Thermal decomposition study on Li2O2 for Li2NiO2 synthesis as a sacrificing positive additive of lithium-ion batteries

Article abstract of DOI:10.3390/molecules24244624

Herein, thermal decomposition experiments of lithium peroxide (Li₂O₂) were performed to prepare a precursor (Li₂O) for sacrificing cathode material, Li₂NiO₂. The Li₂O₂ was prepared by a hydrometallurgical reaction between LiOH·H₂O and H₂O₂. The overall reaction during annealing was found to involve the following three steps: (1) dehydration of LiOH·H₂O, (2) decomposition of Li₂O₂, and (3) pyrolysis of the remaining anhydrous LiOH. This stepwise reaction was elucidated by thermal gravimetric and quantitative X-ray diffraction analyses. Furthermore, over-lithiated lithium nickel oxide (Li₂NiO₂) using our lithium precursor was synthesized, which exhibited a larger yield of 90.9% and higher irreversible capacity of 261 to 265 mAh g^?1 than the sample prepared by commercially purchased Li₂O (45.6% and 177 to 185 mAh g^?1, respectively) due to optimal powder preparation conditions.

Full text of DOI:10.3390/molecules24244624

molecules
Communication
Thermal Decomposition Study on Li2O2 for Li2NiO2
Synthesis as a Sacrificing Positive Additive of
Lithium-Ion Batteries
Jaekwang Kim, Hyunchul Kang, Keebum Hwang and Songhun Yoon *
Department of Nanomaterials Science and Engineering, School of Integrative Engineering,
Chung-Ang University, 84, Heukseok-ro, Dongjak-Gu, Seoul 06974, Korea; kjk9025@hanmail.net (J.K.);
guscjf763@gmail.com (H.K.); hwang1443@naver.com (K.H.)
* Correspondence: yoonshun@cau.ac.kr; Tel.: +82-2-820-5769
Academic Editor: Gregorio F. Ortiz
Received: 8 November 2019; Accepted: 14 December 2019; Published: 17 December 2019
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Herein, thermal decomposition experiments of lithium peroxide (Li2O2) were performed to
prepare a precursor (Li2O) for sacrificing cathode material, Li2NiO2. The Li2O2 was prepared by a
hydrometallurgical reaction between LiOH
was found to involve the following three steps: (1) dehydration of LiOH
·
H2O and H2O2. The overall reaction during annealing
H2O, (2) decomposition of
·
Li2O2, and (3) pyrolysis of the remaining anhydrous LiOH. This stepwise reaction was elucidated
by thermal gravimetric and quantitative X-ray diffraction analyses. Furthermore, over-lithiated
lithium nickel oxide (Li2NiO2) using our lithium precursor was synthesized, which exhibited a larger
yield of 90.9% and higher irreversible capacity of 261 to 265 mAh g−1 than the sample prepared by
commercially purchased Li2O (45.6% and 177 to 185 mAh g−1, respectively) due to optimal powder
preparation conditions.
Keywords: Li2O2; thermal decomposition; Li2O; Li2NiO2; Li source; Li-ion battery
1. Introduction
Within Li-ion (Li+) batteries (LIBs), organic electrolytes are easily decomposed due to the highly
negative potential of the anode materials during first few cycles. This decomposition can be favorable
in one sense, since the solid electrolyte interphase (SEI) passivates the surface of the electrode and
prevents further decomposition of the electrolyte. The stabilized SEI film resulted in reasonable
coulombic efficiency and cycle performance [1–4]. However, the electrolyte decomposition is not
beneficial with respect to the specific energy of LIBs. Since decomposition consumes electric charges
and Li+ during the first few charging cycles, an overload of the positive electrode material is inevitable
to compensate the Li+ and electric charges for the SEI formation [
5
,
6
]. The carbonaceous materials,
such as soft carbons, hard carbons, graphite, etc. have been widely used as anode materials for
LIBs for recent device applications such as electric vehicle (EV) and hybrid EV [ 15]. At present,
7–
moreover, high-capacity materials are on use (ex. SiO) in commercial LIBs to increase energy density.
Unlike to a graphite (>90%), the SiO exhibits a quite low coulombic efficiency about 45% in the initial
few cycles because of a charge consumption originated from the irreversible formation of the SEI.
However, this high-capacity material have been applied to meet the demands of higher energy density
cells. Then, the compensation of charge (or Li+) consumption should become a critical issue in the
future market of LIBs [3,16,17]. For this reason, the role of a sacrificing positive electrode additive has
been widely discussed. Researchers who studied the sacrificing materials reported that the materials
should be of a light weight for better energy density, easily decomposed near the charging potential
Molecules 2019, 24, 4624; doi:10.3390/molecules24244624
www.mdpi.com/journal/molecules

