c) and (d)). To evaluate the effects of chemical treatment on the surface change further, SSA of the prepared materials was measured. Fig. 3
summarizes the SSA of prepared materials. For Li1.16Ni0.37Mn0.47O2, the SSA increased as the used (NH4)2SO4 content increased. The same change as that of Li1.16Ni0.37Mn0.47O2 was confirmed for Li1.2Ni0.2Mn0.6O2. The changes in the morphology and SSA due to chemical treatment indicate that the Li2O was extracted from the cathode surface [38, 41]. The effects of treatment with (NH4)2SO4 on the crystal structure were investigated using XRD. Fig. 4 shows the XRD patterns of the prepared materials. The main diffraction peaks for the H-N0 were assigned to the hexagonal α-NaFeO2 structure (space group: R-3m). The small diffraction peaks at 2θ = 20−22° were attributed to the superlattice ordering of lithium and manganese in the transition metal layer [42]. The diffraction patterns of H-N2.5, H-N5, and H-N10
were the same as that of H-N0. The diffraction patterns were not changed due to the treatment with (NH4)2SO4 for the Li1.2Ni0.2Mn0.6O2 (insert in Fig. 4), These results indicate that the α-NaFeO2 structure was maintained after the chemical treatment.
The results obtained using the ICP-AES, SEM, BET, and XRD indicate the treatment with
(NH4)2SO4 can extract Li2O from the cathode surface without changing the crystal structure,
8
regardless of the metal composition for based materials.
3.2. Effects of treatment with (NH4)2SO4 on the electrochemical performance The effects of treatment with (NH4)2SO4 on the electrochemical performance for each material were investigated. Fig. 5 shows the charge and discharge curves in the first and second
cycles for each cathode. In the first cycle for H-N0 (Fig. 5(a)), the charge potential increased up to
4.4 V, and then a plateau was observed. During discharging, the potential smoothly decreased to 3.6
V and then rapidly decreased to 2.5 V. From the first (Fig. 5(a)) to the second (Fig. 5(b)) cycle, the
plateau during charging (> 4.4 V) vanished, but the discharge curves changed slightly. Although
there was large irreversible capacity in the first cycle (Fig. 5(a)), there was little irreversible capacity
in the second cycle (Fig. 5(b)). For the treated samples (H-N2.5, H-N5, and H-N10), irreversible
capacity was observed in the first cycle (Fig. 5(a)), but little irreversible capacity was observed in the
second cycle (Fig. 5(b)). In contrast, for the treated sample, the following three changes were
observed in the charge and discharge curves. (i) The charge potential of the sloping region (< 4.4 V)
in the first cycle increased (Fig. 5(a)). Conversely, no difference was observed in the charge curve
between non-treated and treated samples in the second cycle (Fig. 5(b)). (ii) The capacity ratio
between sloping (< 4.4 V) and plateau (> 4.4 V) regions in the first cycle was changed (Fig. 5(a)).
(iii) A new discharge reaction occurred around 2.9 V in both the first and second cycles (Fig 5(a) and
9
(b)). The aforementioned three changes in the charge and discharge curves due to the treatment with
(NH4)2SO4 were observed for Li1.2Ni0.2Mn0.6O2 (insert in Fig 5(a) and (b)), indicating that these changes occurred regardless of the metal composition for based materials.
The cause of the three changes after chemical treatment was investigated. First, the factor
of potential increase in the sloping region (< 4.4 V) in the first cycle was investigated. We expected
that the lithium ions were partially extracted in the present state for the cathode treated with
(NH4)2SO4; therefore, higher polarization might be required to extract lithium ions with the treated cathode than with the non-treated one. The OCPs for each cathode were measured to determine
whether or not the increase in the charge potential (Fig. 5(a)) could be attributed to the polarization.
Fig. 6 shows the OCPs during charging in the first cycle for each cathode. The OCPs in the initial
phase of the charge reaction for H-N2.5, H-N5, and H-N10 were coincident with that of H-N0.
Moreover, the OCP for the treated cathodes (C-N2.5, C-N5, and C-N10) overlapped with that of the
non-treated cathode (C-N0) (insert in Fig. 6). These results indicate that the increase in the potential
of the sloping region (< 4.4 V) with the chemical treatment could be attributed to the polarization. In
the second cycle, no difference was observed in the charge potential between the non-treated and
treated samples (Fig. 5(b)). This indicates that once lithium ions were compensated during
discharging, the charge reaction for the treated samples proceeded the same potential as that for the
non-treated sample. Thus, large polarization was not observed for the treated samples in the second
10
cycle.
Second, the cause of the change in the charge capacity ratio between sloping (< 4.4 V) and
plateau (> 4.4 V) regions was investigated. It has been reported that the oxidation of nickel
contributes to the charge reaction in the sloping region (< 4.4 V). In contrast, the oxidation of
oxygen and Li2O extraction occurs in the plateau region (> 4.4 V) [43−45]. The charge capacity below and above 4.4 V for each cathode was summarized to evaluate the effect of chemical
treatment on the charge reaction in the sloping and plateau regions, respectively. Fig. 7 plots the
charge capacities over the potential ranges of 2.5−4.4, 4.4−4.6, and 2.5−4.6 V for each cathode,
respectively. The charge capacities over the potential range of 2.5−4.4 V for H-N2.5, H-N5, and H-N10 were almost the same as that for H-N0. In contrast, as the used (NH4)2SO4 content increased, the charge capacity over the potential range of 4.4−4.6 V decreased, and then the total charge
capacity (2.5−4.6 V) also decreased. A similar change was observed for Li1.2Ni0.2Mn0.6O2 (C-N0, C-N2.5, C-N5, and C-N10). These results indicate that the chemical treatment slightly affected the
charge reaction contributed to the redox of nickel (sloping region; < 4.4 V), but it reduced the charge
reaction that contributed to the Li2O extraction (plateau region; > 4.4 V), which is the cause of irreversible capacity.
