Strong Lewis-acid coordinated PEO electrolyte achieves 4.8 V-class all-solid-state batteries over 580 Wh kg⁻¹

All-solid-state Li metal batteries (ASSLBs) using polymer electrolytes are highly promising for high energy density and improved safety. Polyethylene oxide (PEO)-based solid polymer electrolytes offer advantages in processability and good physical contact, but their poor oxidation stability at high voltages hinders their use. The interface between state-of-the-art layered Ni-rich materials and PEO-based electrolytes presents a significant challenge. While protective methods like inorganic fillers, cathode coating, and molecular grafting have been explored to extend voltage windows, reaching the performance of 4.8 V-class ASSLBs remains a challenge. The inherent low oxidative potential of ether oxygen (EO) chains in PEO limits its use at ultra-high voltages. High-concentration salt strategies have improved high-voltage resistance, but low ionic conductivity compromises Li⁺ migration. Metal ion-coordinated polymers offer faster Li⁺ conduction, but incompatibility with high-voltage cathodes persists. This research aims to decouple oxidative stability and ionic conductivity by regulating the coordination mode in the electrolytes.

Numerous studies have focused on improving the high-voltage stability of PEO-based solid-state batteries. Research efforts have explored various strategies, including the incorporation of inorganic fillers to enhance the mechanical and electrochemical properties of the electrolyte [20, 21], the application of cathode coatings to protect the cathode surface from direct contact with the electrolyte [22-24], and the molecular grafting of functional groups onto the PEO chains to improve their oxidative stability [25, 26]. High-concentration salt electrolytes have also been investigated to improve the oxidative stability of PEO-based electrolytes [30, 31], although these electrolytes often suffer from reduced ionic conductivity [34]. The use of metal ion-coordinated polymers has shown some promise in enhancing the ionic conductivity of PEO-based electrolytes [35], but compatibility issues with high-voltage cathodes remain a significant challenge [35]. Despite these advancements, achieving high-voltage stability and high energy density simultaneously in PEO-based solid-state batteries remains a significant hurdle. This study builds upon this existing body of knowledge by introducing a novel Lewis-acid coordinated strategy to address the limitations of existing approaches.

The study employed a multi-faceted approach combining experimental and computational techniques. PEO-based electrolytes were synthesized using solution casting and hot pressing methods. The Lewis-acid coordinated electrolyte (PEO-Mg-Al-LITFSI) was prepared by incorporating Mg(ClO₄)₂ and Al(ClO₄)₃ additives into the PEO-LITFSI electrolyte. The optimal molar ratio of Mg/Al:LITFSI and the molecular weight of PEO were determined based on conductivity and oxidation potential measurements. The morphology and composition of the electrolytes were characterized using SEM, Raman spectroscopy, and Karl-Fisher titration. Operando synchrotron transmission X-ray microscopy (TXM) and X-ray absorption spectroscopy (XAS) were employed to investigate the interface evolution between the electrolyte and Ni-rich cathodes at various voltages. Molecular dynamics (MD) simulations were performed to analyze the coordination configurations of the electrolytes and understand the influence of Lewis acid additives on Li⁺ transport. Density functional theory (DFT) calculations were carried out to determine the adsorption energy and electrostatic potential distribution to understand the interaction between the electrolyte and the cathode surface. Electrochemical performance was evaluated using coin-type and pouch-cell batteries with Ni-rich LiNi₀.₈₃Co₀.₁₂Mn₀.₀₅O₂ (Ni83) and Li-rich Li₁.₁₄Ni₀.₁₃₆Co₀.₁₃₆Mn₀.₅₄₂O₂ cathodes. Linear sweep voltammetry (LSV) was used to measure the oxidation potential. Electrochemical impedance spectroscopy (EIS) was used to analyze the interfacial impedance. Industrial-scale electrolyte membrane production using a continuous slurry-casting process was also demonstrated. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of the solid electrolyte interphase (SEI) and cathode-electrolyte interphase (CEI) layers.

Operando TXM revealed the formation of a "Ni-poor" layer at the PEO-cathode interface at voltages above 4.3 V, indicating Ni dissolution due to the strong interaction between PEO and the high-valence Ni in the cathode. The introduced Mg²⁺ and Al³⁺ in the PEO-Mg-Al-LITFSI electrolyte effectively weakened the interaction between EO chains and the Ni-rich cathode. Raman spectroscopy and MD simulations showed that the Lewis-acid additives modified the coordination environment, facilitating Li⁺ diffusion. DFT calculations revealed that the PEO-Mg-Al-LITFSI electrolyte exhibited a lower HOMO energy and weaker interaction with the cathode compared to PEO-LITFSI. LSV tests demonstrated a significantly improved oxidation potential (>5 V) for the PEO-Mg-Al-LITFSI electrolyte. The PEO-Mg-Al-LITFSI electrolyte exhibited higher ionic conductivity (0.23 mS cm⁻¹ at room temperature) and Li⁺ transference number (t⁺ = 0.67) than PEO-LITFSI. Full cells using the PEO-Mg-Al-LITFSI electrolyte demonstrated excellent cycling stability over 300 cycles at 4.8 V. Pouch cells achieved an energy density exceeding 586 Wh kg⁻¹ with good cycling performance. Industrial-scale production of the electrolyte membrane was successfully demonstrated, producing a 50-meter roll of high-quality membrane. Synchrotron TXM and X-ray nano-tomography after cycling showed a homogeneous state of charge (SOC) and an inorganic-dominant interphase for the PEO-Mg-Al-LITFSI cathode, resulting in improved interface stability and preventing continuous electrolyte oxidation. XPS confirmed the presence of LiF, MgF₂, and AlF₃ in the cathode's interphase, further supporting the formation of a stable, inorganic-rich interphase.

The results demonstrate that the strong interaction between PEO chains and Ni-rich cathodes is a major cause of high-voltage battery failure in PEO-based electrolytes. The Lewis-acid coordination strategy successfully mitigated this interaction, leading to significantly improved high-voltage stability. The synergistic effects of Mg²⁺ and Al³⁺ enhanced both ionic conductivity and oxidation resistance. The formation of a robust, inorganic-rich interphase further contributes to the high performance and stability observed. The successful scaling-up of electrolyte membrane production highlights the practical feasibility of this approach for high-energy-density ASSLBs. The observed improvement in high-voltage stability is directly linked to the weakened interaction between the electrolyte and the cathode, as confirmed by both experimental and computational results.

This study presents a highly effective Lewis-acid coordination strategy for enhancing the performance of PEO-based electrolytes in high-voltage solid-state batteries. The use of Mg²⁺ and Al³⁺ as electron-withdrawing ligands effectively weakened the interaction between the electrolyte and the cathode, resulting in superior high-voltage stability and energy density. The successful demonstration of industrial-scale production of the electrolyte membrane further highlights the practical potential of this technology for widespread application in next-generation ASSLBs. Future research could focus on exploring other Lewis acid candidates and optimizing the electrolyte formulation for even higher voltage stability and energy density.

While the study demonstrated significant improvements in high-voltage stability and energy density, several limitations should be acknowledged. The long-term stability of the pouch cells at higher C-rates needs further investigation. The impact of the electrolyte's moisture sensitivity on the long-term performance requires further study. The cost-effectiveness of the industrial-scale production process should be carefully evaluated. The study primarily focused on Ni-rich cathodes; investigations with other cathode materials are needed to assess the broader applicability of this electrolyte.