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of cathode material, and over-lithiated for maximum irreversible capacity to match the consumed
charges (and Li+).
The over-lithiated lithium nickel oxide (Li2NiO2) has been considered as a promising additive
since it is both over-lithiated to produce the large irreversible capacity during initial cycles and of light
weight [18]. Li2NiO2 is widely synthesized by nickel oxide (NiO) and lithium oxide (Li2O) since they
directly react in solid-state in stoichiometric ratio 1:1. However, as the main precursor of Li2NiO2, Li2O
is still expensive to prepare as it is obtained by direct oxidizing of Li metal. Therefore, the efficient
and economic preparation of the Li2O is highly required. The thermal decomposition of lithium
peroxide (Li2O2) is can be less expensive way to synthesize Li2O, because it only exhales O2 during its
reaction and its precursor (LiOH H2O) is inexpensive. To the best of our knowledge, the systematic
·
study of this thermal decomposition of Li2O2 has not yet been reported. To provide a better insight
into this beneficial synthesis method, the authors have investigated the thermal decomposition of
Li2O2 by applying various decomposition temperatures and reaction times. Quantitative analysis is
performed on the reaction products to elucidate the composition ratios of several lithium compounds
obtained from the reaction. Moreover, two types of Li2NiO2 (L2N) are obtained by using as prepared
or commercially purchased Li2O, and their electrochemical performances are compared.
2. Results and Discussion
The morphologies of the Li2O2 and its thermally decomposed products at 450 ◦C and 600 ◦C
are characterized by the FE-SEM images presented in Figure 1. The 450 ◦C reaction products display
well-defined primary particles compared to the Li2O2. However, as-prepared particles tended to
aggregate, suggesting that their surfaces were slightly sintered to produce a massive secondary
◦
particle. In support of this observation, the secondary particles reacted at 600 C displayed a
highly sintered phase and it is difficult to find any primary particle boundaries. Since the Li2O2
was synthesized by the hydrometallurgical reaction between LiOH
Li2O2 H2O2 3H2O, contamination with LiOH H2O or anhydrous LiOH during vacuum, drying is
inevitable [19]. Furthermore, it is widely known that LiOH H2O and anhydrous LiOH are easily
·
H2O and H2O2 to generate
·
·
·
·
sintered when transformed to the anhydrous phase and Li2O, respectively, via the molten phase [20].
Therefore, it is obvious that an unintended Li impurity exists in the prepared Li2O2 and that its presence
should be attributed to the monolithic phase of a reacted product during the thermal decomposition.
Figure 1. FE-SEM images of (a) Li2O2 and its decomposed products at (b) 450 ◦C and (c) 600 ◦C.
For further investigation, XRD and TGA analyses were conducted on the prepared materials.
The XRD spectra of the materials prepared at 350, 450, 600 ◦C are presented in Figure 2a. The materials
were exposed to individual reaction temperature for 60 min. The red, navy and yellow columns
represent the XRD peak indices of Li2O2 (PDF-01-073-1640), LiOH (PDF-01-076-0911) and LiOH
(PDF-01-076-1073), respectively. The vertical dotted lines indicate the locations of Li2O (lithia) peaks
with an Fm3m cubic structure (PDF-01-076-9262). Since dehydration of LiOH H2O occurs between 90
and 200 ◦C, there is no trace of an LiOH
H2O crystalline signal in any of the spectra; instead, anhydrous
LiOH peaks are observed on each XRD profile [21 22]. As the reaction temperature increased, the Li2O2
remaining in the 350 ◦C sample disappeared, the majority of it being transformed into Li2O.
·H2O
·
·
,

