Ultra-stable solid-state lithium metal batteries with ferroelectric oxide-enhanced PVDF-based hybrid solid electrolytes

Abstract

Polymer electrolytes are promising for solid-state lithium metal batteries, while intrinsic limitations such as low room-temperature ion conductivity and moderate electrochemical stability exist. The introduction of inorganic particles provides limited improvement in ionic conductivity and fails to alleviate dendrite formation, which severely compromises battery stability. Herein, we present a highly conductive hybrid solid electrolyte (HSE) composed of polyvinylidene fluoride (PVDF), Ga/Nb-doped Li_6.4Ga_0.2La₃Zr_1.6Nb_0.4O₁₂ as the active filler, and ferroelectric BaTiO₃ as the functional filler, referred to as PLBO. Under electric bias, BTO particles generate reverse electric fields to dissociate Li salts to boost ion migration. Also, BTO with a high dielectric constant equalizes the potential difference at the electrolyte–electrode interface for homogeneous Li deposition. As a result, the hybrid electrolyte exhibits a high lithium transference number (t_Li⁺ = 0.413) and ionic conductivity of 0.74 mS cm⁻¹ at a temperature of 25 °C. Additionally, both the electrochemical and the cycling performance of Li//Li symmetric, LiFePO₄ (LFP)‖Li and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)‖Li batteries could be significantly improved when PLBO electrolytes are utilized. Our work validates the potential of ferroelectric materials in hybrid solid electrolytes to alleviate dendrite formation and enhance the performance of all-solid-state lithium batteries.

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1. Introduction

Modern battery technologies are rapidly advancing to meet the increasing demand for sustainable and effective energy storage.¹ Among these innovations, solid-state batteries have gained significant attention due to high energy density and decreased safety hazards.^2,3 Solid-state electrolytes, as alternatives to liquid electrolytes, not only mitigate safety concerns, but also enhance theoretical energy density, particularly by leveraging lithium metal anodes with high theoretical specific capacity (3860 mA h g⁻¹).^4–8 Solid-state polymer electrolytes are particularly attractive because of their inherent flexibility, ease of processing, and intimate contact with electrodes.^9–12 In this context, polyvinylidene fluoride has emerged as a widely studied polymer matrix for solid-state batteries, thanks to its excellent Li⁺ solvating ability and reliable mechanical properties.^13–15 However, PVDF electrolytes suffer from low ionic conductivities (10⁻⁷ to 10⁻⁵ S cm⁻¹) at room temperature due to the dissociated lithium salts and impeded ion migration.^16,17 Furthermore, porous structures are generated within the electrolyte during the drying process, which results in uneven ion flux, rapid growth of lithium dendrites and ultimately an enhanced risk of short circuits.¹⁸

Recent efforts have focused on reducing the crystallinity of the polymer matrix and enhancing Li⁺ ion diffusion by incorporating inorganic fillers.^19,20 In addition to the moderate enhancement of ion conductivity to approximately 5.0 × 10⁻⁴ S cm⁻¹,²¹ the challenge of lithium dendrite formation persists.²² A space charge layer between the polymer and inorganic particles blocks ion migration. Additionally, inorganic particles accumulate an electric field with high localized surface charge density, which further promotes dendrite formation.^23–25 Ferroelectric ceramics with high dielectric constants (ε_r) are anticipated to homogenize the internal electric field and realize the balanced rate between ion migration and electron transfer, and BaTiO₃ (BTO) with ε_r ∼ 10³ is polarized under electric bias through the displacement of Ti⁴⁺ and electrons of O²⁻, which induces a sufficient surface charge density that is opposite to the electric field.²⁶ The generated reverse electric field would alleviate the surface charge layer and dissociate Li salts, increasing the concentration of free Li⁺ and boosting ion migration. Additionally, the high ε_r effectively reduces polarization and minimizes the potential difference at the electrolyte–electrode interface, equalizing the distribution of the electric field and cation flux.^27–29 Overall, the optimized process of ion insertion and extraction would boost the electrochemical performance and stability of the battery system.^30–33

