Dissociating high concentration lithium salts in LLZTO-based high dielectric polymer electrolytes for low temperature Li metal batteries

Abstract

Incorporating high concentrations of lithium salts into solid polymer electrolytes can enhance the electrochemical performance of Li metal batteries. However, this approach is often obstructed by the reduced mechanical properties and limited lithium salt dissociation capacity. To address these challenges, we coupled a rigid inorganic solid electrolyte, Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO), with a high-dielectric-constant polymer, polyvinylidene-trifluoroethylene-trifluoroethylene chloride. The resulting composite solid electrolyte (named PTCL-1.5) significantly improves Li⁺ transport at low temperatures. The assembled Li|PTCL-1.5|Li cell demonstrates remarkable cycling stability, operating for over 4350 hours at −20 °C and 0.1 mA cm⁻². The PTCL-1.5 electrolyte exhibits excellent compatibility with various cathodes. Specifically, the Li|PTCL-1.5|LiNi_0.8Co_0.1Mn_0.1O₂ cell achieves a capacity of 127.69 mA h g⁻¹ at −20 °C, while the Li|PTCL-1.5|LiFePO₄ cell shows exceptional cycle stability, exceeding 750 cycles. Our work offers a promising approach for developing solid-state electrolytes with high electrochemical stability at low temperatures.

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1. Introduction

With the continuous growth of global energy demand, advanced and high-safety energy storage technologies are becoming increasingly important.^1,2 Lithium-ion batteries (LIBs), a leading energy storage technology, are widely used across various fields.³ Solid-state lithium metal batteries (LMBs) have gained significant attention due to their enhanced safety compared to traditional lithium-ion batteries that use flammable liquid electrolytes (LEs).^4–7 Among solid-state electrolytes (SSEs), solid-state polymer electrolytes (SPEs) are particularly noteworthy.^8,9 SPEs offer promising application prospects due to their adjustable molecular structure, excellent processability, flexibility, and outstanding interface compatibility.^10,11 Additionally, SPEs are considered potential alternatives to LEs.¹² However, unlike LEs, which exhibit high fluidity and ionic conductivity, SPEs are characterized by high crystallinity and a low dielectric constant, both at room temperature and low temperatures. This results in inefficient dissociation of lithium salts and hindered rapid transport of Li⁺ ions.^13,14 Incorporating nanoscale inorganic fillers into polymer electrolytes can mitigate crystallinity and enhance Li⁺ mobility.^14,15 Yet, excessive filler loading may lead to uneven dispersion within the polymer, limiting improvements in ion conductivity.¹⁶ Developing polymer electrolytes with high-concentration salts can effectively reduce polymer crystallinity and improve battery performance.¹⁷ However, commonly used polymers, such as poly(ethylene oxide), have low dielectric constants, making it challenging to efficiently dissociate lithium salts at high concentrations.¹⁸ Moreover, excessive salt addition can significantly weaken SPEs and reduce their resistance to lithium dendrite growth.¹⁷ These issues have substantially limited the application of SPEs in solid-state LMBs.¹⁴

Poly(vinylidene fluoride) (PVDF)-based solid polymer electrolytes (SPEs) have gained significant attention due to their high dielectric constants (ε_r ∼ 10) and wide electrochemical windows.^11,19 When combined with active fillers such as Li₇La₃Zr₂O₁₂ (LLZO) and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO), PVDF-based composite solid electrolytes (CSEs) demonstrate the enhanced capacity for suppressing lithium dendrites and expanding the electrochemical stability window.^20,21 Additionally, residual N,N-dimethylformamide (DMF) in CSEs have been found to facilitate the rapid migration of [Li(DMF)_x]⁺ complexes, thus enhancing ionic conductivity.²² However, there is a significant discrepancy in the dielectric constant between PVDF and LEs, making it challenging to fully dissociate lithium salts in high-concentration systems and achieve high ionic conductivity.²³ The copolymer poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE), or PTC) has a superior dielectric constant (30–70) compared to PVDF.^4,17 When combined with LLZTO of Lewis base, PTC allows efficient dissociation of lithium salts and promotes rapid ion transport. Moreover, PTC can undergo a dehydrochlorination reaction in an alkaline environment due to the presence of basic impurities (such as Li₂CO₃ and LiOH) on the surface of LLZTO. This reaction increases the proportion of the highly polar β-phase in PTC, further elevating its dielectric constant and enhancing its ability to dissociate lithium salts.^24–26

