Stability of Li2CO3 in cathode of lithium ion battery and its influence on electrochemical performance

Article abstract of DOI:10.1039/c6ra00648e

Lithium carbonate is an unavoidable impurity at the cathode side. It can react with LiPF₆-based electrolyte and LiPF₆ powder to produce LiF and CO₂, although it presents excellent electrochemical inertness. Samples of Li₂CO₃-coated and LiF-coated LiNi_0.8Co_0.1Mn_0.1O₂ were prepared to compare their influence on a cathode's behavior. After 200 cycles at 1C, in contrast to 37.1% of capacity retention for the Li₂CO₃-coated material, the LiF-coated LiNi_0.8Co_0.1Mn_0.1O₂ retained 91.9% of its initial capacity, which is similar to the fresh sample. This demonstrates that decomposition of Li₂CO₃ can seriously deteriorate cyclic stability if this occurs during working.

Full text of DOI:10.1039/c6ra00648e

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Stability of Li2CO3 in Cathode of Lithium Ion Battery and Its
Influence on Electrochemical Performance
Yujing Bia,b,d, Tao Wangc, Meng Liua, Rui Dua,b, Wenchao Yanga, Zixuan Liua, Zhe Penga, Yang Liua,
Deyu Wanga,*, Xueliang Sund
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Lithium carbonate is one of the unavoidable impurities in cathode cathode side, and its influence on electrochemical performance
side. It could chemically react with LiPF6 based electrolyte and of LiNi0.8Mn0.1Co0.1O2. Although it possesses the excellent
LiPF6 powder to produce LiF & CO2, although it presents the electrochemical inertness, the exposed Li2CO3 particles are
excellent electrochemical inertness. The samples of Li2CO3- easily reacted with LiPF6 based electrolyte and LiPF6 powder to
coated and LiF-coated LiNi0.8Co0.1Mn0.1O2 are prepared to compare generate LiF, CO2 and POF3. To compare the influence of this
their influence on cathode’s behavior. After 200 cycles at 1C, in reaction, the samples of Li2CO3ꢀcoated and LiFꢀcoated
contrast to 37.1% of capacity retention for the Li2CO3- LiNi0.8Co0.1Mn0.1O2 are prepared. In contrast to the poor cyclic
coated material, the LiF-coated LiNi0.8Co0.1Mn0.1O2 still retains 91.9% stability of Li2CO3ꢀcoated material, LiFꢀcoated sample presents
of the initial capacity, which is similar to the fresh sample. much better cyclic stability, which is similar to the fresh
It demonstrates that Li2CO3‘s decomposition could seriously pristine materials.
deteriorate the cyclic stability if it is taken place during working.
Experimental section
Introduction
LiPF6 (Kanto Chemical Co., Inc.， 99%), LiTFSI (MMM, Ltd.,
Lithium ion battery (LIB) was the most successfully electrochemical 98%), and Li2CO3 (Aladin, >99%) were dried at 120 °C under
energy storage technology in the past decades. As a chemical vacuum in transfer chamber of Arꢀglove box for 48 h before used.
system, its performance should be significantly influenced by the The electrolyte of 1 molLꢀ1 LiPF6 dissolved in the mixed solution of
impurities, including the species brought by the components or inꢀ propyl carbonate (PC) and dimethyl carbonate (DMC) (1:1，v/v)
situ generated during working.1ꢀ5 The good understanding on their (Guotai Huarong Chemical New Material Co., Ltd.) was directly
chemical behavior and influence on cell performance should be used as received.
particularly important to develop highꢀquality commercial batteries.
