Single-Ion Conducting Polymer Electrolytes for Lithium Metal Polymer Batteries that Operate at Ambient Temperature Luca Porcarelli,† Alexander S. Shaplov,‡ Federico Bella,† Jijeesh R. Nair,† David Mecerreyes,*,§ and Claudio Gerbaldi*,† †
Department of Applied Science and Technology (DISAT), Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences (INEOS RAS), Vavilov str. 28, 119991 Moscow, Russia § POLYMAT, University of the Basque Country UPV/EHU, Joxe Mari Korta Center, Avenida Tolosa 72, 20018 Donostia-San Sebastian, Spain ‡
S Supporting Information *
ABSTRACT: Safety issues rising from the use of conventional liquid electrolytes in lithium-based batteries are currently limiting their application to electric vehicles and large-scale energy storage from renewable sources. Polymeric electrolytes represent a solution to this problem due to their intrinsic safety. Ideally, polymer electrolytes should display both high + lithium transference number (tLi ) and ionic conductivity. Practically, + strategies for increasing tLi often result in low ionic conductivity and vice versa. Herein, networked polymer electrolytes simultaneously displaying t+Li approaching unity and high ionic conductivity (σ ≈ 10−4 S cm−1 at 25 °C) are presented. Lithium cells operating at room temperature demonstrate the promising prospect of these materials.
I
very low, in the range of 0.2 < t+Li < 0.5, hence limiting the maximum deliverable current.9,11 With the aim to increase t+Li, polymeric single-ion conductors were introduced in the early 90s, namely, polyelectrolytes where the anions are covalently tethered to the polymer backbone and the counter lithium ions are the only mobile species.12 Conversely to conventional polymer electrolytes, single-ion conductors displayed t+Li approaching unity but simultaneously resulted in a drop of ionic conductivities (below 10−8−10−7 S cm−1 at 25 °C).13 Although considerable improvements in ionic conductivity have been achieved by the introduction of weakly coordinating anionic species (approaching 10−6 S cm−1 at 25 °C)14,15 and the optimization of the macromolecular architecture via random and block copolymerization with various neutral monomers,16,17 battery operation for single-ion conducting solid polymer electrolytes has been demonstrated only at moderately high temperatures. For instance, Bouchet et al.18 described anionic BAB triblock copolymers showing good performance in all-solid-state lithium metal cells (∼150 mAh g−1 at C/8), but the
n the last decades, development of reliable and highperforming electrolytes for the next generation of lithium based batteries has been a major challenge of the scientific community and industry.1−3 In this respect, polymer electrolytes, commonly representing a lithium salt associated with a polar neutral polymer or with an ion-conducting polymer matrix, have been extensively studied due to their nonvolatility and low flammability and represent a safer alternative to conventional liquid electrolytes.4 As an additional advantage, good mechanical properties of polymer electrolytes allow the fabrication of thinfilm batteries with enhanced energy density, free from leakage issues, and with increased design flexibility.5,6 However, obstacles have yet to be overcome before their widespread use. On the one hand, typical ionic conductivities (σ ≈ 10−6−10−5 S cm−1 at 25 °C) are several orders of magnitude lower than their liquid counterparts (σ ≈ 10−2−10−3 S cm−1 at 25 °C), thus precluding ambient-temperature operation.7 On the other hand, polymer electrolytes are usually characterized by a low lithium transference number (tLi+ < 0.6) and consequently are more susceptible to polarization phenomena that eventually limit the power delivery during battery discharge. The addition of liquid plasticizers,8 room-temperature ionic liquids,9 or inorganic particles10 has proved to be a viable strategy for increasing the ionic conductivity. However, t+Li values for these electrolytes are © XXXX American Chemical Society
Received: June 20, 2016 Accepted: September 6, 2016
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DOI: 10.1021/acsenergylett.6b00216 ACS Energy Lett. 2016, 1, 678−682
Letter
http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters
one maximum, thereby confirming the homogeneity of the films at the DMTA scale. It was found that the increase of LiMTFSI content in SIPE was accomplished by a small rise of the α relaxation temperature (Tg or Tα) from −69 to −66 °C. As for the storage modulus, two common temperature domains could be observed for SIPE3 (Figure S1a). At low temperature, the SIPE film was in the glassy, state and the storage modulus E′ (∼2400 MPa) slightly decreased until approximately −80 °C. Then a strong decay of E′ was observed during the α relaxation, where tan δ displays its maximum (−70 °C). Afterward, in the elastic part, the modulus again showed a slow decrease. To highlight the role of PC within the macromolecular architecture, a sample of SIPE3 was prepared without