A novel electrolyte additive for improving the interfacial stability of LiMn2O4 cathode lithium-ion batteries at elevated temperature

Article abstract of DOI:10.1039/c8ra08355j

Methanesulfonic acid 2,2,3,3-tetrafluoropropyl (TFPMS) is newly explored as a protection additive to improve the interfacial stability of LiMn₂O₄ cathode/electrolyte at an elevated temperature. At 1C rate, the addition of 0.5 vol% TFPMS improves the capacity retention of an LiMn₂O₄/Li cell from 56.9% to 72.9% after 200 cycles at 55 °C. Electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM) results suggest that the electrolyte with 0.5 vol% TFPMS forms a thinner and less resistive interface. X-ray photoelectron spectroscopy (XPS) analysis reveals that the addition of TFPMS restrains the formation of LiF and Li₂CO₃. Moreover, X-ray diffraction (XRD) and inductively coupled plasma (ICP) analysis confirms the effectiveness of TFPMS-enhanced structural stability of LiMn₂O_4.

Full text of DOI:10.1039/c8ra08355j

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A novel electrolyte additive for improving the
interfacial stability of LiMn O cathode lithium-ion
2
4
Cite this: RSC Adv., 2018, 8, 38831
batteries at elevated temperature†
T. Huang, X. Zheng, G. Fang, Y. Pan, W. Wang and M. Wu
*
Methanesulfonic acid 2,2,3,3-tetrafluoropropyl (TFPMS) is newly explored as a protection additive to improve
the interfacial stability of LiMn cathode/electrolyte at an elevated temperature. At 1C rate, the addition of
.5 vol% TFPMS improves the capacity retention of an LiMn /Li cell from 56.9% to 72.9% after 200 cycles at
C. Electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM) results
2 4
O
0
2 4
O
ꢀ
5
5
Received 9th October 2018
Accepted 5th November 2018
suggest that the electrolyte with 0.5 vol% TFPMS forms a thinner and less resistive interface. X-ray
photoelectron spectroscopy (XPS) analysis reveals that the addition of TFPMS restrains the formation of LiF
DOI: 10.1039/c8ra08355j
2 3
and Li CO . Moreover, X-ray diffraction (XRD) and inductively coupled plasma (ICP) analysis confirms the
rsc.li/rsc-advances
effectiveness of TFPMS-enhanced structural stability of LiMn O
2 4.
the new lithium salts due to some of their own drawbacks have
not fully replaced LiPF in LiMn -based batteries in recent
Introduction
6
2 4
O
LiMn O is an ideal high-power cathode material for lithium- years.
2
4
ion batteries because of its high thermal stability, low
Using a functional additive is the most economical and
manufacturing cost, environmentally friendly nature, high effective method. W. T. Li et al. mentioned that the addition of
energy density and long life cycles. However, capacity fading of 3% DMAc to an electrolyte increases the thermal stability of
LiMn
for practical applications. The reasons for capacity fading and sition of electrolyte decomposition products on LiMn
poor thermostability are the following possible mecha- particles. Y. K. Li et al. reported that the stability of batteries
2 4
O
-based batteries at high temperatures is still a barrier the electrolyte and reduces both surface corrosion and depo-
O
2 4
1
7
1
–5
nisms : (1) LiPF -based carbonate electrolyte decomposition, using electrolytes with 0.1 wt% heptamethyldisilazane can be
(
6
3
+
2+
4+
ꢀ
18
2) manganese dissolution: 2Mn (s) ¼ Mn (aq.) + Mn (s) improved aer storage at 60 C for 7 days. L. Li et al. used tris
on the surface of the LiMn O electrode, (3) irreversible crystal (trimethylsilyl) borate (TMSB) as an effective SEI electrolyte
2
4
phase transition (Jahn–Teller distortion), and (4) oxygen additive to improve the cycling performance of an LiMn
2
O
4
19
ꢀ
deciency.
lithium-ion battery at both room temperature and 55 C.
