Regulating Lewis Acid–Base Interaction in Poly (ethylene oxide)‐Based Electrolyte to Enhance the Cycling Stability of Solid‐State Lithium Metal Batteries

Solid polymer electrolytes (SPE) offer an outstanding choice because of their lightweight, flexibility, and excellent thin‐film forming ability. However, the low ionic conductivity and poor lithium ion transfer number (tLi+) restrict its application in all‐solid‐state lithium batteries (ASSLBs). Herein, UIO66‐X metal‐organic frameworks with controllable Lewis basicity, acidity, or neutrality functional groups are synthesized successfully and then incorporated into the poly (ethylene oxide) (PEO) matrix to fabricate SPE. The influence of different organic ligands on the interface interaction between PEO and LiTFSI is investigated by solid‐state nuclear magnetic resonance, Fourier transform infrared spectoscopy and Raman tests, as well as density functional theory calculations. The Lewis acidity group plays a key role in enhancing the ionic conductivity and tLi+. As a result, the constructed Li–Li symmetrical cells retain stable cycling for 2300 h and the LiFePO4‐based ASSLBs deliver outstanding electrochemical properties with 147 mAh g−1 of reversible capacity after 500 cycles at 1C and 60 °C, 131 mAh g−1 after 150 cycles at 0.1C and 30 °C. The fabricated SPEs are self‐standing and flexible with good mechanical stability, demonstrating the great potential for practical application. The results can guide choosing the inorganic filler to prepare high‐performance SPE.


Introduction
With the rapid development of electric vehicles, portable electronic devices, and energy integration systems, the development of Li-ion batteries (LIB) with good safety and higher energy density, as well as longer cycle life, is eagerly required. [1]ithium metal anode possesses ultrahigh theoretical specific capacity (3860 mAh g À1 ) and low electrochemical potential (À3.04 V), which enables the possibility of high-energydensity lithium batteries. [2,3]However, the uncontrollable dendrite growth will puncture the polypropylene (PP) separator with low mechanical modulus and give rise to severe safety hazards, including thermal runaway, combustion, and explosion originating from the inflammable and unstable features of liquid electrolyte, which limits the application of lithium metal batteries (LMBs) based on conventional liquid electrolytes greatly. [4,5]8][9] Solid polymer electrolytes (SPE) with lightweight, flexible, and excellent thin-film forming ability in comparison with ceramic electrolytes show considerable advantages in solid-state batteries. [10]In recent years, polyethylene oxide (PEO) is the most researched polymer matrix due to its outstanding performance in dissolving Li salts and low glass transition temperature. [11]The transmission Solid polymer electrolytes (SPE) offer an outstanding choice because of their lightweight, flexibility, and excellent thin-film forming ability.However, the low ionic conductivity and poor lithium ion transfer number (t Li þ ) restrict its application in all-solid-state lithium batteries (ASSLBs).Herein, UIO66-X metal-organic frameworks with controllable Lewis basicity, acidity, or neutrality functional groups are synthesized successfully and then incorporated into the poly (ethylene oxide) (PEO) matrix to fabricate SPE.The influence of different organic ligands on the interface interaction between PEO and LiTFSI is investigated by solid-state nuclear magnetic resonance, Fourier transform infrared spectoscopy and Raman tests, as well as density functional theory calculations.The Lewis acidity group plays a key role in enhancing the ionic conductivity and t Li þ .As a result, the constructed Li-Li symmetrical cells retain stable cycling for 2300 h and the LiFePO 4 -based ASSLBs deliver outstanding electrochemical properties with 147 mAh g À1 of reversible capacity after 500 cycles at 1C and 60 °C, 131 mAh g À1 after 150 cycles at 0.1C and 30 °C.The fabricated SPEs are self-standing and flexible with good mechanical stability, demonstrating the great potential for practical application.The results can guide choosing the inorganic filler to prepare