Synergistic Enhancement of Ion Transport and Cycling Stability in Composite Solid Electrolytes via Inert/Active Dual-Ceramic Fillers

Abstract

Poly(ethylene oxide) (PEO)-based solid electrolytes are promising candidates for solid-state lithium metal batteries because of their flexibility and ease of processing. However, their practical application is limited by insufficient mechanical strength and poor interfacial stability. Conventional single-filler strategies typically improve either ionic conductivity or mechanical robustness, making it challenging to simultaneously optimize both properties. In this work, a dual-ceramic strategy is proposed that integrates inert and active ceramic fillers with complementary roles to construct a polymer electrolyte that is both mechanically robust and ionically conductive. The inert ceramic filler promotes lithium-salt dissociation and Li⁺ transport, whereas the active ceramic filler enhances structural integrity and suppresses lithium dendrite growth, enabling a synergistic balance between ionic transport and cycling stability. As a representative implementation, paraelectric SrTiO₃ and Li⁺-conducting Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) are incorporated into the PEO/LiTFSI matrix to construct a composite solid electrolyte (PLLS). The optimized PLLS electrolyte, containing 8 wt% STO and 5 wt% LLZTO, exhibits a high ionic conductivity of $4.48\times {10}^{-4}\mathrm{S}{\mathrm{cm}}^{-1}$, an increased Li⁺ transference number of 0.20, and a wide electrochemical stability window of 5.165 V versus Li/Li⁺ at 60 °C. Li/Li symmetric cells demonstrate stable lithium plating/stripping for nearly 2000 h at a current density of$0.2\mathrm{mA}{\mathrm{cm}}^{-2}$. Furthermore, LiFePO₄/Li full cells retain 92.1% of their initial capacity after 500 cycles at 1 C, and stable cycling performance is also achieved with high-voltage LiCoO₂ cathodes. These results demonstrate that the proposed dual-ceramic synergistic strategy offers an effective and potentially generalizable approach to enhancing the durability of PEO-based solid electrolytes for long-life solid-state lithium metal batteries.

1. Introduction

Poly(ethylene oxide) (PEO)-based solid electrolytes are regarded as one of the most promising polymer electrolyte systems for all-solid-state lithium metal batteries (ASSLMBs) due to their excellent flexibility, superior interfacial wettability, strong lithium-ion solvation ability, and mature processing technology [1]. These advantages collectively enable good electrode–electrolyte contact and improved safety [2]. However, two major challenges still limit their practical application. First, the intrinsic mechanical strength of PEO is insufficient to effectively suppress lithium dendrite growth during long-term cycling or under high current densities, leading to potential short-circuit risks. Strategies such as incorporating high-strength polymers or inorganic fillers (e.g., aramids, SiO₂, and ZnO) can enhance tensile strength and thermal stability, yet complete dendrite prevention remains difficult [3,4,5]. Second, attempts to increase ionic conductivity by raising lithium-salt concentration or introducing inorganic fillers may compromise structural stability. Excessive filler loading and reduced PEO-chain flexibility can increase interfacial impedance and hinder uniform ion transport [6,7].

Facing these challenges, recent research has increasingly focused on incorporating single-type inorganic fillers into PEO-based solid electrolytes to enhance both ionic conductivity and mechanical stability. Two main strategies have emerged. The first involves inert or dielectric fillers, such as SiO₂, Al₂O₃, and TiO₂, which can effectively disrupt the crystallinity of the PEO matrix, increase the amorphous phase, and promote lithium-salt dissociation through Lewis acid–base interactions at the polymer–filler interface. These mechanisms facilitate faster Li⁺ transport by weakening Li⁺–anion coordination and enabling enhanced segmental motion of polymer chains, thereby improving ionic conductivity [8,9,10]. The second strategy utilizes active or rigid fillers, including fast-ion-conducting ceramics such as perovskite-type oxides (Li_0.33La_0.56TiO₃, LLTO) and NASICON-type materials (Li_1.3Al_0.3Ti_1.7(PO₄)₃, LATP). These fillers not only provide additional Li⁺ conduction pathways through their intrinsic ionic channels but also serve as a structural backbone to reinforce mechanical integrity and suppress lithium dendrite growth [11,12,13]. Nevertheless, in composite polymer electrolytes, achieving high ionic conductivity while maintaining long-term cycling stability remains a challenge. The interfacial interactions between fillers and the polymer matrix play a critical role in governing both ion transport and structural integrity, such that gains in ionic mobility are often accompanied by reduced stability [14]. Inert fillers promote ion transport by disrupting polymer crystallinity but provide limited enhancement to interfacial stability, whereas active fillers strengthen structural integrity but restrict polymer-chain dynamics, thereby reducing ionic conductivity [15]. This intrinsic trade-off has inspired the development of multifunctional and hybrid fillers to synergistically optimize ion conduction and cycling durability [16,17,18].

