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Salt-with-Salt, a novel strategy to design the flexible solid electrolyte membrane for highly safe lithium metal batteries
Journal Pre-proof Salt-with-salt, a novel strategy to design the flexible solid electrolyte membrane for highly safe lithium metal batteries Nan Meng, Hongnan Zhang, Shuyi Lianli, Fang Lian PII:
S0376-7388(19)33440-4
DOI:
https://doi.org/10.1016/j.memsci.2019.117768
Reference:
MEMSCI 117768
To appear in:
Journal of Membrane Science
Received Date: 8 November 2019 Revised Date:
18 December 2019
Accepted Date: 19 December 2019
Please cite this article as: N. Meng, H. Zhang, S. Lianli, F. Lian, Salt-with-salt, a novel strategy to design the flexible solid electrolyte membrane for highly safe lithium metal batteries, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117768. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Statement Nan Meng: Investigation, Data Curation, Validation, Writing - Original Draft, Visualization. Hongnan Zhang: Methodology, Formal analysis, Investigation. Shuyi Lianli: Validation, Data Curation. Fang Lian: Conceptualization, Writing - Review & Editing, Supervision, Project administration.
Salt-with-Salt, A Novel Strategy to Design the Flexible Solid Electrolyte membrane for Highly Safe Lithium Metal Batteries
Nan Meng, Hongnan Zhang, Shuyi Lianli, Fang Lian*
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
*Corresponding author: E-mail address: [email protected] (Dr. F.Lian) Tel.: +86 10 82377985 Fax: +86 10 82377985.
1
ABSTRACT As one of the outstanding candidates for solid polymer electrolytes (SPEs), Poly(vinyl formal) (PVFM) based SPEs are facing higher requirements toward practical application such as further improving ambient ionic conductivity, lithium ion transference number, and suppressing interfacial passivation on the electrode. Based on lithiated PVFM single lithium-ion conductor (LiPVFM), a novel salt-with-salt concentrated flexible SPEs membrane have been prepared herein via intermolecular design as Dual-Li SPEs for highly safe solid-state lithium metal batteries. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dissolves in LiPVFM matrix to obtain the composite unexpectedly with a decreased glass transition temperature (Tg) and an inherited amorphous structure. Moreover, lithium ions are dissociated by the dominant ether ring groups in LiPVFM, improving the charge concentration of the electrolyte. Therefore, Dual-Li SPEs is achieved with high ionic conductivity of 5.7 × 10−4 S cm−1 at 25 °C, high lithium ion transference number of 0.79, excellent elasticity and stability on the interface of lithium metal anode. Li/ Dual-Li SPEs/ LiCoO2 batteries process a high reversible capacity in 200 cycles, high coulombic efficiency of 99% and high safety in bending and punching test. The work paves a new way for developing solid polymer electrolyte membrane with ideal integrated properties for lithium metal batteries. Keywords: solid-state batteries; polymer electrolyte membrane; lithium metal anode; safety 1. Introduction Applying of solid-state electrolyte (SSE) is an effective route to promote the safety of the high-energy density batteries using lithium metal as anode [1-4]. Among the SSEs, the solid polymer electrolytes (SPEs) attract more attention because (1) their light weight is necessary to further improve the gravimetric energy density (Wh kg-1); (2) good flexibility to develop roll to roll processing, and (3) easy to form films and good adhesion for allowing a low-impedance contact with 2
the solid electrodes. So far SPEs are usually achieved via dissolving lithium salt in the polymer matrix (known as “salt-in-polymer”). Poly(ethylene oxide) (PEO) is one of the most popular species, which has been extensively studied to address its key problem of low ionic conductivity at room temperature due to its high crystallinity [5-7], such as introducing inorganic powders to improve the mobility of polymer segments [8-10]. Moreover, the interaction of Li+ ions with the molecular chain of SPEs accelerates the dissolution of lithium salts and dissociation of Li+ ions. As known, an increase of concentration of lithium salt up to M:O (the mole ratio of salt cation to ether oxygen) values of ~1:16 facilitates access to the maximum conductivity of PEO-based electrolyte [11, 12]. The effect of further increase of the Li+ concentration above 1.3 mol L-1 will be unfortunately offset by a decrease of ionic mobility, since the viscosity especially glass transition temperature (Tg) of the mixture rises rapidly with the salt content [13]. Additionally, such salt-in-polymer electrolyte have dual ion conductivity of the anion X and the lithium ions, in which Li+ diffusion induced by segmental motion is much more difficult than the counter anions. About 80% of the ionic current is carried by anions, leading to the high concentration polarization and low Li+ ion transference number of salt-in-polymer electrolyte [14]. In recent years, novel single-ion conductors has been prepared by fixing the anions such as alkyl carboxylate (RCO2−) [15] or sulfonate (RSO3−) [16] to the polymeric backbones, in which the lithium-ion transference number (tLi+) is close to one. However, these single-ion conductors still suffer from a low ionic conductivity due to their low carrier concentration and lack of Li+ conductive group. Therefore in order to improve the ionic conductivity of single-ion polymer electrolyte, ether oxygen groups were introduced in recent studies as ionic conducting group into the matrix by PEO
3
blending or EO grafting [17, 18]. However, there is still difficulty to increase simultaneously the carrier concentration and the mechanical strength with high deformation capacity of SPEs. Poly(vinyl formal) (PVFM) polymer is an outstanding candidate for SPEs due to its low crystallinity, unique film-forming properties and tunable intramolecular structure. In our previous study, PVFM based polymer electrolytes have been achieved via the crosslinking and in-situ polymerizing, which dramatically improves their chemical and electrochemical stabilities [19, 20]. Furthermore, PVFM based single-ion SPEs (shorted for LiPVFM) have been proposed with wide electrochemical stability window and high tLi+ of 0.87, in which the Li+ coordinated PVFM via coupling suitable amount of oxalate-chelated borate units to vinyl hydroxyl in the host polymer backbone [21, 22]. Nevertheless, the ionic conductivity at room temperature 3.3×10−5 S cm−1 should be further improved to meet an essential requirement for the ambient one ≥10−4 S cm−1. Moreover, LiPVFM membrane is deserved to be lack of elastic deformation even the stress is performed up to 30 MPa, which is ascribed to ether ring as the major group accounting for 62.3% in the structure and the resultant tight structural network. In this work, it is proved that the dominant ether ring groups enable LiPVFM to be capable of a high
solubility
and
good
dissociation
of
lithium
salts.
Herein,
lithium
bis(trifluoromethylsulfonyl)imide (LiTFSI) with high degree of dissociation and extensive negative charge delocalization is employed to dissolve in the single-ion conductive salt LiPVFM to achieve “Salt-with-Salt” highly concentrated flexible solid electrolyte membrane, named as Dual-Li SPEs. The dissolved lithium salts decrease Tg of the composites, relax the original tight structure and activate the segmental motion in LiPVFM. So the Dual-Li SPEs are achieved with a high ionic conductivity at ambient temperature, high lithium-ion transference number, and good elastic 4
performance. Moreover, the Dual-Li SPEs shows the stable interface with lithium metal anode over a long cycle life, ending up with a highly safe lithium metal battery. 2. Experimental 2.1. Preparation of solid polymer electrolyte membrane The single ion conductor LiPVFM is obtained as reported in our previous work [21, 22] and described in the Supplementary information. Subsequently, LiPVFM and a certain amount of LiTFSI (Aladdin) were dissolved into dimethyl sulfoxide (DMSO, Sinopharm Chemical Reagent), in which the molar ratio of vinyl acetal in PVFM and LiTFSI is optimized to be 30:4. The above solution was stirred at 50 °C for 12 h, cooled to room temperature, and then cast into a membrane. Finally, the SPEs membranes were dried under vacuum at 70 °C for 24 h to evaporate the solvent and transferred to an argon-filled glove box, marked as Dual-Li SPEs. Additionally, the membrane consisting of LiPVFM with less or without LiTFSI prepared via the aforementioned method has also been shown in which the molar ratio of vinyl acetal and LiTFSI is fixed at 30:3, 30:2, 30:1 and 30:0, named as Dual-Li-3, Dual-Li-2, Dual-Li-1 and LiPVFM, respectively. The thickness of SPEs has been listed in Table S2. While, the self supporting membranes can’t be achieved when higher content LiTFSI is introduced in LiPVFM-based composite electrolyte. For comparison, the LiTFSI-PEO polymer electrolyte was prepared by dissolving PEO (molecular weight of ~300,000, Sigma Aldrich) and LiTFSI in acetonitrile (J&K Scientific) with a PEO/ LiTFSI mole ratio of 12 to ensure that LiTFSI-PEO has the same lithium concentration of 1.5 mol L−1 as the aformentioned Dual-Li samples. Then, the mixture was stirred continually at 65 °C, coated on polytetrafluoroethylene plate with a doctor blade and dried in an argon-filled glove box for about 12 h to obtain LiTFSI-PEO polymer membrane. Finally, the above electrolyte membranes 5
were punched into circular pieces (d = 16 mm) for assembling cells. 2.2. Material analysis The morphology of the samples was characterized by a field emission scanning electron microscope (FESEM, Carl Zeiss SUPRA55). The X-ray diffraction (XRD) profile was obtained by XD2618N X-ray diffraction analyzer in scanning range between 10° to 60°. Fourier transform infrared spectroscopy (FT-IR) experiment of polymer electrolytes was performed on NEXUS FT-IR670 spectrometer with a frequency range of 400-4000 cm−1. The electrostatic potential of the ether ring was calculated by ORCA [23] and Multiwfn [24] as described in Supplementary information. Thermogravimetric analysis (TGA, TGA2050) experiments were carried out in the temperature range from room temperature to 600 °C under argon atmosphere at a heating rate of 10 °C min−1. Dynamic mechanical analysis (DMA, Waters Q800) were carried out in the temperature range from -100 to 300 °C at frequency of 1 Hz. The stress and strain curves of polymer electrolytes with a size of 1 cm × 5 cm were carried out on a tensile testing apparatus (INSTRON, 4465) at a stretching speed of 10 mm min−1 at room temperature. 2.3. Electrochemical characterization The sandwich cells of SS (stainless steel)/ SPEs/ SS were used for the test of ionic conductivity by using the alternating current impedance method. Impedance data were obtained with an electrochemical working station VersaSTAT3 in the frequency range of 10−2-105 Hz. The ionic conductivity of the SPEs was calculated based on the following equation (1): σ=L/(RbS)
(1)
Where σ is the ionic conductivity, Rb is the bulk resistance and L and S are the thickness and area of the electrolyte , respectively. 6