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Figure 2.
differential thermal gravimetric (DTG, navy color).
(a b) TGA (black) and
) XRD spectra of the materials decomposed at 350, 450, 600 ◦C and (
The TGA and differential thermal gravimetric (DTG) results are also plotted in Figure 2b to
elucidate the change in the Li2O2 sample during thermal decomposition. The analysis was performed
between room temperature and 900 ◦C at a heating rate of 5 ◦C min−1. The full reaction consists of the
following three steps: (1) dehydration of LiOH H2O, (2) decomposition of Li2O2, and (3) pyrolysis of
·
anhydrous LiOH, as indicated by arrows in the plot. At the beginning of the DTG profile, a minute
amount of signal was detected between 90 and 200 ◦C, the temperature at which the dehydration of
LiOH
and absence of LiOH
340 ◦C and ended at 420 ◦C, which is in agreement with the results of the other group who studied
the decomposition of Li2O2 [23
24]. A broad and small signal appearing between 420 and 550 ◦C was
·
H2O occurs [21
,
22]. Note that this is consistent with the FE-SEM results for the sintered material
·
H2O peaks in the aforementioned XRD. The Li2O2 decomposition started at
,
probably related to the pyrolysis of the anhydrous LiOH reported by Kiat et al. [25]. However, since
their results were obtained using a vacuum annealing atmosphere (which is different from our N2
atmosphere), this would need to be verified.
In order to verify the full decomposition of Li2O2, several thermal annealing experiments were
conducted. The gravimetric composition of an obtained Li compound was quantitively analyzed via
XRD Rietveld refinement, as listed in Table 1. While a large amount of unreacted Li2O2 remained in
the 350 ◦C samples, the content of Li2O nevertheless increased with increasing thermal exposure time.
On the other hand, the Li2O2 reacted thoroughly after 60 min at 400 ◦C to form the other Li compounds.
Even if the quantity of Li hydroxides deviates, however, it is noteworthy that they were not found after
the reaction at 500 ◦C, as expected from the TGA and DTG results. Moreover, the reaction trend of the
Li2O shows consistency irrespective of the hydroxides. Thus, the gravimetric information for Li2O in
Table 1 was converted to mole fraction of reacted product and listed in Table 2 for detailed investigation.
The mole (phase) fraction of Li2O within a decomposed product was plotted against exposure
time at each reaction temperature, as shown in Figure 3a. The linear profiles depicted over the dotted
results represent the best-fitted results of thermal conditions using the least-square method. While the
line surges adversely to a high phase fraction at 350 ◦C, the others rise somewhat slowly. Since their
slopes equal the reaction rate of Li2O in Equation (1), the reaction rate was plotted against the reaction
temperature for better insight:
1
2
Li2O2(s) → Li2O(s) + O2(g)
(1)