PC, and the resulting curve is shown for comparison (Figure S1a). The Tg of this sample increased by 11 °C with respect to its PC encompassing counterpart, and the plot of E′ (the values of which were always higher with respect to those of the PC-based sample) evidence the increased mechanical properties of the polymer film and the plasticizing role of the PC. Overall, the prepared samples (SIPE1−SIPE5) were fully amorphous, showing high mechanical properties and single Tg well below room temperature. The thermal stability of SIPEs was evaluated by thermogravimetric analysis (TGA) under an inert atmosphere (see Figure S1b). All of the samples showed a two-step degradation process referred to the stability of the different components used for sample preparation. The first degradation step attributed to the loss of PC started at 100 °C and reached 50% of the weight loss (the amount of PC added) at 150 °C. The second step was observed at around 240−250 °C and was assigned to the degradation of the polymeric matrix, which is in full agreement with the thermal stability of methacrylic architectures22 and poly(ionic liquid)s.23,24 Overall, as evidenced by the results of DMTA and TGA analysis, the newly proposed polymer electrolytes can be safely implemented in lithium-based battery systems in a wide temperature range (−60 to 100 °C). Ionic conductivities of the SIPEs were measured by electrochemical impedance spectroscopy (EIS) in the temperature range between 0 and 80 °C (Figure 2a). The conductivity value obtained at 25 °C for SIPE3 was as high as 1.1 × 10−4 S cm−1. As a comparison, the ionic conductivity of the PC-free sample was
tests were limited to 60−80 °C. In previous work, we reported on the synthesis of anionic block copolymers19 and their successful utilization as truly solid electrolytes in Li metal/LiFePO4 cells at 70 °C (130 mAh g−1 at C/15). Recently, Rohan et al.20 succeeded with the synthesis of single-ion conducting membranes based on anionic polysiloxane networks. However, the reported membranes needed to be swelled in mixtures of carbonate solvents before use. To date, the development of polymer electrolytes showing both high lithium transference number (approaching unity) and high ionic conductivity at ambient temperature remains a major intriguing and challenging task. Herein, we report on a new class of single-ion conducting polymer electrolytes (SIPEs) simultaneously exhibiting high lithium transference number, high ionic conductivities at room temperature, and good mechanical robustness. Such a unique combination of properties allowed their use as thin-film separators in lithium metal batteries operating at room temperature. The new polymer electrolytes were prepared by simple onestep radical copolymerization of lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethylsulfonyl)imide (LiMTFSI) with poly(ethylene glycol) methyl ether methacrylate (PEGM) and bifunctional poly(ethylene glycol) methyl ether dimethacrylate (PEGDM), in the presence of propylene carbonate (PC) as a plasticizer (Figure 1). This type of electrolyte is usually defined as a gel polymer electrolyte due to the presence of the PC.21
Figure 1. General pathway for the preparation of SIPE films.
A series of samples (SIPE1−SIPE5; see Table 1) were prepared, for which the ratio between LiMTFSI and PEGM Table 1. Sample Composition in Weight for SIPEsa
a
sample
LiMTFSI [w/w]
PEGM [w/w]
PEGDM [w/w]
PC [w/w]
SIPE1 SIPE2 SIPE3 SIPE4 SIPE5
6.4 7.5 9.0 11.3 15.0
38.6 37.5 36.0 33.7 30.0
5 5 5 5 5
50 50 50 50 50
AIBN 3% w/w of the monomers.
monomers was varied while the amounts of PEGDM and PC were kept constant (at 5 and 50% w/w, respectively). The proposed in situ synthesis allowed direct incorporation of PC in the polymer electrolyte with the formation of the cross-linked network. Despite the substantial amount of PC in the polymer electrolyte (50% w/w), the prepared films were found to be selfstanding, flexible, and easy to handle. The membranes were then investigated by dynamic mechanical thermal analysis (DMTA), and the results are demonstrated by example of SIPE3 in Figure S1a (see the Supporting Information). For all SIPEs, the plots of the mechanical loss tangent (tan δ) versus temperature show only
Figure 2. (a) Ionic conductivity versus inverse temperature for SIPE3 and PC-free SIPE3. (b) Ionic conductivity versus the LiMTFSI content dependence for SIPE1−SIPE5. (c,d) Electrochemical stability windows for SIPE3 obtained by CV at 25 (left) and 70 °C (right) with a scan rate of 0.2 mV s−1. 679
DOI: 10.1021/acsenergylett.6b00216 ACS Energy Lett. 2016, 1, 678−682
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ACS Energy Letters
Figure 3. (a) Nyquist plots representing the time evolution of the impedance profile corresponding to the change in the interfacial resistance for symmetrical lithium metal cells (Li/SIPE3/Li) at 25 (top) and 70 °C (bottom). (b) Potential as a function of the current rate for symmetrical lithium metal cells (Li/SIPE3/Li) at 25 and 70 °C.
