Ordered mesogenic units-containing hyperbranched star liquid crystal all-solid-state polymer electrolyte for high-safety lithium-ion batteries

Article abstract of DOI:10.1016/j.electacta.2017.10.163

Functional liquid crystal polymers have attracted much attention as new all-solid-state polymer electrolytes due to their ability of orientation. In this paper, a hyperbranched star liquid crystal polymer (HSLCP) electrolyte for lithium-ion batteries (LIBs) is developed for the first time. This type of HSLCP bears a hyperbranched poly(glycidol) (HPG) as core, poly-ε-caprolactone (PCL) and poly (4-cyano-biphenyl methylmethacrylate) ((LC)_x) compose the block arms of the HSLCP. (LC)_x segments, one part of the arms, are assisted to promote transmission of Li-ion via orientation and PCL segments are used to dissociate lithium salt and efficiently migrate charge carriers. The good self-standing all-solid-state polymer electrolyte film composed of HSLCP and lithium bis(trifluoromethanesulfonimide (LiTFSI) can be formed by solution casting method. It is demonstrated that such a HSLCP exhibits good thermal stability (368°C), the HSLCP-based electrolyte shows high ionic conductivity (5.98 × 10^?5 S cm^?1 at 30°C), outstanding ion transference number (0.63), and wide electrochemical window (5.12 V). In addition, LiFePO₄/Li batteries employing the electrolyte deliver good cycling performance and rate capability. Simultaneously, the corresponding half-cell can still power a LED lamp at room temperature. Thus, this is a great progress for liquid crystal-based polymer electrolytes to be applied to high energy LIBs.

Full text of DOI:10.1016/j.electacta.2017.10.163

Accepted Manuscript
Ordered mesogenic units-containing hyperbranched star liquid crystal all-solid-state
polymer electrolyte for high-safety lithium-ion batteries
Shi Wang, Ailian Wang, Xu Liu, Hao Xu, Jie Chen, Liaoyun Zhang
PII:
S0013-4686(17)32309-5
10.1016/j.electacta.2017.10.163
EA 30549
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 7 September 2017
Revised Date: 24 October 2017
Accepted Date: 25 October 2017
Please cite this article as: S. Wang, A. Wang, X. Liu, H. Xu, J. Chen, L. Zhang, Ordered mesogenic
units-containing hyperbranched star liquid crystal all-solid-state polymer electrolyte for high-safety
lithium-ion batteries, Electrochimica Acta (2017), doi: 10.1016/j.electacta.2017.10.163.
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ACCEPTED MANUSCRIPT
Ordered Mesogenic Units-containing Hyperbranched
Star Liquid Crystal All-solid-state Polymer Electrolyte
for High-safety Lithium-ion Batteries
Shi Wang,[a] Ailian Wang,[a] Xu Liu,[a] Hao Xu,[a] Jie Chen,[a] Liaoyun Zhang*[a]
a College of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences, Beijing, 100049, China
Corresponding author:
Liaoyun Zhang
Emails：zhangly@ucas.ac.cn
Table of Content

ACCEPTED MANUSCRIPT
Ordered Mesogenic Units-containing Hyperbranched
Star Liquid Crystal All-solid-state Polymer Electrolyte
for High-safety Lithium-ion Batteries
Shi Wang,[a] Ailian Wang,[a] Xu Liu,[a] Hao Xu,[a] Jie Chen,[a] Liaoyun Zhang*[a]
a College of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences, Beijing, 100049, China
Corresponding author:
Liaoyun Zhang
Emails：zhangly@ucas.ac.cn
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Abstract: Functional liquid crystal polymers have attracted much attention as new all-solid-state polymer
electrolytes due to their ability of orientation. In this paper, a hyperbranched star liquid crystal polymer (HSLCP)
electrolyte for lithium-ion batteries (LIBs) is developed for the first time. This type of HSLCP bears a
hyperbranched poly(glycidol) (HPG) as core, poly-ε-caprolactone (PCL) and poly (4-cyano-biphenyl
methylmethacrylate) ((LC)x) compose the block arms of the HSLCP. (LC)x segments, one part of the arms, are
assisted to promote transmission of Li-ion via orientation and PCL segments are used to dissociate lithium salt
and efficiently migrate charge carriers. The good self-standing all-solid-state polymer electrolyte film composed
of HSLCP and lithium bis(trifluoromethanesulfonimide (LiTFSI) can be formed by solution casting method. It is
demonstrated that such a HSLCP exhibits good thermal stability (368 oC), the HSLCP-based electrolyte shows
o
high ionic conductivity (5.98×10-5 S cm-1 at 30 C), outstanding ion transference number (0.63), and wide
electrochemical window (5.12 V). In addition, LiFePO4/Li batteries employing the electrolyte deliver good
cycling performance and rate capability. Simultaneously, the corresponding half-cell can still power a LED lamp
at room temperature. Thus, this is a great progress for liquid crystal-based polymer electrolytes to be applied to
high energy LIBs.
Keywords: Liquid crystal, Hyperbranched star liquid crystal polymer, Orientation, All-solid-state polymer
electrolytes, Lithium ion batteries
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1. Introduction
Lithium batteries are among the best accepted technologies to meet the energy storage needs for regenerative
energy use [1]. The traditional organic electrolytes for commercial lithium-ion batteries (LIBs) are
well-established in the consumer good market due to their excellent interfacial contact with electrodes and
electrochemical performance [2]. However, the well-known problems of liquid electrolytes in LIBs are
flammable and volatile [3-5], which severely limits their further large-scale application. To solve these problems,
considerable attention has been paid to all-solid polymer electrolytes (SPEs) in the field of LIBs, because SPEs
are safer, more flexible [4, 5], have better electrochemical stability and interfacial compatibility and even can
suppress the dendrite growth under certain conditions [6, 7].
