Article pubs.acs.org/JACS
Synergistic Coupling between Li6.75La3Zr1.75Ta0.25O12 and Poly(vinylidene fluoride) Induces High Ionic Conductivity, Mechanical Strength, and Thermal Stability of Solid Composite Electrolytes Xue Zhang, Ting Liu, Shuofeng Zhang, Xin Huang, Bingqing Xu, Yuanhua Lin, Ben Xu, Liangliang Li, Ce-Wen Nan,* and Yang Shen* State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: Easy processing and flexibility of polymer electrolytes make them very promising in developing all-solid-state lithium batteries. However, their low roomtemperature conductivity and poor mechanical and thermal properties still hinder their applications. Here, we use Li6.75La3Zr1.75Ta0.25O12 (LLZTO) ceramics to trigger structural modification of poly(vinylidene fluoride) (PVDF) polymer electrolyte. By combining experiments and first-principle calculations, we find that La atom of LLZTO could complex with the N atom and CO group of solvent molecules such as N,N-dimethylformamide along with electrons enriching at the N atom, which behaves like a Lewis base and induces the chemical dehydrofluorination of the PVDF skeleton. Partially modified PVDF chains activate the interactions between the PVDF matrix, lithium salt, and LLZTO fillers, hence leading to significantly improved performance of the flexible electrolyte membrane (e.g., a high ionic conductivity of about 5 × 10−4 S cm−1 at 25 °C, high mechanical strength, and good thermal stability). For further illustration, a solid-state lithium battery of LiCoO2|PVDF-based membrane|Li is fabricated and delivers satisfactory rate capability and cycling stability at room temperature. Our study indicates that the LLZTO modifying PVDF membrane is a promising electrolyte used for all-solid-state lithium batteries.
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INTRODUCTION Solid-state lithium-ion electrolytes have attracted ever-increasing attention as they can be used to solve the safety issue for current lithium-ion batteries caused by the leakage, flammability, and poor chemical stability of liquid electrolytes.1−3 Solid-state electrolytes include inorganic electrolytes such as sulfides and oxides and polymer electrolytes such as poly(ethylene oxide) (PEO)-based electrolytes that can transfer Li+ by the local relaxation and segmental motion of polymer chains.4−7 In comparison, polymer electrolyte membranes are flexible and easy to process. Nevertheless, the low roomtemperature conductivity of the polymer electrolytes restricts their applications. To enhance their conductivity, several strategies such as cross-linking,8 forming block copolymer,9 adding plasticizers,10 and introducing ceramic fillers11 have been intensively explored. Among these attempts, dispersing ceramic fillers in polymer matrix to synthesize composite polymer electrolytes (CPEs) has attracted great interest because it can effectively enhance not only ionic conductivity but also mechanical properties and thermal stability of polymer electrolytes. The ceramic fillers in CPEs could be divided into inert fillers such as SiO2,12 TiO2,13 and Al2O314 and active inorganic electrolyte fillers such as Li0.3La0.557TiO3 (LLTO)15 and Li1.3Al0.3Ti1.7(PO4)3.16 Comparatively, the active fillers could © 2017 American Chemical Society
more effectively enhance the electrochemical performance of the CPEs since the fillers allow the Li+ migration. Among the active fillers, garnet-type Li7La3Zr2O12 (LLZO) is a promising candidate due to its high ionic conductivity and excellent chemical stability in contact with Li metal.17 As the polymer matrix in the CPEs, PEO is most employed. PEO incorporated with lithium salts was proposed as polymer electrolytes in 1979.18 Afterward, PEO-based CPEs blending with TiO2 and Al2O3 nanofillers were extensively investigated because the ceramic fillers could reduce the crystallization of PEO and hence enhance ionic conductivity of the CPEs.13,14 Recently, the PEO-based CPE membranes consisting of LLZO ceramics have been studied a lot,17,19,20 and it was shown that LLZO ceramic particles are a benefit for improving the performance of the electrolytes. Nevertheless, the PEO-LLZO CPEs still cannot meet the requirements of solid-state lithium batteries, due to low ionic conductivity at room temperature and low thermal stability. Also, PEO shows higher viscosity and poor film-forming ability. In comparison, poly(vinylidene fluoride) (PVDF) could be better than PEO for use in CPEs because high polarization of the PVDF is effective in dissociating lithium salt and probably enhances ionic Received: June 19, 2017 Published: September 12, 2017 13779
