A Self‐Healing and Nonflammable Cross‐Linked Network Polymer Electrolyte with the Combination of Hydrogen Bonds and Dynamic Disulfide Bonds for Lithium Metal Batteries

The self‐healing solid polymer electrolytes (SHSPEs) can spontaneously eliminate mechanical damages or micro‐cracks generated during the assembly or operation of lithium‐ion batteries (LIBs), significantly improving cycling performance and extending service life of LIBs. Here, we report a novel cross‐linked network SHSPE (PDDP) containing hydrogen bonds and dynamic disulfide bonds with excellent self‐healing properties and non‐flammability. The combination of hydrogen bonding between urea groups and the metathesis reaction of dynamic disulfide bonds endows PDDP with rapid self‐healing capacity at 28 °C without external stimulation. Furthermore, the addition of 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) improves the ionic conductivity (1.13 × 10−4 S cm−1 at 28 °C) and non‐flammability of PDDP. The assembled Li/PDDP/LiFePO4 cell exhibits excellent cycling performance with a discharge capacity of 137 mA h g−1 after 300 cycles at 0.2 C. More importantly, the self‐healed PDDP can recover almost the same ionic conductivity and cycling performance as the original PDDP.

dimers, the SHSPE can heal damage within 1 h at room temperature. To further improve the mechanical properties and thermal stability of SHSPEs, Xue et al. [39] introduced polyethylene glycol-bis-carbamate dimethacrylate (PEGBCDMA) to copolymerize with PEGMA and UPyMA to prepare a dual-network SHSPE (DN-SHSPE). Physically cross-linked ureido-pyrimidinone hydrogen bonds act as the first network, giving DN-SHSPE excellent self-healing properties. The improved mechanical strength of DN-SHSPE is attributed to chemically crosslinked PEGBCDMA acting as a second network. In 2021, Pu et al. [40] prepared a SHSPE by the polymerization of poly(ethyleneglycol) diamine and 1,3,5-triformylbenzene. Dynamic imine bonds endow the SHSPE with excellent self-healing capability, and it can self-heal damage within 24 h at room temperature. So far, most reported SHSPEs have based on single healing mechanism, either supramolecular interactions or dynamic covalent bonds. [37] Recently, it has found that supramolecular interactions and dynamic covalent bonds are complementary. Supramolecular interactions with lower bonding energy generally give materials higher healing efficiency, even at room temperature. [41][42][43] Dynamic covalent bonds can significantly improve the mechanical properties of materials due to higher bond energy and more stability. [44] The combination of supramolecular interactions and dynamic covalent bonds is expected to prepare SHSPEs with both good mechanical performance and high healing efficiencies, but there is a large gap in related research. [45] Herein, we report a novel cross-linked network SHSPE (PDDP) with excellent self-healing capability and non-flammability fabricated by thiol-ene click reaction. The combination of hydrogen bonds and dynamic disulfide bonds gives PDDP fabulous mechanical and selfhealing properties. The introduction of ionic liquids effectively improves the ionic conductivity and non-flammability. Therefore, PDDP achieves high ionic conductivity (over 10 −4 S cm −1 at 28°C) and wide electrochemical window (up to 4.8 V). The assembled Li/ PDDP/LiFePO 4 (LFP) cells exhibit stable cycling performance and excellent rate capability. Moreover, the Li/LFP cells assembled with the selfhealed PDDP can recover almost the same ionic conductivity and cycling performance as the original one.

Results and Discussion
The cross-linked network polymer matrix was prepared by thiol-ene click polymerization between PEGDA, DDB, DODT and PETMP ( Figure  1a,c). DDB was synthesized by the reaction of 4-aminophenyl disulfide with 2-isocyanatoethyl methacrylate and characterized by NMR (Figures S1 and S2, Supporting Information). The FTIR spectra of PEGDA, PETMP, DODT, DDB and PDDP are displayed in Figure 1b. The absorption bands at 1650 cm −1 in the spectra of PEGDA and DDB are associated to C=C stretching vibration. The absorption bands at 2553 cm −1 in the spectra of PETMP and DODT correspond to S−H stretching vibration. These absorption bands disappear in the spectrum of PDDP, indicating that the thiol-ene click polymerization occurred and the crosslinked network polymer was successfully prepared.
