2.1 Preparation of Solid Polymer Electrolyte
The SPE film consisted of three monomers of 1-allyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide (IL), 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol diacrylate (OFHDODA) and vinyl ethylene carbonate (VEC) blended with lithium salt (LiTFSI), and photoinitiator (IRGACURE 819) was prepared by solution casting and UV light curing sequentially, as schematically illustrated in Fig. 1a (see the Experimental Section for details). Supplementary Fig. 1 shows the colorless, transparent electrolyte turned from liquid into a solid after UV light curing. The SPE with IL is nonflammable as shown in Supplementary Video 1. Figure 1b shows the Fourier transform infrared (FTIR) spectra of the electrolyte before and after UV light curing. The peak at 1640 cm− 1 assigned to the stretching vibration of C = C bonds became invisible after UV light curing. Thermogravimetric analysis (TGA) of the electrolyte before and after UV light curing is shown in Fig. 1c. Before UV light curing, the electrolyte exhibited a significant mass loss of 16.6% between 125 and 200°C. In contrast, the electrolyte presented a single decomposition temperature of 325℃ after UV light curing. Combined with both FTIR spectra and TGA, it demonstrates that IL, OFHDODA and VEC monomers of the SPE were copolymerized by the functional groups of C = C bonds after UV light curing, which provides a higher thermal stability.
2.2 Ionic Conductivity of Solid Polymer Electrolyte
The ionic conductivity of the SPE was sensitive to the ratio of each monomer. Here, a molar ratio of [LiTFSI] to [the monomers] was set as 1:2. The molar ratio of [IL] to [OFHDODA] was tuned from 10.5:2 to 2.5:2 at 25°C, which reached a maximum ionic conductivity of 0.16 mS cm-1 under 8.5:2 (Fig. 2a). The ionic conductivity further increased from 0.49 to 1.37 mS cm-1 by incorporating VEC with a molar ratio of [IL]:[OFHDODA] to [VEC] from 8.5:2:1 to 8.5:2:3 at 25 ℃ (Fig. 2b). However, the ionic conductivity decreased with a higher VEC molar ratio. The conductivity of P(IL-OFHDODA-VEC) with optimized molar ratio is comparable to that of state-of-the-art SPEs.18–20, 22,26 The Li+ transference number (tLi+) ~ 0.40 at 25 ℃ is the ratio of Li+ transport to the total ions (cations and anions) that transport in SPEs, which can be obtained from chronoamperometry (CA) and electrochemical impedance spectrum (EIS) tests (Fig. 2c). The superior conductivity and decent Li+ transport number of SPEs will benefit a high capacity and superior rate performance of LMBs.
The effects of VEC on the superior ionic conductivity of the SPE have been investigated via Raman spectroscopy, which is sensitive to the bonding state of TFSI-.27 Fig. 2d shows Raman spectra of stretching vibrations of N‒S bonds in TFSI- with different SPE compositions. Specifically, the Raman shifts of N‒S bonds in 1 M LiTFSI/DME and LiTFSI crystals are regarded as references for dissociation and undissociated LiTFSI salt states, respectively, which guide the fitting of the spectra of
P(IL-OFHDODA) and P(IL-OFHDODA-VEC) to distinguish different states of TFSI- (Fig. 2d). The ratio of free state TFSI- to undissociated state TFSI- in the SPE increases from 34:66 without VEC to 67:33 with VEC. This result indicates that more dissociation state LiTFSI of P(IL-OFHDODA-VEC) than that of P(IL-OFHDODA) can be attributed to the interaction between VEC and Li+, leading to a higher ionic conductivity.
The superior ionic conductivity of the SPE with VEC was also attributed to the low Li+ transport energy barrier, as shown in Fig. 2e. The activation energy (Ea) of P(IL-OFHDODA) and P(IL-OFHDODA-VEC) extracted from the temperature dependence of ionic conductivities from 25 to 75°C was 0.43 eV and 0.26 eV, respectively. The thermal transition was investigated by differential scanning calorimetry (DSC, Supplementary Fig. 2). P(IL-OFHDODA-VEC) exhibited a lower glass transition temperature of -63°C than that of P(IL-OFHDODA) at -59°C, which indicates that VEC can plasticize the P(IL-OFHDODA) segment to activate segment movement and increase the free volume for a low Li+ transport energy barrier.