In this study, we developed a highly conductive, ferroelectric-enhanced polymer-based HSE by incorporating PVDF with Ga/Nb-substituted Li-garnet Li_6.4Ga_0.2La₃Zr_1.6Nb_0.4O₁₂ (Ga/Nb-LLZO) and ferroelectric BTO particles, referred to as PLBO (Fig. 1). The HSE exhibits an improved room-temperature ion conductivity of 0.74 mS cm⁻¹, an increased transference number of Li⁺ (t_Li⁺) of 0.413, and enhanced electrochemical performances in Li‖Li symmetrical, LiFePO₄ (LFP)‖Li and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)‖Li batteries. The ferroelectric BTO balances internal electric fields, reduces polarization, and suppresses lithium dendrite growth. The uniform deposition of Li could be verified by the enhanced electrochemical stability window of 4.95 V and ultra-stable cycling of lithium symmetric cells for over 2000 h at 0.2 mA cm⁻². Impressively, LiFePO₄ cells employing the PLBO solid-state electrolyte exhibit 92% capacity retention after 500 cycles at 1C, while the high-voltage NCM811 battery exhibits stable cycling performance with 81% capacity retention after 700 cycles at 2C. Our work presents a promising avenue to construct high-performance solid-state electrolytes for solid-state lithium batteries.

Fig. 1 Illustration of the Li salt state in PLBO and PLO electrolytes and SEM images of cycled Li anodes.

2. Materials and methods

2.1 Materials

PVDF 761 (Arkema, Kynar 761), tetragonal phase BTO (Jiangsu XFNANO Materials Tech Co., Ltd, China) and the laboratory synthesized Ga/Nb-LLZO powders (the synthesis route is similar to that reported in our previous work²⁵) were all dried under vacuum at 80 °C for 24 h before use.

2.2 Synthesis of PLBO, PLO and PVDF electrolytes

The PVDF-based HSE is synthesized using a slurry casting and drying method. In a typical preparation procedure, LiFSI (lithium bis(fluorosulfonyl)imide) is initially dissolved and uniformly dispersed in DMF (N,N-dimethylformamide) within an inert glove box. Subsequently, 187 mg LiFSI and 704 mg PVDF were added and dissolved in 15 ml DMF solvent. For the HSE, comprising 90 wt% PVDF, 8 wt% LLZO and 2 wt% BTO (denoted as PLBO), pre-weighed Ga/Nb-LLZO and BTO powders were mixed and subjected to ultrasonication for approximately 15 min and then stirred at a constant temperature of 50 °C for 6 hours to yield a homogeneous slurry. Using a doctor blade, this slurry was poured onto a glass slide. Following a 24 hour vacuum oven drying process at 55 °C to remove any remaining solvent for dissolution, an electrolyte film with a thickness of approximately 100 μm was obtained.

For comparison, PVDF and PLO (i.e., without the BTO filler) membranes were prepared using the same method and served as control samples of PLBO.

2.3 Materials characterization

X-ray diffraction (XRD, Bruker AXS, D2 Phaser, Karlsruhe, Germany), Fourier transform infrared (FTIR, Nicolet IS 50/6700, Thermo Electron Corporation, USA) and Raman spectra (Horiba Scientific-LabRAM HR Evolution, France) were used to characterize the compositions and structures of the polymer electrolytes. Scanning electron microscopy (SEM, FEI-Quanta FEG 250, USA) was used to evaluate the morphology. Thermogravimetric analysis (TGA, PerkinElmer Inc., Waltham, MA, USA) was carried out in a N₂ environment. An atomic force microscope (AFM, Bruker Dimension Icon, Germany) was used to examine the surface roughness of the polymer electrolytes. The dielectric properties of the samples were examined by piezoelectric force microscopy (PFM). Broadband dielectric spectroscopy (BDS, BALAB DMS-1000, Wuhan, China) was conducted to measure the relative permittivity at different temperatures and frequencies.