To determine the optimal content of EMIMTFSI in PDDP, we prepared three electrolyte membranes PDDP-5, PDDP-10 and PDDP-15. As shown in Figure S3b-d, Supporting Information, the surfaces of PDDP-5 and PDDP-10 are smooth and trace-free, while PDDP-15 has many cracks. Furthermore, when the three electrolyte membranes were wound on the glass rod, PDDP-5 remains intact, but obvious cracks appear in the interior of PDDP-10 and PDDP-15, indicating that the tensile strength of the electrolyte membranes decreases with the increase in EMIMTFSI ( Figure S3f-h, Supporting Information). The deterioration of tensile strength is further confirmed by tensile testing experiments. The stress-strain curves are displayed in Figure S4, Supporting Information, the tensile strength of PDDP-5 is 0.105 MPa, while that of PDDP-10 and PDDP-15 decreases to 0.087 and 0.038 MPa, respectively. Ultimately, the addition of EMIMTFSI is determined to be appropriate at 5 wt%.
The thermal stability of PDDP-5 was evaluated by TGA. As shown in Figure 2a, PDDP-5 begins to weight loss at 150°C and a 5 wt% weight loss at 284°C, which associate with evaporation and degradation of EMIMTFSI. In contrast, PDDP-0 without the addition of EMIMTFSI does not begin to decompose until 300°C, indicating that the cross-linked network structure has high thermal stability. These results indicate that PDDP-5 has high thermal stability and meet the thermal safety requirements of SPEs. The thermal behavior of PDDP-5 was further studied by DSC. The glass transition temperature (T g ) of PDDP-0 is −43.49°C (Figure 2b), and the crystalline melting temperature (T m ) is 127.62°C, while the T g and T m of PDDP-5 are −47.35 and 108.68°C, respectively. These results suggest that the addition of EMIMTFSI can reduce T g and T m . The low T g indicates that the disordered cross-linked network structure endows PDDP with good segment mobility, which is beneficial to the transport of Li + .
To investigate the self-healing ability of PDDP-5, the electrolyte membrane (about 900 μm) was split into two pieces with a sterile scalpel, and the two cut surfaces were in contact for the healing tests ( Figure 2c). The two pieces can be joined together and support its own weight after 3 s without any external stimulus at 28°C (Figure 2d), although the interface is still clearly visible. After 24 h, the healed PDDP-5 may bear a weight of 100 g without sundering (Figure 2e, Figure S5, Supporting Information). The superb self-healing ability of PDDP-5 is owing to the combination of dynamic disulfide bonds and hydrogen bonds (Figure 2c). To confirm the existence of multiple hydrogen bonds in PDDP-5, in situ temperature-dependent FTIR spectroscopy was used to examine the change in the characteristic C=O band, which acts as a hydrogen bond acceptor ( Figure S6, Supporting Information). With increasing temperature, the C=O band shifted from 1733.79 to 1735.27 cm −1 caused by the dissociation and recombination of hydrogen bonds, indicating the existence of hydrogen bond interaction in PDDP-5 ( Figure S6b, Supporting Information). [46] Low-energy hydrogen bonds enable PDDP-5 to selfheal at 28°C through efficient reversible cleavage and remodeling, [47,48] and dynamic disulfide bonds can improve the mechanical properties of PDDP-5 while improving the self-healing ability. [41] To compare with PDDP-5, we prepared PMBP-5 containing only hydrogen bonds and PDBP-5 containing only disulfide bonds (see Appendix S1, Supporting Information; Figures S7-S11, Supporting Information), and their film-forming and self-healing properties were investigated. PDBP-5 shows a stringy appearance when peeled from the plate, indicating poor mechanical properties and film formation ( Figure S12, Supporting Information). Although PMBP-5 exhibits good film-forming properties and can also support its own weight without any external stimulus after contacting the two cut surfaces for 3 s at 28°C, but cracks appear on the contact surface after 1 min. In addition, the healed PMBP-5 cannot bear the weight of 100 g after 24 h ( Figure S13, Supporting Information). The comparative experiments demonstrate the importance of combining dynamic disulfide bonds and hydrogen bonds for improving self-healing ability. PDDP-5 exhibits excellent flame retardancy due to the addition of Energy Environ. Mater. 2023, 6, e12568 2 of 8 non-flammable EMIMTFSI. [49] PDDP-5 was exposed to blaze, it melted but did not burn after 3 s (Figure 2f), indicating that PDDP-5 can reduce the risk of fire caused by battery short circuit and improve the safety of LIBs.