In this work, we combine the active Li⁺ filler LLZTO, high concentration of bis(trifluoromethanesulfonyl)imide lithium (LiTFSI), and high dielectric constant polymer (PTC) to prepare CSEs with high lithium salt dissociation capacity, designated as PTCL-1.5. The synergy between the high dielectric constant of PTC and the Lewis basicity of LLZTO facilitates the dissociation of lithium salts in PTCL-1.5, leading to a substantial generation of active Li⁺ ions. Additionally, the incorporation of LLZTO, with its intrinsic ion channels, enhances the suppression of Li dendrite formation and establishes multiple Li⁺ transport pathways, enabling efficient ion transport (Fig. 1a and b). The PTCL-1.5 exhibits an ionic conductivity of 4 × 10⁻⁴ S cm⁻¹ at RT, an oxidation stability window of 4.53 V, and excellent suppression of Li dendrites. Consequently, the Li|PTCL-1.5|Li cell can cycle for 4350 h at −20 °C. Furthermore, PTCL-1.5 shows exceptional rate performance with different cathodes, such as LiFePO₄ (LFP), LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), and LiNi_0.9Co_0.05Mn_0.05O₂ (NCM9055). The assembled Li|PTCL-1.5|NCM9055 cell can consistently cycle at 1C for 150 cycles at RT. The Li|PTCL-1.5|NCM811 cell achieves 127.69 mA h g⁻¹ at −20 °C. Due to its outstanding electrochemical stability and high safety characteristics, the PTCL-1.5 pouch cell remains fully functional and can illuminate a sign even after being folded and cut multiple times, demonstrating its remarkable electrochemical stability and safety.

Fig. 1 Diagram of Li-ion transport of PTC-1.5 (a) and PTCL-1.5 (b). (c) Schematic diagram of LiTFSI dissociation by PVDF-TrFE-CTFE. (d) and (e) FTIR of electrolyte membranes with different lithium content in different ranges, respectively. (f) SEM image of PTCL-1.5.

2. Experimental section

2.1. Materials

PTC (VDF : TrFE : CTFE = 64 : 27 : 9) was supplied by Dongguan Hongfu Plastics Trading Department. LLZTO (500 nm) was purchased from Hefei Kejing Materials Technology Co., Ltd. Bis(trifluoromethane) sulfonamide lithium (LiTFSI) and the electrolyte used for wetting the electrode/electrolyte interface, model LBED11-230522, were provided by Shanghai Songjing New Energy Technology Co., Ltd. N,N-Dimethylformamide (DMF, Adamas) was used as solvents. All chemicals were directly used without further purification.

2.2. Preparation of composite solid electrolytes

PTCL-1.5 was prepared by a simple solution casting method. Specifically, 0.4 g of PTC and 0.6 g of LiTFSI were weighed and dissolved in 2.5–2.6 g of DMF solvent and stirred thoroughly to form a homogeneous and transparent viscous solution. Then, 0.06 g of LLZTO was added to the prepared PTC/LiTFSI solution, and it was heated at 45 °C and stirred vigorously for 2 h. After being transferred to RT and stirred overnight, the precursor solution was vacuum-evaporated to remove air bubbles and then coated onto a smooth glass plate. After drying at 50 °C for 12 h, it was cut into 16 mm diameter discs and placed in a glove box for storage. The preparation method of PTC-1.5 was simply not adding LLZTO, and the rest of the steps were the same. In this work, the electrolyte film used as a control was labeled according to the mass ratio of LiTFSI to PTC, with the mass of PTC fixed at 0.4 g. When 0.8 g, 0.6 g, 0.4 g, and 0.2 g of LiTFSI were added, they were named PTC-2, PTC-1.5, PTC-1, and PTC-0.5 respectively.