6
–9
10–12
Methods such as element doping and surface coating
Park et al. added 2 wt% uoroethylene carbonate (FEC) into an
have been proposed in previous studies. The electrolyte is one of electrolyte (EC/DEC/PC with 1 M LiPF
). When 2 wt% FEC was
6
ꢀ
the main factors for serious deterioration of LiMn
2
O
4
-based added, the capacity retention at 60 C aer 130 cycles signi-
2
0
batteries at elevated temperatures. Recently, researchers have cantly improved by about 20%. W. Li et al. reported that
used new lithium salts and functional electrolyte additives to trimethyl borate (TPB) can stabilize the LiMn /carbonate-
based electrolyte interface. Nan et al. evaluated the effec-
2 4
O
2
1
improve cyclic stability.
It is reported that lithium diuoro(oxalato)borate, instead tiveness of 0.5 wt% MMDS in LiMn
2
O
4
-based LIBs on the
ꢀ
of LiPF
cathode signicantly aer 100 cycles at 25 C and 60 C. X. J. 85 C.
Huang et al. reported that LiFNFSI as a single electrolyte salt
In this study, methanesulfonic acid 2,2,3,3-tetrauoropropyl
at 60 C. Some new lithium (TFPMS) was synthesized and used as a new SEI-forming addi-
salts, such as LiTFSI, LiFAP, and LiBETI, with properties tive for lithium-ion batteries. It was found to be effective for
6
, can improve the capacity retention of an LiMn
2
O
4
cycling performance at 60 C and capacity retention storage at
ꢀ
ꢀ
13
ꢀ
22
ꢀ
14
2 4
improves the stability of LiMn O
1
5,16
superior to those of LiPF have been evaluated.
However, stabilizing the interface between LiMn
2
O
4
and a carbonate-
6
ꢀ
based electrolyte at 55 C. The effect of TFPMS was investi-
gated using electrochemical impedance spectroscopy (EIS), X-
ray photoelectron spectroscopy (XPS), transmission electron
microscopy (TEM), nuclear magnetic resonance spectroscopy
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002,
P. R. China. E-mail: mxwu@irsm.ac.cn
(
(
NMR), X-ray diffraction (XRD) and inductively coupled plasma
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra08355j
ICP).
1
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1
atmosphere glove box. H NMR data of the electrolytes were
Experimental
Preparation of electrolyte
tested by NMR (AVANCE III, Bruker Biospin) using a CD CN
3
solvent. The crystal structure was identied by X-ray diffraction
TFPMS was synthesized in our laboratory. 2,2,3,3-Tetrauoro-1- (XRD, Bruker D8 ADVANCE) using Cu Ka radiation. ICP analysis
propanol (176 g, 1.33 mol) and triethylamine (157 g, 1.56 mol) was carried out on IRIS Intrepid II XSP. The cycled lithium
ꢀ
were added to a round ask, stirred and cooled to 10 C in an ice electrodes for ICP analyses were rinsed with DMC solvent three
3
bath. Then, under stirring, methylanesulfonyl chloride (145 g, times and dissolved in 5.0 wt% HNO .
1
.26 mol) was added dropwise from a dropping funnel with the
ꢀ
reaction mixture temperature controlled below 10 C. Next, the
mixture was stirred at 50 C for another 6 h. Then, the mixture
ꢀ
Results and discussion
was cooled to room temperature and deionized water (300 ml)
was added. The organic layer was washed with deionized water 6
times and dried over anhydrous magnesium sulphate. Meth-
anesulfonic acid 2,2,3,3-tetrauoropropyl ester was obtained by
Fig. 1 presents the cyclic stability of LiMn O electrodes in
2
4
ꢀ
electrolytes without and with 0.5 vol% TFPMS at 1.0C at 55 C
aer 200 cycles. The discharge capacity of LiMn without an
but became
9.7 mA h g aer 200 cycles. Aer adding 0.5 vol% TFPMS
2 4
O
ꢁ
1
additive at the rst cycle was 105 mA h g
fractional distillation under vacuum with 185 g (70%) yield.