Third, the trigger of a new discharge reaction around 2.9 V was investigated. It has been
reported that the redox of transition metal and oxygen contribute to the charge and discharge
11
reactions for Li1.2NiaMnbCo0.8−a−bO2; therefore, high capacity was obtained [43−45]. Manganese and oxygen were electrochemically activated due to the Li2O extraction occurring in the plateau region (> 4.4 V) [43−45]. In other words, only redox of nickel contributed to the charge and discharge reaction without Li2O extraction, and high capacity was not obtained below 4.4 V. Previously, our group investigated the effects of charge cut-off potential on the discharge capacity for
Li1.2Ni0.2Mn0.6O2 [33]. When Li1.2Ni0.2Mn0.6O2 was charged and discharged below the plateau (2.5−4.4 V), the discharge reaction mainly progressed above 3.6 V, and the discharge capacity was low (< 100 Ah kg−1). In contrast, when Li1.2Ni0.2Mn0.6O2 was charged above the plateau region (2.5−4.6 V), the discharge capacity below 3.6 V greatly increased, and high discharge capacity (> 250 Ah kg−1) was obtained. This result supports the finding that discharge reaction below 3.6 V occurred after the Li2O extraction. We expected that the discharge reaction below 3.6 V might proceed without charging
above the plateau region (> 4.4 V) because the treatment with (NH4)2SO4 can extract Li2O before charging. Fig. 8 shows the charge and discharge curves over a potential range of 2.5−4.4 V in the first and second cycle for each cathode. For the H-N0 (Fig. 8(a)), the discharge reaction mainly
progressed above 3.6 V. From H-N0 to H-N2.5, a new discharge reaction appeared below 3.6 V, and
the discharge capacity below 3.6 V increased as the treatment content increased. In the second cycle
for H-N2.5, H-N5, and H-N10 (Fig. 8(b)), a new charge reaction occurred below 3.6 V. For
12
Li1.2Ni0.2Mn0.6O2 (insert in Fig. 8(a) and (b)), the treatment causes the same phenomena observed for Li1.16Ni0.37Mn0.47O2. Regardless of the metal composition for based materials, a new discharge reaction below 3.6 V was observed without charging above the plateau region (> 4.4 V).
When the charge cut-off potential was limited to 4.4 V, no discharge reaction below 3.6 V
was observed for the non-treated samples (H-N0 and C-N0) (Fig. 8(a) and (b)). In contrast, a
discharge reaction below 3.6 V was observed (Fig. 5(a) and (b)) when the non-treated samples were
charged up to 4.6 V. However, no discharge reaction around 2.9 V was observed for the non-treated
samples (H-N0 and C-N0), regardless of the charge cut-off potential (4.4 and 4.6 V) (Figs. 5 and 8).
This means that the discharge reaction around 2.9 V was characteristic for the chemical Li2O extraction. The occurrence of a new discharge reaction due to the oxygen deficiency was reported
for LiNi0.5Mn1.5O4 [46, 47]. The occurrence of the new discharge reaction in this compound was related to the redox of manganese that was activated due to the oxygen deficiency [46, 47]. For
Li1.16Ni0.37Mn0.47O2 and Li1.2Ni0.2Mn0.6O2, the new discharge reaction around 2.9 V (Figs. 5 and 8) was observed below the OCP in the pristine state (3.0−3.4 V) (Fig. 6). If a similar phenomenon observed for oxygen-deficient LiNi0.5Mn1.5O4 was occurred for Li1.16Ni0.37Mn0.47O2 and Li1.2Ni0.2Mn0.6O2 due to chemical Li2O extraction, the new discharge reaction around 2.9 V was observed without charging. Prepared cathodes were discharged without charging to evaluate whether
or not the discharge reaction around 2.9 V occurred due to treatment with (NH4)2SO4. Fig. 9 shows
13
the discharge curves for each cathode. For H-N0, little discharge reaction was observed. In contrast,
the discharge reaction around 2.9 V appeared for H-N2.5, and the discharge capacity increased as the
treatment content increased. The same changes were observed for the Li1.2Ni0.2Mn0.6O2 (insert in Fig. 9).
These results indicate that the chemical Li2O extraction with (NH4)2SO4 was effective in activating a redox of manganese and oxygen without electrochemical Li2O extraction. In addition, the chemical Li2O extraction causes the discharge reaction around 2.9 V, which does not occur due to the electrochemical Li2O extraction.
3.3. Effects of treatment with (NH4)2SO4 on the rate and cycling performance Finally, the effects of treatment on the rate and cycling performance were investigated. The
rate and cycling performance for C-N0, C-N5, H-N0, and H-N5 was evaluated, and the effects of
metal composition and chemical treatment for each cathode on the electrochemical performance
were discussed.
Fig. 10 shows the discharge curves at 0.2 and 1 C for each cathode. As the operating C-rate
increased in C-N0 (Fig. 10(a)), the discharge capacity greatly decreased, and the reaction potential
also decreased. Although the discharge capacities for C-N5 at 0.2 and 1 C were higher than that for
C-N0, a decrease in the discharge capacity and potential due to increasing the C-rate was observed
14
for C-N5.
The decrease in the discharge capacity and potential due to the change in the operating
C-rate from 0.2 to 1 C for H-N0 (Fig. 10(b)) was smaller than that for C-N0 (Fig. 10(a)). The
discharge capacity and potential at 0.2 and 1 C for H-N5 were similar to those for H-N0. These
results indicate that the Li1.16Ni0.37Mn0.47O2 exhibited higher rate performance than Li1.2Ni0.2Mn0.6O2, and the high rate performance for Li1.16Ni0.37Mn0.47O2 was maintained after the treatment with (NH4)2SO4. Next, the effects of treatment with (NH4)2SO4 on the cycling performance were investigated. Fig. 11 shows the discharge and dQ/dV curves for C-N0, C-N5, H-N0, and H-N5
before and after hundred cycles. For all samples, the shapes of discharge curves changed after
hundred cycles (insert in Fig. 11(a) and (b)). The dQ/dV curves for each material were compared to
evaluate the detailed shape changes in the discharge curves. For C-N0, the peak area above 3.6 V
decreased due to cycling, and that below 3.6 V shifted to the lower potential side (Fig. 11(a)). From
C-N0 to C-N5, the peak area above 3.6 V before cycling changed slightly, but that below 3.6 V
increased. The changes in the dQ/dV curve due to cycling for C-N5 below and above 3.6 V showed
similar changes to those observed for C-N0 (Fig. 11(a)). For H-N0 and H-N5, the peak area above
3.6 V decreased due to cycling, and that below 3.6 V shifted to the lower potential side (Fig. 11(b)).
However, the changes in the dQ/dV curves due to cycling for H-N0 and H-N5 were much smaller
15
than those for C-N0 and C-N5.
These results indicate that the potential shift during cycling was suppressed due to the
composition change from Li1.2Ni0.2Mn0.6O2 to Li1.16Ni0.37Mn0.47O2, and this effect was maintained after the treatment with (NH4)2SO4. Migration of reduced manganese ions reportedly causes the potential shift during cycling [48]. For C-N0, the discharge reaction below 3.6 V increased due to
chemical treatment (Fig. 11(a)), indicating that the reduction of manganese ions during discharging
proceeded due to the treatment with (NH4)2SO4. The increase in the reduced manganese ions with the chemical treatment caused the potential to shift further. In contrast, the discharge capacity below
3.6 V for H-N0 was much smaller than that for C-N0 (Fig. 5). Furthermore, the discharge capacity
below 3.6 V for H-N0 changed slightly after chemical treatment (Fig. 5). For H-N0 and H-N5, the
contribution of manganese redox to the discharge reaction was small; therefore, the potential shift
during cycling for H-N0 and H-N5 was small.