2.4 Electrochemical tests and cell assembly

Ionic conductivity was measured using stainless steel (SS)‖PLBO‖SS cells. Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) were recorded using an electrochemical workstation (Autolab PGSTAT 302N) between 10 MHz and 0.01 Hz. Li‖PLBO‖Li symmetrical cells were assembled to assess t_Li⁺via impedance spectra and direct current (DC) polarization. Li deposition and stripping behavior and critical current density (CCD) tests of PLBO were performed using Li‖PLBO‖Li symmetrical cells. Full-cell performance, including long-term cycling and rate performance, was tested on LFP‖PLBO‖Li and NCM811‖PLBO‖Li CR 2032-type coin cells. The cathodes had a loading density of 2.5–3.0 mg cm⁻² for both LFP and NCM811, consisting of 80 wt% active materials, 10 wt% PVDF and 10 wt% Super P. In a glove box filled with Ar, all of the cells were built. LFP‖PLBO‖Li coin cells were tested in a potential range of 2.4–4.2 V (vs. Li⁺/Li) from 0.1C to 2C (1C = 140 mA h g⁻¹). NCM811‖PLBO‖Li coin cells were tested in an estimated range of 3.0–4.3 V (vs. Li⁺/Li) at rates ranging from 0.1C to 2C (1C = 180 mA h g⁻¹). Each electrochemical test was run at ambient temperature.

3. Results and discussion

3.1 PVDF-based electrolyte membranes

Micro-sized Ga/Nb-LLZO powders were synthesized through a detailed process, as elaborated in the ESI.† The XRD pattern in Fig. S1a† confirms the cubic garnet phase structure and high purity of the material. A series of PVDF-based HSE membranes were prepared using a simple solution casting method, incorporating micro-sized Ga/Nb-LLZO and/or nanosized BTO (Fig. S1b†) in specific proportions. To ensure better dispersion, Ga/Nb-LLZO and BTO were premixed before blending with PVDF, as illustrated in Fig. S2.† SEM images show that the HSE membranes (comprising PVDF, 2% BTO and 8% LLZO, denoted as PLBO) have a uniform and smooth surface (Fig. 2a). In contrast, the surfaces of the other two electrolytes – PVDF with 10% Ga/Nb-LLZO (denoted as PLO) and pure PVDF – exhibit noticeable surface pits (Fig. S3†). This observation is corroborated by AFM analysis, which demonstrates that the surface roughness (R_a) of the PLBO electrolyte is significantly lower (49.6 nm, Fig. 2c) compared to PLO (78.9 nm) and pure PVDF (144 nm, Fig. S4†). The addition of inorganic nanoparticles provides extra nucleation sites, reducing the likelihood of phase separation and producing a more uniform surface texture.²⁰ This improvement also aids in alleviating the occurrence of small voids on the surface, which are typically caused by the evaporation of DMF solvent during the formation of electrolyte membranes.³⁴ Elemental mapping (Fig. 2b) reveals the uniform distribution of Ga/Nb-LLZO and BTO inorganic nanoparticles within the PVDF/LiFSI composite. Cross-sectional SEM analysis (Fig. S5†) confirms that the membranes have a consistent thickness of approximately 100 μm and exhibit homogeneously distributed nanoparticles, which create effective pathways for lithium-ion transport.

Fig. 2 Preparation and characterization of HSEs. (a) Top-view SEM images of PLBO HSE. (b) EDS maps of F, Ti and La in the sample marked. (c) AFM image and height profile (inset) of the PLBO membrane. (d) XRD spectra. (e) FTIR spectra and (f) Raman spectra. (g) Hysteresis loop obtained from piezoelectric force microscopy analysis of the PLBO HSE. (h) Real part of relative permittivity as a function of temperature at different frequencies for the PLBO membrane. (i) as a function of frequency at different temperatures for the PLBO membrane.