The effects of VEC on the low Li+ transport energy barrier of the SPE have been further explored via 6Li solid-state nuclear magnetic resonance (NMR), which is sensitive to the local chemical environment of Li+.28,29 The 6Li NMR spectra of P(IL- OFHDODA) and P(IL-OFHDODA-VEC) are shown in Fig. 2f. For P(IL- OFHDODA), three peaks representing different lithium environments were identified by fitting the 6Li NMR spectrum. Among them, the light blue peak at -1.2 ppm has the largest proportion of ~ 73%, which has the same chemical shift as the 6Li NMR peak of the LiTFSI crystal. For P(IL-OFHDODA-VEC), five peaks representing different lithium environments were identified by fitting the 6Li NMR spectrum peaks. Compared to the 6Li NMR spectrum of P(IL-OFHDODA), three peaks emerge at -0.2, -1.7 and − 2.9 ppm. Among them, the red peak at -0.2 ppm has the largest proportion of ~ 55%, which indicates that a larger proportion of Li+ changes from strongly bound states to weakened bound strength.
Since the interaction between Li+ and polar atoms of the functional group of the SPE segments was an important factor affecting the chemical environment of Li+, density functional theory (DFT) calculations of the adsorption energy of Li+ on polar atoms of different monomers or dimers were carried out. As shown in the optimized geometry models in Supplementary Fig. 3, the Li+ always have the O atom to be the closest neighbor for different monomers or dimers to achieve adsorption energy minimalization. Two types of interaction between Li+ and O atoms were considered. One is the interaction between Li+ and a polar atom of monomers (Li+-O), and the other is the bridging interaction between Li+ and two polar atoms of monomers or dimers (O-Li+-O). Generally, adsorption energy of O-Li+-O type is more negative than that of Li+-O, as listed in Supplementary Table 1.
As the small adsorption energy of Li+ on O leads to a chemical shift of Li close to 0 ppm, the 6Li NMR peaks of P(IL-OFHDODA-VEC) can be distinguished.30 The red fitting peak of P(IL-OFHDODA-VEC) in Fig. 2f with a lower chemical shift originates from the interaction between Li+ and the O atom in the VEC side chain (Li+-OVEC), which exhibited the lowest adsorption energy of -238.47 kJ mol− 1. Compared to the 6Li NMR spectrum of P(IL-OFHDODA), the emerging peak of P(IL-OFHDODA-VEC) at -1.7 ppm originate from the interaction between Li+ and the O atoms in the VEC-VEC side chains (OVEC-Li+-OVEC) with adsorption energy of -262.46 kJ mol− 1. While the emerging peak of P(IL-OFHDODA-VEC) at -2.9 ppm is attributed to the interaction between Li+ and the O atoms in the VEC-IL(OVEC-Li+-OIL) with adsorption energy of -431.37 kJ mol− 1. The largest proportion of Li+ interacting with the VEC side chain demonstrates that incorporation of VEC into the polymer segment contributes to the dissociation of LiTFSI, which is consistent with the results obtained by the Raman spectroscopy. As a result, the interaction of Li+ with the VEC side chain is beneficial to achieve the high ionic conductivity at 25℃.
2.3 Electrochemical Stability of Solid Polymer Electrolytes
The ESW of the SPE, which is a critical factor in working with a high-voltage cathode, was investigated. Figure 3a shows the LSV of an asymmetrical cell of Li|P(IL-OFHDODA-VEC)|carbon at 25 ℃, which shows no obvious oxidation current before 5.08 V (vs. Li+/Li), indicating a wide ESW over 5 V. To understand the contribution of polymer components to the ESW, LSVs of PIL, POFHDODA and PVEC and each pair of them were measured as shown in Supplementary Fig. 4. Figure 3b shows that the extracted ESW of PIL, POFHDODA and PVEC is 5.51 V, 5.13 V, and 4.35 V (vs. Li+/Li), respectively. Among them, PVEC possesses the narrowest ESW. Meanwhile, the ESW of P(IL-VEC), P(OFHDODA-VEC) and P(IL-OFHDODA) is 4.39 V, 5.12 V, and 5.31 V (vs. Li+/Li), respectively. It is noted that the ESW of P(OFHDODA-VEC) was close to that of P(IL-OFHDODA-VEC) and significantly higher than that of P(IL-VEC), indicating that OFHDODA plays an important role in improving electrochemical stability of the SPE.
The effect of each component on the ESW of the SPE can be understood from the chemical structure. For example, PIL has a stable N-containing five-membered heterocyclic structure without unsaturated bonds in the cation, benefiting its wide ESW over 5 V. The carbonate group on the VEC segment has a high electron density, which may easily lose electrons and be oxidized, resulting in the narrow ESW. Although the POFHDODA segments also contain ester groups, many fluorine atoms on the segment have a strong electron-withdrawing effect, which effectively reduces the electron cloud density of ester groups, resulting in a wider ESW. In addition, this electron-withdrawing effect of polyfluorinated group can transmit through the carbon chain to reduce the electron density of other side chains on the copolymer, such as VEC. By substitution nonfluorinated crosslinkable HDODA for OFHDODA, the ESW of SPE reduced to 4.55 V (vs. Li+/Li) as seen in Supplementary Fig. 5. While replacing OFHDODA by fluorinated non-crosslinkable OFPMA, the ESW of SPE ~ 4.71 V (vs. Li+/Li) was slightly higher than that with HDODA but lower than that with OFHDODA as shown in Supplementary Fig. 5. These facts indicate that not only the polyfluorinated groups with strong electron-withdrawing effect improves oxidation resistance of the SPE, but also the crosslinked structure may inhibit sidechain movement to reduce possibility of contact between oxygen containing polar groups and electrodes induced oxidation as supported by a higher glass transition temperature (Supplementary Fig. 2). In addition, the inductive electron-withdrawing effect of polyfluorinated segments in crosslinked network may have more chance to reduce electron density of adjacent oxygen containing polar groups, resulting in further enhanced oxidation resistance of the SPE.
