Robust Succinonitrile-Based Gel Polymer Electrolyte for Lithium-Ion

Subscriber access provided by University of South Dakota

Energy, Environmental, and Catalysis Applications

A robust succinonitrile-based gel polymer electrolyte for lithium ion batteries withstanding mechanical folding and high temperature Pengfei Lv, Yongsheng Li, Yuhan Wu, Guobiao Liu, Hao Liu, Shaomin Li, Changyu Tang, Jun Mei, and Yuntao Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06800 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

A robust succinonitrile-based gel polymer electrolyte for lithium ion batteries withstanding mechanical folding and high temperature Pengfei Lva,b, Yongsheng Lib, Yuhan Wub, Guobiao Liub, Hao Liub, Shaomin Lib, Changyu Tangb*, Jun Meib, Yuntao Lia* a. College of Material Science and Engineering, Southwest Petroleum University, Chengdu 610500, China b. Chengdu Green Energy and Green Manufacturing Technology R&D Center, China Academy of Engineering Physics, Chengdu 610200, China

Abstract: Fabrication of gel polymer electrolyte containing succinonitrile (GPE-SN) with high mechanical strength is quite challenging because SN electrolyte always suppresses the formation of polymer networks during in-situ polymerization. In this work, a mechanically robust GPE-SN was successfully prepared by using a solution immersion method. During fabrication, paste-like SN electrolyte was transformed into liquid SN electrolyte with low viscosity by heating at 50 oC and then infiltrated into the UV-cured highly cross-linked polyurethane acrylate (PUA) skeleton. The resulted GPE-SN film exhibits superior tensile strength (6.5 MPa) compared to the one (0.5 MPa) prepared by in-situ polymerization (GPE-SN-IN). The high mechanical strength of the GPE-SN-IM film enables the LiCoO2/Li4Ti5O12 film battery to withstand 100-cycle folding without electrolyte damage and capacity loss. Besides, the GPE-SN presents a high ionic conductivity (1.63 × 10-3 S·cm-1 at 25 oC), which is comparable to GPE with commercial liquid electrolyte (GPE-LE). Due to good thermal stability of the GPE-SN, the LiCoO2/Li cell with this electrolyte shows better charge-discharge cycling stability than that with GPE-LE at high temperature (55 °C). Thus, the GPE-SN prepared by our method could be a promising polymer electrolyte offering better safety and reliability for lithium ion batteries. Keywords: gel polymer electrolyte, lithium ion battery, succinonitrile, mechanical strength, thermal stability

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

1. Introduction With rapid development of electric vehicles and portable electronic devices, conventional lithium-ion batteries with liquid organic electrolytes have been facing serious safety issues such as leakage, volatilization, and combustion.1-4 Solid polymer electrolytes (SPEs) consisting of lithium salts and polar polymers have been considered to be safer alternatives to the liquid electrolytes for lithium ion batteries.5-7 They can perform as separators in batteries so that flexible batteries could be fabricated for emerging applications such as wearable electronic devices and flexible sensors.8-12 Unfortunately, the ion conductivities of the SPEs (below 10-6 S•cm-1) are much lower than those of liquid electrolytes (over 10-3 S•cm-1) at room temperature.13-14 Gel polymer electrolytes (GPEs), another type of polymer electrolyte, can exhibit high ionic conductivity comparable to liquid electrolytes. In the GPEs, organic liquid electrolytes

are

encapsulated

in

various

polar

polymer

matrices

(e.g.,

polymethylmethacrylate and polyvinylidenefluoride) by either in-situ polymerization or solution immersion to provide good conducting phases for lithium ions.15-17 However, the presence of flammable and volatile liquid solvents (e.g., ethylene carbonate and dimethyl carbonate) in these GPEs still brings safety issues for batteries.18-19 Succinonitrile (SN, NC–CH2–CH2–CN), which is a solid plastic crystal with plastic crystal behavior between -40 °C (plastic crystal transition point) and 60 °C (melting point), shows many advantages such as good solubility for lithium salt, high boiling

2


Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


point (267 °C), and low flammability.20-24 Thus, SN is a promising candidate to make safer polymer electrolytes. Upon complexing with lithium salt, SN exhibits a lower melting point and transforms into a paste-like (viscous) electrolyte.25 Although the GPEs with lithium salt-doped SN electrolyte (GPE-SN) fabricated by in-situ polymerization (blending) method can possess high conductivities over 10-3 S•cm-1, their mechanical strength are poor.26-28 The presence of SN electrolyte inevitably suppresses crystallization and network formation of polymer matrices and thus seriously degrades the mechanical strength of GPEs.29 Compared to in-situ polymerization method, solution immersion method is to infiltrate liquid electrolyte into the established polymer networks so that GPEs can be fabricated with higher mechanical strength.30 However, the preparation of the GPE-SN by solution immersion method has not been achieved because SN electrolyte is too viscous to penetrate into polymer matrix. Herein, a simple route is deployed to make a mechanically robust GPE-SN with high ionic conductivity by solution immersion method. Firstly, polyurethane acrylate (PUA) was UV-cured to form an elastomeric film. Then, the resulted PUA film was immersed into the melted SN electrolyte at 50 oC so that the liquid-like SN electrolyte with low viscosity can infiltrate into the PUA network. After cooling down to room temperature, SN electrolyte changed back to paste-like state and was successfully secured in the polymer skeleton. The resulted GPE-SN shows a high ionic conductivity of 1.63 × 10-3 S·cm-1 at 25 oC and much higher mechanical strength than GPE-SN

prepared

by

in-situ

polymerization.