3. Results and discussion

3.1. Synthesis and characterization

The electrolyte PTCL-1.5, with fast Li⁺ transport properties, was manufactured using the solution casting method. As shown in Fig. 1c, PTC has a higher dielectric constant than PVDF, generating a large number of negative dipoles to attract Li⁺ and facilitate the dissociation of lithium salts. The electrolyte membrane observed under a scanning electron microscope (SEM) (Fig. 1f) presents a dense and flat surface morphology. The LLZTO in the PTC is uniformly dispersed, as verified by energy dispersive spectrometer (EDS) characterization (Fig. S1, ESI†). Compared to the low Li-salt-containing PTCL-0.5 (Fig. S2a and b, ESI†), PTCL-1.5 has a more compact morphology. The thickness of PTCL-1.5 was measured using a thickness measurement instrument, and the results show it is only 52 μm (Fig. S3, ESI†). Despite the addition of a large amount of LiTFSI and inorganic filler LLZTO, the flexibility of PTCL-1.5 remains unaffected, as demonstrated in Fig. S4 (ESI†). PTCL-1.5 exhibits excellent flexibility, recovering its original state after being folded twice. The property makes it well-suited for current mainstream roll-to-roll battery manufacturing technology.²⁷

Fourier transform infrared spectroscopy (FTIR) analysis (Fig. 1d and e) shows that the peak at approximately 673 cm⁻¹ corresponds to the binding of DMF with Li⁺ ions, appearing consistently across different LiTFSI contents.²² The stretching vibration peak of free DMF is absent in the polymer electrolyte membrane containing lithium salts. The vibration peak of the α phase in PTC is represented by the vibration at 1402 cm⁻¹.²⁸ The peaks at 1072 cm⁻¹ and 1186 cm⁻¹ correspond to the stretching vibration of the –CF₂ group in PTC. As the proportion of LiTFSI increases, these two peaks continuously shift towards lower frequencies, indicating an interaction between PTC and LiTFSI, which strengthens with higher Li-salt concentrations.²⁹ The vibrations at 1352 and 1125 cm⁻¹ originate from the –SO₂ group and the stretching vibration of –CF in LiTFSI, and are not present in pure PTC. The bands around 841 and 879 cm⁻¹ (Fig. 1e) represent the amorphous region of PTC, and the peaks at these locations shift to different degrees as various lithium salt concentrations are added.²⁹ These results indicate that increasing lithium salt concentration and introducing LLZTO enhance the proportion of the amorphous region in PTC, thereby improving Li⁺ transport ability.³⁰

Further understanding of the effect of increasing lithium salt concentration on the crystalline peaks of PTC can be obtained through X-ray diffraction (XRD) analysis. As lithium salt concentration increases, the ratio of the amorphous region in PTC also increases (Fig. S5, ESI†).³¹ The results indicate that increasing lithium salt concentration and introducing LLZTO enhance the proportion of the amorphous region in PTC, thereby improving Li⁺ transport ability.

To further understand the advantages of the synergistic effect of PTC and LLZTO in improving the dissociation ability of lithium salts, we prepared three electrolytes: PTCL-1.5, PTC-1.5, and an electrolyte membrane using PVDF as the polymer skeleton (PVL-1.5). Raman spectroscopy was employed to test the dissociation of Li⁺ ions in these systems (Fig. 2a–c). The results demonstrate that the peak observed at 741 cm⁻¹ can be simulated to synthesize three distinct peaks: a solvent-separated ion pair (SSIP), a contact ion pair (CIP), and an ion aggregate (AGG).^32,33 In the PTCL-1.5 electrolyte, SSIP accounted for 73.9%, highlighting its superior ability to dissociate lithium salts. In contrast, the electrolyte without LLZTO showed a lower SSIP proportion of 61.9%. Furthermore, PVL-1.5, prepared with PVDF as the skeleton, exhibited the lowest SSIP ratio, accounting for only 34.7%. The results verify that polymers with high dielectric constants, when used synergistically with Lewis-basic LLZTO, effectively dissociate lithium salts and produce large amounts of free Li⁺ ions.