The chemical stability of Li2CO3 was evaluated by immersing the
In cathode side, lithium carbonate is one of the unavoidable powder material (1 g) into 5 mL electrolyte, sealed into Alꢀplastic
°
impurities due to the carbonateꢀbased electrolyte and the metaꢀ packages, and then heated at 30, 55, 80 and 100 C for 12 h in Arꢀ
stability of cathode materials.6ꢀ9 Li2CO3 possessed the good glove box respectively. The powder of Li2CO3 (0.49 g) and LiPF6 (2
electrochemical inertness, so that it was considered as one of the g) were also mixed and sealed into Alꢀplastic packages to heat at 80
component to construct the cathode’s protective layer.10ꢀ12 In other °C for 12 h. To check the interactions between water and LiPF6, 10
hand, its appearance was often accompanied with the serious mL H2O is slowly dropped onto 2 g LiPF6 powder. Moreover, 2 g
capacity fading of cathode materials, especially for nickel rich layer LiPF6 powder is fast dissolved into 10 mL H2O.
oxides.13ꢀ16 This contradiction, namely the role of lithium carbonate,
LiNi0.8Co0.1Mn0.1O2 was synthesized by the highꢀtemperature
solidꢀstate method as published.16,17 The Li2CO3ꢀcoated
was not carefully investigated till date in our best knowledge.
Here we research the stability of lithium carbonate on LiNi0.8Co0.1Mn0.1O2 was obtained by storing LiNi0.8Co0.1Mn0.1O2 at
55 °C with saturated stream (15 kPa) in desiccator for one month.
The LiFꢀcoated LiNi0.8Co0.1Mn0.1O2 was prepared by immersing the
a.Ningbo Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, Ningbo, 315201, China
Li2CO3ꢀcoated sample into the electrolyte and heat at 80 °C for 12 h.
b.University of Chinese of Academy of Sciences, Beijing, 100049, China
c. Shanghai Institute of Space Power Source, Shanghai, 200245, China.
d.Nanomaterial and Energy Lab, Department of Mechanical and Materials
Engineering, University of Western Ontario, London, Ontario, N6A 5B9, Canada
* Corresponding author: wangdy@nimte.ac.cn
The electrode was composed of cathode material, super P and
polyvinylidenefluoride (PVdF) at the weight ratio 8:1:1 suspended in
NMP and the slurry was casted on aluminum foil current collectors.
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Electrode was dried at 120 °C under vacuum for 12 h and then
punched into φ16 mm disks. CR2032 coin cells were assembled with
the prepared electrode as cathode, lithium foil as anode, Celgard
2550® as separator and 1 molLꢀ1 LiPF6 in mixture solution of poly
carbonate (PC)/dimethyl carbonate (DMC) (1:1，v/v) as electrolyte.
The cells assembly was performed in Arꢀfilled MꢀBraun glove box,
where H2O and O2 were controlled less than 1 ppm. Linear sweep
voltammetry between 2.5ꢀ5.5 V vs. Li/Li+ at 0.1 mV/s rate is carried
out on electrochemical work station (Solartron, 1470E). The
electrochemical measurements were performed on battery tester of
Land 2001A (Wuhan, China) between 2.8ꢀ4.3 V vs. Li/Li+ at 25 °C
under various rates.
(a) Heated at 30 oC for different time
4 weeks
2 days
pure Li2CO3
pure LiF
30
40
50
60
o
( )
2θ
Li2CO3 + Electrolyte
(b)
: LiF
*
#
Heated at different temperature
: Li2CO3
*
*
100 o
80 o
55 o
C
The powders were characterized by Xꢀray diffraction with Bruker
D8 advanced diffractometer using CuKα (λ= 1.5406Å) radiation
(Bruker AXS, D8 Advance) between 10ꢀ60° by 0.02° step. The
analysis of microscopy was performed with scanning electron
microscopy (SEM, Hitachi, S4800) and transmission electron
microscopy (TEM, FEI, Tecnai F20, 200 kV). Surface analysis was
conducted with a PHI 3056 Xꢀray photoelectron spectrometer (XPS)
which was excited by an Mg Kα radiation at a constant power of 100
W (15 kV and 6.67 mA). The gases species were analyzed on gas
chromatograph mass spectrometer (GCꢀMS, Agilent 7890Bꢀ5977A,
EI). The column of GC is capillary column chromatograph (Agilent,
HPꢀPLOT/U).