The ionic conductivity of PDDP membranes at different temperatures was tested. The ionic conductivity of PDDP-0 is 1.86 × 10 −5 S cm −1 at 28°C (Figure 3a). Ionic liquid can promote the dissociation of LiTFSI and improve the ionic conductivity. [50,51] After the addition of EMIMTFSI, the ionic conductivity of PDDP-5 is increased to 1.13 × 10 −4 S cm −1 at 28°C. It is an order of magnitude higher compared with PDDP-0. The ionic conductivity of PDDP-5 increases to 1.44 × 10 −3 S cm −1 when the temperature increases to 100°C. The relationship between the temperature and Li + conductivity of PDDP from 28 to 100°C is in good agreement with the Arrhenius equation: where A and T represent the pre-exponential factor and the corresponding temperature; E a and R are the activation energy and the ideal gas constant, respectively. The values of PDDP membranes were calculated from the Arrhenius equation. As shown in Table S1, Supporting Information, the E a value of PDDP-5 is lower than that of PDDP-0, indicating that the addition of EMIMTFSI accelerates the migration of ions in the polymer matrix.
Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were performed to evaluate the electrochemical stability of PDDP-5. The electrochemical window of PDDP-0 is 4.0 V versus Li/Li + ( Figure S14, Supporting Information). Benefiting from the high electrochemical stability of EMIMTFSI, the electrochemical window of PDDP-5 increases to 4.8 V versus Li/Li + (Figure 3b). [52,53] The CV curves of Li/ PDDP-5/LFP cell show oxidation/reduction peaks at 3.71 and 3.15 V (Figure 3c). The peaks in the first cycle are not completely symmetrical due to the consumption of trace electrolyte to form solid electrolyte interface (SEI). The CV curves in the 2nd to 4th cycles almost overlap, suggesting the good reversibility of the Li/PDDP-5/LFP cell.
The lithium-ion transference numbers (t Li þ ) is also an important parameter to evaluate the performance of SPEs. [54] Figure 3d shows the chronoamperometry and electrochemical impedance of Li/PDDP-5/Li cell before and after steady state. Using Equation (3), the t Li þ of PDDP-5 is calculated to be 0.39. It is higher compared with PEO-based SPEs. [55,56] The reason for the higher t Li þ is attributed to the fact that EMIMTFSI promotes the dissociation of LiTFSI and the cross-linked network structure hinders the migration of TFSI − .
The interface resistance of Li/PDDP-5/Li symmetrical cell was performed over time to assess the interfacial stability between PDDP-5 and lithium metal. The resistance (R a ) and interfacial resistance (R i ) of PDDP-5 are displayed in Figure 4a,b. Due to the interfacial wetting between lithium metal anode and PDDP-5, [57] R a decreases from 200 Ω at fresh to 194 Ω, R i decreases from 1020 to 961 Ω after 1 d. R a and R i stabilize at 100 and 860 Ω after 5 d, respectively, indicating the formation of a stable SEI layer on lithium metal surface. The interfacial stability was further evaluated by the galvanostatic cycling measurements of lithium plate/strip performance in Li/PDDP-5/Li symmetric cell under Assembling Li/LFP cells to assess performance of PDDP in solid-state batteries. Galvanostatic cycling measurements were performed at 28°C to explore the performance of Li/LFP cells at 2.5-3.8 V. As shown in Figure 5a, to compared with Li/PDDP-0/LFP cell, Li/PDDP-5/LFP cell at 0.1 C not only has higher initial discharge capacity (139.6 mAh g −1 ), but also the discharge capacity stabilizes at 141.4 mAh g −1 after 150 cycles, indicating the addition of EMIMTFSI is beneficial to improve the long-circulating performance of the cell. Figure 5b shows the Li/PDDP-5/LFP cell exhibites an initial specific capacity of 160, 150, 130, 100 and 80 mAh g −1 at 0.1, 0.2, 0.5, 1.0 and 2.0 C, respectively. The discharge capacity completely recovers when the current rate is restored to 0.1 C, demonstrating the Li/PDDP-5/LFP cell has excellent rate capability. Figure 5c displays the polarization charge/discharge platforms of Li/PDDP-5/LFP cell at 0.1 and 0.2 C are 0.11 and 0.15 V, respectively. The polarization voltage stabilizes at 0.11 V when the current rate recovers from 2 to 0.1 C. As shown in Figure 5d,e, even after 300 cycles, the discharge capacity of Li/ PDDP-5/LFP cell can still reach 137.6 mAh g −1 at 0.2 C, the average coulombic efficiency is 99.9%, indicating the cell has excellent long cycle performance.
Assembling Li/healed PDDP-5/LFP cells to test the performance of the healed PDDP-5 in solid-state batteries. The first lap discharge of Li/healed PDDP-5/LFP cell is <139.6 mAh g −1 at 0.1 C (Figure 5f). However, the discharge specific capacities of Li/healed PDDP-5/LFP cell and Li/PDDP-5/LFP cell are similar after 10 cycles, indicating that the healed PDDP-5 can still be effectively applied to solid-state batteries.
