A novel PVdF-based composite gel polymer electrolyte doped with ionomer modified graphene oxide

Abstract

In this paper, a novel composite gel polymer electrolyte (CGPE) was prepared by doping ionomer-modified-graphene (IMGO) into poly(vinylidenefluoride) (PVdF) according to a phase separation technique. As to the doped IMGO, the original sp² equipotential network of the GO component can be damaged with the grafted ionomer, while, due to the sheet-like morphology, the prepared IMGO can act as the ion-conducting channel in the final CGPE system. The effect of IMGO on the morphology, mechanical and electrochemical performance of the prepared CGPE were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), a differential scanning calorimeter (DSC) and thermogravimetry (TGA), a mechanical tensile test, AC-impedance, linear sweep voltammetry (LSV), and so on. When the relative content of IMGO was at 10 wt%, the prepared CGPE showed the highest ionic conductivity (3.35 × 10⁻³ S cm⁻¹), and the electrochemical window could be stable up to 4.8 V. The assembled polymer lithium-ion batteries based on it delivered the highest charge–discharge capacity (168.4 mA h g⁻¹) and the capacity retention of the cells (LiCoO₂/CGPE/Li) could be maintained by up to 90% after 50 cycles.

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1 Introduction

Gel polymer electrolytes (GPEs) have many good properties, such as a wide electrochemical window, high ionic conductivity, good compatibility between electrode materials, and good mechanical properties.^1,2 Polymer lithium-ion batteries based on polymer electrolytes have gained an unprecedented importance as power sources for electronic equipment such as cell phone, electric vehicles and other portable devices.^3–5

Among various polymer materials, poly(vinylidenefluoride) (PVdF) has received great attention as a favorable polymer matrix for composite polymer electrolytes with regard to its outstanding properties such as high mechanical strength, good thermal stability, chemical resistance, and high hydrophobicity.^6–9 Furthermore, GPEs based on PVdF are currently being explored as potential candidates in application for lithium-ion batteries. However, crystallographic studies have shown that PVdF based GPE system consists of two crystalline phases, namely, the swollen polymer chains and the electrolyte retained in the cavities of the composite porous polymer.¹⁰ Therefore, it is expected that the interaction between ion carriers and the polymer matrix absorbing liquid electrolyte contributes to ionic conduction activity of GPE. However, the crystalline property of pristine PVdF can inhibit lithium ion diffusion in the GPE system.

In order to prepare high-performance PVdF-based GPE, a lot of efforts had been done. I. S. Eleshmawi et al. prepared LiCl metal halide fillers and studied its application in PVdF-based composite polymer electrolyte with thermal phase separation method.¹¹ Yusong Zhu et al. prepared PVdF-based composite gel polymer electrolyte containing a single-ion conducting polyelectrolyte, lithium polyacrylic acid oxalate borate (LiPAAOB) by electrospinning technique for lithium-ion polymer batteries.¹² However, ionic conductivity of the prepared composite gel polymer electrolyte was very low at ambient temperature. Our group had also made a lot of effects in this area for these years, and the key idea was to synthesize the ionomer and dope them into the PVdF based polymer matrix by various methods. The result showed that the prepared CGPEs possessed high ionic conductivity, wide electrochemical window, and good charge/discharge cycle performance.^13–15

It is generally known that the graphene is a two-dimensional (2-D) and one-atom-thick planar nanomaterial consisting of sp² hybridized carbon atoms combined in a honeycomb crystal lattice and constitutes the basic material of 0D carbon nanotubes and 3D graphite.¹⁶ Because of its unique structure and outstanding properties, such as electrical, mechanical and thermal conductivity properties, it has drawn the attention of scientists in recent years.^17–21 As graphene oxide (GO) exhibits many physical properties similar to that of graphene, in addition, its preparation method is moderately easy, they are being considered to be applied in electronic, sensor, and catalytic areas.^22–25 Especially, GO also had been studied to be applied into CGPEs. As GO contains various oxygen functional groups, it can increase ionic conductivity because the dissociation of the lithium salt can be increased by the interactions between the functional groups of GO and lithium ions.²⁶ In addition, the modified GOs exhibit insulating properties because sp² equipotential networks are disrupted by the grafted groups.^27,28