high-performance SPE.mechanism of Li þ in PEO matrix is the repeated hopping and coordinating between Li þ and ether oxygens in the amorphous phase of PEO. [12]However, the high molecular weight of PEO tends to crystallize, which leads to low ionic conductivity at room temperature. [13]Meanwhile, there is still a lot of room for improvement in the Li þ transference number (t Li þ ) of PEObased solid-state electrolytes (SSEs), which represents the ratio of charge transportation by lithium ions to the overall charge transport quantitatively.The low cation transference number leads to concentration polarization phenomenon, precipitation of salt and rapid dendrite growth at the anode side, and depletion of electrolytes at the cathode side. [14,15]Therefore, extensive studies have focused on reducing the crystallization and enhancing the t Li þ of PEO.Copolymerization is an attractive way to introduce the rational functional groups to immobilize the anion of Li salts and then enhance the t Li þ . [14]However, the process will disrupt the structure of the polymer for transporting lithium ions, which will reduce the ion conductivity.[18] MOFs are ideal fillers in composite polymer electrolytes due to their diverse and adjustable structures. [19,20]For example, Liu et al. synthesized the Zn 4 O clusters with Lewis acidic characteristic by tailoring the stoichiometric ratio of metal salt in the preparation process of MOF-5, which was then mixed into PEO as a filler to enhance the dissolution of LiTFSI through the Lewis acid-base interaction. [21]UIO66 with high chemistry stability and tunable surface functional groups is introduced as a functional filler to modify the performance of PEO-based electrolytes.Sun et al. reported PEO-based SPEs by treating the UIO66 with CH 3 I to form -N þ CH 3 charging center for immobilizing anion through electrostatic interaction, which was used to construct dendrite-free LMBs. [22]Huang et al. proved that the -OH groups with electronegativity in UIO66-2OH could bind Li þ and serve as hopping sites to promote ion transportation. [23]Although pioneering works demonstrate the effectiveness of MOF in improving the ionic conductivity of PEO-based SSE, the local interaction between the surface organic functional groups of MOF and the PEO matrix or Li salts still remains ambiguous.Understanding the mechanism of interfacial chemistry interaction is helpful to select the rational MOF particles to design highperformance PEO-based SPE.
Herein, UIO66 (an MOF of Zr 4þ and terephthalic acid coordination) with different surface functional groups (Lewis basicity, acidity, and neutrality) is synthesized by selecting the organic ligands elaborately.Both the Lewis acidic and basic groups can enhance charge carrier concentration by interfacial chemistry interaction with Li salts.Specifically, the UIO66-2CO 2 H shows Lewis acidic characteristics, and the polar carboxyl group on its surface would interact with the TFSI À anion and enhance the dissociation of the LiTFSI.However, the UIO66-2NH 2 shows a Lewis basic feature, the interaction between the amidogen group and the Li þ cation will limit the Li þ cation transportation.As for UIO66, the anion and cation will reassociate with each other, and the actual concentration of the charge carrier is lower.Moreover, the random distribution of MOF particles can reduce the crystallinity of the PEO matrix, which will increase the ion conductivity of SPE.Profiting from the above special features, the designed PEO-UIO66-2CO 2 H SPE shows high ion conductivity of 5.03 Â 10 À4 S cm À1 and high Li-ion transference number (t Li þ ) of 0.55 at 60 °C.And the constructed Li-Li symmetrical cells matched with PEO-LiTFSI-UIO66-2CO 2 H, SPE exhibit high critical current density of 0.3 mA cm À2 and stable plating/stripping at 0.1 mA cm À2 and 0.1 mAh cm À2 for 2300 h at 60 °C.When matched with LiFePO 4 cathode and lithium anode, the prepared all-solid-state lithium batteries (ASSLBs) deliver high reversible capacities of 147 mAh g À1 after 500 cycles at a current density of 1C at 60 °C with an outstanding capacity retention of 91.1% and of 131 mAh g À1 after 150 cycles even at 0.1C and 30 °C.