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Table 1. Total gravimetric ratio of the Li compound after thermal reaction.
Temp (◦C)
350
Time (min)
Li2O2
Li2O
LiOH
LiOH·H2O
Li2CO3
Sum (wt%)
30
60
90
85.7
84.7
75.9
70.7
2.1
5.8
14.3
18.9
11.7
9.2
10.1
9.9
0.5
0.2
0.3
0.4
0
0
0
0
100
99.9
100
120
99.9
30
60
90
25.8
0.1
0.2
0
59.5
80.1
89.2
81
14.0
17.4
7.8
0.5
2.3
2.6
2.1
0.2
0.2
0.3
0
100
100.1
100.1
100
400
450
500
600
120
16.9
30
60
90
0
0
0
0
82.4
83.5
87.5
89.5
11.3
12.2
12.2
9
6.4
0.3
0.3
1.5
0
0
0
0
100.1
100
100
120
100
30
60
90
0
0
0
0
82.8
85.9
87.7
87.7
10.5
8.7
6.3
5.9
6.7
5.4
6
0
0
0
0
100
100
100
100
120
6.4
30
60
90
0
0
0
0
97.9
99.4
99.1
99.5
1.6
0.2
0.2
0.2
0
0.5
0
0
100
100
100.1
100
0.4
0.8
0.3
120
0
Table 2. Mole fraction of Li2O among the product of thermal reaction (mol%).
350 ◦C
400 ◦C
450 ◦C
500 ◦C
600 ◦C
30 min
60 min
90 min
120 min
12.3
33.3
52.7
59.9
76.8
77.7
88.4
78.2
81.5
84.4
85.0
87.9
82.2
85.4
87.8
88.0
97.8
99.5
99.2
99.5
The profile in Figure 3b shows excessive reaction between 350 and 400 ◦C, whereas the other
regions exhibit small and broad reaction profiles. Moreover, these phenomena and fitted slopes agree
well with the DTG profiles in Figure 2b. More interestingly, the small and broad reaction profile
in Figure 3b can be attributed to the pyrolysis of Li hydroxides, since a slight trace of hydroxide
compounds was found after the samples were annealed at 600 ◦C. Consequently, it can be said that the
broad and small peak observed in Figure 2b originated from the pyrolysis of anhydrous LiOH under
an N2 atmosphere. To summarize the full decomposition reaction, the transformation ratios of Li2O
presented in Table 2 were plotted on a three-dimensional graph (Figure 3c) with each color division
representing a mole fraction of 20%.
To estimate the performance of the prepared Li2O (decomposed at 450 ◦C for 10 h under N2 flow)
as a precursor of the L2N sacrificing additive material of a LIB cathode, two types of L2N compound
were synthesized using the prepared material (P-L2N) and commercially purchased material (C-L2N).
Spherical shaped NiO particles were used for the synthesis. Figure S1 shows the SEM images of the
two L2N samples. It appears that more sintered particles are found in the P-L2N compound than
in the C-L2N. Moreover, spherical NiO particles are more abundant in the SEM image of the C-L2N
presented in Figure S1b. XRD was performed on the synthesized L2N and the diffraction patterns are
presented in Figure S2. While both materials exhibited the characteristic peaks of L2N, there may be
some difference in signal intensity. Nevertheless, residual NiO and Li2O signals are observed for both
materials. Therefore, Rietveld refinement was conducted on both XRD patterns to indicate that 90.9%
and 45.6% of L2N exist in the P-L2N and C-L2N, respectively.

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Figure 3.
(a
) Mole fraction of Li2O within a decomposed product according to reaction time. (
b) Reaction
rate of each exposure temperature derived from the slope of mole fraction of Li2O. (
summary of Li2O transformation ratio (wt%) obtained from Table 1.
c) 3D graphical
Figure 4 presents the galvanostatic charge/discharge profiles of the prepared L2N, as recorded
between 3.0 and 4.25 V with a 0.1 C current injection. For the first cycle of charge/discharge, three
identically conditioned cells were tested for repeatability. As a promising sacrificing positive additive
providing surplus Li-ion to a counter (negative) electrode, an irreversible capacity of the first cycle is
generally considered as a key point index for the L2N material [3]. The P-L2N and C-L2N showed 261
to 265 and 177 to 185 m Ah g−1 of irreversible capacity at the first cycle. Therefore, the yield of the L2N
synthesis can be regarded as significant for its performance. In this regard, the Li2O material produced
by thermal decomposition of Li2O2 can be beneficial for the L2N additive synthesis.
Figure 4. Galvanostatic charge/discharge profiles obtained from three identically conditioned cells of
(a) P-L2N and (b) C-L2N.