The

3


thermal

stability

and


electrochemical performance of the GPE-SN were also investigated. 2. Experimental 2.1. Materials. PUA (7219, molecular weight of 6000, and viscosity of 50000-60000 mPa·s) was purchased from Jiangmen Hengguang New Material Co., Ltd (Jiangmen, China). Anhydrous acetonitrile (ACN, 99.8%, H2O: ≤0.005%), succinonitrile (SN, 99%), acetone (99%), chloroform（99%）, lithium bis(trifluoromethane sulfonimide) (LiTFSI, 99%), trimethylbenzoyldi-phenylphosphinate (TPO, photo-initiator, 97%), and lithium cobalt oxide (LiCoO2) were purchased from Aladdin (Shanghai, China). LiTFSI was vacuum dried at 80°C for 24 hours before use. Lithium titanate (Li4Ti5O12) was purchased from Hefei Kejing Materials Technology Co., Ltd. Solvent mixture of ethylene carbonate (EC) and dimethyl carbonates (DMC) (1:1, v/v) containing 1 M LiPF6 was purchased from Guotai-huarong New Chemical Materials Co., Ltd (Zhangjiagang, China) and was used as organic liquid electrolyte. Carbon black (Conductive agent) and polyvinylidenefluoride (PVDF) binder were purchased from Sinopharm Chemical Reagent Co., Ltd. All the experiments were carried out in glove box (H2O and O2 < 0.1 ppm) under argon atmosphere. 2.2. Fabrication of polymer electrolyte films. PUA oligomer and TPO (98:2 in mass ratio) were dissolved in ACN to form a homogenous solution. Subsequently, the mixture solution was poured into a polytetrafluoroethylene (PTFE) mold. After removing the solvent by room temperature evaporation, the residue mixture was irradiated under UV-light (365 nm in wavelength and 200 mW·cm−2 in light intensity) for 50 seconds to form a cross-linked PUA film with a thickness of about 70 µm. 4


Page 4 of 31

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Since it is found that the ionic conductivity of SN electrolyte is dependent on lithium salt concentration and reaches maximum value at 1 M LiTFSI (Figure S1), 1 M LiTFSI was added into SN to form a paste-like SN electrolyte. While heated at 50 oC, the paste-like SN electrolyte was melted to be a liquid state with low viscosity (5.2 mPa·s). Then, as-prepared PUA film was immersed into the melted SN electrolyte at 50 oC for 2 hours so that the liquid SN electrolyte can infiltrate into the PUA matrix (Scheme 1). After cooling down to room temperature, SN electrolyte changed back to its paste-like state and was successfully localized in the polymer skeleton. The PUA/SN electrolyte film prepared by the above method is denoted as GPE-SN-IM. The SN content in GPE-SN-IM is 74 wt%. By using our previous reported route, 29 the blend of SN electrolyte and PUA oligomer with TPO can be cured through in-situ photopolymerization to form a polymer electrolyte denoted as GPE-SN-IN. For comparison, the SN content in the GPE-SN-IN is fixed at 74 wt%, which is the same as that in GPE-SN-IM. Conventional GPE containing commercial liquid electrolyte (GPE-LE) was also prepared as a reference by immersing the UV-cured PUA film in 1M LiPF6/EC/DMC (viscosity: 1.4 mPa·s) at room temperature.

5



Page 6 of 31

Scheme 1. A schematic representation of GPE-SN-IM prepared by immersing UV-cured PUA film in melted SN electrolyte at 50 oC. 2.3 Material Characterization. Chemical structures and the conversion of double bonds ( α ) of the samples were measured by using FTIR (Nicolet-iS10, Thermo Fisher Company, USA) in the range of 4000-400 cm-1. The electrolyte uptake (η) of the PUA film was calculated by the following equation:

η=

W1 − W0 ×100% W0

（1）

Where W0 and W1 are the weights of PUA films before and after the absorption of electrolyte, respectively. The network characterization of the cross-linked PUA in GPE-SN-IN and GPE-SN-IM films were performed by swelling equilibrium method.31 The films were firstly washed with chloroform to remove SN electrolyte and then vacuum dried at 60 o

C for 24 h. After that, the samples were immersed in acetone and weighed (m2) after

swelling equilibrium. The swollen samples were then vacuum dried at 80 °C until the residual solvent was removed completely. The weight of the dried sample was recorded (m1). The swelling degree ( Q ), molecular weight ( M c ) between two cross-linking points, and cross-linking density ( ρc ) of the samples can be calculated by the following equations, respectively.

Q=

Mc =

m2 / ρ 2 + (m2 − m1 ) / ρ 2 m2 / ρ 2

(2)

Q 5 / 3 ρ1V1 1 / χ1 − 1

(3)

6


Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ρc =

1 2M c

(4)

Where ρ1 and ρ 2 are the density of PUA and acetone, respectively; V1 is the molar volume of acetone (75 L/mol) and χ1 is Flory−Huggins interaction parameter of PUA/acetone (0.34). Thermal behavior of samples was investigated by using a differential scanning calorimetry (DSC) instrument (Mettler Toledo DSC3+, Switzerland) scanned from -80 to 60 °C with a heating rate of 10 °C·min-1. Dried nitrogen was vented into the furnace with a constant flow rate of 50 mL·min-1. A scanning electron microscopy (SEM, XL-30-ESEM-FEG instrument, FEI PHILIPS, Netherland) was used to observe the surface morphology of cathode. The mechanical properties of samples were measured on an MTS universal testing machine (with a 200 N load cell) with a tensile rate of 50 mm·min-1 at room temperature. The dimensions of samples are 75.0 mm×4.0 mm×0.1 mm.