Results and Discussion
The different UIO66 MOFs with Lewis acidity (-CO 2 H), basicity (-NH 2 ), or neutrality are synthesized by changing the ligand elaborately and then used as fillers to assemble corresponding PEO-based SPE.The -CO 2 H and -NH 2 groups would enhance the dissociation of lithium salts through the Lewis acid-basic interaction and then increase the ion conductivity of SPE.Meanwhile, the -CO 2 H group possesses a strong electronwithdrawing ability.Differently, UIO66-2NH 2 is Lewis basicity because of the lone pair electrons of the N atom.Thus, as shown in Figure 1, the -NH 2 group can interact with Li þ , which will impede the transportation of Li þ .Conversely, the -CO 2 H group can immobilize the anion of Li salt through Lewis acid-basic interaction and then promote the t Li þ .The SEM and TEM images of the synthesized UIO66-X are shown in Figure S1 and S2, Supporting Information.All UIO66-X particles are about 200 nm in size and disperse uniformly.The similar morphology demonstrates that the effect of the introduced different functional groups on morphology can be neglected.The X-ray diffractometer (XRD) patterns of UIO66-X are displayed in Figure S3, Supporting Information, wherein the peaks at 7.3°, 8.5°, and 25.6°are observed and assigned to (110), (200), and (600) planes, respectively.For UIO66-NH 2 and UIO66-2CO 2 H, the peak intensity decreases after ligand exchange, which may be due to the enhanced steric-hindrance after chelating 1,2,4,5-benzenetetracarboxylic acid and 2,5-diamino-1,4-benzene.The pore structure of UIO66-X is studied through BET testing, as shown in Figure S4, Supporting Information.The specific surface area of UIO66 is 769.3 m 2 g À1 , which is higher than UIO66-2NH 2 (451.6 m 2 g À1 ) and UIO66-2CO 2 H (444.5 m 2 g À1 ).And the mesopores centered at about 4 nm in UIO66-2CO 2 H and macropores centered at about 60 nm in UIO66-2NH 2 are observed, which is ascribed to the change of topology structure due to enhanced steric hindrance after introducing functional groups in to ligands.The decreased specific surface area indicates that the micropore is decorated with additional functional groups successfully, which can regulate the ion transport within the micropore and enhance the interaction between UIO66-X and PEO.The Fourier transform infrared spectoscopy (FT-IR) absorption peaks at 1367.2 and 1585.2 cm À1 in UIO66 belong to the vibration modes of the carboxyl group (Figure S5, Supporting Information).In contrast, these peaks from UIO66-2NH 2 shift to 1343.6 and 1579.9 cm À1 .The blue-shift phenomenon demonstrates that the electron-donor amino functional groups will enhance the electron cloud of the carboxyl group.Differently, these peaks of UIO66-2CO 2 H shift to 1378.8 and 1589.0 cm À1 , which confirms the electron-withdrawing functional group can reduce the electron cloud density of the carboxyl group.Besides, the distinctive C-N stretching absorption peak at 1320 cm À1 can be observed as shown in Figure S5b, Supporting Information, which evidences that the UIO66-2NH 2 has been synthesized successfully.These differences in functional groups and structures have the potential to affect the performance of the designed composite electrolyte.
After incorporating UIO66-X into the PEO matrix as filler to fabricate composite electrolytes, the SEM and corresponding cross-section images of the produced PEO-UIO66-X are shown in Figure 2 and S6, Supporting Information.The surface of SPEs is relatively smooth and flat, revealing that UIO66-X is dispersed uniformly in the system, which implies good contact with the cathode and Li anode.The PEO-LiTFSI SPE illustrates the distinct spherical crystal (Figure S6, Supporting Information) due to the high molecular weight of PEO polymer, which will impede Li þ migration along the PEO chain segment.When UIO66-X is incorporated into PEO-LiTFSI SPE, the size of PEO spherical crystals decreases due to the random distribution of MOF and the interaction between polymer and fillers.Particularly, the crystal sizes of both PEO-UIO66-2CO 2 H and PEO-UIO66-2NH 2 are smaller than PEO-UIO66, which demonstrates the introduced functional groups would enhance the interaction between PEO and fillers, proving an optimum effect for Li þ migration.And the thicknesses of the different SPEs are all about 100 μm through cross-section SEM analysis.The uniform thickness proves controllable preparation of the UIO66-based SPEs.The PEO-UIO66-2CO 2 H membrane made by tape casting is uniform and self-standing with 8 Â 7.5 cm in size (Figure S7, Supporting Information), showing the potential for practical application.The mechanical properties of the PEO-UIO66-2CO 2 H SPEs are also shown by simple stretching and bending tests (Figure S8, Supporting Information).The PEO-UIO66-2CO 2 H can be stretched without cracking and maintains a flat state after bending, which suggests good flexibility that can maintain interface stability.
