Enhanced Electrochemical Properties and Optimized Li+ Transmission Pathways of PEO/LLZTO‐Based Composite Electrolytes Modified by Supramolecular Combination

Poly(ethylene oxide) (PEO) and Li6.75La3Zr1.75Ta0.25O12 (LLZTO)‐based composite polymer electrolytes (CPEs) are considered one of the most promising solid electrolyte systems. However, agglomeration of LLZTO within PEO and lack of Li+ channels result in poor electrochemical properties. Herein, a functional supramolecular combination (CD‐TFSI) consisting of active β‐cyclodextrin (CD) supramolecular with self‐assembled LiTFSI salt is selected as an interface modifier to coat LLZTO fillers. Benefiting from vast H‐bonds formed between β‐CD and PEO matrix and/or LLZTO, homogeneous dispersion and tight interface contact are obtained. Moreover, 6Li NMR spectra confirm a new Li+ transmission pathway from PEO matrix to LLZTO ceramic then to PEO matrix in the as‐prepared PEO/LLZTO@CD‐TFSI CPEs due to the typical cavity structure of β‐CD. As a proof, the conductivity is increased from 5.3 × 10−4 S cm−1 to 8.7 × 10−4 S cm−1 at 60 °C, the Li+ transference number is enhanced from 0.38 to 0.48, and the electrochemical stability window is extended to 5.1 V versus Li/Li+. Li‖LiFePO4 CR2032 coin full cells and pouch cells prove the practical application of the as‐prepared PEO/LLZTO@CD‐TFSI CPEs. This work offers a new strategy of interface modifying LLZTO fillers with functional supramolecular combination to optimize PEO/LLZTO CPEs for solid lithium batteries.


Introduction
In terms of safety and energy density issues, solid-state lithium batteries (SSLBs) hold dreams to create a new world powered by new energy.6] There are three primary categories of SSEs so far: solid inorganic electrolytes (SIEs), solid polymer electrolytes (SPEs), and composite polymer electrolytes (CPEs). [7]The former mainly includes ceramic oxides and sulfides, which own relatively high ionic conductivities at room temperature (>10 −4 S cm −1 ) and wide electrochemical windows.10] In contrast, SPEs are flexible and possess excellent processability and good interfacial compatibility, whereas, the low ionic conductivity (10 −6 -10 −5 S cm −1 ) results in sluggish Li + transfer kinetics, the weak mechanical property represents potential hazards resulting from uncontrolled lithium dendrites. [11]With polymer electrolyte as skeleton and inorganic as fillers, CPEs combine the merits of both SIEs and SPEs, which have been regarded as the most promising SSEs family closing to commercial application. [12,13]s for CPEs, numerous polymers can be used as flexible electrolytes, including polyethylene oxide (PEO), [14] polyvinylidene fluoride (PVDF), [15] poly(methyl methacrylate) (PMMA), [16] polyacrylonitrile (PAN), [17] polypropylene carbonate (PPC), [18] and polyether sulfone (PESf); [19] inorganic nanoparticles can be usually selected as fillers, such as passive SiO 2 , Al 2 O 3 , ZrO 2 , active garnet-type Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO), NASICON-type Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) and Li 1.3 Al 0.3- Ti 1.7 (PO 4 ) 3 (LATP), perovskite-type Li 0.33 La 0.5 TiO 3 (LLTO), etc. [20][21][22] Among them, PEO/LLZO CPEs are currently the most widely investigated SSEs.PEO is flexible, nonflammable and has good lithiophilicity and can transfer Li + through chain segment movement.Garnet-type LLZTO ceramic electrolytes can act as plasticizers to disorder the crystallization of PEO matrixes, facilitate the dissociation of lithium salts, and thus improve its conductivity, electrochemical window, and mechanical stability. [23]With different LLZTO contents, Chen et al. [24] reported "polymer in ceramic" and "ceramic in polymer" PEO/LLZTO CPEs, a high ionic conductivity (σ Li þ > 10 −4 S cm −1 at 55 °C) and excellent cycling stability can be obtained.Zhang prepared a Li-salt-free PEO electrolyte incorporating with 40 nm LLZO, which delivers an ionic conductivity of 2.1 × 10 −4 S cm −1 at room temperature. [25]Falco reported an UV-cross-linked PEO-LLZO CPE, which exhibits high σ Li þ exceeding 0.1 mS cm −1 at 20 °C. [26]Although significant achievements have been obtained in above-mentioned PEO/LLZTO CPEs, some drawbacks still need to be well addressed, including poor interfacial contact between PEO and LLZTO due to worse wetting of