2.4. Electrochemical characterization. AC Impedance, linear sweep voltammetry, and DC polarization measurements were performed at ambient temperature using electrochemical workstation (Zahner Zennium, Germany). The ionic conductivity of polymer electrolyte was evaluated using the electrochemical impedance spectroscopy (EIS) measurement under an AC voltage of 10 mV amplitude with frequency range of 1~106 Hz. The ionic conductivity (σ) can be calculated by the following equation:

σ=

d Rb S

(5)

where d and S are the thickness and the effective area of polymer electrolyte,

7



Page 8 of 31

respectively; Rb is the bulk impedance of electrolyte film. The electrochemical stability windows of GPE-LE and GPE-SN-IM were determined by linear sweep voltammetry at the potential range from 2.5 to 6.0 V with a scan rate of 1.0 mV · s−1 at room temperature. Lithium ion transference number ( t Li + ) of GPEs was measured using symmetric cell of Li/GPE/Li by DC polarization method combined with EIS method. The t Li + can be calculated according to the following equation: t Li + =

I s ( ∆V − R0 I 0 ) I 0 ( ∆V − Rs I s )

(6)

where I 0 and I s are the initial and steady current, respectively; R0 and Rs are the initial interfacial and steady-state resistances, respectively; ∆V is the applied DC voltage. The lithium ion half-cell was assembled by sandwiching the GPE between LiCoO2 cathode and Li anode in the glove box. The flexible film battery was fabricated by laminating LiCoO2/GPE/Li4Ti5O12 (4 cm × 4 cm) between two polyethylene terephthalate (PET) films via a lamination machine. The copper tapes were bonded onto the battery electrodes as tabs for both anode and cathode. The cathode (LiCoO2) and the anode (Li4Ti5O12) were prepared by mixing the active materials, carbon black, and PVDF binder in the weight ratio of 8:1:1. The slurry was then coated onto aluminum foils followed by drying. Constant current charge-discharge test was investigated by using a CT2001A cell test instrument (LAND CT2001A, Wuhan Jinnuo Electronics). The cycling performances of LiCoO2/GPE/Li cells were tested at a current density of 0.2C (=0.055 mA·cm-2) under a voltage of 2.75-4.25 V. The 8


Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cycling performance of flexible batteries was evaluated at a current density of 0.2C under a voltage of 1.0-3.0 V.

3. Results and discussion 3.1 Preparation of GPEs. The formation of PUA networks via photo-polymerization of PUA oligomer can be confirmed by FTIR spectra (Figure 1a). In FTIR spectrum of PUA oligomer, the strong absorption at 1730 cm−1 is ascribed to C=O groups and the peaks at 1636 and 1619 cm−1 are ascribed to C=C of acrylic groups. The C=C bands of acrylic groups disappear after UV exposure, indicating that the UV cross-linking reaction of PUA oligomer occurs via double bond cleavage.32 Due to unique hard and soft segment structures,33-34 the cured PUA elastomer can not only exhibit good mechanical performance but also act as a matrix to hold liquid electrolyte in its network. Upon heating at 50 oC, paste-like SN electrolyte transforms into liquid electrolyte (Figure1b). After immersing in melted SN electrolyte, the diameter of the cured PUA film increases from 16.0 to 20.2 mm (Figure 1c), indicating that the PUA film can be swollen by the melted SN electrolyte. Since SN electrolyte returns to its paste-like state after cool-down, the resulted GPE-SN film presents solid-like characteristic at room temperature and can be stretched and folded without physical damage (Figure 1d).

9



Figure 1. (a) FTIR spectra of PUA oligomer before and after UV-irradiation. (b) Photographs of SN electrolytes kept at 25 °C (left) and 50 °C (right); the paste-like SN electrolyte cannot flow at 25 °C. (c) Photographs of PUA, GPE-LE, and GPE-SN-IM films. (d) The foldable and stretchable features of GPE-SN-IM film. Figure 2a shows the electrolyte uptake capacities of PUA films immersed in SN and EC/DMC electrolytes, respectively. Although the liquid uptakes of the PUA film in both electrolytes saturate at the same time (40 min), PUA film has higher uptake capacity in EC/DMC electrolyte (400%) than that in SN electrolyte (285%). SN electrolyte has a higher viscosity than EC/DMC so that it is more difficult to be penetrated into PUA film. However, the high viscosity and high boiling point of SN electrolyte offer GPE-SN-IM excellent high temperature performance. As shown in Figure 2b, the electrolyte retention of GPE-SN-IM does not obviously decrease with 10


Page 10 of 31

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


time at 55°C. Thus, the GPE-SN-IM can be used for lithium ion battery operating at elevated temperature. In contrast, the electrolyte retention of GPE-LE containing EC/DMC sharply decreases by 71% after 5 h storage at the same temperature due to high volatility of the solvent.

