- Email: [email protected]
Preparation and characterization of nanocomposite ionic liquid-based gel polymer electrolyte for safe applications in solid-state lithium battery
Solid State Ionics 321 (2018) 48–54
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Preparation and characterization of nanocomposite ionic liquid-based gel polymer electrolyte for safe applications in solid-state lithium battery ⁎
T
⁎
Qingpeng Guo , Yu Han , Hui Wang, Shizhao Xiong, Shuangke Liu, Chunman Zheng, Kai Xie College of Aerospace Science and Engineering, National University of Defence Technology, Changsha, Hunan 410073, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ionic liquid-based gel polymer electrolyte PVDF-HFP Safety Compatibility Solid-state lithium battery
Gel electrolyte is one of the most direct and appropriate ways to solve the safety problem of battery before the most challenging technological hurdles to all solid-state batteries have not yet been overcame. A major challenge towards gel polymer electrolyte (GPE) is making an electrolyte that is effectively combine with electrochemical performance, thermal safety and the stability to lithium. Here, PVDF-HFP-LiTFSI/SiO2/EMITFSI polymer electrolyte composite membrane (ILGPE) is prepared via solution casting method, in which nano-silica and ionic liquid positively effect on the performance of polymer electrolytes, such as ionic conductivity, electrochemical stability and thermal safety stability. It remarkably enhances the interface compatibility between ILGPEs and lithium metal. Significantly, we characteristically elucidate the possible transmission mechanism and interaction within the materials of ILGPE. Furthermore, Li/LiFePO4 battery based on such ILGPEs can exhibit fascinating interfacial stability and cycling performance. Herein, cells based on ILGPEs can overcome the drawbacks of solid electrolytes and volatile organic liquid electrolytes, which suggest a promising method for highly secure lithium batteries with appreciably enhanced performance.
1. Introduction Polymer electrolytes (PEs) with excellent properties such as light weight, safety, easy processing and other advantages have been proposed for implementation in popular power sources [1–6]. However, solid polymer electrolytes (SPEs) can hardly apply in large-scale commercial electronic equipment effectively due to the low ion conductivity and other defects [7]. Fortunately, the polymer-gel electrolytes (PGEs) are formed by introducing significant amount of organic solvents in SPEs, which can offer advantages in terms of high ionic conductivity and good interfacial compatibility with electrodes [8]. Compared with recent organic solvents, ionic liquids (ILs) is constituted by ions with unique properties such as high ionic conductivity, nonvolatility, high thermal stability and wide electrochemical windows [9,10]. Therefore, the replacement of volatile organic solutions with ILs entrapped in a polymer host forming ionic liquid based gel polymer electrolytes (ILGPEs), which combine with the advantages of IL and polymer electrolytes are regarded as promising electrolytes [9,11]. Poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly (ethylene oxide) (PEO), poly(vinylidenedifluoride) (PVDF) and its copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are the most common polymer host used in the electrolyte matrix [12–16]. Among them, the PVDF-HFP matrix has excellent
⁎
electrochemically stable owing to the strong electron-withdrawing group-CF [17–26]. Meanwhile, the higher dielectric constant (ε = 8.4) of this polymer will help to dissociate the lithium salt and increase the carrier concentration [27,28]. Thus, PVDF-HFP is widely regarded as a preferred matrix for gel polymer electrolytes. In this paper, a family of ILGPEs based on LiTFSI and SiO2 nanoparticles dispersed in PVDF-HFP is proposed. Meanwhile, the polymer molecules are swelled by IL, the ions can therefore transport in the space provided by the free volume around the polymer host, which means ILGPE possesses the advantages of both IL and solid polymer electrolytes, including high ionic conductivity, stable electrochemical window, especially, good thermal stability and non-volatile. Hereafter, the interplay between properties and structures is investigated, and the feasibility of solid-state lithium battery using this type electrolyte is characterized as well. All of these results suggest that the ILGPE has great promise for safe applications in solid-state lithium battery. 2. Experimental 2.1. Preparation of ILGPE The ILGPE membranes were prepared using solution-casting technique. PVDF-HFP was completely dissolved in butanone (which was
Corresponding authors. E-mail addresses: [email protected] (Q. Guo), [email protected] (Y. Han).
https://doi.org/10.1016/j.ssi.2018.03.032 Received 31 January 2018; Received in revised form 18 March 2018; Accepted 29 March 2018 0167-2738/ © 2018 Published by Elsevier B.V.
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
diffraction peaks polymer matrix to a lesser degree. It is illustrated the inferior miscibility of LiTFSI with the semi crystalline PVDF-HFP. Furthermore, a dramatic decrease on peaks intensity especially the diffraction peaks at 2θ = 20 and 40° can be observed with the increase of IL. These findings indicate that IL can effectively reduce the degree of crystallinity of PVDF-HFP through fully swelling the molecular chain and making molecular chains almost in the amorphous state. Thus, we can speculate that the ionic conduction of polymer electrolytes should be enhanced with the amorphous areas of polymer obviously increased. The thermal stability of the electrolytes is studied by TGA as shown in Fig. 2c. As noticed, the decomposition of the polymer matrix PVDFHFP (LiTFSI) starts at the lower onset temperature, and the lost weight is 80% when the temperature rises to 500 °C. The TGA curves of ILEMITFSI show that the mass loss begins at about 352.7 °C. Therefore, this result indicates incorporation of IL could increase the thermal stability of polymer electrolytes. The degradation processes of ILGPE can be divided into two-step. A small amount of weight loss is observed in the range from 75 to 300 °C, corresponding to the decomposition of the lithium salt and the polymer PVDF-HFP. The decomposition of mass loss in the temperature range of 300–460 °C is the second step, which is more noticeable and related to the decomposition of EMITFSI. It is worth noting that the thermal stability of ILGPE is greatly improved. Meanwhile, the effect by the content of ILs on the thermal stability of ILGPEs is compared, which further proved that IL played an important role in safe electrolytes. At the same time, we further judge whether the gel electrolyte is flammable or not from another index-the limiting oxygen index (LOI), which is an useful method to judge the relative flammability through evaluating minimum oxygen concentration [29]. Generally, the value of LOI is higher than 27 indicating that the material is flame retardant [30]. The LOI values of 1#–8# are shown in the Fig. 2d, 1# (Celgard membrane) and 2# (polymer electrolyte is not contains IL) exhibit combustible (LOI = 22) and flame retardant (LOI = 39) respectively. However, ILGPEs with different contents of IL show a higher LOI value, which can reach above 56, exhibiting absolutely flame retardant. In contrast, the calorific values of ILGPEs reflect a lower value compared to the samples of 1# and 2#, which reflect good thermal stability and non-flammability. As we know, the electrolyte membrane shrinkage problem also represents a safety hazard especially in high temperature conditions, which leads to a large area of the battery short circuit. In order to verify the above phenomenon sufficiently, the ability of suppressing thermal shrinkage experiments of membranes are carried out (Fig. 2e). The Celgard membrane is happened crimping phenomenon under the sustained heating, owing to the structure of polymer matrix changes in the high temperature. However, there is no crimping phenomenon of the ILGPEs with different content of IL under continuous heating process, indicating that ILGPE has significant advantages in terms of heat resistance.