The incorporation of inorganic materials proves effective in reducing the crystallinity of PVDF while boosting its ionic conductivity. XRD results in Fig. 2d suggest that the phases of cubic garnet Ga/Nb-LLZO and BTO remain stable within the PVDF-based electrolyte membranes, whereas the intensity of PVDF-related peaks diminishes. This decrease in peak intensity indicates a significant reduction in the crystallinity of the polymer phase. Additionally, Fig. 2e presents the FTIR spectra of pure PVDF, PLO, and PLBO electrolytes to further analyze the phases within the PVDF matrix. The spectra display peaks at 811 cm⁻¹, 835 cm⁻¹, and 1232 cm⁻¹, signifying the presence of γ-phase PVDF in these electrolytes. Furthermore, no FTIR signals associated with free DMF (658 cm⁻¹) were observed in any of these three electrolytes, indicating the complete absence of free DMF. Instead, residual DMF appears in a bound form as [Li(DMF)_x]⁺ (673 cm⁻¹). The vibrational peak at 1657 cm⁻¹ (Fig. S6a†) corresponds to the C=O stretching mode in DMF, and its reduced intensity following the addition of Ga/Nb-LLZO indicates strong coordination between Ga/Nb-LLZO and the C=O groups of DMF. A redshift in the position of the S–N–S peak (Fig. S6b†) suggests that the inclusion of BTO facilitates the dissociation of the lithium salt in the electrolyte, thus increasing the concentration of Li⁺ ions.

To elucidate the structural changes occurring during the polymerization process, Raman spectra were collected for PVDF, hybrid PLO and PLBO, as illustrated in Fig. 2f. The spectra of PLBO and PLO reveal two primary differences compared to the PVDF electrolyte. First, the peak at 2975 cm⁻¹, which represents the typical bending vibration mode of CH₂, nearly vanishes, suggesting CH₂ deprotonation after the introduction of inorganic nanoparticles. Second, two significant peaks at 1121 cm⁻¹ and 1510 cm⁻¹, representing the C=C stretching vibration modes in polyene, signify the dehydrofluorination of the PVDF chain. These structural alterations align with those observed in PVDF after alkali treatment, implying that the incorporation of Ga/Nb-LLZO might induce alkaline-like conditions, resulting in partial dehydrofluorination of the PVDF chain.^35,36 The following factors support high-voltage stability: the generated polymer networks with a high proportion of C=C bonds exhibit limited reactivity during charge/discharge cycling, while the enhanced ionic conductivity optimizes electrochemical performances. Also, the inorganic fillers homogenize the electric field within the electrolyte, further enhancing its electrochemical stability and enabling the electrolyte to withstand high voltages of up to 4.95 V without degradation.

A detailed examination of the dielectric properties of the electrolyte membranes revealed that the PLBO electrolyte exhibited a distinct rectangular hysteresis loop, indicative of modulated phase transitions and a large polarization window (Fig. 2g). The phase angle of the PLBO electrolyte under an applied bias voltage ranging from −60° to 120°, covering a total span of 180°, highlights the strong dielectric properties attributed to the inclusion of BTO. Broadband dielectric spectroscopy (BDS) analysis (Fig. 2h and i) demonstrates that ε_r initially increases with temperature, peaking at 416, 30 °C and a frequency of 10 Hz, likely due to the enhanced mobility of dipoles at elevated temperatures. However, beyond 30 °C, the dielectric constant gradually decreases and remains stable up to 60 °C. This decline is attributed to the Curie transition from the relaxor ferroelectric phase to the paraelectric phase. Notably, even following this phase transition, the dielectric constant remains relatively high, indicating that high-dielectric PLBO electrolyte membranes are well-suited for practical applications at elevated temperatures.