We conclude that the irreversible capacity of Li1.16Ni0.37Mn0.47O2 was reduced due to the treatment with (NH4)2SO4 without losing characteristics such as high rate and cycling performance. Therefore, due to the treatment with (NH4)2SO4, Li1.16Ni0.37Mn0.47O2 exhibits high rate, high cycling, and low irreversible capacity.
4. Conclusion
16
Li2O was extracted using (NH4)2SO4 before charging to reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2. The ICP-AES, SEM, BET and XRD results indicate that the Li2O was extracted from the cathode surface without changing the crystal structure. The charge and discharge
measurement results indicate that the chemical Li2O extraction was effective in suppressing the irreversible Li2O extraction that proceeded during the first charging, and then the irreversible capacity was reduced. These effects were observed in not only Li1.16Ni0.37Mn0.47O2 but also conventional Li1.2Ni0.2Mn0.6O2 composition, indicating that treatments with (NH4)2SO4 can be useful for lithium-rich layer-structured cathodes comprising different Li/(Ni+Mn) and Ni/Mn ratios.
Furthermore, the effects of treatment on the rate and cycling performance were evaluated.
High rate performance and small potential shift during cycling for Li1.16Ni0.37Mn0.47O2 were maintained after the treatment with (NH4)2SO4. These results indicate that the chemical treatment with (NH4)2SO4 was effective to reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2 without harming this material’s advantages.
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Figure captions Fig. 1.
Li/(Ni+Mn) ratio for each cathode.
21
Fig. 2.
SEM images of (a) H-N0, (b) H-N10, (c) C-N0, and (d) C-N10.
Fig. 3.
SSA of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10.
Fig. 4.
XRD patterns of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10.
Fig. 5.
Charge and discharge curves of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and
C-N10 over a potential range of 2.5−4.6 V in the (a) first and (b) second cycles.
Fig. 6.
Open circuit potentials of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10
during charging in the first cycle.
Fig. 7.
Summary of charge capacities for H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and
C-N10 over potential ranges of 2.5−4.4, 4.4−4.6, and 2.5−4.6 V in the first cycle.
Fig. 8.
Charge and discharge curves of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and
C-N10 over a potential range of 2.5−4.4 V in the (a) first and (b) second cycles.
22
Fig. 9.
Fig. 10.
Discharge curves of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10.
Discharge curves of (a) C-N0 and C-N5 and (b) H-N0 and H-N5 at 0.2 and 1 C.
Fig. 11. Discharge and dQ/dV curves of (a) C-N0, (b) C-N5, (c) H-N0, and (d) H-N5 over a
potential range of 2.5−4.6 V before and after hundred cycles.
23
1.5
Li/(Ni+Mn)
1.4
1.3
Li1.2Ni0.2Mn0.6O2 Li1.16Ni0.37Mn0.47O2
1.2
1.1
0
2
4 6 8 (NH4)2SO4 (wt%)
10
(a)
+
Potential (V (vs. Li/Li ))
4.5
4.0
3.5
3.0
C-N0 0.2 C C-N5 0.2 C
1C 1C
2.5 0
50
100
150
200
250
-1
Capacity (Ah kg ) (b)
+
Potential (V (vs. Li/Li ))
4.5
4.0
3.5
3.0
H-N0 0.2 C H-N5 0.2 C
1C 1C
2.5 0
50
100
150
200 -1
Capacity (Ah kg )
250
0
C-N0 before C-N5 before
after
-1
-1
dQ/dV (Ah kg V )
-100
(a) after
+
Potential (V (vs. Li/Li ))
-200
-300
-400 2.5
3.0
3.5
4.5 4.0 3.5 3.0 2.5 0
100 200 300 -1 Capacity (Ah kg )
4.0
4.5
+
Potential (V (vs. Li/Li ))
0
(b)
+
-200
H-N0 before H-N5 before Potential (V (vs. Li/Li ))
-1
-1
dQ/dV (Ah kg V )
-100
-300
-400
after after
4.5 4.0 3.5 3.0 2.5 0
2.5
100 200 300 -1 Capacity (Ah kg )
3.0
3.5
4.0 +
Potential (V (vs. Li/Li ))
4.5
4
2
-1
SSA (m g )
5
Li1.2Ni0.2Mn0.6O2 Li1.16Ni0.37Mn0.47O2
3
2 0
2
4 6 8 (NH4)2SO4 (wt%)
10
20 30 70
40 50 2q (°) (018) (110) (113)
60
(104) (015)
40 50 2q (°)
(107)
30 (006) (012)
20 (101)
(003)
Intensity (a.u.) Intensity (a.u.)
C-N10
C-N5
C-N2.5
C-N0
H-N10
H-N5
H-N2.5
H-N0
60 70
(a)
+
Potential (V (vs. Li/Li ))
4.5
3.0
+
3.5
Potential (V (vs. Li/Li ))
4.0
H-N0 H-N2.5 H-N5 H-N10 4.5 4.0 3.5 3.0 2.5 0
2.5 0
50
C-N0 C-N2.5 C-N5 C-N10
100 200 300 -1 Capacity (Ah kg )
100
150
200
250
300
-1
Capacity (Ah kg ) (b)
+
Potential (V (vs. Li/Li ))
4.5
3.0
+
3.5
Potential (V (vs. Li/Li ))
4.0
H-N0 H-N2.5 H-N5 H-N10 4.5 4.0 3.5 3.0 2.5 0
2.5 0
50
C-N0 C-N2.5 C-N5 C-N10
100 200 300 -1 Capacity (Ah kg )
100
150
200 -1
Capacity (Ah kg )
250
300
+
4.0
+
Potential (V (vs. Li/Li ))
Potential (V (vs. Li/Li ))
4.5
H-N0 H-N2.5 H-N5 H-N10
3.5
3.0 0
100
4.5 4.0
C-N0 C-N2.5 C-N5 C-N10
3.5 3.0 0
100 200 300 -1 Capacity (Ah kg )
200
300 -1
Capacity (Ah kg )
-1
Charge capacity (Ah kg )
350 300 250 200 150 100 50 0
Li1.2Ni0.2Mn0.6O2 2.5–4.4 V 4.4–4.6 V Li1.16Ni0.37Mn0.47O2 2.5–4.4 V 4.4–4.6 V
0
2
2.5–4.6 V 2.5–4.6 V
4 6 8 (NH4)2SO4 (wt%)
10
4.5
H-N0 H-N2.5 H-N5 H-N10
4.0
3.5
+
Potential (V (vs. Li/Li ))
+
Potential (V (vs. Li/Li ))
(a)
3.0
4.5 4.0 3.5
C-N0 C-N2.5 C-N5 C-N10
3.0 2.5 0
2.5 0
40 80 120 -1 Capacity (Ah kg )
50
100
150
200
-1
Capacity (Ah kg )
4.5
H-N0 H-N2.5 H-N5 H-N10
4.0
3.0
2.5 0
+
3.5
Potential (V (vs. Li/Li ))
+
Potential (V (vs. Li/Li ))
(b)
4.5 4.0 3.5
C-N0 C-N2.5 C-N5 C-N10
3.0 2.5 0
40 80 120 -1 Capacity (Ah kg )
50
100
150 -1
Capacity (Ah kg )
200
3.2
+
Potential (V vs. Li/Li )
+
Potential (V vs. Li/Li )
3.4
3.0
3.4 C-N0 C-N2.5 C-N5 C-N10
3.2 3.0 2.8 2.6 2.4 0
5 10 15 20 -1 Capacity (Ah kg )
2.8 H-N0 H-N2.5 H-N5 H-N10
2.6
2.4 0
5
10
15 -1
Capacity (Ah kg )
20
• Li2O was extracted due to chemical treatment without changing crystal structure. • Irreversible capacity for Li1.16Ni0.37Mn0.47O2 was reduced due to chemical treatment. • Advantages for Li1.16Ni0.37Mn0.47O2 was maintained after chemical treatment.
