English

• 0.   Introduction

  Solid polymer electrolytes (SPEs) have attracted increasing attention due to their lightweight, exceptional mechanical flexibility, ease of processing, and low electrolyte/electrode interfacial impedance^[1-3]. On one hand, SPEs hold tremendous potential to replace the traditional separator-liquid electrolyte system in lithium-ion batteries, addressing safety concerns related to the high flammability and potential leakage of organic liquid electrolytes^[4-6]. On the other hand, solid-state electrolytes offer an opportunity to develop next‐generation high‐energy‐density lithium metal batteries (LMBs) with highly reactive lithium metal anode^[7-8].

  SPEs are generally comprised of a polymer matrix and lithium salts^[9-12]. Currently, polyethylene oxide (PEO)-based SPEs have garnered extensive research interest due to their excellent interface compatibility with lithium metal, mechanical flexibility, and ease of processing^[13-15]. However, PEO-based SPEs often suffer from low ionic conductivity (approximately 10^-7 S·cm^-1) at room temperature attributed to their high crystallinity, since lithium-ion transport mainly relies on the movement of the PEO chain segments in the amorphous phase. Moreover, PEOs suffer from poor film-forming ability, especially at lower molecular weights^[16-17]. Many strategies such as polymer blending^[18-19], filler incorporation^[20-21], and polymer grafting^[22-24] are employed to lower the crystallinity of the polymer matrix. Introducing inorganic fillers into the polymer matrix is a simple and effective approach to enhance ionic conductivity, which often brings broadened stable voltage windows, improved thermodynamics, and enhanced mechanical properties for SPEs^[25-27]. Fillers can be classified into active fillers and inert fillers. Active fillers, such as Li₇La₃Zr₂O₁₂ (LLZO), Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP), and Li_0.33La_0.55TiO₃ (LLTO), provide additional lithium-ions to enhance the ionic conductivity of SPEs. Inert fillers, such as SiO₂, TiO₂, and Al₂O₃, have drawn considerable attention due to their low cost and abundant resources^[28]. Vermiculite is a layered clay with ample reserve, low cost, and environmentally friendly. A 2D porous silica nanosheet (PSN) with a negatively charged surface can be obtained through conducting ion exchange on vermiculite crystals followed by shear exfoliation and selective etching. The introduction of PSN fillers in PEO-based SPE can create additional migration pathways for lithium-ions and reduce the crystallinity of the polymer matrix, greatly improving the performance of the as-prepared polymer electrolyte^[29-30].

  To explore the influence of surface charge on the 2D PSN, (3-aminopropyl)triethoxysilane (APTES) was utilized to modify the 2D PSN for gaining positively charged 2D PSN (denoted as PSN⁺). The PSN⁺ was employed as a filler to prepare PEO-based SPE through the solution casting method. Results show that the PSN⁺ not only reduces the crystallinity of the SPEs but more importantly, the interaction between the positive surface charges and the anions promotes the lithium salt dissociation. Consequently, the SPE exhibited a higher ionic conductivity of 7.5×10^-5 S·cm^-1 at 50 ℃, a decent lithium-ion transference number (t_Li⁺) of 0.30, and an electrochemical stable voltage window of 4.41 V. Moreover, the assembled lithium symmetric cells demonstrated stable lithium plating/stripping for 400 h at 50 ℃. Furthermore, the assembled LiFePO₄ (LFP)||Li battery exhibited a discharge-specific capacity of 155.7 mAh·g^-1 at 0.2C, and even after 100 cycles at 50 ℃, the capacity remained as high as 151.2 mAh·g^-1, with a capacity retention rate of 97.1%.

  1.   Experimental

  1.1   Chemicals

  Commercial PEO (M_w=6×10⁵), lithium bis(trifluoromethanesulfonyl) imide (LiN(SO₂CF₃)₂, LiTFSI, 99.5%), and APTES, and vermiculite were purchased from Beijing InnoChem Science & Technology Co., Ltd.; All these polymers were desiccated in a vacuum oven at 60 ℃ before use; Acetonitrile and N-methyl pyrrolidone (NMP) were purchased from Shanghai Dingmiao Chemical Technology Co., Ltd.; LFP was purchased from Guangdong Canrd New Energy Technology Co., Ltd.; Vermiculite was purchased from Sigma-aldrich; Hydrochloric acid was purchased from Chongqing Chuandong Chemical Co., Ltd.