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Journal of the American Chemical Society conductivity.21 Furthermore, PVDF has good electrochemical stability and much better thermal and mechanical properties than PEO.4 Earlier, PVDF was mainly used in gel polymeric electrolytes due to its affinity for liquid electrolyte.22,23 However, the mechanical strength of gel electrolytes is poor, and membrane processing is difficult. Recently, PVDF-based solid composite electrolytes with inert ceramic particles such as SiO2 and TiO2 have been studied and showed improved mechanical properties but still unsatisfactory ionic conductivities.21 To make high-performance CPEs, dispersing active ceramic fillers LLZO in the PVDF matrix may be a favorable choice. So far, the CPEs made up of PVDF and LLZO have not been reported, and the interaction between them is not clear, which is critical for improving the performance of the CPEs. In this work, we synthesized flexible CPE membranes composed of PVDF matrix and Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO) fillers using a simple solution-casting method, denoted as the PVDF/LLZTO-CPEs. After introducing LLZTO fillers, the color of the CPEs changed, indicating the chemical structure change in the PVDF backbone. The chemical reaction and Li+ transport mechanisms in this PVDF-based CPEs were investigated based on the Fourier transform infrared (FTIR), Raman, and 1H NMR spectra and first-principles calculations. The LLZTO modifying PVDF membrane presented greatly improved electrochemical performance, mechanical properties, and thermal stability. For further illustration, a solid-state cell based on the PVDF/ LLZTO-CPEs membrane was fabricated to demonstrate its excellent performance.
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RESULTS AND DISCUSSION Physicochemical Properties of Prepared CPEs. The PVDF/LLZTO-CPEs were prepared in free-standing form via a simple solution-casting method. The as-prepared LLZTO powder used in the CPEs exhibits a crystalline structure of pure cubic phase (see Figure 1a), which is the high-conductivity phase of LLZO.19 The comparison in the X-ray diffraction (XRD) patterns shows that the crystallinity of LLZTO does not change after embedded in the PVDF-based CPE membranes. To determine the phase of the PVDF matrix, the XRD and FTIR profiles of pure PVDF, PVDF-SPE, and PVDF/LLZTOCPE were obtained and exhibited in Figure 1a,b, respectively. Obviously, the XRD pattern has diffraction peaks around 18, 20, and 40° (Figure 1a), and the FTIR spectrum shows the peaks at 811, 835, and 1232 cm−1 (Figure 1b), which indicates the existence of γ-phase PVDF in the CPE.24 After LLZTO and lithium salt were added, the degree of crystallinity of the polymer decreased (see Figure 1a). The scanning electron microscopy (SEM) images of as-prepared LLZTO, pure PVDF, and PVDF/LLZTO-CPE are, respectively, shown in Figures 1c, S1, and 1d. Uniformly micron-sized LLZTO particles are observed (Figure 1c); pure PVDF shows spherulite-type structure (Figure S1); after the addition of LLZTO and lithium salt, distributed interconnecting microstructures appear in the CPE (see Figure 1d), which might facilitate ion hopping and hence enhance ionic conductivity.25 SEM image of the PVDF-based membrane also shows bits of pores, which may be ascribed to phase separation between the polymer matrix and the solvent during the evaporation (see Supporting Information for the quantitative analysis). The energy-dispersive spectral (EDS) mapping images of the PVDF/LLZTO-CPE are presented in Figure 1e, which confirms the uniformity of the LLZTO dispersion in the CPE membranes. As shown in Figure
Figure 1. (a) XRD patterns of PVDF-based membrane and LLZTO powders. (b) FTIR spectra of pure PVDF, PVDF-SPE, and PVDF/ LLZTO-CPE. (c) SEM image of the as-prepared LLZTO powder. (d) Top-view and cross-sectional (inset) SEM images of PVDF/LLZTOCPE. (e) EDS maps of F and Zr in the sample marked (e). Photographs of (f) a piece of PVDF-SPE and a piece of PVDF/ LLZTO-CPE and (g) bent PVDF/LLZO-CPE membrane.