S1572-6657(19)30760-X
DOI:
https://doi.org/10.1016/j.jelechem.2019.113492
Reference:
JEAC 113492
To appear in:
Journal of Electroanalytical Chemistry
Received Date: 7 April 2019 Revised Date:
28 August 2019
Accepted Date: 13 September 2019
Please cite this article as: H. Konishi, S. Terada, T. Okumura, Suppression of irreversible capacity for Li1.16Ni0.37Mn0.47O2 due to the chemical treatment with (NH4)2SO4 in lithium ion batteries, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/j.jelechem.2019.113492. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
Suppression of irreversible capacity for Li1.16Ni0.37Mn0.47O2 due to the chemical treatment with (NH4)2SO4 in lithium ion batteries
Hiroaki Konishi, Shohei Terada, Takefumi Okumura
Research & Development Group, Hitachi Ltd.
7-1-1 Omika-cho, Hitachi, Ibaraki 319-1292, Japan
Corresponding author Hiroaki Konishi
Research & Development Group, Hitachi Ltd.
7-1-1 Omika-cho, Hitachi, Ibaraki 319-1292, Japan
E-mail: [email protected]
Keywords: lithium ion battery; lithium-rich; chemical treatment; rate; potential shift; cathode
1
Abstract A lithium-rich layer-structured cathode material (Li1.2Ni0.2Mn0.6O2) is promising for lithium ion batteries due to its high practical capacity; however, it has several issues such as low rate
performance, potential shift during cycling, and large irreversible capacity. Previously, our group
reported that the change in the metal composition from Li1.2Ni0.2Mn0.6O2 to Li1.16Ni0.37Mn0.47O2 was effective in improving rate performance and suppressing potential shift; however, the large
irreversible capacity could not be addressed by the composition change. In this study, Li2O extraction, which occurs during initial charging and causes irreversible capacity, progressed using
(NH4)2SO4 before charging to reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2. The results obtained in the composition, morphology, specific surface area, and crystal structure analyses
indicate that the (NH4)2SO4 treatment can extract Li2O from the particle surface without changing the crystal structure. The results obtained in the electrochemical measurements indicate that the
irreversible capacity was reduced due to the chemical treatment. Moreover, the high rate and cycling
performance for the Li1.16Ni0.37Mn0.47O2 was maintained after the treatment. The treatment with (NH4)2SO4 can reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2 without harming this material’s advantages such as high rate and cycling performance.
1. Introduction
2
Lithium ion batteries (LIBs) exhibit higher energy density than other batteries such as lead
batteries and nickel hydrogen batteries [1, 2]; therefore, LIBs are used for many electronic devices
and vehicles. LiCoO2 and LiNixMnyCo1−x−yO2 are mainly used as cathode materials for existing LIBs. A variety of materials such as LiMn2O4, LiNi0.5Mn1.5O4, LiFePO4, LiMnPO4, nickel-rich LiNixMnyCo1−x−yO2, and Li1.2NiaMnbCo0.8−a−bO2 have been developed to meet future demand such as high energy and high safety for LIBs [3−17]. Li1.2Ni0.2Mn0.6O2 is one of the promising cathode materials for these batteries because it exhibits high capacity and high thermal stability [18].
However, Li1.2Ni0.2Mn0.6O2 has several issues to be resolved. The discharge capacity drastically decreases as the operating current increases [19]. The potential of charge and discharge reactions
shifts to a low potential as the charge-discharge cycling proceeds [19, 20]. In addition, large
irreversible capacity occurs between charging and discharging in the first cycle [19, 20]. Various
attempts have been made to resolve these issues [21−32]. The elemental substitution was reported to
be useful technique to improve the rate and potential shift during cycling [21−26]. The surface
coating was also the useful method to address these issues [27−32]. Our group has studied another approach and found that the rate performance and potential
shift during cycling were improved by changing the metal composition [33]. The increase in the
Ni/Mn ratio for the conventional lithium-rich cathode (Li1.2Ni0.2Mn0.6O2) improved the rate performance and suppressed the potential shift during cycling. However, it decreased the discharge
3
capacity at a low C-rate [33]. The Li/(Ni+Mn) ratio was controlled to increase the discharge capacity
for a high-Ni/Mn-ratio cathode (Li1.2Ni0.35Mn0.45O2). The decrease in the Li/(Ni+Mn) ratio from 1.2/0.8 (Li1.2Ni0.35Mn0.45O2) to 1.16/0.84 (Li1.16Ni0.37Mn0.47O2) was effective in increasing the discharge capacity without harming the high rate performance and small potential shift [34].
However, the composition change was not effective in reducing the irreversible capacity between
charging and discharging in the first cycle.
The cause of the large irreversible capacity for Li1.2NiaMnbCo0.8−a−bO2 was reportedly attributed to the irreversible Li2O extraction that occurred above 4.4 V during initial charging [35−37]. Subsequently, chemical treatments, which can extract Li2O before charging, have been found to be a useful technique to reduce the irreversible extraction of Li2O during charging [38−40]. Yu et al. reported that, due to the chemical treatment with (NH4)2SO4, Li2O can be extracted from the cathode according to the formula ((NH4)2SO4 + 2Li + 1/2O2 → Li2SO4 + 2NH3 + H2O (i)). The Li2O extraction before charging was effective in reducing irreversible capacity [38]. Zheng et al. reported that Li[Li0.2Mn0.54Ni0.13Co0.13]O2 treated with Na2S2O8 exhibited low irreversible capacity [39]. Xu et al. found that the irreversible capacity of a cathode was reduced after a mild acid
treatment [40]. Previously, our group prepared Li1.2Ni0.2Mn0.6O2 using metal acetates as raw material and found that the irreversible capacity of Li1.2Ni0.2Mn0.6O2 was reduced after a chemical treatment with (NH4)2SO4 [41]. Our group has changed raw material from metal acetates to metal carbonates
4
to reduce the particle size of the cathode, and Li1.2Ni0.2Mn0.6O2 and Li1.16Ni0.37Mn0.47O2, which both exhibit high capacity, were obtained [33, 34].