  1.2   Preparation of 2D PSN⁺

  Secondary ion exchange: First, 10 g of vermiculite in 450 mL of a saturated sodium chloride solution was heated at 80 ℃ for 22-26 h while stirring, then filtered and washed the obtained sample (sample A). Next, sample A was dispersed in 450 mL of a saturated lithium chloride solution and the mixture was heated at 80 ℃ while stirring for 22-26 h. Subsequently, the mixture was dialyzed until all chloride ions were completely removed (confirmed by AgNO₃ testing) to obtain a 2D layered vermiculite.

  Secondary hydrochloric acid etching: the 2D vermiculite layers were added into 50 mL of concentrated hydrochloric acid solution and heated with stirring for 48 h, then rinsed with deionized water through vacuum filtration until the filtrate was neutral. The acid etching process was repeated once more, and the resulting suspension was centrifuged at 2 000 r·min^-1 for 5 min to obtain a dispersion of 2D PSN. The PSN was obtained by freeze-drying the dispersion.

  Positive charge modification: 100 mL of the dispersion of 2D PSN (4 mg·mL^-1) was taken and added into 100 mL of ethanol while stirring for 30 min. Then, 10 mL of APTES was added and the mixture was heated at 80 ℃ for 8 h in an oil bath. Afterward, the mixture was washed 9-12 times with a mixture of deionized water and ethanol (1∶2, V/V) using vacuum filtration. Finally, the mixture was centrifuged at 1 000 r·min^-1 for 2 min to obtain a dispersion of PSN⁺. The PSN⁺ was obtained by freeze-drying the dispersion.

  1.3   Preparation of the SPEs

  PEO (0.400 g) and PSN⁺ (0.002, 0.004, 0.012 g) were dispersed in a solvent of acetonitrile at 40 ℃ under magnetic stirring for 8 h, then LiTFSI (0.200 g) was added to the solution and magnetically stirred for 2 h at 40 ℃. Then, the solution was poured into a polytetrafluoroethylene (PTFE) mold and allowed to rest at room temperature for 5 h before transferring to a vacuum drying oven at 60 ℃ for 12 h to completely remove the solvent, resulting in the creation of CPE-0.5%PSN⁺, CPE-1%PSN⁺, and CPE-3%PSN⁺ (corresponding the mass of PSN⁺ of 0.002, 0.004, 0.012 g). Next, the dried CPE-1%PSN⁺ was sliced into small circular pieces with a diameter of 16 mm. Additionally, a similar method was used to prepare the SPE without PSN⁺, the as-resulted samples were named CPE-P.

  1.4   Preparation of LFP cathodes

  Following a mass ratio of 8∶1∶1, LFP, polyvinylidene fluoride (PVDF), and carbon black (Super P) were precisely measured. Subsequently, the three components were ground in a mortar for 30 min, then NMP was introduced and the mixture was continued grinding for an additional 10 min. Afterward, the slurry was applied onto the coated aluminum foil using a coater, followed by vacuum drying at 60 ℃ for 12 h. Once dried, a slicing machine was used to cut the material into small circular pieces with a diameter of 12 mm and readied for further use.

  1.5   Assembly of cells

  Assembly of symmetric/asymmetric cells: Three types of symmetric/asymmetric cells were assembled based on CPE-1%PSN⁺ or CPE-P polymer electrolytes. The symmetric cell configurations included a symmetric stainless-steel cell (SS||SPEs||SS) for testing the volume resistance of the SPE membrane, and a symmetric lithium cell (Li||SPEs||Li) for evaluating the interface resistance of the composite SPEs membrane. Additionally, the asymmetric cell (SS||SPEs||Li) was employed to assess the stable voltage window of the SPE membrane.