To overcome this intrinsic trade-off, a rational synergistic design requires the integration of fillers with clearly differentiated yet complementary functions, rather than a simple combination of multiple components. Ideally, the selected fillers should independently target ion transport and structural stability without introducing adverse interference to each other [8]. From this perspective, paraelectric SrTiO₃ (STO) and lithium-ion-conducting Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) represent a particularly suitable dual-filler combination for PEO-based solid electrolytes. STO, as a paraelectric ceramic with a relatively high dielectric constant and low polarization hysteresis, has been demonstrated to facilitate lithium-salt dissociation and promote Li⁺ migration through dielectric polarization effects, thereby enhancing ionic conductivity without significantly stiffening the polymer matrix [19,20]. In contrast, LLZTO possesses a rigid oxide framework and intrinsic lithium-ion conductivity, which can provide mechanical reinforcement and interfacial stabilization while contributing additional Li⁺ transport pathways [21,22,23]. Importantly, the functions of STO and LLZTO are fundamentally distinct and largely decoupled, enabling the possibility of synergistic performance enhancement when they are incorporated into a single polymer electrolyte system [24,25].

Based on the above considerations, this study introduces a dual-ceramic synergistic modification strategy for PEO-based solid electrolytes, as schematically illustrated in Figure 1, by deliberately integrating inert and active ceramic fillers with distinct yet complementary functions. Paraelectric SrTiO₃ (STO) and lithium-ion-conducting Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) are concurrently incorporated into a PEO/LiTFSI matrix to construct a composite solid electrolyte, referred to as PLLS. Through systematic optimization of the dual-ceramic composition, the PLLS electrolyte is demonstrated to simultaneously sustain fast Li⁺ transport, with ionic conductivity maintained in the ${10}^{-4}\mathrm{S}{\mathrm{cm}}^{-1}$ range (approaching $5\times {10}^{-4}\mathrm{S}{\mathrm{cm}}^{-1}$ at 60 °C), and robust interfacial stability against lithium metal, enabling long-term and stable lithium plating/stripping.In addition, solid-state full cells employing the PLLS electrolyte exhibit high-capacity retention over prolonged cycling and stable operation with both conventional and high-voltage cathodes. Furthermore, LiFePO₄/Li full cells employing the PLLS electrolyte exhibit a high capacity retention of 92.1% after 500 cycles, demonstrating excellent long-term cycling stability. Collectively, these results establish that a rationally designed dual-ceramic architecture can effectively mitigate the inherent trade-off between ionic conductivity and durability in PEO-based solid electrolytes, thereby advancing their practical applicability in long-life solid-state lithium metal batteries.

2. Materials and Methods

2.1. Materials

Polyethylene oxide (PEO) with an average molecular weight of $M_{\mathrm{w}}\approx \mathrm{1,000,000}$ was purchased from Aladdin and used as the polymer matrix for solid electrolyte preparation. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, ≥99% purity, Aladdin, Shanghai, China) was employed as the lithium salt without further purification.

Paraelectric strontium titanate (SrTiO₃, STO) nanopowder with an average particle size below 100 nm and a purity of 99.5% was obtained from Macklin (Shanghai, China). The garnet-type lithium-ion-conducting ceramic Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) powder was synthesized in our laboratory via a conventional solid-state reaction method. Stoichiometric amounts of Li₂CO₃ (Aladdin, Shanghai, China) (with a 10 wt% excess to compensate for lithium loss), La₂O₃ (Aladdin, Shanghai, China), ZrO₂ (Aladdin, Shanghai, China), and Ta₂O₅ (Aladdin, Shanghai, China) were used as starting materials. La₂O₃ was pre-dried at 900 °C prior to use to remove absorbed moisture and carbonates. The precursor powders were thoroughly mixed by ball milling, followed by calcination at elevated temperature to obtain the garnet-phase LLZTO. After calcination, the resulting powder was ground and collected for use as the active ceramic filler. Prior to electrolyte preparation, the LLZTO powder was dried under vacuum to eliminate residual moisture.

Finally, all materials were stored and handled in a dry atmosphere to minimize moisture contamination during electrolyte preparation.