Figure 2. (a) Liquid electrolyte uptakes of PUA films immersed in LiPF6/EC/DMC at 25 °C and LiTFSI/SN at 50 °C, respectively. (b) Electrolyte retention of GPE-LE and GPE-SN-IM films at 55 °C.

3.2 Mechanical properties and structural characteristics of GPEs. Since polymer electrolyte can act as a separator in lithium battery, its mechanical performance is very critical for the safety of the battery. Figure 3a shows the stress-strain curves of PUA, GPE-LE, GPE-SN-IM, and GPE-SN-IN films. All three GPE films have much lower tensile strength than PUA film (14.7 MPa) due to the plasticizing effect from EC/DMC and SN electrolytes.35 The tensile strength and elongation at break of the GPE-SN-IM (6.5 MPa and 52%) is significantly higher than those of GPE-SN-IN (0.5 MPa and 23%) and GPE-LE (1.5 MPa and 34%). This result demonstrates that our method achieved a combination of relatively high mechanical strength and good flexibility in the GPE-SN-IM, which could be used for fabrication of flexible 11



batteries. To understand the effect of EC/DMC and SN on mechanical behavior of GPEs, the structural characteristics of GPEs were investigated by using DSC, FTIR, and swelling equilibrium testing. The glass transition temperature ( Tg ) observed in DSC profiles (Figure 3b) reflects the mobility of polymer chain segments. A low Tg of UV-cured PUA at -45.4 °C suggests high chain mobility in PUA’s network, which facilitates liquid electrolyte to penetrate into polymer skeleton. All GPEs have lower

Tg s than PUA due to the plasticizing effect from the addition of liquid electrolytes into the system. The plasticizing effect can significantly decrease the mechanical strength of polymer matrix.25, 36-37 Due to the strong plasticizing effect of EC/DMC, the Tg of GPE-LE (-64.0 °C) is lower than that of GPE-SN-IM (-61.0 °C). Besides, the melting point (33.4 °C) of SN is much higher than that of EC (14.7 °C) and DMC (3.8 °C). Thus, SN keeps its solid-like state at room temperature and doses not seriously degrade the mechanical strength of polymer matrix like liquid solvent (EC/DMC). With the same SN electrolyte content (74 wt%), GPE-SN-IN shows lower Tg (-69.5 °C) than GPE-SN-IM (-61.0 °C). This decrease should be attributed to the structural difference between two PUA networks obtained by different fabrication methods. Table 1 shows structure parameters of cross-linked PUA networks in GPE-SN-IN and GPE-SN-IM, which are measured by FTIR and swelling equilibrium method. The conversion of double bond ( α ) after cure is only 23% in GPE-SN-IN film, which is much lower than that in GPE-SN-IM film (89%), indicating that the presence of SN electrolyte greatly inhibits the cross-linking 12


Page 12 of 31

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reaction of PUA oligomer during in-situ polymerization process. In GPE-SN-IM film, PUA has been already UV cured before the addition of SN so that its crosslinking level is high and the polymer network is well established. Accordingly, the PUA network of the GPE-SN-IN film presents extremely low cross-linking network density ( ρ c ) and high molecular weight ( M c ) between cross-linking points. These results suggest that the SN electrolyte inevitably suppresses cross-linking reaction of PUA during in-situ photo-polymerization and thus leads to low cross-linking degree and low mechanical strength of the GPE-SN-IN film.27

13



Page 14 of 31

Figure 3. (a) Stress–strain curves of PUA, GPE-LE, GPE-SN-IN, and GPE-SN-IM films. Inset shows stress–strain curve of GPE-SN-IN film. (b) DSC profiles demonstrating glass transition temperature and melting point of PUA, GPE-LE, GPE-SN-IM, and GPE-SN-IN film. The endothermic peak at -36.5 °C corresponds to the transition temperature (from solid crystal to plastic crystal phase) of SN.

Table 1 Structure parameters of cured PUA networks of GPE-SN-IM and GPE-SN-IN. Samples

α (%)

Q (%)

M c (g/mol)

ρ c (mol/cm3)

GPE-SN-IM

89

10.3

1803

2.77×10-4

GPE-SN-IN

23

36.5

14911

3.35×10-5

14


Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3 Flammability. The flammability of polymer electrolyte is another key factor associated with the safety of batteries. The flammability of GPEs with SN and EC/DMC is evaluated by ignition test, as shown in Figure 4. GPE-LE with EC/DMC can be quickly burned upon contacting the flame. Under the same test condition, GPE-SN-IM does not burn but produce small amount of white smog only. With low flammability, GPE-SN-IM can be used as a much safer electrolyte for lithium ion batteries.

Figure 4. Flame ignition tests for (a) GPE-LE and (c) GPE-SN-IM. After contacting the flame, (b) GPE-LE quickly burns but (d) GPE-SN-IM does not burn.

3.4 Electrochemical performance. Figure 5a shows the temperature-dependent ionic conductivities of GPE-LE and GPE-SN-IM films. The ionic conductivities of both films increase with the increase of temperature, which follows Arrhenius-type behavior. Although the liquid electrolyte uptake of GPE-SN-IM is lower than that of 15



Page 16 of 31

GPE-LE, the ionic conductivity of GPE-SN-IM (1.63×10-3 S·cm-1 ) is comparable with that of the GPE-LE (1.92×10-3 S·cm-1) at room temperature . It is proved that GPE-SN-IM benefits from the excellent ionic conductivity of SN in its structure. The electrochemical stability windows of GPE-SN-IM and GPE-LE were examined by linear

sweep

voltammetry

(Figure

Li/electrolyte/stainless steel cell.