Among all the polymers, polyethers such as linear poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO),
are the widely studied conventional polymers for SPEs due to the favorable properties for salts solvation and
ionic conduction [8]. However, the characteristics of semi-crystallinity of PEO and strong complexation between
EO segments and lithium ions cause the low ionic conductivity of linear polyethers-based SPEs at room
temperature (RT) [9-11]. SPEs based on linear PEO also show narrow electrochemical window and poor
mechanical strength, which limits their further application [12]. Furthermore, as reported by researchers, the
lithium-ion transference numbers of typical PEO/lithium salts are in the range of 0.2 to 0.5, it is because that the
anionic dynamics generally supersedes the cationic dynamics [13, 14]. As ion transmission performance is
related to the ionic conductivity and ion transference number, therefore, in order to achieve fast ion transport in
SPEs, one strategy is to suppress the crystalline of the low-Tg polyethers for obtaining high ionic conductivity.
The approaches include copolymerization modification of PEO [15], crosslinking modification of PEO [16, 17]
and bending modification of PEO with inorganic substance [18]. Unfortunately, lithium-ion transference
numbers of PEO-based polymer electrolytes are still low.
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Recently, non-polyether materials, like polycarbonates and polyesters, have attracted much more attention as
SPEs because of their higher cationic transference numbers and wider electrochemical windows [19-21].
However, both of them display lower ionic conductivity than PEO at RT due to the chain structures are more
rigid [21, 22]. It is reported that the combination of trimethylene carbonate (TMC) and ε-caprolactone (CL) via
random co-polymerization can significantly enhance in overall ionic conductivity [20, 23], yet the structure of the
polymer is hard to be controlled and the ionic conductivity does not attain sufficiently high levels (10-7～10-5
S
cm-1). Obviously, further molecular design is needed to ulteriorly improve the overall performance of these
SPEs. Here, cost-effective and environment friendly advantages of poly-ε-caprolactone (PCL) urge us to further
optimize its electrochemical properties. Our research group [24, 25] and other researchers [26, 27] recently
demonstrated that design and synthesis of hyperbranched star polymers can be a promising method to perfect
the performance of SPEs. Concurrently, one of the most promising approaches to further improve the
performance of SPEs is the introduction of liquid crystals (LCs) into the polymer electrolytes. It has been
reported that polymers decorated by functional liquid crystals can construct ion-conductive pathways, which
can effectively improve the ionic conductivity of SPEs [28-30]. However, the other electrochemical properties
such as electrochemical stability, interfacial property, cycling performance and rate capacity in LIBs are rarely
further investigated [31].
Our intention here is to combine the advantages of hyperbranched star polymers and LC for developing a novel
solid electrolyte (used in LIBs). Compared with linear copolymers of LC, the hyperbranched star structure can
restrict the crystallization of electrolyte matrix owing to less macromolecular chain winding, which can probably
make the transportation of Li+ ions more efficient, and poly-ε-caprolactone (PCL) segments are constructed as
the arms of the as-prepared hyperbranched star polymer for lithium salt dissociation and efficient transport of
Li+ ions. Meanwhile, biphenyl-4-carbonitrile-based liquid crystal is chosen and introduced into the arms of the
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hyperbranched star polymer due to its strong ability of regular arrangement, which can promote ion-conductive
channel formation of the SPE [32]. Moreover, nitrile group at the end of LC not only shows electrochemical
stability [6], but also favorably interact with the Li-ion of lithium salt, which is beneficial to the dissolution of salt
and the migration of charge carries [33]. Specifically, PCL segments were introduced to the hyperbranched
polyether (HPG) core via ring-open polymerization (ROP). Then LC was attached to the end of PCL through
atom transform radical polymerization (ATRP). In our study, we found that the existence of mesogenic groups
at the end of the synthesized hyperbranched copolymer arms can completely restrain the crystallization of PCL
in the arms and have the potential to promote migration of charge carriers via orientation, thereby improving the
batteries performance.
To the best of our knowledge, there has no report about hyperbranched star liquid crystal polymer electrolytes
for LIBs. Although star-shaped LC polymers were synthesized and used as SPEs [32], the arms of the star
polymers were very limited, which restricted the dissolution of lithium salt. Herein, we report HSLCPs which are
served as SPEs. Both cycling performance and rate capacity have been successfully achieved based on coin
cell LiFePO4 /SPE/ Li.
2. Experimental Section
2.1. General Procedures
1H NMR spectra were collected on a JNM-ECZ400S (JEOL, 400 MHz) spectrometer at RT using CDCl3 as
solvent and tetramethylsilane (TMS) as the internal standard. Fourier transform infrared (FTIR) spectra were
conducted on a Thermo Nicolet AVATAR 360 infrared instrument using the attenuated total reflectance (ATR)
technique from 4000 to 500 cm-1. Gel permeation chromatography (GPC) measurement was obtained using a
o
PL-GPC50 equipped with a refractive index detector at 40 C, using polystyrene as the standard and THF or
DMF as the fluent at a flow rate of 1.0 mL min-1. Thermal stability analyses were performed on a thermal
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gravimetric analyzer from 30 to 600 oC using a Perkin–Elmer TGA 1 series instrument with the heating rate of
20 oC min-1. Differential scanning calorimetry (DSC) studies were carried out on a Q2000 calorimeter
manufactured by TA Instruments at nitrogen atmosphere with a constant heating/cooling rate of 10 oC min-1 to
determine phase-transition temperature, and the data was obtained from the second heating curve. The
samples (4 mg) were sealed at the aluminium pans during the measurements. Atomic Force Microscope
images were conducted with a Ntegra Prima AFM (NT-MDT) under the ambient condition using the taping
mode (silicon wafers were used as the matrixes and samples were casted on the surface by solution casting
method). Polarizing optical microscopy (POM) (Leica DM2700P polarizing optical microscope equipped) with
an LNP95 heating and cooling stage was used to observe texture structures of HSLCPs electrolyte matrixes at
different temperature with a heating or cooling rate of 10 oC min-1. The sample was sandwiched between two
circular slides (the diameter and thickness of the circular slide are 15 and 0.14 mm, respectively). X-ray
diffraction (XRD) measurements were evaluated on an Automated Multipurpose X-ray Diffractometer Smartlab
9kW operating at 45 kV and 200 mA with a copper target (λ= 1.54 Å) and at a scanning rate of 1o min-1.