Information). The transference number value at 25 °C was found to be as high as 0.86 ± 0.02, while at 70 °C, it slightly increased up to 0.90 ± 0.02, thus comparable within the range of experimental error. Although the theoretical value of single-ion conductors should be equal to 1, the practical value is usually measured to be between 0.85 and 0.96 (see Table S2). Two main reasons can be adducted to explain why the transference number cannot reach unity: (1) anions are attached to the main polymer chain by a flexible spacer that likely allows for short-range motion of negative charges and (2) additional motions of negative charges may arise from segmental motion of the polymer backbone as tests were conducted well above the glass transition temperature of the polymer. Summarizing, our presently reported transference number values approaching unity prove that, unlike a traditional binary electrolyte, the bulk of the ionic current in SIPE3 is carried exclusively by lithium cations. Such a result is particularly interesting in view of application of our newly proposed material in safe lithium metal batteries. In fact, the use of polymer electrolytes having a high lithium ion transference number has been proposed to prevent the formation of lithium dendrites,18 therefore improving the cycling stability and safety of these devices. Next, in order to understand the compatibility of the SIPEs with lithium metal electrodes, two lithium symmetric cells (Li/ SIPE3/Li) were assembled and the evolution of the interfacial resistance with time was studied at 25 and 70 °C. Measurements were carried out by means of EIS, and the resulting profiles are shown in Figure 3a. The incomplete semicircle at high frequency is attributed to the bulk resistance of the polymer electrolyte, while the semicircle at lower frequency is attributed to the interfacial resistance with the electrode, along with the impedance arising from the formation of the SEI layer. The deviations of the bulk resistance (Rb, high-frequency intercept) and interfacial resistance (Ri, low-frequency intercept) are shown as a function of time (days). Symmetric cells tested at 25 °C showed an increase in Ri value with time, which was stabilized after 10 days and remained stable for a long time. In the case of the cell tested at 70 °C, Ri increased with time and stabilized after 3 days. Then, the resistance values remained stable, serving as an indication that the prepared electrolyte can form a stable interfacial layer with lithium metal, which further accounts for its safe operation in lithium metal batteries. Constant current steps at different current intensities were performed on symmetrical lithium cells in order to determine the change in overpotential related to the lithium plating−stripping
also measured, and a 2 orders of magnitude decrease in σ was observed all over the temperature interval (Figure 2a). Thus, it can be concluded that the addition of PC markedly increases the mobility of the lithium ion in the polymer matrix, therefore overcoming one of the major drawback of single-ion conducting electrolytes.25 Further on, the dependence between σ and the Li+ concentration was investigated by the example of SIPE2−SIPE5. It was observed (Figure 2b) that σ increases with a rise in LiMTFSI content, reaching its maximum at a 9% w/w concentration. Such behavior can be readily explained by the increase in concentration of the charge carriers in the electrolyte. However, the subsequent growth in the LiMTFSI content resulted in the decrease of σ, which can be explained by the reduced lithium ion mobility due to aggregation in ion clusters26 or by reduced flexibility of the polymer film due to the higher anionic monomer content.19 Thus, the highest ionic conductivity equal to 1.2 × 10−4 S cm−1 at 25 °C was obtained for the SIPE3 film. It is worth pointing out that the described trend was generally respected in the entire temperature range considered in this study (0−80 °C) and is in good agreement with the previous reports on SIPEs.26,27 In summary, the obtained ionic conductivities of the SIPE1−SIPE5 films approach values required for practical operation in lithium ion batteries at ambient temperature and markedly outperform the reported values for other known single-ion polymer electrolytes.16,18 The electrochemical stability window of SIPE3 as the best film in terms of highest ionic conductivity was investigated by cyclic voltammetry (CV) at 25 and 70 °C (Figure 2c,d). At both temperatures, cathodic scans showed a couple of reversible redox peaks between −0.5 and 0.6 V versus Li+/Li that were associated with reversible lithium plating and stripping onto/from the copper electrode. Despite a roughly 10-fold order of magnitude difference between the current densities at 70 and 25 °C, the plating− stripping process was highly efficient in both cases. During anodic scans, no appreciable oxidation currents were observed up to 5.5 V versus Li+/Li. Commonly, the anodic decomposition of electrolytes is connected to the oxidation of anions.28 In the case of SIPE3, anions are covalently bonded to the polymer network and can be oxidized only at the electrolyte interface,18 hence accounting for the notable electrochemical stability in the wide temperature interval between 25 and 70 °C. The method proposed by Evans and Vincent29 was used to measure the lithium ion transference number of the proposed SIPE3 electrolyte and the results of EIS and polarization tests (both at 25 and 70 °C) are given in Table S1 (see the Supporting 680
DOI: 10.1021/acsenergylett.6b00216 ACS Energy Lett. 2016, 1, 678−682
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Figure 4. (a) Schematic representation of the in situ polymerization process on the LiFePO4 electrode sheets and LiFePO4/SIPE3/Li cell assembly. (b) Discharge capacity of the cell according to the discharge rate compared at 25 and 70 °C. (c) Specific capacity of the cell against the cycle number at a constant 0.1C rate at 25 °C.