2.2. Preparation of Dual-Ceramic PLLS Solid Electrolytes

The dual-ceramic-modified PEO-based solid electrolytes were prepared via a solution-casting method. PEO and LiTFSI were first dissolved in anhydrous acetonitrile (Aladdin, Shanghai, China) under magnetic stirring to form a homogeneous polymer–salt solution with a fixed EO/Li molar ratio. Subsequently, paraelectric SrTiO₃ (STO) and garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) powders were added into the solution.

The STO content was fixed at 8 wt%, while the LLZTO content was systematically varied between 5, 8, and 10 wt% to investigate its influence on electrolyte performance. This composition design was based on our previous study [26], in which the STO content in PEO/LiTFSI electrolytes was systematically optimized and 8 wt% STO was identified as an effective loading that balances ion transport and morphological uniformity. The STO-only system (denoted as PLS) delivered an ionic conductivity of $6.1\times {10}^{-4}$ S cm⁻¹ at 60 °C, providing a reliable performance baseline. Based on this optimized composition, LLZTO was introduced as a second ceramic phase to construct a dual-ceramic electrolyte and to further examine the trade-off between ion-transport performance and structural/interfacial stability.

The resulting suspensions were subjected to ultrasonic treatment followed by continuous magnetic stirring to ensure uniform dispersion of the inorganic fillers within the polymer matrix. Afterward, the well-dispersed slurry was cast onto a polytetrafluoroethylene (PTFE) substrate and slowly evaporated to remove the solvent. The obtained electrolyte films were further dried under vacuum to completely eliminate residual solvent and moisture, yielding free-standing PLLS solid electrolyte membranes.

For comparison, the pristine PEO/LiTFSI electrolyte (denoted as PL) and the single-ceramic-modified electrolyte containing only LLZTO (denoted as PLL) were prepared using the same procedure.

2.3. Structural and Physicochemical Characterization

The morphology and microstructure of the solid electrolyte membranes were characterized by scanning electron microscopy (SEM), performed using a Phenom Pharos microscope (Phenom-World B.V., Eindhoven, The Netherlands), to examine both surface and cross-sectional features. Energy-dispersive X-ray spectroscopy (EDS) mapping was conducted on the same instrument to analyze the spatial distribution of the inorganic fillers and to evaluate the dispersion uniformity of STO and LLZTO within the PEO matrix.

Electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) to determine the ionic conductivity of the electrolytes. The measurements were performed with stainless steel blocking electrodes over a frequency range from 1 MHz to 1 Hz with a small AC perturbation voltage. The ionic conductivity was calculated from the bulk resistance obtained from the Nyquist plots according to the electrolyte thickness and electrode area.

The electrochemical stability window of the electrolytes was evaluated by linear sweep voltammetry (LSV) using stainless steel as the working electrode and lithium metal as the counter/reference electrode. The potential was swept at a constant scan rate to assess the oxidative stability of the solid electrolytes.

2.4. Electrochemical Measurements

Lithium symmetric cells were assembled using lithium metal foils as both the working and counter electrodes, with the solid electrolyte membrane sandwiched in between them. The cells were sealed in coin-cell configurations and subjected to galvanostatic lithium plating/stripping tests to evaluate interfacial stability and cycling durability.

The symmetric cells were evaluated over a range of current densities, and long-term cycling was performed at 0.2 mA cm⁻² to assess electrolyte stability during repeated lithium deposition and dissolution.

LiFePO₄/Li full cells were assembled to further evaluate the practical applicability of the solid electrolytes. The LiFePO₄ cathodes were prepared by coating a slurry containing LiFePO₄ active material, conductive carbon, and polymer binder onto aluminum foil current collectors. The cathode loading was controlled to ensure consistent electrochemical evaluation. The full cells were cycled within an appropriate voltage window to investigate the long-term cycling performance of the electrolytes.

All electrochemical measurements were carried out at a constant temperature to minimize the influence of thermal fluctuations on cell performance.

3. Results and Discussion

3.1. Optimization of Dual-Ceramic Composition in PLLS Electrolytes

To optimize the composition of the dual-ceramic-modified PEO-based solid electrolytes, a series of PLLS samples were prepared with different LLZTO contents while maintaining a fixed STO content of 8 wt%. The influence of LLZTO incorporation on both ionic conductivity and film morphology was systematically investigated to identify a suitable balance between ion transport and electrolyte integrity.