In this work, a novel ionomer composite dopant, which named as IMGO, was synthesized by combining GO and ionomer with covalent bond. Then, PVdF-based CGPEs doped with IMGO were prepared by thermal phase inversion method and activated by being immersed in to liquid electrolyte at room temperature. The morphology, physical and electrochemical properties of CGPEs affected by the doped IMGO were tested and discussed in detail.

2 Experiment

2.1 Materials

Graphite powders (Aladdin) was employed to provide graphite oxide. Concentrated sulfuric acid (H₂SO₄), hydrochloric acid (HCl), thionylchloride (SOCl₂), glycerin (A.R.) and N,N-dimethylformamide (DMF) (A.R.) were purchased from Sinopharm Chemical. Poly(vinylidene fluoride) (PVdF, M_n, ca. 6 × 10⁵, Shanghai Ofluorine Chemical) was used as polymer matrix. Polyacrylic acid (PAA) (Aladdin, M_w; 50 000), boric acid (H₃BO₃) (Sinopharm), lithium hydroxide (LiOH) (Sinopharm) and oxalic acid (H₂C₂O₄) (Sinopharm) were used as received. Organic liquid electrolyte (EC/PC/DMC 1 : 1 : 1 (w/w/w) LiClO₄ 1 mol l⁻¹) was purchased from Guotai-Huarong Chemical.

2.2 Chlorination of graphite oxide

Graphite oxide (GO) was prepared from flake graphite powders based on modified Hummers method as described in the other literatures.^29,30 The prepared GOs was rinsed with dilute HCl and deionized water alternately for several times to remove the redundant ions, then, a homogeneous GO dispersed solution was obtained. After being filtrated and dried under vacuum at 60 °C overnight, the obtained dried GO was scattered in mixed solvents (DMF : thionylchloride = 1 : 10, by volume) for 24 h at 70 °C under nitrogen atmosphere.³¹ Finally, the solution was poured onto a glass dish and product was dried under normal pressure at 60 °C for 24 h to obtain GO-Cl.

2.3 Modifying GO with ionomer

IMGO was manufactured according to Scheme 1. Firstly, boric acid and PAA were dissolved into the deionized water at 1 : 2 (by mole rate), then the mixture were stirred and heated at 80 °C for 6 h to obtain a homogeneous and transparent solution 1. Then the solution was cast on a flat glass plate and heated at 80 °C to remove the solvent and get the dried PAAOB. Secondly, PAAOB and GO-Cl were dissolved in DMF at mass rate of 20 : 1, the mixture were kept string for 6 h at 70 °C. After that, LiOH and H₂C₂O₄ (LiOH : H₂C₂O₄ : PAAOB = 0.9 : 0.9 : 1 (by mol rate)) were added and refluxed for another 12 h. Finally, the cooled reaction solution was poured into PTFE centrifuge tube, the product IMGO was dried under vacuum for 24 h at 60 °C after being separated with fast speed centrifugation.

Scheme 1 The process of preparation IMGO.

2.4 Preparation of PVdF-based composite gel polymer electrolyte

PVdF based composite gel polymer electrolytes were prepared by thermal phase separation method. At first, a suitable amount of PVdF and IMGO were dissolved in mixed solvents (DMF : glycerin = 8 : 1 by volume) and stirred for 6 h at room temperature. The polymer solution was then cast on a flat glass plate with a drawknife and dried at 80 °C under vacuum for 24 h to remove the solvent. The dried composite polymer membranes were then transferred into the glove box and immersed in the organic liquid electrolyte (1 M LiClO₄-EC/PC/DMC) for 1 h. After being activated, the gelled composite polymer electrolytes were removed from liquid electrolyte and excess electrolyte solution on the surface was wiped with a filter paper. For convenience, the dried composite polymer membranes and obtained composite gel polymer electrolytes were simply marked as CPM-IMGO-X and CGPE-IMGO-X, respectively, where X are 5, 10, and 20 based on the mass ratio of IMGO to PVdF polymers.

