Cyclophosphazene-based hybrid polymer electrolytes obtained: Via epoxy-amine reaction for high-performance all-solid-state lithium-ion batteries

Article abstract of DOI:10.1039/c9ta05028k

A polymer electrolyte (PE) is considered as a kind of promising candidate in lithium batteries due to its high energy density and no leakage issue compared to the liquid electrolyte. However, poly(ethylene oxide) (PEO)-based electrolytes also have some pr

Full text of DOI:10.1039/c9ta05028k

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DOI: 10.1039/C9TA05028K
Journal Name
ARTICLE
Cyclophosphazene-based hybrid polymer electrolyte via epoxy-
amine reaction for high-performance all-solid-state lithium-ion
batteries
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
Cai Zuo , Mengling Yang , Zhijun Wang, Ke Jiang, Sibo Li, Wen Luo, Dan He, Chengmei Liu,* Xiaolin
Xie and Zhigang Xue*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Polymer electrolyte (PE) is considered as a kind of promising candidates in lithium batteries due to its high energy density
and no leakage issue compared to the liquid electrolyte. However, poly(ethylene oxide) (PEO)-based electrolytes also bring
some problems such as inferior mechanical strength, low ionic conductivity, poor thermal stability, and flammability. The
flaws often trigger the short circuit due to the growth of the lithium dendrites, restrict the potential application of lithium-
ion batteries and cause severe safety problems as well. Herein, we propose a strategy to overcome these defects. Hexa(4-
aminophenoxy)cyclotriphosphazene (HAmCP) is utilized as the core, and polyethylene glycol diglycidyl ether (PEGDE) is
employed as the cross-linking agent via the facile amino-epoxy reaction. The structure and purity of the substrate were
confirmed by Fourier transform infrared (FTIR), and nuclear magnetic resonance ( H-, 13C-, 31P NMR) characterizations. The
formed network polymer electrolyte is mechanically robust and flexible owing to the hybrid cyclophosphazene core and the
1
o
cross-linked structure. The PE is thermally stable under the temperature of 368 C with a 5% decomposition of the polymer
matrix and can be stretched up to 127% elongation without a break. Meanwhile, the PE also exhibits the fire-retardant ability
to some extent compared with the pure PEO-based polymer matrix formed via the radical polymerization of polyethylene
glycol diacrylate (PEGDA). Furthermore, the all-solid-state PE owns an ionic conductivity up to 10 -4 S cm-1 at 30 C and
excellent interfacial stability with the lithium anode, contributing to stable cycling for high-performance batteries.
o
inorganic filler to form composite polymer electrolyte and
1
1-28
cross-linked network structure.
2
SiO is the most extensively
1
. Introduction
investigated nanofiller due to the excellent ionic conductivity,
but the aggregation of inorganic filler would lead to polymer
electrolytes brittle and easy to creak, and the unevenly
dispersed inorganic filler would result in the growth of lithium
dendrites in which mechanical strength is not tough enough to
Lithium-ion batteries (LIBs) as energy storage devices are
considered the most widely used due to the boosting expansion
of the consumer electronics market, increasing demands for
wearable, portable intelligent devices and applications for
electric vehicles.1
-6
Polymer electrolytes are promising
2
9-33
suppress the lithium dendrites.
Compared to the inorganic
alternatives for LIBs as they can solve the leakage problem and
fillers, the strategy of inorganic-organic hybrid offers a better
solution to distribute evenly nanofiller and retain characteristics
of inorganic core. Li and co-workers fabricated hybrid polymer
electrolytes with excellent ionic conductivity and robust
mechanical strength via a facile one-pot reaction of octakis(3-
glycidyl-oxypropyldimethylsiloxy)octasilsesquioxane
amine-terminated poly(ethylene glycol) (PEG).
7
-
offer a higher energy density compared to liquid electrolytes.
1
0
However, polymer electrolytes still have some depressing
defects. First, poly(ethylene oxide) (PEO)-based electrolytes
usually suffer poor mechanical strength due to the fragile
nature of PEO. Second, PEO-based electrolytes typically exhibit
low ionic conductivity at room temperature, since the
and
3
4-36
+
crystalline PEO prevents the migration of the Li due to the
Besides, PEO-based electrolytes usually start to decompose
at about 250 C with 95% residue left and are readily to combust
slowed down polymer chain dynamics. Third, the low
decomposition temperature of PEO-based electrolyte may lead
to safety issues such as inflammation and even explosion.