In
this
study,
a
chemical
treatment
with
(NH4)2SO4
was
conducted
with
Li1.16Ni0.37Mn0.47O2 to obtain cathode which exhibits high rate performance, small potential shift, and low irreversible capacity. Li1.2Ni0.2Mn0.6O2 treated with (NH4)2SO4 was also prepared as a reference, and the effects of the chemical treatment on the electrochemical properties for both
materials were investigated.
2.
Experimental
2.1. Preparation of active materials
Li2CO3, NiCO3･2Ni(OH)2･4H2O, MnCO3 were mixed in proportion of 1.2: 0.2: 0.6 or 1.16: 0.37: 0.47 (Li: Ni: Mn (mol)). The resulting mixture was pre-annealed at 500°C for 12 hr in air
and then re-annealed at 1000°C (Li1.2Ni0.2Mn0.6O2) or 950°C (Li1.16Ni0.37Mn0.47O2) for 12 hr in air, and Li1.2Ni0.2Mn0.6O2 and Li1.16Ni0.37Mn0.47O2 were obtained. Then, 2.5, 5, or 10 wt% (NH4)2SO4 was mixed with Li1.2Ni0.2Mn0.6O2 or Li1.16Ni0.37Mn0.47O2 in distilled water at 120°C until the water was evaporated. After that, the residual materials were annealed at 300°C for 6 hr in air and washed
with distilled water. Next, the residue materials were dried at 120°C for 6 hr in vacuum to remove
residual water. The samples prepared using 0, 2.5, 5, and 10 wt% (NH4)2SO4 for conventional
5
Li1.2Ni0.2Mn0.6O2 and high-Ni/Mn-ratio Li1.16Ni0.37Mn0.47O2 cathodes were classified as C-N0, C-N2.5, C-N5, C-N10, H-N0, H-N2.5, H-N5, and H-N10, respectively.
2.2. Evaluation of metal composition, morphology, specific surface area, crystal structure, and
electrochemical properties for prepared materials
Inductively coupled plasma atomic emission spectroscopy (ICP-AES; PerkinElmer,
OPTIMA8300) was used to measure the metal composition of the prepared materials. Scanning
electron microscope (SEM; Hitachi 4300) was used to observe morphology of the prepared materials.
Gas adsorption method based on the Brunauer–Emmett–Teller theory (BET theory; MircotracBEL
BELSORP-6) was used to evaluate the specific surface area (SSA) of the prepared materials. X-ray
diffraction (XRD; Rigaku Rint-2200) with Cu Kα radiation was used to evaluate their crystal
structure. Three-electrode electrochemical cells were used to evaluate the electrochemical properties
for the prepared materials. The cathode was comprised of active material (85 wt%), acetylene black
(10 wt%), and binder (5 wt%). Lithium metal was used as the anode and reference electrode. A
mixture of ethylene carbonate (20 vol%), ethylmethyl carbonate (40 vol%), and dimethyl carbonate (40 vol%) with dissolved LiPF6 (1 mol dm−3) was used as an electrolyte. The charge and discharge measurements were conducted under two potential ranges (2.5−4.6 and 2.5−4.4 V (vs. Li/Li+)). The charge was conducted using constant current (CC) and constant voltage mode (operating current
6
condition: 0.05 C (1 C: 260 A kg−1) and cut-off current condition: 0.005 C). The discharge was conducted using CC mode (operating current condition: 0.05, 0.2, and 1 C). The open circuit
potential (OCP), which was defined as the potential 5 hr after charging and discharging were stopped,
during the first charging was measured for each cathode. The cycling test was conducted as follows.
The electrochemical cells were cycled at 0.05 C two times, at 1 C hundred times, and then at 0.05 C
one time.
3. Results and discussions 3.1. Effects of treatment with (NH4)2SO4 on the physical properties Effects of chemical treatment on the physical properties such as composition, morphology,
SSA, and crystal structure were evaluated. The metal composition of non-treated and treated
materials was measured using ICP-AES to determine the progress of Li2O extraction (shown in formula (i)) with (NH4)2SO4 treatment for Li1.16Ni0.37Mn0.47O2 and Li1.2Ni0.2Mn0.6O2. Fig. 1 summarizes the Li/(Ni+Mn) ratio for each material. The Li/(Ni+Mn) ratio decreased as the used
(NH4)2SO4
content
increased
for
Li1.16Ni0.37Mn0.47O2.
The
same
change
as
that
of
Li1.16Ni0.37Mn0.47O2 was observed for Li1.2Ni0.2Mn0.6O2. These results indicate that the progress of the Li2O extraction was affected by the used (NH4)2SO4 content. The effects of treatment with (NH4)2SO4 on the morphology were evaluated using SEM.
7
Fig. 2 shows the particles of H-N0, H-N10, C-N0, and C-N10. The surface of H-N10 (Fig. 2(b))
particle was more coarse than that of H-N0 particle (Fig. 2(a)). The surface of C-N0 particle was
roughened due to treatment with (NH4)2SO4. (Fig. 2(c) and (d)). To evaluate the effects of chemical treatment on the surface change further, SSA of the prepared materials was measured. Fig. 3
summarizes the SSA of prepared materials. For Li1.16Ni0.37Mn0.47O2, the SSA increased as the used (NH4)2SO4 content increased. The same change as that of Li1.16Ni0.37Mn0.47O2 was confirmed for Li1.2Ni0.2Mn0.6O2. The changes in the morphology and SSA due to chemical treatment indicate that the Li2O was extracted from the cathode surface [38, 41]. The effects of treatment with (NH4)2SO4 on the crystal structure were investigated using XRD. Fig. 4 shows the XRD patterns of the prepared materials. The main diffraction peaks for the H-N0 were assigned to the hexagonal α-NaFeO2 structure (space group: R-3m). The small diffraction peaks at 2θ = 20−22° were attributed to the superlattice ordering of lithium and manganese in the transition metal layer [42]. The diffraction patterns of H-N2.5, H-N5, and H-N10
were the same as that of H-N0. The diffraction patterns were not changed due to the treatment with (NH4)2SO4 for the Li1.2Ni0.2Mn0.6O2 (insert in Fig. 4), These results indicate that the α-NaFeO2 structure was maintained after the chemical treatment.