  LFP||SPEs||Li cell assembly: The CR2032-type coin cells were meticulously assembled in the glove box following the sequence of lithium disc, CPE-1%PSN⁺ or CPE-P polymer electrolyte, LFP cathodes electrode sheet, and steel disc. The glove box environment was strictly regulated to maintain oxygen and moisture levels (volume fraction) below 10^-8.

  1.6   Characterizations

  Field emission scanning electron microscopy (FESEM, ZEISS Gemini 300, 10 kW) and transmission electron microscope (TEM, JEOL JEM 2100F, 100 kW) were used to characterize the surface morphology and microstructures of samples. ζ potential measurement was conducted by ZETA SIZER (Nano S90). The nitrogen adsorption-desorption isotherms of the samples were recorded on a gas adsorption apparatus (Belsorp Max Ⅱ, MicrotracBEL, Japan) to produce the specific surface area and pore size distribution. Differential scanning calorimetry (DSC) curves were obtained by differential scanning calorimeter (TA Q20). X-ray diffraction (XRD) patterns were obtained on a Bruker D8 VENTURE diffractometer using Cu Kα radiation (40 kV, 100 mA, λ=0.154 nm, 2θ=5°-90°). Raman spectra were obtained by using a laser Raman tester with 532 nm of wavelength (HORIBA, LabRam HR800).

  1.7   Electrochemical characterizations

  The electrochemical properties of the SPE membranes were studied on an electrochemical workstation (CHI 604D). The ionic conductivities (σ) of the SPE membranes were measured in SS||SPEs||SS symmetrical cells using electrochemical impedance spectroscopy (EIS). The ionic conductivity was calculated using the following equation:

  $ \sigma=\frac{D}{R S} $

  (1)

  where R is the bulk resistance, D is the thickness of the SPE membrane in the symmetric stainless-steel cell cells, and S is the area of the SS blocking electrode.

  The plot of lg σ as a function of T^-1 for the PEO/PVDF-HFP-based polymer electrolyte was exhibited in Eq.2.

  $ \sigma=\delta_0 \exp \left[E_{\mathrm{a}} /(R T)\right] $

  (2)

  Where σ is ionic conductivity at different temperatures of SPEs, δ₀ is the pre-exponential factor, E_a is the activation energy (kJ·mol^-1), and R is the ideal gas constant (8.314 J·mol^-1·K^-1).

  Li||SPEs||Li symmetrical cells were assembled to determine the lithium-ion transference number. A constant polarization voltage of 10 mV was applied to the cell, and the currents from the initial to the steady state were measured. The lithium-ion transference number ($ t_{\mathrm{Li}^{+}}$) was calculated according to Eq.3:

  $ t_{\mathrm{Li}^{+}}=\frac{I_{\mathrm{s}}\left(\Delta V-I_0 R_0\right)}{I_0\left(\Delta V-I_{\mathrm{s}} R_{\mathrm{s}}\right)} $

  (3)

  where ΔV is the dc (direct current) voltage, I₀, and I_S are the initial and steady-state currents, respectively, and R₀ and R_S are the initial and steady-state resistances, respectively.

  Linear sweep voltammetry (LSV) was used to measure the electrochemical stability windows of the SPE membranes in SS||SPEs||Li asymmetric cells at a scan rate of 10 mV·s^-1 from 2 to 6 V. The charge-discharge test of coin cells (CR 2032) were conducted using the Neware battery testing system (Neware Electric Co., China) over a voltage range of 2.2-4.2 V at 0.2C (34 mA·g^-1) and different rates (0.2C, 0.5C, 1C, 2C, 5C, and 10C).

  2.   Results and discussion

  2.1   Characterization and analysis

  The colloid solutions of PSN and PSN⁺ had the ζ potential of 11 and -20 mV (Fig. 1a), respectively, indicating the surface charge of PSN has been successfully altered from negative to positive by introducing APTES. The PSN⁺ had a large specific surface area of 120 m²·g^-1 (Fig. 1b). SEM images (Fig. 1c-1d) showed that PSN⁺ still possessed well-defined 2D morphology.