1f, the PVDF-based polymer electrolyte membranes without LLZTO fillers (denoted as PVDF-SPE) are transparent. However, after LLZTO is introduced, the color of the composite electrolyte membrane turns dark brown. The bent CPE membrane in Figure 1g maintains good flexibility. Reaction Mechanism in the CPEs. The color change observed in the PVDF/LLZTO-CPE may imply the structural modification of the polymers. In order to clarify the reaction mechanism, Raman spectra of the PVDF-SPE and PVDF/ LLZTO-CPE were measured and are shown in Figure 2. Apparently, there are two main differences in the spectrum of the PVDF/LLZTO-CPE in comparison with that of the PVDFSPE. The first one is that the peak at 2980 cm−1 that is characteristic of CH2 bending vibration mode almost disappears for the PVDF/LLZTO-CPE, indicating the deprotonation of CH2.26 The second one is the appearance of two peaks at 1121 and 1510 cm−1 corresponding to the CC stretching vibration modes of polyenes,27 suggesting the dehydrofluorination of PVDF chains. These variations in chemical structure are in accordance with those of PVDF after alkaline treatment, which means that the adding of LLZTO is likely to create an alkalinelike condition for PVDF and hence cause the partial dehydrofluorination of PVDF.26−29 To further confirm this analysis, more tests were conducted, and we found that the 13780
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of DMF, respectively, can be observed.30 Along with the addition of LLZTO, a distinct shoulder appears at ∼688 cm−1 next to the peak related to the OC−N group, which may be due to the interaction of LLZTO particles with the OC−N group of DMF.31 Meanwhile, with the increase of LLZTO content in the mixture of DMF/LLZTO, the intensity of the peak at 1668 cm−1 decreases, which means that LLZTO interacts with the CO group of DMF.32 Analogous to the interaction between different lithium salts and DMF reported previously,31−33 our results above reveal the association behavior between LLZTO and the N atom and CO group of the DMF molecule. 1 H NMR measurement on pure DMF and the DMF/ LLZTO mixture was carried out to further demonstrate the aggregation state of DMF and LLZTO, as presented in Figure 3c. The spectrum of pure DMF shows two typical peaks at ∼3.0 ppm for the methyl protons and a shoulder at 7.131 ppm for the HCO proton.34 Comparatively, the two resonances of methyl protons for DMF/LLZTO shifts to upfield, indicating the increase in the electron density of the amide group resulting from the association between N atoms and LLZTO. The peak corresponding to the HCO proton shifts a little downfield, which is attributed to the complexation between LLZTO and carbonyl group of DMF or the H-bonding interactions of DMF molecules.35 Based on the analysis of FTIR and NMR tests above, it is concluded that the reaction occurs between LLZTO and the N atom and CO group with the amide group of DMF in a high-electron-density state. To further study the electron-transfer pathway between the N atom and LLZTO, the interaction between DMF and LLZO
Figure 2. Raman spectra of PVDF-SPE and PVDF/LLZTO-CPE membranes.