2.5 Structure and thermodynamics test

FT-IR measurement was carried out on BRUKER VECTOR-22 spectrometer. The morphology of the prepared GO was observed by transmission electron microscope (TEM) on a LIBRA 120 with an acceleration voltage of 120 V. The surface morphologies of the dried CPMs obtained by thermal phase separation method were observed by scanning electron microscope (SEM) (XL30-ESEM, PHILIPS). X-ray diffraction analysis of the dried CPMs were performed by using Bruker D8 with 2θ values between 10° and 60°. Thermogravimetry analysis (TGA) was performed in the temperature range of 35–700 °C at heating rate of 10 °C min⁻¹ by using Perkin Elmer Thermogravimetric Analyzer (TGA7). DSC measurement was carried out by using DSC Q100 (TA Instrument) (TA Instruments, USA) in the temperature range of 25–200 °C at a scan rate of 10 °C min⁻¹. The crystallinity (χ_c) was obtained based on the following eqn (1):where ΔH_f and represent the fusion enthalpy of the dried CPM and pure PVdF with 100% crystallinity, respectively.

Electrolyte uptake values for different CPMs were calculated by eqn (2) after the CPMs were immersed into liquid electrolyte for 1 h in the glove box.

where W₀ and W are weight of dry and wet CPMs, respectively.

The porosity of the CPMs were measured by immersing the dried CPMs in n-butanol for 2 h, and calculated by eqn (3):

where, ρ_a and ρ_b are the density of n-butanol and the composite membranes, respectively. m_a and m_b are the mass of the membrane after immersion in n-butanol and the dried membrane, respectively.

Tensile properties of different CPMs were measured by a 10 kN electromechanical tensile testing machine (CMT5104, China) at room temperature.

2.6 Electrochemical measurement

The ionic conductivities (σ) of the CGPE-IMGO-X were measured by AC impedance spectroscopy (EG&G Model 273A potentiostat). The electrochemical batteries of SS/CGPE-IMGO-X/SS were fabricated by sandwiching CGPE-IMGO-X between the two stainless steel electrodes. Impedance data were obtained in the frequency range of 100 kHz to 100 mHz between 25–85 °C. Ionic conductivity (σ) was then calculated from bulk resistance (R_b) obtained from the fitting of the Nyquist plot at high frequency region, the polymer electrolytes thickness (l), and the stainless electrode area (A) according to the eqn (4).

σ = 1/AR_b (4)

The lithium-ion transference number (t_Li⁺) was measured by using electrochemical batter of Li/CGPE-IMGO-X/Li according to DC polarization combined with AC impedance method as described by Bruce and Vincent.³² It was calculated by eqn (5):

where I₀ and I_s are the initial and steady current, respectively. R₀ and R_s are the initial interfacial impedance and steady-state resistance, respectively. ΔV is the applied DC voltage.

The electrochemical stability was characterized by linear sweep voltammetry (LSV) measurement on a PARST2273 electrochemical workstation at a scanning rate of 5 mV s⁻¹ form the voltage of 3–6 V by using a stainless steel and lithium foil as the counter (Li/CGPE-IMGO-X/SS).