two surfaces, incomplete understanding of conductive mechanisms of PEO/LLZTO CPEs, etc. [27,28] Surface modification with an organic layer on the surface of LLZTO ceramic is an effective strategy to optimize poor interfacial contact between LLZTO and PEO. [29,30]Usually, the organic layer has excellent compatibility with both PEO and LLZTO, and can minimize the surface energy difference between them.Because of a highstrength yet reversible coordination bond with LLZTO surfaces and a covalent bond on PEO surfaces generated at the same time, polydopamine (PDA) is introduced to form uniform a dispersion of LLZTO in the PEO matrix. [27]Stearic acid (SA) is also reported as a coating material to protect LLZTO by bonding with surface hydroxyls, and enhance its compatibility with PEO matrix. [31]These researches suggest that the physical and electrochemical properties of the PEO/LLZTO CPEs can be affected by the nano-interface modifier; however, PDA and/or SA are inert for Li + transmission, which has almost no influence on the Li + pathway within the PEO/LLZTO CPEs.
As one of the amazing supramolecular materials, cyclodextrins (CDs) have been extensively studied to act as host molecules forming inclusion complexes with a variety of guest molecules. [32]Brunklaus and co-workers found that extra nanochannels for fast Li + transport can be formed by CD threaded on PEO chains.The as-prepared solid polymer electrolyte has an impressive σ Li þ of 1.0 × 10 −3 S cm −1 at 60 °C and a superior oxidative electrochemical stability of up to 4.7 V versus Li/Li + . [33,34]erein, we report our findings in the development of PEO/LLZTO CPEs.β-cyclodextrin (CD) supramolecular with self-assembled LiTFSI salt (CD-TFSI) is selected as an interface modifier to coat LLZTO nanoparticles.As shown in Scheme 1, LiTFSI is self-assembled in the cavity of β-CD via a host-guest interaction, and β-CD is bound to LLZTO through hydrogen bonding.In the as-prepared PEO/ LLZTO@CD-TFSI CPEs, on the one hand, the anion TFSI − (-SO 2 CF 3 group) can weaken the interaction between Li + and ether oxygen functional group of PEO through competitive coordination, to fast Li + movement in PEO matrix; on the other hand, it will also affect the lithium environment of LLZTO.Meanwhile, the special cavity structure of β-CD can provide a high-speed Li + diffusion channel, which will make it possible for Li + migration from PEO matrix to LLZTO ceramic to PEO matrix, and finally construct a new Li + pathway in PEO/ LLZTO@CD-TFSI CPEs.Furthermore, the nano supramolecular βCD-TFSI shell on the surface of LLZTO helps to reduce its surface energy of nanoparticles, contributing to the good dispersion of LLZTO in PEO, and the optimized interfacial contact between LLZTO and PEO.As proof, the as-assembled PEO/LLZTO@CD-TFSI CPEs deliver a promising Li + conductivity of 8.7 × 10 −4 S cm −1 at 60 °C, an enhanced Li + transfer number of 0.48 and an enlarged electrochemical window of <5.1 V (vs Li/Li + ).

Results and Discussion
FTIR spectra and 19 F NMR measurements were carried out to confirm that LiTFSI was successfully self-assembled into the cavity of the β-CD supermolecule.Figure S1a, Supporting Information shows FTIR spectra of LiTFSI, β-CD, and CD-TFSI.Compared with pure LiTFSI, there are some significant peak shifts happening in the CD-TFSI with weakened strength, such as the asymmetric stretching vibration absorption peaks of CF 3 (ν a CF 3 ) shift from 1205 to 1196 cm −1 , the asymmetric stretching vibration absorption peaks of S-N-S (ν a SNS) negatively shift from 1066 to 1053 cm −1 . [35][38] FTIR results preliminarily suggest that LiTFSI was successfully self-assembled into the β-CD to form a supramolecular combination, and the process did not destroy the cavity structure of β-CD.As shown in Figure S1b, Supporting Information, 19 F NMR results of LiTFSI and CD-TFSI further verified the supramolecular interaction between TFSI − anion and β-CD.It can be found that the 19 F signal positively shifts from −78.73 ppm in LiTFSI into −78.7 ppm in CD-TFSI, the phenomenon results from the increased electron cloud density in the β-CD cavity caused by the self-assembled TFSI − . [39,40]Combining FTIR and 19 F NMR results, we can conclude that TFSI − was successfully self-assembled into the cavity of β-CD through host-guest interaction.