The relationships between the complex impedance, dielectric constant (εr), dielectric loss (εi), and loss tangent (tan δ) were calculated as in Supplementary information [25, 26]. The electrochemical stability window of the SPEs was evaluated by linear sweep voltammetry (LSV) in the range from open circuit voltage to 5.5 V at the rate of 5.0 mV s−1 with SS/ SPEs/ Li cells. Lithium-ion transference number was calculated by the chronoamperometry test on the Li/ SPEs/ Li cells with an applied voltage of 10 mV and was determined as in Supplementary information. The symmetrical Li/ SPEs/ Li cells was assembled to process Li stripping-deposition measurements (Wuhan Land Electronic Co. Ltd., CT2001A, China) at 0.10 mA cm-2 and a stripping-deposition time of 20 min. In the assembling process of the cells, propylene carbonate (PC) about 5.0 wt% of the related membrane was dropped to allow the polymer electrolyte membrane spreading out and attachable to the electrode at room temperature. Interfacial resistance of the lithium and SPEs was analyzed by impedance tests before and after cycles with potential amplitude of 5.0 mV from 105 Hz to 10−2 Hz, respectively. The solid electrolyte interphase (SEI) between Dual-Li SPEs and lithium metal anode was characteriazed by X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTRADLD). The solid-state cells (CR2032) were assembled for electrochemical test using lithium metal anode and LiCoO2 cathode. The LiCoO2 electrode was composed of 70 wt% LiCoO2, 15 wt% PVDF, 5.0 wt% Dual-Li SPEs and 10 wt% carbon black, in which the areal density of the active material was 2.8 mg cm−2. The Li/ SPEs/ LiCoO2 batteries were charged and discharged between 3.0 and 4.4 V at 0.10 C current density on LAND testing system (Wuhan LAND electronics Co., Ltd.) at 25 °C. The C rates in the electrochemical measurements are defined on the base of 1.0 C = 170 mA g−1. 2.4. Battery safety testing 7
The soft-package lithium batteries were assembled for safety test using lithium metal anode and LiCoO2 cathode. The pouch cells were charged to 4.3 V at 0.10 mA cm−2 current density on LAND testing system at 25 °C. The cells were folded three times while texting its voltage. A commercial blue light-emitting diode (LED) lamp was lighted up during folding. In the punching test, the pouch cells with LiCoO2 cathode and lithium metal anode were cycled for 20 cycles between 3.0 and 4.4 V at 0.10 C on LAND testing system at 25 °C. The steel needle was punched into the cells while monitoring the voltage and temperature of the cells. In addition, the needle was pulled out as long as it pierced the cell. 3. Results and discussion
Scheme 1. A schematic of the preparation of Dual-Li SPEs membrane and solid state lithium metal cells. 8
The schematic of the preparation process of “Salt-with-Salt” highly concentrated flexible solid electrolytes derived from single-ion conductive salt LiPVFM is illustrated in Scheme 1. The optimal molar ratio of vinyl acetal in LiPVFM and LiTFSI is at 30:4, that is, the M:O value (the mole ratio of total Li+ from both LiPVFM and LITFSI to oxygen atom in vinyl acetal) is 1:12 in the composites. The dense, transparent and freestanding Dual-Li SPEs membranes were obtained with no detectable difference with LiPVFM in morphology. The typical images of surface and cross-section of the Dual-Li SPEs membranes are demonstrated in Fig. 1a-d. The membranes exhibit smooth surface, dense and homogeneous cross-section morphology as shown between two red dash lines in Fig. 1c and its enlarged view in Fig. 1d. From the energy dispersive spectrometer (EDS) mapping of the cross-section (Fig. 1e and f) it can be seen that the yellow and pink dots with bright color assigned to the S and F elements respectively are uniformly distributed. The structure of the SPEs has been detected by XRD and has been shown in Fig. 1g and Fig. S1 in supplementary information. A broad diffraction peak near 20° and no diffraction peak of LiTFSI are detected in the XRD patterns, indicating the amorphous state of LiPVFM and Dual-Li SPEs, which is very different from that of PEO-based electrolytes owning several intense peaks and high crystallinity at room temperature [27]. The result indicates that LiTFSI dissolves completely and coordinates in LiPVFM to form an ionic transfer-convenient amorphous system. FT-IR spectra and zoom-in image with a frequency range of 1000-1300 cm−1 are shown in Fig. S2 and Fig. 1h, respectively. The peak at 3491 cm−1 is assigned to –OH stretching of alcoholic hydroxyls in LiPVFM. The characteristic frequency at 1733 cm−1 is ascribed to the C=O stretching in the vinyl acetate groups of LiPVFM [28]. The bands at 1243, 1179, 1133 and 1066 cm−1 represent the stretching mode of the C-O-C-O-C bonds in ether ring of LiPVFM. However, the locations of these bands shift due to 9
the inclusion of LiTFSI, demonstrating that the interaction between LiTFSI and the ether ring occurs. The oxygen in the ether ring with lone pair electrons facilitates its complexation with Li+. The electrostatic potential of the ether ring was calculated by Density Functional Theory. As shown in Fig. 1i the yellow areas representing negative electrostatic potential reveals that the oxygen atom of the ether ring groups in LiPVFM induces the interaction with Li+, which contributes to a high solubility and good dissociation of lithium salts in LiPVFM.
Fig. 1. Image and micrograph of the Dual-Li SPEs membrane. (a) Digital image, (b) surface, (c) (d) cross-section; EDS mapping of the portion selected: (e) S, (f) F. (g) XRD spectra of SPEs, (h) FT-IR spectra of the LiPVFM and Dual-Li SPEs series, (i) the calculated electrostatic potential of the ether ring. 10
Fig. 2. The physical properties of Dual-Li SPEs membrane. (a) DSC, (b) and (c) DMA. (d) TGA, (e) Stress-strain curves and (f) Stretching test. Differential scanning calorimetry (DSC) [29] and DMA was employed to analyze Tg of the “Salt-with-Salt” system. It is important to note that unlike the traditional “salt-in-polymer” PEO-based SPEs, the Dual-Li series shows a decreasing Tg value with an increase of lithium salt concentration (Fig. S3). As shown in Fig. 2a LiPVFM presents Tg = 62 °C, while Dual-Li SPEs exhibit the complete amorphous state at ~20 °C. The temperature-sweep test of DMA in Fig. 2b and c indicates the maximum of the loss factor for LiPVFM and Dual-Li SPEs appears at the temperature of 63.3 °C and 25.5 °C, respectively, which is in agreement with Tg analysis by DSC. Moreover, as tan δ is associated with the movement of the polymer segment and the relaxation transition peak reflects the glass-rubber transition, the broad relaxation transition peak in tan δ curves demonstrates that the much more amorphous phase dominates in Dual-Li SPEs. The result corresponds to an increasing ratio of flexible chain segments and an improved ionic migration kinetics, which is contributed to the intermolecular interaction between LiPVFM and LiTFSI in Dual-Li SPEs. 11
The thermal stability of SPEs is a crucial parameter to ensure the battery safety. Fig. 2d and Fig. S5 demonstrate the TGA of SPEs. The liquid electrolyte, namely, 1.0 M LiPF6 in ethylene carbonate/ dimethyl carbonate/ diethyl carbonate (EC: DMC: DEC) = 1: 1: 1 volume ratio evaporates rapidly at the beginning of heating, whereas the weight loss in Dual-Li SPEs is observed above 140 °C. The slope near 265 °C represents the decomposition of the oxalate-chelated borate structure. Moreover, the weight loss at around 325 °C ascribes to the decomposition of LiTFSI. The results show that Dual-Li SPEs possess the superior thermal stability. Moreover, the decrease of decomposition temperature in compare with LiTFSI (375 °C) and LiPVFM (380 °C) further confirms their strong interaction in Dual-Li SPEs. The tensile stress/ strain curves of the membranes in Fig. 2e and Fig. S4 demonstrate that all the SPEs samples possess a high mechanical strength. LiPVFM shows the high ultimate tensile strength ≥ 40 MPa but a low elongation-at-break value ≤ 6.0% [22]. While, the Dual-Li SPEs exhibit an ultimate tensile strength 25 MPa and a significant improved elongation-at-break value 266.7%. In detail, Dual-Li SPEs membranes can stretch about 2.5 times their original length before break as shown in Fig. 2f, which exhibits a prospect in application as the flexible solid electrolyte. The Young modulus of Dual-Li SPEs is determined to 2.0 GPa by nano indentation as shown in Fig. S6. The amorphous state and flexible chain induced by strong interaction of LiPVFM with LiTFSI is responsible for an excellent elastic property and a high structural compliance of the Dual-Li SPEs membrane. Fig. 3a shows the ionic conductivity of SPEs varied from 25 to 60 °C. Symmetric cells were utilized for the measurements, consisting of two identical stainless steel foils as blocking electrodes. The Arrhenius plots of ionic conductivity were obtained correspondingly along with a well-fined 12
electrochemical impedance spectroscopy (EIS) spectrum (Fig. S7). Dual-Li SPEs exhibits the high ionic conductivity of 5.7 × 10−4 S cm−1 at 25 °C and 1.4 × 10−3 S cm−1 at 60 °C. In compare with the traditional “salt-in-polymer” electrolytes such as LiTFSI-PEO and LiTFSI-LLZTO-PEO in the references, the Dual-Li SPEs exhibits notable improvements in ambient ionic conductivity as shown in Fig. 3b. What is more, the “salt-in-polymer” LiTFSI-PEO system with the same M:O value is confirmed in the reference to show relatively low ionic conductivity, since the concentrated Li salt astricts the mobility of ether group and significantly increases Tg of PEO [30]. While, as shown in Table S2, the ionic conductivity of the “salt-with-salt” electrolyte is improved with an increase of LiTFSI concentration, which is very different from traditional “salt-in-polymer” system.
Fig. 3. (a) Conductivity and the fitted results of SPEs between 25 and 60 °C, (b) A comparison of conductivity of polymer electrolytes in the representative works [30-32]. (c) Current-time plots of Li symmetrical cells with Dual-Li SPEs, (c inside) The impedance spectra before and after potential polarization, (d) A comparison of Li+ transference number of polymer electrolytes in the representative works [33]. Variation of (e) ε’ and (f) tan δ for different SPEs as a function of the 13
frequency at 30 °C (ε: dielectric constant). The dielectric permittivity including the stored and loss charges in polymer electrolytes was studied as in Fig. 3e and f. For Dual-Li SPEs, the relaxation peak shifts to high frequencies, even higher than 105 Hz, meanwhile the real part (ε’) of the dielectric constant is significantly higher than that of LiPVFM and LiTFSI-PEO, which presents evidence of an enhanced segmental mobility and concentrated charge carrier of Dual-Li SPEs [25]. Furthermore, over the frequency range of 10−2-102 Hz, the oscillating dipoles in the macromolecules tend to orient in the derection of the applied field, while the oscillating dipoles could not rotate rapidly with the rising of frequency, so no excess ion diffuses in the direction of the applied field, ending up with polarization because of the charge accumulation decrease [34], which leads to a drop in the value of dielectric constant. The low-frequency dispersion of ε’ aggravates in Dual-Li SPEs, declaring that the polymer possess the typical Li ion transport mechanism i.e. ionic hopping in couple with segment motion of the polymer [26]. The electrochemical window is also crucial for the polymer electrolyte especilly in compatibility with high voltage caothde and lithium metal anode. By a LSV scan of the SPEs that sandwiched between stainless steel and Li metal (Fig. S9), the Dual-Li SPEs are stable up to 4.5 V vs Li+/Li from the anodic scan part. Additionally, the steady-state current method was used to measure the tLi+ by sandwiching the electrolyte membrane between two lithium metal electrodes. The time dependence response of direct current polarization of SPEs membranes is depicted in Fig. S10 and 3c. The measured values of the parameters are summarized in Table S1. Generally, tLi+ is about 0.20 for the conventional liquid electrolyte (in detail, 0.22 for 1M LiPF6-EC/ DEC) and about 0.35 for the generally obtained PEO based polymer electrolyte [35]. By contrast, Dual-Li SPEs shows a quasi 14
single-ion conductive state of tLi+ = 0.79, which is very different to the dual ion conductors [36]. For comparison, the “salt-in-polymer” type LiTFSI-PVFM composite has been prepared via mixing LiTFSI with PVFM polymer and modulating the total Li+ molar content to be 1.5 mol L−1, which demonstrates a half tLi+ value 0.40 of Dual-Li SPEs.
