Stable Cycling of All‐Solid‐State Lithium Metal Batteries Enabled by Salt Engineering of PEO‐Based Polymer Electrolytes

Poly (ethylene oxide) (PEO)‐based polymer electrolytes show the prospect in all‐solid‐state lithium metal batteries; however, they present limitations of low room‐temperature ionic conductivity, and interfacial incompatibility with high voltage cathodes. Therefore, a salt engineering of 1, 1, 2, 2, 3, 3‐hexafluoropropane‐1, 3‐disulfonimide lithium salt (LiHFDF)/LiTFSI system was developed in PEO‐based electrolyte, demonstrating to effectively regulate Li ion transport and improve the interfacial stability under high voltage. We show, by manipulating the interaction between PEO matrix and TFSI−‐HFDF−, the optimized solid‐state polymer electrolyte achieves maximum Li+ conduction of 1.24 × 10−4 S cm−1 at 40 °C, which is almost 3 times of the baseline. Also, the optimized polymer electrolyte demonstrates outstanding stable cycling in the LiFePO4/Li and LiNi0.8Mn0.1Co0.1O2/Li (3.0–4.4 V, 200 cycles) based all‐solid‐state lithium batteries at 40 °C.


Introduction
[6] Impressively, Li-metal polymer batteries with PEO-based electrolytes have been applied in the electric vehicle by Bollor e at 60-80 °C.Nevertheless, the PEO-based SPEs often possess a high proportion of crystalline phase below 60 °C, which significantly curtails the ion transport and solvation, presenting low ionic conductivities (10 À7 -10 À5 S cm À1 ) at room temperature. [7,8]Additionally, the PEO-based SPEs also suffer from the unfavorable oxidative degradation above 4 V, [9] which limits their further coupling with high-voltage cathode materials (> 4.3 V) such as layered LiNi x Co y Mn z O 2 , (x + y + z = 1, x > 0.6), to achieve a high energy density of the batteries.The high oxidability of the layered material at high voltage triggers severe parasitic reactions with the PEO-SPE, generating an unstable cathode-electrolyteinterphase (CEI) at the interface.This CEI overgrows during cycling, leading to interfacial instability and degeneration, and eventually the poor performance of the battery. [10]Therefore, it is essential to unlocking the PEO-based SPE's potential for iontransporting and interfacial compatibility with high voltage cathode, paving way for further development of high-energy all-solid-state batteries.
Because the interactions (including cation-anion, polymer-ion, and Lewis acid-base) intensively affect the ion conductivity and mobility in the PEO-based SPEs, adding a second phase to the SPEs is an effective way to enhance their Li + conductivity and mechanical strength. [11,12]nert fillers, such as Al 2 O 3 , SiO 2 , TiO 2 nanoparticles, and MOFs [5,13,14] are applied to improve the ionic conduction by reducing the crystallinity of the PEO matrix or providing interfacial transporting channels. [15]Besides, employing active fillers, such as garnet, NASICON, and perovskite, [6,12,16] can offer the percolation path for Li + transport to improve the conductivity.Nonetheless, the high content of the fillers in the SPEs is still a concern that may reduce the energy density.Another feasible solution is adding the plasticizers to promote the conductivity by being rubbery in the PEO matrix and thus enhancing the ionic solvation and association. [17][20] These plasticizers are indeed significantly improved the conductivity, but their flammability and vaporability may arouse safety concerns. [15]ailoring additives for the PEO-SPEs have been demonstrated to be an effective way to enhance the Li + conductivity and improve the interfacial stability.Recently, Li 2 S x , LiN 3 , and MgClO 4 [13,15,21,22]   have been proved to be effective additives to facilitate the lithium-ion transport and stabilize the lithium anode, eventually improving the performance of all-solid-state batteries.However, the interfacial compatibility of the electrolyte with the high-voltage cathode has not been well addressed, needing further improvement.As the main salt (LiTFSI) in the SPE is always well-established, researchers are used to optimize the electrolyte by additive and fillers.Some salts are added in SPEs as the role of sacrificial components or additives, such as dual-salt systems of LiTFSI-LiBOB, LiTFSI-LiODFB, LiPF 6 -LiTFSI [23][24][25] to improve the performance.However, the salt engineering (saltcomponents optimization) is rarely