Figure 1a presents SEM image of the as-modified LLZTO@CD-TFSI particles.It can be seen that the size of the secondary particle is 200-400 nm, which is similar with the pristine LLZTO particles (Figure S2, Supporting Information).TEM image displayed in Figure 1b shows that a well core-shell structure is obtained with LLZTO as a core and CD-TFSI as shell after surface modification.The thickness of CD-TFSI shell is ~15 nm.In addition, element mapping images of EDS suggest that CD-TFSI supramolecular combination is homogeneously distributed in LLZTO ceramics (Figure 1c), which contributes to forming a continuous and tight interfacial contact with the PEO matrix during the preparation process of CPEs.As shown in Figure 1d, XRD patterns of LLZTO and LLZTO@CD-TFSI indicate that surface modification of CD-TFSI did not change the crystallinity of LLZTO with the standard cubic garnet phase (JCPDS 80-457).FTIR demonstrated in Figure S3, Supporting Information and Figure 1e further confirmed the existence of CD-TFSI. Figure 1f,g show the C 1s and F 1s high-resolution XPS spectrum of the pristine LLZTO and the as-prepared LLZTO@CD-TFSI, respectively.The peaks located at 284.8 and 289.8 eV can be assigned into the adsorbed hydrocarbon impurities and trace Li 2 CO 3 , respectively. [41,42]t can be observed that the peak associated with Li 2 CO 3 is greatly weakened in the as-prepared LLZTO@CD-TFSI, suggesting that CD-TFSI interface coating effectively avoids the formation of Li 2 CO 3 .The peaks related to C-O-C (286.4 eV) and O-C-O (288.1 eV) come from the β-CD structure, and the peak of C-F (292.3 eV) is associated with the LiTFSI in LLZTO@CD-TFSI, respectively. [43][46] Li-F bond (684.4 eV) mainly comes from the pristine LLZTO.Figure S4, Supporting Information exhibits TG curves of both LLZTO and LLZTO@CD-TFSI; it can be calculated that the weight percent of CD-TFSI in LLZTO@CD-TFSI is ~8.0%.
Previously reported calculations based on the Vickers indentation test show that the surface energy of LLZO with 98% relative density is ~3.0 J m −2 , while that of the typical PEO polymer is ~0.044J m −2 .Because of the huge difference of surface energy between LLZTO and PEO/acetonitrile solution, LLZTO particles tend to agglomerate during the drying process of electrolyte membrane, which will bring some adverse effects on conductivity and mechanical strength of the asprepared CPEs due to heterogeneous microstructure. [27]Here, sedimentation experiments were firstly performed to contrast their compatibilities of both the pristine LLZTO and the LLZTO@CD-TFSI with acetonitrile solvent.0.16 g LLZTO and LLZTO@CD-TFSI particles were ultrasonically dispersed in 20 mL acetonitrile, then kept static settlement for 24 h, respectively.As shown in Figure 2a, the pristine LLZTO is complete precipitation stratification after 8 h; in contrast, the LLZTO@CD-TFSI solution is still turbid after 24 h.The phenomenon suggests better dispersibility of the LLZTO@CD-TFSI in acetonitrile.
Figure 2b presents the contact angle between a 5 wt.%PEO in acetonitrile solution droplet and the LLZTO wafer with and without CD-TFSI surface modification.The contact angle of the PEO solution on the pristine LLZTO tablet is 83°, while that on the LLZTO@CD-TFSI substrate is 55°.The difference in the contact angle suggests that the CD-TFSI coating contributes to reducing the surface energy mismatches between LLZTO and PEO, and finally enhances the wettability.We predict that the strong adhesion owns to newly formed H bond between -OH in β-CD and ether oxygen functional group of PEO.