The charge–discharge performances of LiCoO₂/CGPE-IMGO-X/Li 2016-type coin cell were carried out by using Land battery test system (Wuhan Land electronic Co. Ltd. China). LiCoO₂ working electrode was prepared by coating a N-methyl-2-pyrrolidone (NMP)-based slurry containing LiCoO₂ (DLG Battery Co. Ltd., China), carbon black, and poly(vinylidene fluoride) (PVdF) in a weight ratio of 8 : 1 : 1.³³ The slurry was on aluminum foil substrates at 120 °C for vacuum to remove the solvent. Then the polymer lithium-ion battery was assembled by sandwiching CGPE between LiCoO₂ cathode and lithium anode in the argon-filled box. Charge–discharge testing was measured at a constant current density of 0.1C rate between the voltage range of 2.5–4.25 V at room temperature.

3 Results and discussion

Fig. 1S† presented FTIR spectra of GO-Cl, ionomer (LiPAAOB) and IMGO, respectively. For GO-Cl, the absorption peaks at 3420 cm⁻¹ and 676 cm⁻¹ are assigned to stretching vibration of –OH and C–Cl, respectively. The characteristic absorption peaks at 3469 cm⁻¹, 1394 cm⁻¹, 1051 cm⁻¹ and 626 cm⁻¹ for PAAOB are attributed to stretching vibration of B–OH, anti-symmetric stretching of B–O, vibration of B–O–C and vibration of B–O, respectively.¹ From line c, the characteristic absorption peaks at 3100 cm⁻¹ (V_C–OH), 1394 cm⁻¹ (V_B–O), 1051 cm⁻¹ (V_B–O–C), and 626 cm⁻¹ (V_B–O) are observed, which indicates LiPAAOB has been grafted onto GO-Cl, and IMGO has been obtained.

Fig. 1 displays the transmission electron microscopy (TEM) image of GO-Cl sheets used in this work.

Fig. 1 TEM image of GO-Cl.

Two-dimensional (2D) structure is formed with a single-layer sheet of the prepared GO-Cl.

The macroscopic appearance and SEM images of the dried CPMs varied with the content of IMGO are presented in Fig. 2. From these images, the surface of these membranes displays a uniform and dense porous morphology. No apparent aggregation of IMGO in all the membranes indicates good compatibility between the doped IMGO and PVdF polymer matrix. The micropores size of the different CPMs increases with the content IMGO. Lamellar morphology of IMGO and polarity difference between the two components may lead to this phenomenon. Larger size of microspores in the CPMs lead to more efficient liquid electrolyte uptake and gelation when the CPMs are soaked in the organic liquid electrolyte.³⁴ However, the excessively loose structure may deteriorate mechanical property of the prepared CPMs.

Fig. 2 The macroscopic and SEM images of the dried CPMs varied with the content of IMPG (a) CPM-IMGO-5, (b) CPM-IMGO-10, (c) CPM-IMGO-20.

The change in the degree of crystallinity of the CPMs varied with the content of doped IMGO was examined by DSC measurement. The results are summarized in Table 1, and all these CPMs show a glass transition temperature (T_g) at around 127 °C which attributes to the segmental motion of the ionomer from IMGO. In addition, melting enthalpies (ΔH_f) of CPMs decrease with the increased content of IMGO. When the content of IMGO is at 10 wt%, the crystallinity of CPMs is 39.55%, which show the lowest degree of crystallinity. The evenly distributed IMGO may damage the structured arrangement of PVdF polymer chains. Amorphous area of the polymer phase benefits to the adsorption of the liquid electrolyte and the movement of ions, which provides good condition for higher ionic conductivity.³⁵

Table 1 Thermodynamic properties of the dried composite polymer membranes varied with the content of IMGO^a

Sample	Tg/°C	Tm/°C	ΔHm/J g^−1	χ/%
a χ = ΔHm/ΔHm0(PVdF).
CPM-IMGO-5	127.1	163.6	40.03	38.22
CPM-IMGO-10	127	162	38.40	36.67
CPM-IMGO-20	127	163	41.42	39.55