Fig. 2 Raman spectra of (a) PTCL-1.5, (b) PTC-1.5, and (c) PVL-1.5 at 720–770 cm⁻¹. (d) and (e) ⁷Li MAS NMR spectra of PTCL-1.5 and PTC-1.5. (f) LSV of Li|PTCL-1.5|SS and Li|PTC-1.5|SS. (g) Temperature-dependent ionic conductivity of PTCL-1.5 and PTC-1.5. Current–time profile during the polarization of (h) Li|PTCL-1.5|Li and (i) Li|PTC-1.5|Li symmetrical cells.

The local chemical environment and kinetics of Li⁺ in PTCL-1.5 and PTC-1.5 were further studied using ⁷Li solid-state nuclear magnetic resonance (ss-NMR). As shown in Fig. 2d and e, the ss-NMR data revealed two distinct peaks: one at 0.23 ppm in PTCL-1.5 and another at 0.05 ppm in PTC-1.5.³⁴ The addition of LLZTO caused the ⁷Li spectrum of PTCL-1.5 to shift to a lower field, indicating a weaker charge environment around Li⁺ in PTCL-1.5 compared to PTC-1.5. This weaker charge environment is more favorable for the fast migration of Li-ions.³⁵ The results are reflected in the ionic conductivity at different temperatures. PTCL-1.5 exhibits higher ionic conductivities than PTC-1.5 (Fig. 2g). The ionic conductivity of PTCL-1.5 is 4 × 10⁻⁴ S cm⁻¹ at RT, while PTC-1.5 is 6.85 × 10⁻⁵ S cm⁻¹ even at −10 °C, PTCL-1.5 maintains an ionic conductivity of 1.57 × 10⁻⁴ S cm⁻¹, whereas PTC-1.5 is 2.28 × 10⁻⁵ S cm⁻¹. The linear sweep voltammetry (LSV) test further investigates the electrochemical window of PTCL-1.5. As shown in Fig. 2f, PTCL-1.5 has an oxidation stability window of up to 4.53 V, compared to 4.22 V for PTC-1.5. Additionally, the addition of LLZTO facilitates the construction of multiple Li⁺ transmission paths. Consequently, the Li⁺ transference number (t_Li⁺) of PTCL-1.5 reaches 0.4, while that of PTC-1.5 is only 0.19 (Fig. 2h). A higher t_Li⁺ reduces concentration polarization during charge and discharge, improving battery rate performance and facilitating rapid Li⁺ transmission.³⁶