Figure 2c shows the surface morphology of the as-prepared PEO/LLZTO CPEs and PEO/ LLZTO@CD-TFSI CPEs to reveal the particle distribution in the CPEs.Obviously, there is serious agglomeration of LLZTO particles in PEO/ LLZTO CPEs.In contrast, the distribution of ceramic particles in PEO/LLZTO@CD-TFSI CPEs was more homogeneous.To further assess the dispersibility of LLZTO particles in two CPEs, Figure 2d displays AFM images in the cross-sectional view.It can be seen that the PEO/LLZTO CPEs are rough and uneven with obvious particle clusters, while the PEO/LLZTO@CD-TFSI CPEs are relatively smooth.All results indicate that CD-TFSI surface modification is beneficial to reducing surface energy difference and optimizing particle distribution.
To discuss the effect of CD-TFSI interface modifier on the Li + conduction, ionic conductivity of the PEO/LLZTO@CD-TFSI CPEs was measured by EIS.Here, the PEO/LLZTO CPEs and the PEO/LLZTO@CD (CD without self-assembled TFSI) CPEs were also tested at the same condition as the control groups, respectively.Equation ( 1), the corresponding Li + conductivities (σ Li þ ) are calculated.The PEO/LLZTO@CD-TFSI CPEs delivers σ Li þ values of 6.5 × 10 −5 S cm −1 at 25 °C and 8.7 × 10 −4 S cm −1 at 60 °C, which are much higher than that of the PEO/LLZTO CPEs (2.7 × 10 −5 S cm −1 at 25 °C and 5.3 × 10 −4 S cm −1 at 60 °C).On "micro" level, the enhanced conductivity originated from the uniform distribution of LLZTO in PEO matrix, and the improved compatibility between ceramic fillers and polymer.Because the PEO chain end is relatively inert at low temperature, and the formed H bonding between the -OH group in β-CD and the ether oxygen functional group of PEO limits the PEO chain end movement, [47] the PEO/LLZTO@CD CPEs demonstrate lower ionic conductivity at low temperature.
Figure 3b depicts the Arrhenius curves of asprepared CPEs.For comparison, PEO/LiTFSI and PEO/CD-TFSI polymer electrolytes without LLZTO fillers, Composite electrolyte membrane composed of PEO, LLZTO, LiTFSI, and CD-TFSI by mechanical mixing method (noted as PEO/ LLZTO/CD-TFSI) were also fabricated and measured.It can be seen that all samples with LLZTO fillers show bigger σ Li þ than both PEO/ LiTFSI and PEO/CD-TFSI at all testing temperatures.In addition, the PEO/LLZTO@CD-TFSI CPE also exhibits higher σ Li þ than the PEO/ LLZTO/CD-TFSI CPE.Based on the Arrhenius equation of σ = Aexp(−E a /RT) and the slope of the fitted lines, the active energy E a of each CPE can be calculated to evaluate its Li + conduction kinetics.The corresponding E a values are 0.63, 0.69, 0.61, and 0.56 eV for the PEO/ LLZTO CPE, the PEO/LLZTO@CD CPE, the PEO/LLZTO/CD-TFSI CPE, and the PEO/LLZTO@CD-TFSI CPE, respectively.The lower activation energy implies the fast Li + migration kinetics.This suggested that more channels were probably involved in Li + transmission after interfacial modification by CD-TFSI.In addition, from the Arrhenius plots, it can be observed that there appear obvious inflection points at 50 °C for all as-prepared CPEs except the PEO/LLZTO@CD CPE, the plot of the PEO/ LLZTO@CD-TFSI CPE is relatively smooth.The phenomenon suggests that there reflect different conductive mechanisms for the three samples.As we all know, the conductivity of PEO mainly comes from the swing of its chain end, the low glass transition temperature (T g ) and crystallization of PEO chains benefit its conductivity. [23]Figure S5, Supporting Information shows the differential scanning calorimetry (DSC) results of the CPE with and without CD-TFSI interfacial modification, respectively.Notably, the PEO/LLZTO@CD-TFSI CPE demonstrates reduced T g (−44 °C) and melt (50 °C).Furthermore, many previous literatures have reported that there is an optimized percolation threshold of ~10 wt.% LLZTO for the PEO/LLZTO CPE to obtain the highest conductivity. [24]In order to explore the possibility of increasing ceramic fillers, the PEO/30LLZTO@CD-TFSI CPE (the mass ratio of LLZTO@CD-TFSI is 30 wt.