The density functional theory (DFT) calculations with the ALKEMIE platform were conducted to investigate the interaction of the -NH 2 and -CO 2 H groups with LiTFSI in the hybrid electrolytes. [24]The modified UIO66-based MOF, LiTFSI structure, and the interaction mechanism diagram are shown in Figure 3a,b and S9, Supporting Information.The DFT calculation is in line with the fundamental principle of SSEs systems.Specifically, the Li þ cation should be immobilized on the surface of the -NH 2 group due to the lone pair electron of N. Differently, TFSI À anion is immobilized by the strong electron-withdrawing ability of -CO 2 H.The dissociation energy of LiTFSI to Li þ and TFSI À with and without UIO66-X are calculated and shown in Figure 3c.Based on Lewis acid-base interaction, the dissociation energy of LiTFSI decreases to 4.01 and 3.75 eV for UIO66-2CO 2 H and UIO66-2NH 2 , respectively, both of which are lower than that of pure LiTFSI (5.52 eV).The decreased dissociation energy indicates that LiTFSI is easier to dissociate, hence offering more free cation and anion of Li salt in the hybrid electrolyte.
The FT-IR, Raman, and solid-state nuclear magnetic resonance (NMR) are used to further discover the interaction mechanism between UIO66-X and LiTFSI, as shown in Figure 3d-f and S10-S12, Supporting Information.The FT-IR peaks located at 1058.2 and 1095.8 cm À1 from the PEO-LiTFSI electrolyte belong to the C-O-C stretching motion in PEO. [25]The peaks shift to 1056.8 and 1095.3 cm À1 , which evidences the hydrogen bond interaction between UIO66-X and PEO.And the peak at 1232.7 cm À1 corresponds to asymmetric stretching of -CF 3 in Li salt anion.When UIO66 is added into PEO-LiTFSI, the peak at 1232.7 cm À1 almost has no shift.However, the peak shifts to 1228.9 and 1228.4 cm À1 when added UIO66-2NH 2 and UIO66-2CO 2 H, respectively.Meanwhile, a similar shift of -CF 3 symmetric stretching peak from 1188.9 to 1187.45 and 1186.0 cm À1 can be seen when added UIO66-2NH 2 and UIO66-2CO 2 H.Such a red shift shows that the -NH 2 and -CO 2 H groups can enhance the dissociation of Li salt through Lewis acid-base interaction. [17,25,26]And the shift degree reaches the maximum value when UIO66-2CO 2 H is added, implying that the -CO 2 H group has the strongest interaction with the Li salts.These results are also certified by Raman shift as shown in Figure 3e.The Raman peak at 743.3 cm À1 indicates the interaction between Li salt cation and anion.When added UIO66 to PEO-LiTFSI, the peak also stands at 743.3 cm À1 .However, after adding UIO66-2NH 2 and UIO66-2CO 2 H, the peak shifts to 741.6 and 739.5 cm À1 , respectively.The Raman and FT-IR results also demonstrate that the -NH 2 and -CO 2 H groups can enhance the dissociation of Li salt to release more free Li þ and TFSI À .The solid-state NMR is employed to measure the chemical environment of 7 Li, as shown in Figure 3f.The 7 Li NMR signals in PEO and PEO-UIO66 are all located at À0.96 ppm, which indicates that the added UIO66 has a weak influence on the chemical environment of Li.Differently, after adding UIO66-2NH 2 and UIO66-2CO 2 H, the 7 Li NMR signals shift to À1.11 and À1.15 ppm, respectively, demonstrating that the interaction between PEO and Li þ is weakened through Lewis basicity and acidity of the -NH 2 and -CO 2 H groups.The solid-state NMR results further indicate that the introduced -NH 2 and -CO 2 H functional groups can enhance the dissociation of Li salt and weaken PEO and Li þ interaction and then enhance the Li-ion conductivity through Lewis acid-base interaction.Thus, combining with the above calculation and spectroscopy results, the -NH 2 and -CO 2 H functional groups can combine with the Li salt cation and anion to enhance the dissociation of Li salt and regulate the chemical environment of Li þ .