ꢁ
1
5
1
H-NMR (CDCl , ppm): 5.95 (tri, 1H); 4.55 (tri, 2H); 3.12 (s,
3
into the electrolyte, the cyclic stability of LiMn
2
O
4
improved,
1
3
3
H). C-NMR (CDCl , ppm): 115.87 (tri); 113.38 (tri); 111.47 (tri);
3
ꢁ1
becoming 76.6 mA h g . The coulombic efficiencies of 0.5 vol%
110.88 (tri); 108.99 (tri); 106.50 (tri); 63.65 (tri); 37.84.
TFPMS-containing Li/LiMn O cells were better than that of the
2
4
6
Electrolytes of 1 M LiPF /EC + DMC + EMC (Dongguan
cell with no additive. Apparently, TFPMS could provide protec-
Shanshan Battery Materials Co., Ltd.) with and without 0.5 vol%
TFPMS were prepared in an argon atmosphere glove box (the
oxygen and water contents were less than 5 ppm).
tion for LiMn O at an elevated temperature. The charge–
2
4
ꢀ
discharge proles of the LiMn O /Li cells cycled at 55 C are
2 4
shown in ESI 2.† The change in the charge–discharge platform
of the cell without an additive was larger than that observed for
the cell containing TFPMS additive. Unlike the cell with the
electrolyte without additive, the cell with the electrolyte con-
taining 0.5 vol% TFPMS additive exhibited negligible polariza-
tion and smooth charge–discharge proles.
Cell assembly
2 4
The LiMn O electrode with an active mass loading of about
ꢁ
2
2
.3 mg cm was prepared by combining 80 wt% LiMn
2 4
O ,
10 wt% polyvinylidene uoride (PVDF) and 10 wt% acetylene
The discharge curves and corresponding differential capacity
dQ/dV) plots of LiMn O /Li cells cycled in electrolyte with and
2 4
black. Also, 2025-type coin cells were assembled in an argon
atmosphere glove box using a lithium sheet as the anode, the
(
without TFPMS aer different cycles are shown in Fig. 2. From
Fig. 2(a) and (c), we can see that the discharge capacity and
voltage plateau of the electrode in the electrolyte without the
additive fade quickly compared with those of the electrode in
the electrolyte with 0.5 vol% TFPMS aer 200 cycles, indicating
that the latter exhibits smaller electrochemical polarization.
Differential capacity plots exhibit two main peaks located at
around 3.98 V and 4.1 V at the 1st cycle, as shown in Fig. 2(b)
and (d). All these peaks shi negatively with cycling in the
electrolyte with and without the additive. This shows that the
2 4 6
above LiMn O electrode as the cathode, 1 M LiPF /EC + DMC +
EMC with and without 0.5 vol% TFPMS as electrolytes (ESI 1†),
and Celgard 2325 porous polypropylene as a separator.
Electrochemical measurements
Cycling performance testing of LiMn
2 4
O /Li cells was carried out
on a Land cell tester, CT2001A. The formation cycles were as
ꢁ1
follows: 0.1C (1C ¼ 148 mA g ) three times, 0.2C three times,
ꢀ
0
.5C three times, and 1.0C three times at 25 C under constant
current conditions from 3.0 to 4.3 V. The LiMn O /Li cells were
2
4
ꢀ
then cycled at 1.0C at 55 C for the cycling performance test. The
LiMn
2
O
4
/Li cells were then cycled at 0.1C, 0.2C, 0.5C, 1C, and
ꢀ
5C at 25 C for rate performances. The EIS measurements were
tested through a frequency response analyzer (VSP, Bio-logic).
EIS measurements were tested at the fully delithiated state of
4
.3 V for LiMn O /Li cells. The frequency range was from 100
2 4
kHz to 10 MHz, and the amplitude was 5 mV.
Physical characterizations
2 4
The cycled LiMn O was disassembled in a glove box and
washed three times with DMC solvent to remove the residual
electrolyte. TEM measurements were performed with JEM-2010,
JEOL. The XPS measurements were obtained with an X-ray
photoelectron spectrometer (ESCALAB 250, USA) to investigate
the interfacial components of the electrodes. The electrolyte
Fig. 1 The cyclic stability of LiMn
2
O
4
electrodes in the electrolytes
ꢀ
was taken from the cells, which were disassembled in an argon without and with 0.5 vol% TFPMS at 1.0C at 55 C after 200 cycles.