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PVdF can crystallize in four polymorphs (α, β, γ and δ) and each crystal structure have different polymorphs.³⁶ The crystalline phases of pure PVdF membrane and the composite polymer membranes various with the contents of IMGO were investigated by means of X-ray diffraction (XRD) (Fig. 3). XRD patterns can be interpreted by considering the criterion of a correlation between the height of the peak and the degree of crystallinity established by Hodge et al.³⁷ A strong diffraction peak for all samples is observed at 2θ = 20.4° corresponding to β(110) and β(200) planes, which exhibits a semicrystalline structure for the prepared CPMs. Obviously, the crystallinity of PVdF decreased with the content of IMGO, the ordinary arrangement of polymer chains may be disturbed by uniformly distribution of IMGO. In addition, IMGO shows the peaks at 14.5°, 19.6°, 28.1°, 30.8° and 36.7, respectively, which are consistent with the previous results.¹ Interestingly, the crystalline behavior of IMGO may not affected by the surrounding polymer chains when it is doped. The peak at 36.7° which attributes to IMGO don't disappear in all the CPMs, on the contrary, its intensity increases with the doped IMGO. So, the doped IMGO doesn't dissolve into the surrounding polymer chains, and also, it doesn't aggregate in the system. Combine with the results of SEM and DSC, they may distributed in the polymer membranes evenly and form the so called “ion-conducting channel” to promote ion conduction in the system.

Fig. 3 XRD patterns of the IMGO, pure PVDF membrane and the dried CPMs varied with the content of IMGO.

The doped IMGO may improve thermo-tolerance of the composite polymer membranes effectively. TGA curve of pure PVdF membrane and the composite polymer membranes varied with the contents of IMGO are presented in Fig. 3S.† For pure PVdF polymer membrane, there is only one weight loss platform starting from 420 °C, which is high enough to be applied in the lithium-ion battery. When IMGO is doped into the composite polymer membrane, there are a little change in thermogravimetric behavior. When the testing temperature is at 200 °C, the grafted ionomer of IMGO begins to decompose. However, the doped GO component may benefit to the thermal stability of the prepared composite polymer membranes. As is seen in Fig. 3S,† the main weight loss platform of PVdF component increases over 30 °C. Due to GO component has good heat resistance, the interaction between PVdF polymer chains and IMGO component may be favorable for the improvement of thermal stability of the prepared composite polymer membranes when IMGO is dispersed into PVdF component at nanoscale.

Tensile properties of different CPMs were measured by electromechanical tensile testing machine. Stress–strain curves for different dried CPMs varied with the content of IMGO are present in Fig. 4S,† and their mechanical properties are summarized in Table 2. The elongation at break of the different CPMs increases with the content of IMGO. Due to being homodispersed in the CPM at nanoscale, the rigid lamellar morphology of IMGO may reinforce the mechanical strength of the prepared CPMs. In addition, the hydrogen bonding and strong polar forces between PVdF and the doped IMGO may also be the reason for the increased mechanical properties of the prepared composite polymer membranes. For CPM-IMGO-20, the maxima of stress and elongation at break are 7.82 Mpa and 24.67%, respectively.

Table 2 Mechanical properties of composite polymer membranes with different content of IMGO

Samples	Stress/Mpa	Strain/%
CPM-IMGO-5	6.01	17.02
CPM-IMGO-10	6.72	17.65
CPM-IMGO-20	7.82	24.67

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Table 3 lists the values of the porosity and electrolyte uptake of the prepared composite polymer membrane at room temperature. For pure PVdF based CPM, the porosity and liquid electrolyte uptake are only 31.21% and 175.67%, respectively. According to the result of SEM, the pore size of the CPMs increase with the content of IMGO. Organic liquid electrolyte can be absorbed into the CPMs easily, then, the absorbed liquid electrolyte will be gelated due to similar polarity between the absorbed liquid electrolyte and ionomer. When the content of IMGO is 20%, the porosity and the electrolyte uptake of CPMs at room temperature can reach 50.43% and 260.67%, respectively.