%) was prepared for comparison.It can be observed that the high LLZTO@CD-TFSI content does not bring a positive impact on conductivity.FTIR measurements were carried out to study the possible reason for the phenomenon, as displayed in Figure S6, Supporting Information.Pure PEO shows characteristic double peaks and three peaks located at ~1360, ~1340, and ~1100 cm −1 , corresponding to bending vibration of -CH 2 and symmetric stretching vibration of C-O-C in the PEO chain, respectively.The intensity of these peaks reflects the crystallinity of PEO. [48,49]It can be observed that the introduction of CD-TFSI can change the crystallinity of PEO, but the content of CD-TFSI should be controlled <1.4 wt.% (vs PEO) to obtain low crystallinity.In the optimized PEO/LLZTO@CD-TFSI CPE and the contrast sample of PEO/30LLZTO@CD-TFSI CPE, the relative content of CD-TFSI is 0.75 wt.% and 2.9 wt.%, respectively.In addition, we can conclude that the introduction of LLZTO can also effectively reduce the crystallinity of PEO by comparing FTIR of pure PEO and PEO/LLZTO.
Electrochemical stability and Li + transference number (t Li þ ) are two important parameters to assess electrolyte membrane.Figure 3c shows the LSV plots of as-prepared CPEs, including the PEO/LLZTO, the PEO/ LLZTO@CD-TFSI, the PEO/LLZTO/CD-TFSI, PEO/CD-TFSI, and PEO/ LiTFSI, respectively.As for the traditional PEO/LiTFSI electrolyte, the LSV plot exhibits the typical oxidation behavior of PEO, there appear three regions on its: voltage range of 3.5-3.9V with slight current change due to the migration of TFSI − , [50] Voltage range of 3.9-4.5V with significant current change coming from the oxidation of PEO and voltage over 4.8 V with drastic current increase resulting in deep oxidation. [51]In the case of the LLZTO@CD-TFSI CPE, a significantly reduced oxidation current of PEO can be observed, and an extended electrochemical window up to 5.1 V are achieved, indicating that CD-TFSI surface coating plays the role of isolating protection and improving electrochemical stability.Figure 3d presents the t Li þ of the PEO/ LLZTO@CD-TFSI CPE.An enhanced t Li þ of 0.48 is obtained, which is much higher than that of the PEO/LiTFSI (t Li þ = 0.21), the PEO/CD-TFSI (t Li þ = 0.27), the PEO/LLZTO CPE (t Li þ = 0.38), and the PEO/ LLZTO/CD-TFSI (t Li þ = 0.41), as shown in Figure S7, Supporting Information.This can be ascribed to fast Li + movement in PEO matrix due to the weakened interaction between Li + and ether oxygen functional group of PEO through competitive coordination of TFSI − anion.Figure 3e and Table S1, Supporting Information summarized σ Li þ at 60 °C, t Li þ and electrochemical window of the LLZTO@CD-TFSI CPE, and compared them with reported electrolyte membranes based on PEO polymer.[54][55][56][57][58][59][60][61][62][63][64][65][66] To get a deeper understanding on inspiring performances of the asprepared LLZTO@CD-TFSI CPE, potential Li + transmission channels have been proposed and discussed.Normally, there are three possible pathways for Li + diffusion within "ceramic-in-polymer" type PEO/ LLZTO CPE, as shown in Figure 4a: pathway 1, through the PEO phase; pathway 2, along the interfacial region of the PEO polymer and LLZTO ceramic; pathway 3, from PEO matrix to LLZTO ceramic to PEO matrix.The former two Li + transport pathways have been verified by many literatures. [28,67,68]However, the latter has not yet been proven and adopted due to unexpected Li 2 CO 3 impurity on the surface of LLZTO and/or high concentration difference of Li + between PEO matrix and LLZTO. [68,69]In this work, functional CD-TFSI supramolecular combination is introduced to create a bridge to directly connect PEO and LLZTO and then form a new highway for Li + transmission, as schematically displayed in Figure 4b.In order to confirm our assumption, 6 Li NMR spectra have been characterized.Figure 4c shows that the 6 Li NMR peak associated with 48g + 96h sites of LLZTO particles is narrowed along with a slight peak shifting after modification of CD-TFSI supramolecular combination, suggesting the Li + migration in 48g + 96h sites is reinforced in LLZTO@CD-TFSI powders. [70]Figure 4d systematically compares 6 Li NMR spectra of LiTFSI salt, PEO/LiTFSI (molar ratio: 18:1) film, PEO/LLZTO (molar ration of PEO: LiTFSI = 18:1, 10 wt.