The results obtained using the ICP-AES, SEM, BET, and XRD indicate the treatment with
(NH4)2SO4 can extract Li2O from the cathode surface without changing the crystal structure,
8
regardless of the metal composition for based materials.
3.2. Effects of treatment with (NH4)2SO4 on the electrochemical performance The effects of treatment with (NH4)2SO4 on the electrochemical performance for each material were investigated. Fig. 5 shows the charge and discharge curves in the first and second
cycles for each cathode. In the first cycle for H-N0 (Fig. 5(a)), the charge potential increased up to
4.4 V, and then a plateau was observed. During discharging, the potential smoothly decreased to 3.6
V and then rapidly decreased to 2.5 V. From the first (Fig. 5(a)) to the second (Fig. 5(b)) cycle, the
plateau during charging (> 4.4 V) vanished, but the discharge curves changed slightly. Although
there was large irreversible capacity in the first cycle (Fig. 5(a)), there was little irreversible capacity
in the second cycle (Fig. 5(b)). For the treated samples (H-N2.5, H-N5, and H-N10), irreversible
capacity was observed in the first cycle (Fig. 5(a)), but little irreversible capacity was observed in the
second cycle (Fig. 5(b)). In contrast, for the treated sample, the following three changes were
observed in the charge and discharge curves. (i) The charge potential of the sloping region (< 4.4 V)
in the first cycle increased (Fig. 5(a)). Conversely, no difference was observed in the charge curve
between non-treated and treated samples in the second cycle (Fig. 5(b)). (ii) The capacity ratio
between sloping (< 4.4 V) and plateau (> 4.4 V) regions in the first cycle was changed (Fig. 5(a)).
(iii) A new discharge reaction occurred around 2.9 V in both the first and second cycles (Fig 5(a) and
9
(b)). The aforementioned three changes in the charge and discharge curves due to the treatment with
(NH4)2SO4 were observed for Li1.2Ni0.2Mn0.6O2 (insert in Fig 5(a) and (b)), indicating that these changes occurred regardless of the metal composition for based materials.
The cause of the three changes after chemical treatment was investigated. First, the factor
of potential increase in the sloping region (< 4.4 V) in the first cycle was investigated. We expected
that the lithium ions were partially extracted in the present state for the cathode treated with
(NH4)2SO4; therefore, higher polarization might be required to extract lithium ions with the treated cathode than with the non-treated one. The OCPs for each cathode were measured to determine
whether or not the increase in the charge potential (Fig. 5(a)) could be attributed to the polarization.
Fig. 6 shows the OCPs during charging in the first cycle for each cathode. The OCPs in the initial
phase of the charge reaction for H-N2.5, H-N5, and H-N10 were coincident with that of H-N0.
Moreover, the OCP for the treated cathodes (C-N2.5, C-N5, and C-N10) overlapped with that of the
non-treated cathode (C-N0) (insert in Fig. 6). These results indicate that the increase in the potential
of the sloping region (< 4.4 V) with the chemical treatment could be attributed to the polarization. In
the second cycle, no difference was observed in the charge potential between the non-treated and
treated samples (Fig. 5(b)). This indicates that once lithium ions were compensated during
discharging, the charge reaction for the treated samples proceeded the same potential as that for the
non-treated sample. Thus, large polarization was not observed for the treated samples in the second
10
cycle.
Second, the cause of the change in the charge capacity ratio between sloping (< 4.4 V) and
plateau (> 4.4 V) regions was investigated. It has been reported that the oxidation of nickel
contributes to the charge reaction in the sloping region (< 4.4 V). In contrast, the oxidation of
oxygen and Li2O extraction occurs in the plateau region (> 4.4 V) [43−45]. The charge capacity below and above 4.4 V for each cathode was summarized to evaluate the effect of chemical
treatment on the charge reaction in the sloping and plateau regions, respectively. Fig. 7 plots the
charge capacities over the potential ranges of 2.5−4.4, 4.4−4.6, and 2.5−4.6 V for each cathode,
respectively. The charge capacities over the potential range of 2.5−4.4 V for H-N2.5, H-N5, and H-N10 were almost the same as that for H-N0. In contrast, as the used (NH4)2SO4 content increased, the charge capacity over the potential range of 4.4−4.6 V decreased, and then the total charge
capacity (2.5−4.6 V) also decreased. A similar change was observed for Li1.2Ni0.2Mn0.6O2 (C-N0, C-N2.5, C-N5, and C-N10). These results indicate that the chemical treatment slightly affected the
charge reaction contributed to the redox of nickel (sloping region; < 4.4 V), but it reduced the charge
reaction that contributed to the Li2O extraction (plateau region; > 4.4 V), which is the cause of irreversible capacity.
Third, the trigger of a new discharge reaction around 2.9 V was investigated. It has been
reported that the redox of transition metal and oxygen contribute to the charge and discharge
11
reactions for Li1.2NiaMnbCo0.8−a−bO2; therefore, high capacity was obtained [43−45]. Manganese and oxygen were electrochemically activated due to the Li2O extraction occurring in the plateau region (> 4.4 V) [43−45]. In other words, only redox of nickel contributed to the charge and discharge reaction without Li2O extraction, and high capacity was not obtained below 4.4 V. Previously, our group investigated the effects of charge cut-off potential on the discharge capacity for
Li1.2Ni0.2Mn0.6O2 [33]. When Li1.2Ni0.2Mn0.6O2 was charged and discharged below the plateau (2.5−4.4 V), the discharge reaction mainly progressed above 3.6 V, and the discharge capacity was low (< 100 Ah kg−1). In contrast, when Li1.2Ni0.2Mn0.6O2 was charged above the plateau region (2.5−4.6 V), the discharge capacity below 3.6 V greatly increased, and high discharge capacity (> 250 Ah kg−1) was obtained. This result supports the finding that discharge reaction below 3.6 V occurred after the Li2O extraction. We expected that the discharge reaction below 3.6 V might proceed without charging
above the plateau region (> 4.4 V) because the treatment with (NH4)2SO4 can extract Li2O before charging. Fig. 8 shows the charge and discharge curves over a potential range of 2.5−4.4 V in the first and second cycle for each cathode. For the H-N0 (Fig. 8(a)), the discharge reaction mainly
progressed above 3.6 V. From H-N0 to H-N2.5, a new discharge reaction appeared below 3.6 V, and
the discharge capacity below 3.6 V increased as the treatment content increased. In the second cycle
for H-N2.5, H-N5, and H-N10 (Fig. 8(b)), a new charge reaction occurred below 3.6 V. For
12
Li1.2Ni0.2Mn0.6O2 (insert in Fig. 8(a) and (b)), the treatment causes the same phenomena observed for Li1.16Ni0.37Mn0.47O2. Regardless of the metal composition for based materials, a new discharge reaction below 3.6 V was observed without charging above the plateau region (> 4.4 V).