  Figure 1

  

  Figure 1.  (a) ζ potentials of PSN and PSN⁺; (b) Nitrogen adsorption-desorption isotherm of PSN⁺; SEM images of (c) PSN⁺ and (d) PSN

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  2.2   Morphology and composition analysis of polymer electrolyte membranes

  SPEs are prepared by the solution pouring method (referring to the experimental section). The blank SPEs without the PSN⁺ were referred to as CPE-P. The XRD patterns (Fig. 2a) showed two sharp diffraction peaks at 19.1° and 23.0° on these SPEs, which correspond to the characteristic diffraction peaks of crystalline PEO. Upon the introduction of PSN⁺ filler, the intensity of PEO characteristic peaks diminished, indicating the reduction in crystallinity due to the filler incorporation. It is well-known that lithium ions in polymer electrolytes predominantly migrate through the amorphous regions of the polymer host. The decreased crystallinity helps enhance the ionic conductivity of the polymer electrolytes. To further investigate the impact of PSN⁺ on the crystallinity of the polymer electrolyte, DSC measurement was conducted. The glass transition temperatures (T_g) (Fig. 2b) of CPE-P and CPE-1%PSN⁺ were -39.2 and -40.9 ℃, respectively. The lower T_g of CPE-1%PSN⁺ implies its lower crystallinity, indicating the addition of PSN⁺ decreases the crystallinity of the polymer host, which is beneficial for enhancing lithium-ion migration within the polymer electrolyte. The ionic conductivities of the CPE-0.5%PSN, CPE-1%PSN, and CPE-3%PSN were measured via EIS in SS||SPEs||SS symmetric cells at 50 ℃ (Fig. 2c). The ionic conductivities can be calculated according to Eq.1. It is found that CPE-1%PSN⁺ has superior ionic conductivities to that of others. The corresponding ionic conductivities of these membrane electrolytes at 50 ℃ are compared in Fig. 2d. Since CPE-1%PSN⁺ had the highest ionic conductivity, this PSN content was used for further study.

  Figure 2

  

  Figure 2.  (a) XRD patterns and (b) DSC curves of CPE-P and CPE-1%PSN⁺; (c) Nyquist plots and (d) the corresponding ionic conductivities of SS||SPE||SS symmetric cells with different membranes; (e, f) Raman spectra of CPE-P (e) and CPE-1%PSN⁺ (f)

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  To further elucidate the interaction between the PSN⁺ filler, lithium-ions, and the polymer host, Raman spectra analysis was conducted on these polymer electrolyte membranes. The two peaks situated at 739 and 745 cm^-1 respectively signify the unbound TFSI^- and the ion cluster [Li(TFSI)₂]^-[31-32]. As depicted in Fig. 2e, the CPE-P displayed a pronounced ion cluster peak, validating the abundant presence of [Li(TFSI)₂]^- within the pure SPEs. In Fig. 2f, such an ion cluster peak diminished, while the percentage area of free TFSI^- peaks noticeably increased. This observation indicates that the PSN⁺ filler effectively promotes the dissociation of lithium salts, leading to enhanced Li⁺ concentration within the SPEs, thus ameliorating the electrochemical performance of the SPEs.

  2.3   Electrochemical properties of polymer electrolyte membranes

  EIS was conducted on the assembled symmetrical SS||SPEs||SS cell. Fig. 3a and 3b present the Nyquist plots of the solid electrolyte at various temperatures (30-70 ℃), and the corresponding ionic conductivities were calculated using Eq.1 and summarized in Table 1. The ionic conductivity of CPE-1%PSN⁺ increased from 2.8×10^-5 to 1.7×10^-4 S·cm^-1 within the temperature range of 30-70 ℃. At 50 ℃, the ionic conductivity of CPE-1%PSN⁺ reached 7.5×10^-5 S·cm^-1, whereas CPE-P exhibited only 4.3×10^-5 S·cm^-1, showcasing a notable enhancement in ionic conductivity. This enhancement signifies that the addition of PSN⁺ can effectively elevate the ionic conductivity of SPEs. This effect can be attributed to the positively charged surface of PSN⁺ that can engage with the TFSI^- anion groups in LiTFSI, thus promoting the dissociation of lithium salts and elevating lithium-ion concentration within the polymer electrolyte, and finally amplifying the ionic conductivity of the polymer electrolyte. The relationship between the ionic conductivity and the temperature is plotted in Fig. 3c. Activation energies of CPE-P and CPE-1%PSN⁺ were calculated using Eq.2, yielding 0.42 kJ·mol^-1 for CPE-P and 0.39 kJ·mol^-1 for CPE-1%PSN⁺. The lower activation energy of CPE-1%PSN⁺ indicates the facilitated migration of lithium-ions due to PSN⁺ incorporation, which can effectively improve the electrochemical performance of SPEs.