To address the first issue, extensive efforts have been made
to enhance the mechanical strength of polymer electrolytes
such as copolymerization with a functional monomer, adding an
o
which may cause severe safety problems. Moreover, the
flammability of polymer electrolytes is indispensable factors
which may bear the risk of combustion and explosion. Hence, it
is crucial to fabricate a polymer electrolyte to solve the
aforementioned issues to achieve safe and high-performance
LIBs. The design of flexible solid polymer electrolytes which
possess simultaneously high thermal stability, robust
mechanical strength, good electrochemical performances and
safety performances is very challenging for lithium batteries.
Meanwhile, various flame-retardant materials are emerging for
flame resistant applications. The resistance to fire initiation,
propagation and its extinguishment are influenced by the
Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry
of Education, School of Chemistry and Chemical Engineering, Huazhong University
of
Science
and
Technology,
Wuhan
430074,
China.
E-mail:
zgxue@mail.hust.edu.cn, liukui@mail.hust.edu.cn.
a
These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
presence of bromine, phosphorus, antimony, tin and boron in room temperature for 15 min under inert atmosphere.
View Article Online
flame retardant compounds. Although the halogenated
additives exhibit high effectiveness of flame retardancy, they
pose a possible threat due to the release of highly toxic and
carcinogenic gases during combustion. In comparison to the
brominated derivatives, the organophosphous is considered as
a good alternative because of enhanced char yield and
unharmful decomposition. Thus, the novel halogen-free
phosphazene was extensively investigated to improve the
functionalization of flame-retardant materials due to its
DOI: 10.1039/C9TA05028K
2 3
Preheated K CO (29.9 g, 216 mmol) was rapidly added to the
solution and the mixture was heated at reflux under vigorous
stirring for 4 days under argon atmosphere. After that, the
solvent was evaporated under reduced pressure and the
concentrated solution was poured into the excessive water. The
suspension was filtered and the residue was washed with water
for three times. The white powder was harvested after drying
o
1
under vacuum at 50 C for 2 days (12.5 g, Yield: 93%). H-NMR
thermal stability, flame-retardancy and ease of chemical (DMSO-d
6
, TMS, ppm): 9.92 (1H, -NH), 6.79-7.45 (4H, dd, Ar-H),
3
7-45
13
substitution of P-Cl bond.
2.04 (3H, -CH3). C-NMR (DMSO-d , TMS, ppm): 168.0 (C=O),
6
3
1
Notably, Allcock et al. reported the pioneering work 144.9 (C-O), 136.4 (C-N), 120.5 (CH), 119.9 (CH), 23.8 (CH ). P-
3
regarding polyphosphazene and cyclophosphazene with
pending short PEG chain applying for polymer electrolyte. To
this work, cyclophosphazene-based hybrid polymer electrolytes
NMR (DMSO-d
.3 Synthesis of hexa(4-aminophenoxy)cyclotriphosphazene
HAmCP)
HAcCP (25.0 g, 26.6 mmol) was dissolved in 300 mL of methanol.
0 mL of aqueous solution of NaOH (57.96 g, 1.449 mol) was
6
, ppm): 9.18.
2
(
(CHPEs) were designed and fabricated for the first time for all-
solid-state lithium-ion batteries by a facile one-pot reaction
with hexa(4-aminophenoxy)cyclotriphosphazene (HAmCP) and
different molecular weights of polyethylene glycol diglycidyl
ether (PEGDE). The novel CHPE was designed considering as
follow. First, the PEO main chains are anticipated to transport
lithium ion via polymer chain dynamics. Second, the thermal of distilled water for three times. The resulting white powder
stability and flame retardancy of CHPE are endowed by hybrid was obtained after drying under vacuum at 50 C for 2 days
6
slowly added to the solution under inert atmosphere, and the
o
mixture was refluxed at 70 C under vigorous stirring for 72 h.
After that the suspension was filtered and washed with excess
o
cyclophosphazene core. Meanwhile, the cyclophosphazene (14.6 g, Yield: 80%).