Fig. 4. (a) Impedance spectra of Li/ Dual-Li SPEs/ Li symmetric battery for different aging time at room temperature. (b) Impedance spectra of Li/ Dual-Li SPEs/ Li symmetric battery before cycling and after 100 cycles. XPS characterization of SEI film at the lithium surface after 10 cycles of plating and stripping in Li/ Li symmetrical cell. F 1s spectra of the SEI film in (c) Dual-Li SPEs and (d) LiTFSI-PVFM SPEs. (e) Voltage profiles of the lithium plating/ stripping in a Li/ Li symmetrical cell with LiTFSI-PVFM and Dual-Li SPEs at current density of 0.10 mA cm−2, at 30°C. SEM images of Li plating on lithium metal with (f) Dual-Li SPEs and (g) LiTFSI-PVFM SPEs in Li/ Li cells. (h) Schematic of the electrochemical deposition behavior of the Li metal anode with Dual-Li SPEs. 15
To study the compatibility of the “salt-with-salt” Dual-Li SPEs and the lithium anode, the interfacial stability has also been verified by EIS after long aging time (Fig. 4a). The bulk resistance (Rb) is stable before and after interior stabilization, while the interfacial resistance (Ri) decreases in 40 days and remains stable till 100 days. Furthermore, the EIS of Li/ Dual-Li SPEs/ Li cells before and after plating/ stripping in Fig. 4b shows no obvious change in Rb and Ri, indicating the stable interfacial contact between Dual-Li SPEs and Li metal electrodes. Therefore, the Dual-Li SPEs has an outstanding stability with aging time and an excellent interfacial compatibility toward Lithium metal electrodes. As the XPS spectra shown in Fig. 4c and d, only one peak of LiF (684.79 eV) emerges in the SEI between Dual-Li SPEs and lithium metal anode. While, -C-F peak at 688.80 eV is also observed on SEI with LiTFSI-PVFM polymer. The fundamental difference indicates that the complete coordination of LiTFSI into the polymer matrix of Dual-Li SPEs ,leading to the suppressed TFSI− anion diffusion and its side reaction accordingly in the electrolyte, which is in agreement with the obtained high Li ion transference number of Dual-Li SPEs and the improved interfacial compatibility between Dual-Li SPEs and Li metal anode. The long-term electrochemical stability of Dual-Li SPEs solid electrolytes with lithium metal anode was evaluated by using plating/ stripping test of symmetrical Li/ Li cells at a constant current density of 0.10 mA cm−2. The cell with the Dual-Li SPEs exhibited excellent stability with a nearly constant voltage polarization of 80 mV during the 1500 h cycling at 30 °C (Fig. 4e). A smooth surface of lithium metal is observed in Li/ Li cells after the repeated plating and stripping with Dual-Li SPEs membrane (Fig. 4f). However, the surface of Li shows a typical dendrite-like morphology in Li/ Li cell with LiTFSI-PVFM (Fig. 4g), leading to an accelerated cell death less than 16
420 h. The ion transport near the anode is driven by the electric field according to the space-charge theory, thus the suppressed anion mobility in SPEs inhibits the spatial gradient of charge and favorites much more homogeneous distribution of Li+ ion on the interface [37, 38], ending up with a uniform Li deposition on the lithium anode [33, 39]. Moreover, the “salt-with-salt” electrolyte with the concentrated lithium ions contributes to the significantly delayed dendrite nucleation under a certain current density based on Sand’s time mode [40, 41]. As shown in Fig. 4h, the Dual-Li SPEs rubbery membrane keep intimate contact with lithium anode, the homogeneous nucleation of lithium deposition is significantly promoted on the interface.
Fig. 5. Performance of Li/ Dual-Li SPEs/ LiCoO2 coin cell at a rate of 0.10 C between 3.0-4.4 V: (a) Charge-discharge voltage profiles for the first cycle. (b) Discharge capacity and coulombic efficiency for 200 cycles. (c) Impedance spectra of Li/ Dual-Li SPEs/ LiCoO2 coin cells and cycle. The rate performance of Li/ Dual-Li SPEs/ LiCoO2 coin cell between 3.0-4.4 V at 0.10 C, 0.20 C and 0.50 C: (d) charge-discharge voltage profiles and (e) discharge capacity. The Li/ Dual-Li SPEs/ LiCoO2 batteries were assembled and charged and discharged between 17
3.0 and 4.4 V at 0.10 C current density (where 1.0 C = 170 mAh g−1) as shown in Fig. 5a and b. The cells exhibit an initial discharge capacity of 165 mAh g−1 and high initial coulombic efficiency of 97%, moreover the cells show a high capacity retention of 76% after 200 cycles. In addition, a pair of weak redox peaks presents at high voltage in the charge and discharge curves may result from phase transition between ordered and disordered lithium ion arrangements in the CoO2 framework of LiCoO2 electrode [42]. Moreover, the EIS measurements of the Li/ Dual-Li SPEs/ LiCoO2 cells before and after cycle were performed as shown in Fig. 5c. Bulk resistance of the cell is stable during cycling. The irregular semicircles in the middle frequency ascribes to the charge-transfer resistance at interface, which decreases as the contact with electrodes is improved with the electrodes at initial cycling. However, the interface resistance increases in the subsequent cycles, which leads to the gradual decline of discharge capacity. The rate performance of the Li/ Dual-Li SPEs/ LiCoO2 batteries has been tested between 3.0-4.4 V at 0.10 C, 0.20 C and 0.50 C for five cycles respectively, and finally cycled at 0.10 C, as shown in Fig. 5d and e. The cell delivers the discharge capacity of 153 mAh g-1 at 0.20 C and 142 mAh g-1 at 0.50 C, which remains 94.4% and 87.7% of the discharge capacity at 0.10 C, respectively.
18
Fig. 6. Photographs for the soft package Li/ Dual-Li SPEs/ LiCoO2 battery that powers light-emitting diode (LEDs) (a) before and (b) after bending; (c) the voltage curve of battery during bending. Photographs for the soft package Li/ Dual-Li SPEs/ LiCoO2 battery that powers LEDs (d) before and (e) after punching; (f) the voltage and temperature curve during punching of batteries after 20 cycles. The soft-package lithium batteries using lithium metal anode have been assembled and tested as shown in Fig. 6a and b. When the battery was bended or severely wrinkled, it can still be able to light up a commercial blue LEDs lamp without any electrical failure and the cell voltage remains stable around 3.92 V. During bending, the strong and high elastic Dual-Li SPEs will maintain integrity and keep intimately connected with cathode and anode, avoiding the short circuit of the battery. Moreover, the fully charged pouch cells with LiCoO2 cathode and lithium metal anode after 20 cycles between 3.0 and 4.4 V at 0.10 C (shown in Fig. S10) performed the nail penetration tests. No smoke or burning occurs during punching the steel needle into the pouch cell with Dual-Li SPEs. The cell voltage and surface temperature profiles of the pouch cells during the nail penetration test are shown in Fig. 6f. The temperature of the cell kept stable during the punching. Additionally, the 19
voltage of the battery dropped to nearly zero when the needle was punched through the cell, and then recovered to the initial value of 3.94 V after the needle was removed. The pierced soft-package lithium battery is still able to light up LEDs lamp as shown in Fig. 6d and e. The phenomena demonstrate that the Dual-Li SPEs with high deformation capacity shows self-healing properties during punching, preventing the lithium metal from the open air and avoiding severe side reaction. Moreover, compact lithium deposition guided by the Dual-Li SPEs membrane during cycling is also responsible for the improved safety of the lithium metal batteries. Dual-Li SPEs based on single lithium-ion conductive salt LiPVFM show a significant increase on concentration of the mobile charge carriers, contributing to high ionic conductivity and transference number tLi+ of nearly unity. Additionally, there are other outstanding properties as the electrolytes for solid state lithium metal batteries, including (1) the electrochemical stability with respect to the cathodes, (2) intimate contact with electrode temporally, (3) homogeneous distribution of spatial ionic current guiding the compact lithium planting, and (4) high mechanical strength suppressing the dendrite growth. This work offers a novel strategy to develop flexible solid electrolytes membrane for lithium metal batteries. 4. Conclusion We proposed a “Salt-with-Salt” strategy to achieve high lithium concentration flexible solid electrolyte membrane based on single lithium-ion conductor LiPVFM. The dominant ether ring groups in LiPVFM are confirmed to possess a high solubility and good dissociation of lithium salts. The strong interaction between LiPVFM and LiTFSI improves polymer chain relaxation, but also suppresses TFSI− anion diffusion. Therefore, the introduction of LiTFSI improves the concentration and migration kinetics of the charge carriers Li+ in the composite electrolyte, leading to high ionic 20
conductivity of 5.7 × 10−4 S cm−1 at 25 °C, high lithium ion transference number of 0.79. Applying Dual-Li SPEs in Li/ LiCoO2 metal batteries, they show the cycle stability in 200 cycles between 3.0 and 4.4 V, which is contributed to the chemical and electrochemical stability of Dual-Li SPEs between high voltage LiCoO2 cathode and Lithium metal anode. Moreover the Li/ Dual-Li SPEs/ LiCoO2 metal pouch cells shows high flexibility and safety in the bending and nail penetration tests due to the excellent elasticity and deformation strength of Dual-Li SPEs. The “Salt-with-Salt” Dual-Li SPEs provides a promising candidate of the electrolytes membrane for high-energy-density lithium metal batteries. Acknowledgements This work was financially supported by the National Key R&D Program of China (Grant No. 2018YFB0104300) and the Beijing Municipal Science and Technology Project (Grant No. Z181100004518003). References [1] D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries, Nat. Nanotechnol., 12 (2017) 194-206. https://doi.org/10.1038/nnano.2017.16 [2] X. Shen, X. B. Cheng, P. Shi, J. Q. Huang, X. Q. Zhang, C. Yan, T. Li, Q. Zhang, Lithium-matrix composite anode protected by a solid electrolyte layer for stable lithium metal batteries, J. Energy Chem., 37 (2019) 29-34. https://doi.org/10.1016/j.jechem.2018.11.016 [3] X. B. Cheng, C. Z. Zhao, Y. X. Yao, H. Liu, Q. Zhang, Recent advances in energy chemistry between solid-state electrolyte and safe lithium-metal anodes, Chem, 5 (2019) 74-96. https://doi.org/10.1016/j.chempr.2018.12.002 [4] H. L Dai, K. Xi, X. Liu, C. Lai, S. Q. Zhang, Cationic Surfactant-Based Electrolyte Additives for 21
Uniform Lithium Deposition via Lithiophobic Repulsion Mechanisms, J. Am. Chem. Soc., 140 (2018) 17515-17521. https://doi.org/10.1021/jacs.8b08963 [5] R. P. Liu, P. He, Z. R. Wu, F. Guo, B. Huang, Q. Wang, Z. Y. Huang, C. A. Wang, Y. T. Li, PEO/hollow mesoporous polymer spheres composites as electrolyte for all solid state lithium ion battery, J. Electroanal. Chem., 822 (2018) 105-111. https://doi.org/10.1016/j.jelechem.2018.05.021 [6] A. Manthiram, X. Yu, S. Wang, Lithium battery chemistries enabled by solid-state electrolytes, Nat. Rev. Mater., 2 (2017) 16103. https://doi.org/10.1038/natrevmats.2016.103 [7] C. W. Sun, J. Liu, Y. D. Gong, D. P. Wilkinson, J. J. Zhang, Recent advances in all-solid-state rechargeable
lithium
batteries,
Nano
Energy,
33
(2017)
363-386.
http://doi.org/10.1016/j.nanoen.2017.01.028 [8] X. Y. Ban, W. Q. Zhang, N. Chen, C. W. Sun, A High-Performance and Durable Poly(ethylene oxide)-Based Composite Solid Electrolyte for All Solid-State Lithium Battery, J. Phys. Chem. C, 122 (2018) 9852-9858. https://doi.org/10.1021/acs.jpcc.8b02556 [9] S. Chen, J. Wang, Z. Zhang, L. Wu, L. Yao, Z. Wei, Y. Deng, D. Xie, X. Yao, X. Xu, In-situ preparation of poly(ethylene oxide)/Li3PS4 hybrid polymer electrolyte with good nanofiller distribution for rechargeable solid-state lithium batteries, J. Power Sources, 387 (2018) 72-80. https://doi.org/10.1016/j.jpowsour.2018.03.016 [10] W. Q. Zhang, J. H. Nie, F. Li, Z. L. Wang, C. W. Sun, A durable and safe solid-state lithium battery
with
a
hybrid
electrolyte
membrane,
Nano
Energy,
45
(2018)
413-419.
https://doi.org/10.1016/j.nanoen.2018.01.028 [11] Q. Zhou, J. Ma, S. M. Dong, X. F. Li, G. L. Cui, Intermolecular Chemistry in Solid Polymer Electrolytes for High-Energy-Density Lithium Batteries, Adv. Mater., (2019) 1902029. 22
https://doi.org/10.1002/adma.201902029 [12] L. Long, S. Wang, M. Xiao, Y. Meng, Polymer electrolytes for lithium polymer batteries, J. Mater. Chem. A, 4 (2016) 10038-10069. https://doi.org/10.1039/c6ta02621d [13] T. Itoh, Y. Mitsuda, T. Ebina, T. Uno, M. Kubo, Solid polymer electrolytes composed of polyanionic
lithium
salts
and
polyethers,
J.
