Phase regulation enabling dense polymer-based composite electrolytes for solid-state lithium metal batteries

The increasing demand for high-energy-density, long cycle life, and safe batteries necessitates advanced electrolytes compatible with lithium (Li) metal anodes and high-voltage cathodes like LiNi₀.₈Co₀.₁Mn₀.₁O₂ (NCM811). Conventional organic liquid electrolytes suffer from flammability, Li dendrite growth, and uncontrollable side reactions. Solid-state electrolytes (SSEs), particularly solid polymer electrolytes (SPEs), offer a safer alternative due to their inherent safety and ability to suppress dendrite growth. Poly(vinylidene fluoride) (PVDF)-based electrolytes are attractive due to their mechanical strength, thermal stability, and ionic conductivity. The presence of a small amount of N,N-dimethylformamide (DMF) solvent, interacting strongly with the Li salt and PVDF, facilitates Li salt dissociation and ion transport. However, PVDF-based electrolytes face challenges such as porous structures due to phase separation between polymer and solvent, leading to uneven ion flux and dendrite growth. DMF's side reactions with Li metal and poor oxidation resistance also limit the electrochemical stability window. While strategies like using additives, adjusting Li salts and solvents, regulating solvent content, and anchoring solvent with fillers have been explored, achieving dense PVDF-based electrolytes with high ionic conductivity remains a significant challenge. This research aims to develop a novel strategy for creating dense PVDF-based electrolytes with superior ion transport and stable interfaces.

Previous research has focused on improving the interfacial compatibility and ionic conductivity of PVDF-based electrolytes. Studies have employed electrolyte additives, adjusted Li salt and solvent types, regulated solvent content, and anchored solvents with fillers to mitigate side reactions and improve performance. Xu et al. showed that PVDF-based electrolytes with a local high concentrated (LHC) structure, formed by LiTFSI and DMSO, can mitigate interfacial side reactions. However, solvent decomposition at high current densities, rates, and potentials limits cycle life and capacity. Other studies introduced active and inactive fillers or modified polymer structures to decrease PVDF crystallinity and enhance conductivity, but the results remained unsatisfactory. The challenge of obtaining dense PVDF-based electrolytes for practical applications persists, highlighting the need for innovative strategies to address this limitation. This work leverages the inherent properties of two-dimensional transition metal dichalcogenides (TMDs) to overcome these limitations.

This study synthesized MoSe₂ nanosheets via an in situ selenization route. Free-standing and flexible PVDF and PVMS composite electrolytes were prepared using a solution-casting method with DMF solvent and LiFSI. Various weight percentages of MoSe₂ (10%, 15%, 20%) were incorporated into the PVDF electrolyte. The materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy-nano-infrared spectroscopy (AFM-nano-IR), thermogravimetric analysis (TGA), solid-state nuclear magnetic resonance (ss-NMR), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and density functional theory (DFT) calculations. Electrochemical properties were evaluated through electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and galvanostatic charge/discharge tests. Cryogenic scanning transmission electron microscopy (cryo-STEM) was used to characterize the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI). The ionic conductivity, activation energy, electrochemical stability window, and Li metal compatibility were determined. Li||Li symmetric cells and Li||NCM811 full cells, including pouch cells, were assembled and tested under various conditions (current densities, temperatures, loadings). The β-phase PVDF content was calculated using the Lambert-Beer law. Computational methods including Density Functional Theory (DFT) and Climbing Image Nudged Elastic Band (CI-NEB) were employed to study the interactions between MoSe2 and PVDF, and the diffusion pathways and barriers in the SEI components.

The incorporation of MoSe₂ sheets into the PVDF electrolyte resulted in a dense composite electrolyte, eliminating the porous structure observed in the PVDF-only electrolyte. DFT calculations revealed strong interactions between MoSe₂ and PVDF, leading to an increase in the β-phase PVDF content (from 39% to 77% with increasing MoSe₂ content). This β-phase enrichment significantly increased the dielectric constant (εr) from 9.6 to 21.1, facilitating Li⁺ ion dissociation and forming solvent-separated ion pairs (SSIPs) for enhanced ion transport. The enhanced ionic conductivity of the PVMS-15 electrolyte (6.4 × 10⁻⁴ S cm⁻¹) compared to the PVDF electrolyte (2.1 × 10⁻⁵ S cm⁻¹) was achieved with a substantially lower activation energy (0.07 eV vs 0.26 eV). In-situ reactions between MoSe₂ and Li metal formed Li₂Se in the SEI, leading to a smoother, denser SEI with improved Li⁺ ion transport kinetics compared to the rougher, thicker SEI formed by the PVDF electrolyte. The Li₂Se acted as a fast ionic conductor, evidenced by a migration energy of only 0.056 eV, while other SEI components like LiOH, Li₂O, and Li₂CO₃ exhibited higher energy barriers. The improved SEI resulted in enhanced Li metal compatibility with higher critical current densities (CCDs) (2.3 mA cm² and 8.3 mA cm²) and exchange current density (ECD) (0.245 mA cm²) compared to the PVDF electrolyte. Li||Li symmetric cells exhibited robust cycling stability for 480 h at 1 mA cm⁻². Li||NCM811 full cells demonstrated superior performance at high rates (3C), high loadings (2.6 mAh cm⁻²), and even in a pouch cell configuration, showcasing a significant improvement in cycle life and capacity retention compared to cells using the PVDF electrolyte. Analysis of the cycled electrodes revealed that the PVMS-15 electrolyte mitigated DMF decomposition and the formation of a uniform CEI with improved properties on the cathode.

The findings demonstrate that the phase regulation strategy using MoSe₂ successfully addressed the limitations of PVDF-based electrolytes. The dense structure, enhanced ionic conductivity, and improved interfacial stability of the PVMS-15 electrolyte collectively contributed to the superior electrochemical performance observed in both Li||Li and Li||NCM811 full cells. The formation of Li₂Se in the SEI played a crucial role in enhancing Li metal compatibility and suppressing side reactions. The results highlight the importance of optimizing both the bulk electrolyte properties and the interfacial chemistry for achieving high-performance solid-state LMBs. This work provides a practical and scalable approach for developing advanced solid-state electrolytes, overcoming previous challenges associated with PVDF-based electrolytes.

This study successfully developed a high-performance, dense PVDF-based composite electrolyte using a phase regulation strategy. The incorporation of MoSe₂ nanosheets enhanced the β-phase PVDF, leading to a high dielectric constant, optimized solvation structures, high ionic conductivity, and low activation energy. The in-situ formation of Li₂Se within the SEI further enhanced Li metal compatibility and cycling stability. The superior electrochemical performance demonstrated in Li||Li and Li||NCM811 full cells, including pouch cells, under practical conditions validates this approach. Future research could explore other 2D materials and polymer matrices to further improve electrolyte performance and expand the applicability of this strategy to other battery chemistries.

While this study demonstrates significant improvements in PVDF-based electrolytes, further research is needed to optimize the long-term stability at very high current densities and extremely high temperatures. The scalability and cost-effectiveness of the MoSe₂ synthesis and electrolyte fabrication need to be assessed for industrial applications. Exploring the potential impact of Mo and Mo-containing species in the long term needs further investigation, although the current study suggests a negligible effect on electronic conductivity. A more comprehensive analysis of the degradation mechanisms at different operating conditions would improve understanding of the electrolyte's long-term performance.