To further investigate the effect of polyfluorinated crosslinking on the ESW of SPE, some other SPEs containing oxygen-bearing polar groups with and without POFHDODA have been tested. As shown in Supplementary Fig. 6, these SPEs all exhibited wider ESW after incorporating polyfluorinated crosslinking agent.
2.4 Anode Interface Compatibility
The stability of the SPE to lithium metal plays an important role in determining the cycle life of LMBs. Figure 4a shows periodically charged and discharged curves for 1 h in Li||Li symmetric cells. The SPE with OFHDODA crosslinking agent-based cell exhibited a flat polarization curve with a constant polarization as low as 43 mV for up to 2500 h. In contrast, the SPE without OFHDODA-based cell short-circuited after 388 h. Supplementary Fig. 7 shows the voltage response of P(IL-OFHDODA-VEC) from 0.1 mA cm− 2 to 0.8 mA cm− 2, which maintains excellent interface compatibility even under a current density as high as 0.8 mA cm− 2.
To understand the improvement of interface stability between the Li and the SPE by adding OFHDODA, the SEM images of lithium deposition morphology on the Li from disassembled Li|P(IL-OFHDODA-VEC)|Li and Li|P(IL-VEC)|Li cells after cycling were compared with each other (Fig. 4b). It is obvious that lithium deposited unevenly and dendrites are formed when OFHDODA is not added. For Li|P(IL-OFHDODA-VEC)|Li, however, the morphology of the lithium deposition was rather uniform without lithium dendrites. Moreover, nanomechanical images of P(IL-VEC) and P(IL-OFHDODA-VEC) were measured. As seen in Fig. 4c, Young's modulus is uniformly distributed, and P(IL-OFHDODA-VEC) ~ 142 MPa is much higher than that of P(IL-VEC) ~ 27 MPa, indicating the improved mechanical strength of the SPE by introduction of the crosslinking agent. As a result, the growth of lithium dendrites can be effectively resisted with P(IL-OFHDODA-VEC).
The cycling stability of polymer electrolytes against the Li also depends on the solid electrolyte interphase (SEI) properties of the Li|SPE interface. X-ray photoelectron spectroscopy (XPS) was conducted to investigate the surface compositions of the Li metal before and after long-term cycling. As shown in Fig. 4d, the F 1s spectrum of the Li metal exhibits no obvious fluorine product before cycling, but a significant Li-F signal is found at 685.5 eV after cycling.31 Meanwhile, the S 2p spectrum of the Li metal shows no obvious signal, while six peaks at 160.5 eV, 161.7 eV, 166.0 eV, 167.9 eV, 168.6 eV and 169.3 eV ascribed to the S 2p3/2 characteristic peaks of Li2S, Li2S2, RSO2Li, RSO3Li, Li2SO3 and Li2SO4, respectively, emerge after cycling (Fig. 4e).32,33 The N 1s spectrum shows that two peaks at 398.4 eV and 401.1 eV reflect the composition of Li2N-SO2− and Li3N after cycling.34 In contrast, the Li metal exhibits no obvious Li3N before cycling (Fig. 4f).
The evidence that the species of containing sulfur, e.g., Li2S, etc., appearing after cycling suggests that the stable SEI layer mainly originates from the decomposition of TFSI−. Notably, the characteristic peaks of Li3N and LiF indicate that they also participate in the construction of a stable SEI. The presence of Li3N and Li2S in the SEI layer has also been reported to have beneficial effects for fast Li+ ion transfer.35,36 LiF in the SEI layer is effective in suppressing the continuous decomposition of the SPE induced by the interfacial electrochemical reaction.37 Therefore, the SEI containing Li3N, Li2S and LiF is beneficial to inhibit the further decomposition of the SPE and ensure uniform current density during cycling.