stirred at 50 °C for 30 min) to obtain 6.7 wt% of polymer clear solution. The lithium bis(trifluoromethane)sulfonamide (LiTFSI, 99.9%, Aldrich), silicon dioxide (SiO2) and 1-ethyl-3-methylimidazolium triluoromethanesufonate (EMITFSI) with different weight ratio (the values of mPVDF-HFP:mLiTFSI:mEMITFSI:mSiO2 changed from 5:5:7:0.5 to 5:5:7:1.75) were added into the clear solution under argon atmosphere. The resulting mixture was magnetic stirring for 3 h and ultrasonically agitated for 20 min to from a homogeneous casting solution in a sealed container. Then, the solution was casted onto a Teflon plate to from the wet membrane and dried for 12 h at 60 °C in vacuum condition. Finally, the free standing ILGPE membrane was cut into circles with diameters of 19 mm. 2.2. Characterization of the ILGPE membrane The surface microstructures of the ILGPEs were observed by HITACHI S-4800 scan electron microscopy (SEM). The crystal structure of the membranes was obtained using X-ray diffraction (XRD) with Cu Kɑ radiation in the range of 10–60°. The Fourier transform infrared (FTIR) spectra were acquired on a Bruke V70 spectrometer. Raman spectra was recorded on a Bruker V70-FRA106 Raman module. The thermal properties of the electrolytes evaluated by Differential Scanning Calorimetric (DSC) and Thermogravimetric analysis (TGA) were carried out at a scan rate of 10 °C min−1 under N2 atmosphere. The ionic conductivity of the ILGPEs was measured by the AC impedance method using symmetrical stainless steel electrodes at different temperature range from 0 to 100 °C. The electrochemical stability of the ILGPEs was measured by linear sweep voltammetry (LSV) and cyclic voltammograms (CV) using the cell structure of Li/ILGPE/SS at the scanning rate of 0.1 mV s−1. They were measured from 2.5 V to 6 V (LSV, vs. Li+/Li) and between −0.5 V to 2.5 V (CV, vs. Li+/Li) respectively. The LiFePO4 electrode was prepared by the mixed slurries coated on Al foils, the slurries contain LiFePO4 powders, carbon black and PVDFHFP which was contained in ingredient of ILGPE-20%SiO2 at 8:1:1 weight ratio in N-methylpyrrolidone (NMP) solvent. The loading density of LiFePO4 cathode was controlled to be 1.5 mg cm−2. The composite cathode was dried in a vacuum oven for 12 h. The battery performance of the ILGPE based on lithium anode and LiFePO4 cathode was evaluated by galvanostatic charging-discharging tests. Cells were assembled in an argon-filled glove box and cyclic voltammetry (CV) of the cell was measured at a scanning rate of 0.1 mV s−1 between 2.5 and 4.5 V. The testing experiments were conducted in the voltage range of 2.7–3.85 V at room temperature according to the LAND CT2001A. 3. Results and discussion 3.1. Morphology of the ILGPE membrane
3.3. Electrochemical properties of ILGPE and interfacial stability against electrode
The schematic diagram of preparation process about ILGPE and the solid-state battery is shown in the Fig. 1a. Because of the swelling of IL, the dissociation of the lithium salt and the movement of the polymer molecular chains are improved, relating properties of the electrolyte are also changed. The prepared membrane looks semi-transparent and flexible self-retaining (Fig. 1b). And the surface is homogeneous and crack-free, as shown in SEM images (Fig. 1b). On the one hand, the SiO2 particles are disperse in the PVDF-HFP polymer matrix uniformly, on the other hand, the IL entrapped around polymer matrix making the hybrid electrolyte shows no obvious porous structure from the interior to the surface.
The temperature dependence of ionic conductivity of ILGPEs with different contents of lithium salts, IL and filler SiO2 are shown in Fig. 3a and b. All the ionic conductivities are gradually changed with the temperature increased. The highest ionic conductivity is observed for the mass ratio of PVDF-HFP:LiTFSI at 1:1 and PVDF-HFP:IL at 5:7, respectively, being the value of 0.67 mS cm−1 and 0.70 mS cm−1 at 25 °C·This observation shows that doping ILGPE with a larger amount of LiTFSI does not result in higher ionic conduction. This phenomenon can be explained by the formation of contact ions and salt aggregates, which decrease the actual concentration of the effective carrier of Li+, thus, corresponding a relative decrease in the free-moving Li+ and migration number [31]. In addition, the ionic conductivity of ILGPE (mPVDF-HFP:mLiTFSI = 1:1, PVDF-HFP:IL = 5:7) with the SiO2 content at 20% (names as ILGPE-20%SiO2) have a relatively maximum ionic
3.2. Crystalline and thermal stability of the ILGPE The XRD patterns of the ILGPE membranes with different ingredients contents are shown in Fig. 2a–b. Compared to the pure semicrystalline copolymer of PVDF-HFP, the lithium salt can weaken 49
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 1. a) The schematic diagram of preparation process about ILGPE and the solid-state battery. b) The photograph of the membrane and SEM images of ILGPE with 20 wt% SiO2 content.
conductivity of 0.74 mS cm−1 at 25 °C. Furthermore, the ionic conductivity is greatly attenuated with increased the filler content. That is because the extra inert fillers become obstacle to hinder the effective migration of Li+ in the membrane. However, the certain amount of inorganic fillers could use its Lewis acid effect to reduce the negative polarity of the F atom in the polymer matrix and further improve the
ionic conductivity of the electrolyte. Meanwhile, we can deduce that the dispersion of IL in polymer electrolyte results in multi-directional swelling of the polymer molecular chain, forming more amorphous regions. These usually change the transfer style of lithium ion in electrolytes, which can be proved in the later. However, when the mass ratio of IL exceeds 0.7 to 0.8, the ionic conductivity of ILGPE increased 50
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 2. a) and b) The XRD patterns of the ILGPE membranes with different ingredients contents. c) TGA curves of electrolytes with different ingredients, IL1GPE(mPVDF-HFP:mEMITFSI = 5:4), IL2GPE-(mPVDF-HFP:mEMITFSI = 5:7), IL3GPE-(mPVDF-HFP:mEMITFSI = 5:10). d) Limiting oxygen index of different electrolytes. The code names of 1#–8# were introduced to denote the electrolytes of Celgard membrane, PVDF-HFP:EC/PC(5:7,m/m)-LiTFSI-20%SiO2, ILGPE-(mPVDF-HFP:mEMITFSI = 5:3), ILGPE-(mPVDF-HFP:mEMITFSI = 5:4), ILGPE-(mPVDF-HFP:mEMITFSI = 5:5), ILGPE-(mPVDF-HFP: mEMITFSI = 5:6), ILGPE-(mPVDF-HFP:mEMITFSI = 5:7), ILGPE-(mPVDFHFP:mEMITFSI = 5:10). e) The ability of suppressing thermal shrinkage experiments.