3.2 Performance of Li–Li symmetric cells

To examine the effect of varying ratios of Ga/Nb-LLZO and BTO nanoparticles on the electrolyte membrane, the ionic conductivity at room temperature was characterized for various compositions (Fig. S7†). Among them, the PLBO electrolyte exhibits the highest ionic conductivity, reaching 0.74 mS cm⁻¹. This significantly surpasses that of pure PVDF (0.17 mS cm⁻¹) and other electrolytes, including PBO (PVDF with 10% BTO) and PLO (PVDF with 10% Ga/Nb-LLZO, Fig. 3a). Additionally, EIS spectroscopy was conducted on SS symmetrical cells with different inorganic nanoparticle ratios at varying temperatures (Fig. S8†). The activation energy (E_a) for Li⁺ transport was determined using the Arrhenius equation by analyzing the corresponding Arrhenius plots (Fig. 3b).³⁷ With the inclusion of both Ga/Nb-LLZO and BTO fillers, the E_a value decreased from 0.196 eV to 0.16 eV, suggesting enhanced Li⁺ mobility.

Fig. 3 Physical properties and dendrite suppression of HSEs. (a) Ionic conductivities of the PVDF-based electrolytes at 25 °C. (b) Arrhenius plots of the PVDF-based electrolytes. (c) Comparison of LSV curves of PLBO, PLO and PVDF electrolytes. (d) The t_Li⁺ of the PVDF, PLO, and PLBO HSEs. (e) Current density (CCD) values of PLBO, PLO and PVDF electrolytes.

Additionally, as shown in Fig. 3c, the incorporation of inorganic nanoparticles in the PLBO electrolyte extends the electrochemical stable window from 4.64 V (pure PVDF) to 4.95 V, demonstrating excellent compatibility with high-voltage cathodes. High-voltage-stable inorganic nanoparticles enhance ionic conductivity and absorb DMF to enhance interfacial stability with the lithium anode. Besides, ferroelectric BTO homogenizes the distribution of the electric field and Li⁺ within the electrolyte, which further decreases localized overpotentials that could induce decomposition. To further explore the impact of inorganic fillers on Li⁺ transport, the Li⁺ transference number (t_Li⁺) was measured. According to Fig. S9a–c,† the t_Li⁺ value increases from 0.132 in pure PVDF to 0.235 in PLO and 0.413 in PLBO, illustrating a substantial improvement in ion transport. Furthermore, we conducted Raman analysis to examine the state of the FSI⁻ anion. As depicted in Fig. S9d–i,† the content of free FSI⁻ in PLBO was found to be 76%, which is substantially higher than that in PLO (57%) and PVDF (28%). This suggests that the addition of BTO in the PLBO electrolyte enhances the concentration of free FSI⁻. These data effectively demonstrate that the activation of lithium salt dissociation by BTO leads to an increased formation of mobile Li⁺ ions.

This enhancement is attributed to the synergistic effect of ferroelectric BTO and Ga/Nb-LLZO in PVDF-based electrolytes: enhanced dissociation of LiFSI by BTO, boosted ionic conductivity by Ga/Nb-LLZO and decreased crystallinity of PVDF by both the inorganic particles (Fig. 3d). Moreover, the interfacial resistance of the Li‖PLBO‖Li symmetric cell (116 Ω cm²) is significantly lower compared to that of Li‖PBO‖Li (159 Ω cm²) and Li‖PVDF‖Li (358 Ω cm²) (Fig. S10†), indicating superior interfacial contact for the PLBO electrolyte. To further investigate its electrochemical performance, CCD measurements were conducted on lithium symmetric cells. The CCD of the Li‖PLBO‖Li symmetric cell reached 1.5 mA cm⁻², considerably higher than that of PVDF (0.8 mA cm⁻²) and PLO (1.2 mA cm⁻²) (Fig. 3e). The high t_Li⁺ and low interfacial resistance of PLBO enhance Li⁺ migration, showing that the electrolyte not only improves ion mobility but also mitigates surface charge accumulation on the lithium anode. The incorporation of BTO further improves the interfacial kinetics between the PLBO electrolyte and the lithium anode, facilitating more uniform Li⁺ diffusion and deposition.