HPG-PCL-(LC)n/2 was dissolved in THF and dropped on the silicon wafer for XRD test (the unannealed film was
dried at RT in order to avoid orientation while the annealing temperature for the film was 80 oC, the thickness of
the film is about 80 ꢀm)
2.2. Materials
4-Hybdroxybiphenyl-4-Carbonitrile (99 %, Adamas-beta), Methacryloyl chloride (95 %, Adamas-beta), obtained
from Adamas Reagent Co., Ltd., were used as received. ε-Caprolactone (CL, 99 %, Alfa Aesar),
N,N,N’,N’’,N’’-pentamethyldiethylenetria-mine (PMDETA, 99 %, Aladdin, China), N,N-Dimethylformamide
（DMF,
≥
99.5 %, Sinopharm Chemical Reagent Co., Ltd) were dried with CaH2 and distilled under reduced
pressure before use. Tetrahydrofuran (THF) and toluene were purchased from Beijing Chemical Works and
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dried with CaH2, then, distilled before use. Triethylamine (TEA, 99 %, Aladdin, China) was dried by 4 Å
molecular sieves for 24 h before use. Copper(I) bromide (CuBr, 99 %, Aladdin, China) was used after washing
with acetic acid, methanol, and ether, respectively. Tin(II) 2-ethylhexanoate (95%, Aladdin, China),
2-bromo-2-methylpropionyl bromide (98%, Aladdin, China), potassium methoxide (5 % w/v in methanol,
Adamas-beta), 2-ethyl-2(hydroxymethyl)-1, 3-propanediol (TMP, Adamas-beta) and glycidol (96%, Acros)
were used without further purification. Lithium bis(trifluoromethanesulfonimide) (LiTFSI, 99 %, Aladdin, China)
was dried at 80 °C under vacuum for 24 h before use .
2.3. Preparation of HPG-PCL-(LC)x (x = n/3, n/2, n)
General approaches employed for the synthesis of LC, HPG, HPG-PCL, HPG-PCL-Br, HPG-PCL-(LC)x are
outlined in Scheme 1.
2.4. Synthesis of monomer 4-biphenyl-4-Carbonitrile methylmethacrylate (LC)
4-Hybdroxybiphenyl-4-Carbonitrile (9.36 g, 0.048 mol) was dissolved in a mixture of TEA (5.34 g, 0.053 mol)
and dry THF (100 mL). A solution of methacryloyl chloride (5.01 g, 0.048 mol) in 50 mL THF was added
dropwise to the obtained mixture. After stirring for 12 h at RT, the white solid which was removed by filtration
and the solution was evaporated at reduced pressure. White crystals could be got after recrystallizing the
residue in 96% ethanol. 1H NMR (400 MHz, CDCl3, δ) : 7.72 (d, J = 8 Hz, 2H; Ar H), 7.66 (d, J = 8 Hz, 2H; Ar H),
7.22 (d, J = 8 Hz, 2H; Ar H), 6.38 (s, 1H, CH2), 5.79 (s, 1H, CH2), 2.07 (s, 3H, CH3).
2.5. Synthesis of hyperbranched poly(glycidol) (HPG)
HPG was synthesized according to our group previously published report [34]. To the dry 50 mL three-necked
flask with a magnetic stir bar was firstly degassed by three freeze-pump-thaw cycles. Then, 1 mL potassium
methoxide (5 % w/v in methanol) was added. Methanol was removed through evaporation after stirring for 0.5 h.
5.15 mL glycidol was slowly added into the reaction mixture at 100 oC within 12 h, and then the reaction was
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continued for another 12 h at 100 C. A transparent and highly viscous liquid was obtained after the product
was washed with 20 mL methanol and dried in a vacuum oven at 45 oC for 12 h.
2.6. Synthesis of hyperbranched HPG-PCL via ROP
HPG-PCL was prepared by ROP of CL using HPG as macroinitiator and Sn(Oct)2 as catalyst. Typically, HPG
(0.3 g, ~ 4 mmol hydroxyl functionalities), a catalyst amount ([Catalyst]/[hydroxyl functionalities] = 1/20) of
o
Sn(Oct)2 [35] and DMF (5 mL) were charged into a 50 mL flask and heated to 130 C under N2 atmosphere.
After HPG was completely dissolved in DMF, 5 mL of ε-caprolactone was introduced in 0.5 h to start the
polymerization. The reaction mixture was continued to stir for 24 h at 130 oC. After cooling to 30 oC and DMF
was distilled under reduced pressure, the crude product was precipitated into 80 mL of cold methanol to
o
remove residual monomers and catalyst. The final product was dried at 50 C in a vacuum oven to afford a
white powder.
2.7. Synthesis of hyperbranched star HPG-based macrointiator (HPG-PCL-Br)
HPG-PCL (0.5 g, ~0.8 mmol of hydroxyl functionalities) and TEA (0.164 g, 1.62 mmol) were dissolved in dry
THF (10 mL) and cooled to 0 oC in an ice-water bath. 2-Bromoisobutyryl bromide (0.37 g, 1.6 mmol) was added
dropwise in 0.5 h. The reaction mixture was stirred at 0 oC for another 2 h and then at 30 oC for 72 h. Then the
insoluble salt was removed by filtration, the filtrate was concentrated on a rotary evaporator and precipitated
into 50 mL of cold methanol. After washing and filtrating for three times, the final product was dried in a vacuum
oven at 50 oC for 12 h to afford a faint yellow powder [35, 36].