% LLZTO) film, and the as-prepared PEO/LLZTO@CD-TFSI CPE, respectively.Here, we firstly declare that there are not 6   Li resonance peaks from LLZTO fillers because of low content (10 wt.%).When the content of LLZTO@CD-TFSI increased to 30 wt.% in the CPE (sample of PEO/30LLZTO@CD-TFSI), a 6 Li resonance peak of LLZTO located at 0.14 ppm can be observed, and a weak should peak at −0.17 ppm can be fitted, which reflects the existence of interface. [71]Compared with pure LiTFSI, the 6 Li resonance positively shifts from −1.23 to −0.64 ppm within PEO/LiTFSI.The dissociation of LiTFSI and a new coordination between Li + and the ether oxygen bond at the end of PEO chain attribute to the phenomenon. [57]The result also suggests the  6 Li NMR spectra of pristine LLZTO and LLZTO@CD-TFSI powders.d) 6 Li NMR spectra of pure LiTFSI salt, PEO/LiTFSI, PEO/LLZTO, PEO/LLZTO@CD-TFSI CPE, and PEO/30LLZTO@CD-TFSI CPE, respectively.
Energy Environ.Mater.2024, 7, e12498 pathway 1 is feasible.After normal LLZTO fillers are added, the 6 Li resonance in PEO/LLZTO film steadily shifts to −0.56 ppm, and the resonance coming from PEO is seriously weakened.More Li + transfer alone the interface between PEO and LLZTO, the pathway 2 is predominant in PEO/LLZTO film.Differently, the 6 Li resonance in the PEO/ LLZTO@CD-TFSI CPE is widened and shifts back to −0.60 ppm.More importantly, the strengths of the 6 Li resonance associated with both interface and PEO chains are obviously enhanced.The phenomenon indicates that Li + belonging to the LLZTO body involves transmission in PEO/LLZTO@CD-TFSI CPE.The bridge constructed by CD-TFSI supramolecular combination opens pathway 3. Figure 5 demonstrates polarization curves of the Li‖PEO/ LLZTO@CD-TFSI CPE‖Li symmetric cells at different conditions to study electrochemical compatibility and stability between the PEO/ LLZTO@CD-TFSI CPE and the Li metal anode.As shown in Figure 5a, the PEO/LLZTO CPE just operates ~750 h at 0.2 mA cm −2 then a short circuit occurs.Conversely, the PEO/LLZTO@CD-TFSI CPE successfully works over 2000 h without distinct voltage fluctuation.Figure 5b indicates the PEO/LLZTO@CD-TFSI CPE can withstand higher current shocks, even at 0.7 mA cm −2 , a relative stable hysteresis of <360 mV is obtained, the performance is better than that of most similar works based on PEO/LLZTO CPE (Table S2, Supporting Information).
Li‖PEO/LLZTO@CD-TFSI CPE‖LiFePO 4 CR2032 coin full cells were firstly assembled and measured to explore its possibility of practical application.As displayed in Figure 6a, standard charge/discharge plots of LiFePO 4 cathode are obtained with typical specific discharge capacities of 160.1 and 156.6 mAh g −1 for the PEO/LLZTO@CD-TFSI CPE and the PEO/LLZTO CPE, respectively.Figure 6b shows that the PEO/ LLZTO@CD-TFSI CPE owns better rate capability; the specific discharge capacities are 156.9,155.3, 149.3, 135.1, and 153.8 mAh g −1 at 0.1, 0.2, 0.5, 1.0, and 0.2 C, respectively.Figure 6c presents the charge/discharge cycling performances at 0.2 C. The PEO/LLZTO@CD-TFSI CPE delivers an inspiring discharge capacity retention ratio of 92.5% after 200 cycles.As a contrast, the PEO/LLZTO CPE only keeps 88% after 170 cycles with severe fluctuation.Although the PEO/LLZTO@CD-TFSI CPE exhibits lower polarization resistance after long-term cycling, EIS results shown in Figure 6d indicate there is still interface deterioration.