Table 3 Porosity and liquid electrolyte uptake of the pure PVdF membranes and the composite polymer membranes

Sample	Porosity (%)	Liquid electrolyte uptake (%)
Pure PVDF	31.21	175.67
CPM-IMGO-5	41.56	210.24
CPM-IMGO-10	48.55	240.45
CPM-IMGO-20	50.43	260.67

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Ionic conductivities of pure PVdF based gel polymer electrolyte (GPE) and the composite gel polymer electrolytes (CGPE-IMGO-X) were measured by AC impedance measurement. Fig. 4a shows the typical Nyquist plot for all the samples. At the range of high frequency (close to 100 kHz), the corresponding values of the intercept on the real axis (X axis) represent the intrinsic resistances of CPGEs as the ohmic resistance of the testing device can be negligible.³⁸ Based on the data obtained from AC impedance measurement, the bulk resistance (R_b) of CGPE (or PVdF based GPE) can be observed from the enlarged drawing in Fig. 4a. The ionic conductivities at room temperature (25 °C) are 1.41 × 10⁻³ S cm⁻¹ (pure PVDF based GPE), 2.54 × 10⁻³ S cm⁻¹ (CGPE-IMGO-5), 3.53 × 10⁻³ S cm⁻¹ (CGPE-IMGO-10) and 2.01 × 10⁻³ S cm⁻¹ (CGPE-IMGO-20), respectively.

Fig. 4 (a) AC impedance spectra of pure PVDF-based GPE and different CGPE varied with the content of IMGO at room temperature. (b) Dependence of conductivity on the reciprocal of temperature for pure PVDF-based GPE and CGPE varied with the content of IMGO.

Interestingly, a small amount of IMGO leads to a considerable increase in ionic conductivity of CGPE, which would be beneficial to obtain charge–discharge performance of the lithium-ion battery. Firstly, the grafted ionomer can destruct the sp² equipotential of GO to inhibit its electron transfer, which will exclude operational problems such as short-circuit and self-discharge of the lithium-ion polymer battery.³⁹ Secondly, more liquid electrolyte can be absorbed into the IMGO due to good compatibility between liquid electrolyte and the grafted ionomer. Besides the grafted ionomer, COOH and OH groups of the GO can also interact with lithium salt by Lewis acid–base interactions, thereby further promoting the dissociation of lithium salt into free ions. The combination of salt dissociation from the grafted ionomer and ion mobility enhancements due to lamellar morphology of IMGO appear to be highly effective to form the “ion-conducting channel” in the CGPE system, which results in easier movement of conductive ions in the system. In the electrical field, the free conductive lithium-ions in the CGPEs could move in the absorbed liquid electrolyte. In addition, by complexation effect and unlocking the complexation effect, the conductive ions would move due to the movement of polymer chains. It should be highlight that with the grafted ionomer, the doped IMGO can also promote the lithium-ion transmission.⁴⁰ The ionic conductivity of CGPE reaches the maximum when the content of IMGO is at 10 wt%. However, with the content of IMGO increasing, excessive stacked IMGO may restrain the segmental movement of polymer chains in the amorphous region due to its rigid structure, which will make the ionic conductivity of the CGPE decrease.

Temperature dependence of ionic conductivity of different CGPEs in the range from 25 °C to 85 °C is presented Fig. 4b. The appeared linear plot of CGPEs suggests that their ion conduction behavior obeys Arrhenius equation:

σ = σ₀ exp(−E_a/RT)

where σ₀ is the pre-exponential index, E_a is the activation energy, and T is the testing absolute temperature. The calculated activated energy E_a of pure PVDF-based GPE, CGPE-IMGO-5, CGPE-IMGO-10 and CGPE-IMGO-20 are 15.1, 12.5, 10.2 and 13.5 kJ mol⁻¹, respectively. The polymer chains can obtain more energy at higher temperature, which will promote the movement of conducting ions, so the ionic conductivity of the testing CGPEs will increase with the temperature.