5b)

and

cyclic

voltammetry

with

a

GPE-LE exhibits electrochemical stability up to

4.7 V versus Li+/Li and begins to decompose with further increasing voltage. GPE-SN-IM keeps electrochemical stability at higher voltage (5.3 V), indicating that GPE-SN-IM can act as a promising polymer electrolyte for high-voltage lithium ion batteries.38 Furthermore, the distinct reduction and oxidation peaks can be observed in each cycle at a potential range of -1.0-1.0 V (vs Li/Li+) from the cyclic voltammetry (CV) profiles

(Figure S2), representing the reversible plating and stripping of

metallic lithium on stainless steel electrode.39 There is no noticeable oxidation current peak observed in GPE-LE (before 4.7 V) and GPE-SN-IM (before 5.3 V). Thus, both GPE-LE and GPE-SN-IM should be electrochemically stable enough to meet the requirements for rechargeable lithium ion batteries. The lithium ion transference number ( t Li + ) of polymer electrolyte plays a significant role in charging and discharging performance of lithium batteries, which is measured by DC polarization method (Figure 5c and 5d). According to the equation (2), the t Li + of GPE-SN-IM is calculated to be 0.59, which is much higher than that of GPE-LE ( t Li + =0.33). The high t Li + facilitates reducing concentration polarization of battery during charge and discharge. 40-41 16


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) Temperature-dependent ionic conductivity of GPEs. (b) Linear scan voltammetry of GPE-LE and GPE-SN-IM at a scanning rate of 1 mV·s−1 at 25 °C. Current-time curves of (c) the Li/GPE-LE/Li and (d) Li/GPE-SN-IM/Li cells under a DC voltage of 10 mV. Insets show the corresponding Nyquist impedance of the cells before and after DC polarization. Figure 6a shows the discharge capacities and columbic efficiencies of two kinds of half cells based on LiCoO2/GPE-LE/Li and LiCoO2/GPE-SN-IM/Li at 25 °C with a constant rate of 0.2C. The cell with GPE-SN-IM presents a high columbic efficiency of over 99% and a reversible discharge capacity of 136.7 mAh·g-1, which is close to the value of the cell with GPE-LE (142.5 mAh·g-1). The discharge capacities of both cells slowly decrease with the increase of charge−discharge cycles. After 100 charge−discharge cycles, the cell with GPE-LE (Figure 6b) delivers a discharge 17



capacity of 132.1 mAh·g-1 with a capacity retention of 92.7%. The cell with GPE-SN-IM shows a lower capacity retention of 87.6%. It can be attributed to GPE-LE’s higher ionic conductivity and better compatibility with electrodes. At higher temperature (55 °C), the discharge capacity of the cell with GPE-LE (Figure 6c and 6d) remarkably decreases to 108 mAh·g-1 with a low retention of 66% after 100 cycles, while the cell with GPE-SN-IM can still deliver the discharge capacity of 128.3 mAh·g-1 with a much higher capacity retention of 89.4%. Figure 6e and 6f show the rate capabilities of the half cells at different temperatures. The discharge capacities of both cells decrease with the increase of discharge rate and present high retention of over 75% at high current density (2C). Their discharge capacities of both cells can almost return to their initial values when the current density decreases to 0.1 C, indicating that both cells exhibit good reversibility. At elevated temperature (55 °C), the rate capacity of the cell with GPE-SN-IM is still reversible. However, discharge capacity of the cell with GPE-LE cannot recover to its initial capacity at 0.1C. It is obvious that that the cell with GPE-SN-IM shows much better cycling stability than that with GPE-LE at high temperature and high current density. The enhanced electrochemical stability of the cell at high temperature could be attributed to the good thermal stability of GPE-SN-IM. Furthermore, to check the compatibility between GPE and electrode, LiCoO2 cathodes in cells with different electrolytes were observed by SEM (Figure 7) after 100 charge-discharge cycles at 55 °C. The surface of fresh cathode is smooth. Only carbon black can be found around LiCoO2 particles. For the cell with GPE-LE, a layer 18


Page 18 of 31

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of impurities appear on LiCoO2 particles after 100 cycles, which should be caused by the decomposition of liquid electrolyte based on EC/DMC.42 The evaporation and decomposition of organic solvent at high temperature can lead to the higher interfacial resistance between electrode and electrolyte and more Ohmic polarization, which will degrade the cycling performance of the cell. No decomposed product was observed on LiCoO2 particles from the cell with the GPE-SN-IM at high temperature, indicating that good thermal stability of the GPE-SN-IM bring the cell with good cycling stability at high temperature.

Figure 6. Charge−discharge profiles of LiCoO2/Li cells assembled with GPE-LE and 19



GPE-SN-IM at a current density of 0.2C at (a) 25 °C and (c) 55 °C. Discharge capacities of LiCoO2/Li cells assembled with GPE-LE and GPE-SN-IM during cycling at a current density of 0.2 C at (b) 25 °C and (d) 55 °C. Rate performance of LiCoO2/Li cells using GPE-LE and GPE-SN-IM operated at (e) 25°C and (f) 55 °C.