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Fig. 2 Selected discharge curves and corresponding dQ/dV plots of
2 4
LiMn O electrode in the electrolytes without (a and b) and with
0
.5 vol% TFPMS (c and d).
peak position shi becomes less signicant in the TFPMS-
containing electrolyte compared to that for the electrolyte
without the additive aer cycling, which can be ascribed to the
protection provided by the cathode interface lm due to TFPMS.
The rate capability of the cathode is inuenced by polariza-
tion. A rate performance study is carried out, as shown in Fig. 3.
It can be seen that the cell with 0.5 vol% TFPMS displays better
rate performance than that without the additive (especially at
Fig. 4 EIS of LiMn
2
ꢀ
O
4
/Li cells with and without 0.5 vol% TFPMS after
ꢀ
(a) formation at 25 C and (b) 200 cycles at 55 C.
5C), suggesting that the incorporation of TFPMS can decrease
polarization.
(
(
59.53 U) is smaller than that of the cell without the additive
EIS measurements were obtained at the fully delithiated
state of 4.3 V for LiMn
mainly include bulk resistance (R
63.51 U), and the lower R value implies that SEI due to TFPMS
f
2
O
4
/Li cells. The EIS spectra of the cells
), SEI resistance (R ), and
can be thinner. Rct of the cell with TFPMS (81.31 U) is signi-
cantly smaller than that of the cell without the additive (116.6
U), showing that the charge transfer is easier through the lm;
this conrms the ability of TFPMS in hindering the increase in
electrode polarization. This is in agreement with the results
shown in Fig. 2.
e
f
charge transfer resistance (Rct). Fig. 4 presents the EIS of
LiMn O /Li cells with different electrolytes. Re is the bulk
2
4
resistance of the cell, which reects the electric conductivity of
the electrolyte, separator, and electrodes. Also, 0.5 vol%
concentration of TFPMS in the electrolyte was shown to have
a negligible effect on the electrical conductivity of the electro-
lytes. Thus, the values of R
the same. Aer the formation cycle at 25 C, it can be found that
1
Fig. 5 shows H NMR data of the electrolytes with 0.5 vol%
ꢀ
ꢀ
TFPMS before and aer formation at 25 C and 200 cycles at 55 C.
The peaks at 5.9–6.2 ppm were assigned to –CF H of TFPMS, and
the peak appearing at 3.17 ppm corresponded to –CH of TFPMS.
The TFPMS peaks disappeared aer 200 cycles. Based on EIS and
NMR results, we believe that SEI on the LiMn surface is formed
e
with different electrolytes are nearly
ꢀ
2
3
R
f
and Rct with different electrolytes are nearly the same.
ꢀ
However, aer 200 cycles at 55 C, R of the cell with TFPMS
f
2 4
O
gradually during cycling due to TFPMS.
1
Fig. 3 The rate performances of LiMn
trolytes with and without TFPMS at different charge/discharge rates.
2
O
4
/Li cells cycled in the elec- Fig. 5 H NMR data of the electrolytes with 0.5 vol% TFPMS before (a)
ꢀ
ꢀ
and after formation at 25 C (b) and 200 cycles at 55 C (c).
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TEM images (Fig. 6) show the edges of LiMn
aer being cycled with different electrolytes at 55 C. Aer LiMn O cathode electrode with TFPMS is covered with less
2
O
4
particles smaller than those without the additive, suggesting that the
ꢀ
2
4
cycling, in the electrolyte without the additive, the LiMn O
inorganic degradation products. This leads to decreased R and
f
2
4
particles are covered with a thick deposit, as shown in Fig. 6(a). Rct of the cells with 0.5 vol% TFPMS. The C 1s peaks located at
22
The TEM image in Fig. 6(b) shows that a thin and uniform lm 290.3 eV and 285.7 eV correspond to the PVDF binder. The
2
9
29
30
exists on LiMn
has been formed due to TFPMS.
Fig. 7 presents the XPS spectra of the observed elements on differences seen, which is in accordance with the O 2p spectra.