3.2. Electrochemical properties of lithium anode

To further investigate the stability of PTCL-1.5 against lithium metal, we assembled a symmetrical Li|PTCL-1.5|Li cell. At 0.1 mA cm⁻², the cell demonstrated extremely stable cycling performance without short-circuits at RT for more than 600 h (Fig. 3a). In contrast, the Li|PTC-1.5|Li cell, exhibited extremely high polarization after a short cycle of just tens of hours. The introduction of high-concentration salts and LLZTO significantly decreases the crystallinity of PTC. Combined with the synergistic effect of LLZTO and PTC on the dissociation of high-content lithium salt, allows PTCL-1.5 to exhibit excellent kinetic performance even at −20 °C. The Li|PTCL-1.5|Li cell can cycle for 4350 h at −20 °C and 0.1 mA cm⁻². It indicates that PTCL-1.5 is still capable of efficient lithium dissociation and Li⁺ transport at low temperatures. Moreover, the critical current density (CCD) of Li||Li symmetrical cells was tested. As shown in Fig. 3b, the CCD of PTCL-1.5 is 3 mA cm⁻², while that of PTC-1.5 is only 1 mA cm⁻². These results indicate that PTCL-1.5 has high stability for lithium metal. To further measure the lithium metal stability of PTCL-1.5, a Tafel test was conducted (Fig. 3e). The results show that PTCL-1.5 has a higher corrosion current density (0.159 mA cm⁻²) compared to PTC-1.5 (0.063 mA cm⁻²), further verifying that PTCL-1.5 has better Li metal stability.³⁷ Both PTCL-1.5 and PTC-1.5 were assembled into symmetrical Li cells and cycled for 50 h at 0.1 mA cm⁻². After cycling, the Li tablets were observed by SEM. The Li|PTCL-1.5|Li cell shows a relatively smooth and flat morphology (Fig. 3f), indicating that Li⁺ ions are deposited homogeneously. However, the lithium tablet in the Li|PTC-1.5|Li cell exhibits a rough and irregular surface topography (Fig. 3g), indicating an uneven lithium deposition process and violent side reactions. As illustrated in Fig. S6 (ESI†), the surface of Li metal in the Li|PTC-1.5|Li cell was uneven and accompanied by the generation of dendritic lithium. In contrast, the lithium deposition in the Li|PTCL-1.5|Li cell presented a relatively uniform and dense morphology. These results further show that PTCL-1.5 has high-efficiency ion transport and ultra-high stability against Li metal.³⁸

Fig. 3 (a) Cycle comparison of Li|PTCL-1.5|Li and Li|PTC-1.5|Li cells at 0.1 mA cm⁻² (RT). The cycle of (b) Li|PTCL-1.5|Li, (d) Li|PTC-1.5|Li under various current densities, and (c) Li|PTCL-1.5|Li cells at 0.1 mA cm⁻² (−20 °C). (e) Tafel of Li|PTCL-1.5|Li and Li|PTC-1.5|Li cells. SEM image of (f) Li|PTCL-1.5|Li and (g) Li|PTCL-1.5|Li cells after cycling.

3.3. Full cell performances

The introduction of LLZTO facilitated the establishment of multiple ion transfer channels. Combined with the high-dielectric-constant polymer PTC, these materials synergistically dissociate lithium salts, generating a substantial amount of active Li⁺ ions. Li⁺ ions can travel along the amorphous region in the PTC polymer and the intrinsic ion transport channels of LLZTO, demonstrating exceptional ionic coupling transport efficiency. When paired with various cathodes, the PTCL-1.5 electrolyte shows excellent cycling stability and rate performance. As shown in Fig. 4a, the assembled Li|PTCL-1.5|LFP cell shows excellent cycling stability in the 0.1C–5C rate test, and all different rates excited higher capacities than the Li|PTCL-1.5|LFP cell. To further investigate its endurance at 4.3 V, rate cycling tests were conducted with two high nickel-containing ternary cathodes, NCM811 (Fig. 4b) and NCM9055 (Fig. 4c). The assembled Li|PTCL-1.5|NCM811 cell exhibits capacities of 223.3, 199.51, 179.46, 153.67, 138.57, 119.19, and 97.92 mA h g⁻¹ at 0.1C to 5C. Upon reducing the rate to 1C, the capacity reaches 186.52 mA h g⁻¹. However, the Li|PTCL-1.5|NCM811 cell demonstrates a capacity of only 79.3 mA h g⁻¹ at 5C, indicating that the polarization at high-rate cycling with LLZTO-added PTCL-1.5 is much lower than that of PTCL-1.5 without LLZTO.

Fig. 4 Rate performance of (a) Li|PTCL-1.5|LFP and Li|PTC-1.5|LFP, (b) Li|PTCL-1.5|NCM811 and Li|PTC-1.5|NCM811, and (c) Li|PTCL-1.5|NCM9055 and Li|PTC-1.5|NCM9055 cells. Cycling performance of (d) Li|PTCL-1.5|NCM9055 cell at 1C at RT, (e) Li|PTCL-1.5|NCM811 cell at 0.2C at −20 °C. (f) Li|PTCL-1.5|LFP and Li|PTC-1.5|LFP cells at 1C at RT. (g) The voltage–capacity curves of Li|PTCL-1.5|LFP cell from 5th to 750th.