To further demonstrate the potential application of the as-prepared PEO/LLZTO@CD-TFSI CPEs in practice, Li‖PEO/LLZTO@CD-TFSI CPE-s‖LiFePO 4 pouch cells with a size of 5.0 cm × 5.0 cm were fabricated and tested.From Figure 7a, an initial specific discharge capacity as high as 139 mAh g −1 is obtained.The pouch cell sustained for 20 cycles with a terminal capacity of 97 mAh g −1 (Figure 7b). Figure 7c suggests that the pouch cells can keep the LED light on even after being folded and cut.Considering the excellent flexibility of the PEO/ LLZTO@CD-TFSI CPEs, a laminated pouch cell with two single batteries in series has been assembled and demonstrated, which delivered an open circuit voltage as high as 5.97 V, and successfully powered a row of LED lights shaped like hearts and a fan, as shown in Figure S8, Movies S1 and S2, Supporting Information, respectively.

Conclusion
In conclusion, β-CD-TFSI supramolecular combination was prepared to surface modify LLZTO particles, and a PEO/LLZTO@CD-TFSI CPEs was fabricated and measured.An enhanced ionic conductivity of 8.7 × 10 −4 S cm −1 at 60 °C with a t Li þ of 0.48 is obtained.Moreover, the PEO/LLZTO@CD-TFSI CPEs show excellent electrochemical compatibility and stability with Li metal anode.As for application evaluation, the as-prepared Li‖PEO/LLZTO@CD-TFSI CPEs‖LiFePO 4 coin full cells delivered an initial specific discharge capacity of 160.2 mAh g −1 and successfully worked for 200 cycles with a discharge capacity retention ratio of 92.5%.These results shed lights on the development of PEO/ LLZTO CPEs and their application in SSLBs.
Fabrications of LLZTO@CD-TFSI: The self-assembly process of LiTFSI in β-CD was the following: 0.126 g of LITFSI was added to 20 mL of deionized water with 0.5 g β-CD and stirred for 12 h at room temperature (RT).The supramolecular CD-TFSI powder was finally obtained after being lyophilized for 24 h.
The as-prepared CD-TFSI (0.1 g) was dissolved in 10 mL N,Ndimethylformamide (DMF), then 2 g LLZTO powder was added.The surface modification process was performed in the glove box by magnetically stirring for 12 h.After centrifugation, it is transferred to vacuum oven at 60 °C for drying for 24 h.The final CD-TFSI modified LLZTO product was denoted as LLZTO@CD-TFSI.
Preparations of PEO/LLZTO@CD-TFSI CPEs: PEO/LLZTO@CD-TFSI CPEs were prepared and controlled by solution evaporation and hot pressing processes. [3]PEO, LLZTO@CD-TFSI, and LiTFSI were mixed in anhydrous acetonitrile (CH 3 CN, AR grade, 99.9%; Sigma) with an [EO]: [Li] ratio of 18:1 and 10 wt.% LLZTO@CD-TFSI.The solution was stirred in an Ar-filled glovebox for 12 h to form a uniform slurry and then vacuum dried at 60 °C for 24 h in a culture dish.The PEO/LLZTO@CD-TFSI CPEs with a thickness of ~200 μm were finally obtained by hot pressing for 2 min at 60 °C under 10 MPa, then cut into wafers with a diameter of 19 mm and stored in an Ar-filled glove box to further evaporate solvent.