Figure 7. SEM images of fresh LiCoO2 cathode (a) and LiCoO2 cathodes from the cell with (b) GPE-LE and (c) GPE-SN-IM after 100 charge -discharge cycles at 55°C. LiCoO2/GPE-SN-IM/Li4Ti5O12 cell (4 cm×4 cm) was laminated between 20


Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


polyethylene terephthalate (PET) films for a flexible film battery, which can light up a red light-emitting diode (LED) light (~2.2V) (Figure 8a). The capacities of LiCoO2 and Li4Ti5O12 are 5.95 mAh and 6.50 mAh, respectively. The obtained laminated lithium ion battery is mechanically robust and can still light up the LED light even after folding for 100 times (Figure 8b). For comparison, GPE-LE and GPE-SN-IN films were fabricated into the film batteries with the same way. All three batteries exhibit good cyclic performance while been charged and discharged at a current density of 1C for 20 cycles (Figure 8c). After 180 degree-folding for 100 times (Figure 8d), the discharge capacity of the battery with GPE-SN-IN sharply decreases to zero. The cycling performance of the battery with GPE-LE deteriorated significantly, too. However, the battery with GPE-SN-IM exhibits very stable cycling performance against mechanical folding. This result is consistent with the mechanical performance of GPE films. Figure 8e and 8f show photographs of GPE-SN-IN, GPE-LE, and GPE-SN-IM films before and after folding. GPE-SN-IN film is easily broken (Figure 8f) after folding which may lead to electrical short of the battery. The folding process also makes the liquid electrolyte leak in GPE-LE film, which causes the capacity decay of the battery. As expected, GPE-SN-IM film with superior mechanical strength can keep structural integrity of the flexible battery against folding damage.

21



Figure 8. (a) A laminated LiCoO2/GPE-SN-IM/ Li4Ti5O12 full battery connected to a LED light. (b) Flexible battery with GPE-SN-IM can still light up a LED light after 180 degree-folding for 100 times. Discharge capacities (at a current density of 1C) of the flexible batteries with three different GPEs (c) before and (d) after 180 degree-folding for 100 times. Photographs of GPE-SN-IN, GPE-LE, and GPE-SN-IM 22


Page 22 of 31

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


films (e) before and (f) after 180 degree-folding for 100 times. Blue rubber substrate is wetted by the liquid electrolyte leaking from GPE-LE film after folding.

4. Conclusion In summary, a mechanically robust gel polymer electrolyte containing succinonitrile with high ionic conductivity (1.63 × 10-3 S· cm-1 at 25 oC) was successfully fabricated by solution immersion method. The melted SN electrolyte with low viscosity can be infiltrated into the cross-linked PUA network at 50 oC to form a GPE-SN. The tensile strength of GPE-SN-IM (6.5 MPa) is much higher than that of GPE-SN-IN (0.5 MPa) prepared by in-situ polymerization and GPE-LE (1.5 MPa). The high mechanical strength of GPE-SN-IM enables the fabricated flexible film battery to withstand 100-cycle folding without degrading the discharge capacity of battery. Due to the good thermal stability of GPE-SN-IM, the cell with GPE-SN-IM shows better cycling stability at 55 oC than that with GPE-LE. Thus, GPE-SN prepared by solution immersion method could be a promising polymer electrolyte for safer lithium ion batteries. ASSOCIATED CONTENT Supporting Information includes AC impedance spectra of GPE-LE and PUA/SN electrolytes with different lithium salt concentrations at 25 °C together with cyclic voltammetry profiles. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Changyu Tang: [email protected] 23



Page 24 of 31

*Yuntao Li: [email protected]

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (51573217).

References 1.

Xue, Z.; He, D.; Xie, X., Poly(ethylene oxide)-based electrolytes for lithium-ion

batteries. J. Mater. Chem. A 2015, 3 (38), 19218-19253. 2.

Manthiram, A.; Yu, X.; Wang, S., Lithium battery chemistries enabled by

solid-state electrolytes. Nat. Rev. Mater 2017, 2 (4), 16103. 3.

B. Puthirath, A.; Patra, S.; Pal, S.; M, M.; Puthirath Balan, A.; S, J.; Tharangattu

N, N., Transparent flexible lithium ion conducting solid polymer electrolyte. J. Mater. Chem. A 2017, 5 (22), 11152-11162.

4.

Prasanth, R.; Shubha, N.; Hng, H. H.; Srinivasan, M., Effect of poly(ethylene

oxide)

on

ionic

conductivity

and

electrochemical

properties

of

poly(vinylidenefluoride) based polymer gel electrolytes prepared by electrospinning for lithium ion batteries. J. Power Sources 2014, 245, 283-291. 5.

Manuel Stephan, A.; Nahm, K. S., Review on composite polymer electrolytes for

lithium batteries. Polymer 2006, 47 (16), 5952-5964. 6.

Long, L.; Wang, S.; Xiao, M.; Meng, Y., Polymer electrolytes for lithium

polymer batteries. J. Mater. Chem. A 2016, 4 (26), 10038-10069. 7.

Schnell, J.; Günther, T.; Knoche, T.; Vieider, C.; Köhler, L.; Just, A.; Keller, M.;

Passerini, S.; Reinhart, G., All-solid-state lithium-ion and lithium metal batteries – 24


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


paving the way to large-scale production. J. Power Sources 2018, 382, 160-175. 8.