1
H-NMR (DMSO-d , TMS, ppm): 6.41-6.52
6
core can suppress the forming of crystalline PEO which would
be harmful for the migration of lithium ion. Third, the cross-
linking structure can offer strong mechanical strength to
prevent the risk of breakage. The novel CHPE exhibits excellent
stretchability with a stretch of 127% of the original length and a
tension of 3.3 MPa. Compared to PEO-based electrolyte, the
CHPE500 is enabled by self-extinguishment. Over 1200 h cycling
of Li/CHPE/Li symmetrical cell indicates that the lithium
dendrites are suppressed efficiently. Furthermore, the
13
(
2 6
4H, dd, Ar-H), 4.91 (2H, Ar-NH ). C-NMR (DMSO-d , TMS,
31
ppm): 145.3 (C-N), 140.5 (C-O), 120.6 (CH), 113.9 (CH). P-NMR
DMSO-d , ppm): 10.67.
.4 Synthesis of polyethylene glycol diglycidyl ether (PEGDE)
The PEGDE was synthesized according to the reported
procedures with slight modification. The epichlorohydrin
2.78 g, 0.03 mol) was added dropwise under vigorous stirring
to a mixture of melting polyethylene glycol diol (M = 2000)
(
2
6
4
7
(
n
LiFePO
under the charge and discharge rate of 0.1 C and 0.2 C.
4
/CHPE/Li cell exhibits excellent cycling performance (10.0 g, 0.005 mol), distilled water (0.12 g, 0.12 mL) and pellets
of sodium hydroxide (0.12 g, 0.03 mol). Then, the mixture was
o
stirred at 40 C for 5 h. After that, the mixture was filtered in
vacuum and washed with dichloromethane three times (3 × 30
mL). The combined filtration was dried with sodium sulfate and
2
2
. Experimental
.1 Materials
Lithium bis(trifluromethanesulfonyl)imide (LiTFSI, Aladdin), concentrated under reduced pressure to obtain solid powder
hexachlorocyclotriphosphazene (HCCP, 98%, Aladdin), 4- without further purification (8.35 g, Yield: 79.0%). The resulting
1
acetamidophenol (98%, Aladdin), polyethylene glycol diacrylate product was denoted as PEGDE2000. H-NMR (chloroform-d,
(
PEGDA, M
diglycidyl ether (PEGDE500, M
as received. Polyethylene glycol diol (M
was purchased from Alfa Aesar. Anhydrous tetrahydrofuran molecular weight of polyethylene glycol diol for 5 g and 20 g,
THF) and acetone were distilled under reduced pressure prior respectively.
n
= 600, 98%, Aladdin) and polyethylene glycol TMS, ppm): 3.81-3.78 (m, 2H), 3.63 (s, 226H), 3.43 (s, 2H), 3.16
= 500, 98%, Aladdin) were used (s, 2H), 2.79 (s, 2H), 2.60 (s, 2H). PEGDE1000 and PEGDE4000 were
= 1000, 2000, or 4000) prepared following the same procedures except using different
n
n
(
to use. Epichlorohydrin, methanol, potassium carbonate 2.5 Preparation of the cyclophosphazene-based hybrid
benzophenone and all other chemicals were purchased from polymer electrolytes
Tianjin Chemical Reagent Factory (China) and used as received. LiTFSI (807.2 mg), 1.5 g of PEGDE4000 and equivalent molar
2
.2 Synthesis of hexa(4-acetylphenoxy)cyclotriphosphazene amount of HAmCP (97.9 mg) were mixed in 6 mL of THF, and
HAcCP)
then subjected to sonication for 15 min to ensure sufficient
The HAcCP was synthesized according to the reported dissolution. Subsequently, the mixture solution (6 mL) was
(
4
6
procedures with slight modification. Briefly, preheated HCCP added to the mold via a pipette and heated for another 12 h
5.0 g, 14.4 mmol) and 4-acetamidophenol (17.4 g, 115.2 mmol) after the addition to enable the evaporation of THF and the
(
were dissolved in 200 mL of anhydrous acetone with stirring at
o
accomplishment of amine-epoxy reaction at 100 C to obtain prepared following the same procedures except using different
the CHPE4000 film. CHPE500, CHPE1000 and CHPE2000 were weight of PEGDE for 0.188 g, 0.375 g and 0.750 g, respectively.