2.8. Synthesis of HSLCP electrolyte matrixes (HPG-PCL-(LC)x (x = n/3, n/2, n) )
HPG-PCL-(LC)x was prepared by the ATRP of LC using HPG-PCL-Br as the macroinitiator [32, 36]. In a typical
polymerization, HPG-PCL-Br (0.5 g, ~0.7 mmol Br moieties), LC ( 0.158 g, 0.6 mmol), PMDETA (0.062g, 0.36
mmol), and dry toluene (10 mL) were charged into a schlenk flask with a magnetic stir bar and degassed before
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CuBr (0.051g, 0.36 mmol ) was introduced under N2. The solution was degassed three times with
o
freeze-pump-thaw cycles. After thermostatting at 60 C in an oil bath and stirring for 10 h, the reaction was
exposed to air and diluted with THF. The mixture was then passed through a column of neutral alumina to
remove the copper catalyst. The product was precipitated from an excess of ether twice, filtered, and vacuum
dried at 45 ºC to give a product of HPG-PCL-(LC)n/2. HPG-PCL-(LC)n/3 and HPG-PCL-(LC)n were similarly
obtained only by changing the contents of LC to 0.105g (0.40 mmol) and 0.316 g (1.2 mmol), respectively.
2.9. Preparation of polymer electrolytes
The all-solid-state polymer electrolytes were prepared by solution-casting method. HSLCPs and LiTFSI were
dissolved in THF through intense agitation. The solution was poured into the PTFE (Polytetrafluoroethylene)
mould and THF was slowly evaporated at RT. Then, samples were placed into a vacuum oven at 50 oC for 12 h
to remove the remaining solvent and water. The dried polymer electrolyte films were tested in time in order to
reduce the absorption of vapour.
2.10. Electrochemical measurements
The ionic conductivity of samples was tested on a ZahnerEnnium Electrochemical workstation through
electrochemical impedance spectroscopy using a sealed stainless steel mold with AC amplitude of 5 mV, the
frequency range of 1 Hz to 1 MHz, and the temperature range from 30 to 80 oC via a two-probe method, and
the samples were held at each temperature for 30 min before measurement. The ionic conductivity was
calculated by
ꢁ
ꢀ = ꢂꢃ
(1)
Where L is the thickness of SPE film, R is the bulk resistance of polymer electrolyte, and S is the contact area
of electrode and electrolyte. The electrochemical stability window of HPG-PCL-(LC)n/2 / LiTFSI (25 wt%) was
measured using the electrochemical working station CHI660C (ChenHua) with a Li/electrolyte/stainless steel
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coin cell, and the linear sweep voltammogram was performed from 3 to 6 V (vs. Li+/Li) at a scan rate of 1 mV s-1
at 30 oC. Li+ transference number (t+) of HSLCP electrolyte at 60 oC was measured by AC impedance and DC
polarization measurements using a symmetric cell of Li/ electrolyte/Li and calculated using Equation (2) [16,
17].
ꢉ
ꢎ
ꢎ
ꢇ
∆ꢊꢋꢇ ꢃ
ꢌ
ꢈ
ꢍꢌ
ꢍꢈ
ꢄꢁꢆꢅ
=
(2)
ꢉ
ꢇ
∆ꢊꢋꢇ ꢃ
ꢈ
ꢌ
The initial (Rlo) , steady (Rls) resistances (ꢁ) of the interfacial layers of electrolyte and Li metal electrode were
obtained before and after polarization by impedance spectra of the cell in the frequency between 10 mHz and
10 MHz with an oscillation voltage of 5 mV. Simultaneously, the initial (Io) and steady (Is) current flowing
through the cell were measured by DC polarization with a voltage ((∆V = 10 mV). Interfacial compatibility was
o
performed by monitoring the interfacial resistance of cell Li/electrolyte/Li at 30 C at various times. The tests
were conducted on Zennium Electrochemical workstation in the frequency range from 10 mHz to 1 MHz with an
oscillation voltage of 5 mV. LiFePO4/polymer electrolyte/Li cells were assembled to evaluate the
electrochemical performance of HSLCP electrolyte. All electrochemical measurements of the samples were
conducted without annealing except ionic conductivity. The charge-discharge cycles of all-solid-state cells were
carried out on LANHE CT2001A battery testing system and the voltage range used for battery cycling was 2.3
to 4.0 V. All cathodes were consisted of 80 wt% LiFePO4, 10 wt% super P and 10 wt% binder (PVDF), and the
active substance content of cathode is about 1-1.5 mg cm-2.
3. Results and Discussion
3.1. Synthesis and characterization of HSLCP electrolyte matrixes
The synthetic route of the HSLCPs (HPG-PCL-(LC)x) is shown in Scheme 1. Hyperbranched polyether (The
1HNMR characterization was showed in supporting information), was synthesized via the anionic
polymerization of glycidol. HPG-PCL was synthesized via the ROP of ε-caprolactone (CL) using HPG as the
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macroinitiator and Sn(Oct)2 as the catalyst. The well appeared characteristic proton NMR peaks (Figure 1a) at
4.03 (CH2-O-CO-), 2.30 (-CO-CH2-), 1.64-1.37 (-CH2-CH2-CH2) ppm suggest that monomer of CL was
polymerized to HPG through ROP.
<Scheme 1>
The ATRP macroinitiator, HPG-PCL-Br, was synthesized by subsequent esterification reaction of terminal
hydroxyl groups of HPG-PCL with 2-bromo-2-methylpropionyl bromide. After esterification reaction, the peak e
(Figure 1a) at 3.61 ppm characteristic of methylene protons adjacent to the terminal hydroxyl groups is moved
to 4.18 ppm (Figure 1b), the still remained small peak g can be attributed to the core of HPG. Moreover, the
appearance of a new peak f (Figure 1b) at 1.92 ppm is methyl protons originate from 2-bromoisobutyryl moiety,
indicating that 2-bromo-2-methylpropionyl bromide has been almost quantitative attached to HPG-PCL. GPC
can further illustrate the above results (see table S1), as HPG has an Mn of 5.79 × 103 g mol-1 (Mw/Mn = 1.68),
and HPG-PCL increases to 4.79 × 104 g mol-1 (Mw/Mn = 1.45). Meanwhile, compared with HPG-PCL, the Mn of
HPG-PCL-Br increases to 5.71×104 g mol-1, with an Mw/Mn of 1.21 as revealed by GPC analysis (see table S1).