The XRD and mechanical stability are tested to further demonstrate the interaction between UIO66-X and PEO matrix, as presented in Figure 3g,h.The diffraction peaks of UIO66-X (2θ = 7.42 and 8.62) are found in the XRD patterns of different SPEs, which illustrate that the UIO66-X has been embedded into the PEO matrix successfully.Furthermore, the diffraction intensities of peaks at 19.1°and 22.9°, corresponding to (120) and (112) crystal faces of PEO, are significantly weakened, suggesting that the crystallinity of PEO decreases after adding the UIO66-X.These results suggest that the crystalline area is transformed into the amorphous zone mostly, which can increase the ionic conductivity resultantly.The mechanical test illustrates that all SPEs experience a softening process until the stress reaches the yield strength (Figure 3i).The consequence exhibits that the PEO-UIO66-2CO 2 H SPE has the maximal strength of 6.65 MPa than the PEO-UIO66-2NH 2 (5.8 MPa) and PEO-UIO66 (5.4 MPa).And the PEO-UIO66-2CO 2 H and PEO-UIO66-2NH 2 SPEs can bear a 900% and 770% strain state, respectively.However, the crack and stress failure occur when the strain increases to 530% in PEO-UIO66 SPE.The mechanical stability of PEO-UIO66-2CO 2 H is obviously improved, which results from that the O atom in UIO66-2CO2H can act as a bridge to enhance the hydrogen bond interaction between polymer molecular chains.
Figure 4a and S13-S15, Supporting Information, show the ion conductivity and impedance spectra of the produced different SPEs.According to the impedance results evaluated by Equation ( 1), the ion conductivity of PEO-UIO66-2CO 2 H can reach 1.38 Â 10 À5 S cm À1 at 30 °C and 5.03 Â 10 À4 S cm À1 at 60 °C, which are highest than the PEO-UIO66-2NH 2 (1.03 Â 10 À5 S cm À1 at 30 °C and 1.84 Â 10 À4 S cm À1 at 60 °C) and PEO-UIO66 (3.26 Â 10 À6 S cm À1 at 30 °C and 6.26 Â 10 À5 S cm À1 at 60 °C).Undoubtedly, the ion conductivities of both PEO-UIO66-2CO 2 H and PEO-UIO66-2NH 2 are higher than PEO-UIO66, which is due to that the introduced -NH 2 and -CO 2 H functional groups can enhance the dissociation of Li salt and regulate the chemical environment of Li þ through Lewis acid-base interaction.Figure 4b shows the linear sweep voltammetry (LSV) curves of the produced different SPEs.The PEO-UIO66 SPE maintains oxidative stability at about 4.0 V due to the poor oxidation resistance of ether oxygen in the PEO framework.Benefiting from the interaction between -NH 2 or -CO 2 H with PEO, the ability of oxidation resistance enhances to 4.4 V and 4.2 V, respectively.4e, the voltage profiles of the cell with PEO-UIO66-2CO 2 H SPE are stable over 2300 h at 0.1 mA cm À2 , 0.1 mAh cm À2 , and 60 °C.However, a sharp downshift of overpotential appears after 730 and 840 h for the cells with PEO-UIO66 and PEO-UIO66-2NH 2 , respectively.The lithium deposition morphology is closely related to the electrochemical performance.
Figure 5 shows the lithium deposition morphology on Cu with different SPEs at 0.1 mAh cm À2 .It's clear that lithium deposits unevenly with flower-like morphology when using PEO-UIO66 SPE.For the PEO-UIO66-NH 2 SPE, the morphology of the lithium deposition is rather uniform and flat but with a loose structure.While the PEO-UIO66-2CO 2 H SPE promotes homogeneous lithium deposition with a flat and dense structure, which is ascribed to that of the -CO 2 H group can enhance the dissociation of lithium salts, improve the t Li þ , and reduce the crystallinity of the PEO framework, and then increase the ion conductivity and promote the uniform deposition of Li metal finally.
