Structural characterization of PTADOL. The liquid-state 13C NMR spectra in Supplementary Fig. 1 reveal DOL has two characteristic peaks at 64.1 and 94.5 ppm corresponding to -O-CH2-CH2-O- and -O-CH2-O- units. After the addition of 0.8 wt% tris(pentafluorophenyl)borane (TFB) as initiator and 1.5M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and stayed for 24 h, the obtained PDOL sample in Fig. 1b exhibits two new peaks at 66.9 and 95.2 ppm, indicating successful polymerization of DOL molecules. And the residual DOL can be calculated to 10.11% from the integrated area of the PDOL spectra, implying an incomplete polymerization reaction. The PTADOL was prepared by mixing TTMAP, DOL, LiTFSI with TFB as the initiator (Supplementary Fig. 2,). It was found that the obtained PTADOL is insoluble in most of deuterated solvents even after stirring for 24 h, and no characteristic peak of PDOL or DOL was detected in liquid NMR spectra (Supplementary Fig. 3). These results demonstrate superior structural stability of PTADOL, which is quite different from PDOL SPE (Supplementary Fig. 4). The solid-state NMR characterization was used to reveal the crosslinked polymerization of the PTADOL electrolyte. As shown in Fig. 1d, chemical resonance shifts from 171.0 ppm (Fig. 1c) to 176.6 ppm corresponded to C = O from the TTMAP groups. And the downfield displacement of chemical shifts indicates the weakened shielding effect on the electronic environment of 13C nuclei, which contributes to the redistribution of the electron density of the carbonyl groups.[22, 23] Furthermore, chemical shifts at 70.5 and 76.2 ppm in the PTADOL sample are assigned to short ether chains compare to that of pure TTMAP. The tremendous change in the chemical environment of oxygen atoms implies the alterant O-Li+ coordination, which affects Li+ transportation in PTADOL.
Furthermore, Fourier transform infrared spectroscopy (FTIR) measurement was conducted to verify the structure of PTADOL (Supplementary Fig. 5). Compared to PDOL, the absence of -OH peak in PTADOL indicates that the terminal hydroxyl of ether chains was end-capped by highly reactive aziridine from TTMAP via copolymerization. Besides, the X-ray diffraction (XRD) result (Supplementary Fig. 6) shows broad diffusion peaks between 10° and 35° in PTADOL, indicating the amorphous state and complete dissolution of LiTFSI.
Structure-dependent ion transport properties. The temperature-dependent ionic conductivity of the electrolytes was demonstrated in Fig. 2a. As is well known that the ionic conductance of SPEs with network structure is always inferior to that of the polymers with linear configuration due to the limited motion of conductive segments in crosslink structure. However, interestingly, the ionic conductivity of PTADOL at room temperature is 1.48 × 10− 4 S cm− 1, which is even slightly larger than that of the PDOL sample (1.29 × 10− 4 S cm− 1). And the curve for PTADOL appears less steep than that of PDOL, suggesting lower activation energy for ion motion (Supplementary Table 1).[24] To study the Li+ transportation in PTADOL electrolyte, FTIR spectroscopy was carried out in the range of 725 − 760 and 800 − 1400 cm− 1. As shown in Fig. 2b, the peaks at 741.2 and 746.8 cm− 1 correspond to the free ions and ion-pairs, respectively, which represents the intensity of the interaction between Li+ and TFSI−.[25, 26] Obviously, PTADOL has a larger ratio of free Li+ than that of PDOL, indicative of higher carrier concentration in the polymer system. Figure 2c shows that the characteristic peak of the long-chained PDOL at 845 cm− 1 cannot be observed in PTADOL sample, demonstrating the alternative copolymerization of DOL and TTMAP. And the enhanced C-H out-of-plane peaks at 915 cm− 1 indicate a spatial polymeric structure formed in PTADOL. In addition, for PDOL sample, vibrational peaks at about 1000 cm− 1 are assigned to the C-O-C stretching; these peaks dramatically shift to 1060 cm− 1 in PATDOL, suggesting a great change of O-Li+ interaction in the network structure. Moreover, the electrostatic potential calculated by Density Functional Theory (DFT) in Supplementary Fig. 7–8 demonstrates that the red areas in ester group from TTMAP and ether group from PDOL represent uniform negative electrostatic potential, which induces the interaction with Li+ and contributes to high solubility and good dissociation of lithium salts in PTADOL.