Fig. 3. a) The lithium salt and ionic liquid content dependence of ionic conductivities at 25 °C. b) Arrhenius plots of the composite electrolytes with various SiO2 concentrations. c) CV and LSV curves of ILGPEs at 25 °C. d) The time evolution of interfacial resistance of Li/ILGPE-20%SiO2/Li non-blocking symmetric electrode battery. e) Time evolution of the interfacial resistances of the cell for different storage time. f) Voltage profile of the lithium plating/striping cycling with a current density of 0.1 mA/cm2.
rapidly at 4.8 V, which means that ILGPE-20%SiO2 has occurred oxidative decomposition reaction. Thus, the upper limited anode stabilization voltage of ILGPE-20%SiO2 is 4.8 V, which also indicates that the ILGPE-20%SiO2 can be assembled with the common LIBs cathode material, even the high voltage cathodes for high energy lithium batteries. The interface compatibility between ILGPE and lithium metal directly affects the specific capacity and cycling life of solid state lithium battery. Fig. 3d illustrated the time evolution of interfacial resistance of Li/ILGPE-20%SiO2/Li non-blocking symmetric electrode battery as a function of storage time at 25 °C. The plots consist of two semicircles at high and lower frequency corresponding to the bulk resistance (Rb) and
not obvious, and the mechanical properties of ILGPE reduces markedly with the further increase of IL content. The above results indicate that the mass ratio of IL should be controlled in an optimal amount. Thus, the ILGPE is maintained in a better state with good mechanical and electrochemical properties. LSV and CV are often used to characterize the electrochemical stability of the electrolytes, and the results are shown in Fig. 3c. For the cathodic scan, a reductive current occurred around 0 V (vs. Li/Li+). This result indicate that the Li+ can reductive deposition on the electrode, and the reaction don't affect the electrochemical stable of ILGPE20%SiO2 until 0 V. During anodic scan, the response current increase 51
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 4. a) Raman spectra of the binary IL electrolyte in the region of 730–755 cm−1. b) Raman spectra of ILGPE with different proportions of ingredients in the region of 730–770 cm−1. c) FTIR spectrum of PVDF-HFP and PVDF-HFP + LiTFSI. d) XPS patterns of the electrolyte with different components. The code names of (1)–(3) were introduced to denote PVDF-HFP, PVDF-HFP-LiTFSI, PVDF-HFP-LiTFSI-EMITFSI. e) The possible mechanism illustration of Li+ movement in ILGPE.
[32]. Fig. 3f shows the time-dependent voltage profile of the cell obtained with ILGPE-20%SiO2 and cycled at current density of 0.1 mA/ cm2. The voltage keeps increasing with cycling in the initial because of sustained reaction happened between the electrolyte and the metal lithium. Thus, we can call this kind of electrolyte requiring an induction period of several cycles to achieve a steady-state voltage [33]. Subsequently, the cell displays a much lower voltage and keeps stable voltage profiles during subsequent cycles, indicating uniform lithium deposition and a thin interfacial film formed. And the cell also exhibits stable state in voltage with increasing time to 280 h, showing good interfacial stability during cycling.
interface charge transfer resistance (Ri) respectively. Combination Fig. 3e, we can see that the Ri increases first and then can remain a stable value during the testing period. These results illustrate that the ILGPE-20%SiO2 would react with lithium metal forming the SEI film. And this layer of SEI film is constantly thickened as the reaction continues to occur, resulting interfacial resistance increase. When a stable interface is formed on the contact surface between ILGPE and lithium metal, the chemical reaction is inhibited and the interface impedance increases slowly until keeping stable. Lithium electrodeposition and interfacial stability are investigated by the other method of plate-strip procedure in symmetric Li/Li cells 52
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 5. a) Schematic diagram represents the solid-state battery of LiFePO4/Li. b) Cyclic voltammograms of the LiFePO4/ILGPE-20%SiO2/Li cell. c) Cycling performance of LiFePO4/ILGPE-20%SiO2/Li, insets indicate the moderate changes in the impedance of the battery. d) Typical charge-discharge curves.
3.4. Spectrum study and possible mechanism analysis
3.5. Cell performance
Raman spectroscopy is used to study the interactions within the materials and the effects of the relative amount of the ingredients on the interactions. Firstly, the spectroscopy of PVDF-HFP, EMITFSI and their mixtures are performed and demonstrate that the presence of PVDFHFP has a significant effect on the spectrum of EMITFSI. As shown in the Fig. 4a, the obviously shift and intensity changes of diagnostic functional group in EMITFSI is TFSI−, this fact suggests that there has a significant interaction between EMITFSI and LiTFSI. In addition, as regard to the changes of ring deformation vibration mode of EMITFSILiTFSI system (contrast Fig. 4a and Fig. 4b) can be seen that the interaction between Li+ and solvent (EMITFSI) become insignificant with the effect of PVDF-HFP. In contrast, the changes of interaction between Li+ and the polymer matrix can also be observed by FT-IR spectrum and XPS spectra in ILGPE (Fig. 4c and d). Firstly, the peak at 1200 and 510, 475 cm−1 of PVDF-HFP that are characteristic of CeF stretching vibration mode and swing mode change for the PVDF-HFP-LiTFSI [34], indicating the strong negative polarity of F atoms in PVDF-HFP polymer molecular could impede the migration of Li+ through forming interactions between the two different atoms. Meanwhile, the XPS spectra further confirm this analysis, as presented in Fig. 4d, F 1s spin-orbital splitting photoelectron in PVDF-HFP polymer is located at the binding energy of 687.8 eV. Comparatively, the CeF binding energy shifts to 688.5 eV when LiTFSI was mixed with the polymer, which is attributed to the interaction between F element and Li+. Interestingly, with the appears of ionic liquid-EMITFSI in the mixture of PVDF-HFP-LiTFSI, the binding energy of CeF decreased and almost returns to 687.8 eV again, indicating that there exist a competition reaction between polymer and ionic liquid on the association of lithium ions. Accordingly, the interaction of F and Li+ is weakened, from the macroscopic, what can be indicated is that IL can also undermine the coordination between Li+ and PVDF-HFP, thus making more and more Li+ migrate in the shortterm gel state instead of the long-range leaping movement along the chain of the polymer (As shown in the schematic illustration of Fig. 4e) [35]. In summary, it is shown that the polymer matrix and IL have a mutual influence on the side of the association with Li+, resulting in a better dissociation of lithium salt and a faster transmission of Li+.