To evaluate the electrochemical performance of SPEs in full cells, the ASSLBs are assembled and measured by pairing the LiFePO 4 (LFP) cathode and Li metal anode.The intercalation/ deintercalation voltage profiles of LFP-based ASSLBs with PEO-UIO66-2CO 2 H and PEO-UIO66 SPEs at 1 and 5C are displayed in Figure 6a.The polarization voltage of ASSLBs with PEO-UIO66-2CO 2 H SPE is lower than that with PEO-UIO66 both at 1 and 5C, which is ascribed to the faster electrochemical reaction kinetics due to higher ion conductivity of PEO-UIO66-2CO 2 H SPE. The cyclic voltammetry (CV) curves of these two ASSLBs are shown in Figure S17, Supporting Information.The peak at about 3.20 V is related to the deintercalation of Li þ in the anodic scan, and the peak at about 3.65 V in the cathodic scan is related to the intercalation of Li þ .In contrast, the anodic peak and cathodic peak shift to 3.71 and 3.15 V with PEO-UIO66-SPE, which implies the lower ion conductivity of PEO-UIO66 SPEs.The CV results are in correspondence with the intercalation/deintercalation voltage profiles.
The rate properties of ASSLBs with different SPEs are shown in Figure 6b.Specifically, the cell with PEO-UIO66 SPE delivers reversible capacities of about 160, 112, 70, 40 mAh g À1 at 0.1, 1, 3, and 5C, respectively.When the current density returns to 0.1C, the capacity recovers to 147 mAh g À1 .The specific capacity reaches 90 mAh g À1 at 5C when paired with PEO-UIO66-2CO 2 H SPE, which is more than two times higher than that with PEO-UIO66 SPE.And the ASSLBs with PEO-UIO66-2CO 2 H SPE display high coulombic efficiency and large specific capacity of 147 mAh g À1 after 500 cycles at 1C with a superior capacity retention of 91.1% (Figure 6c), which is obviously better than the ASSLBs with PEO-UIO66 and with PEO-UIO66-2NH 2 SPE.In addition, the cell can also deliver a reversible capacity of 75 mAh g À1 at 3C after 1000 cycles with a high capacity retention of 61.2% (Figure S18, Supporting Information).
To evaluate the practical application of the produced PEO-UIO66-2CO 2 H, the ASSLBs with PEO-UIO66-2CO 2 H SPE is tested at 30 °C.The reversible capacity for the first cycle reaches 128 mAh g À1 at 0.1C.Due to the wetting process between the SPE and electrodes, the capacity increases slightly in the first 20 cycles, and a high reversible capacity of about 131 mAh g À1 is acquired after 150 cycles.Furthermore, to further demonstrate the flexibility, high safety, and practicality of PEO-UIO66-2CO 2 H SPE, pouch cells with LFP cathode Li anode were assembled and tested.Figure S19, Supporting Information, shows the charge-discharge curves of the pouch cell at 0.2C.A high specific capacity of 161 mAh g À1 with 99.4% of CE and a low polarization voltage of 0.1 V are obtained in the first cycle.And the specific capacity of 150 mAh g À1 is achieved after 70 cycles as shown in Figure 6e.In addition, the open-circuit voltage can be maintained even the pouch cell is bent by 90°or 180°, which demonstrates the good flexible feature of PEO-UIO66-2CO 2 H SPE.Moreover, the pouch cell can light up a light-emitting diode (LED) successfully even though the cell is at the cutting state, demonstrating the excellent safety performance.
The surface SEI components of PEO-UIO66-2CO 2 H and PEO-UIO66 SPE after cycling are researched by XPS.In  peak at 291.1 eV and F 1s peak at 687.1 eV correspond to the C-F bonds of LiTFSI. [13,27,28]After cycling, LiF (683.4 eV), Li 2 O (530.9 eV), and ROLi (286.2, 528.0 eV) are found at the SPEs surface. [13]The signal intensities of ROLi are negligible in both two SPEs, which indicates less PEO decomposition during cycling. [29]he LiTFSI signal intensities decrease after cycling, which indicates the reduction of LiTFSI to LiF.And the signal intensity of LiF in the PEO-UIO66-2CO 2 H SPE sample is stronger than that in the PEO-UIO66 SPE, which illustrates the more LiF content and thinner SEI layer.After etching 200s by Ar þ , the peak of ROLi disappears in the PEO-UIO66-2CO 2 H SPE sample.However, the peak of ROLi persists in the PEO-UIO66 SPE, which also demonstrates the SEI layer is much thicker and looser compared to that generated in the PEO-UIO66-2CO 2 H SPE. These results evidence the better structural stability of PEO-UIO66-2CO 2 H SPE.