For a better understanding of the ion transfer dynamics and molecule structure, Molecular Dynamics (MD) simulations were performed to elucidate the subtle interactions of Li+ ions with the polymer host in PTADOL and PDOL electrolytes. The atomic position and the trajectory of the ions were convoluted because of the thermal effect; therefore, deconvolution of the ionic trajectories was necessary to show clearly the ion interactions. Figure 2d-g show the snapshots of the simulated PTADOL and PDOL electrolytes, respectively. The snapshots of the local coordination structure in Fig. 2f demonstrate that Li+ tends to coordinate with four oxygen atoms from the ether group in PTADOL. By contrast, five oxygen atoms from the ether group coordinate with one Li+ in PDOL (Fig. 2g). Corresponding average bond distances of O-Li+ bond in PTADOL (2.223 Å) are longer than that of PDOL (2.207 Å), suggesting that PTADOL coordinates less strongly to Li+ than PDOL does (Supplementary Table 2). The introduction of trident TTMAP crosslinker with ester group weakened the O-Li+ coordination in PTADOL compared to that of the PDOL sample, and the weaker interaction strength of nearby Li+ is beneficial to enhancing the local diffusion of Li+ along the polymer segments.[27] To further investigate the local coordination structural properties of SPE interacting with Li+, the radial distribution functions (g(r), solid lines) and coordination numbers (CN, dash lines) of O-Li+ were calculated. The g(r) plots in Fig. 2h and 2i display the dominated peaks at 2.1 Å for O-Li+ (TFSI− and polymer chain) in both SPE systems, and the first Li+ solvation sheath (≈ 3 Å) is primarily composed of the oxygen atom from ether groups and TFSI− anions, which implies the oxygen atom is the major type to coordinate with cation (Supplementary Fig. 9). And the g(r) of PTADOL shows a sharper peak, demonstrating a well-defined coordination sphere for Li+ in the SPE system.[28] Besides, all peaks vanish at longer distances, indicating only very local ordering in this system. Furthermore, it can be seen directly from the CN of O-Li+ in the first solvation sheath (Fig. 2i) that the CN in PDOL is significantly higher than that in PTADOL, which indicates a much stronger polymer-Li+ coordination effect in PDOL. In contrast, stereoscopic PTADOL coordinates to Li+ more weakly, allowing for a higher fraction of the applied potential to lead to cation transport.
Electrochemical characterization and intrinsic property of PTADOL. As mentioned above, rational O-Li+ coordination results in the Li+ ions being less immobilized and therefore facilitates faster lithium transport, which will favor an increase of tLi+ in SPEs.[2, 17] Accordingly, the tLi+ of PTADOL and PDOL were measured by chronoamperometry using Li||SPE||Li symmetric cell, and the chronoamperometry curves and the AC impedances before and after polarization of the cells are shown in Fig. 3a and Supplementary Fig. 10. There is a significant augment in tLi+ from 0.24 of PDOL to 0.76 of PTADOL (Supplementary Table 3), which is comparable to that of single-ion conductors (≥ 0.8). Moreover, from the DFT calculation (Supplementary Fig. 11), the adsorption energy for TFSI− of TTMAP group in PTADOL was − 0.556 eV, which is favorable to the immobilization of anions and further improves the tLi+ of PTADOL. As shown in Fig. 3b, the ESW of the PTADOL can be stabilized up to 4.6 V with a standard leakage current of 20 µA, which is much higher than that of PDOL (4.0 V). It has been theoretically and experimentally confirmed that the terminal hydroxyl of ion-conductive polymers will be firstly oxidized at higher voltage, which is the main factor that limits the improvement of the ESW.[29, 30] Compared to PDOL, the aziridine groups in TTAMP could react with terminal hydroxyl groups and reduce the chain length of polyether groups, which significantly improved the oxidation stability of PTADOL.[31]
The formation of 3D crosslinked amorphous polymer networks enables PTADOL to possess superior film-forming capability, transparent appearance, high mechanical flexibility, and excellent thermal stability. As shown in Supplementary Fig. 12–13, an ultrathin and flexible PTADOL film (~ 7.2 µm) has been fabricated by incorporating with polyethylene (PE) separator. The tensile strength of the as-prepared solid electrolytes was assessed by stress-strain curves (Fig. 3c). The elastic limit of PDOL electrolyte reaches 9.5 MPa, similar to that of pure PE separator (8.8 MPa). While for PTADOL sample, significant improvement of elastic limit (19.4 MPa) reveals that three-dimension crosslink structure of PTADOL could better resist elastic deformation under the influence of the external environment.