reported.Actually, the salt-saltpolymer interactions will strongly affect the properties of the SPEs.Tailoring the salt-components means tuning the interactions and taking advantage of synergistic effects in the SPE, which is not only favoring the ion transport but also resulting in robust interface generation.As there are multitude kinds and components of the salts for selection, salt engineering is an effective and promising strategy to improve the SPEs for high performance all solid state batteries.In this work, we investigate the 1, 1, 2, 2, 3, 3-hexafluoropropane-1, 3disulfonimide lithium salt (LiHFDF)/LiTFSI dual-salt-system for the salt engineering of PEO-based SPEs.LiHFDF, [26] with a large anion, is able to be dissociated in the PEO matrix and presents a notable plasticizing effect.It also can interact with the PEO matrix and LiTFSI salt, promoting ionic conductivity.More importantly, the system enlarges the electrochemical window and enables the generation of stable CEI for NMC811/SPE and LiF-based SEI for the Li/SPE interfaces, enhancing the cyclic capability and suppressing the dendrite growth.The concentration-optimized SPE (Li 1.8 T 1.0 H 0.8 /P(EO) 18 ) achieves maximum Li + conduction of 1.24 9 10 À4 S cm À1 at 40 °C.Li/Li symmetric cells illustrate a long and stable lithium-plating/stripping up to 1000 h at 40 °C, with a maximum current density of 0.2 mA cm À2 .Moreover, the critical current density is enhanced to 0.96 mA cm À2 .Impressively, this novel SPE (Li 1.8 T 1.0 H 0.8 /P(EO) 18 ) coupled with LiFePO 4 and NMC811cathodes display the outstanding electrochemical performances at 40 °C, all-solid-state NMC811/ lithium battery performs a superior cycle ability (capacity retention of 85.7% over 200 cycles under 4.4 V).

Results and Discussion
SPEs with various LiTFSI and LiHFDF contents are fabricated by a solvent-free hot-roller-casting of PEO and salts in an Ar-filled glovebox. [27]The prepared membranes exhibit optically transparent and homogeneous (Figure S1a,b, Supporting Information).The typical SEM images also present uniform and smooth surfaces, revealing that the salts are well solvated and incorporated in the PEO-polymer matrix (Figure S1c,d, Supporting Information). [28]Thermal stability is one of the paramount features of the SPE.Indeed, Thermogravimetric analysis (TGA) curves in Figure S1e, Supporting Information, demonstrate that the onset mass loss temperature (5%) are beyond 360 °C for both the SPEs, revealing the excellent thermal tolerance. [29]Figure 1a,b present the EIS spectra at 40 °C and Arrhenius plots of various SPE membranes, respectively.Comparatively, all the SPEs containing LiHFDF possess higher Li + conductivity than the pure SPEs.The Li 1.8 T 1.0 H 0.8 /P(EO) 18  sample, which has an EO/Li + ratio of 10, exhibits the highest Li + conductivity of 1.24 9 10 À4 S cm À1 at 40 °C.This conductivity value is almost 3 times the Li 1.8 T 1.8 /P(EO) 18 (4.73 9 10 À5 S cm À1 ) with the same EO/Li + ratio, even higher than that of pure LiHFDF based-Li 1.8 H 1.8 /P(EO) 18 sample (5.07 9 10 À5 S cm À1 ).Nevertheless, the higher content of LiHFDF (Li 2.07 T 1.0 H 1.07 /P(EO) 18 ) leads to a slight decrease in the conductivity.Figure 1c presents the XRD patterns for the various samples.In the pure LiTFSI-based Li 1.0 T 1.0 /P(EO) 18 membrane, the pattern appears with two typical peaks at 19.2 o and 23.3 o , indexed as the (120) planes and (112) planes, respectively.The (120) planes parallel to the PEO chain direction, while the (112) planes intersect the chain direction. [30]When the content of LiHFDF is increasing, the (120) peak shifts toward the lower angle side, indicating the enhancement of the d-spacing of the (120) planes.And it is intriguing that the peaks of (112) planes disappear, while the amount of LiHFDF (z) exceeds 0.8(Li 1.8 T 1.0 H z /P(EO) 18 ), revealing the LiHFDF strongly limits the size of (112) planes and therefore reduced or even eliminated their diffractions. [30]For comparison, both the Li 1.8 T 1.8 /P(EO) 18 and Li 1.8 H 1.8 /P(EO) 18 become apparent to be amorphous.These results demonstrate the intensive interactions among the HFDF À , TFSI À and polymer matrix, forming an anisotropic ordering.And this unique anisotropic ordering may facilitate the Li-ion transport.