It is well known that higher nickel content in cathode materials increases the likelihood of active material cracking along grain boundaries during cycling. This phenomenon leads to irreversible side reactions between the cathode and the electrolyte, degrading overall battery performance. When the cathode with a nickel content of up to 90%, specifically NCM9055, was assembled into a Li|PTCL-1.5|NCM9055 cell, the capacity at a 0.1C rate reached 217.18 mA h g⁻¹. After the 5C test, the capacity recovered to 166.01 mA h g⁻¹ at the 1C rate. However, the Li|PTCL-1.5|NCM9055 cell shows overcharging when cycled back to 1C. To verify the ion transfer capability and kinetic performance of PTCL-1.5 at −20 °C (Fig. 4e), Li|PTCL-1.5|NCM811 cell was assembled, achieving 127.69 mA h g⁻¹ at 0.2C. It is well demonstrated that the construction of multiple ion transport paths and the high dissociation of lithium salts by PTCL-1.5 facilitates the rapid Li⁺ transport, even at low temperatures (−20 °C), with excellent applicability. To evaluate the stability of PTCL-1.5 at 4.3 V, a Li|PTCL-1.5|NCM9055 cell was assembled (Fig. 4d). At RT, the cell demonstrates remarkable cycling stability, maintaining performance over 150 cycles at a 1C rate. The olivine structure cathode, with the most representative example being the LFP cathode, is crucial for evaluating the long-term cycling capability of LMBs. The Li|PTCL-1.5|LFP cell maintained an 80% capacity retention rate after 750 cycles at 1C. In contrast, the Li|PTC-1.5|LFP cell experiences a short circuit shortly after the initial few cycles (Fig. 4f). Furthermore, from its discharge–charge curve (Fig. 4g), the polarization potential at the 750th cycle was only 45.8 mV higher than at the 5th cycle. It indicates that the cell has high electrochemical stability and capacity retention during the cycling process.

To analyze the composition of cathode surface products after cell cycling, two electrolytes were used to assemble full cells with a lithium metal anode and NCM811 cathode. X-Ray photoelectron spectroscopy (XPS) is employed to analyze the elements present on the cathode surface following 15 cycles at 1C. In Fig. 5a and d, both cells after cycling show peaks of LiF and Li₃N/Li–O, in the Li 1s spectra.³⁹ In the XPS spectra of N 1s (Fig. 5b and e), three peaks corresponding to LiN_xO_y, C–N, and Li₃N appear at the PTCL-1.5|NCM9055 interface. However, at the PTC-1.5|NCM9055 interface, only two peaks (LiN_xO_y and C–N) are observed, with a higher proportion of LiN_xO_y compared to PTCL-1.5. It is known that LiN_xO_y has poor oxidation stability, indicating that a higher content of LiN_xO_y suggests a more unstable interface. The peak at 397.4 eV corresponds to Li₃N, which can significantly reduce the charge transfer impedance at the cathode electrolyte interface (CEI).⁴⁰ In the F 1s spectra (Fig. 5c and f), two peaks are observed at the CEI: Li_xPO_yF_z at 688.18 eV and LiF at 685.58 eV.⁴¹ LiF plays a dominant role at the PTCL-1.5|NCM9055 interface, while the content of Li_xPO_yF_z is higher than LiF at the PTC-1.5|NCM9055 interface.⁴² Rich LiF at the interface is beneficial for reducing irreversible side reactions during cycling.⁴³ After cycling, a uniform CEI of 4 nm is found at the PTCL-1.5|NCM9055 interface (Fig. 5g), while the CEI at the PTC-1.5|NCM9055 interface is thicker (up to 10 nm) and uneven (Fig. 5h). It indicates that there are fewer side reactions with PTCL-1.5|NCM9055 compared to PTC-1.5|NCM9055 and that PTCL-1.5 has higher oxidation stability.⁴⁴