Figure 2 shows the ionic conductivity of the PLLS electrolytes as a function of LLZTO content. As the LLZTO loading increases from 5 to 10 wt%, a gradual decrease in ionic conductivity is observed. This trend can be attributed to the rigid ceramic framework of LLZTO, which partially restricts polymer chain mobility and reduces the effective segmental motion of the PEO matrix. Nevertheless, all PLLS samples maintain ionic conductivities on the order of ${10}^{-4}$ S cm⁻¹ at 60 °C, indicating that efficient ion transport is preserved within the dual-ceramic system despite the increasing LLZTO content.

In addition to ionic conductivity, the morphology and structural integrity of the electrolyte films are strongly affected by the LLZTO content, as evidenced by the SEM images shown in Figure 3. At lower LLZTO loadings, the electrolyte films exhibit relatively smooth and uniform surfaces with no apparent particle agglomeration, suggesting good compatibility between the ceramic fillers and the PEO matrix. With increasing LLZTO content, the films become progressively rougher and less homogeneous, and slight undulating (wave-like) surface features can be observed at higher loadings (e.g., 10 wt%). This morphological evolution is primarily associated with the increased ceramic fraction, where stronger particle–particle interactions and localized aggregation reduce the self-leveling ability of the PEO matrix during solvent evaporation and film formation, leading to local surface topography irregularities. Such increased surface heterogeneity may in turn reduce electrode–electrolyte interfacial conformity and aggravate interfacial polarization during cycling, highlighting the need to balance morphology and mechanical robustness in the dual-ceramic design. By jointly considering the ionic conductivity results and the morphological observations, an LLZTO content of 5 wt% is identified as the optimal composition for the dual-ceramic system. This composition provides a favorable compromise between maintaining sufficient ion transport and ensuring good film uniformity and structural integrity. Given that the STO-only baseline (8 wt% STO, without LLZTO) has been established in our previous work [26], lower LLZTO contents are expected to behave close to this baseline in bulk ion transport while providing limited reinforcement; Therefore, 5 wt% was selected as the minimum effective LLZTO loading to make the influence of the second ceramic phase clearly observable. Accordingly, the PLLS electrolyte containing 8 wt% STO and 5 wt% LLZTO (denoted as PLLS−5) was selected for subsequent structural characterization and electrochemical performance evaluation.

3.2. Structural Integrity and Lithium Metal Compatibility of PLLS Electrolytes

After determining the optimized dual-ceramic composition, the structural integrity and electrochemical compatibility of the PLLS electrolytes were evaluated through morphological and structural characterization as well as lithium symmetric-cell tests.

3.2.1. Structural Characteristics and Physicochemical Properties

Figure 4 presents the SEM morphology and EDS elemental distribution of the PLLS membrane. The cross-sectional SEM image shows a continuous composite film with an overall thickness of approximately 110 $\mathsf{\mu }$m. The small bright dots observed in both the surface and cross-sectional SEM images correspond to dispersed inorganic ceramic particles (SrTiO₃ and LLZTO) embedded within the PEO/LiTFSI matrix, which appear brighter than the polymer phase due to the higher atomic-number contrast. Consistently, the EDS maps show C and F signals originating from the PEO/LiTFSI matrix, Sr signals from SrTiO₃, and Zr signals from LLZTO. Because Figure 4a,b show the surface and cross-section of the same membrane, the elemental compositions are intrinsically identical between the two views, and the uniform Sr and Zr distributions confirm the homogeneous dispersion of the dual ceramics throughout the membrane thickness. Notably, the low-magnification cross-sectional images indicate a slightly reduced surface smoothness of the membrane after dual-ceramic incorporation, which is consistent with the inherent characteristics of blending multiple heterogeneous ceramic fillers into a polymer matrix [27,28].

The crystalline structure and thermal stability of PLL and PLLS electrolytes were systematically investigated by XRD and TGA, as shown in Figure 5. For the PLL electrolyte (Figure 5a), two diffraction peaks characteristic of crystalline PEO are observed at 2$\theta$ = 19° and 23°, together with the diffraction peaks associated with LLZTO located at 2$\theta$ = 17° and 31°. After the incorporation of STO, as shown in Figure 5b, the PLLS electrolyte exhibits an additional diffraction peak at 2$\theta$ = 34°, confirming the successful incorporation of STO into the dual-ceramic system. Meanwhile, the LLZTO-related peak shows a slight shift from 2$\theta$ = 30.5° in PLL to 30.7° in PLLS, suggesting that the presence of polarized STO influences the local structural environment of LLZTO. It is speculated that the electrostatic polarization effect of STO partially weakens the interaction between PEO and LLZTO, leading to a slight recovery of the LLZTO crystalline structure, which may be beneficial for lithium-ion transport through the LLZTO lattice [29,30]. The weak features observed below 16° are attributed to background scattering and short-range structural correlations in the polymer-based composite film rather than distinct crystalline phases, which is consistent with grazing-incidence/operando scattering observations in PEO–LiTFSI electrolytes where low-angle diffuse features mainly reflect local structural heterogeneity instead of new Bragg reflections [31].