2.5 Cathode Interface Compatibility
The interface compatibility between SPE and cathode also plays an important role in determining the cycling stability of LMBs, especially at a high cutoff voltage. Here, NCM523 was selected to assemble the cell due to its superior stability at a high cutoff voltage of ~ 4.5 V. The morphology of the NCM523 cathode particles was observed by high-resolution transmission electron microscopy (HRTEM), and it is found that there is an amorphous cathode electrolyte interphase (CEI) layer of 2 nm before cycling, which increase to 4 nm after 200 cycles at a cutoff voltage of 4.5 V (Fig. 5a). Combined with time-of-flight secondary-ion mass spectrometry (TOF-SIMS) (Fig. 5b) and energy dispersive X-ray spectroscopy (EDX) (Supplementary Fig. 8), it is confirmed that the CEI layer contained a large amount of LiF and gradually became homogeneous and dense with cycling, resulting in a slight increase in interfacial impedance (Supplementary Fig. 9). The LiF-rich CEI layer not only has good electronic insulation but also avoids the direct contact of the SPE with the cathode surface, which can prevent the high valence nickel from catalyzing further decomposition of electrolyte under high voltage.38,39
To understand the formation mechanism of LiF, XPS was employed to investigate the change in composition of the NCM523 cathode before and after cycling. The F 1s spectrum exhibits a higher peak intensity at 685.5 eV corresponding to Li-F after cycling than that before cycling (Fig. 5c), which is consistent with the increased thickness of LiF as measured by TOF-SIMS and EDX. In addition, the peak intensity of the C 1s spectrum at 293.3 eV corresponding to CF3 decreases, and the N 1s spectrum at 401.1 eV corresponding to Li2N-SO2− appears after cycling (Fig. 5d). In contrast, the C 1s spectrum also reveals that the characteristic peaks of C-C/C-H, C-O, CH2-CF2, C = O and CF2 at 284.8 eV, 285.7 eV, 287.6 eV, 288.5 eV and 290.4 eV, respectively,40 show little change before and after cycling.
The formation of LiF before cycling may be due to the photoinduced substitution reaction between F-containing radicals and LiOH on the cathode surface. The increased thickness of LiF after cycling originates from the decomposition of TFSI−, which is consistent with the decreased C 1s peak at 293.3 eV corresponding to the CF3 group of TFSI−.41 Meanwhile, the characteristic peaks of C-C/C-H, C-O, CH2-CF2, C = O and CF2 exhibit little change before and after the cycle, indicating that the polymer main chain and side chain maintain high stability. In addition, the spectrum of N 1s shows that TFSI− is decomposed to Li2N-SO2− after cycling. However, the C-N+ of the pyrrole side chain in the electrolyte does not change significantly, which further indicates that the polymer is stable (Fig. 5e).
2.6 Performance of Solid Polymer Electrolyte based Li Metal Cell
Based on the SPE with the high ionic conductivity, wide ESW, superior mechanical strength and excellent interface compatibility, Li|P(IL-OFHDODA-VEC)| NCM523 batteries (1C = 180 mAh g− 1 ) were assembled to evaluate electrochemical performance. Figure 6a and 6b shows the galvanostatic charge‒discharge curves of the full cell with a cutoff voltage of 4.5 V at 30°C, which exhibits a high initial discharge capacity of 164.20 mAh g− 1 at 0.5 C and outstanding cycling stability with a discharge capacity of 146.96 mAh g− 1 even after 200 cycles. To the best of our knowledge, this is one of the fewest polymer electrolytes with self-supporting properties for 4.5 V class LMBs with a high capacity retention of ~ 90% after 200 cycles (Supplementary Table 2). Figure 6c shows the rate performance of the full cell as the current densities increased from 0.1 C to 2 C. The full cell exhibits a superior specific capacity of 186.79 mAh g− 1 at 0.1 C, and ~ 84.39 mAh g− 1 at 2 C, which is attributed to the high ionic conductivity and Li+ transference number.
The universality of the SPE was further confirmed by assembling Li|P(IL- OFHDODA-VEC)|LiFePO4 (LFP) batteries (1C = 165 mAh g− 1 ). The LFP cell with a cutoff voltage of 4.0 V at 30 ℃ exhibits a high initial discharge capacity of 161.15 mAh g− 1 at 0.5 C and retains a discharge capacity of 140.72 mAh g− 1 even after 600 cycles (Supplementary Fig. 10). The LFP cell also exhibits a superior rate performance with a specific capacity of 170 mAh g− 1 at 0.1 C, and ~ 122.28 mAh g− 1 at 2 C (Supplementary Fig. 10)
Li|P(IL-OFHDODA-VEC)|NCM523 pouch cell have been assembled for practical application exploration (Fig. 6d-f). The pouch cell can simultaneously light up a SINANO logo composed of 70 blue light-emitting diode (LED) lamps connected in parallel. When the pouch cell was cut or bent in a fully charged state, it can still light up the logo, indicating a high safety and superior mechanical stability.