mixture of DMF and LLZTO caused the alkaline-like environment (see Supporting Information for details). In order to explore the origin of the alkaline-like feature of the blend of DMF and LLZTO, FTIR, 1H NMR spectroscopy, and first-principles calculations were performed with the results shown in Figure 3. FTIR spectra were taken for different mixing ratios of LLZTO to elucidate the interaction between LLZTO and DMF (Figure 3a,b). Two vibrational peaks at 1668 and 657 cm−1, corresponding to CO and OC−N stretching modes
Figure 3. FTIR spectra of DMF and DMF mixing with different concentrations of LLZTO at the wavenumbers of (a) ∼600−900 cm−1 and (b) ∼1550−1750 cm−1. (c) 1H NMR spectra of pure DMF and DMF coupling LLZTO. (d) Computed charge density difference of DMF-absorbed LLZO-001. The yellow and blue isosurfaces represent charge accumulation and depletion in the space, respectively. The isovalue is 0.00014 au. 13781
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Journal of the American Chemical Society was explored by first-principles calculations. We constructed five kinds of typical surface models of LLZO, and the La/Li-coterminated (001) surface was found to be the most stable surface structure (see Supporting Information for details). The structure of DMF adsorption on the La/Li-co-terminated (001) surface was optimized and taken for the investigation of charge density difference. As displayed in Figure 3d, the exchange and transfer of charges between the DMF molecule and the (001) surface of LLZO are observed. The obvious accumulation of electrons centered around the N atom and the charge depletion region around O atom of DMF are consistent with the results of 1H NMR test. There is a large region of the charge depletion around the La atom adjacent to the N−CO group, indicating that the La atom is likely to interact with the N−CO group of DMF and causes the electrons to gather around the N atom. Thus, La atoms in LLZTO can complex with N atoms and CO groups in DMF with N atoms in the high-electrondensity state. Like Lewis bases,36,37 electron-enriched N atoms in DMF caused the alkaline-like environment. According to these analyses, a reaction mechanism for the PVDF-based systems is proposed and displayed in Figure 4a, which is in accord with the results of FTIR and Raman tests mentioned above. As is shown, electron-enriched N atoms of DMF, acting
as Lewis bases, donate pairs of electrons and induce the partial dehydrofluorination of PVDF. Partially modified PVDF would affect the complex mechanisms in the CPEs, hence influencing the ionic transport behavior in the CPE membranes. By comparison, chemical structures around Li and F in PVDF/ LLZTO-CPE are more complicated than those in PVDF-SPE (see Figure S5). The possible complex structures in the PVDF/ LLZTO-CPEs are proposed and presented in Figure 4b. Partial dehydrofluorination of PVDF with activated region around CC38,39 could facilitate the acid−base interactions between PVDF, LiClO4, and LLZTO in the CPEs. Ionic Conductivities of the CPEs. The Nyquist spectra of PVDF-based electrolyte membranes measured at room temperature are presented in Figure S6. Depressed semicircle from high to intermediate frequencies is attributed to the parallel combination of bulk resistance and bulk capacitance, which is associated with the bulk resistance and properties of the polymer electrolyte.40 The linear behavior at low frequencies is ascribed to the double-layer capacitance formed at the interface between the electrode and the electrolyte.41 The ionic conductivity σ data were calculated by the equation σ = L/ RS, where R is the bulk resistance obtained by the simulation according to the equivalent circuit shown in Figure S6b and L and S are the thickness and area of the solid electrolyte membrane, respectively. The ionic conductivity of as-prepared LLZTO determined from a dense ceramic disk is about 0.9 mS cm−1 at room temperature42 (see Figure S6a). The ionic conductivities for the PVDF-based polymer electrolytes with various LLZTO contents (from 0 to 40 wt %) at different temperatures were measured and are shown in Figure 5a. With the increase of the weight fraction of LLZTO, the ionic conductivities of the PVDF/LLZTO-CPE increase initially and reach the maximum at 10 wt % and then decrease. At 25 °C, the ionic conductivities of the PVDF/LLZTO-CPE with 10 wt % LLZTO particles can reach as high as about 5 × 10−4 S cm−1, about 7 times higher than that for the PVDF-SPE without LLZTO. The ionic conductivities of PVDF-based polymer electrolytes that have been reported recently are compared and summarized in Figure S7. Obviously, the