Additional experiments were conducted to assess the interfacial stability between the HSEs and the lithium anode. At a current density of 0.2 mA cm⁻², the Li‖PLBO‖Li cell demonstrated stable cycling for over 2000 hours (Fig. 4a). In comparison, cells using PVDF and PLO showed severe polarization after 250 hours and 1200 hours, respectively. Notably, the Li‖PBLO‖Li cell displayed a lower overpotential than the Li‖PLO‖Li cell, which is likely due to the suppression of surface charge accumulation on the lithium anode over time. This suppression promotes more uniform Li⁺ diffusion and deposition, thereby reducing impedance and preventing dendrite formation. SEM images of lithium foils after cycling (Fig. 4b–e) reveal that, following 200 hours of cycling, the lithium foils in the Li‖PLBO‖Li cell exhibit a relatively smooth surface, while mossy lithium dendrites are evident on the lithium foils cycled with PLO and PVDF electrolytes.³⁸ This indicates that the Li‖PLBO‖Li cell promotes uniform Li⁺ deposition, leading to a smoother lithium surface.^39,40

Fig. 4 Evaluation of compatibility with Li metal. (a) Galvanostatic voltage profiles of Li/Li symmetric cells using PLBO, PLO and PVDF electrolytes at 0.2 mA cm⁻² and 0.2 mA h cm⁻². The insets show zoomed-in views. Surface SEM images of (b) the lithium anode before cycling, (c) the lithium anode from the Li‖PLBO‖Li battery after cycling, (d) the lithium anode from the Li‖PLO‖Li battery after cycling and (e) the lithium anode from the Li‖PVDF‖Li battery after cycling for 200 hours at 0.2 mA cm⁻². (f) Schematic illustration of the Li plating/stripping behaviors of LMBs without/with BTO.

The incorporation of Ga/Nb-LLZO and BTO into the PVDF matrix plays a crucial role in effectively suppressing dendrite growth, contributing to the achievement of ultra-stable cycling performance. Fig. 4f presents a comparative model of lithium deposition, highlighting the impact of ferroelectric BTO in the electrolyte. After multiple cycles, electrolytes lacking BTO experience severe dendritic formation caused by uneven interfacial diffusion. In contrast, BTO-containing electrolytes facilitate uniform Li⁺ diffusion and deposition by generating a more uniform electric field. This promotes the formation of a stable solid-electrolyte interphase (SEI) layer, which significantly enhances long-term cycling performance by preventing dendritic growth and ensuring smoother lithium deposition.

The outstanding performance of the symmetric cells can be attributed to several key factors. First, when ionically conductive LLZO particles are immersed in DMF, partial dehydrofluorination of PVDF takes place, resulting in the formation of additional C=C bonds. This reaction, along with the activated region around the PVDF, strengthens the interactions between Ga/Nb-LLZO and LiFSI in the electrolyte. These enhanced interactions promote the dissociation of Li⁺ and FSI⁻ ion pairs, increasing the concentration of free Li⁺ and improving their transport. Second, the ferroelectric BTO nanoparticles further aid in the dissociation of Li⁺, since the polarization and surface charge enable enhanced electrostatic interaction for FSI⁻ coordination. Besides, the resultant more uniform electric field boosts Li⁺ transport and alleviates the uneven distribution of Li⁺, which also facilitates LiFSI dissociation. Third, the incorporation of Ga/Nb-LLZO and BTO particles reduces the crystallinity of PVDF, thereby increasing its segmental mobility, which further facilitates Li⁺ migration. Additionally, the inclusion of ferroelectric BTO nanoparticles generates a uniform distribution of internal electric fields, which greatly suppresses lithium dendritic growth and promotes uniform lithium deposition.