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3. Materials and Methods
3.1. Preparation of Lithium Peroxide (Li2O2)
Li2O2 powder was prepared by a simple precipitation method. For the precipitation, lithium
hydroxide monohydrate (1.25 g, LiOH H2O, Junsei Chemical Co. Ltd., Tokyo, Japan) was poured into
·
100 mL of vigorously stirred hydrogen peroxide (30% H2O2 (aq), Samchun Chemicals, Seoul, Korea).
The stirring was continued for an hour until a yellowish precipitate was obtained. The precipitate
was filtered using a cellulose filter paper and dried in a vacuum oven under ambient temperature
conditions until parched.
3.2. Thermal Decomposition of Lithium Peroxide (Li2O2) to Lithium Oxide (Li2O)
The Li2O2 powder (10 g) was placed in an alumina (Al2O3) crucible and heated in a tube furnace
with a continuous flow of N2 (99.999%) gas into the tube. The ramping rate was 5 ◦C/min and the
reaction temperature was 350–600 ◦C. After reaching the reaction temperature, the thermal exposure
time was varied from 30 to 120 min to find the economically optimal reaction time while obtaining
highly pure Li2O.
3.3. Preparation of the L2N
The Li2NiO2 (L2N) was synthesized by a solid-state reaction described elsewhere [3,18]. Two type
of L2N were synthesized by using different Li2O samples (as prepared and commercially purchased
sample). For this synthesis, we decomposed the Li2O2 at 450 ◦C for 2 h. Stoichiometrically mixed NiO
and Li2O were placed in an alumina crucible and heated at 650 ◦C for 12 h under an N2 atmosphere.
The obtained L2N powder was handled under an argon atmosphere in a glove box because it would
be unstable in air.
3.4. Material Characterization
The Morphology of the material was characterized by field emission scanning electron
microscopy (FE-SEM 500, ZEISS Sigma, Oberkochen, Germany). In addition, crystallographic
analysis was conducted by X-ray diffraction (New D8-advance, Bruker-AXS, Karlsruhe, Germany) from
0 to 130 degrees and the quantitative analysis was performed by HighScore (Malvern Panalytical, Great
Malvern, UK). Moreover, thermalgravimetric analysis (TGA) was conducted on the prepared Li2O2.
3.5. Electrochemical Analysis
For the electrochemical performance estimation, a 2032 type coin cell was used as a half-cell.
An active material (L2N), binder and conductive agent (carbon black) were homogeneously mixed in
N-methyl-2-pyrrolidone (NMP) solvent to form a slurry, which was then coated onto an aluminum
foil current collector, pressed and dried overnight in a vacuum oven at 120 ◦C. The 2032 type coin
cells were assembled with the Li2NiO2 working electrode, Li foil counter electrode and 1.0 M LiPF6 in
ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (1:2 v/v%) electrolyte. The cells were assembled
under an argon atmosphere (H2O: ~0.1 ppm) in a glove box. After assembly, the cells were rested for
10 h.
To measure the electrochemical performance of the assembled cells, galvanostatic charge-discharge
was conducted on three cells for each material with constant-current/constant-voltage (CCCV) mode
between a potential window of 3.0–4.25 V (vs. Li/Li+) by applying a 0.1 C current rate.
4. Conclusions
In summary, various thermal decomposition experiments of Li2O2 were conducted to find the
optimal reaction conditions to produce Li2O. The full reaction during annealing was found to involve
the following steps: (1) dehydration of LiOH H2O, (2) decomposition of Li2O2 and (3) pyrolysis of
·

Molecules 2019, 24, 4624
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the remaining anhydrous LiOH. This stepwise reaction was elucidated by the TGA, and Rietveld
refinement analysis using XRD. Using the prepared precursor and commercial one, two types of
Li2NiO2 were synthesized and the Li2NiO2 synthesized from the prepared Li2O showed a larger
yield of 90.9% and higher irreversible capacity of 261 to 265 mAhg−1 than the sample synthesized by
commercially purchased Li2O (45.6% and 177 to 185 mAh g−1, respectively). Consequently, it was
confirmed that the Li2O decomposed from Li2O2 was highly suitable precursor in the sacrificing
cathode material.
Supplementary Materials: The following are available online, Figure S1: SEM images of (a) P-L2N and (b) C-L2N,
Figure S2: XRD patterns of C-L2N (black) and P-L2N (blue), Figure S3: Quantitative analysis result of P-L2N by
Rietveld refinement, Figure S4: Quantitative analysis result of C-L2N by Rietveld refinement.
Author Contributions: Conceptualization, J.K., and S.Y.; methodology, H.K.; formal analysis, K.H.; writing—review
and editing, S.Y.
Funding: This work receive no external funding.
Acknowledgments: This research was supported by the Chung-Ang University Research Grants in 2018. Also,
this work was supported by industrial Strategic Technology Development Program Tongyang incorporated
(20006777, Development of SiOxartificial graphite composite for anode material by using binder and coating pitch)
funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).
Conflicts of Interest: The authors declare no conflict of interest.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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