The thermal stability of the electrolytes was further evaluated by TGA (Figure 5c,d). Compared with PLL, the first weight-loss plateau associated with PEO decomposition shifts from 365 °C to 374 °C in PLLS, while the second plateau corresponding to LiTFSI decomposition increases from 439 °C to 442 °C. These results suggest that the incorporation of polarized STO into the LLZTO-containing electrolyte enables additional electrostatic interactions with the PEO matrix, thereby enhancing the thermodynamic stability of the composite electrolyte system.

3.2.2. Electrochemical Performance and Lithium Metal Compatibility

The ionic transport properties and electrochemical stability of the PLL and PLLS electrolytes were systematically investigated by electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and constant-voltage polarization measurements, as shown in Figure 6. In the EIS analysis, the ionic conductivity was determined from the bulk resistance ($R_{\mathrm{b}}$) extracted from the high-frequency intercept of the Nyquist plots using the measured electrolyte thickness and the identical electrode area. Therefore, under the same cell configuration and testing conditions, a smaller $R_{\mathrm{b}}$ corresponds directly to a higher ionic conductivity. At 60 °C, the PLLS electrolyte exhibits a higher ionic conductivity of $4.48\times {10}^{-4}$ S cm⁻¹ compared with that of PLL ($3.31\times {10}^{-4}$ S cm⁻¹), which is mainly attributed to the polarized STO promoting further lithium-salt dissociation and generating more free charge carriers, thereby enhancing ionic transport in the solid electrolyte [26]. In addition, LSV results indicate that PLLS maintains a relatively wide electrochemical stability window of 5.165 V versus Li/Li⁺ after STO incorporation, suggesting that STO primarily enhances the oxidative electrochemical stability of the PLLS electrolyte.

The lithium-ion transport behavior was further evaluated by constant-voltage polarization measurements (Figure 6c,d). The Li⁺ transference number of PLL is determined to be 0.14, whereas the PLLS electrolyte exhibits a markedly increased transference number of 0.20. This improvement is attributed to the STO-induced weakening of EO–Li coordination, which increases the proportion of mobile Li⁺ carriers and is beneficial for the long-term cycling performance of PLLS-based solid-state cells.

The long-term lithium plating/stripping behavior was evaluated using Li symmetric cells assembled with different solid electrolytes, as summarized in Figure 7 and Figure 8. At a current density of 0.1 mA cm⁻² with a capacity of 0.1 mAh cm⁻² (Figure 7a), the Li/PLLS/Li cell exhibits stable cycling for over 1100 h, whereas the Li/PLL/Li cell shows a sudden voltage drop after approximately 200 h, indicative of lithium dendrite penetration through the PLL electrolyte. Correspondingly, the PLL electrolyte displays a relatively high polarization voltage of about 38 mV with pronounced fluctuations (Figure 7b,c), while the PLLS cell maintains a much lower and stable polarization voltage of approximately 21 mV, reflecting improved interfacial stability against lithium metal.

When the current density is increased to 0.2 mA cm⁻² (Figure 8a), the Li/PLL/Li cell exhibits a rapidly rising polarization voltage approaching 400 mV and fails after around 100 h. In contrast, the Li/PLLS/Li cell shows an initial polarization voltage of only ∼46 mV and sustains stable cycling for nearly 2000 h without short-circuiting. Under identical conditions, the Li/PL/Li and Li/PLS/Li cells exhibit significantly shorter lifetimes of approximately 60 h and 1200 h, respectively. As shown in Figure 8b,c, the polarization voltage of the Li/PLLS/Li cell remains relatively stable, increasing only slightly from ∼47 mV at 500 h to ∼57 mV at around 1500 h. These results demonstrate that the PLLS electrolyte effectively integrates the polarization effect of STO with the activation and reinforcement effects of LLZTO, thereby markedly enhancing lithium metal interfacial stability and suppressing dendrite-induced failure [32,33].