So it can be calculated that HPG-PCL or HPG-PCL-Br star copolymer possesses 61 PCL arms. Then,
HPG-PCL-(LC)n/2 was synthesized by the ATRP of LC using HPG-PCL-Br as macroinitiator, as shown in Figure
1c, characteristic proton NMR peaks at 4.19, 4.03 (CH2-O-CO-), 2.30 (-CO-CH2-), 1.64 - 1.37 (-CH2-CH2-CH2-)
ppm of PCL and characteristic peaks of aromatic ring protons at 7.69 - 7.00 ppm of LC are all well present.
GPC analysis of the products reveals Mn of 6.92×104 (Mw/Mn = 1.44) for HPG-PCL-(LC)n/3, 8.08 × 104 (Mw/Mn
<Figure 1>
= 1.73 ) for HPG-PCL-(LC)n/2 and 9.63 × 104 (Mw/Mn = 1.39) for HPG-PCL-(LC)n (see table S1), confirming that
the polymerization processes were well controlled. Furthermore, IR spectra (Figure S2) show that the intensity
of OH bond at 3373 cm-1 of HPG (Figure S2a) obviously declines after ROP of ε-caprolactone (CL), as the new
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absorption peak (C=O) at 1715 cm-1 which can be assigned to PCL segments have extra high absorption
intensity. The still remained peak at about 3373 cm-1 is the terminal hydroxyls of HPG-PCL (Figure S2b). The
characteristic peaks of CN (2226 cm-1) and aromatic C-H (1600 cm-1) (Figure S2c, d, and e) appear after
ATRP of LC using HPG-PCL-Br as macroinitiator, and the intensity of peaks become stronger with increasing
the contents of LC (also no hydroxyls can be found), further demonstrating the successful synthesis of
hyperbranched star copolymers containing mesogenic units.
3.2. Thermal behaviors and liquid crystallinity
Thermal stability is a crucial index for application of SPEs in LIBs. Inset of Figure 2a presents the DTA
(Derivative Thermogravimetry) curves of LC, HPG-PCL, HPG-PCL-(LC)n/3
,
HPG-PCL-(LC)n/2 and
HPG-PCL-(LC)n. The thermal decomposition temperature (Td, temperature for 5 wt% loss of samples) values of
HPG-PCL, HPG-PCL-(LC)n/3, HPG-PCL-(LC)n/2 and HPG-PCL-(LC)n are 364, 366, 368, and 377 oC (Figure 2a),
respectively, suggesting that the HSLCPs are thermally stable. However, LC shows a much low degradation
o
o
temperature of 251 C. Meanwhile, a second decomposition stage (Degradation temperature of 366 C) can
also be seen at higher temperature. The groups including CH2=CH- and -C(O)-O- show much lower thermal
stability than aromatic ring and –CN group, so the first decomposition stage is mainly attributed to CH2=CH-
and -C(O)-O- while the second stage is the decomposition of other parts. The thermal behaviors of the series
materials were further studied using DSC, and the results are shown in Figure 2b. The melting temperature
(Tm) and Melt enthalpy (
△
H) of HPG-PCL-(LC)n/3 are 39.7 oC and 47.8 J g-1, respectively, which is lower than
o
that of HPG-PCL (Tm = 47.0 C,
△
H = 72.0 J g-1) on heating (Figure 2b), revealing that LC can reduce the
crystallization of HPG-PCL by suppressing the neat arrangement of PCL segments. Such phenomenon can be
observed more clearly with increase the contents of LC. As can be seen in Figure 2b, both HPG-PCL-(LC)n/2
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and HPG-PCL-(LC)n only appear glass-transition temperature (Tg) at -43.8 and -43.3 oC, respectively. That is to
say the materials are completely amorphous, which is beneficial to the transport of lithium-ion [9].
<Figure 2>
To investigate the liquid crystal characteristic of the related materials, the cross-polarized optical microscopy
(POM) tests were conducted at different temperature. LC exhibits the enantiotropic liquid crystallinity at 30 oC,
as shown in Figure 3a of the microphotograph. After cooling from 120 oC, strong orientation of smectic phase
LC can be directly observed at Figure 3b. Both HPG-PCL-(LC)n (Figure 3c and d) and HPG-PCL-(LC)n/2
(Figure 3e and f) show optical anisotropy with birefringent textures when heated or cooled, owing to strong
orientation ability of canobiphenyl mesogenic units. It can be clearly observed in Figure 3c, the higher contents
of LC on HPG-PCL (compared with HPG-PCL-(LC)n/2) result in significant liquid crystal characteristic. Even
HPG-PCL-(LC)n was annealed from 190 oC to 30 oC, the texture structure is kept without much change (Figure
3d). The only change seems that the texture structure is arranged in a more ordered manner (Figure 3d),
because LC has strong ordered arrangement ability. With the decrease of grafting contents of LC on HPG-PCL
(HPG-PCL-(LC)n/2), a new wavy texture is observed (Figure 3e) and the ordered feature can be observed more
clearly after annealing from 80 oC to 30 oC (Figure 3f). Although HPG-PCL-(LC)n/2 has less contents of LC, the
ordering is higher than HPG-PCL-(LC)n after annealing. This is probably caused by steric-hinerance effect:
When the contents of LC at the end of the hyperbranched polymer are too high, the orientation of LC is
impeded, while the soft CL segments may be stacked inside; For PG-PCL-(LC)n/2, the space is more suitable
for the orientation of LC, so the CL segments can be more extended and the polymer shows higher orderliness.
<Figure 3>
AFM is one of the most crucial methods to be used for observing the local microcosmic morphology of samples.
The AFM images of HSLCP (HPG-PCL-(LC)n/2) electrolyte matrix films before and after annealing are
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presented in Figure 4a, b and Figure 4c, d, respectively. The height map in Figure 4a indicates that the surface
of the film is very smooth before annealing. Some local orientation can also be observed at the corresponding
phase diagram of the unannealed film in Figure 4b, which probably can be attributed to the strong orientation
ability of canodiphenyl mesogenic units. The height map of the annealed film in Figure 4c not only shows more
smooth surface morphology but also more ordered arrangement. The orientation of the annealed film is
presented more clearly in Figure 4d of the phase diagram. Generally speaking, the AFM images further confirm
that the HSLCP electrolyte matrix film can be oriented more remarkable after annealing.