To determine the interfacial composition of Li/PDDP-5/LFP cell, the electrode surfaces were analyzed by XPS after 300 cycles.  Figure 6b shows the analysis results of SEI layer on the surface of lithium metal anode. The positions of the bands in the C 1s, F 1s, S 2p, and O 1s spectrum are almost the same as those of the CEI layer. The band at 397.19 eV in the N 1s spectrum belongs to Li 3 N, which and LiF can effectively inhibit the growth of lithium dendrites. [58][59][60][61] S-S is also detected in the S 2p spectrum, indicating that PDDP-5 is also involved in the formation of the SEI layer.

Conclusions
We have designed a self-healing and nonflammable cross-linked network polymer electrolyte (PDDP-5) based on supramolecular interactions and dynamic covalent bonds. Owing to the combination of hydrogen bonds and dynamic disulfide bonds, PDDP-5 exhibits excellent self-healing capability and mechanical properties. In addition, the addition of EMIMTFSI not only improves the ionic conductivity (1.13 × 10 −4 S cm −1 at 28°C), but also endows PDDP-5 with excellent nonflammability. The cycling performance and rate capability of the assembled Li/PDDP-5/ LFP cells are good at 28°C. Furthermore, the LFP cell assembled with the healed PDDP-5 can still exhibit stable cycling performance. Our preliminary results show that combining supramolecular interactions and dynamic covalent bonds is an efficient method to prepare SPEs with both high mechanical properties and high self-healing ability.
Materials characterization: Nuclear magnetic resonance (NMR) spectrum of DDB was obtained with a Bruker Avance III HD (500 MHz) instrument. Fourier transform infrared (FTIR) spectroscopy was recorded on a Nicolet 6700 spectrometer. Differential scanning calorimetry (DSC) measurement was recorded on a NETZSCH DSC 200F3 analyzer at a rate of 10°C min −1 in the −80 to 200°C temperature range under nitrogen atmosphere. Thermogravimetric analysis (TGA) was conducted on a Henven T15-114 at a rate of 10°C min −1 in the 30 to 600°C temperature range. The tensile properties of PDDP were conducted using the electronic universal testing machine at a crosshead speed of 20 mm min −1 . Scanning electron microscopy (SEM) measurement was performed on a Hitachi S5500.
Electrochemical measurements: Electrochemical impedance spectroscopy (EIS) measurements of stainless steel in diameter of 16 mm (SS)/PDDP/ SS cells were conducted on a PARSTAT4000 (P4000) electrochemical workstation changing the frequency range from 1 to 10 6 Hz. Calculating the ionic conductivity is as follows: where S, L, R b , and σ denote area, thickness, bulk resistance and ionic conductivity of PDDP, respectively. CV and LSV tests were performed by the P4000 with a scan rate of 0.1 mV s −1 . The CV of PDDP were conducted with Li/PDDP/LFP cells at the potential range of 2.5-3.8 V. The LSV of PDDP were performed with Li/PDDP/SS cells from OCV to 6.0 V (vs Li/Li + ). The t Li þ of PDDP were measured with Li/PDDP/Li cells at a polarization voltage (ΔV) of 0.01 V. Using the Bruce-Vincent-Evans equation to calculate the t Li þ of PDDP.
where R o and R s represent the impedance spectra before and after steady-state current; I o and I s are the initial and steady-state current, respectively. The interfacial stability of PDDP to lithium dendrites were evaluated by monitoring Li/PDDP/Li cells charged/discharged for 1 h in each cycle at a constant current density of 0.05 mA cm −2 , where diameter of the lithium foil was 12 mm.
Assembly and Testing of LFP Cells: The cathode consists of 80 wt% LFP, 10 wt% C350 and 10 wt% PVDF. The slurry was made by dispersing LiFePO 4 (1.6 g), C350 (0.2 g) and PVDF (0.2 g) in 6.0 mL of NMP, the evenly mixed slurry was knife-coated onto an aluminum foil and transferred to a 120°C vacuum oven to dry for 12 h, and the LFP mass loading was 1.5 mg cm −2 . Solid-state Li/LFP cells (CR2025) assembled with PDDP were subjected to constant current charge and discharge tests in the LAND CT2001A battery test system. X-ray photoelectron spectroscopy (XPS, ESCALab 220i-XL) tests were performed on the urface of lithium