3.3. Full-Cell Performance and Practical Applicability

The practical applicability of the PLLS electrolyte was first evaluated in solid-state full cells assembled with LiFePO₄ (LFP) cathodes with a loading of 2.0 mg cm⁻², as shown in Figure 9a. The LFP/PLLS/Li cell delivers stable long-term cycling at 1 C, retaining 92.1% of its initial discharge capacity after 500 cycles, with the Coulombic efficiency remaining close to 99.9%. In contrast, the LFP/PLL/Li cell exhibits rapid capacity decay after approximately 150 cycles and eventually undergoes electrochemical failure, indicating inferior interfacial and structural stability.

The performance of the PLLS electrolyte was further evaluated in solid-state full cells employing high-voltage, high-loading LiCoO₂ (LCO) cathodes (4.7 mg cm⁻²) with a cutoff voltage of 4.3 V. Figure 9b,d illustrate that a pronounced difference in the charge–discharge behavior is observed between the LCO/PLLS/Li and LCO/PLL/Li cells at 0.2 C. The LCO/PLL/Li cell exhibits severe voltage noise during charging (Figure 9d), characterized by continuous voltage fluctuations and difficulty in reaching the cutoff voltage, suggesting abnormal interfacial instability and parasitic reactions. By contrast, the LCO/PLLS/Li cell delivers a smooth and well-defined charge–discharge profile with a discharge capacity of 115.3 mAh g⁻¹ and a Coulombic efficiency close to 99% (Figure 9b).

Furthermore, the LCO/PLLS/Li cell maintains stable cycling for more than 50 cycles without the occurrence of voltage noise, as shown in Figure 9c. These results collectively demonstrate that the synergistic combination of the dielectric polarization effect of STO and the activation effect of LLZTO effectively enhances the stability of the PEO-based electrolyte system. In particular, the improved cathode-side stability enables preliminary compatibility of PEO-based solid electrolytes with high-voltage, high-loading cathodes [34,35]. The synergistic effects of the two ceramic fillers jointly contribute to the overall stability enhancement of the PEO matrix [36]. Rate capability in PEO-based solid polymer electrolytes is generally constrained by the segmental-motion-mediated transport mechanism, and the cell performance tends to deteriorate at elevated current densities due to increased polarization. In our previously reported STO-modified system (PLS) [26], the LFP/PLS/Li full cell maintained stable cycling at 1 C with 85.3% capacity retention after 850 cycles, and it could further sustain cycling at 2 C for over 250 cycles with a capacity retention of 66.3%. In contrast, several control polymer-electrolyte systems failed quickly at 2 C [26], highlighting the intrinsic difficulty of high-rate operation in such polymer electrolytes. Accordingly, the present work emphasizes enhancing interfacial stability and long-term cycling robustness through a dual-ceramic design under practically relevant moderate current densities.

4. Conclusions

In this work, a dual-ceramic synergistic strategy based on the functional integration of inert and active ceramic fillers was proposed to address the intrinsic trade-off between ionic transport and cycling stability in PEO-based solid electrolytes. Using paraelectric SrTiO₃ (STO) and lithium-ion-conducting Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) as representative fillers, a composite solid electrolyte (PLLS) with balanced electrochemical performance was successfully constructed by incorporating both ceramics into a PEO/LiTFSI matrix. Systematic composition optimization and structural characterization reveal that an appropriate LLZTO content enables effective mechanical reinforcement while preserving the STO-induced enhancement of ion transport. As a result, the optimized PLLS electrolyte delivers a high ionic conductivity of $4.48\times {10}^{-4}$ S cm⁻¹ at 60 °C and maintains a wide electrochemical stability window of 5.165 V versus Li/Li⁺. Constant-voltage polarization measurements further indicate an increased Li⁺ transference number, reflecting improved lithium-ion migration behavior. Benefiting from the synergistic effects of dielectric polarization from STO and structural activation from LLZTO, the PLLS electrolyte shows excellent lithium metal compatibility. Li/Li symmetric cells achieve stable lithium plating/stripping for nearly 2000 h at 0.2 mA cm⁻² with low and stable polarization. Furthermore, full-cell evaluations confirm the practical applicability of the PLLS electrolyte, delivering a high capacity retention of 92.1% after 500 cycles in LiFePO₄/Li cells. In addition, favorable cycling performance is also achieved when the PLLS electrolyte is matched with high-voltage, high-loading LiCoO₂ cathodes. Overall, this study demonstrates that a rational dual-ceramic synergistic design is an effective and generalizable approach to enhance the stability and durability of PEO-based solid electrolytes.