[33] As indicated in Figure 1d, the spectra within the regions of 900-980, 1230-1300, 1300-1380, and 1000-1160 cm À1 can be indexed as the stretching (m), twisting (s) and wagging (x) of the -CH 2 -, and the symmetrical stretching (m s ) of the C-O-C, respectively.With the increase of LiHFDF amounts, the peaks at 1279 and 1241 cm À1 become broadened and decrease in intensities.The distinct peaks at 906 cm À1 for the membranes consisting LiHFDF in the -CH 2stretching (m) regions, can be assigned to the CF 2 rocking.The variation in the peak position, shape, intensity and new peak generation of these C-H bands is suggestive of the strong interactions between the HFDF À , TFSI À , and PEO. [15]The doublet peaks at 1342 and 1359 cm À1 for the Li 1.0 T 1.0 /P(EO) 18 are assigned to the wagging (x) of the -CH 2 -, indicating the existence of crystalline regions in the PEO.When the amount of LiHFDF raises, these two peaks merge into a single peak at 1352 cm À1 .The combination of these two peaks reveals the presence of a more amorphous phase in the PEO.These results also can be seen in the Li 1.8 T 1.8 /P(EO) 18 with high LiTFSI content.In the m s (C-O-C) region, the pure PEO matrix indicates triplet peaks of 1145, 1100, and 1060 cm À1 .After adding the pure LiTFSI, the shape, intensity, and position of the membrane (Li 1.8 T 1.8 /P(EO) 18 ) slightly change, which is due to the mediation of the LiTFSI and PEO.In sharp contrast, added the pure LiHFDF, the membrane (Li 1.8 H 1.8 /P(EO) 18 ) exhibits significantly varied triplet peaks of 1030 (blue *), 1090 (blue %), and 1170 cm À1 (blue &), revealing the strong interaction between LiHFDF and PEO.When the membranes contain dual salts, the curves alter the shape, intensity, and position.Impressively, the Li 1.8 T 1.0 H 0.8 /P(EO) 18 membrane presents red-shifted peaks (blue *, %, &, and green #) compared to the pure Li 1.8 H 1.8 /P(EO) 18 and Li 1.8 T 1.8 /P(EO) 18 , verifying the strong intermolecular interaction of TFSI À -HFDF À -PEO. [4,31]Also the Li + transference number is the fraction of the total electric current carried in the electrolyte by Li + .Although possessing the same EO/Li + ratio of 10/1, the Li 1.8 T 1.0 H 0.8 /P(EO) 18 (t Li + = 0.36) has a much higher transference number than the Li 1.8 T 1.8 /P(EO) 18 (t Li + = 0.21), which agrees well with the HFDF À -PEO-TFSI À interactions (Figure S2, Supporting Information).These results are highly consistent with the observation of the 6 Li NMR. Figure 2a compares the 6 Li NMR spectra of the Li 2 of 8 peak-fitting results also reveal the larger mobile-Li + component (Gaussian peak, 62.9%) for the Li 1.8 T 1.0 H 0.8 /P(EO) 18 , while the Li 1.8 T 1.8 /P (EO) 18 only contains that of 56.1%. [14,15]These results also are consistent with FTIR spectra (Figure S3a,b, Supporting Information), evidenced by the ion-paired component (around 745 cm À1 ) of the Li 1.8 T 1.0 H 0.8 /P(EO) 18 (30.8%)being much smaller than that of the Li 1.8 T 1.8 /P(EO) 18 (42%). [13]e also calculated the binding energies of the LiTFSI-PEO, LiHFDF-PEO, LiTFSI/LiHFDF-PEO, suggesting the strong molecular interactions of the LiTFSI/LiHFDF-PEO system (Figure S3c-e, Supporting Information).In the dual-salt system, the quite strong but different interactions in LiTFSI/LiHFDF with PEO segments will lead to anisotropic ordering.The anisotropic ordering may provide more favorable 'active sites' or 'paths' for Li + transport.Furthermore, the strong interactions will facilitate dissociation of the salts, and restrict the motion of the anions, allowing more 'free' Li + ions to hop in the SPEs and thus improving the Li + conductivity.Since the interaction between the PEO matrix and HFDF À , TFSI À anions is enhanced, more free-Li + -ions are released for transportation.Based on the favorable interaction, the electrochemical stability of the SPE can be considerably promoted.Remarkably, the higher voltage stability of the Li 1.8 T 1.0 H 0.8 /P(EO) 18 membrane is indicated in the LSV profiles (Figure 2b). [6]ymmetric Li/Li 1.8 T 1.0 H 0.8 /P (EO) 18 /Li and Li/Li 1.8 T 1.8 /P (EO) 18 /Li cells are assembled and cycled at 40 °C to evaluate the stability of SPEs/lithium anode interface. [16,22]Figure 3a illustrates the long-term cycling of the Li/Li 1.8 T 1.0 H 0.8 /P(EO) 18 / Li cell under 0.05, 0.1, 0.15 and 0.2 mA cm À2 .As indicated in the figure, the overpotential increases with the current density.The symmetric cell