<Figure 4>
To further demonstrate the effective orientation of HSLCP and the solid polymer electrolyte, HPG-PCL-(LC)n/2
and the corresponding electrolyte are chosen and characterized by XRD. Compared with the as-cast films of
HPG-PCL-(LC)n/2 and HPG-PCL-(LC)n/2 electrolyte (25 wt% LiTFSI), the XRD diffractograms of the films
annealed at 80 oC show two obvious reflection at 2Ɵ = 3.9 and 4.2, a small new peak at 2.1 oC (corresponding
to d-spacing) represents the local ordered arrangement of the polymer (Figure 5). According to the Bragg
equation, both the annealed polymer and electrolyte have a local interlamellar spacing of 42 Å. The result
suggests that the liquid crystal-based electrolyte forms local ordered structure. Moreover, the diffractograms of
the polymer and electrolyte films have the same peak position and intensity, suggesting that the orientation of
HSLCP is not destroyed after doping with lithium salt. It is worth to mention that the ordered arrangement of
HSLCP may improve the ionic conductivity of the related polymer electrolyte. As liquid crystal (LC)x segments,
one part of the arms, are beneficial to form ion conductive pathway via orientation, and PCL segments can be
used to dissociate lithium salt and efficiently migrate charge carriers.
<Figure 5>
3.3. Electrochemical properties
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The insets (Figure 6a and b) show that both HPG-PCL-(LC)n/2 and HPG-PCL-(LC)n electrolytes can form
free-standing films by solution casting method. However, HPG-PCL-(LC)n/3 is very viscous and unable to form
free-standing film. Compared with other polymers with higher percentages of LC, the contents of rigid benzene
rings in the HPG-PCL-(LC)n/3 are lower, therefore, the HPG-PCL-(LC)n/3 is very viscous and unable to form
free-standing film. Although higher contents of rigid LC can improve the mechanical strength of hyperbranched
star liquid crystal polymer and the film-forming ability can be improved, however, the film will be easy to break
once the contents of rigid segments are too higher. Only when the rigid (LC) and soft (PCL) segments are
appropriate, a flexible film with good film-forming ability can be obtained. In addition, when LC is grafted into the
end of PCL, the ordered arrangement of PCL is interrupted and the crystallinity of PCL becomes lower. With the
increase of the contents of grafting LC, the crystallization ability of the sample decreased. So we focus our
study on HPG-PCL-(LC)n/2 and HPG-PCL-(LC)n.
Ionic conductivity of the solid electrolyte is one of the important parameters which can reflect ion migration
ability. Figure 6a and b show the relation of ionic conductivity at different temperature with corresponding
weight content of LiTFSI. As can be observed, the increase of temperature causes a corresponding increase in
ionic conductivity of the polymer electrolytes at the same content of LiTFSI. One reason is that the movement of
polymer segments is enhanced at higher temperature, which can improve the ions transmission capacity of
polymer chains significantly at the same time interval at higher temperature. Another reason is that the easier
dissociation of LiTFSI at higher temperature provides more free ions.
<Figure 6>
In addition, ionic conductivity of SPEs can also be strongly influenced by the content of lithium salt. As also
depicts in Figure 6a, the ionic conductivity of HPG-PCL-(LC)n/2 increases at first with the increase of lithium salt
content, and then reaches a maximum value with 25 wt% LiTFSI at the same temperature (range from 30 to 80
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oC), further increasing the weight content of LiTFSI causes the decrease of ionic conductivity. Generally, the
phenomenon is because addition of lithium salt would increase the free ions concentration in the case of low
content of lithium salt, thus the conductivity improved, while too high proportion of lithium salt would hinder the
dissociation of lithium salt and the migration of charge carries [24, 37]. The interaction of lithium salt with
HPG-PCL-(LC)n/2 was further studied by DSC. As shown in Figure 7, the amorphous state is kept with different
weight contents of LiTFSI, but the Tg increases once 15 wt% of LiTFSI is added. This change is probably
caused by the coordination between LiTFSI and ester groups[38], resulting in lower movement of the CL chains.
With further increasing the content of lithium salt from 15 to 25 wt%, Tg of the electrolyte decreases, because
lithium salt has plasticizing effect. Then Tg increases again as the LiTFSI content is too high, because the high
content of LiTFSI causes larger ionic aggregation, which can restrict the movement of the molecular chains.
Usually, lower Tg is beneficial to higher ionic conductivity [39, 40]. In this study, the DSC test of HSLCP
electrolyte is consistent with the ionic conductivity analysis, as both the highest ionic conductivity and the
lowest Tg of the electrolyte reach at 25 wt% of LiTFSI. For HPG-PCL-(LC)n (Figure 6b), it has the same result
except for the highest ionic conductivity reaches at 35 wt% of LiTFSI (the DSC data is not shown). It is
reasonable that the optimal content of LiTFSI increases, as the higher contents of benzene-containing LC in
HPC-PCL-(LC)x probably influence migration of charge carriers. Thus, results in the higher optimal content of
lithium salt and lower ionic conductivity. The Nyquist plots corresponding to the optimal ionic conductivity of
HPG-PCL-(LC)n/2 (25 wt% LiTFSI) and HPG-PCL-(LC)n (35 wt% LiTFSI) at different temperature are also
presented in Figure 6c and d, respectively. It can be clearly observed that both of the bulk resistances of the
electrolytes decrease with increasing temperature. Especially, the bulk resistance of HPG-PCL-(LC)n/2 (25 wt%
LiTFSI) decreases from 1673 to 97 ꢁ when temperature is increased from 30 to 80 oC. As for HPG-PCL-(LC)n
(35 wt% LiTFSI), the bulk resistance decreases from 5433 to 223 ꢁ when temperature is increased from 30 to
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o
80 C, which is much higher than HPG-PCL-(LC)n/2 (25 wt% LiTFSI). Thus, the change of bulk resistances of
the polymer electrolytes is consistent with the variation of the corresponding ionic conductivity.