C
#
#
C
30
40
50
60
2θ
(ο)
1# : Electrolyte
(c)
POF3
2# : Li CO + Electrolyte
2
3
1#
1#
2#
2#
Before
Heating
CO2
2#
1#
After
Heating
Results and Discussion
0
2
4
6
8
10
12
14
16
The purchased lithium carbonate could be attributed to the C2/c
space group of monoclinic system with particles size at 2~10 ꢁm, as
shown in Fig.1a. It presents a good electrochemical inertness, which
decomposition reaction is peaked at ~5.3 V vs. Li/Li+ as shown in
Fig.1b. Obviously, Li2CO3 is electrochemically stable inside of
lithium ion cells in current working conditions.
Time (min)
Figure 2 (a) XRD patterns comparison of Li2CO3 immersed in
electrolyte at 30 °C for 2 days and 4 weeks; (b) XRD patterns
comparison of Li2CO3 heating in electrolyte at 55, 80, 100 °C for 12
h; (c) GC-MS results and digital pictures of Al-plastic package
before and after heating (1# , 2#); (d) digital pictures of LiPF6
(a)
crystal dissolved by different ways (
dissolved in to water,
crystal)
①
② water is slowly dropped onto LiPF6
LiPF6 powder is fast
Li2CO3: JCPDS 22-1141
20
40
60
80
o
The chemical stability of Li2CO3 is evaluated via the experiment
that the mixture of 1 g Li2CO3 and 5 mL electrolyte was sealed in an
Alꢀplastic package and heated at 30, 55, 80, 100 °C, respectively.
When treated at 30 °C, the signal of Li2CO3 is gradually weakened
as the time prolonged, as shown in Fig. 2a. In contrast, LiF peaks are
detectable after 2 days storage, and their intensities are obviously
strengthened after 30 days. As the temperature increasing to 55 °C,
lithium fluoride is one of the dominant phases after 12 h heat
treatment. As the sample treated at 80 °C and 100 °C after 12 h, LiF
becomes the single crystalline phase in Fig. 2b. It indicates that
Li2CO3 should chemically react with LiPF6 to generate lithium
fluoride as the solid product.
2θ ( )
140
120
100
80
(b)
Oxidation Peak Potential 5.3V
Li2CO3
Blank
60
40
20
0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Votage (V)
Figure 1 Characterization of commercial Li2CO3 powder (a) XRD
pattern and SEM image (b) linear sweep voltammetry with
Li2CO3 electrode as working electrode, Li foil as counter and
reference electrode, and scanned with 0.1 mVs-1
2 | J. Name., 2012, 00, 1-3
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The generated gas in the package is tested by GCꢀMS. In all tested contains some insoluble stuff, as demonstrated in Fig. 2dꢀ2, which is
packages, CO2 and POF3 are the extremely dominant gaseous lithium fluoride identified by XRD. The pH value of solution is ~2,
species, which take a ratio more than 90%, as demonstrated in Fig. indicating H+ was generated during dissolving. It demonstrates that
2c. Carbon dioxide should be decomposed from Li2CO3, and POF3 LiPF6 tends to decompose with small amount of H2O to generate H+.
ꢀ
ꢀ
could be attributed to the reaction of PF6 group with the trace The probable reason for this phenomenon should be that PF6 ion
amount of moisture.18 Small chromatographic peaks at 9, 13 min and group couldn’t form the stable hydration layer with small amount of
16.2 min would be caused by the corrosion of HF. Combined with H2O.21ꢀ24 Therefore trace amount of HF may catalyze the
the XRD results, lithium carbonate should be spontaneously reacted decomposition of lithium carbonate with H+ as the catalyst, as shown
with electrolyte to generate LiF as the solid product, CO2 and POF3 in Pathway 2. This mechanism is still possible although we tend to
as the gaseous species.
believe Patherway1 is more reasonable.
To further explore the probable reason of this reaction, the Pathway2: 2H++ Li2CO3 → 2Li+ + CO2 +H2O
mixture of Li2CO3 and LiPF6 (1:1 in mole) powder were sealed and
heated at 80 °C for 12 h. The tested packages were inflated quickly.