Kim, J.-S.; Lee, Y.-H.; Lee, I.; Kim, T.-S.; Ryou, M.-H.; Choi, J. W., Large area

multi-stacked lithium-ion batteries for flexible and rollable applications. J. Mater. Chem. A 2014, 2 (28), 10862-10868.

9.

De, B.; Yadav, A.; Khan, S.; Kar, K. K., A Facile Methodology for the

Development of a Printable and Flexible All-Solid-State Rechargeable Battery. ACS Appl. Mater. Interfaces 2017, 9 (23), 19870-19880.

10. Wang, Y.; Chen, C.; Xie, H.; Gao, T.; Yao, Y.; Pastel, G.; Han, X.; Li, Y.; Zhao, J.; Fu, K. K.; Hu, L., 3D-Printed All-Fiber Li-Ion Battery toward Wearable Energy Storage. Adv. Funct. Mater. 2017, 27 (43), 1703140. 11. Yang, L.; Wang, Z.; Feng, Y.; Tan, R.; Zuo, Y.; Gao, R.; Zhao, Y.; Han, L.; Wang, Z.; Pan, F., Flexible Composite Solid Electrolyte Facilitating Highly Stable “Soft Contacting” Li-Electrolyte Interface for Solid State Lithium-Ion Batteries. Adv. Energy Mater. 2017, 1701437.

12. Nyholm, L.; Nystrom, G.; Mihranyan, A.; Stromme, M., Toward flexible polymer and paper-based energy storage devices. Adv. Mater. 2011, 23 (33), 3751-3769. 13. Kuo, P. L.; Wu, C. A.; Lu, C. Y.; Tsao, C. H.; Hsu, C. H.; Hou, S. S., High performance of transferring lithium ion for polyacrylonitrile-interpenetrating crosslinked polyoxyethylene network as gel polymer electrolyte. ACS Appl. Mater. Interfaces 2014, 6 (5), 3156-3162.

14. Chen, L.; Li, Y.; Li, S.-P.; Fan, L.-Z.; Nan, C.-W.; Goodenough, J. B., PEO/garnet

composite

electrolytes

for

solid-state

25


lithium

batteries:

From


Page 26 of 31

“ceramic-in-polymer” to “polymer-in-ceramic”. Nano Energy 2018, 46, 176-184. 15. Zhu, Y.; Wang, F.; Liu, L.; Xiao, S.; Chang, Z.; Wu, Y., Composite of a nonwoven fabric with poly(vinylidene fluoride) as a gel membrane of high safety for lithium ion battery. Energy Environ. Sci. 2013, 6 (2), 618-624. 16. Nirmale, T. C.; Karbhal, I.; Kalubarme, R. S.; Shelke, M. V.; Varma, A. J.; Kale, B. B., Facile Synthesis of Unique Cellulose Triacetate Based Flexible and High Performance Gel Polymer Electrolyte for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9 (40), 34773-34782.

17. Fasciani, C.; Panero, S.; Hassoun, J.; Scrosati, B., Novel configuration of poly(vinylidenedifluoride)-based

gel

polymer

electrolyte

for

application

in

lithium-ion batteries. J. Power Sources 2015, 294, 180-186. 18. Min-Kyu Song, J.-Y. C., Byung Won Cho, Hee-Woo Rhee, Characterization of UV-cured gel polymer electrolytes for rechargeable lithium batteries. J. Power Sources 2002, 110, 209-215.

19. Zhu, Y.; Xiao, S.; Shi, Y.; Yang, Y.; Wu, Y., A trilayer poly(vinylidene fluoride)/polyborate/poly(vinylidene fluoride) gel polymer electrolyte with good performance for lithium ion batteries. J. Mater. Chem. A 2013, 1 (26), 7790-7097. 20. Fan, L. Z.; Hu, Y. S.; Bhattacharyya, A. J.; Maier, J., Succinonitrile as a Versatile Additive for Polymer Electrolytes. Adv. Funct. Mater. 2007, 17 (15), 2800-2807. 21. Alarco, P. J.; Abu-Lebdeh, Y.; Abouimrane, A.; Armand, M., The plastic-crystalline phase of succinonitrile as a universal matrix for solid-state ionic conductors. Nat. Mater. 2004, 3 (7), 476-481. 26


Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


22. Kim, Y.-S.; Kim, T.-H.; Lee, H.; Song, H.-K., Electronegativity-induced enhancement of thermal stability by succinonitrile as an additive for Li ion batteries. Energy Environ. Sci 2011, 4 (10), 4038-4045.

23. Zhu, X.; Zhao, R.; Deng, W.; Ai, X.; Yang, H.; Cao, Y., An All-solid-state and All-organic Sodium-ion Battery based on Redox-active Polymers and Plastic Crystal Electrolyte. Electrochim. Acta 2015, 178, 55-59. 24. Hu, P.; Chai, J.; Duan, Y.; Liu, Z.; Cui, G.; Chen, L., Progress in nitrile-based polymer electrolytes for high performance lithium batteries. J. Mater. Chem. A 2016, 4 (26), 10070-10083.

25. Das, S.; Prathapa, S. J.; Menezes, P. V.; Row, T. N.; Bhattacharyya, A. J., Study of ion transport in lithium perchlorate-succinonitrile plastic crystalline electrolyte via ionic conductivity and in situ cryo-crystallography. J. Phys. Chem. B 2009, 113 (15), 5025-5031. 26. Chopade, S. A.; Au, J. G.; Li, Z.; Schmidt, P. W.; Hillmyer, M. A.; Lodge, T. P., Robust

Polymer

Electrolyte

Membranes

with

High

Ambient-Temperature

Lithium-Ion Conductivity via Polymerization-Induced Microphase Separation. ACS Appl. Mater. Interfaces 2017, 9 (17), 14561-14565.