2
| J. Name., 2012, 00, 1-3
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2
.6 Characterization of the polymer electrolytes
The stretchable test was conducted using an electronic
View Article Online
DOI: 10.1039/C9TA05028K
1
The H NMR spectra were recorded on a Bruker AV400 nuclear universal testing apparatus (Shenzhen San Testing Machine co.,
3 6
magnetic resonance instrument using CDCl and d -DMSO as CMT4104) with the sample fixed on both sides and stretched at
-1
the solvents and TMS as the internal reference. The FT-IR a speed of 2 mm min .
spectroscopy was carried out on an FT interferometer (Bruker 2.8 Interfacial stability of electrolyte/electrodes
-1
Equinox 55) over the range of 4000-400 cm . The glass The interfacial stability between polymer electrolyte and
lithium foil was evaluated by a Li/CHPE/Li symmetric cell with a
transition temperature (T
g
) of polymers was evaluated by
-2
constant current density of 0.1 mA cm . The symmetric cells
were alternatively charged and discharged for 3 h in turn for
each cycle.
differential scanning calorimetry (DSC, Q2000) with a heating
o
-1
o
o
rate of 10 C min from -80 C to 100 C under nitrogen
atmosphere. The thermal stability was investigated by
thermogravimetric analyzer (PerkinElmer 4000) under nitrogen 2.9 Battery fabrication and evaluation
o
-1
o
The slurry, consisted of LiFePO , carbon black and PVDF with the
atmosphere with a heating rate of 10 C min from 30 C to 800
4
o
weight ratio of 8:1:1, were coated to aluminum foil and dried in
C. A dynamic mechanical analysis (DMA) was measured by
EXSTAR with a heating rate of 30 C min and a frequency of 21
Hz. The test was conducted using compressed mode, the
dimensions being 1 × 1 cm . The crystal structure of the CHPEs
and PEGDE was characterized by X-ray diffraction (SmartLab-
SESmartLab-SE) data using Cu Kα radiation (40 mA/40 kV) over
the range of 5 - 80 with a scanning rate of 5 min .
The ionic conductivity of the CHPEs was investigated by using
o
o
-1
vacuum oven at 80 C overnight and used as cathode materials.
The CR2032 coin cell batteries were assembled with a sequence
of LiFePO
cycling performance was conducted using a battery apparatus
LANHE CT2001A) with a potential range from 2.5 to 4.2 V at 60
4
cathode, CHPE and Li anode. The charge-discharge
2
(
o
C.
o
o
o
-1
3
. Results and discussion
an electrochemical workstation (Autolab PGSTAT302N, The novel polymer electrolytes with different molecular
o
Netherlands) with the temperature from 30 to 100 C with an weights of PEGDE were prepared by facile amino-epoxy curing
o
interval of 10 C. The recording data were measured over a reaction (Scheme 1). These polymer electrolytes are referred as
range from 1 MHz to 10 Hz. The polymer electrolyte films were CHPEx, where x represents the molecular weight of PEGDE. The
clamped by two stainless steel electrodes. The membranes NMR spectra of HAcCP, HAmCP, PEGDE are listed in Figs. S1-S9
were held for 5 min before measurement at predetermined (Supporting Information). After the substitution of HCCP, the
temperature. The ionic conductivity of the CHPE films was characteristic peaks in
calculated by using the following equation, σ = L/(R S), in which ppm assigned to benzene moieties are clear, meanwhile, the
L (cm) represents the thickness of the polymer electrolyte film, characteristic peaks in 31P NMR spectroscopy at 9.22 ppm
(Ω) is the bulk resistance, and S is the effective contact area indicates that the phosphorus is in the same chemical
1
H NMR spectroscopy at 7.46 and 6.82
b
R
b
of the electrode. The linear sweep voltammetry (LSV) was environment, which demonstrates the substitution of 4-
carried out to investigate the electrochemical stability window acetamidophenol is complete. After the deacetylation is
of the polymer electrolyte with Li/CHPE/stainless steel. The accomplished, the amino group shows up at 4.91 ppm in
range of the voltage sweep was from 0 to 6 V. The lithium ion NMR spectroscopy, and the unimodal peaks in 31P NMR
transference number (t ) was measured using a Li/CHPE/Li cell. spectroscopy at 10.05 ppm reveals that the completely
1
H
+
The cell was polarized at 10 mV (ΔV) and time-current deacetylation is established.
dependence was recorded until the current was steady. The
initial current and the steady state current was denoted as I
and I . The impedance of the cell was measured before and after
the polarization and denoted as R and R . The equation t
(ΔV- I )/I (ΔV- I ) was utilized to calculate the t
.7 Flame-retardant and stretchable test
0
S
0
s
+
=
I
S
R
0 0
0
R
s s
+
.