<Figure 7>
The properties of ordered arrangement of HSLCPs urge us to explore whether the ionic conductivity of
HSLCPs-based electrolytes can be improved through annealing. Interestingly, as shown in table S2, without
annealing, the ionic conductivity of HPG-PCL-(LC)n and HPG-PCL-(LC)n/2 electrolytes is 2.11×10-6 and
o
6.85×10-6 S cm-1 at 30 C, respectively. After annealing, the highest ionic conductivity of HPG-PCL-(LC)n and
o
HPG-PCL-(LC)n/2 reaches to 3.71 × 10-6 and 5.98×10-5 S cm-1 at 30 C, respectively. Both of the values are
higher than the ionic conductivity of the unannealed samples and also higher than HPG-PCL electrolyte (7.13 ×
10-7 S cm-1, 30 oC). The improved ionic conductivity of HSLCPs-based electrolytes proofs our image that
orientation of HSLCPs can increase the ionic conductivity. Thus the improved ionic conductivity can probably
be attributed to the high-orientating of mesogenic units at the end of hyperbranched copolymer, as orientation
favors the rapid transmission of ions [29, 30, 32]. According to the characterizations of POM, AFM, XRD and
ionic conductivity measurements before and after annealing, an ion transport model for the oriented
HSLCP-based electrolytes is proposed in Figure 8. The ionic transport mechanism can be described that the
layer formed from mesogenic groups LC can afford an ordered ion transport pathway, while the layer generated
from PCL segments can dissociate lithium salt and effectively migrate charge carriers by the movement of the
CL segments.
<Figure 8>
The interfacial compatibility of the solid polymer electrolyte with electrode was also investigated by
electrochemical impedance spectroscopy (EIS). The impedance plot for Li/HPG-PCL-(LC)n/2 electrolyte (with
25 wt% LiTFSI )/Li symmetric cell is shown in Figure 9a. The interfacial resistance (Rintf) which consists of the
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charge transfer resistance (Rct) and the passivation film resistance (Rpf) can be calculated from the semicircle in
the medium frequency range [41]. Rintf of the fresh cell at 60 oC is 2.35 × 105 ꢁ, and then decreases to 4.46 ×
104 ꢁ with the increase of the storage time. After 30 days of storage, Rintf approaches to a steady value of 4.47
× 104 ꢁ (Figure 9b). Considering that solid polymer electrolyte require more time than organic electrolyte to
maintain contact with electrode and forms passivation by the reaction of constituents in electrode and
electrolyte [42], it is reasonable to take 30 days for the Rintf reaches a steady value after the formed passivation
layer hinder the further reaction between the two interfaces, denoting the good interfacial compatibility of the
prepared solid polymer electrolyte with electrode.
<Figure 9>
Electrochemical stability of the solid electrolytes is considered to be an important factor for safety performance
of LIBs. Electrochemical stability window for HPG-PCL-(LC)n/2 with 25 wt% of lithium salt at 30 oC was studied
by linear sweep voltammetry. The result indicates that the electrochemically stable window of the polymer
electrolyte is as high as 5.12 V (Shown in Figure 9c), which is favorable for possible application in
high-potential LIBs and also higher than other reports [43-45].
The impedance spectra before and after constant voltage polarization at 60 oC can be seen in Figure 9d. The
o
chronoamperometry result for Li /HPG-PCL-(LC)n/2/ Li cell with 25 wt% of lithium salt obtained at 60 C is
demonstrated inset of Figure 9d. In addition, the lithium-ion transference number ( t+ ) was calculated using
Equation (2) and the corresponding value was tabulated in table 1. As shown in table 1, a high t+ = 0.63 is
obtained, which is much higher than the reported electrolytes [24, 25, 46] and also indicates the advantage of
using HSLCP as SPE matrix. The high t+ of the HSLCP electrolyte is mainly due to the excellent
electrochemical performance of PCL [20, 23]. Moreover, the presence of mesogenic units at the end of
hyperbranched copolymer, which can also overall weaker binding between the coordinating carbonyl groups
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and the Li+ ions [20]. The other reason is that the strong electron withdrawing capability of nitrile groups can
interact with TFSI-, which is also probably beneficial to the higher t+ [47].
<Table 1>
3.4. Battery Performance
To further study the electrochemical performance of the HSLCP-based electrolyte, coin cells based on the
electrolyte were assembled using a LiFePO4 cathode and a lithium anode. Figure 10a exhibits the charge and
o
discharge property of the Li/HPG-PCL-(LC)n/2 SPE (25% LiTFSI) /LiFePO4 cell cycled at 60 C. The initial
discharge capacity is 114 mAh g-1, and the dischage capacity increases to 121 mAh g-1 after 50 cycles. The
discharge capacity increases after 50 cycles probably due to the requirement of a longer activation time for the
electrolyte. After cycling for ~80 cycles, the discharge capacity declines slightly. The phenomenon may be
attributed to the harmful interfacial reactions between electrodes and electrolyte, which causes the unwanted
resistive layer formed at the interface and impedes ionic transport. However, the discharge capacity is still kept
at 112 mAh g-1 after 100 cycling, resulting in 98% of capacity retention. Figure 10b further illustrates the cycling
performance of the cell with a c-rate of 0.1 C at 60 oC, an average discharge capacity of around 116 mA h g-1 is
obtained for the 100 cycles with the coulombic efficiency close to 100%.