Power
Sources,
189
(2009)
531-535.
https://doi.org/10.1016/j.jpowsour.2008.10.113 [14] Z. Xue, D. He, X. Xie, Poly(ethylene oxide)-based electrolytes for lithium-ion batteries, J. Mater. Chem. A, 3 (2015) 19218-19253. https://doi.org/10.1039/c5ta03471j [15] S. S. Zhang, G. X. Wan, Single-ion conduction and lithium battery application for ionomeric cross-linking
polymer,
J.
Appl.
Polym.
Sci.,
48
(1993)
405-409.
https://doi.org/10.1002/app.1993.070480304 [16] Y. Feng, R. Tan, Y. Zhao, R. Gao, L. Yang, J. Yang, H. Li, G. Zhou, H. Chen, F. Pan, Insight into fast ion migration kinetics of a new hybrid single Li-ion conductor based on aluminate complexes for solid-state Li-ion batteries, Nanoscale, 10 (2018) 5975-5984. https://doi.org/10.1039/c8nr00573g [17] Y. Kim, S. J. Kwon, H. Jang, B. M. Jung, S. B. Lee, U. Choi, High ion conducting nanohybrid solid polymer electrolytes via single-ion conducting mesoporous organosilica in poly(ethylene oxide), Chem. Mater., 29 (2017) 4401-4410. https://doi.org/10.1021/acs.chemmater.7b00879 [18] Y. Chen, Y. Tian, Z. Li, N. Zhang, D. Zeng, G. Xu, Y. Zhang, Y. Sun, H. Ke, H. Cheng, An AB alternating diblock single ion conducting polymer electrolyte membrane for all-solid-state lithium metal
secondary
batteries,
J.
Membr.
Sci.,
566
(2018)
181-189.
https://doi.org/10.1016/j.memsci.2018.09.013 [19] Y. Ren, F. Lian, Y. Wen, H. Y. Guan, A novel PVFM based membranes prepared via phase 23
inversion
method
and
its
application
in
GPE,
Polymer,
54
(2013)
4807-4813.
http://doi.org/10.1016/j.polymer.2013.07.013 [20] H. Y. Guan, F. Lian, K. Xi, Y. Ren, J. L. Sun, R. Kumar, Polyvinyl formal based gel polymer electrolyte prepared using initiator free in-situ thermal polymerization method, J. Power Sources, 245 (2014) 95-100. https://doi.org/10.1016/j.jpowsour.2013.06.120 [21] F. Lian, H. Y. Guan, Y. Wen, X. R. Pan, Polyvinyl formal based single-ion conductor membranes as polymer electrolytes for lithium ion batteries, J. Membr. Sci. 469 (2014) 67-72. http://doi.org/10.1016/j.memsci.2014.05.065 [22] H. Zhang, F. Lian, L. Bai, N. Meng, C. Xu, Developing lithiated polyvinyl formal based single-ion conductor membrane with a significantly improved ionic conductivity as solid-state electrolyte
for
batteries,
J.
Membr.
Sci.
552
(2018)
349-356.
https://doi.org/10.1016/j.memsci.2018.02.032 [23] F. Neese, The ORCA program system, Wiley Interdiscip. Rev.-Comput. Mol. Sci., 2 (2012) 73-78. https://doi.org/10.1002/wcms.81 [24] T. Lu, F. W. Chen, Multiwfn: A multifunctional wavefunction analyzer, J. Comput. Chem., 33 (2012) 580-592. https://doi.org/10.1002/jcc.22885 [25] S. Das, A. Ghosh, Effect of plasticizers on ionic conductivity and dielectric relaxation of PEO-LiClO4
polymer
electrolyte,
Electrochim.
Acta,
171
(2015)
59-65.
https://doi.org/10.1016/j.electacta.2015.04.178 [26] H. Y. Guan, Z. B. Guo, J. J. Ding, F. Lian, Ionic conductivity and dielectric behavior of novel poly(vinyl formal)-based single-ion-conductor polymer electrolytes, J. Appl. Polym. Sci., 133 (2016) 43510. https://doi.org/10.1002/app.43510 24
[27] C. H. Wang, Y. F. Yang, X. J. Liu, H. Zhong, H. Xu, Z. B. Xu, H. X. Shao, F. Ding, Suppression of lithium dendrite formation by using LAGP-PEO (LiTFSI) composite solid electrolyte and lithium metal anode modified by PEO (LiTFSI) in all-solid-state lithium batteries, ACS Appl. Mater. Interfaces, 9 (2017) 13694-13702. https://doi.org/10.1021/acsami.7b00336 [28] F. L. Huang, Y. F. Xu, B. Peng, Y. F. Su, F. Jiang, Y. L. Hsieh, Q. F. Wei, Coaxial electrospun cellulose-core fluoropolymer-shell fibrous membrane from recycled cigarette filter as separator for high performance lithium-ion battery, ACS Sustain. Chem. Eng., 3 (2015) 932-940. https://doi.org/10.1021/acssuschemeng.5b00032 [29] H. P. Wu, Y. Cao, H. P. Su, C. Wang, Tough gel electrolyte using double polymer network design for the safe, sable cycling of lithium metal anode, Angew. Chem.-Int. Edit., 57 (2018) 1361-1365. https://doi.org/10.1002/anie.201709774 [30] K. Pozyczka, M. Marzantowicz, J. R. Dygas, F. Krok, Ionic conductivity and lithium transference number of poly(ethylene oxide):LiTFSI system, Electrochim. Acta, 227 (2017) 127-135. https://doi.org/10.1016/j.electacta.2016.12.172 [31] Z. Q. Zhu, M. L. Hong, D. S. Guo, J. F. Shi, Z. L. Tao, J. Chen, All-solid-state lithium organic battery with composite polymer electrolyte and pillar[5]quinone cathode, J. Am. Chem. Soc., 136 (2014) 16461-16464. https://doi.org/10.1021/ja507852t [32] K. Fu, Y. H. Gong, J. Q. Dai, A. Gong, X. G. Han, Y. G. Yao, C. W. Wang, Y. B. Wang, Y. N. Chen, C. Y. Yan, Y. J. Li, E. D. Wachsman, L. B. Hu, Flexible, solid-state, ion-conducting membrane with 3D garnet nanofiber networks for lithium batteries, Proc. Natl. Acad. Sci. U. S. A., 113 (2016) 7094-7099. https://doi.org/10.1073/pnas.1600422113 [33] C. Z. Zhao, X. Q. Zhang, X. B. Cheng, R. Zhang, R. Xu, P. Y. Chen, H. J. Peng, J. Q. Huang, Q. 25
Zhang, An anion-immobilized composite electrolyte for dendrite-free lithium metal anodes, Proc. Natl. Acad. Sci. U. S. A., 114 (2017) 11069-11074. https://doi.org/10.1073/pnas.1708489114 [34] S. Ramesh, K. C. Wong, Conductivity, dielectric behaviour and thermal stability studies of lithium ion dissociation in poly(methyl methacrylate)-based gel polymer electrolytes, Ionics, 15 (2009) 249-254. https://doi.org/10.1007/s11581-008-0268-2 [35] Q. Ma, H. Zhang, C. W. Zhou, L. P. Zheng, P. F. Cheng, J. Nie, W. F. Feng, Y. S. Hu, H. Li, X. J. Huang, L. Q. Chen, M. Armand, Z. B. Zhou, Single lithium-ion conducting polymer electrolytes based on a super-delocalized polyanion, Angew. Chem.-Int. Edit., 55 (2016) 2521-2525. https://doi.org/10.1002/anie.201509299 [36] W. Liu, N. Liu, J. Sun, P. C. Hsu, Y. Z. Li, H. W. Lee, Y. Cui, Ionic conductivity enhancement of polymer electrolytes with ceramic nanowire fillers, Nano Lett., 15 (2015) 2740-2745. https://doi.org/10.1021/acs.nanolett.5b00600 [37] M. D. Tikekar, S. Choudhury, Z. Y. Tu, L. A. Archer, Design principles for electrolytes and interfaces
for
stable
lithium-metal
batteries,
Nat.
Energy,
1
(2016)
1-7.
https://doi.org/10.1038/nenergy.2016.114 [38] Z. Y. Tu, M. J. Zachman, S. Choudhury, S. Y. Wei, L. Ma, Y. Yang, L. F. Kourkoutis, L. A. Archer, Nanoporous hybrid electrolytes for high-energy batteries based on reactive metal anodes, Adv. Energy Mater., 7 (2017) 1602367. https://doi.org/10.1002/aenm.201602367 [39] C. Q. Niu, J. Liu, G. P. Chen, C. Liu, T. Qian, J. J. Zhang, B. K. Cao, W. Y. Shang, Y. B. Chen, J. L. Han, J. Du, Y. Chen, Anion-regulated solid polymer electrolyte enhances the stable deposition of lithium
ion
for
lithium
metal
batteries,
J.
Power
Sources,
417
(2019)
70-75.
https://doi.org/10.1016/j.jpowsour.2019.02.004 26
[40] R. Akolkar, Mathematical model of the dendritic growth during lithium electrodeposition, J. Power Sources, 232 (2013) 23-28. https://doi.org/10.1016/j.jpowsour.2013.01.014 [41] L. L. Fan, X. F. Li, Recent advances in effective protection of sodium metal anode, Nano Energy, 53 (2018) 630-642. https://doi.org/10.1016/j.nanoen.2018.09.017 [42] J. N. Reimers, J. R. Dahn, Electrochemical and In Situ X-ray Diffraction Studies of Lithium Intercalation
in
LixCoO2,
J.
Electrochem.