In order to study the effect of ILGPE on the performance of cells, Li/ ILGPE-20%SiO2/LiFePO4 half-cells were assembled and the cyclability was investigated. Particularly, we introduced a small amount of the composition of electrolyte into the cathode with the purpose of reducing the interfacial impedance between the positive particles and improve the ion transport performance of the positive electrode (Schematic illustration of the cell design in Fig. 5a). Firstly, the cyclic voltammetry (CV) curves of the ILGPE-20%SiO2 in the LiFePO4/Li battery are shown in Fig. 5b. Comparing to the first scan, the cathodic peak shifts to the positive direction and the anodic peak shifts to the negative direction in the following scans, respectively. These redox peaks indicate that the battery configuration with the electrolyte of ILGPE-20%SiO2 could retains the electrochemical properties as we expected. As shown in Fig. 5c, the fabricated cell was galvanostatically charge and discharge at a current rate of 0.05C. The first specific discharge capacity is as high as 131.8 mAh g−1, which gradually increases over the first few cycles and maintains 117.9 mAh g−1 after the 50th cycle with a capacity retention of 89%. The coulombic efficiency increases after the first cycle and maintains at a relatively high value after cycles, indicating that the ILGPE-20%SiO2 can sustain the cycling stability of the cell with the positive performance such as the relatively high ionic conductivity and electrochemical stability of the electrolyte itself. Meantime, the cell impedance mainly consists of a superimposed semicircle in the high frequency to medium frequency range (see insets of Fig. 5c). It is note that the total cell impedance increases after cycles, indicating an interfacial film is generated on the surface of the electrode, and the increased interphasial resistance is responsible for the cell polarization with cycling as shown in Fig. 5d.
4. Conclusions The ILGPE was fabricated via combining the polymer matrix PVDFHFP with IL and silica nanoparticles. The hybrid electrolyte shows not only high ionic conductivity but also electrochemical stability and good compatibility with lithium electrode. Especially, with a mount of IL embedded in the GPE, ILGPE have high thermal safety stability. The XRD patterns, Raman spectroscopy, FT-IR spectrum and XPS spectra were carried out to demonstrate that the IL can also inhibit 53
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
crystallization and undermine the coordination between Li+ and PVDFHFP chain, which also promotes dissociation of lithium salt and improves the ion conductivity of GPEs. In addition, Li/LiFePO4 battery based on such ILGPE-20%SiO2 exhibits fascinating interfacial stability and cycling stability. Thus, this strategy makes it is possible for us to fabricate solid-state metallic lithium battery with high safety and appreciably enhanced performance.
(2015). [13] Y. Qi, L. Pan, L. Ma, P. Liao, J. Ge, D. Zhang, Q. Zheng, B. Yu, Y. Tang, D. Sun, J. Mater. Sci. Mater. Electron. 24 (2013) 1446. [14] Y.A. Samad, A. Asghar, R. Hashaikeh, Renew. Energy 56 (2013) 90. [15] B. Sun, J. Mindemark, K. Edström, D. Brandell, Electrochem. Commun. 52 (2015) 71. [16] I. Gurevitch, R. Buonsanti, A.A. Teran, B. Gludovatz, R.O. Ritchie, J. Cabana, N.P. Balsara, J. Electrochem. Soc. 160 (2013) A1611. [17] C.L. Yang, Z.H. Li, W.J. Li, H.Y. Liu, Q.Z. Xiao, G.T. Lei, Y.H. Ding, J. Membr. Sci. 495 (2015) 341. [18] Z. Tang, Q. Li, G. Gao, Polym. Adv. Technol. 21 (2010) 153. [19] F. Wu, N. Chen, R. Chen, Q. Zhu, J. Qian, L. Li, Chem. Mater. 28 (2016) 3. [20] J.H. Shin, S.S. Jung, K.W. Kim, H.J. Ahn, J.H. Ahn, J. Mater. Sci. Mater. Electron. 13 (2002) 727. [21] J.K. Kim, C.R. Shin, J.H. Ahn, A. Matic, P. Jacobsson, Electrochem. Commun. 13 (2011) 1105. [22] Y. Liu, B. Li, H. Kitaura, X. Zhang, M. Han, P. He, H. Zhou, ACS Appl. Mater. Interfaces 7 (2015) 17307. [23] S.N. Syahidah, S.R. Majid, Electrochim. Acta 175 (2015) 184. [24] P. Mukherjee, A. Kundu, S. Samanta, S. Roy, A.K. Nandi, J. Phys. Chem. B 117 (2013) 1458. [25] K. Mishra, S.A. Hashmi, D.K. Rai, J. Solid State Electrochem. 18 (2014) 2255. [26] S. Das, P. Kumar, K. Dutta, P.P. Kundu, Appl. Energy 113 (2014) 169. [27] Y. Shen, X. Qiu, J. Shen, J. Xi, W. Zhu, J. Power Sources 161 (2006) 54. [28] R. Chen, W. Qu, X. Guo, L. Li, F. Wu, Mater. Horiz. 3 (2016). [29] P.L. Kuo, C.H. Tsao, C.H. Hsu, S.T. Chen, H.M. Hsu, J. Membr. Sci. 499 (2016) 462. [30] B. Chen, W. Gao, J. Shen, S. Guo, Compos. Sci. Technol. 93 (2014) 54. [31] E.E. Paillard, Z. Qian, W.A. Henderson, G.B. Appetecchi, M. Montanino, S. Passerini, J. Electrochem. Soc. 16 (2009) 51. [32] N.W. Li, Y.X. Yin, J.Y. Li, C.H. Zhang, Y.G. Guo, Adv. Sci. (Weinheim, Ger.) 4 (2016) 1600400. [33] Y. Lu, K. Korf, Y. Kambe, Z. Tu, L.A. Archer, Angew. Chem. 53 (2014) 488. [34] Z. Li, G. Su, D. Gao, X. Wang, X. Li, Electrochim. Acta 49 (2004) 4633. [35] F. Bertasi, E. Negro, K. Vezzù, G. Nawn, G. Pagot, V.D. Noto, Electrochim. Acta 175 (2015) 113.
Acknowledgements This work was financially supported by the Research Project of National University of Defense Technology (ZDYYjCYj20140701). References [1] Y. Zhao, Z. Huang, S. Chen, B. Chen, J. Yang, Q. Zhang, F. Ding, Y. Chen, X. Xu, Solid State Ionics 295 (2016) 65. [2] C. Wang, Y. Yang, X. Liu, H. Zhong, H. Xu, Z. Xu, H. Shao, F. Ding, ACS Appl. Mater. Interfaces 9 (2017) 15. [3] J. Zhang, Q. Kong, Z. Liu, S. Pang, L. Yue, J. Yao, X. Wang, G. Cui, Solid State Ionics 245 (2013) 49. [4] A.J.F. Romero, J.P.T. Guisao, Electrochim. Acta 176 (2015) 1447. [5] B. Sun, J. Mindemark, K. Edström, D. Brandell, Solid State Ionics 262 (2014) 738. [6] S.Y. Chew, J. Sun, J. Wang, H. Liu, M. Forsyth, D.R. Macfarlane, Electrochim. Acta 53 (2008) 6460. [7] S. Gao, J. Zhong, G. Xue, B. Wang, J. Membr. Sci. 470 (2014) 316. [8] Q. Zhang, F. Ding, W. Sun, L. Sang, RSC Adv. 5 (2015) 65395. [9] M. Li, B. Yang, Z. Zhang, L. Wang, Y. Zhang, J. Appl. Electrochem. 43 (2013) 515. [10] J. Jin, Z. Wen, X. Liang, Y. Cui, X. Wu, Solid State Ionics 225 (2012) 604. [11] Q. Hu, S. Osswald, R. Daniel, Y. Zhu, S. Wesel, L. Ortiz, D.R. Sadoway, J. Power Sources 196 (2011) 5604. [12] B.E. Kidd, S.J. Forbey, F.W. Steuber, R.B. Moore, L.A. Madsen, Macromolecules 48
54
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Preparation and characterization of nanocomposite ionic liquid-based gel polymer electrolyte for safe applications in solid-state lithium battery ⁎
T
⁎
Qingpeng Guo , Yu Han , Hui Wang, Shizhao Xiong, Shuangke Liu, Chunman Zheng, Kai Xie College of Aerospace Science and Engineering, National University of Defence Technology, Changsha, Hunan 410073, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ionic liquid-based gel polymer electrolyte PVDF-HFP Safety Compatibility Solid-state lithium battery
Gel electrolyte is one of the most direct and appropriate ways to solve the safety problem of battery before the most challenging technological hurdles to all solid-state batteries have not yet been overcame. A major challenge towards gel polymer electrolyte (GPE) is making an electrolyte that is effectively combine with electrochemical performance, thermal safety and the stability to lithium. Here, PVDF-HFP-LiTFSI/SiO2/EMITFSI polymer electrolyte composite membrane (ILGPE) is prepared via solution casting method, in which nano-silica and ionic liquid positively effect on the performance of polymer electrolytes, such as ionic conductivity, electrochemical stability and thermal safety stability. It remarkably enhances the interface compatibility between ILGPEs and lithium metal. Significantly, we characteristically elucidate the possible transmission mechanism and interaction within the materials of ILGPE. Furthermore, Li/LiFePO4 battery based on such ILGPEs can exhibit fascinating interfacial stability and cycling performance. Herein, cells based on ILGPEs can overcome the drawbacks of solid electrolytes and volatile organic liquid electrolytes, which suggest a promising method for highly secure lithium batteries with appreciably enhanced performance.