  Figure 3

  

  Figure 3.  Nyquist plots of symmetric stainless-steel cells with (a) CPE-P and (b) CPE-1%PSN⁺; (c) Arrhenius curves of different membranes; Chronoamperometry profiles of symmetric lithium cell with (d) CPE-P and (e) CPE-1%PSN⁺; (f) LSV curves of different electrode membranes

  Inset in d and e: Nyquist plots of the Li||SPEs||Li symmetric cell before and after polarization.

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  Table 1

  

  Table 1.  Ionic conductivities of CPE-1%PSN⁺ and CPE-P at 30, 40, 50, 60, and 70 ℃  S·cm^-1

  下载: 导出CSV

  Sample	30 ℃	40 ℃	50 ℃	60 ℃	70 ℃
  CPE-P	1.5×10-5	2.7×10-5	4.3×10-5	5.8×10-5	1.0×10-4
  CPE-1%PSN+	2.8×10-5	4.4×10-5	7.5×10-5	9.7×10-5	1.7×10-4

  Fig. 3d and 3e depict the current-time curves of CPE-P and CPE-1%PSN⁺ at 50 ℃ obtained through chronoamperometry. Utilizing Eq.3, t_Li⁺ was computed as 0.12 for CPE-P and 0.30 for CPE-1%PSN⁺. Compared with CPE-P, the higher lithium-ion transference number of CPE-1%PSN⁺ suggests that PSN⁺ can enhance t_Li⁺. The stable voltage window of CPE-P and CPE-1%PSN⁺ was determined by assembling asymmetrical SS||SPEs||Li cells and conducting LSV tests, as depicted in Fig. 3f. At a voltage of 4.24 V, CPE-P exhibited a rapid increase in current, indicating oxidative decomposition of the electrolyte; whereas CPE-1%PSN⁺ started to decompose only at 4.41 V, underscoring the improved electrochemical stability conferred by CPE-1%PSN⁺. This improved stability is potentially attributed to the protective effect of PSN⁺ on the PEO chain segments, particularly under elevated voltages.

  2.4   Investigation of the polymer electrolyte/lithium metal interfacial compatibility

  To explore the interfacial compatibility of CPE-P and CPE‐1%PSN⁺ with lithium metal, symmetrical Li||SPEs||Li cells were assembled for a constant current charge-discharge cycling tests at 50 ℃ and a current density of 0.1 mA·cm^-2. As illustrated in Fig. 4a, Li||CPE-1%PSN⁺||Li exhibited stable cycling for over 400 h, while the Li||CPE-P||Li cell experienced a sharp voltage drop after 117 h of charge-discharge cycling, indicating a short circuit caused by lithium dendrite penetration through the electrolyte. This outcome is attributed to the enhanced interfacial compatibility of the polymer electrolyte with lithium metal due to the incorporation of PSN⁺. Fig. 4b shows the surface morphology of the lithium metal before cycling, Fig. 4c and 4d present SEM images of lithium metal surfaces from the Li||CPE-P||Li and Li||CPE-1%PSN⁺||Li symmetrical cells respectively after 50 h of cycling at 50 ℃ and a current density of 0.1 mA·cm^-2. The lithium metal surface of the Li||CPE-1%PSN⁺||Li cell appeared smoother compared to that of Li||CPE-P||Li, further substantiating the ability of PSN⁺ to ameliorate the interfacial compatibility between the polymer electrolyte and lithium metal.