Soc.,
139
(1992)
2091-1097.
https://doi.org/10.1149/1.2221184
27
●Salt-with-salt concentrated SPEs was proposed on base of single Li+ conductor LiPVFM ●LiTFSI coordinates in LiPVFM matrix to obtain the composite with a decreasing Tg ●The dissolved LiTFSI exhibits strong interaction with LiPVFM via ether ring groups ●Dual-Li SPEs show high ionic conductivity, tLi+, elasticity and stable with Li metal
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0376-7388(19)33440-4
DOI:
https://doi.org/10.1016/j.memsci.2019.117768
Reference:
MEMSCI 117768
To appear in:
Journal of Membrane Science
Received Date: 8 November 2019 Revised Date:
18 December 2019
Accepted Date: 19 December 2019
Please cite this article as: N. Meng, H. Zhang, S. Lianli, F. Lian, Salt-with-salt, a novel strategy to design the flexible solid electrolyte membrane for highly safe lithium metal batteries, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2019.117768. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Statement Nan Meng: Investigation, Data Curation, Validation, Writing - Original Draft, Visualization. Hongnan Zhang: Methodology, Formal analysis, Investigation. Shuyi Lianli: Validation, Data Curation. Fang Lian: Conceptualization, Writing - Review & Editing, Supervision, Project administration.
Salt-with-Salt, A Novel Strategy to Design the Flexible Solid Electrolyte membrane for Highly Safe Lithium Metal Batteries
Nan Meng, Hongnan Zhang, Shuyi Lianli, Fang Lian*
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
*Corresponding author: E-mail address: [email protected] (Dr. F.Lian) Tel.: +86 10 82377985 Fax: +86 10 82377985.
1
ABSTRACT As one of the outstanding candidates for solid polymer electrolytes (SPEs), Poly(vinyl formal) (PVFM) based SPEs are facing higher requirements toward practical application such as further improving ambient ionic conductivity, lithium ion transference number, and suppressing interfacial passivation on the electrode. Based on lithiated PVFM single lithium-ion conductor (LiPVFM), a novel salt-with-salt concentrated flexible SPEs membrane have been prepared herein via intermolecular design as Dual-Li SPEs for highly safe solid-state lithium metal batteries. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dissolves in LiPVFM matrix to obtain the composite unexpectedly with a decreased glass transition temperature (Tg) and an inherited amorphous structure. Moreover, lithium ions are dissociated by the dominant ether ring groups in LiPVFM, improving the charge concentration of the electrolyte. Therefore, Dual-Li SPEs is achieved with high ionic conductivity of 5.7 × 10−4 S cm−1 at 25 °C, high lithium ion transference number of 0.79, excellent elasticity and stability on the interface of lithium metal anode. Li/ Dual-Li SPEs/ LiCoO2 batteries process a high reversible capacity in 200 cycles, high coulombic efficiency of 99% and high safety in bending and punching test. The work paves a new way for developing solid polymer electrolyte membrane with ideal integrated properties for lithium metal batteries. Keywords: solid-state batteries; polymer electrolyte membrane; lithium metal anode; safety 1. Introduction Applying of solid-state electrolyte (SSE) is an effective route to promote the safety of the high-energy density batteries using lithium metal as anode [1-4]. Among the SSEs, the solid polymer electrolytes (SPEs) attract more attention because (1) their light weight is necessary to further improve the gravimetric energy density (Wh kg-1); (2) good flexibility to develop roll to roll processing, and (3) easy to form films and good adhesion for allowing a low-impedance contact with 2
the solid electrodes. So far SPEs are usually achieved via dissolving lithium salt in the polymer matrix (known as “salt-in-polymer”). Poly(ethylene oxide) (PEO) is one of the most popular species, which has been extensively studied to address its key problem of low ionic conductivity at room temperature due to its high crystallinity [5-7], such as introducing inorganic powders to improve the mobility of polymer segments [8-10]. Moreover, the interaction of Li+ ions with the molecular chain of SPEs accelerates the dissolution of lithium salts and dissociation of Li+ ions. As known, an increase of concentration of lithium salt up to M:O (the mole ratio of salt cation to ether oxygen) values of ~1:16 facilitates access to the maximum conductivity of PEO-based electrolyte [11, 12]. The effect of further increase of the Li+ concentration above 1.3 mol L-1 will be unfortunately offset by a decrease of ionic mobility, since the viscosity especially glass transition temperature (Tg) of the mixture rises rapidly with the salt content [13]. Additionally, such salt-in-polymer electrolyte have dual ion conductivity of the anion X and the lithium ions, in which Li+ diffusion induced by segmental motion is much more difficult than the counter anions. About 80% of the ionic current is carried by anions, leading to the high concentration polarization and low Li+ ion transference number of salt-in-polymer electrolyte [14]. In recent years, novel single-ion conductors has been prepared by fixing the anions such as alkyl carboxylate (RCO2−) [15] or sulfonate (RSO3−) [16] to the polymeric backbones, in which the lithium-ion transference number (tLi+) is close to one. However, these single-ion conductors still suffer from a low ionic conductivity due to their low carrier concentration and lack of Li+ conductive group. Therefore in order to improve the ionic conductivity of single-ion polymer electrolyte, ether oxygen groups were introduced in recent studies as ionic conducting group into the matrix by PEO
3
blending or EO grafting [17, 18]. However, there is still difficulty to increase simultaneously the carrier concentration and the mechanical strength with high deformation capacity of SPEs. Poly(vinyl formal) (PVFM) polymer is an outstanding candidate for SPEs due to its low crystallinity, unique film-forming properties and tunable intramolecular structure. In our previous study, PVFM based polymer electrolytes have been achieved via the crosslinking and in-situ polymerizing, which dramatically improves their chemical and electrochemical stabilities [19, 20]. Furthermore, PVFM based single-ion SPEs (shorted for LiPVFM) have been proposed with wide electrochemical stability window and high tLi+ of 0.87, in which the Li+ coordinated PVFM via coupling suitable amount of oxalate-chelated borate units to vinyl hydroxyl in the host polymer backbone [21, 22]. Nevertheless, the ionic conductivity at room temperature 3.3×10−5 S cm−1 should be further improved to meet an essential requirement for the ambient one ≥10−4 S cm−1. Moreover, LiPVFM membrane is deserved to be lack of elastic deformation even the stress is performed up to 30 MPa, which is ascribed to ether ring as the major group accounting for 62.3% in the structure and the resultant tight structural network. In this work, it is proved that the dominant ether ring groups enable LiPVFM to be capable of a high
solubility
and
good
dissociation
of
lithium
salts.
Herein,
lithium
bis(trifluoromethylsulfonyl)imide (LiTFSI) with high degree of dissociation and extensive negative charge delocalization is employed to dissolve in the single-ion conductive salt LiPVFM to achieve “Salt-with-Salt” highly concentrated flexible solid electrolyte membrane, named as Dual-Li SPEs. The dissolved lithium salts decrease Tg of the composites, relax the original tight structure and activate the segmental motion in LiPVFM. So the Dual-Li SPEs are achieved with a high ionic conductivity at ambient temperature, high lithium-ion transference number, and good elastic 4
performance. Moreover, the Dual-Li SPEs shows the stable interface with lithium metal anode over a long cycle life, ending up with a highly safe lithium metal battery. 2. Experimental 2.1. Preparation of solid polymer electrolyte membrane The single ion conductor LiPVFM is obtained as reported in our previous work [21, 22] and described in the Supplementary information. Subsequently, LiPVFM and a certain amount of LiTFSI (Aladdin) were dissolved into dimethyl sulfoxide (DMSO, Sinopharm Chemical Reagent), in which the molar ratio of vinyl acetal in PVFM and LiTFSI is optimized to be 30:4. The above solution was stirred at 50 °C for 12 h, cooled to room temperature, and then cast into a membrane. Finally, the SPEs membranes were dried under vacuum at 70 °C for 24 h to evaporate the solvent and transferred to an argon-filled glove box, marked as Dual-Li SPEs. Additionally, the membrane consisting of LiPVFM with less or without LiTFSI prepared via the aforementioned method has also been shown in which the molar ratio of vinyl acetal and LiTFSI is fixed at 30:3, 30:2, 30:1 and 30:0, named as Dual-Li-3, Dual-Li-2, Dual-Li-1 and LiPVFM, respectively. The thickness of SPEs has been listed in Table S2. While, the self supporting membranes can’t be achieved when higher content LiTFSI is introduced in LiPVFM-based composite electrolyte. For comparison, the LiTFSI-PEO polymer electrolyte was prepared by dissolving PEO (molecular weight of ~300,000, Sigma Aldrich) and LiTFSI in acetonitrile (J&K Scientific) with a PEO/ LiTFSI mole ratio of 12 to ensure that LiTFSI-PEO has the same lithium concentration of 1.5 mol L−1 as the aformentioned Dual-Li samples. Then, the mixture was stirred continually at 65 °C, coated on polytetrafluoroethylene plate with a doctor blade and dried in an argon-filled glove box for about 12 h to obtain LiTFSI-PEO polymer membrane. Finally, the above electrolyte membranes 5
were punched into circular pieces (d = 16 mm) for assembling cells. 2.2. Material analysis The morphology of the samples was characterized by a field emission scanning electron microscope (FESEM, Carl Zeiss SUPRA55). The X-ray diffraction (XRD) profile was obtained by XD2618N X-ray diffraction analyzer in scanning range between 10° to 60°. Fourier transform infrared spectroscopy (FT-IR) experiment of polymer electrolytes was performed on NEXUS FT-IR670 spectrometer with a frequency range of 400-4000 cm−1. The electrostatic potential of the ether ring was calculated by ORCA [23] and Multiwfn [24] as described in Supplementary information. Thermogravimetric analysis (TGA, TGA2050) experiments were carried out in the temperature range from room temperature to 600 °C under argon atmosphere at a heating rate of 10 °C min−1. Dynamic mechanical analysis (DMA, Waters Q800) were carried out in the temperature range from -100 to 300 °C at frequency of 1 Hz. The stress and strain curves of polymer electrolytes with a size of 1 cm × 5 cm were carried out on a tensile testing apparatus (INSTRON, 4465) at a stretching speed of 10 mm min−1 at room temperature. 2.3. Electrochemical characterization The sandwich cells of SS (stainless steel)/ SPEs/ SS were used for the test of ionic conductivity by using the alternating current impedance method. Impedance data were obtained with an electrochemical working station VersaSTAT3 in the frequency range of 10−2-105 Hz. The ionic conductivity of the SPEs was calculated based on the following equation (1): σ=L/(RbS)
(1)
Where σ is the ionic conductivity, Rb is the bulk resistance and L and S are the thickness and area of the electrolyte , respectively. 6