Materials characterizations: Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a TENSOR 27 equipment (Bruker Optics, Germany).Xray diffraction (XRD) patterns were collected by a Bede D1 X-ray diffractometer (UK, Bede Scientific Ltd; Cu Kα radiation; Worked 40 mA and 40 kV).Scanning electron microscope (SEM, Hitachi SU8010) and transmission electron microscope (TEM, FEI Tecnai-G2 F20 operating at 200 kV) were employed to visualize the morphology of samples.X-ray photoelectron spectroscopy (XPS) was carried out on Thermo Fisher Escalab 250Xi with the Al Ka line as the excitation source to study the surface chemical compositions of sample.Thermogravimetric analysis (TGA) was performed on TG/DTA7300 (SII NanoTechnology) at a heating rate of 10 °C min −1 from 20 to 600 °C in static air.The contact angles of pristine LLZTO and LLZTO@CD-TFSI on 5 wt.% PEO solution were measured on a contact angle instrument (JGW-360DZ). 6Li solid-state NMR spectra were surveyed on an AVANCEIII/WB-400 spectrometer with a 3.2 mm DVT H/F/X triple-resonance probe. 19F NMR spectra were recorded at 400 MHz with a AVANCE NEO NMR spectrometer at 25 °C.
SS‖PEO/LLZTO@CD-TFSI CPEs‖SS (stainless steel) CR2032-coin asymmetric cells were assembled and measured to evaluate ionic conductivity by EIS.EIS data was collected in a frequency range of 1 MHz to 0.1 Hz at 25-70 °C.The ionic conductivity (σ) is calculated according to Equation (1): [72] where l is the thickness (cm) of the CPEs, R is the measured resistance (Ω), and S is the effective contact area between electrolyte and electrode (cm 2 ).Direct current (DC) polarization with EIS was measured by Li‖PEO/LLZTO@CD-TFSI CPEs‖Li CR2032coin asymmetric cells to calculate the lithium-ion transference number (t Li þ ) according to Equation ( 2): [73] t Li þ ¼ I ss ΔVÀI 0 R i 0 À Á where I 0 is the initial current, I ss is the steady state current, and ΔV is the direct current polarization voltage with a value of 10 mV applied to the Li‖PEO/LLZTO@CD-TFSI CPEs‖Li symmetrical cells.R i 0 is the initial charge-transfer resistance and R i ss is the steady-state charge-transfer resistance of the CPEs.
Li‖PEO/LLZTO@CD-TFSI CPEs‖SS CR2032-coin half cells were prepared and tested by LSV to estimate the electrochemical window from open circuit voltage (OCV) to 6.0 V at a scanning rate of 5 mV s −1 at 60 °C.
Electrochemical compatibility and stability between the PEO/LLZTO@CD-TFSI CPEs and the Li metal anode was detected via galvanostatic charge-discharge cycling test at different current densities (0.05-0.7 mA cm 2 ) and a fixed 2 h for each cycle at 60 °C by Li‖PEO/LLZTO@CD-TFSI CPEs‖Li CR2032-coin symmetric cells on LAND CT2001A testing system.
Li‖PEO/LLZTO@CD-TFSI CPEs‖LiFePO 4 CR2032 coin full cells were tested to explore its possibility of practical application.The thickness of the CPEs was controlled as ~100 μm.The loading of LiFePO 4 was ~1.6 mg cm −2 .The charge-discharge performance was tested on LAND CT 2001A testing system at 60 °C between 2.9 and 3.8 V.
Scheme 1. Illustration of the process for preparing CD-TFSI supramolecular combination modified LLZTO.

Figure 3 .
Figure 3. a,b) Nyquist plots and Arrhenius curves from electrochemical impedance spectroscopy of different CPEs.c) LSV plots of as-prepared CPEs at 60 °C.d) DC polarization currents as a function of time at 60 °C, insets are AC impedances of the PEO/LLZTO@CD-TFSI CPEs before and after DC polarization.e) Comparisons of the PEO/LLZTO@CD-TFSI CPE with reported similar CPEs on σ Li þ at 60 °C, t Li þ and electrochemical window, respectively.

Figure 5 .
Figure 5. Galvanostatic cycling curves of the Li symmetric cells with PEO/ LLZTO@CD-TFSI CPE and PEO/LLZTO CPE at different current densities.

Figure 7 .
Figure 7. a,b) Initial charge/discharge plots and cycling stability of pouch cells.c) Photographs of the pouch cells lighting LEDs after folded and cut.