LiPF6 + 2Li+ + H2O → POF3 +2H+ + 3LiF
The solid remnant is LiF and the dominant gaseous products are
The chemical stability of Li2CO3 with LiTFSI electrolyte and
POF3 and CO2, indicating this reaction between solids is same as
LiTFSI powder was also evaluated to compare the influence of
Li2CO3 with electrolyte. This result probably indicates that the
cations. No obvious inflation was observed after storing at 80 °C for
Li2CO3 decomposition could be preceded with the direct attack of
12 h, indicating Li2CO3 decomposition should be tightly related to
LiPF6 without the assistance of moisture, as shown in Pathway 1,
ꢀ
PF6 group. Since no candidate exhibits the possibility to totally
because both salts are extremely dried before the test.
replace LiPF6 in commercial system, the chemical decomposition of
lithium carbonate seems unavoidable if it was directly exposed to
electrolyte. The good understanding of its influence on cathode’s
Pathway1: LiPF6 + Li2CO3 → POF3 + CO2 + 3LiF
According to LiPF6 production procedure, it should contain trace performance should be helpful to construct the high quality cells. As
amount of HF,19 which could react with lithium carbonate to produce Li2CO3 is relatively easy to generate on surface of nickel rich
water. Although they are capable to react with water in organic content cathodes, LiNi0.8Co0.1Mn0.1O2 was selected as the target to
ꢀ
media, PF6 groups are quite stable in aqueous solution, e.g., NaPF6 prepare Li2CO3ꢀcoated and LiFꢀcoated samples in the following
ꢀ
aqueous solution utilized as the source of PF6 in the synthesis of comparison.
ionic liquids.20 The stability of LiPF6 powder with small amount of
In order to accelerate the Li2CO3 generation rate,
LiNi0.8Co0.1Mn0.1O2 sample was placed in glass dish and stored in
desiccator with water at 55 °C for one month. The glass jar is opened
water, the situation in this case, is not discussed in our best
knowledge.
Here dissolution behavior of LiPF6 salt (2 g) in water (10 mL) is daily to ensure the CO2 content. As show in Fig. 3a & 3b, the signal
compared with the experiments of quickly pouring LiPF6 powder of Li2CO3 was detected with XRD and XPS after humidity
into water and slowly dropping water into LiPF6 powder. In the first corrosion. And TEM identifies this decomposing layer with a
case, it forms the clear solution with a pH value of ~5, as shown in thickness of ~18 nm in Fig. 3g. It demonstrates that the sample after
Fig. 2dꢀ1. In the second case, smoke is released when water was humidity corrosion is Li2CO3ꢀcoated LiNi0.8Co0.1Mn0.1O2.
encountered to LiPF6 powder. After dropping, the solution still
(a)
(c)
(b)
C
-
Standard
LiF
684.9 eV
Sample
284.8 eV
Li2CO3
*
#
LiF
F 1s
C1s
289.6 eV
Water
LiF coated
Li2CO3
LiF-coated
LiF-coated
Li2CO3-coated
Fresh
*
Li2CO3 coated
Fresh
Li2CO3-coated
Fresh
#
20
30
40
50
60
295
290
285
280
680
685
690
695
21
22 44 45 46
(o)
o
Binding Energy (eV)
2
θ
Binding Energy (eV)
2θ ( )
Figure 3 (a) XRD patterns and C1s (b) and F1s (c) XPS spectrum of fresh, Li2CO3-coated and LiF-coated LiNi0.8Co0.1Mn0.1O2 cathode material;
SEM and HRTEM images of (d), (e) fresh, (f), (g) Li2CO3-coated and (h), (i) LiF-coated material
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According to the previous results, the Li2CO3ꢀcoated sample was
mixed with electrolyte and heated at 80 °C for 12 h to prepare LiFꢀ
coated sample. As shown in Fig. 3a, Li2CO3 is totally disappeared,
and LiF becomes the only detectable impurity after this treatment.
The weak broaden peak from 687.5 ꢀ 689.0 eV in Fig. 3c may be
attributed to LixPOyFz during the heat process in electrolyte.25
Combined with the TEM images (Fig. 3g, 3i), the dense Li2CO3
layer changes to the porous LiF membrane. These results indicate
that LiFꢀcoated LiNi0.8Co0.1Mn0.1O2 was obtained after the
electrolyte attack, which is consistent with the previous results.