1. Introduction Polymer electrolytes (PEs) with excellent properties such as light weight, safety, easy processing and other advantages have been proposed for implementation in popular power sources [1–6]. However, solid polymer electrolytes (SPEs) can hardly apply in large-scale commercial electronic equipment effectively due to the low ion conductivity and other defects [7]. Fortunately, the polymer-gel electrolytes (PGEs) are formed by introducing significant amount of organic solvents in SPEs, which can offer advantages in terms of high ionic conductivity and good interfacial compatibility with electrodes [8]. Compared with recent organic solvents, ionic liquids (ILs) is constituted by ions with unique properties such as high ionic conductivity, nonvolatility, high thermal stability and wide electrochemical windows [9,10]. Therefore, the replacement of volatile organic solutions with ILs entrapped in a polymer host forming ionic liquid based gel polymer electrolytes (ILGPEs), which combine with the advantages of IL and polymer electrolytes are regarded as promising electrolytes [9,11]. Poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly (ethylene oxide) (PEO), poly(vinylidenedifluoride) (PVDF) and its copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are the most common polymer host used in the electrolyte matrix [12–16]. Among them, the PVDF-HFP matrix has excellent
⁎
electrochemically stable owing to the strong electron-withdrawing group-CF [17–26]. Meanwhile, the higher dielectric constant (ε = 8.4) of this polymer will help to dissociate the lithium salt and increase the carrier concentration [27,28]. Thus, PVDF-HFP is widely regarded as a preferred matrix for gel polymer electrolytes. In this paper, a family of ILGPEs based on LiTFSI and SiO2 nanoparticles dispersed in PVDF-HFP is proposed. Meanwhile, the polymer molecules are swelled by IL, the ions can therefore transport in the space provided by the free volume around the polymer host, which means ILGPE possesses the advantages of both IL and solid polymer electrolytes, including high ionic conductivity, stable electrochemical window, especially, good thermal stability and non-volatile. Hereafter, the interplay between properties and structures is investigated, and the feasibility of solid-state lithium battery using this type electrolyte is characterized as well. All of these results suggest that the ILGPE has great promise for safe applications in solid-state lithium battery. 2. Experimental 2.1. Preparation of ILGPE The ILGPE membranes were prepared using solution-casting technique. PVDF-HFP was completely dissolved in butanone (which was
Corresponding authors. E-mail addresses: [email protected] (Q. Guo), [email protected] (Y. Han).
https://doi.org/10.1016/j.ssi.2018.03.032 Received 31 January 2018; Received in revised form 18 March 2018; Accepted 29 March 2018 0167-2738/ © 2018 Published by Elsevier B.V.
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
diffraction peaks polymer matrix to a lesser degree. It is illustrated the inferior miscibility of LiTFSI with the semi crystalline PVDF-HFP. Furthermore, a dramatic decrease on peaks intensity especially the diffraction peaks at 2θ = 20 and 40° can be observed with the increase of IL. These findings indicate that IL can effectively reduce the degree of crystallinity of PVDF-HFP through fully swelling the molecular chain and making molecular chains almost in the amorphous state. Thus, we can speculate that the ionic conduction of polymer electrolytes should be enhanced with the amorphous areas of polymer obviously increased. The thermal stability of the electrolytes is studied by TGA as shown in Fig. 2c. As noticed, the decomposition of the polymer matrix PVDFHFP (LiTFSI) starts at the lower onset temperature, and the lost weight is 80% when the temperature rises to 500 °C. The TGA curves of ILEMITFSI show that the mass loss begins at about 352.7 °C. Therefore, this result indicates incorporation of IL could increase the thermal stability of polymer electrolytes. The degradation processes of ILGPE can be divided into two-step. A small amount of weight loss is observed in the range from 75 to 300 °C, corresponding to the decomposition of the lithium salt and the polymer PVDF-HFP. The decomposition of mass loss in the temperature range of 300–460 °C is the second step, which is more noticeable and related to the decomposition of EMITFSI. It is worth noting that the thermal stability of ILGPE is greatly improved. Meanwhile, the effect by the content of ILs on the thermal stability of ILGPEs is compared, which further proved that IL played an important role in safe electrolytes. At the same time, we further judge whether the gel electrolyte is flammable or not from another index-the limiting oxygen index (LOI), which is an useful method to judge the relative flammability through evaluating minimum oxygen concentration [29]. Generally, the value of LOI is higher than 27 indicating that the material is flame retardant [30]. The LOI values of 1#–8# are shown in the Fig. 2d, 1# (Celgard membrane) and 2# (polymer electrolyte is not contains IL) exhibit combustible (LOI = 22) and flame retardant (LOI = 39) respectively. However, ILGPEs with different contents of IL show a higher LOI value, which can reach above 56, exhibiting absolutely flame retardant. In contrast, the calorific values of ILGPEs reflect a lower value compared to the samples of 1# and 2#, which reflect good thermal stability and non-flammability. As we know, the electrolyte membrane shrinkage problem also represents a safety hazard especially in high temperature conditions, which leads to a large area of the battery short circuit. In order to verify the above phenomenon sufficiently, the ability of suppressing thermal shrinkage experiments of membranes are carried out (Fig. 2e). The Celgard membrane is happened crimping phenomenon under the sustained heating, owing to the structure of polymer matrix changes in the high temperature. However, there is no crimping phenomenon of the ILGPEs with different content of IL under continuous heating process, indicating that ILGPE has significant advantages in terms of heat resistance.