conductivity of PVDF/LLZTO-CPE is higher than that of other PVDF or PVDF copolymer-based solid electrolytes and even higher than that of some PVDFbased gel electrolytes.43−46 Furthermore, the ionic conductivity of the PVDF/LLZTO-CPE with 10% LLZTO fillers is also higher than those reported previously for other solid composite electrolytes based on PEO or polyacrylonitrile (PAN) matrix with nanosized ceramic fillers such as LLZTO or LLTO (see Figure S8).16,17,20,41,47 The Arrhenius plots of the PVDF-based polymer electrolytes with various contents of LLZTO are shown in Figure 5b. The activation energy Ea was calculated according to the classical Arrhenius equation σ(T) = A exp(−Ea/RT), where T is the absolute temperature and A is a pre-exponential factor. With the introduction of LLZTO, Ea for the PVDF/LLZTO-CPE with 10 wt % LLZTO and 20 wt % LLZTO decreased to 0.20 and 0.24 eV, respectively, which are lower than that of PVDFSPE. Lower Ea indicates faster Li+ migration. For PVDF/ LLZTO-CPE with 30 and 40 wt % LLZTO, Ea values are 0.34 and 0.32 eV, which are higher than that of the PVDF-SPE. For the PVDF/LLZTO-CPE with 10 wt % LLZTO, substantialy enhanced ionic conductivity and lower Ea in comparison with the PVDF-SPE might be ascribed to three reasons. First, as shown in Figure 4b, partially dehydrofluori-
Figure 4. Proposed (a) reaction schemes consistent with the chemical signatures detected by Raman, FTIR, and 1H NMR; (b) possible complex structures in the PVDF/LLZTO-CPEs, where blue clusters denote LLZTO. 13782
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Figure 5. (a) Conductivities of the PVDF-based composite electrolytes as a function of the weight percentages of LLZTO at different temperatures. (b) Arrhenius plots of the PVDF-based polymer electrolytes and LLZTO disk. (c) Stress−strain curves and (d) TGA curves of the pure PVDF, PVDF-SPE, and PVDF/LLZTO-CPE with 10 wt % LLZTO.
mechanical properties with a Young’s modulus of 30.8 MPa and a tensile strength of 5.92 MPa, which are superior to those of the electrolyte membranes reported previously.9,13,49 Thermal stability, another crucial characteristic of the electrolytes used for lithium batteries, was also investigated for our membranes and is shown in Figure 5d. As presented in TGA curves, the thermal degradation of pure PVDF is about 500 °C. Along with the addition of LiClO4 and LLZTO, the thermal stability of the polymer decreased. The minor weight loss of PVDF-based membranes before thermal decomposition is ascribed to the trapped moisture. For the PVDF-based SPE and CPE, the weight loss of the membrane starting before 300 °C corresponds to the polymer melting and gradual degradation, which may be due to the increased amorphous fraction in the polymer electrolytes resulting from the interaction between LLZTO, polymer matrix, and lithium ions.50,51 The complete thermal decomposition of PVDF/ LLZTO-CPE began at 310 °C. Nonetheless, thermal stability of the PVDF/LLZTO-CPEs is still good enough for the applications in the lithium batteries. Cell Performances. To evaluate the electrochemical performance of the composite electrolytes, a CR2032 cointype cell with a LiCoO2-based cathode, a Li metal anode, and the optimal electrolyte membrane of PVDF/LLZTO-CPE with 10 wt % LLZTO was assembled in the glovebox. The fabricated LiCoO2|PVDF/LLZTO-CPE|Li cell was galvanostatically charged and discharged at 25 °C between 3.0 and 4.2 V at a current density of 54 mA·g−1 (i.e., 0.4 C-rate). The cycling
nated PVDF and the introduced LLZTO particles could complex with Li+ via acid−base interaction, which can effectively dissociate lithium salt to increase Li+ carrier density for conduction.6,21,48 Second, the dehydrofluorinated amorphous region can promote the segmental motion of the PVDF chains and the interactions between LLZTO fillers, and F atoms in the CPE can reduce crystallinity of the PVDF matrix, both of which can accelerate ionic migration.21,39 Third, the cubic structure of LLZTO and/or the complex structure of LLZTO and partially dehydrofluorinated PVDF in the CPE might be in favor of faster ionic hopping in available sites.17,21 With increasing LLZTO contents, the decline of the ionic conductivity and the increase of Ea at high filler loading are due to the aggregation of LLZTO particles, which results in the decreased miscibility between the filler and matrix, phase separation, and depressed ion migration.17,49 Mechanical and Thermal Properties. Good mechanical properties are critical for solid electrolyte membranes used in lithium batteries. Young’s modulus, tensile strength, and elongation of the CPEs were characterized and are summarized in Figure 5c. Along with the increase of LLZTO, the tensile strength and Young’s modulus for the PVDF/LLZTO-CPE are dramatically improved, and the elongation of the membranes is decreased, which can be attributed to the adhesion effect between the PVDF matrix and inorganic LLZTO particles.49 It is worth noting that the PVDF/LLZTO-CPE membrane containing 10 wt % LLZTO that possesses an excellent room-temperature ionic conductivity also shows the favorable 13783