  Figure 4

  

  Figure 4.  (a) Lithium plating/stripping of Li||CPE-P||Li and Li||CPE-1%PSN⁺||Li symmetrical cells at 0.1 mA·cm^-2 at 50 ℃; SEM images of the lithium metal anode surface of original surface (b), and the symmetric lithium cells with CPE-P (c) and CPE-1%PSN⁺ (d)

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  2.5   Electrochemical performance of the cells

  To substantiate the prospective application of CPE-1%PSN⁺ in solid-state lithium-ion batteries, the prepared SPEs were paired with LFP and lithium metal anodes to assemble CR2032 coin cells for the electrochemical test at 50 ℃. As depicted in Fig. 5a, the EIS of assembled LFP||CPE-P||Li and LFP||CPE-1%PSN⁺||Li lithium-ion batteries before cycling is shown. These Nyquist plots comprised a semicircle in the high-frequency region and a diagonal line in the mid-to-high-frequency region. The semicircle corresponds to the charge transfer resistance (R_ct) of the cell, while the diagonal line indicates the ionic diffusion resistance (Warburg) within the cell. Notably, the impedance of CPE-1%PSN⁺ was lower than that of CPE-P, underscoring the facilitation of lithium-ion migration due to PSN⁺ integration.

  Figure 5

  

  Figure 5.  (a) Nyquist plots, (b) cyclic performances, (c) rate performance, and (d, e) GCD curves of the LFP||CPE-P||Li and LFP||CPE-1%PSN⁺||Li cells

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  Fig. 5b presents the cycling performance of LFP||CPE-P||Li and LFP||CPE-1%PSN⁺||Li within the voltage range of 2.2-4.2 V at a rate of 0.2C. The findings illustrate that LFP||CPE-1%PSN⁺||Li attained an initial discharge-specific capacity of 155.7 mAh·g^-1 and retained a capacity of 151.2 mAh·g^-1 after 100 cycles, corresponding to a capacity retention of 97.1%. In comparison, LFP||CPE-P||Li showcased a peak discharge specific capacity of 152.8 mAh·g^-1 and maintained a capacity of 146.0 mAh·g^-1 after 100 cycles, with a capacity retention of 95.5%. The rate performance of the assembled LFP||SPEs||Li cell is depicted in Fig. 5c. The outcomes reveal the exceptional discharge-specific capacity of the LFP||CPE-1%PSN⁺||Li at various rates. While its capacity at 1C during the first cycle was lower than that of the LFP||CPE-P||Li cell, upon returning from high to low rates, its discharge-specific capacity at 1C significantly surpasses that of the LFP||CPE-P||Li cell. This phenomenon is likely attributed to the initial voltage spike and limited cycle count in the first cycle, which hindered the complete manifestation of the performance of the LFP||CPE-1%PSN⁺||Li cell. It is worth noting that at 10C, the specific discharge capacity of LFP||CPE-1%PSN⁺||Li was much higher than that of LFP||CPE-P||Li, which may indicate that the positive charge-modified sample is more suitable for operation at high rates. By analyzing the galvanostatic charge-discharge (GCD) curves of LFP||CPE-P||Li and LFP||CPE-1%PSN⁺||Li at 0.2C (Fig. 5d, 5e), it became evident that the cell assembled with CPE-1%PSN⁺ demonstrated a diminished polarization voltage (0.14 V). This reduction is attributed to the relatively lower bulk resistance of CPE-1%PSN⁺, thereby establishing a favorable interfacial compatibility with the lithium metal anode.

  To delve into the utility and safety of the CPE, we undertook the assembly of LFP||CPE-1%PSN⁺||Li pouch cell. The open circuit voltage (OCV) of the pouch cell maintained a steady equilibrium at 3.462 V (Fig. 6a). Moreover, this cell effectively illuminates a light-emitting diode (LED) lamp (Fig. 6b). It is worth highlighting that even under the folding and corner-cut conditions exemplified in Fig. 6c and 6d, the cell continued to operate seamlessly, providing further substantiation of the high safety and practicality demonstrated by this electrolyte.