process while applying different current regimes (from 3 μA cm−2 to 0.5 mA cm−2 at 25 and 70 °C). As can be elucidated from Figures 3b and S2, the change in potential at high current density was minimal for the cell tested at 70 °C, while it was more pronounced when the Li/SIPE3/Li device was tested at 25 °C. Finally, SIPEs were tested for their electrochemical behavior in lithium metal cells having LiFePO4 positive electrodes. A weight composition of 80% LiFePO4, 10% PVdF, and 10% carbon black was used for the cathodes’ preparation with the doctor-blade technique (active mass loading of about 5 mg cm−2) on aluminum current collectors. To ensure the best interfacial contact between the positive electrode and the polymer electrolyte,30 the latter was synthesized in situ on the surface of a LiFePO4 composite electrode film, as shown schematically in Figure 4a. The assembled LiFePO4/SIPE3/Li cell demonstrated the ability to reversibly operate at both 70 and 25 °C. Figure 4b shows the plot of the specific discharge capacity at different C rates and different temperatures. The cell operating at 70 °C was able to deliver 143 mAh g−1 at 0.1C, corresponding to 84% of the theoretical capacity of LiFePO4. At higher current rates, the capacity retention was still high, with 110 mAh g−1 at 2C. A substantial decrease in the discharge capacity was observed only at very high rates, which is definitely attractive for a SIPE. It is necessary to pinpoint that the cell operating at 25 °C was able to deliver as high of a specific capacity as 126 mAh g−1 at 0.1C, 110 mAh g−1 at 0.2C and 92 mAh g−1 at 0.5C. The measured discharge capacity dropped only at a relatively high 1C current rate. The cycling stability of the LiFePO4/SIPE3/Li cells was investigated by prolonged cycling at the 0.1C rate (Figure 4c). The cells were able to operate with excellent stability for 100 cycles at room temperature, and the capacity retention was found to exceed 98% of the initial capacity.
To demonstrate further the promising prospects of these materials as safe and reliable polymer electrolytes, prolonged cycling tests were performed at 70 °C with comparable results (see Figure S3). Potential versus specific discharge capacity profiles obtained at different current rates are shown in Figure S4 (see the Supporting Information). At both operating temperatures, distinctive discharge curves of LiFePO4 were readily detected even when the current was increased up to 0.2C. When higher current rates were applied, the typical potential plateau of LiFePO4 turned into a slope, along with an increased overpotential, which was more evident at lower temperature. Overall, the excellent operation of the LiFePO4/SIPE3/Li cells at ambient temperature, with stable and reproducible capacity values upon long-term cycling, can be definitely considered as a remarkable result for a single-ion polymer electrolyte and outperforms most of the recent literature reports in this field. In conclusion, in this Letter, innovative quasi-solid SIPEs are presented. They were prepared by simple in situ radical copolymerization of the novel lithium sulfonamide methacrylic monomer, PEGM, and PEGDM in the presence of PC. Unlike previous reports, these materials showed a unique combination of high lithium transference number (approaching unity) and high ionic conductivity at ambient temperature (σ ≈ 10−4 S cm−1). Among other striking advantages of the suggested SIPEs are (a) good mechanical robustness (storage modulus E′ = 2400 MPa), (b) no PC leakage even after prolonged testing/aging, (c) low Tg values of around −70 °C, (d) sufficient thermal stability (Tonset = 100 °C), and (e) wide electrochemical stability window versus Li+/Li (5 V). The remarkable performance of lithium metal polymer cells (LiFePO4/SIPE3/Li) operating at room temperature suggests that these materials may represent an important step toward the development of the next generation of 681
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safe, cost-effective, and environmentally friendly lithium polymer batteries.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00216. TGA traces for SIPEs; plot of the specific capacity for a Li/ SIPE3/LiFePO4 cell at 70 °C; table of transference numbers reported in the literature; plots of potential profiles for Li/SIPE3/Li and Li/SIPE3/LiFePO4 cells at different C rates; and measured lithium transference numbers of SIPE3 at different temperatures (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (D.M.). *E-mail:
[email protected] (C.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (Project No. 14-29-04039_ofi_m) and by the European Commission (Project No. 318873 IONRUN). J.R.N. gratefully acknowledges financial support from the MARS-EV project (FP7/2007-2013, under Grant Agreement No. 609201). The manuscript was written through contributions of all authors that have given approval to the final version of the Letter.
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REFERENCES
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DOI: 10.1021/acsenergylett.6b00216 ACS Energy Lett. 2016, 1, 678−682