27. Ha, H.-J.; Kwon, Y. H.; Kim, J. Y.; Lee, S.-Y., A self-standing, UV-cured polymer networks-reinforced plastic crystal composite electrolyte for a lithium-ion battery. Electrochim. Acta 2011, 57, 40-45. 28. Kim, S.-H.; Choi, K.-H.; Cho, S.-J.; Kil, E.-H.; Lee, S.-Y., Mechanically compliant and lithium dendrite growth-suppressing composite polymer electrolytes 27



Page 28 of 31

for flexible lithium-ion batteries. J. Mater. Chem. A 2013, 1 (16), 4949-4955. 29. Lv, P.; Yang, J.; Liu, G.; Liu, H.; Li, S.; Tang, C.; Mei, J.; Li, Y.; Hui, D., Flexible

solid

electrolyte

based

on

UV

cured

polyurethane

acrylate/succinonitrile-lithium salt composite compatibilized by tetrahydrofuran. Compos Part B 2017, 120, 35-41.

30. Guan, J.; Li, Y.; Li, J., Stretchable Ionic-Liquid-Based Gel Polymer Electrolytes for Lithium-Ion Batteries. Ind. Eng. Chem. Res. 2017, 56 (44), 12456-12463. 31. Barlkani M, H. C., Determination of Crosslink Density by Swelling in the Castable Polyurethane Elastomer Based on 1/4 - Cyclohexane Diisocyanate and para Phenylene Diisocyanate. Iranian J. Polym. Sci. Technol 1992, 1, 1-5. 32. Li, Y.; Shao, H.; Lv, P.; Tang, C.; He, Z.; Zhou, Y.; Shuai, M.; Mei, J.; Lau, W.-M., Fast preparation of mechanically stable superhydrophobic surface by UV cross-linking of coating onto oxygen-inhibited layer of substrate. Chem. Eng. J. 2018, 338, 440-449. 33. Yu, R.; Bao, J.-J.; Chen, T.-T.; Zou, B.-K.; Wen, Z.-Y.; Guo, X.-X.; Chen, C.-H., Solid polymer electrolyte based on thermoplastic polyurethane and its application in all-solid-state lithium ion batteries. Solid State Ionics 2017, 309, 15-21. 34. Zhai, Y.; Xiao, K.; Yu, J.; Ding, B., Fabrication of hierarchical structured SiO2/polyetherimide-polyurethane nanofibrous separators with high performance for lithium ion batteries. Electrochim. Acta 2015, 154, 219-226. 35. Ha, H.-J.; Kil, E.-H.; Kwon, Y. H.; Kim, J. Y.; Lee, C. K.; Lee, S.-Y., UV-curable semi-interpenetrating polymer network-integrated, highly bendable 28


Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


plastic crystal composite electrolytes for shape-conformable all-solid-state lithium ion batteries. Energy Environ. Sci 2012, 5 (4), 6491-6499. 36. He, R.; Echeverri, M.; Ward, D.; Zhu, Y.; Kyu, T., Highly conductive solvent-free polymer electrolyte membrane for lithium-ion batteries: Effect of prepolymer molecular weight. J. Membrane Sci 2016, 498, 208-217. 37. Z Zhou, D.; He, Y.-B.; Liu, R.; Liu, M.; Du, H.; Li, B.; Cai, Q.; Yang, Q.-H.; Kang, F., In Situ Synthesis of a Hierarchical All-Solid-State Electrolyte Based on Nitrile Materials for High-Performance Lithium-Ion Batteries. Adv. Energy Mater.

2015, 5 (15), 1500353. 38. Chen, R.; Liu, F.; Chen, Y.; Ye, Y.; Huang, Y.; Wu, F.; Li, L., An investigation of functionalized electrolyte using succinonitrile additive for high voltage lithium-ion batteries. J. Power Sources 2016, 306, 70-77. 39. Liu, K.; Ding, F.; Liu, J.; Zhang, Q.; Liu, X.; Zhang, J.; Xu, Q., A Cross-Linking Succinonitrile-Based Composite Polymer Electrolyte with Uniformly Dispersed Vinyl-Functionalized SiO2 Particles for Li-Ion Batteries. ACS Appl. Mat. Interfaces

2016, 8 (36), 23668-23675. 40. Armand, J.-M. T. M., Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414 (6861), 359-367.

41. Cha, J.; Han, J.-G.; Hwang, J.; Cho, J.; Choi, N.-S., Mechanisms for electrochemical performance enhancement by the salt-type electrolyte additive, lithium difluoro(oxalato)borate, in high-voltage lithium-ion batteries. J. Power Sources 2017, 357, 97-106. 29



42. Yang, Q.; Zhang, Z.; Sun, X. G.; Hu, Y. S.; Xing, H.; Dai, S., Ionic liquids and derived materials for lithium and sodium batteries. Chem. Soc. Rev. 2018, 47 (6), 2020-2064.

30


Page 30 of 31

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of contents graphic

31


Robust Succinonitrile-Based Gel Polymer Electrolyte for Lithium-Ion

Recommend Documents