3.3 Properties of NCM811‖PLBO‖Li solid-state batteries

To evaluate the performance of the PLBO electrolyte in full cells, solid-state LFP‖Li and NCM811‖Li cells were assembled. Focusing on the high-voltage NCM811 cathode, as shown in Fig. 5a and b, the NCM811‖PLBO‖Li cell operates stably within a high voltage range of 3 V to 4.3 V, delivering impressive discharge capacities of 184.9, 156.0, 124.9, 95.3, and 63.6 mA h g⁻¹ at rates of 0.1C, 0.2C, 0.5C, 1C, and 2C, respectively. In its initial charge and discharge cycles, the cell also exhibits a high initial coulombic efficiency of up to 81%, indicating excellent charge and discharge performance. The superior ionic conductivity and low interfacial resistance of the PLBO electrolyte are key contributors to this outstanding performance (Fig. S11†), surpassing that of both NCM811‖PVDF‖Li and NCM811‖PLO‖Li cells. Furthermore, the specific discharge capacity nearly returns to its original value when the current density is reduced back to 0.5C, demonstrating strong rate capability recovery.

Fig. 5 Electrochemical performance of the NCM811‖Li solid-state batteries. (a) Rate performance of the NCM811‖Li solid-state batteries. (b) Charge/discharge curves at different rates of the NCM811‖PLBO‖Li solid-state battery. (c) CV measurements of the NCM811‖PLBO‖Li solid-state battery at a scan rate of 0.05 mV s⁻¹. Cycling stability of the NCM811‖Li solid-state batteries at (d) 0.5C and (e) 2C under 25 °C.

The cyclic voltammetry (CV) plot demonstrates consistent behavior during the first three cycles, indicating excellent reversible redox characteristics and fast kinetics (Fig. 5c). This consistency highlights the exceptional interface stability between the NCM811 cathode and Li metal anode. After 200 cycles at 25 °C and 0.5C, the NCM811‖PLBO‖Li battery retains 84.8% of its initial capacity (Fig. 5d). In stark contrast, the NCM811‖PLO‖Li battery shows significantly lower capacity retention, only 29.1%, under the same conditions. Remarkably, the NCM811‖PLBO‖Li cell even surpasses a batch of liquid-state NCM811 batteries, which retain 80.7% of their capacity at 0.5C (Fig. S12a†). Even more impressive is the fact that the NCM811‖PLBO‖Li cell maintains stable cycling for 700 cycles at an elevated rate of 2C, retaining more than 80% of its specific capacity (Fig. 5e). In contrast, the performance of two other counterparts rapidly declines, failing to sustain stability at higher cycling rates. The detailed surface chemistry of the interfacial layer before and after cycling is shown in Fig. S13.†

Additionally, solid-state lithium metal batteries based on the LFP cathode were also assembled. Demonstrating the high compatibility of the PLBO electrolyte with the LFP cathode (Fig. S14–S16†). As shown in Fig. S14a and b,† the LFP‖Li cell with the PLBO electrolyte exhibits excellent rate capability recovery and cycling stability, retaining 98% of its capacity after 200 cycles at 0.5C. Fig. S14c† further illustrates that the PLBO-based cell maintains a longer and more stable working voltage plateau compared to the PLO and PVBF-based cells. The high ionic conductivity of the PLBO solid-state electrolyte enhances transport kinetics within the LFP cathode, reducing voltage hysteresis. Specifically, the LFP‖PLBO‖Li cell shows a significantly lower overpotential of just 171 mV at 2C, compared to 576 mV for LFP‖PVDF‖Li and 310 mV for LFP‖PLO‖Li, underscoring the substantial benefits of the PLBO electrolyte in improving electrochemical performance.