stirred at 50 °C for 30 min) to obtain 6.7 wt% of polymer clear solution. The lithium bis(trifluoromethane)sulfonamide (LiTFSI, 99.9%, Aldrich), silicon dioxide (SiO2) and 1-ethyl-3-methylimidazolium triluoromethanesufonate (EMITFSI) with different weight ratio (the values of mPVDF-HFP:mLiTFSI:mEMITFSI:mSiO2 changed from 5:5:7:0.5 to 5:5:7:1.75) were added into the clear solution under argon atmosphere. The resulting mixture was magnetic stirring for 3 h and ultrasonically agitated for 20 min to from a homogeneous casting solution in a sealed container. Then, the solution was casted onto a Teflon plate to from the wet membrane and dried for 12 h at 60 °C in vacuum condition. Finally, the free standing ILGPE membrane was cut into circles with diameters of 19 mm. 2.2. Characterization of the ILGPE membrane The surface microstructures of the ILGPEs were observed by HITACHI S-4800 scan electron microscopy (SEM). The crystal structure of the membranes was obtained using X-ray diffraction (XRD) with Cu Kɑ radiation in the range of 10–60°. The Fourier transform infrared (FTIR) spectra were acquired on a Bruke V70 spectrometer. Raman spectra was recorded on a Bruker V70-FRA106 Raman module. The thermal properties of the electrolytes evaluated by Differential Scanning Calorimetric (DSC) and Thermogravimetric analysis (TGA) were carried out at a scan rate of 10 °C min−1 under N2 atmosphere. The ionic conductivity of the ILGPEs was measured by the AC impedance method using symmetrical stainless steel electrodes at different temperature range from 0 to 100 °C. The electrochemical stability of the ILGPEs was measured by linear sweep voltammetry (LSV) and cyclic voltammograms (CV) using the cell structure of Li/ILGPE/SS at the scanning rate of 0.1 mV s−1. They were measured from 2.5 V to 6 V (LSV, vs. Li+/Li) and between −0.5 V to 2.5 V (CV, vs. Li+/Li) respectively. The LiFePO4 electrode was prepared by the mixed slurries coated on Al foils, the slurries contain LiFePO4 powders, carbon black and PVDFHFP which was contained in ingredient of ILGPE-20%SiO2 at 8:1:1 weight ratio in N-methylpyrrolidone (NMP) solvent. The loading density of LiFePO4 cathode was controlled to be 1.5 mg cm−2. The composite cathode was dried in a vacuum oven for 12 h. The battery performance of the ILGPE based on lithium anode and LiFePO4 cathode was evaluated by galvanostatic charging-discharging tests. Cells were assembled in an argon-filled glove box and cyclic voltammetry (CV) of the cell was measured at a scanning rate of 0.1 mV s−1 between 2.5 and 4.5 V. The testing experiments were conducted in the voltage range of 2.7–3.85 V at room temperature according to the LAND CT2001A. 3. Results and discussion 3.1. Morphology of the ILGPE membrane
3.3. Electrochemical properties of ILGPE and interfacial stability against electrode
The schematic diagram of preparation process about ILGPE and the solid-state battery is shown in the Fig. 1a. Because of the swelling of IL, the dissociation of the lithium salt and the movement of the polymer molecular chains are improved, relating properties of the electrolyte are also changed. The prepared membrane looks semi-transparent and flexible self-retaining (Fig. 1b). And the surface is homogeneous and crack-free, as shown in SEM images (Fig. 1b). On the one hand, the SiO2 particles are disperse in the PVDF-HFP polymer matrix uniformly, on the other hand, the IL entrapped around polymer matrix making the hybrid electrolyte shows no obvious porous structure from the interior to the surface.
The temperature dependence of ionic conductivity of ILGPEs with different contents of lithium salts, IL and filler SiO2 are shown in Fig. 3a and b. All the ionic conductivities are gradually changed with the temperature increased. The highest ionic conductivity is observed for the mass ratio of PVDF-HFP:LiTFSI at 1:1 and PVDF-HFP:IL at 5:7, respectively, being the value of 0.67 mS cm−1 and 0.70 mS cm−1 at 25 °C·This observation shows that doping ILGPE with a larger amount of LiTFSI does not result in higher ionic conduction. This phenomenon can be explained by the formation of contact ions and salt aggregates, which decrease the actual concentration of the effective carrier of Li+, thus, corresponding a relative decrease in the free-moving Li+ and migration number [31]. In addition, the ionic conductivity of ILGPE (mPVDF-HFP:mLiTFSI = 1:1, PVDF-HFP:IL = 5:7) with the SiO2 content at 20% (names as ILGPE-20%SiO2) have a relatively maximum ionic
3.2. Crystalline and thermal stability of the ILGPE The XRD patterns of the ILGPE membranes with different ingredients contents are shown in Fig. 2a–b. Compared to the pure semicrystalline copolymer of PVDF-HFP, the lithium salt can weaken 49
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 1. a) The schematic diagram of preparation process about ILGPE and the solid-state battery. b) The photograph of the membrane and SEM images of ILGPE with 20 wt% SiO2 content.
conductivity of 0.74 mS cm−1 at 25 °C. Furthermore, the ionic conductivity is greatly attenuated with increased the filler content. That is because the extra inert fillers become obstacle to hinder the effective migration of Li+ in the membrane. However, the certain amount of inorganic fillers could use its Lewis acid effect to reduce the negative polarity of the F atom in the polymer matrix and further improve the
ionic conductivity of the electrolyte. Meanwhile, we can deduce that the dispersion of IL in polymer electrolyte results in multi-directional swelling of the polymer molecular chain, forming more amorphous regions. These usually change the transfer style of lithium ion in electrolytes, which can be proved in the later. However, when the mass ratio of IL exceeds 0.7 to 0.8, the ionic conductivity of ILGPE increased 50
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 2. a) and b) The XRD patterns of the ILGPE membranes with different ingredients contents. c) TGA curves of electrolytes with different ingredients, IL1GPE(mPVDF-HFP:mEMITFSI = 5:4), IL2GPE-(mPVDF-HFP:mEMITFSI = 5:7), IL3GPE-(mPVDF-HFP:mEMITFSI = 5:10). d) Limiting oxygen index of different electrolytes. The code names of 1#–8# were introduced to denote the electrolytes of Celgard membrane, PVDF-HFP:EC/PC(5:7,m/m)-LiTFSI-20%SiO2, ILGPE-(mPVDF-HFP:mEMITFSI = 5:3), ILGPE-(mPVDF-HFP:mEMITFSI = 5:4), ILGPE-(mPVDF-HFP:mEMITFSI = 5:5), ILGPE-(mPVDF-HFP: mEMITFSI = 5:6), ILGPE-(mPVDF-HFP:mEMITFSI = 5:7), ILGPE-(mPVDFHFP:mEMITFSI = 5:10). e) The ability of suppressing thermal shrinkage experiments.