keeps stable cycling for 1000 h and exhibits an overpotential of 0.28 V at 0.2 mA cm À2 without short-circuiting and hysteresis potential increasing, revealing the formation of a stable and robust interface between the SPEs and the lithium anode. [6,13]or comparison, the baseline Li/ Li 1.8 T 1.8 /P(EO) 18 /Li presents a high over-potential of 0.59 V at 0.1 mA cm À2 with obvious hysteresis potential increasing and quickly forms lithium dendrites for only 230 h.Such poor performance can be ascribed to the low ionic conductivity and large interfacial resistance arising from the generated unstable interface. [9]igure 3b compares the galvanostatic cycling of Li 1.8 T 1.0 H 0.8 /P(EO) 18 and Li 1.8 T 1.8 /P(EO) 18 with a rate-performance test to determine the critical current densities.According to expectation, the voltage enhances with the increasing current densities for each SPE.The  Energy Environ.Mater.2024, 7, e12580 Li 1.8 T 1.8 /P(EO) 18 possesses a critical current density of 0.6 mA cm À2 , while Li 1.8 T 1.0 H 0.8 /P(EO) 18 has a high critical current density of 0.96 mA cm À2 .To further investigate the interfacial properties, electrochemical impedance spectra (EIS) before and after cycling were performed.Before cycling, the cell resistance for the Li/Li 1.8 T 1.8 /P (EO) 18 /Li is ~890 Ω，then considerably increases to ~1218 Ω after cycling, revealing the overgrowth of solid-state electrolyte interphase (SEI).In contrast, the cell resistance for the Li/Li 1.8 T 1.0 H 0.8 /P(EO) 18 / Li is only ~455 Ω, with a small variation to ~468 Ω after cycling, demonstrating the generation of a stable and compatible interface.The SEM images (inset pictures in Figure 3c,d) after cycling also identify the distinct surface morphology.The lithium metal anode with Li 1.8 T 1.8 /P(EO) 18 appears loose and moss-shaped surface, suggesting the unstable Li + plating/stripping and dendrite growth.In sharp contrast, the Li anode with Li 1.8 T 1.0 H 0.8 /P(EO) 18 illustrates a dense and smooth surface with layered shaped, without any crack, whisker, and moss-shaped deposition.This could be ascribed to the formed robust and insulated SEI on the surface of anode, which induces the lithium growth along the parallel direction rather than vertical.As indicated in XPS C1s spectra of Figure S4, Supporting Information, the C-C species has been significantly increased in the Li 1.8 T 1.8 /P(EO) 18 membrane after cycling, which implies the degeneration of the SPE/Li anode interface.In contrast, the C-C species generation has been suppressed in the Li 1.8 T 1.0 H 0.8 /P(EO) 18 . [21]urthermore, to demonstrate the practicality, two typical commercial cathode chemistries, LiFePO 4 and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathodes are assembled in the full cells and evaluated with the various SPEs (Li 1.8 T 1.0 H 0.8 /P(EO) 18 and Li 1.8 T 1.8 /P(EO) 18 ) (Figure 4).Galvanostatic cycling of the LiFePO 4 /Li 1.8 T 1.0 H 0.8 /P(EO) 18 /Li exhibits the discharge capacities of 174.5, 151.7, 135.6, 98.3 and 72.7 mAh g À1 at the C-rates of 0.1, 0.2, 0.3, 0.5 and 1C, respectively.And the discharge also can recover to 174 mAh g À1 after the C-rate baking to 0.1 C. (Figure 4a,b) These values are higher than those of the LiFePO 4 / Li 1.8 T 1.8 /P(EO) 18 /Li, revealing the kinetic superiority of the Li 1.8 T 1.0 H 0.8 /P(EO) 18 (Figure S5a, Supporting Information).The LiFePO 4 /Li 1.8 T 1.0 H 0.8 /P(EO) 18 /Li keeps an impressive capacity retention of 95.5% (300th/1st) after 300 cycles at 0.2 C, with columbic efficiencies close to 100% during cycling, demonstrating the considerable stability and compatibility in this full cell with novel dual-salt SPE system.For comparison, the LiFePO 4 /Li 1.8 T 1.8 /P(EO) 18 /Li only can maintain a capacity retention of 75% after 300 cycles (Figure 4e, Figure S6a,b, Supporting Information).The interface stability of the LiFePO 4 /Li 1.8 T 1.8 /P(EO) 18 /Li is also verified by the EIS spectra after 1st and after 200th (Figure S6c, Supporting Information).The resistance of the LiFePO 4 /Li 1.8 T 1.8 /P(EO) 18 /Li has significantly changed after 200 times cycling.While the resistances after 1st and after 200th for the LiFePO 4 /Li 1.8 T 1.0 H 0.8 /P(EO) 18 /Li cell are almost overlapped, highlighting that the robust and protective interface has been generated (Figure S6d, Supporting Information).