The thermal stability of SPEs directly determines the battery safety. Figure 3d shows the thermal gravity analysis (TGA) thermograms of polymer electrolytes. TGA curve of PDOL electrolyte show a sharp weight loss started at 107°C, indicating disaggregation of PDOL chains (Supplementary Fig. 14). Since the decomposition temperature of pure PDOL is about 300°C, incorporation of LiTFSI greatly reduce the thermal stability of PDOL electrolyte.[32] This can be attributed to electropositive carbon atom between the two electronegative oxygen atoms was vulnerable to be attacked by TFSI− anions at elevated temperature.[33] In contrast, PTADOL suffer no obvious weight drop until 400°C due to significant decline of terminal hydroxyl of polymer chains, demonstrating superior thermal stability of the network structure. To further simulate the thermal performance of the polymer electrolytes, the PTADOL and PDOL samples were sealed in glass fiber substrate, and then were put into an oven at 120°C for 20 mins. As seen in Fig. 3e and 3f, the PDOL sample quickly melted into liquid, while PTADOL maintained the original state. Furthermore, even at room temperature, PDOL slowly turned from solid into translucent liquid in 5 days and could not become solid again at room temperature (Supplementary Fig. 15), indicating an irreversible depolymerization of PDOL sample.[32] All these features indicate that the PTADOL is much more thermally stable than that of PDOL, emphasizing the importance of the network structure induced by TTMAP.
Characterization of the Li/PTADOL interface. Li symmetrical cells were assembled with PTADOL and PDOL to investigate the compatibility with lithium metal anode. Figure 4a displays interfacial impedance versus storage time of PTADOL based Li symmetric batteries. It is worth noting that the cells exhibit no obvious increase for bulk/interfacial resistances during 10 days of storage time, indicating superior chemical stability of the PTADOL/Li interface in the batteries. To evaluate the critical current density (CCD) of the PTADOL, rate performance test with a Li/Li symmetric cell was performed at current densities ranging from 0.1 to 2.6 mA cm− 2 for 1 h per half cycle, the results of which are given in Fig. 4b. The PTADOL based battery shows an extremely high CCD in excess of 2 mA cm− 2, which can ascribe to the improvement of mechanical strength and tLi+ of the electrolyte. The long-term electrochemical compatibility of PTADOL with lithium metal was characterized by galvanostatic charging and discharging for 1 h at 0.25 mA cm− 2 in Li symmetric cells. The cycling curves in Fig. 4c of PTADOL cell show small overpotential of 20 mV, and excellent long-term stability for over 1300 h, which reveals a stable interface during the stripping/plating process. By contrast, PDOL battery suffered sharp overpotential augment after 700 h, and the heavy fluctuation of voltage hysteresis from insert graph implies uncontrolled growth of dendrites and continuous side reactions of Li at the anode/electrolyte interface.[34]
Moreover, high-resolution X-ray photoelectron spectroscopy (XPS) was conducted to characterize the composition and structure of SEI on cycled lithium metal in these two electrolytes and study the SPE/Li interfacial compatibility. The deconvolution of C 1s spectra in Fig. 4d reveals five peaks in PTADOL sample, representing C-C (284.8 eV), C-O (286.3 eV), O-C-O (288.8 eV), COOR (289.7 eV), and -CF3 (292.5 eV, from TFSI−).[20] Even after 1000 h cycling, the peak intensities of C-O, C-O-C, and COOR in PTADOL are significantly lower than that of PDOL sample after 200 h cycling (Fig. 4e), indicating a suppressed polymer degradation in PTADOL compared to the ether groups decomposition in PDOL that generates abundant organic species in SEI.[35] Besides, with the extension of etching time, -CF3 concentrations in F 1s spectra of these two samples sharply decrease from 0 to 180 s as the LiF increases.[36, 37] Not coincidentally, PTADOL sample exhibits less -CF3 and LiF peak intensities than that of PDOL, demonstrating suppressed TFSI− anion diffusion and its decomposition accordingly in the network structure, which is in agreement with the obtained high tLi+ of PTADOL and the improved interfacial compatibility between PTADOL and lithium metal electrode.