2
O
4
particles, indicating that a protective SEI lm peaks at 284.7 eV, 288.5 eV, and 286.5 eV are assigned to
graphite, C]O and C–O, respectively. There are only minor
ꢀ
31
the LiMn O electrode cycled in different electrolytes at 55 C. However, there is a small peak at 292.7 eV from C–F, which
In the Mn 2p spectra, three main peaks can be found. The peaks indicates the existence of C–F due to the decomposition of
located at 641.7 eV, 642.9 eV, and 653.6 eV correspond to Mn , TFPMS on the cathode surface lm. This is because C–F is the
. The O 1s spectrum is dominated by the functional group of the TFPMS additive. The existence of S
peak of O–Mn (529.6 eV ) in metal oxide, C]O (531.3 eV ), indicates that TFPMS contributes to the surface lm of the
2 4
3
+
4
+
1/2 23
Mn , and Mn 2p
24
25,26
25,26
25
C–O (533.3 eV ), and Li
2
CO
3
(532 eV ). There are two char- cathode. The S 2p spectrum at 169 eV in our analysis can be
27
32,33
acteristic peaks in the F 1s spectra: LiF (684.5eV ) and PVDF assigned to ROSO
(
2
Li.
The possible reaction paths of TFPMS
28
34
687.6 eV ). The Mn peaks of Mn 2p and Mn–O of O 1s for the are presented in Fig. 8.
electrode with TFPMS are stronger than that for the electrode
without the additive, conrming that the CEI lm from elec- different electrolytes at 55 C. The intensities of all the LiMn O
Fig. 9 presents the XRD spectra of LiMn
2
O
4
electrodes cycled in
ꢀ
2
4
trolyte decomposition is thinner due to TFPMS. When 0.5 vol% diffraction peaks of the electrodes become weaker compared to
TFPMS is added, the intensity of the C]O peak of O 1s that of the pristine electrode, which may be due to the dissolution
2+
19,35,36
decreases, whereas the intensity of the C–O peak of O 1s of Mn in the electrolyte at an elevated temperature.
increases, suggesting that TFPMS can efficiently hinder the found that the peak intensities of the electrodes with TFPMS are
polymerization of EC solvent. The intensities of Li CO of O 1s stronger than those of the electrode without the additive. The
It can be
2
3
and LiF of F 1s on the LiMn
2
O
4
electrode with TFPMS are much diffraction peaks (111) of the cycled cathodes shi to higher
angles, whereas the extent of the shi in the peaks of the electrode
without the additive is much larger than that with the additive.
This indicates that the extent of degradation of the spinel structure
of LiMn O in the electrode with TFPMS is much weaker than that
2
4
without the additive, which is in accordance with the Mn 2p
spectra as discussed above.
Manganese metal dissolved from LiMn
2 4
O is also deposited
on the anode, which can be conrmed by analyzing the
ꢀ
2 4
O cathodes after 200 cycles at 55 C in
Fig. 6 TEM images of LiMn
the electrolyte (a) without and (b) with 0.5 vol% TFPMS.
Fig. 8 The possible reaction paths of TFPMS on the cathode.
Fig. 7 The XPS spectra of LiMn
electrolytes at 55 C.
O
2 4
cathodes cycled in different Fig. 9 The XRD spectra of LiMn
O
2 4
cathodes cycled in different
ꢀ
ꢀ
electrolytes at 55 C.
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elemental contents on the lithium electrode. The contents of 11 X. Q. Wang, O. Tanaike, M. Kodama and H. Hatori, J. Power
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1
the lithium electrode cycled in the electrolyte without TFPMS
Electrochim. Acta, 2013, 108, 727–735.
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In summary, TFPMS can be used as an effective electrolyte
additive for the formation of a stable SEI layer on the LiMn O
2
4
ꢀ
electrode at 55 C. The addition of 0.5 vol% TFPMS can increase
the cycling and rate performance of LiMn
2
O
4
/Li cells. Aer 200
ꢀ
18 Y. Li, R. Zhang, J. Liu and C. Yang, J. Power Sources, 2009,
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from 56.9% to 72.9% with TFPMS. The stable SEI layer derived
from TFPMS is thinner and less resistive, and we also observe
enhanced structural stability of LiMn O .
1
1
2
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