The relationships between the complex impedance, dielectric constant (εr), dielectric loss (εi), and loss tangent (tan δ) were calculated as in Supplementary information [25, 26]. The electrochemical stability window of the SPEs was evaluated by linear sweep voltammetry (LSV) in the range from open circuit voltage to 5.5 V at the rate of 5.0 mV s−1 with SS/ SPEs/ Li cells. Lithium-ion transference number was calculated by the chronoamperometry test on the Li/ SPEs/ Li cells with an applied voltage of 10 mV and was determined as in Supplementary information. The symmetrical Li/ SPEs/ Li cells was assembled to process Li stripping-deposition measurements (Wuhan Land Electronic Co. Ltd., CT2001A, China) at 0.10 mA cm-2 and a stripping-deposition time of 20 min. In the assembling process of the cells, propylene carbonate (PC) about 5.0 wt% of the related membrane was dropped to allow the polymer electrolyte membrane spreading out and attachable to the electrode at room temperature. Interfacial resistance of the lithium and SPEs was analyzed by impedance tests before and after cycles with potential amplitude of 5.0 mV from 105 Hz to 10−2 Hz, respectively. The solid electrolyte interphase (SEI) between Dual-Li SPEs and lithium metal anode was characteriazed by X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTRADLD). The solid-state cells (CR2032) were assembled for electrochemical test using lithium metal anode and LiCoO2 cathode. The LiCoO2 electrode was composed of 70 wt% LiCoO2, 15 wt% PVDF, 5.0 wt% Dual-Li SPEs and 10 wt% carbon black, in which the areal density of the active material was 2.8 mg cm−2. The Li/ SPEs/ LiCoO2 batteries were charged and discharged between 3.0 and 4.4 V at 0.10 C current density on LAND testing system (Wuhan LAND electronics Co., Ltd.) at 25 °C. The C rates in the electrochemical measurements are defined on the base of 1.0 C = 170 mA g−1. 2.4. Battery safety testing 7
The soft-package lithium batteries were assembled for safety test using lithium metal anode and LiCoO2 cathode. The pouch cells were charged to 4.3 V at 0.10 mA cm−2 current density on LAND testing system at 25 °C. The cells were folded three times while texting its voltage. A commercial blue light-emitting diode (LED) lamp was lighted up during folding. In the punching test, the pouch cells with LiCoO2 cathode and lithium metal anode were cycled for 20 cycles between 3.0 and 4.4 V at 0.10 C on LAND testing system at 25 °C. The steel needle was punched into the cells while monitoring the voltage and temperature of the cells. In addition, the needle was pulled out as long as it pierced the cell. 3. Results and discussion
Scheme 1. A schematic of the preparation of Dual-Li SPEs membrane and solid state lithium metal cells. 8
The schematic of the preparation process of “Salt-with-Salt” highly concentrated flexible solid electrolytes derived from single-ion conductive salt LiPVFM is illustrated in Scheme 1. The optimal molar ratio of vinyl acetal in LiPVFM and LiTFSI is at 30:4, that is, the M:O value (the mole ratio of total Li+ from both LiPVFM and LITFSI to oxygen atom in vinyl acetal) is 1:12 in the composites. The dense, transparent and freestanding Dual-Li SPEs membranes were obtained with no detectable difference with LiPVFM in morphology. The typical images of surface and cross-section of the Dual-Li SPEs membranes are demonstrated in Fig. 1a-d. The membranes exhibit smooth surface, dense and homogeneous cross-section morphology as shown between two red dash lines in Fig. 1c and its enlarged view in Fig. 1d. From the energy dispersive spectrometer (EDS) mapping of the cross-section (Fig. 1e and f) it can be seen that the yellow and pink dots with bright color assigned to the S and F elements respectively are uniformly distributed. The structure of the SPEs has been detected by XRD and has been shown in Fig. 1g and Fig. S1 in supplementary information. A broad diffraction peak near 20° and no diffraction peak of LiTFSI are detected in the XRD patterns, indicating the amorphous state of LiPVFM and Dual-Li SPEs, which is very different from that of PEO-based electrolytes owning several intense peaks and high crystallinity at room temperature [27]. The result indicates that LiTFSI dissolves completely and coordinates in LiPVFM to form an ionic transfer-convenient amorphous system. FT-IR spectra and zoom-in image with a frequency range of 1000-1300 cm−1 are shown in Fig. S2 and Fig. 1h, respectively. The peak at 3491 cm−1 is assigned to –OH stretching of alcoholic hydroxyls in LiPVFM. The characteristic frequency at 1733 cm−1 is ascribed to the C=O stretching in the vinyl acetate groups of LiPVFM [28]. The bands at 1243, 1179, 1133 and 1066 cm−1 represent the stretching mode of the C-O-C-O-C bonds in ether ring of LiPVFM. However, the locations of these bands shift due to 9
the inclusion of LiTFSI, demonstrating that the interaction between LiTFSI and the ether ring occurs. The oxygen in the ether ring with lone pair electrons facilitates its complexation with Li+. The electrostatic potential of the ether ring was calculated by Density Functional Theory. As shown in Fig. 1i the yellow areas representing negative electrostatic potential reveals that the oxygen atom of the ether ring groups in LiPVFM induces the interaction with Li+, which contributes to a high solubility and good dissociation of lithium salts in LiPVFM.
Fig. 1. Image and micrograph of the Dual-Li SPEs membrane. (a) Digital image, (b) surface, (c) (d) cross-section; EDS mapping of the portion selected: (e) S, (f) F. (g) XRD spectra of SPEs, (h) FT-IR spectra of the LiPVFM and Dual-Li SPEs series, (i) the calculated electrostatic potential of the ether ring. 10
Fig. 2. The physical properties of Dual-Li SPEs membrane. (a) DSC, (b) and (c) DMA. (d) TGA, (e) Stress-strain curves and (f) Stretching test. Differential scanning calorimetry (DSC) [29] and DMA was employed to analyze Tg of the “Salt-with-Salt” system. It is important to note that unlike the traditional “salt-in-polymer” PEO-based SPEs, the Dual-Li series shows a decreasing Tg value with an increase of lithium salt concentration (Fig. S3). As shown in Fig. 2a LiPVFM presents Tg = 62 °C, while Dual-Li SPEs exhibit the complete amorphous state at ~20 °C. The temperature-sweep test of DMA in Fig. 2b and c indicates the maximum of the loss factor for LiPVFM and Dual-Li SPEs appears at the temperature of 63.3 °C and 25.5 °C, respectively, which is in agreement with Tg analysis by DSC. Moreover, as tan δ is associated with the movement of the polymer segment and the relaxation transition peak reflects the glass-rubber transition, the broad relaxation transition peak in tan δ curves demonstrates that the much more amorphous phase dominates in Dual-Li SPEs. The result corresponds to an increasing ratio of flexible chain segments and an improved ionic migration kinetics, which is contributed to the intermolecular interaction between LiPVFM and LiTFSI in Dual-Li SPEs. 11
The thermal stability of SPEs is a crucial parameter to ensure the battery safety. Fig. 2d and Fig. S5 demonstrate the TGA of SPEs. The liquid electrolyte, namely, 1.0 M LiPF6 in ethylene carbonate/ dimethyl carbonate/ diethyl carbonate (EC: DMC: DEC) = 1: 1: 1 volume ratio evaporates rapidly at the beginning of heating, whereas the weight loss in Dual-Li SPEs is observed above 140 °C. The slope near 265 °C represents the decomposition of the oxalate-chelated borate structure. Moreover, the weight loss at around 325 °C ascribes to the decomposition of LiTFSI. The results show that Dual-Li SPEs possess the superior thermal stability. Moreover, the decrease of decomposition temperature in compare with LiTFSI (375 °C) and LiPVFM (380 °C) further confirms their strong interaction in Dual-Li SPEs. The tensile stress/ strain curves of the membranes in Fig. 2e and Fig. S4 demonstrate that all the SPEs samples possess a high mechanical strength. LiPVFM shows the high ultimate tensile strength ≥ 40 MPa but a low elongation-at-break value ≤ 6.0% [22]. While, the Dual-Li SPEs exhibit an ultimate tensile strength 25 MPa and a significant improved elongation-at-break value 266.7%. In detail, Dual-Li SPEs membranes can stretch about 2.5 times their original length before break as shown in Fig. 2f, which exhibits a prospect in application as the flexible solid electrolyte. The Young modulus of Dual-Li SPEs is determined to 2.0 GPa by nano indentation as shown in Fig. S6. The amorphous state and flexible chain induced by strong interaction of LiPVFM with LiTFSI is responsible for an excellent elastic property and a high structural compliance of the Dual-Li SPEs membrane. Fig. 3a shows the ionic conductivity of SPEs varied from 25 to 60 °C. Symmetric cells were utilized for the measurements, consisting of two identical stainless steel foils as blocking electrodes. The Arrhenius plots of ionic conductivity were obtained correspondingly along with a well-fined 12
electrochemical impedance spectroscopy (EIS) spectrum (Fig. S7). Dual-Li SPEs exhibits the high ionic conductivity of 5.7 × 10−4 S cm−1 at 25 °C and 1.4 × 10−3 S cm−1 at 60 °C. In compare with the traditional “salt-in-polymer” electrolytes such as LiTFSI-PEO and LiTFSI-LLZTO-PEO in the references, the Dual-Li SPEs exhibits notable improvements in ambient ionic conductivity as shown in Fig. 3b. What is more, the “salt-in-polymer” LiTFSI-PEO system with the same M:O value is confirmed in the reference to show relatively low ionic conductivity, since the concentrated Li salt astricts the mobility of ether group and significantly increases Tg of PEO [30]. While, as shown in Table S2, the ionic conductivity of the “salt-with-salt” electrolyte is improved with an increase of LiTFSI concentration, which is very different from traditional “salt-in-polymer” system.
Fig. 3. (a) Conductivity and the fitted results of SPEs between 25 and 60 °C, (b) A comparison of conductivity of polymer electrolytes in the representative works [30-32]. (c) Current-time plots of Li symmetrical cells with Dual-Li SPEs, (c inside) The impedance spectra before and after potential polarization, (d) A comparison of Li+ transference number of polymer electrolytes in the representative works [33]. Variation of (e) ε’ and (f) tan δ for different SPEs as a function of the 13
frequency at 30 °C (ε: dielectric constant). The dielectric permittivity including the stored and loss charges in polymer electrolytes was studied as in Fig. 3e and f. For Dual-Li SPEs, the relaxation peak shifts to high frequencies, even higher than 105 Hz, meanwhile the real part (ε’) of the dielectric constant is significantly higher than that of LiPVFM and LiTFSI-PEO, which presents evidence of an enhanced segmental mobility and concentrated charge carrier of Dual-Li SPEs [25]. Furthermore, over the frequency range of 10−2-102 Hz, the oscillating dipoles in the macromolecules tend to orient in the derection of the applied field, while the oscillating dipoles could not rotate rapidly with the rising of frequency, so no excess ion diffuses in the direction of the applied field, ending up with polarization because of the charge accumulation decrease [34], which leads to a drop in the value of dielectric constant. The low-frequency dispersion of ε’ aggravates in Dual-Li SPEs, declaring that the polymer possess the typical Li ion transport mechanism i.e. ionic hopping in couple with segment motion of the polymer [26]. The electrochemical window is also crucial for the polymer electrolyte especilly in compatibility with high voltage caothde and lithium metal anode. By a LSV scan of the SPEs that sandwiched between stainless steel and Li metal (Fig. S9), the Dual-Li SPEs are stable up to 4.5 V vs Li+/Li from the anodic scan part. Additionally, the steady-state current method was used to measure the tLi+ by sandwiching the electrolyte membrane between two lithium metal electrodes. The time dependence response of direct current polarization of SPEs membranes is depicted in Fig. S10 and 3c. The measured values of the parameters are summarized in Table S1. Generally, tLi+ is about 0.20 for the conventional liquid electrolyte (in detail, 0.22 for 1M LiPF6-EC/ DEC) and about 0.35 for the generally obtained PEO based polymer electrolyte [35]. By contrast, Dual-Li SPEs shows a quasi 14
single-ion conductive state of tLi+ = 0.79, which is very different to the dual ion conductors [36]. For comparison, the “salt-in-polymer” type LiTFSI-PVFM composite has been prepared via mixing LiTFSI with PVFM polymer and modulating the total Li+ molar content to be 1.5 mol L−1, which demonstrates a half tLi+ value 0.40 of Dual-Li SPEs.