2
The specimens were fabricated via pouring the mixture solution
of HAmCP, PEGDE and LiTFSI into strip mold and curing for 12 h
o
after the evaporation of THF at 100 C. As a comparison,
n
polyethylene glycol diacrylate M = 600 was employed to
dissolve benzophenone and casted into strip mold, and the
compared specimen was obtained after the UV irradiation. The
two specimens were denoted as CHPE strip and P(PEGDA) strip,
respectively. The two strips were hanged using pliers to fix the Scheme 1 Schematic illustration of the structure of CHPE.
strips. Alcohol lamp was removed to evaluate the flame-
retardant ability of the strips after both of the strips were
ignited by alcohol lamp for 3 seconds.
After the coupling reaction with epichlorohydrin, the hydroxyl shown in CHPE500 (Fig. 1). In order to measure the aniline-
group of PEG is substituted by the epoxy group. The epoxide reaction kinetics, DSC was employed to monitor the
characteristic peaks in FTIR spectroscopy at 912 cm-
1
temperature and the completion time of the aniline-epoxide
contributed to the epoxy group of PEGDE are obvious, which are reaction. Firstly, we found that the amine-epoxy reaction is
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allowed to react at about 100 C (Fig. S11). Thus, we raised the
Journal Name
o
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DOI: 10.1039/C9TA05028K
o
-1
temperature rapidly with a heating rate of 20 C min till 100
o
o
C. The amine-epoxy reaction is accomplished at 100 C for
about 60 mins (Fig. S12).
Fig. 3 (a) XRD patterns of CHPE; (b) DSC traces illustrating the T
of CHPE4000 with a ramp of 2 C min .
c
o
-1
The tensile test is implemented to investigate the
mechanical strength of the CHPEs, since stretchability plays an
essential role in the practical application of wearable energy
storage devices. The stretchable test exhibits that CHPEs own
an excellent mechanical strength via the stress-strain curve (Fig.
2). The stress starts to decline from 1.07 MPa to 0.41 MPa, and
the tensile elongation begins to increase from 20.5% to 29.5%,
while PEGDE500, PEGDE1000, and PEGDE2000 were utilized,
respectively. It is rational that PEGDE500 chain is too short to
move freely while the longer PEGDE2000 owns more chain
mobility which is tolerant to elongation when applied to tensile.
Surprisingly, when PEGDE4000 was utilized as the cross-linking
agent, the stress and tensile elongation drastically increase up
to 3.33 MPa and 127.49% (Table 1), respectively. To explain this
extraordinary phenomenon, X-ray diffraction (XRD) is utilized to
characterize patterns of the CHPEs (Fig. 3a). The broad peaks of
the CHPE500, CHPE1000 and CHPE2000 indicate that the PEO matrix
are amorphous compared with the XRD patterns of the PEGDE
Fig. 1 (a) FT-IR spectra of the HCP, PEGDE500 and CHPE500
showing the characteristic peaks with the selected region
between 4000 cm-1 and 400 cm ; (b) the magnified FT-IR
-1
-1
spectra with selected wavenumber region between 960 cm
and 800 cm .
-1
(
Fig. S14), while the CHPE4000 retains the characteristic peaks of
o
o
the PEGDE4000 at 19 and 23 , respectively. We speculate that
these crystalline PEO region would absorb proportional energy
of stress to become an
Fig. 2 The stress-strain profile of CHPE500, CHPE1000, CHPE2000
and CHPE4000
.
Table 1. Tensile elongation, breaking stress and tensile strength
of the CHPEs.
Tensile
elongation
Tensile
strength
(±10%) (MPa)
Breaking stress
CHPE
(
±15%) (MPa)
(
±5) %
CHPE500
CHPE1000
CHPE2000
CHPE4000
20.53
25.62
29.53
127.49
1.07
0.55
0.41
3.33
4.19
2.72
0.78
4.28
4
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DOI: 10.1039/C9TA05028K
Fig. 4 DSC traces (second heating cycle) illustrating the T
g
of four Fig. 5 TGA thermograms of HAmCP, CHPE500, CHPE1000, CHPE2000
types of CHPEs.
and CHPE4000.