Furthermore, morphology of lithium metal disassembled from cycled symmetric Li/Li batteries were characterized by scanning electron microscope (SEM). The surface of the lithium metal anode harvested from the PDOL is covered with Li dendrites, resulting in the deterioration of the interface stability (Fig. 4g). Corresponding cross-section SEM image in Supplementary Fig. 16 exhibits that the thickness of the rough and porous reaction layer of the lithium metal reached up to ~ 40 µm. When PTADOL was employed, effective suppression of Li dendrite growth was observed, and the lithium metal electrode demonstrates a much smoother surface (Fig. 4f). As shown in Supplementary Fig. 17, the surface of cycled lithium electrode harvested from the PTADOL maintained a metallic sheen, while anode from PDOL became darker after cycling. These results demonstrate that PTADOL with homogeneous and stable structure can construct efficient Li+ transport channels in batteries, which effectively inhibit the accumulation of dendritic Li and suppress the expansion stress during the repeated process of stripping/plating.[38] Therefore, the intricate design of PTADOL is quite stable along with the continuous plating/stripping of lithium metal.
Battery Performance of PTADOL with Li anode and cathode. Rate performances of SLMBs with PTADOL and PDOL electrolytes were also tested at 30°C. The coin cells were made with LiFePO4 (LFP) cathodes and lithium metal as anodes. By varying current densities from 0.1 to 2.0 C, LFP||PTADOL||Li cells deliver similar discharge capacities of 157, 150, 143, 137, 128, mAh g− 1 compare to that of LFP||PDOL||Li cells (Fig. 5a and Supplementary Fig. 18). While with the current densities continue to increase, PTADOL cells exhibit much better discharge capacities of 119, and 110 mAh g− 1 than PDOL samples (106, and 52 mAh g− 1). The superior rate capacities can be attributed to the fast Li+ conductance and the excellent interfacial compatibility due to the ultra-stable network structure of PTADOL electrolyte. It is also noteworthy that the PTADOL based battery shows extremely low overpotentials, i.e., 42, 59, 108 and 190 mV at 0.1, 0.2, 0.5, and 1 C, respectively (Fig. 5b). Moreover, overpotentials obtained from the discharge/charge profiles in Fig. 5c change almost-linearly with the discharge rate, which is typical for quasi single-ion conducting batteries.[39] Contrarily, the PDOL battery exhibits great overpotentials augment after 2 C. The differences in overpotentials are attributed to PTADOL battery with immobile anions only suffers from Ohmic overvoltage, while PDOL endure extra obvious concentration polarization at higher current density.[40] The LFP||PTADOL||Li cell exhibits superior cycling stability with 85.6% retention after 300 cycles at 2.5–4.0 V and 0.5 C (Supplementary Fig. 19); and it also shows very stable cycling for more than 200 cycles at 1 C, whereas the LFP||PDOL||Li cell decays dramatically within 100 cycles (Fig. 5d). Moreover, the typical voltage profiles of PTADOL based batteries (Fig. 5e) show negligible increased polarization voltages during long-term cycling than those of PDOL based batteries (Fig. 5f). The strong capability in suppressing Li dendrite growth, and fast Li+ transport capability of PTADOL electrolyte enables us to explore battery performance with higher LFP loading. LFP||PTADOL||Li battery with 7.4 mg cm− 2 loading enables reversible capacity of 154 mAh g− 1 at 0.1 mA cm− 2 and 146 mAh g− 1 at 0.3 mA cm− 2, respectively (Fig. 5g). Furthermore, areal capacity of 3.34 mAh cm− 2 was obtained when the LFP loading increased to 22.8 mg cm− 2 at 0.1 mA cm− 2 (Fig. 5h). More importantly, there is no obvious increase of overpotentials compared to those in the regular testing, and the typical charge/discharge plateaus are clearly identified (Supplementary Fig. 20), indicating great potential of PTADOL for application in high-performance SLMBs.