Lithium ion transference number (t_Li⁺) is also a significant argument to estimate the performance of polymer electrolytes. In this paper, the value of t_Li⁺ for the prepared CGPEs was measured by AC impedance spectra combining with DC polarization test.⁴¹ As is shown in Fig. 5, lithium ion transference numbers (t_Li⁺) of different CGPEs are 0.41 (CGPE-IMGO-5), 0.52 (CGPE-IMGO-10) and 0.42 (CGPE-IMGO-20), respectively. As to pure PVDF-based GPE, the value of t_Li⁺ is only 0.18. Without question, the value is increased by the doped IMGO. As is discussed above, more liquid electrolyte can be absorbed into the IMGO and more cationic dissociation from the grafted ionomer of IMGO lead to the easier ion mobility due to fast ion conduction channel formed by lamellar IMGO, in addition, COOH and OH groups of the GO can also interact with lithium salt by Lewis acid–base interactions, thereby further promoting the dissociation of lithium salt into free ions. When the content of doped IMGO reached 20% (CGPE-IMGO-20), the excessive stacked IMGO may restrain the segmental movement of polymer chains in the amorphous region due to its rigid structure. As the transmission of cations is related to the polymer chain, so the t_Li⁺ value will decrease with more content of IMGO.

Fig. 5 DC polarization plot of (a) pure PVDF-based GPE and CGPEs, (b) CGPE-IMGO-5, (c) CGPE-IMGO-10, (d) CGPE-IMGO-20, the inset are Nyquist plots before and after polarization.

Electrochemical stabilities of CGPEs, which have been measured by linear sweep voltammetry, are presented in Fig. 6. The current in anodic voltage area results from the decomposition process associated with the CGPEs, and the onset voltage relates to the increase of the current can be considered as the upper limit of CGPE.⁴² The onset voltage relates to the increased current of pure PVdF based GPE is at around 4.25 V, while for the prepared CGPEs doped with IMGO show good stability up to 4.7 V. Electrochemical inertia of GO component combining with the effective adsorption of the liquid electrolyte by the grafted ionomer may be the two main reasons for the increased electrochemical stabilities of the prepared CGPEs.

Fig. 6 Electrochemical stability windows of pure PVDF-based GPE and CGPEs varied with the content of IMGO.

Fig. 7a and b show the interfacial stability images of CGPEs (or pure PVdF based GPE) with lithium metal electrode. Fig. 7a shows the initial Nyquist plots of Li/CGPEs (or pure PVDF based GPE)/Li symmetric cells at room temperature, and Fig. 7b illustrates the variation of interfacial resistance with operation time. The impedance spectra show a distorted semicircle at medium frequency region. The interfacial resistance is composed of bulk electrolyte resistance (R_b) at high frequency region and interfacial resistance (R_i) at low frequency regions.¹⁴ In order to study the mechanism of interfacial interaction between CGPEs and lithium electrode, equivalent circuits of pure PVdF-based GPE and CGPEs (circuit cheater 1) are simulated in Fig. 7b. R_cell includes the bulk resistance (R_b), charge transfer resistance in the electronic double-layer (R_dl), passivation film resistance (R_sei1) and interfacial reaction resistance (R_sei2). Among them, R_cell value reaches the maximum at six day, and then trends to be a stable value. The main reason for this phenomenon is that the growth of passivation layer on the surface of lithium electrode, which is associated with the decomposition of electrolyte or the reaction between CGPE and lithium metal electrode.⁴³ Circuit cheater 2 is the equivalent circuits of pure PVdF-based GPE and CGPEs at the six days. As is shown in Fig. 7b, the initial R_cell values are 239 Ω, 208 Ω, 181 Ω and 165 Ω for pure PVdF based GPE, CGPE-IMGO-5, CGPE-IMGO-20 and CGPE-IMGO-10, respectively. After 20 days, the value of R_cell increases according to the order CGPE-IMGO-10 (174 Ω) < CGPE-IMGO-20 (196 Ω) < CGPE-IMGO-5 (225 Ω) < pure PVdF based (266 Ω), which is about 5–11% higher than the initial interfacial resistance. As is discussed above, electrochemical inertia of GO component may inhibit the reaction between CGPEs and lithium metal electrode. When the content of IMGO exceed 10%, the stacked excessive IMGO may restrain the ion conduction activity, which will make the value of R_cell increase during the storage time.