Rate capability is also a crucial criterion to evaluate battery’s performance. The cell shows well-defined
potential plateau at different discharging current density (Figure 10c), however, the gap between the
charge-discharge curves becomes wider as more cycling is performed. This is because the polarization
increases with the increase of the discharging rate capability, which may be ascribed to the charge transport
kinetic limitations associated with the electrode-electrolyte interface.[17] Figure 10d shows the specific capacity
of the cell from 0.1 to 1 C, the discharge capacity of ~107 mAh g-1 is obtained at 0.2 C, which is approximate 96%
of the capacity at 0.1 C, and a discharge capacity of ~ 98 mAh g-1 is achieved even at 0.5 C. The battery also
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has a reversible discharge capacity of about 61 mAh g-1 at 1 C. The discharge capacity is maintained when the
battery is charged and discharged again from 0.5 to 0.1 C. The battery performance results confirm that the
HSLCP-based electrolyte displays not only good cycling performance but also relatively excellent rate
performance. This is primarily due to the improved ionic conductivity at 60 oC and the stable interfacial behavior
and especially the high Li+ transference number of the specific electrolyte. In addition, the LIB based on
HPG-PCL-(LC)n/2 SPE can also power a LED at RT (Figure 10e), further demonstrating the excellent
performance using this special electrolyte.
<Figure 10>
4. Conclusions
For the first time, HSLCP-based solid polymer electrolyte has been developed for LIBs. The electrolyte
composed of LiTFSI and the topological polymer with biphenyl-4-Carbonitrile-based liquid crystal, PCL chain
o
segments, core of HPG. The as-prepared material shows high thermal stability (368 C), the corresponding
electrolyte has good ionic conductivity (5.98 × 10-5 S cm-1), wide electrochemical window (5.12 V), and
outstanding lithium-ion transference number (0.63). The results of battery performance confirm that the
polymer electrolyte displays good cycling performance and rate capability. Moreover, the SPE-based half-cell
can still power a white LED lamp at RT. Thus, combining the advantages of hyperbranched star polymers and
LC for developing a novel HSLCP electrolyte is a worth idea and an approach to guide synthesis and
application of topological structure polymers. Yet, continuous study on molecular design and construction of
conducting pathway of SPEs may be a promising way to further improvement the batteries performance.
Acknowledgements
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The authors express thanks for the supports of the National Natural Science Foundation of China
(No.51073170) and Innovation Program of CAS Combination of Molecular Science and Education.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version.
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Figure and Table captions
Scheme 1. Synthetic routes of LC monomer and HSLCPs.
Figure 1. 1H NMR spectra of the hyperbranched polymers (a) HPG-PCL, (b) HPG-PCL-Br and (c) HPG-PCL-(LC)n/2
.
Figure 2. (a) TGA thermograms of LC and HSLCPs (HPG-PCL-(LC)x (x=n/3，n/2，n)). (b) DSC thermograms of (a) HPG-PCL,
HPG-PCL-(LC)n/3, HPG-PCL-(LC)n/2 and HPG-PCL-(LC)n. Inset of Figure 2a presents the DTA (Derivative Thermogravimetry) curves of
the corresponding samples.
Figure 3. POM pictures of the synthesized compounds: (a) and (b) LC at 30 oC and on cooling from 120 oC to 30 oC (POM images),
respectively; (c) and (d) HPG-PCL-(LC)n at 30 oC and on annealing from 190 to 30 ℃, respectively; (e) and (f) HPG-PCL-(LC)n/2 at 30
oC and on cooling from 80 to 30 ℃, respectively (All scale bars represent 100 µm). According to the POM characterization, the
isotropic temperature of LC, HPG-PCL-(LC)n and HPG-PCL-(LC)n/2 is observed at 120, 190 and 80oC, respectively. This is the gist that
the corresponding annealing temperature was chosen.
Figure 4. Topological (a) and phase (b) AFM images of HPG-PCL-(LC)n/2. Topological (c) and phase (d) AFM images of
HPG-PCL-(LC)n/2 after annealed at 80 oC. All the pictures are 5×5 ꢀm in size.
Figure 5. XRD patterns of HPG-PCL-(LC)n/2, corresponding electrolyte (25 wt% LiTFSI) as-cast films and annealing from 80 oC to 30
oC, respectively.
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Figure 6. Temperature dependence of ionic conductivity of (a) HPG-PCL-(LC)n/2 and (b) HPG-PCL-(LC)n at various weight contents of
LiTFSI. Insets of (a) and (b) are the corresponding films made by solution casting method, respectively. (c) The Nyquist plot of
HPG-PCL-(LC)n/2 (25% weight content of LiTFSI) at different temperature. (d) The Nyquist plot of HPG-PCL-(LC)n (35% weight content
of LiTFSI) at different temperature.
Figure 7. The DSC curves of HPG-PCL-(LC)n/2-based electrolyte with various weight contents of lithium salt recorded under N2 (2nd
heating ) at a scan rate of 10 oC min-1.
Figure 8. The proposed ions transport model of HSLCP-based electrolyte after annealing (According to the characterization of POM,
AFM, XRD and ionic conductivity measurements before and after annealing).
Figure 9. (a) Impedance profile for the Li/SPE/Li cell with 25 wt% LiTFSI at 30 oC. (b) The change in Rintf as a function of storage time at
30 oC. (c) Linear sweep voltammograms of HPG-PCL-(LC)n/2 with 25 wt% of LiTFSI at 30 oC at a scan rate of 1mV s-1. (d) AC
impedance spectra before and after polarization at 60 oC. The inset shows the chronoamperometry of Li/HPG-PCL-(LC)n/2/Li cell with
25 wt% of LiTFSI at a potential step of 10 mV (60 oC).
Figure 10. (a) Charge-discharge curves at 0.1 C rate. (b) Cycle performance of the Li/HPG-PCL-(LC)n/2 SPE/LiFePO4 battery during
galvanostatic cycling at 0.1 C rate. (c) Charge/discharge curves for Li/LiFePO4 cell at different C-rates. (d) Discharge performance at
0.1, 0.2, 0.5 and 1 C rate capability. The above tests were conducted at 60 oC. (e) Schematic of the LIB materials and configuration
(left), image of LIB based on HPG-PCL-(LC)n/2 SPE powering a LED at RT.
Table 1. Lithium ion transference number of HPG-PCL-(LC)n/2 with 25 wt% of lithium salt at 60 oC
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t+
Temperature(oC)
60
Rlo (Ω)
Rls (Ω)
Io (µA)
Is (µA)
664
677
3.9
2.7
0.63