amorphous PEO region when applied to the tensile test. Hence,
the CHPE4000 would suffer more stress and be stretched up
longer compared with CHPE500, CHPE1000 and CHPE2000. The DSC
curves of CHPE4000 were utilized to characterize the
crystallization of CHPE4000. With slower cooling rate of CHPE4000
,
the temperature of crystallization (denoted as T
c
) showed up at
o
1
3.54 C, is coincidence with second scan of cooling profile (Fig.
b). The dynamic mechanical analysis (DMA) is also utilized to
3
calculate the cross-linking density of the CHPEs (Figs. S15-S17
and Table S1). According to the rubber elasticity theory, the
cross-linking density (ρ) of the CHPE is proportional to its
4
8
storage modulus (E') in the rubbery region, and the ρ is able
to be calculated using the following equation, ρ = E'/3RT, R is
the gas constant, and T is the absolute temperature. Cross-
linked by a longer PEO chain, the CHPE4000 shows a better tensile
elongation owing to the elevated storage modulus and cross-
linking density. Besides, the CHPEs film can be bended freely,
and even rolled up without cracking, indicating its superior
flexibility (Fig. S18).
Fig. 6 The ionic conductivities of CHPEs with various molar ratio
+
of EO/Li . (a) CHPE1000; (b) CHPE2000; (c) CHPE4000. (d) Ionic
The glass transition temperature (T
g
) is a key factor for ion
+
conductivities of CHPEs with 12:1 molar ratio of EO/Li .
transport, which is related to the chain mobility of polymer
Apart from the mechanical properties and thermal stability,
the ionic conductivity is an essential factor for polymer
electrolytes. The segmental motion of the polymer chain, and
the coordination and dissociation of lithium ions affect the
lithium-ion conductive mechanism of CHPE. During the process
of ion complexation and dissociation, lithium-ion transport
occurs through intrachain or interchain movement in the CHPE.
7
electrolytes. The T
g
of the CHPEs gradually decreases when the
longer PEGDE is utilized due to the promoted flexibility of the
PEG chain (Fig. 4). The thermal ability of CHPEs is also a crucial
point related to the safety issues of LIBs. The cross-linked
structure is anticipated to enhance the thermal stability of the
CHPE.
To verify the thermal stability, a series of CHPEs composed
Fig.
6 offers the temperature dependence of ionic
+
of HAmCP, PEGDE, and LiTFSI (EO/Li = 12), were fabricated and
conductivities for all four CHPEs doped with LiTFSI in the
characterized by TGA (Fig. 5). Most of them are thermally stable
o
temperature range from 20 to 100 C. As shown in Fig. 6, the
o
above 300 C, which can completely meet the thermal stability
ionic conductivities of all CHPEs with various molecular weights
of PEGDE increase with the temperature increase owing to the
requirement of LIBs. Surprisingly, the thermal decomposition
temperature (T
employed, which contrary to the assumption that the thermal
instable PEO would lower the T of CHPE with the longer PEGDE
d
) gradually increases with the longer PEGDE
49
enhanced segment mobility at higher temperatures. Notably,
the conductivity of these polymer electrolyte increases more or
less with increasing weight percentage of LiTFSI. It goes through
d
employed. The CHPE4000 are thermally stable under the
+
a maximum when EO/Li = 12, and then drops when the high
o
temperature of 368 C with an only 5% decomposition of the
+
salt concentration is employed (EO/Li = 8). The phenomenon
polymer matrix, which may be attributed to the thermal stable
phosphazene core and cross-linking density.
could be due to increase of charge carrier up to the optimum
+
concentration (EO/Li = 12/1) which gives the highest ionic
conductivity among the various CHPEs. In the case of high salt
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concentration, the formation of ion pairs or ion clusters could
restrict the mobility of the charge carriers in the polymer matrix.
View Article Online
DOI: 10.1039/C9TA05028K
+
When the molar ratio of EO/Li is 12:1, CHPEs with long PEG
chains (CHPE2000 and CHPE4000
)
possesses high ionic
conductivities (Fig. 6). The ionic conductivity of CHPE4000 was
-
4
-1
o
-3
-
measured to be 1.04 × 10 S cm at 30 C and 2.69 × 10 S cm
1
o
at 100 C, which is far higher than pure PEO-based polymer
7
electrolyte. During the process of ion transport, lithium ions
dissociate from a soft PEG chain and coordinate with other PEG
chains. Compared to polymer electrolytes with short PEG chain
(CHPE500 and CHPE1000), the long PEG chains in CHPE4000 can
more freely move. Among all the CHPEs, CHPE4000 and CHPE2000
exhibit nearly the highest ionic conductivity, but CHPE4000 owns
better excellent mechanical strength and thermal stability.