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resistance for the cell. After 120 cycles, the rate capability, a vital characteristic for fast-charging lithium ion battery, was also assessed and displayed in Figure 6c. Retained discharging capacities of 140, 136, 133, 130, and 145 mAh·g−1 were obtained at the rate of 1.2, 2, 3.2, 4 C and back to 0.4 C, respectively, demonstrating excellent rate performance of the cell. Although the cell assembled with the PVDF/LLZTO-CPE exhibits superior electrochemical stability during cycling, an impedance increase was still observed in long-term cycling (see Figure 6c), which might be due to the gradual reaction between the polymer and lithium metal. To assess the compatibility between the PVDF-based electrolyte and lithium metal, electrochemical cycling of lithium|lithium symmetric cells was performed and is shown in Figure S10. Obviously, PVDF/ LLZTO-CPE, compared with the PVDF-SPE, delivered a much improved cycling stability in the lithium symmetric cell. These results indicate that the PVDF/LLZTO-CPE membrane is promising for lithium batteries due to its good electrochemical stability.
performance and the corresponding typical charge−discharge curves are presented in Figure 6a,b, respectively. High
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CONCLUSIONS In conclusion, the flexible PVDF/LLZTO composite electrolyte membranes have been successfully prepared using the conventional solution-casting method. La atoms in LLZTO can complex with the N atoms and CO groups of typical solvent molecules, such as DMF, with the N atoms in high-electrondensity state. Analogous to Lewis bases, this complex leads to partial dehydrofluorination in the CPEs and thus enhances the interactions between the PVDF matrix, lithium salt, and LLZTO particles. The structurally modified PVDF/LLZTOCPE with 10 wt % LLZTO particles demonstrates excellent electrochemical performance at ambient temperature, satisfactory mechanical properties, and good thermal stability. As an illustration, a LiCoO2|PVDF/LLZTO-CPE|Li cell presents satisfactory rate capability and cycling stability at room temperature. All these results show that the PVDF/LLZTOCPE have great potential to be used for the electrolyte in solidstate lithium 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/jacs.7b06364. Experimental methods and characterization details; alkaline-like feature exploration; computing section; porosity calculation; photographs of the blend of PVDF and LLZTO in different solvents; Raman spectra; SEM of pure PVDF; comparison of conductivities of PVDF-based electrolytes and composite polymer electrolytes; ssNMR spectra; impedance spectra; electrochemical performance of liquid battery and symmetric lithium|lithium cell (PDF)
Figure 6. (a) Cycle performance at 0.4 C, (b) typical charge− discharge curves, (c) rate capability (0.4−4 C), and impedance spectra (see inset) of a coin-type solid-state cell with a structure of LiCoO2| PVDF/LLZTO-CPE|Li operated at 25 °C.
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Coulombic efficiency and high capacity retention of 98% after 120 cycles (first, 150 mAh·g−1; 120th, 147 mAh·g−1) were obtained as seen from Figure 6a, indicating that the PVDF/ LLZTO-CPE membrane can sustain a stable cycling of the cell. In the charge−discharge voltage curves, the sloping voltage regions of LiCoO2|PVDF/LLZTO-CPE|Li cell are consistent with those of the conventional liquid battery (see Figure S9). Little difference between charge and discharge plateau means minor polarization, as shown in Figure 6b, indicating a low
AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *
[email protected] ORCID
Yang Shen: 0000-0002-1421-0629 Notes
The authors declare no competing financial interest. 13784
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ACKNOWLEDGMENTS This work was supported by the NSF of China (Grant Nos. 51532002, 51572141, and 51625202) and Research Fund of Science and Technology in Shenzhen (JSGG20150331155519130).
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DOI: 10.1021/jacs.7b06364 J. Am. Chem. Soc. 2017, 139, 13779−13785