4.4
4.0
3.6
3.2
2.8
(a)
Initial Discharge at 0.1C
Fresh
Li2CO3-coated
LiF-coated
0
40
80
120
160
200
Capacity ( )
mAhg-1
(b)
200
150
100
Fresh
Li2CO3-coated
Charge Current 0.1C
Li-coated
0.1C
0.5C
1C
2C
1C
0.1C
(c)
5C
Rate
200
150
100
50
Schematic 1 Surface reaction of Li2CO3-coated material in LiPF6
electrolyte
100
80
The electrochemical performances of the asꢀsynthesized, Li2CO3ꢀ
coated and LiFꢀcoated LiNi0.8Co0.1Mn0.1O2 are compared in Fig. 4.
The discharge capacity of the fresh sample is 192.5 mAhgꢀ1. The
discharge capacity of Li2CO3ꢀcoated sample was dropped to 170.5
mAhgꢀ1 (Fig. 4a). Since the discharge plateau is almost overlapped at
the initial part, the lost capacity should be corresponding to the loss
of active material instead of polarization. With this hypothesis, the
decomposition ratio should be ~12%. After the electrolyte attack,
sample capacity was recuperated to 174.2 mAhgꢀ1. It is probably
related to the relatively low molecule weight of lithium fluoride.
Rate performance shares the same trend, as shown in Fig. 4b. The
fresh, Li2CO3ꢀcoated and LiFꢀcoated samples deliver 150.3, 124.1
and 132.0 mAhgꢀ1 at 5 C respectively.
Fresh
Li2CO3-coated
60
LiF-coated
40
0
50
100
150
200
Cycle Number (N)
Figure 4 Electrochemical property of fresh, Li2CO3-coated and LiF-coated
LiNi0.8Co0.1Mn0.1O2 cathode material. (a) the discharge curves in first
cycle at 0.1C; (b) rate capability at 0.1C, 0.2C, 0.5C, 1C, 2C and 5C; (c)
cyclic stability tested at 1C.
Conclusion
All samples are cycled under 1C rate and their results are The role of Li2CO3 in LPF6 electrolyte has been investigated in this
compared in Fig. 4c. The fresh material keeps 149.1 mAhgꢀ1, 87.2% work. Although it is electrochemically stable, Li2CO3 could be
of the initial capacity after 200 cycling. The discharge capacity of chemically decomposed to LiF as the solid product, CO2 & POF3 as
Li2CO3ꢀcoated sample is seriously faded, only 37.1% retained after gaseous species. The generated gaseous species would be absorbed
200th cycle. In contrast, the LiFꢀcoated sample still presents 143.5 on cathode surface, aggregate the discharge polarization and
mAhgꢀ1, namely 91.9% of the initial discharge capacity, in the 200th deteriorate its cyclic stability. Obviously, if Li2CO3 impurity was not
cycle.
able to well control in practical production, the treatment at high
temperature to release the gas should be very helpful to keep the
cell’s cyclic stability.
This improvement on cyclic stability for LiFꢀcoated
LiNi0.8Co0.1Mn0.1O2 should be related to the reaction of Li2CO3
decomposition. According to the results in the former part, if it Acknowledgements
reacts inside of cell, the gaseous species of Li2CO3 decomposition
We thank the 863 project (Grant No. 2013AA050906), the National Natural
should be adsorbed on the surface of cathode material. Gas
adsorption would decrease the active surface area and gradually
aggregate the cell’s polarization. The similar phenomenon is
observed on zinc electrode, where the overꢀpotential is quickly
augmented by the H2 absorption on Zn electrode.26,27 As for LiFꢀ
coated sample, its interface between cathode and electrolyte is not
interfered by the generated gas. Therefore it presents a stable cyclic
stability.
Science Foundation of China (Grant No. 51572273), Hundred Talents
Program of the Chinese Academy of Sciences, Zhejiang Province Key
Science and Technology Innovation Team (Grant No. 2013PT16), and
Ningbo Natural Science Foundation (Grant No.2015A610250). Yujing Bi
thanks the scholarship of International Joint PhD Training Program
sponsored by University of Chinese Academy of Sciences.
4 | J. Name., 2012, 00, 1-3
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TOC
100
80
60
40
20
0
Fresh
LiF coated
Li2CO3 coated
0
50
100
150
200
Cycle number (N)