Fig. 5 XPS spectra for (a) and (d) Li 1s, (b) and (e) N 1s and (c) and (f) F 1s of cycled NCM9055 with PTCL-1.5 and PTC-1.5. The TEM images of NCM9055 particles from (g) Li|PTCL-1.5|NCM9055 and (h) Li|PTCL-1.5|NCM9055 cells after cycling. Optical images of Li|PTCL-1.5|NCM9055 pouch cell in the lit lamp plate experiment at (i) resting, (j) curling, (k) first cut, and (r) secondary cut.

To further investigate the safety and applicability of PTCL-1.5, we conducted a flame test (Fig. S11, ESI†). Unlike the commercial separator absorbed with LE, which quickly ignited upon contact with the flame, PTCL-1.5 did not burn and rapidly carbonized. The demonstrates that PTCL-1.5 has excellent thermal stability and safety. We also assembled a pouch cell with a 50 μm thickness of Li&Cu foil as the anode and an NCM9055 cathode loaded with 10.2 mg cm⁻². The pouch cell can successfully lighten the light plate (Fig. 5i). To further investigate the excellent flexibility and flexibility of the PTCL-1.5, the pouch cell can function normally after folding (Fig. 5j), and the lamp plate is successfully lit, showing that the electrolyte has excellent flexibility. Even after the first cutting (Fig. 5k), the pouch cell still can work normally and light the lamp plate for further cutting. The pouch cell still has no short-circuit phenomenon, and the lamp plate continues to be successfully lit (Fig. 5l), which shows that the pouch cell is highly safe and flexible, and demonstrates fantastic application prospects in the face of various battery preparation processes.

4. Conclusions

This work achieved a significant advancement in the dissociation of lithium salts at high-salt concentrations by combining a high-dielectric-constant polymer (PTC) and an active ionic filler (LLZTO). The designed electrolyte, PTCL-1.5, demonstrates excellent Li⁺ dissociation-transport coupling, significantly enhancing ionic conductivity and reducing interface resistance. These improvements lead to better rate performance and cycling stability of solid LMBs. The Lewis basicity of LLZTO aids in the dissociation of lithium salts in PTC segments while maintaining inherent ion transport channels, creating multiple Li⁺ transport pathways. Compared to the PTC-1.5 electrolyte without LLZTO, PTCL-1.5 exhibits a higher number of active Li⁺ ions, greater lithium salt content, and improved mechanical strength. The ionic conductivity is increased to 4 × 10⁻⁴ S cm⁻¹. The synergistic effect of PTC and LLZTO promotes excellent ion transport efficiency, allowing the Li|PTCL-1.5|Li cell to cycle for 4350 h at −20 °C. PTCL-1.5 shows superior compatibility with LFP, NCM811, and NCM9055 cathodes. The assembled Li|PTCL-1.5|NCM9055 full cell achieves 127.69 mA h g⁻¹ at −20 °C, demonstrating excellent low-temperature applicability. When paired with the classic olivine structure cathode, the Li|PTCL-1.5|LFP full cell demonstrates outstanding cycling performance at 1C, maintaining 80% capacity retention over 750 cycles. This performance is significantly better than that of the Li|PTC-1.5|LFP cell. Furthermore, the voltage polarization of the Li|PTCL-1.5|LFP cell only increases by 45.8 mV at the 750th cycle compared to the 5th cycle, indicating excellent cycling stability. The pouch cell also shows excellent safety and good flexibility. Our strategy effectively increases the dissociation of lithium salts in SPEs and enhances the performance of Li metal batteries at room or low temperature, which is important for the real application of solid-state batteries.

Data availability

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

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (52171198, 51922099), and the Fundamental Research Funds for the Central Universities (buctrc202104).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm00625a

‡ Jiajun Gong and Zhicheng Yao contributed equally to this work.

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