Conclusion
In summary, the addition of MOF particles with specific functional groups can reduce the crystallinity of the PEO matrix due to the hydrogen bond interaction, which will improve the ion conductivity of SPE.Meanwhile, the mechanism of different organic ligands on the interaction with Li salt and PEO matrix is investigated and discussed.The results suggest that the Lewis acidity and basicity ligands will enhance the Li salt dissociation.The Lewis acidity will interact with the anion of Li salt and then improves the t Li þ .Differently, the Lewis basicity will interact with Li þ and then limit its transportation.Additionally, the introduced -CO 2 H and -NH 2 functional organic ligands would enhance the interaction with the PEO matrix.As a result, the PEO-UIO66-2CO 2 H SPE can bear a 900% strain state with a maximum stress of 6.65 MPa than other two counterparts, demonstrating the great potential for practical application.Profiting from the above special features, the designed PEO-UIO66-2CO 2 H SPE exhibits a high ion conductivity of 5.03 Â 10 À4 S cm À1 and t Li þ of 0.55 at 60 °C.The critical current density of Li/Li symmetric batteries can reach up to 0.3 mA cm À2 , which can cycle stably for more than 2300 h at 0.1 mA cm À2 and 0.1 mAh cm À2 under 60 °C.Meanwhile, the LFP|Li ASSLBs deliver a high specific capacity of 147 mAh g À1 with a superior capacity retention of 91.1% after 500 cycles at 1C and 60 °C.Furthermore, the LFP|Li ASSLBs also exhibit good cycling stability even at 30 °C.The assembled pouch batteries show stable cycling performance and high safety.This work can guide the design of the high-performance SPE from the aspect of Lewis acids and bases theory via choosing the rational inorganic filler.
Preparation of SPE Film: The obtained UIO66-X (X = 0, 2CO 2 H, 2NH 2 ), LiTFSI, and PEO (Mw = 6 Â 10 5 ) were added to anhydrous acetonitrile and then stirred for 24 h to form a homogeneous solution.The weight ratio of UIO66-X and PEO was 1:10, and the molar ratio of ether oxygens and Li was 18:1.Then the solution was poured into a polytetrafluoroethylene (PTFE) plate and dried at room temperature at argon atmosphere until the acetonitrile was evaporated mostly.Following, the polymer electrolyte film was dried furtherly in a vacuum oven at 60 °C for 24 h.After cooling down to room temperature, the SPE film was cut into discs with a diameter of 19 mm and stored in an argon-filled glovebox.The obtained SPE films were named as PEO-UIO66, PEO-UIO66-2NH 2 , and PEO-UIO66-2CO 2 H correspondingly.For comparison, the PEO SPE was prepared in the same way without adding UIO66-X.
Preparation of ASSLBs: The as-prepared SPE, LiFePO 4 , and lithium metal were collected to assemble 2025-type coin cells.The LiFePO 4 cathode slurry was composed of 80 wt% of LiFePO 4 , 10 wt% of acetylene black, and 10 wt% of PEO-LiTFSI binder and was coated onto aluminum foil and then dried at 80 °C overnight in a vacuum.The cathode was cut into discs with a diameter of 12 mm, and the mass loading of active material was 1.5-2.0mg cm À2 .The 2032-type Li/ SPE /Li symmetry coin cells were assembled in an Ar-filled glovebox without adding liquid electrolytes.