Fig. 4. (a) Impedance spectra of Li/ Dual-Li SPEs/ Li symmetric battery for different aging time at room temperature. (b) Impedance spectra of Li/ Dual-Li SPEs/ Li symmetric battery before cycling and after 100 cycles. XPS characterization of SEI film at the lithium surface after 10 cycles of plating and stripping in Li/ Li symmetrical cell. F 1s spectra of the SEI film in (c) Dual-Li SPEs and (d) LiTFSI-PVFM SPEs. (e) Voltage profiles of the lithium plating/ stripping in a Li/ Li symmetrical cell with LiTFSI-PVFM and Dual-Li SPEs at current density of 0.10 mA cm−2, at 30°C. SEM images of Li plating on lithium metal with (f) Dual-Li SPEs and (g) LiTFSI-PVFM SPEs in Li/ Li cells. (h) Schematic of the electrochemical deposition behavior of the Li metal anode with Dual-Li SPEs. 15
To study the compatibility of the “salt-with-salt” Dual-Li SPEs and the lithium anode, the interfacial stability has also been verified by EIS after long aging time (Fig. 4a). The bulk resistance (Rb) is stable before and after interior stabilization, while the interfacial resistance (Ri) decreases in 40 days and remains stable till 100 days. Furthermore, the EIS of Li/ Dual-Li SPEs/ Li cells before and after plating/ stripping in Fig. 4b shows no obvious change in Rb and Ri, indicating the stable interfacial contact between Dual-Li SPEs and Li metal electrodes. Therefore, the Dual-Li SPEs has an outstanding stability with aging time and an excellent interfacial compatibility toward Lithium metal electrodes. As the XPS spectra shown in Fig. 4c and d, only one peak of LiF (684.79 eV) emerges in the SEI between Dual-Li SPEs and lithium metal anode. While, -C-F peak at 688.80 eV is also observed on SEI with LiTFSI-PVFM polymer. The fundamental difference indicates that the complete coordination of LiTFSI into the polymer matrix of Dual-Li SPEs ,leading to the suppressed TFSI− anion diffusion and its side reaction accordingly in the electrolyte, which is in agreement with the obtained high Li ion transference number of Dual-Li SPEs and the improved interfacial compatibility between Dual-Li SPEs and Li metal anode. The long-term electrochemical stability of Dual-Li SPEs solid electrolytes with lithium metal anode was evaluated by using plating/ stripping test of symmetrical Li/ Li cells at a constant current density of 0.10 mA cm−2. The cell with the Dual-Li SPEs exhibited excellent stability with a nearly constant voltage polarization of 80 mV during the 1500 h cycling at 30 °C (Fig. 4e). A smooth surface of lithium metal is observed in Li/ Li cells after the repeated plating and stripping with Dual-Li SPEs membrane (Fig. 4f). However, the surface of Li shows a typical dendrite-like morphology in Li/ Li cell with LiTFSI-PVFM (Fig. 4g), leading to an accelerated cell death less than 16
420 h. The ion transport near the anode is driven by the electric field according to the space-charge theory, thus the suppressed anion mobility in SPEs inhibits the spatial gradient of charge and favorites much more homogeneous distribution of Li+ ion on the interface [37, 38], ending up with a uniform Li deposition on the lithium anode [33, 39]. Moreover, the “salt-with-salt” electrolyte with the concentrated lithium ions contributes to the significantly delayed dendrite nucleation under a certain current density based on Sand’s time mode [40, 41]. As shown in Fig. 4h, the Dual-Li SPEs rubbery membrane keep intimate contact with lithium anode, the homogeneous nucleation of lithium deposition is significantly promoted on the interface.
Fig. 5. Performance of Li/ Dual-Li SPEs/ LiCoO2 coin cell at a rate of 0.10 C between 3.0-4.4 V: (a) Charge-discharge voltage profiles for the first cycle. (b) Discharge capacity and coulombic efficiency for 200 cycles. (c) Impedance spectra of Li/ Dual-Li SPEs/ LiCoO2 coin cells and cycle. The rate performance of Li/ Dual-Li SPEs/ LiCoO2 coin cell between 3.0-4.4 V at 0.10 C, 0.20 C and 0.50 C: (d) charge-discharge voltage profiles and (e) discharge capacity. The Li/ Dual-Li SPEs/ LiCoO2 batteries were assembled and charged and discharged between 17
3.0 and 4.4 V at 0.10 C current density (where 1.0 C = 170 mAh g−1) as shown in Fig. 5a and b. The cells exhibit an initial discharge capacity of 165 mAh g−1 and high initial coulombic efficiency of 97%, moreover the cells show a high capacity retention of 76% after 200 cycles. In addition, a pair of weak redox peaks presents at high voltage in the charge and discharge curves may result from phase transition between ordered and disordered lithium ion arrangements in the CoO2 framework of LiCoO2 electrode [42]. Moreover, the EIS measurements of the Li/ Dual-Li SPEs/ LiCoO2 cells before and after cycle were performed as shown in Fig. 5c. Bulk resistance of the cell is stable during cycling. The irregular semicircles in the middle frequency ascribes to the charge-transfer resistance at interface, which decreases as the contact with electrodes is improved with the electrodes at initial cycling. However, the interface resistance increases in the subsequent cycles, which leads to the gradual decline of discharge capacity. The rate performance of the Li/ Dual-Li SPEs/ LiCoO2 batteries has been tested between 3.0-4.4 V at 0.10 C, 0.20 C and 0.50 C for five cycles respectively, and finally cycled at 0.10 C, as shown in Fig. 5d and e. The cell delivers the discharge capacity of 153 mAh g-1 at 0.20 C and 142 mAh g-1 at 0.50 C, which remains 94.4% and 87.7% of the discharge capacity at 0.10 C, respectively.
18
Fig. 6. Photographs for the soft package Li/ Dual-Li SPEs/ LiCoO2 battery that powers light-emitting diode (LEDs) (a) before and (b) after bending; (c) the voltage curve of battery during bending. Photographs for the soft package Li/ Dual-Li SPEs/ LiCoO2 battery that powers LEDs (d) before and (e) after punching; (f) the voltage and temperature curve during punching of batteries after 20 cycles. The soft-package lithium batteries using lithium metal anode have been assembled and tested as shown in Fig. 6a and b. When the battery was bended or severely wrinkled, it can still be able to light up a commercial blue LEDs lamp without any electrical failure and the cell voltage remains stable around 3.92 V. During bending, the strong and high elastic Dual-Li SPEs will maintain integrity and keep intimately connected with cathode and anode, avoiding the short circuit of the battery. Moreover, the fully charged pouch cells with LiCoO2 cathode and lithium metal anode after 20 cycles between 3.0 and 4.4 V at 0.10 C (shown in Fig. S10) performed the nail penetration tests. No smoke or burning occurs during punching the steel needle into the pouch cell with Dual-Li SPEs. The cell voltage and surface temperature profiles of the pouch cells during the nail penetration test are shown in Fig. 6f. The temperature of the cell kept stable during the punching. Additionally, the 19
voltage of the battery dropped to nearly zero when the needle was punched through the cell, and then recovered to the initial value of 3.94 V after the needle was removed. The pierced soft-package lithium battery is still able to light up LEDs lamp as shown in Fig. 6d and e. The phenomena demonstrate that the Dual-Li SPEs with high deformation capacity shows self-healing properties during punching, preventing the lithium metal from the open air and avoiding severe side reaction. Moreover, compact lithium deposition guided by the Dual-Li SPEs membrane during cycling is also responsible for the improved safety of the lithium metal batteries. Dual-Li SPEs based on single lithium-ion conductive salt LiPVFM show a significant increase on concentration of the mobile charge carriers, contributing to high ionic conductivity and transference number tLi+ of nearly unity. Additionally, there are other outstanding properties as the electrolytes for solid state lithium metal batteries, including (1) the electrochemical stability with respect to the cathodes, (2) intimate contact with electrode temporally, (3) homogeneous distribution of spatial ionic current guiding the compact lithium planting, and (4) high mechanical strength suppressing the dendrite growth. This work offers a novel strategy to develop flexible solid electrolytes membrane for lithium metal batteries. 4. Conclusion We proposed a “Salt-with-Salt” strategy to achieve high lithium concentration flexible solid electrolyte membrane based on single lithium-ion conductor LiPVFM. The dominant ether ring groups in LiPVFM are confirmed to possess a high solubility and good dissociation of lithium salts. The strong interaction between LiPVFM and LiTFSI improves polymer chain relaxation, but also suppresses TFSI− anion diffusion. Therefore, the introduction of LiTFSI improves the concentration and migration kinetics of the charge carriers Li+ in the composite electrolyte, leading to high ionic 20
conductivity of 5.7 × 10−4 S cm−1 at 25 °C, high lithium ion transference number of 0.79. Applying Dual-Li SPEs in Li/ LiCoO2 metal batteries, they show the cycle stability in 200 cycles between 3.0 and 4.4 V, which is contributed to the chemical and electrochemical stability of Dual-Li SPEs between high voltage LiCoO2 cathode and Lithium metal anode. Moreover the Li/ Dual-Li SPEs/ LiCoO2 metal pouch cells shows high flexibility and safety in the bending and nail penetration tests due to the excellent elasticity and deformation strength of Dual-Li SPEs. The “Salt-with-Salt” Dual-Li SPEs provides a promising candidate of the electrolytes membrane for high-energy-density lithium metal batteries. Acknowledgements This work was financially supported by the National Key R&D Program of China (Grant No. 2018YFB0104300) and the Beijing Municipal Science and Technology Project (Grant No. Z181100004518003). References [1] D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries, Nat. Nanotechnol., 12 (2017) 194-206. https://doi.org/10.1038/nnano.2017.16 [2] X. Shen, X. B. Cheng, P. Shi, J. Q. Huang, X. Q. Zhang, C. Yan, T. Li, Q. Zhang, Lithium-matrix composite anode protected by a solid electrolyte layer for stable lithium metal batteries, J. Energy Chem., 37 (2019) 29-34. https://doi.org/10.1016/j.jechem.2018.11.016 [3] X. B. Cheng, C. Z. Zhao, Y. X. Yao, H. Liu, Q. Zhang, Recent advances in energy chemistry between solid-state electrolyte and safe lithium-metal anodes, Chem, 5 (2019) 74-96. https://doi.org/10.1016/j.chempr.2018.12.002 [4] H. L Dai, K. Xi, X. Liu, C. Lai, S. Q. Zhang, Cationic Surfactant-Based Electrolyte Additives for 21
Uniform Lithium Deposition via Lithiophobic Repulsion Mechanisms, J. Am. Chem. Soc., 140 (2018) 17515-17521. https://doi.org/10.1021/jacs.8b08963 [5] R. P. Liu, P. He, Z. R. Wu, F. Guo, B. Huang, Q. Wang, Z. Y. Huang, C. A. Wang, Y. T. Li, PEO/hollow mesoporous polymer spheres composites as electrolyte for all solid state lithium ion battery, J. Electroanal. Chem., 822 (2018) 105-111. https://doi.org/10.1016/j.jelechem.2018.05.021 [6] A. Manthiram, X. Yu, S. Wang, Lithium battery chemistries enabled by solid-state electrolytes, Nat. Rev. Mater., 2 (2017) 16103. https://doi.org/10.1038/natrevmats.2016.103 [7] C. W. Sun, J. Liu, Y. D. Gong, D. P. Wilkinson, J. J. Zhang, Recent advances in all-solid-state rechargeable
lithium
batteries,
Nano
Energy,
33
(2017)
363-386.
http://doi.org/10.1016/j.nanoen.2017.01.028 [8] X. Y. Ban, W. Q. Zhang, N. Chen, C. W. Sun, A High-Performance and Durable Poly(ethylene oxide)-Based Composite Solid Electrolyte for All Solid-State Lithium Battery, J. Phys. Chem. C, 122 (2018) 9852-9858. https://doi.org/10.1021/acs.jpcc.8b02556 [9] S. Chen, J. Wang, Z. Zhang, L. Wu, L. Yao, Z. Wei, Y. Deng, D. Xie, X. Yao, X. Xu, In-situ preparation of poly(ethylene oxide)/Li3PS4 hybrid polymer electrolyte with good nanofiller distribution for rechargeable solid-state lithium batteries, J. Power Sources, 387 (2018) 72-80. https://doi.org/10.1016/j.jpowsour.2018.03.016 [10] W. Q. Zhang, J. H. Nie, F. Li, Z. L. Wang, C. W. Sun, A durable and safe solid-state lithium battery
with
a
hybrid
electrolyte
membrane,
Nano
Energy,
45
(2018)
413-419.
https://doi.org/10.1016/j.nanoen.2018.01.028 [11] Q. Zhou, J. Ma, S. M. Dong, X. F. Li, G. L. Cui, Intermolecular Chemistry in Solid Polymer Electrolytes for High-Energy-Density Lithium Batteries, Adv. Mater., (2019) 1902029. 22
https://doi.org/10.1002/adma.201902029 [12] L. Long, S. Wang, M. Xiao, Y. Meng, Polymer electrolytes for lithium polymer batteries, J. Mater. Chem. A, 4 (2016) 10038-10069. https://doi.org/10.1039/c6ta02621d [13] T. Itoh, Y. Mitsuda, T. Ebina, T. Uno, M. Kubo, Solid polymer electrolytes composed of polyanionic
lithium
salts
and
polyethers,
J.