The NMC811 is regarded as high-energy and promising cathode material.Nevertheless, it remains challenging to achieve the stable cycling of PEO-based all-solid-state battery with NMC811 cathode under high voltage (>4.3 V). Figure 4c presents the galvanostatic charge-discharge curves and cycling plot at various C-rates of NMC811/Li 1.8 T 1.0 H 0.8 /P(EO) 18 / Li cell with a high cutoff voltage of 4.4 V. NMC811/ Li 1.8 T 1.0 H 0.8 /P(EO) 18 /Li cell delivers a high discharge curve of 190.6 mAh g À1 at 0.1 C, with a well-defined charge-discharge curve.It shows the discharge capacities of 157.9, 117.6, 76.4 mAh g À1 at 0.2 C, 0.3 C, and 0.5 C, respectively, recovering to 168 mAh g À1 after successive cycling at 0.1 C. Since the superior ionic transporting properties of Li 1.8 T 1.0 H 0.8 /P(EO) 18 (high ionic conductivity and transference number), the NMC811/Li 1.8 T 1.0 H 0.8 /P(EO) 18 / Li cell possesses better ratecapability than that of the NMC811/Li 1.8 T 1.8 /P(EO) 18 /Li cell (Figure S5b, Supporting Information).The NMC811/ Li 1.8 T 1.0 H 0.8 /P(EO) 18 /Li cell also indicates an excellent stable longterm cycling at 0.2 C. A discharge capacity of 114.6 mAh g À1 can be achieved after 200 cycles, with a high capacity-retention of which arise from the decomposition of the electrolyte and dissolution of the transition-metals, strongly highlighting that the formed CEI can effectively protect the electrode and electrolyte from intensive degeneration. [35]EM and XRD results also confirm these results.After cycling with Li 1.8 T 1.0 H 0.8 /P(EO) 18 , the NMC811 secondary particle still maintains intact, with an easily discerned primary particle, which highlights the formation of thin and robust CEI (Figure 5h).While the particle cycled with Li 1.8 T 1.8 /P(EO) 18 presents cracks, shape-distorted, covered with an irregular thick CEI film (Figure 5g). Figure 5i compares the XRD results of the cathode electrodes with two different SPEs after cycling.The intensity ratio I(003)/I(104) is associated with the degree of the rock-salt-formation.The degree of formation is in inverse proportion to the I(003)/I(104) ratio.The ratio for the Li 1.8 T 1.8 /P(EO) 18 (1.48) is much smaller than the Li 1.8 T 1.0 H 0.8 /P(EO) 18 (2.03),revealing more generation of the rock-salt structure, which means that the applied Li 1.8 T 1.0 H 0.8 /P(EO) 18 can maintain the structural integrity of the NMC811.Furthermore, the peak separation between (006) and ( 102) is also an index of structural stability, which can be clearly discriminated in the cathode with Li 1.8 T 1.0 H 0.8 /P(EO) 18 but hardly detected in the cathode with Li 1.8 T 1.8 /P(EO) 18 .
As a chemical-selective and surface-sensitive tool, TOF-SIMS is powerful to probe quantitatively the composition of the CEI. [15,16]TOF-SIMS was employed to quantify and map the CEI of long-term cycling cathodes of NMC811.Figure 6 presents the depth profiles (Figure 6a, b) and 3D-reconstruction images (Figure 6c,d) of the representative species, LiF À , LiF 2 À , and C 2 H 2 O À , which may arise from LiF, organic species containing Li, and F, and the decomposition of PEO, respectively. [13,14]The profiles indicate both the CEI films are multi-layers, with inner layer of LiF.The differences can be clearly visualized with the 3D reconstruction, illustrating the depth distribution fragments (Figure 6c,d).Both of the CEI generated by the SPEs are multi-layered structures and rich in LiF À containing species, which are considered excellent ion-conductor and electronic-insulator. [36,37] The NMC811cathode with Li 1.8 T 1.0 H 0.8 /P(EO) 18 is covered with a continuous and dense LiF-based CEI layer. [21]urthermore, the well-interpenetrated organic species play the role of the 'buffer-zone', relieving the strain stress during repeated chargedischarge, stabilizing the CEI.