Fig. 3. a) The lithium salt and ionic liquid content dependence of ionic conductivities at 25 °C. b) Arrhenius plots of the composite electrolytes with various SiO2 concentrations. c) CV and LSV curves of ILGPEs at 25 °C. d) The time evolution of interfacial resistance of Li/ILGPE-20%SiO2/Li non-blocking symmetric electrode battery. e) Time evolution of the interfacial resistances of the cell for different storage time. f) Voltage profile of the lithium plating/striping cycling with a current density of 0.1 mA/cm2.
rapidly at 4.8 V, which means that ILGPE-20%SiO2 has occurred oxidative decomposition reaction. Thus, the upper limited anode stabilization voltage of ILGPE-20%SiO2 is 4.8 V, which also indicates that the ILGPE-20%SiO2 can be assembled with the common LIBs cathode material, even the high voltage cathodes for high energy lithium batteries. The interface compatibility between ILGPE and lithium metal directly affects the specific capacity and cycling life of solid state lithium battery. Fig. 3d illustrated the time evolution of interfacial resistance of Li/ILGPE-20%SiO2/Li non-blocking symmetric electrode battery as a function of storage time at 25 °C. The plots consist of two semicircles at high and lower frequency corresponding to the bulk resistance (Rb) and
not obvious, and the mechanical properties of ILGPE reduces markedly with the further increase of IL content. The above results indicate that the mass ratio of IL should be controlled in an optimal amount. Thus, the ILGPE is maintained in a better state with good mechanical and electrochemical properties. LSV and CV are often used to characterize the electrochemical stability of the electrolytes, and the results are shown in Fig. 3c. For the cathodic scan, a reductive current occurred around 0 V (vs. Li/Li+). This result indicate that the Li+ can reductive deposition on the electrode, and the reaction don't affect the electrochemical stable of ILGPE20%SiO2 until 0 V. During anodic scan, the response current increase 51
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 4. a) Raman spectra of the binary IL electrolyte in the region of 730–755 cm−1. b) Raman spectra of ILGPE with different proportions of ingredients in the region of 730–770 cm−1. c) FTIR spectrum of PVDF-HFP and PVDF-HFP + LiTFSI. d) XPS patterns of the electrolyte with different components. The code names of (1)–(3) were introduced to denote PVDF-HFP, PVDF-HFP-LiTFSI, PVDF-HFP-LiTFSI-EMITFSI. e) The possible mechanism illustration of Li+ movement in ILGPE.
[32]. Fig. 3f shows the time-dependent voltage profile of the cell obtained with ILGPE-20%SiO2 and cycled at current density of 0.1 mA/ cm2. The voltage keeps increasing with cycling in the initial because of sustained reaction happened between the electrolyte and the metal lithium. Thus, we can call this kind of electrolyte requiring an induction period of several cycles to achieve a steady-state voltage [33]. Subsequently, the cell displays a much lower voltage and keeps stable voltage profiles during subsequent cycles, indicating uniform lithium deposition and a thin interfacial film formed. And the cell also exhibits stable state in voltage with increasing time to 280 h, showing good interfacial stability during cycling.
interface charge transfer resistance (Ri) respectively. Combination Fig. 3e, we can see that the Ri increases first and then can remain a stable value during the testing period. These results illustrate that the ILGPE-20%SiO2 would react with lithium metal forming the SEI film. And this layer of SEI film is constantly thickened as the reaction continues to occur, resulting interfacial resistance increase. When a stable interface is formed on the contact surface between ILGPE and lithium metal, the chemical reaction is inhibited and the interface impedance increases slowly until keeping stable. Lithium electrodeposition and interfacial stability are investigated by the other method of plate-strip procedure in symmetric Li/Li cells 52
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
Fig. 5. a) Schematic diagram represents the solid-state battery of LiFePO4/Li. b) Cyclic voltammograms of the LiFePO4/ILGPE-20%SiO2/Li cell. c) Cycling performance of LiFePO4/ILGPE-20%SiO2/Li, insets indicate the moderate changes in the impedance of the battery. d) Typical charge-discharge curves.
3.4. Spectrum study and possible mechanism analysis
3.5. Cell performance
Raman spectroscopy is used to study the interactions within the materials and the effects of the relative amount of the ingredients on the interactions. Firstly, the spectroscopy of PVDF-HFP, EMITFSI and their mixtures are performed and demonstrate that the presence of PVDFHFP has a significant effect on the spectrum of EMITFSI. As shown in the Fig. 4a, the obviously shift and intensity changes of diagnostic functional group in EMITFSI is TFSI−, this fact suggests that there has a significant interaction between EMITFSI and LiTFSI. In addition, as regard to the changes of ring deformation vibration mode of EMITFSILiTFSI system (contrast Fig. 4a and Fig. 4b) can be seen that the interaction between Li+ and solvent (EMITFSI) become insignificant with the effect of PVDF-HFP. In contrast, the changes of interaction between Li+ and the polymer matrix can also be observed by FT-IR spectrum and XPS spectra in ILGPE (Fig. 4c and d). Firstly, the peak at 1200 and 510, 475 cm−1 of PVDF-HFP that are characteristic of CeF stretching vibration mode and swing mode change for the PVDF-HFP-LiTFSI [34], indicating the strong negative polarity of F atoms in PVDF-HFP polymer molecular could impede the migration of Li+ through forming interactions between the two different atoms. Meanwhile, the XPS spectra further confirm this analysis, as presented in Fig. 4d, F 1s spin-orbital splitting photoelectron in PVDF-HFP polymer is located at the binding energy of 687.8 eV. Comparatively, the CeF binding energy shifts to 688.5 eV when LiTFSI was mixed with the polymer, which is attributed to the interaction between F element and Li+. Interestingly, with the appears of ionic liquid-EMITFSI in the mixture of PVDF-HFP-LiTFSI, the binding energy of CeF decreased and almost returns to 687.8 eV again, indicating that there exist a competition reaction between polymer and ionic liquid on the association of lithium ions. Accordingly, the interaction of F and Li+ is weakened, from the macroscopic, what can be indicated is that IL can also undermine the coordination between Li+ and PVDF-HFP, thus making more and more Li+ migrate in the shortterm gel state instead of the long-range leaping movement along the chain of the polymer (As shown in the schematic illustration of Fig. 4e) [35]. In summary, it is shown that the polymer matrix and IL have a mutual influence on the side of the association with Li+, resulting in a better dissociation of lithium salt and a faster transmission of Li+.
In order to study the effect of ILGPE on the performance of cells, Li/ ILGPE-20%SiO2/LiFePO4 half-cells were assembled and the cyclability was investigated. Particularly, we introduced a small amount of the composition of electrolyte into the cathode with the purpose of reducing the interfacial impedance between the positive particles and improve the ion transport performance of the positive electrode (Schematic illustration of the cell design in Fig. 5a). Firstly, the cyclic voltammetry (CV) curves of the ILGPE-20%SiO2 in the LiFePO4/Li battery are shown in Fig. 5b. Comparing to the first scan, the cathodic peak shifts to the positive direction and the anodic peak shifts to the negative direction in the following scans, respectively. These redox peaks indicate that the battery configuration with the electrolyte of ILGPE-20%SiO2 could retains the electrochemical properties as we expected. As shown in Fig. 5c, the fabricated cell was galvanostatically charge and discharge at a current rate of 0.05C. The first specific discharge capacity is as high as 131.8 mAh g−1, which gradually increases over the first few cycles and maintains 117.9 mAh g−1 after the 50th cycle with a capacity retention of 89%. The coulombic efficiency increases after the first cycle and maintains at a relatively high value after cycles, indicating that the ILGPE-20%SiO2 can sustain the cycling stability of the cell with the positive performance such as the relatively high ionic conductivity and electrochemical stability of the electrolyte itself. Meantime, the cell impedance mainly consists of a superimposed semicircle in the high frequency to medium frequency range (see insets of Fig. 5c). It is note that the total cell impedance increases after cycles, indicating an interfacial film is generated on the surface of the electrode, and the increased interphasial resistance is responsible for the cell polarization with cycling as shown in Fig. 5d.