Hence, CHPE4000 was selected to perform electrochemistry test
in the following part.
o
The linear sweep voltammetry (LSV) at 60 C was conducted Fig. 8 (a) Rate cycling performance of the LFP/CHPE4000/Li cell at
o
by
a
Li/CHPE/Li symmetrical cell to investigate the 60 C. (b) Initial charge and discharge curves of the batteries at
o
electrochemical stability window. The current shown in the different rates at 60 C. (c) Cycling performance of
current-voltage profile is referred as to electrochemically LFP/CHPE4000/Li cell at 0.1 C at 60 C. (d) Cycling performance of
oxidized decomposition of the CHPEs (Fig. 7a). With the longer LFP/CHPE4000/Li cell at 0.2 C at 60 C.
o
o
PEG chain, the obvious oxidative current emerges at 5.1 V, 5.2
Interfacial stability between electrolyte and electrode is an
V, and 5.3 V for CHPE1000, CHPE2000, and CHPE4000, respectively. important parameter for polymer electrolyte. Liquid
This indicates that CHPEs own a wide electrochemical window electrolytes usually have good contact with electrodes but
ascribed to the cross-linking structure, which could be stable suffer side reactions and cannot suppress the growth of lithium
over a broad voltage range for lithium-ion batteries. The lithium dendrites. A galvanostatic cycling test of the symmetric
ion transference number of CHPEs is implemented to be 0.25 Li/CHPE4000/Li cell was conducted to investigate the interfacial
(
Fig. 7b), which is higher than pure PEG-lithium salts stability between Li foil and the CHPE. The cells were charged
5
0
electrolyte.
and discharged for 3 h alternately with a current density of 0.1
-2
o
mA cm to simulate the process of stripping/plating at 60 C. In
Fig. 7c, the symmetrical cell shows deposition potential of 0.17
V and the deposition potential increased up to 0.23 V after
lithium deposition for more than 1400 h. Owing to the excellent
mechanical strength, the polymer electrolyte can suppress the
dendrites attributed to the process of lithium stripping/plating.
The interfacial stability of CHPE/Li was also evaluated using the
change of impedance resistance with predetermined storage
time. The diameter of the semicircle represents the interfacial
f f
resistance (R ) of electrolyte/electrodes (Fig. 7d). The R
decreases slightly with storage time, and becomes stable, and
reaches a stable value of 712  after 21 days, suggesting an
excellent interfacial stability between CHPE4000 and lithium
5
1
metal after long-term storage. The stable interface would
consequently influence the cycle life of LIBs based on the CHPE.
The CHPE was assembled to lithium cells using Li foil as the
anode and LiFePO
of CHPE, the Li/CHPE4000/LiFePO
4
as the cathode. To evaluate the rate capacity
cell was assembled and cycled
4
o
Fig. 7 (a) The linear sweep voltammetry (LSV) of CHPEs at 60 C.
at different rates. Fig. 8a shows the specific capacity with five
+
(
b) The lithium ion transference number (tLi ) of CHPE4000 at 60
cycles performed at each current density. The battery possesses
o
C. (c) Galvanostatic cycling profiles of Li/CHPE4000/Li
symmetrical cell at a current density of 0.1 mA cm at 60 C. (d)
Impedance profiles for the Li/CHPE4000/Li cell at 60 C.
-1
-1
a high capacity of 131 mAh g at 0.1 C and 129 mAh g at 0.2 C,
which is almost the same at specific capacity. When the current
density increases to 0.5 C, the specific capacity slightly
−2
o
o
-1
decreases with a discharge specific capacity of 121 mAh g .
After the current density increases to 1 C, the specific capacity
-1
decreases with a discharge specific capacity of 66 mAh g .
Furthermore, when the current density recovers to 0.1 C, the
-1
specific capacity bound to 128 mAh g with a slight drop down.
6
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Fig. 8b shows the initial charge and discharge curves of the
batteries at 0.1 C, 0.2 C, 0.5 C and 1 C, respectively. Fig. 8c
exhibits the cycle performance of CHPE4000 with a current of 0.1
C. During the third cycle, the discharge specific capacity reached
View Article Online
DOI: 10.1039/C9TA05028K
-1
-1
the maximum of 142 mAh g . After 100 cycles, 129 mAh g of
discharge specific capacity still retained and the coulombic
efficiency was as high as 98.7%. It is disclosed that the CHPE
shows superior stability during cycling performance for lithium
batteries with 90.8% retention of specific capacity. The
-1
discharge capacity was retained at 122 mAh g at 0.2 C after
00 cycles (Fig. 8d), and the capacity retention of the battery is
2
over 90.2%, demonstrating that the highly reversible reactions
occur in the long-term process of charge/discharge.