When the charge cut-off potential was limited to 4.4 V, no discharge reaction below 3.6 V
was observed for the non-treated samples (H-N0 and C-N0) (Fig. 8(a) and (b)). In contrast, a
discharge reaction below 3.6 V was observed (Fig. 5(a) and (b)) when the non-treated samples were
charged up to 4.6 V. However, no discharge reaction around 2.9 V was observed for the non-treated
samples (H-N0 and C-N0), regardless of the charge cut-off potential (4.4 and 4.6 V) (Figs. 5 and 8).
This means that the discharge reaction around 2.9 V was characteristic for the chemical Li2O extraction. The occurrence of a new discharge reaction due to the oxygen deficiency was reported
for LiNi0.5Mn1.5O4 [46, 47]. The occurrence of the new discharge reaction in this compound was related to the redox of manganese that was activated due to the oxygen deficiency [46, 47]. For
Li1.16Ni0.37Mn0.47O2 and Li1.2Ni0.2Mn0.6O2, the new discharge reaction around 2.9 V (Figs. 5 and 8) was observed below the OCP in the pristine state (3.0−3.4 V) (Fig. 6). If a similar phenomenon observed for oxygen-deficient LiNi0.5Mn1.5O4 was occurred for Li1.16Ni0.37Mn0.47O2 and Li1.2Ni0.2Mn0.6O2 due to chemical Li2O extraction, the new discharge reaction around 2.9 V was observed without charging. Prepared cathodes were discharged without charging to evaluate whether
or not the discharge reaction around 2.9 V occurred due to treatment with (NH4)2SO4. Fig. 9 shows
13
the discharge curves for each cathode. For H-N0, little discharge reaction was observed. In contrast,
the discharge reaction around 2.9 V appeared for H-N2.5, and the discharge capacity increased as the
treatment content increased. The same changes were observed for the Li1.2Ni0.2Mn0.6O2 (insert in Fig. 9).
These results indicate that the chemical Li2O extraction with (NH4)2SO4 was effective in activating a redox of manganese and oxygen without electrochemical Li2O extraction. In addition, the chemical Li2O extraction causes the discharge reaction around 2.9 V, which does not occur due to the electrochemical Li2O extraction.
3.3. Effects of treatment with (NH4)2SO4 on the rate and cycling performance Finally, the effects of treatment on the rate and cycling performance were investigated. The
rate and cycling performance for C-N0, C-N5, H-N0, and H-N5 was evaluated, and the effects of
metal composition and chemical treatment for each cathode on the electrochemical performance
were discussed.
Fig. 10 shows the discharge curves at 0.2 and 1 C for each cathode. As the operating C-rate
increased in C-N0 (Fig. 10(a)), the discharge capacity greatly decreased, and the reaction potential
also decreased. Although the discharge capacities for C-N5 at 0.2 and 1 C were higher than that for
C-N0, a decrease in the discharge capacity and potential due to increasing the C-rate was observed
14
for C-N5.
The decrease in the discharge capacity and potential due to the change in the operating
C-rate from 0.2 to 1 C for H-N0 (Fig. 10(b)) was smaller than that for C-N0 (Fig. 10(a)). The
discharge capacity and potential at 0.2 and 1 C for H-N5 were similar to those for H-N0. These
results indicate that the Li1.16Ni0.37Mn0.47O2 exhibited higher rate performance than Li1.2Ni0.2Mn0.6O2, and the high rate performance for Li1.16Ni0.37Mn0.47O2 was maintained after the treatment with (NH4)2SO4. Next, the effects of treatment with (NH4)2SO4 on the cycling performance were investigated. Fig. 11 shows the discharge and dQ/dV curves for C-N0, C-N5, H-N0, and H-N5
before and after hundred cycles. For all samples, the shapes of discharge curves changed after
hundred cycles (insert in Fig. 11(a) and (b)). The dQ/dV curves for each material were compared to
evaluate the detailed shape changes in the discharge curves. For C-N0, the peak area above 3.6 V
decreased due to cycling, and that below 3.6 V shifted to the lower potential side (Fig. 11(a)). From
C-N0 to C-N5, the peak area above 3.6 V before cycling changed slightly, but that below 3.6 V
increased. The changes in the dQ/dV curve due to cycling for C-N5 below and above 3.6 V showed
similar changes to those observed for C-N0 (Fig. 11(a)). For H-N0 and H-N5, the peak area above
3.6 V decreased due to cycling, and that below 3.6 V shifted to the lower potential side (Fig. 11(b)).
However, the changes in the dQ/dV curves due to cycling for H-N0 and H-N5 were much smaller
15
than those for C-N0 and C-N5.
These results indicate that the potential shift during cycling was suppressed due to the
composition change from Li1.2Ni0.2Mn0.6O2 to Li1.16Ni0.37Mn0.47O2, and this effect was maintained after the treatment with (NH4)2SO4. Migration of reduced manganese ions reportedly causes the potential shift during cycling [48]. For C-N0, the discharge reaction below 3.6 V increased due to
chemical treatment (Fig. 11(a)), indicating that the reduction of manganese ions during discharging
proceeded due to the treatment with (NH4)2SO4. The increase in the reduced manganese ions with the chemical treatment caused the potential to shift further. In contrast, the discharge capacity below
3.6 V for H-N0 was much smaller than that for C-N0 (Fig. 5). Furthermore, the discharge capacity
below 3.6 V for H-N0 changed slightly after chemical treatment (Fig. 5). For H-N0 and H-N5, the
contribution of manganese redox to the discharge reaction was small; therefore, the potential shift
during cycling for H-N0 and H-N5 was small.
We conclude that the irreversible capacity of Li1.16Ni0.37Mn0.47O2 was reduced due to the treatment with (NH4)2SO4 without losing characteristics such as high rate and cycling performance. Therefore, due to the treatment with (NH4)2SO4, Li1.16Ni0.37Mn0.47O2 exhibits high rate, high cycling, and low irreversible capacity.
4. Conclusion
16
Li2O was extracted using (NH4)2SO4 before charging to reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2. The ICP-AES, SEM, BET and XRD results indicate that the Li2O was extracted from the cathode surface without changing the crystal structure. The charge and discharge
measurement results indicate that the chemical Li2O extraction was effective in suppressing the irreversible Li2O extraction that proceeded during the first charging, and then the irreversible capacity was reduced. These effects were observed in not only Li1.16Ni0.37Mn0.47O2 but also conventional Li1.2Ni0.2Mn0.6O2 composition, indicating that treatments with (NH4)2SO4 can be useful for lithium-rich layer-structured cathodes comprising different Li/(Ni+Mn) and Ni/Mn ratios.
Furthermore, the effects of treatment on the rate and cycling performance were evaluated.
High rate performance and small potential shift during cycling for Li1.16Ni0.37Mn0.47O2 were maintained after the treatment with (NH4)2SO4. These results indicate that the chemical treatment with (NH4)2SO4 was effective to reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2 without harming this material’s advantages.