Power
Sources,
189
(2009)
531-535.
https://doi.org/10.1016/j.jpowsour.2008.10.113 [14] Z. Xue, D. He, X. Xie, Poly(ethylene oxide)-based electrolytes for lithium-ion batteries, J. Mater. Chem. A, 3 (2015) 19218-19253. https://doi.org/10.1039/c5ta03471j [15] S. S. Zhang, G. X. Wan, Single-ion conduction and lithium battery application for ionomeric cross-linking
polymer,
J.
Appl.
Polym.
Sci.,
48
(1993)
405-409.
https://doi.org/10.1002/app.1993.070480304 [16] Y. Feng, R. Tan, Y. Zhao, R. Gao, L. Yang, J. Yang, H. Li, G. Zhou, H. Chen, F. Pan, Insight into fast ion migration kinetics of a new hybrid single Li-ion conductor based on aluminate complexes for solid-state Li-ion batteries, Nanoscale, 10 (2018) 5975-5984. https://doi.org/10.1039/c8nr00573g [17] Y. Kim, S. J. Kwon, H. Jang, B. M. Jung, S. B. Lee, U. Choi, High ion conducting nanohybrid solid polymer electrolytes via single-ion conducting mesoporous organosilica in poly(ethylene oxide), Chem. Mater., 29 (2017) 4401-4410. https://doi.org/10.1021/acs.chemmater.7b00879 [18] Y. Chen, Y. Tian, Z. Li, N. Zhang, D. Zeng, G. Xu, Y. Zhang, Y. Sun, H. Ke, H. Cheng, An AB alternating diblock single ion conducting polymer electrolyte membrane for all-solid-state lithium metal
secondary
batteries,
J.
Membr.
Sci.,
566
(2018)
181-189.
https://doi.org/10.1016/j.memsci.2018.09.013 [19] Y. Ren, F. Lian, Y. Wen, H. Y. Guan, A novel PVFM based membranes prepared via phase 23
inversion
method
and
its
application
in
GPE,
Polymer,
54
(2013)
4807-4813.
http://doi.org/10.1016/j.polymer.2013.07.013 [20] H. Y. Guan, F. Lian, K. Xi, Y. Ren, J. L. Sun, R. Kumar, Polyvinyl formal based gel polymer electrolyte prepared using initiator free in-situ thermal polymerization method, J. Power Sources, 245 (2014) 95-100. https://doi.org/10.1016/j.jpowsour.2013.06.120 [21] F. Lian, H. Y. Guan, Y. Wen, X. R. Pan, Polyvinyl formal based single-ion conductor membranes as polymer electrolytes for lithium ion batteries, J. Membr. Sci. 469 (2014) 67-72. http://doi.org/10.1016/j.memsci.2014.05.065 [22] H. Zhang, F. Lian, L. Bai, N. Meng, C. Xu, Developing lithiated polyvinyl formal based single-ion conductor membrane with a significantly improved ionic conductivity as solid-state electrolyte
for
batteries,
J.
Membr.
Sci.
552
(2018)
349-356.
https://doi.org/10.1016/j.memsci.2018.02.032 [23] F. Neese, The ORCA program system, Wiley Interdiscip. Rev.-Comput. Mol. Sci., 2 (2012) 73-78. https://doi.org/10.1002/wcms.81 [24] T. Lu, F. W. Chen, Multiwfn: A multifunctional wavefunction analyzer, J. Comput. Chem., 33 (2012) 580-592. https://doi.org/10.1002/jcc.22885 [25] S. Das, A. Ghosh, Effect of plasticizers on ionic conductivity and dielectric relaxation of PEO-LiClO4
polymer
electrolyte,
Electrochim.
Acta,
171
(2015)
59-65.
https://doi.org/10.1016/j.electacta.2015.04.178 [26] H. Y. Guan, Z. B. Guo, J. J. Ding, F. Lian, Ionic conductivity and dielectric behavior of novel poly(vinyl formal)-based single-ion-conductor polymer electrolytes, J. Appl. Polym. Sci., 133 (2016) 43510. https://doi.org/10.1002/app.43510 24
[27] C. H. Wang, Y. F. Yang, X. J. Liu, H. Zhong, H. Xu, Z. B. Xu, H. X. Shao, F. Ding, Suppression of lithium dendrite formation by using LAGP-PEO (LiTFSI) composite solid electrolyte and lithium metal anode modified by PEO (LiTFSI) in all-solid-state lithium batteries, ACS Appl. Mater. Interfaces, 9 (2017) 13694-13702. https://doi.org/10.1021/acsami.7b00336 [28] F. L. Huang, Y. F. Xu, B. Peng, Y. F. Su, F. Jiang, Y. L. Hsieh, Q. F. Wei, Coaxial electrospun cellulose-core fluoropolymer-shell fibrous membrane from recycled cigarette filter as separator for high performance lithium-ion battery, ACS Sustain. Chem. Eng., 3 (2015) 932-940. https://doi.org/10.1021/acssuschemeng.5b00032 [29] H. P. Wu, Y. Cao, H. P. Su, C. Wang, Tough gel electrolyte using double polymer network design for the safe, sable cycling of lithium metal anode, Angew. Chem.-Int. Edit., 57 (2018) 1361-1365. https://doi.org/10.1002/anie.201709774 [30] K. Pozyczka, M. Marzantowicz, J. R. Dygas, F. Krok, Ionic conductivity and lithium transference number of poly(ethylene oxide):LiTFSI system, Electrochim. Acta, 227 (2017) 127-135. https://doi.org/10.1016/j.electacta.2016.12.172 [31] Z. Q. Zhu, M. L. Hong, D. S. Guo, J. F. Shi, Z. L. Tao, J. Chen, All-solid-state lithium organic battery with composite polymer electrolyte and pillar[5]quinone cathode, J. Am. Chem. Soc., 136 (2014) 16461-16464. https://doi.org/10.1021/ja507852t [32] K. Fu, Y. H. Gong, J. Q. Dai, A. Gong, X. G. Han, Y. G. Yao, C. W. Wang, Y. B. Wang, Y. N. Chen, C. Y. Yan, Y. J. Li, E. D. Wachsman, L. B. Hu, Flexible, solid-state, ion-conducting membrane with 3D garnet nanofiber networks for lithium batteries, Proc. Natl. Acad. Sci. U. S. A., 113 (2016) 7094-7099. https://doi.org/10.1073/pnas.1600422113 [33] C. Z. Zhao, X. Q. Zhang, X. B. Cheng, R. Zhang, R. Xu, P. Y. Chen, H. J. Peng, J. Q. Huang, Q. 25
Zhang, An anion-immobilized composite electrolyte for dendrite-free lithium metal anodes, Proc. Natl. Acad. Sci. U. S. A., 114 (2017) 11069-11074. https://doi.org/10.1073/pnas.1708489114 [34] S. Ramesh, K. C. Wong, Conductivity, dielectric behaviour and thermal stability studies of lithium ion dissociation in poly(methyl methacrylate)-based gel polymer electrolytes, Ionics, 15 (2009) 249-254. https://doi.org/10.1007/s11581-008-0268-2 [35] Q. Ma, H. Zhang, C. W. Zhou, L. P. Zheng, P. F. Cheng, J. Nie, W. F. Feng, Y. S. Hu, H. Li, X. J. Huang, L. Q. Chen, M. Armand, Z. B. Zhou, Single lithium-ion conducting polymer electrolytes based on a super-delocalized polyanion, Angew. Chem.-Int. Edit., 55 (2016) 2521-2525. https://doi.org/10.1002/anie.201509299 [36] W. Liu, N. Liu, J. Sun, P. C. Hsu, Y. Z. Li, H. W. Lee, Y. Cui, Ionic conductivity enhancement of polymer electrolytes with ceramic nanowire fillers, Nano Lett., 15 (2015) 2740-2745. https://doi.org/10.1021/acs.nanolett.5b00600 [37] M. D. Tikekar, S. Choudhury, Z. Y. Tu, L. A. Archer, Design principles for electrolytes and interfaces
for
stable
lithium-metal
batteries,
Nat.
Energy,
1
(2016)
1-7.
https://doi.org/10.1038/nenergy.2016.114 [38] Z. Y. Tu, M. J. Zachman, S. Choudhury, S. Y. Wei, L. Ma, Y. Yang, L. F. Kourkoutis, L. A. Archer, Nanoporous hybrid electrolytes for high-energy batteries based on reactive metal anodes, Adv. Energy Mater., 7 (2017) 1602367. https://doi.org/10.1002/aenm.201602367 [39] C. Q. Niu, J. Liu, G. P. Chen, C. Liu, T. Qian, J. J. Zhang, B. K. Cao, W. Y. Shang, Y. B. Chen, J. L. Han, J. Du, Y. Chen, Anion-regulated solid polymer electrolyte enhances the stable deposition of lithium
ion
for
lithium
metal
batteries,
J.
Power
Sources,
417
(2019)
70-75.
https://doi.org/10.1016/j.jpowsour.2019.02.004 26
[40] R. Akolkar, Mathematical model of the dendritic growth during lithium electrodeposition, J. Power Sources, 232 (2013) 23-28. https://doi.org/10.1016/j.jpowsour.2013.01.014 [41] L. L. Fan, X. F. Li, Recent advances in effective protection of sodium metal anode, Nano Energy, 53 (2018) 630-642. https://doi.org/10.1016/j.nanoen.2018.09.017 [42] J. N. Reimers, J. R. Dahn, Electrochemical and In Situ X-ray Diffraction Studies of Lithium Intercalation
in
LixCoO2,
J.
Electrochem.
Soc.,
139
(1992)
2091-1097.
https://doi.org/10.1149/1.2221184
27
●Salt-with-salt concentrated SPEs was proposed on base of single Li+ conductor LiPVFM ●LiTFSI coordinates in LiPVFM matrix to obtain the composite with a decreasing Tg ●The dissolved LiTFSI exhibits strong interaction with LiPVFM via ether ring groups ●Dual-Li SPEs show high ionic conductivity, tLi+, elasticity and stable with Li metal
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.