  Figure 6

  

  Figure 6.  Photos of (a) the LFP||CPE-1%PSN⁺||Li pouch cell showing an OCV of 3.462 V; Photos of (b) the pouch cell lighting up LED lamps; Photos of the cells lighting pouch LED lamps under (c) bending and (d) corner-cut conditions

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  3.   Conclusions

  Using facile ion exchange and acid treatment, 2D PSN was obtained, and after being modified through APTES, 2D PSN⁺ was produced. At 50 ℃, the composite SPE with the PSN⁺ fillers manifests decent ion conductivity of 7.5×10^-5 S·cm^-1 and t_Li⁺ of 0.30, as well as a stable voltage window of 4.41 V. Moreover, the as-obtained Li||CPE-1%PSN⁺||Li symmetrical cell endured stable cycling for over 400 h at 50 ℃ and 0.1 mA·cm^-2. The LFP||CPE-1%PSN⁺||Li cell with the SPE exhibited an initial discharge-specific capacity of 155.7 mAh·g^-1 at 0.2C with a capacity retention of 97.1% after 100 cycles. These commendable performances are ascribed to that the incorporation of PSN⁺ promotes the dissociation of lithium salts, elevates the lithium-ion concentration within the polymer electrolyte, and thus ameliorates the electrochemical performance of the cells. This endeavor holds the promise of propelling the realization of high-energy-density LMBs.

  
  Acknowledgments: This work was financially supported by National Natural Science Foundation of China (Grants No.52062004, 51972070, 52372185), Guizhou Provincial High-Level Innovative Talents Project (Grants No.QKHPTRC-GCC[2022]013-1), Innovation Team for Advanced Electrochemical Energy Storage Devices and Key Materials of Guizhou Provincial Higher Education Institutions (Grants No.QianJiaoJi[2023]054).
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  Figure 1  (a) ζ potentials of PSN and PSN⁺; (b) Nitrogen adsorption-desorption isotherm of PSN⁺; SEM images of (c) PSN⁺ and (d) PSN

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  Figure 2  (a) XRD patterns and (b) DSC curves of CPE-P and CPE-1%PSN⁺; (c) Nyquist plots and (d) the corresponding ionic conductivities of SS||SPE||SS symmetric cells with different membranes; (e, f) Raman spectra of CPE-P (e) and CPE-1%PSN⁺ (f)

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  Figure 3  Nyquist plots of symmetric stainless-steel cells with (a) CPE-P and (b) CPE-1%PSN⁺; (c) Arrhenius curves of different membranes; Chronoamperometry profiles of symmetric lithium cell with (d) CPE-P and (e) CPE-1%PSN⁺; (f) LSV curves of different electrode membranes

  Inset in d and e: Nyquist plots of the Li||SPEs||Li symmetric cell before and after polarization.

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  Figure 4  (a) Lithium plating/stripping of Li||CPE-P||Li and Li||CPE-1%PSN⁺||Li symmetrical cells at 0.1 mA·cm^-2 at 50 ℃; SEM images of the lithium metal anode surface of original surface (b), and the symmetric lithium cells with CPE-P (c) and CPE-1%PSN⁺ (d)

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  Figure 5  (a) Nyquist plots, (b) cyclic performances, (c) rate performance, and (d, e) GCD curves of the LFP||CPE-P||Li and LFP||CPE-1%PSN⁺||Li cells

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  Figure 6  Photos of (a) the LFP||CPE-1%PSN⁺||Li pouch cell showing an OCV of 3.462 V; Photos of (b) the pouch cell lighting up LED lamps; Photos of the cells lighting pouch LED lamps under (c) bending and (d) corner-cut conditions

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  Table 1.  Ionic conductivities of CPE-1%PSN⁺ and CPE-P at 30, 40, 50, 60, and 70 ℃  S·cm^-1

  Sample	30 ℃	40 ℃	50 ℃	60 ℃	70 ℃
  CPE-P	1.5×10-5	2.7×10-5	4.3×10-5	5.8×10-5	1.0×10-4
  CPE-1%PSN+	2.8×10-5	4.4×10-5	7.5×10-5	9.7×10-5	1.7×10-4

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