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Figure captions Fig. 1.
Li/(Ni+Mn) ratio for each cathode.
21
Fig. 2.
SEM images of (a) H-N0, (b) H-N10, (c) C-N0, and (d) C-N10.
Fig. 3.
SSA of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10.
Fig. 4.
XRD patterns of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10.
Fig. 5.
Charge and discharge curves of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and
C-N10 over a potential range of 2.5−4.6 V in the (a) first and (b) second cycles.
Fig. 6.
Open circuit potentials of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10
during charging in the first cycle.
Fig. 7.
Summary of charge capacities for H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and
C-N10 over potential ranges of 2.5−4.4, 4.4−4.6, and 2.5−4.6 V in the first cycle.
Fig. 8.
Charge and discharge curves of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and
C-N10 over a potential range of 2.5−4.4 V in the (a) first and (b) second cycles.
22
Fig. 9.
Fig. 10.
Discharge curves of H-N0, H-N2.5, H-N5, H-N10, C-N0, C-N2.5, C-N5, and C-N10.
Discharge curves of (a) C-N0 and C-N5 and (b) H-N0 and H-N5 at 0.2 and 1 C.
Fig. 11. Discharge and dQ/dV curves of (a) C-N0, (b) C-N5, (c) H-N0, and (d) H-N5 over a
potential range of 2.5−4.6 V before and after hundred cycles.
23
1.5
Li/(Ni+Mn)
1.4
1.3
Li1.2Ni0.2Mn0.6O2 Li1.16Ni0.37Mn0.47O2
1.2
1.1
0
2
4 6 8 (NH4)2SO4 (wt%)
10
(a)
+
Potential (V (vs. Li/Li ))
4.5
4.0
3.5
3.0
C-N0 0.2 C C-N5 0.2 C
1C 1C
2.5 0
50
100
150
200
250
-1
Capacity (Ah kg ) (b)
+
Potential (V (vs. Li/Li ))
4.5
4.0
3.5
3.0
H-N0 0.2 C H-N5 0.2 C
1C 1C
2.5 0
50
100
150
200 -1
Capacity (Ah kg )
250
0
C-N0 before C-N5 before
after
-1
-1
dQ/dV (Ah kg V )
-100
(a) after
+
Potential (V (vs. Li/Li ))
-200
-300
-400 2.5
3.0
3.5
4.5 4.0 3.5 3.0 2.5 0
100 200 300 -1 Capacity (Ah kg )
4.0
4.5
+
Potential (V (vs. Li/Li ))
0
(b)
+
-200
H-N0 before H-N5 before Potential (V (vs. Li/Li ))
-1
-1
dQ/dV (Ah kg V )
-100
-300
-400
after after
4.5 4.0 3.5 3.0 2.5 0
2.5
100 200 300 -1 Capacity (Ah kg )
3.0
3.5
4.0 +
Potential (V (vs. Li/Li ))
4.5
4
2
-1
SSA (m g )
5
Li1.2Ni0.2Mn0.6O2 Li1.16Ni0.37Mn0.47O2
3
2 0
2
4 6 8 (NH4)2SO4 (wt%)
10
20 30 70
40 50 2q (°) (018) (110) (113)
60
(104) (015)
40 50 2q (°)
(107)
30 (006) (012)
20 (101)
(003)
Intensity (a.u.) Intensity (a.u.)
C-N10
C-N5
C-N2.5
C-N0
H-N10
H-N5
H-N2.5
H-N0
60 70
(a)
+
Potential (V (vs. Li/Li ))
4.5
3.0
+
3.5
Potential (V (vs. Li/Li ))
4.0
H-N0 H-N2.5 H-N5 H-N10 4.5 4.0 3.5 3.0 2.5 0
2.5 0
50
C-N0 C-N2.5 C-N5 C-N10
100 200 300 -1 Capacity (Ah kg )
100
150
200
250
300
-1
Capacity (Ah kg ) (b)
+
Potential (V (vs. Li/Li ))
4.5
3.0
+
3.5
Potential (V (vs. Li/Li ))
4.0
H-N0 H-N2.5 H-N5 H-N10 4.5 4.0 3.5 3.0 2.5 0
2.5 0
50
C-N0 C-N2.5 C-N5 C-N10
100 200 300 -1 Capacity (Ah kg )
100
150
200 -1
Capacity (Ah kg )
250
300
+
4.0
+
Potential (V (vs. Li/Li ))
Potential (V (vs. Li/Li ))
4.5
H-N0 H-N2.5 H-N5 H-N10
3.5
3.0 0
100
4.5 4.0
C-N0 C-N2.5 C-N5 C-N10
3.5 3.0 0
100 200 300 -1 Capacity (Ah kg )
200
300 -1
Capacity (Ah kg )
-1
Charge capacity (Ah kg )
350 300 250 200 150 100 50 0
Li1.2Ni0.2Mn0.6O2 2.5–4.4 V 4.4–4.6 V Li1.16Ni0.37Mn0.47O2 2.5–4.4 V 4.4–4.6 V
0
2
2.5–4.6 V 2.5–4.6 V
4 6 8 (NH4)2SO4 (wt%)
10
4.5
H-N0 H-N2.5 H-N5 H-N10
4.0
3.5
+
Potential (V (vs. Li/Li ))
+
Potential (V (vs. Li/Li ))
(a)
3.0
4.5 4.0 3.5
C-N0 C-N2.5 C-N5 C-N10
3.0 2.5 0
2.5 0
40 80 120 -1 Capacity (Ah kg )
50
100
150
200
-1
Capacity (Ah kg )
4.5
H-N0 H-N2.5 H-N5 H-N10
4.0
3.0
2.5 0
+
3.5
Potential (V (vs. Li/Li ))
+
Potential (V (vs. Li/Li ))
(b)
4.5 4.0 3.5
C-N0 C-N2.5 C-N5 C-N10
3.0 2.5 0
40 80 120 -1 Capacity (Ah kg )
50
100
150 -1
Capacity (Ah kg )
200
3.2
+
Potential (V vs. Li/Li )
+
Potential (V vs. Li/Li )
3.4
3.0
3.4 C-N0 C-N2.5 C-N5 C-N10
3.2 3.0 2.8 2.6 2.4 0
5 10 15 20 -1 Capacity (Ah kg )
2.8 H-N0 H-N2.5 H-N5 H-N10
2.6
2.4 0
5
10
15 -1
Capacity (Ah kg )
20
• Li2O was extracted due to chemical treatment without changing crystal structure. • Irreversible capacity for Li1.16Ni0.37Mn0.47O2 was reduced due to chemical treatment. • Advantages for Li1.16Ni0.37Mn0.47O2 was maintained after chemical treatment.