The LFP‖PLBO‖Li solid-state cell also exhibits a significant reduction in interfacial resistance (Fig. S14d†). The cyclic voltammetry plot (Fig. S14e†) demonstrates excellent reversible redox kinetics, with the charge–discharge curves remaining nearly identical after 100 cycles at 1C and only minimal changes occurring after 200 cycles (Fig. S14f†). This minimal capacity decay indicates outstanding cycling stability, indicating that the LFP‖PLBO‖Li solid-state battery offers remarkably long-term cycling performance. For further comparison, we also fabricated an LFP‖PBO‖Li solid-state cell (Fig. S15†) using the PBO solid-state electrolyte. As indicated by the green data points, this configuration exhibits extremely poor cycling efficiency at 0.5C, in stark contrast to the other cells. This poor performance is attributed to the formation of a non-uniform interfacial layer, which obstructs lithium-ion transport at the lithium metal anode, thereby impeding overall electrochemical performance.

Under conditions of 25 °C and 1C, the LFP‖PLBO‖Li cell retains a discharge specific capacity of 119 mA h g⁻¹ after 500 cycles, achieving a high retention rate of 92% (Fig. S16a†). Even at an elevated rate of 2C, the battery maintains 70% of its capacity (Fig. S16b†), whereas the other two batteries quickly fail due to their inability to handle higher rates, primarily due to detrimental interfacial reactions. Overall, the performance of the PLBO-based solid-state cell surpasses that of other PVDF-based systems, regardless of whether NCM811 or LFP is used as the cathode material. Compared to other solid-state electrolytes reported in the literature, the PLBO electrolyte demonstrates performance that is comparable to, or even superior to, its counterparts (Table S1†). Based on a thorough analysis of its performance characteristics, the PLBO electrolyte emerges as one of the most promising polymer-based hybrid solid-state electrolytes for the future development of solid-state lithium metal batteries.

4. Conclusions

In conclusion, we have successfully developed a highly lithium-ion conductive PVDF-based HSE (PLBO) by incorporating uniformly distributed Ga/Nb-LLZO and ferroelectric BTO particles. The inclusion of BTO, known for its strong ferroelectric properties, plays a crucial role in stabilizing the internal electric field under applied voltage, resulting in a more uniform distribution of electron transport and ion migration. Experimental results regarding lithium deposition, lithium-ion conductivity, and transference number show that the addition of BTO ensures consistent electric field distribution, improves lithium-ion migration, and promotes uniform metallic lithium deposition.

With the incorporation of BTO, the ionic conductivity of PLBO reaches 0.74 mS cm⁻¹ at 25 °C. BTO not only enhances ion dissociation but also reduces the crystallinity of PVDF, thereby improving ion migration capability. Symmetrical lithium cells based on PLBO exhibit exceptional cycling stability, lasting over 2000 hours at 25 °C. Furthermore, the PLBO solid-state electrolyte shows excellent compatibility with commercial LFP and NCM811 cathodes, providing remarkable cycling stability.

This study underscores the critical importance of controlling microscopic physical fields within the cell to optimize electric field distribution, ion migration, and metal deposition behavior, all of which are essential for enhancing battery performance. The PLBO electrolyte developed in this work demonstrates significant promise for practical applications in solid-state lithium batteries.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Jie Zhao: investigation, formal analysis, conceptualization, visualization, writing – original draft. Yuyan Zhao: investigation, formal analysis, and writing – original draft. Can Cui: formal analysis, methodology, and writing – review & editing. Yudong Zhang: formal analysis, methodology, and project administration. Haiqin Lin: investigation. Cuijiao Zhao: formal analysis and funding acquisition. Weiji Dai: formal analysis and funding acquisition. Zhuofeng Liu: investigation. Xin Song: formal analysis and validation. Peng Cao: formal analysis, resources, methodology, and writing – review & editing. Saifang Huang: conceptualization, methodology, resources, supervision, writing – review & editing, and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Grant No. 22409075 and 52202093), the Natural Science Foundation of Jiangsu Province (Grant No. BK20241012), the Jiangsu Specially Appointed Professorship Foundation (Grant No. 1064902103), the Natural Science Research of Jiangsu Higher Education Institutions of China (Grant No. 22KJB430021), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYCX24_4107).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta08724k

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