4. Conclusions The ILGPE was fabricated via combining the polymer matrix PVDFHFP with IL and silica nanoparticles. The hybrid electrolyte shows not only high ionic conductivity but also electrochemical stability and good compatibility with lithium electrode. Especially, with a mount of IL embedded in the GPE, ILGPE have high thermal safety stability. The XRD patterns, Raman spectroscopy, FT-IR spectrum and XPS spectra were carried out to demonstrate that the IL can also inhibit 53
Solid State Ionics 321 (2018) 48–54
Q. Guo et al.
crystallization and undermine the coordination between Li+ and PVDFHFP chain, which also promotes dissociation of lithium salt and improves the ion conductivity of GPEs. In addition, Li/LiFePO4 battery based on such ILGPE-20%SiO2 exhibits fascinating interfacial stability and cycling stability. Thus, this strategy makes it is possible for us to fabricate solid-state metallic lithium battery with high safety and appreciably enhanced performance.
(2015). [13] Y. Qi, L. Pan, L. Ma, P. Liao, J. Ge, D. Zhang, Q. Zheng, B. Yu, Y. Tang, D. Sun, J. Mater. Sci. Mater. Electron. 24 (2013) 1446. [14] Y.A. Samad, A. Asghar, R. Hashaikeh, Renew. Energy 56 (2013) 90. [15] B. Sun, J. Mindemark, K. Edström, D. Brandell, Electrochem. Commun. 52 (2015) 71. [16] I. Gurevitch, R. Buonsanti, A.A. Teran, B. Gludovatz, R.O. Ritchie, J. Cabana, N.P. Balsara, J. Electrochem. Soc. 160 (2013) A1611. [17] C.L. Yang, Z.H. Li, W.J. Li, H.Y. Liu, Q.Z. Xiao, G.T. Lei, Y.H. Ding, J. Membr. Sci. 495 (2015) 341. [18] Z. Tang, Q. Li, G. Gao, Polym. Adv. Technol. 21 (2010) 153. [19] F. Wu, N. Chen, R. Chen, Q. Zhu, J. Qian, L. Li, Chem. Mater. 28 (2016) 3. [20] J.H. Shin, S.S. Jung, K.W. Kim, H.J. Ahn, J.H. Ahn, J. Mater. Sci. Mater. Electron. 13 (2002) 727. [21] J.K. Kim, C.R. Shin, J.H. Ahn, A. Matic, P. Jacobsson, Electrochem. Commun. 13 (2011) 1105. [22] Y. Liu, B. Li, H. Kitaura, X. Zhang, M. Han, P. He, H. Zhou, ACS Appl. Mater. Interfaces 7 (2015) 17307. [23] S.N. Syahidah, S.R. Majid, Electrochim. Acta 175 (2015) 184. [24] P. Mukherjee, A. Kundu, S. Samanta, S. Roy, A.K. Nandi, J. Phys. Chem. B 117 (2013) 1458. [25] K. Mishra, S.A. Hashmi, D.K. Rai, J. Solid State Electrochem. 18 (2014) 2255. [26] S. Das, P. Kumar, K. Dutta, P.P. Kundu, Appl. Energy 113 (2014) 169. [27] Y. Shen, X. Qiu, J. Shen, J. Xi, W. Zhu, J. Power Sources 161 (2006) 54. [28] R. Chen, W. Qu, X. Guo, L. Li, F. Wu, Mater. Horiz. 3 (2016). [29] P.L. Kuo, C.H. Tsao, C.H. Hsu, S.T. Chen, H.M. Hsu, J. Membr. Sci. 499 (2016) 462. [30] B. Chen, W. Gao, J. Shen, S. Guo, Compos. Sci. Technol. 93 (2014) 54. [31] E.E. Paillard, Z. Qian, W.A. Henderson, G.B. Appetecchi, M. Montanino, S. Passerini, J. Electrochem. Soc. 16 (2009) 51. [32] N.W. Li, Y.X. Yin, J.Y. Li, C.H. Zhang, Y.G. Guo, Adv. Sci. (Weinheim, Ger.) 4 (2016) 1600400. [33] Y. Lu, K. Korf, Y. Kambe, Z. Tu, L.A. Archer, Angew. Chem. 53 (2014) 488. [34] Z. Li, G. Su, D. Gao, X. Wang, X. Li, Electrochim. Acta 49 (2004) 4633. [35] F. Bertasi, E. Negro, K. Vezzù, G. Nawn, G. Pagot, V.D. Noto, Electrochim. Acta 175 (2015) 113.
Acknowledgements This work was financially supported by the Research Project of National University of Defense Technology (ZDYYjCYj20140701). References [1] Y. Zhao, Z. Huang, S. Chen, B. Chen, J. Yang, Q. Zhang, F. Ding, Y. Chen, X. Xu, Solid State Ionics 295 (2016) 65. [2] C. Wang, Y. Yang, X. Liu, H. Zhong, H. Xu, Z. Xu, H. Shao, F. Ding, ACS Appl. Mater. Interfaces 9 (2017) 15. [3] J. Zhang, Q. Kong, Z. Liu, S. Pang, L. Yue, J. Yao, X. Wang, G. Cui, Solid State Ionics 245 (2013) 49. [4] A.J.F. Romero, J.P.T. Guisao, Electrochim. Acta 176 (2015) 1447. [5] B. Sun, J. Mindemark, K. Edström, D. Brandell, Solid State Ionics 262 (2014) 738. [6] S.Y. Chew, J. Sun, J. Wang, H. Liu, M. Forsyth, D.R. Macfarlane, Electrochim. Acta 53 (2008) 6460. [7] S. Gao, J. Zhong, G. Xue, B. Wang, J. Membr. Sci. 470 (2014) 316. [8] Q. Zhang, F. Ding, W. Sun, L. Sang, RSC Adv. 5 (2015) 65395. [9] M. Li, B. Yang, Z. Zhang, L. Wang, Y. Zhang, J. Appl. Electrochem. 43 (2013) 515. [10] J. Jin, Z. Wen, X. Liang, Y. Cui, X. Wu, Solid State Ionics 225 (2012) 604. [11] Q. Hu, S. Osswald, R. Daniel, Y. Zhu, S. Wesel, L. Ortiz, D.R. Sadoway, J. Power Sources 196 (2011) 5604. [12] B.E. Kidd, S.J. Forbey, F.W. Steuber, R.B. Moore, L.A. Madsen, Macromolecules 48
54