This robust CEI significantly passivates the cathode and shields the electrolyte from decomposition at high voltage, enabling long-term cycling (Figure 4f).In stark contrast, the CEI formed on the cathode with Li 1.8 T 1.8 /P(EO) 18 exhibits a break and sparse CEI, which hardly protects the cathode and SPE from degeneration at high voltage, leading to unstable cycling.

Conclusion
In summary, we have reported the promising salt engineering of the LiTFSI-LiHFDF dual-salts system in PEO-based electrolytes on the performance of lithium metal batteries.The introduction of LiHFDF promotes the ionic conductivity of the SPEs and improves the compatibility of the cathode/electrolyte interface at high voltage by generating robust interphase against high-voltage oxidation.The interaction of HFDF À -TFSI À with the PEO matrix and anisotropic ordering remarkably enhance the ionic conductivity and Li + transference number.Hence, the optimized Li 1.8 T 1.0 H 0.8 /P(EO) 18 electrolyte attains a maximum Li + conductivity of 1.24 9 10 À4 S cm À1 at 40 °C.Owing to the stabilized lithium/electrolyte interface, which suppresses the lithium dendrite growth, Li/Li symmetric cell illustrates a long and stable lithiumplating/stripping up to 1000 h and a significantly enhanced critical current density of 0.96 mA cm À2 at 40 °C.Moreover, the Li 1.8 T 1.0 H 0.8 /P(EO) 18 electrolyte demonstrates outstanding stable cycling in the LiFePO 4 and high voltage LiNi 0.8 Mn 0.1 Co 0.1 O 2 (4.4 V, 200 cycles) based all-solid-state lithium batteries at 40 °C.The in-situ generated LiNi 0.8 Mn 0.1 Co 0.1 O 2 /Li 1.8 T 1.0 H 0.8 /P(EO) 18 interphase mainly consists of dense LiF, stabilizing the interface and protecting the cathode material from degeneration upon high voltage oxidation.Thus, the high-voltage cyclic capability can be considerably promoted.Since we just presented a typical dual-salt case in this work, the salt engineering can further adopt the multiple salt components (≥3) to optimize.Also, the powerful technologies, such as machine learning and high-throughput screening can also be combined.Our results highlight the potential of the salt-engineering strategy to regulate and improve the performance of the polymer electrolytes for future all-solid-state lithium batteries.

Experimental Section
Experimental details are described in the Supporting Information.Energy Environ.Mater.2024, 7, e12580

Figure 1 .
Figure 1.Characterization of SPE membranes.a) EIS spectra at 40 °C, b) Arrhenius plots at various temperatures, c) XRD patterns, and d) FTIR spectra at various wavenumbers.

Figure 3 .
Figure 3. Symmetric Li/SPE/Li cell performance, a) Long-term cyclic performance and b) Critical current density test plots at 40 °C of the symmetric Li/Li 1.8 T 1.8 /P(EO) 18 /Li and Li/Li 1.8 T 1.0 H 0.8 /P(EO) 18 /Li cells, and EIS plots of c) Li/ Li 1.8 T 1.8 /P(EO) 18 /Li cell and d) Li/Li 1.8 T 1.0 H 0.8 /P(EO) 18 /Li at 40 °C.The inset pictures are the SEM images of the corresponding lithium metal anode after cycling with the Li 1.8 T 1.8 /P(EO) 18 and Li 1.8 T 1.0 H 0.8 /P(EO) 18 , respectively.

Figure 5 .
Figure 5. XPS spectra of the NMC811 cathodes with the Li 1.8 T 1.8 /P(EO) 18 and Li 1.8 T 1.0 H 0.8 /P(EO) 18 membranes of C 1s a) and d), O 1s b) and e), and F 1s c, f).SEM images of the NMC811 cathodes with the g) Li 1.8 T 1.8 /P(EO) 18 and h) Li 1.8 T 1.0 H 0.8 /P(EO) 18 membranes after 200 times cycling, respectively.i) XRD patterns of the cycled cathodes.