To investigate the flame retardancy of the separator,
specimen composed of CHPE500 was fabricated to testify the
flammability and P(PEGDA600) was utilized as a comparative test
Fig. 9 (a) The image of the P(PEGDA600) specimen. (b) and (c) The
image of combustion of the P(PEGDA600) specimen. (d) The
image of the CHPE500 specimen. (e) The image of combustion of
the CHPE500 specimen. (f) The image of self-extinguish of the
CHPE500 specimen.
(Fig. 9). Once the samples were lighted up, the two specimens
were observed whether the polymer electrolytes can prevent
the combustion or not. The specimen composed of CHPE500
were self-extinguished after 15 seconds while the P(PEGDA600
)
specimen combusted more intensely (Fig. 9c). The limiting
oxygen index (LOI) was employed to calculate from TGA char
yield at 800 C. The LOI value is a good indication of polymer for
4
. Conclusions
o
flame resistant materials. The polymer has the self-
extinguishing ability when the LOI value in the range to 28-100%. In summary, a panel of cyclophosphazene-based hybrid
polymer electrolytes was designed and fabricated by facile
epoxy-amine reaction between the HAmCP and PEGDE with
different molecular weight. Mechanical strength and thermal
stability were investigated in detail and revealed that the
If the LOI value is in the range to 21−28%, the polymer may
combust slowly. If the LOI value is less than 20.9%, it will burn
easily in air.5
5,56
The LOI value of CHPE500 is a little bit higher than
2
8% (Table 2), which is in accordance with the flame retardancy
o
CHPE4000 possess high thermal stability up to 368 C and can be
test. The specimen composed of CHPE500 showed self-
extinguished ability probably because phosphorus-nitrogen can
inhibit the exothermic process and suppress the combustion via
enhancing the formation of char with volatiles. Based on the
stretched up to 127% tensile elongation owing to the
phosphazene core and the cross-linking structure. In addition,
-4
the CHPE4000 owns excellent ionic conductivity up to 1.04 × 10
-1
o
S cm at 30 C and superior electrochemical stability. Thanks to
formula, LOI = 17.5 + 0.4Yc, the flame retardancy increased with the flame-retardant cyclophosphazene, the CHPE500 exhibits a
the elevated TGA char yield (Yc) attributed to the increasing self-extinguished effect compared with a flammable P(PEGDA)
adoption of HAmCP. On one hand, the longer PEG chains would specimen. Furthermore, the cell prepared using the CHPE4000 as
the separator shows a very stable cycling performance under
.2 C rate. Therefore, the cyclophosphazene-based hybrid
polymer electrolyte provides a promising strategy and
prototype for fabricating all-solid-state lithium-ion batteries.
drastically decrease the content of HAmCP, which is the key
factor for flame retardancy. On the other hand, the longer PEG
chains would endow CHPEs higher ion conductivity.
Consequently, the ion conductivity and flame retardancy of the
CHPEs are trade off in this work.
0
Table 2. The temperature of 5%, 10% and maximum
decomposition rate, char yield and limit oxygen index of the
CHPEs and P(PEGDA).
Conflicts of interest
The authors declare no competing financial interest
1
LOI2
(%)
T
(
5%
oC)
T
( C)
10%
T
( C)
max
Y
c
o
o
(%)
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (51622303, 51703080, and
CHPE500
279.45
327.17
359.44
368.43
306.59
307.95
350.74
378.02
393.26
329.33
349.25
372.30
398.17
430.41
376.84
27.235
18.911
7.571
5.002
0.286
28.39
25.06
20.53
19.50
17.61
CHPE1000
CHPE2000
CHPE4000
P(PEGDA)
5
1433002) and Natural Science Foundation of Hubei Scientific
Committee (2018CFA059).
1
o
2
52-54
Char yield at 800 C. Limiting oxygen index (LOI).
c
LOI = 17.5 + 0.4Y .
Notes and references
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Journal of Materials Chemistry A
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View Article Online
DOI: 10.1039/C9TA05028K
Graphical Abstract
This article reports the cyclophosphazene-based hybrid polymer electrolyte formed
via epoxy-amine reaction for high-performance lithium-ion batteries.