The expanded ESW of PTADOL make it compatible with high-voltage ternary cathodes. As indicated in Fig. 6a, the NMC811||PTADOL||Li cell displayed initial discharge capacity of 209.2 mAh g− 1 at 0.1 C and 193.7 mAh g− 1 at 0.5 C; And the capacity retention was still around 158.4 mAh g− 1 after 150 cycles, which corresponds to a retention of 81.4% of the highest 0.5 C discharge capacity (194.5 mAh g− 1), indicating a stable electrochemical behavior of this PTADOL. Whereas, the NMC811||PDOL||Li cell suffered rapid capacity decay and failed after only 5 cycles. Supplementary Fig. 21 and Fig. 6b present the voltage profiles of the NMC811||PTADOL||Li cell at current density of 0.1 C and 0.5 C, respectively; the curves show clear potential plateaus without abnormal or unstable profiles even at 4.3 V cut-off voltage, demonstrating reversible and stable cycling processes.
Furthermore, operando differential electrochemical mass spectrometry (DEMS) was employed to explore the decomposition mechanism of SPEs using high-voltage cathode materials. A special designed battery mold consisting of NMC811||SPE||Li was set to charge and discharge at cutoff voltage of 4.3 V (Supplementary Fig. 22). As seen in Fig. 6c, no obvious gas generation was detected from PTADOL based battery throughout the whole cycles, demonstrating ultra-stable electrochemical stability of the network structure. In comparison, obvious O2 release occurred during the first cycle of PDOL based battery (Fig. 6d), which can attribute to the decomposition of residual DOL molecules in the electrolyte. Gas generation reoccurred at ~ 4.2 V in the second cycle and continued through the third cycle. As aforementioned, the terminal hydroxyl units in PDOL chain are generally more vulnerable than the units in the middle of molecular chain, which will initiate a chain reaction once the terminal units are decomposed.[41] These results show that the formed network topological structure in PTADOL greatly improve its oxidation stability with transition metal oxide cathode.
Safety assessment of solid-state pouch cells. NMC811||Li solid pouch cell with PTADOL was further fabricated to demonstrate the safety of the battery under various mechanical and thermal abuses. As shown in Supplementary Fig. 23 and Fig. 7a, the pouch cell works well after being folded without noticeable dimming. Subsequently, the cell underwent harsh nail penetration tests and then was cut into pieces. Impressively, the pouch cell can still be able to power the LED array without any flame or explosion (Fig. 7b-c). It is attributed to the excellent adhesive ability of PTADOL induced by TTMAP and the in-situ polymerization method, which is not easy to leak or volatilize, so after the pouch cell was cut, PTADOL maintained the reaction interface of the electrodes/SPE to ensure the normal operation of the cell. Additionally, the PTADOL-wrapped electrode can avoid being directly exposed to the air even after the cell fracture is split (Fig. 7d), thus retarding the side reactions between electrodes and the air and guaranteeing the regular operation of the cell. Furthermore, the flammability tests in Fig. 7e and Supplementary Video 1 show that the PDOL based battery burst into flame as soon as contacting with the fire. In sharp contrast, the PTADOL based battery did not burn even in repeat contact with the flame (Fig. 7f and Supplementary Video 2). The good flame retardancy comes from the superior thermal stability of TTMAP crosslinker, thus effectively preventing the cell burning.[42] Apparently, PTADOL has excellent thermal stability and the battery with PTADOL possess conformal and stable electrode/electrolyte interface.
In summary, a novel in-situ formed PTADOL SPE with ultra-stable network structure is well designed by introducing TTMAP as a multifunctional additive, which delivers high tLi+, significantly expanded ESW, strong mechanical strength, and good flame retardancy. The PTADOL SPE improves the performance of SLMBs in four aspects. (1) Fast ion transportation. Benefited from unique structure of TTMAP, an integrated and stable three-dimensional network with rational O-Li+ coordination forms after the in-situ polymerization, which guarantees abundant movable lithium ions and restricts the anions transportation. (2) Superior compatibility with lithium metal anode. The high mechanical stability induced by the stable network structure and high tLi+ enables uniform Li plating/striping without concentration polarization. (3) A significant expanded ESW. The oxidative stability of PTADOL has increased from 4.0 V to 4.6 V because of the stable network structure and the elimination of terminal hydroxyl, which makes it suitable to match with high-voltage cathodes. And applications of the PTADOL in NMC811||Li and LFP||Li batteries all display excellent electrochemical performances without electrolyte decomposition. (4) Outstanding thermal stability. The formed network structure protects PTADOL from depolymerization caused by TFSI−; and the thermostable TTMAP further improves the flame retardancy of the batteries. This work provides new insights into the design of the advanced SPE system for high performance SLMBs.