Testing and Characterization: Battery Test System (S4008, Newware Electronics) was used to measure galvanostatic cycling and rate performances of the ASSLBs.The crystal structures of the produced samples were characterized by an XRD with Cu-K α radiation at 40 kV and 40 mA.The morphology and microstructure of the as-prepared samples were analyzed by SU-70.Raman spectra were measured on a HORIBA Lab RAM HR Evolution with a 532 nm argon ion laser.The 7 Li solid-state NMR was measured on an AVANCE NEO with 600 MHz.The mechanical stability of SSE films was performed on a microforce tester with a fixed stretching speed of 0.1 mm s À1 .CV at a scanning rate of 0.1 mV s À1 , the LSV curves of SPEs at a scanning rate of 1 mV s À1 from 3.0 to 5.0 V, and the ionic conductivity of the SPEs in the frequency range of 10 MHz-0.01Hz with the amplitude of 5 mV using stainless steel/SPEs/stainless steel sandwiching structure from 30 °C to 70 °C were obtained from an Autolab electrochemical workstation (NOVA 1.9).The ionic conductivity (σ) of SPEs was calculated according to the following Equation R b is obtained from AC impedance spectroscopy, L and S are the thickness and area of the SPEs, and the Li-ion transference number is calculated by the Bruce-Vincent-Evans Equation where I 0 , R 0, I s , and R s are the initial and steady-state DC current and steady-state interface resistances, respectively.ΔV is the applied pulse potential of 10 mV.
DFT Calculations: The first-principles calculations with the projectoraugmented wave method were performed by using the Vienna ab initio simulation package (VASP) code. [30,31]Generalized gradient approximation (GGA) proposed by Perdew-Burke-Ernzerhof (PBE) was adopted for exchange correlation potential.A cutoff energy of 500 eV for a plane-wave basis set was selected.The supercell with four layers was constructed and a vacuum space exceeding 20 Å was set to avoid the interaction between neighboring.The Monkhorst-Pack k-point mesh was set to 2 Â 2 Â 2. The threshold was 10 À5 eV, and 0.02 eV Å À1 for electronic, respectively, during the structure relaxation.The UIO66-2CO 2 H and UIO66-2NH 2 structures were built to calculate the dissociation energy of LiTFSI.

Figure 1 .
Figure1.The schematic illustration of the interaction between various ligands in the synthesized UIO66-X and LiTFSI.

Figure 3 .
Figure 3. a,b) The interaction mechanism diagrams of -NH 2 and -CO 2 H groups with LiTFSI, c) the variation of LiTFSI dissociation energy.d-g) The FT-IR, Raman, solid-state NMR, and XRD spectra of the different SPEs.h) The optical images of PEO-UIO66-2CO 2 H film at initial and tension state.i) The stress-strain curves of different PEO-UIO66-X films.
Figure 4c and S16, Supporting Information, display the Li-ion transference number (t Li þ ) of the produced different SPEs.t Li þ means the contribution of Li-ion in transportation during cycling, which has a great influence on the electrochemical performance of solid Li batteries.The t Li þ of PEO-UIO66-2CO 2 H SPE is as high as 0.55, which profits from the C = O bonding from -CO 2 H functional group possessing the strong electronwithdrawing ability and then restraining the anion movement of Li salt.However, the t Li þ of PEO-UIO66-2NH 2 decreases to

0. 30
due to the interaction between Li þ and -NH 2 group, which will block the transport of Li þ .And the lowest t Li þ of PEO-UIO66 (0.11) is attributed to the strong interaction between Li þ and PEO.The enhanced t Li þ of PEO-UIO66-2CO 2 H is related to the Li þ chemical environment in the SPEs system, which is consistent with solid-state NMR.The high t Li þ can decrease the electrode polarization effectively and favor the uniform deposition of Li metal during cycling.The critical current density, the lithium plating/stripping behavior, and cycling stability are measured in Li/Li symmetric batteries with different SPEs.As shown in Figure4d, the Li/SPE /Li symmetric batteries are measured under 0.02, 0.05, 0.1, and 0.3 mA cm À2 for 30 min at 60 °C.The Li/Li symmetric batteries with PEO-UIO66 SPE exhibit drastic amplitude fluctuation in voltage at 0.3 mA cm À2 .While the Li/Li symmetric batteries with PEO-UIO66-2CO 2 H and PEO-UIO66-2NH 2 SPEs show stable plating/stripping behavior.Moreover, the Li/Li symmetric batteries with PEO-UIO66-2CO 2 H SPEs show the lowest polarization voltage, which implies the enhanced ion conductivity of PEO-UIO66-2CO 2 H.As shown in Figure

Figure 7 ,
the C 1s XPS peaks at 283.1 and 284.8 eV and O 1s peak at 533.0 eV are from C-C, C-H, and C-O of PEO, while the C 1s