Fig. 7 (a) AC impedance behaviors of Li/CGPEs/Li cells: pure PVDF based GPE, CGPE-IMGO-5, CGPE-IMGO-10 and CGPE-IMGO-20 (frequency range from 100 kHz to 10 mHz, amplitude 5 mV); (b) interfacial resistance with storage time of varied with the content of IMGO.

Charge–discharge profiles of polymer lithium-ion battery based on pure PVdF-based GPE and CGPE are shown in Fig. 8. The assembled batteries (LiCoO₂/GPE or CGPE/Li) are charged–discharged at 0.1C rate between 2.5 V and 4.25 V. The initial charge–discharge curves of lithium-ion polymer cell assembled with pure PVdF based GPE and CGPEs are shown in Fig. 8a. The battery deliver discharge capacities at 0.1C are about 161.4 mA h g⁻¹ (pure PVdF based GPE), 165.7 mA h g⁻¹ (CGPE-IMGO-5), 168.4 mA h g⁻¹ (CGPE-IMGO-10), 164.2 mA h g⁻¹ (CGPE-IMGO-20), respectively. Attractively, the initial discharge capacity is increased with the doped IMGO, which may be caused by more conducting ion released by IMGO and the effective ion-conducting channel formed by the random distributed IMGO. The cycling performance and columbic efficiency of the assembled cells are presented in Fig. 8b. It is observed that the capacity retention of the batteries (LiCoO₂/CGPE-IMGO-X/Li) will keep up to 90% after 50 cycles, which is higher than the cells assembled with pure PVdF-based GPE (74%) at 0.1C. Higher ionic conductivity and more stable of interfacial between CGPEs and lithium electrode with the doped IMGO will enhance cycling performance of the assembled polymer lithium-ion battery.

Fig. 8 (a) Initial charge–discharge curves of lithium-ion polymer cell (LiCoO₂/CPGE/Li) assembled with pure PVDF-based GPE and CGPEs varied with the content of IMGO. (b) Cycle stability and columbic efficiency curves of Li/CGPEs/LiCoO₂ cell.

4 Conclusions

In this paper, a novel high-performance of composite gel polymer electrolytes (CGPEs) were prepared by doping the ionomer modified GO (IMGO) into PVdF based gel polymer electrolyte. As the grafted ionomer can destruct the sp² equipotential of GO to inhibit its electron transfer, which will exclude operational problems such as formation of short-circuit and self-discharge. For the prepared CGPEs, more conduction ion are released from the grafted ionomer and the lithium salt due to the Lewis acid–base interactions between GO and them, which will benefit to ionic conduction activity. In addition, a high-effective ion conductive channel can form in the prepared CGPE with lamellar morphology of doped IMGO. The prepared CGPEs present better mechanical properties, thermotolerance, and electrochemical stability. When the content of IMGO is at 10%, the ionic conductivity of the prepared CGPE-IMGO-10 reached 3.82 × 10⁻³ S cm⁻¹ at room temperature and its electrochemical stability window was about 4.7 V. The assembled polymer lithium-ion battery based on it presents good interfacial stability and charge–discharge performance. With these good performance, the prepared CGPEs are very suitable for being applied into polymer lithium-ion battery.

Acknowledgements

This work is funded by the National Natural Science Foundation of China (Grant No. 51673088). State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, NSFC (No. 21201087), and Postdoctoral Foundation of Jiangsu Province (No. 1501091c).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16618k

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