Methylurea-Regulated Low-Concentration (2.6m) Electrolytes for High-Voltage Aqueous Lithium-Ion Pouch Cells Using NbO2 Anodes

Niobium-based oxides exhibit a specific capacity exceeding 300 mAh/g at operating potentials of 1.0–2.0 V vs. Li⁺/Li, significantly outperforming titanium-based oxides. Fully harnessing this high capacity has the potential to significantly enhance the energy density of aqueous lithium-ion batteries. However, the narrow electrochemical stability window of conventional aqueous electrolytes often results in severe hydrogen evolution reactions at low potentials, rendering them incompatible with the reversible lithium intercalation and de-intercalation processes required for high-capacity niobium-based oxides. In contrast to conventional strategies that rely on highly concentrated electrolytes, this study focuses on optimizing the aqueous electrolyte structure by incorporating nonflammable, low-toxicity, and cost-effective methylurea (MU) while maintaining a fixed LiTFSI/H₂O molar ratio. This approach reduces the salt concentration, viscosity, and density of the electrolyte, while simultaneously enhancing ionic conductivity and widening the electrochemical stability window. Leveraging the donor-acceptor amphiphilicity and structural asymmetry of MU, this additive demonstrates high miscibility with LiTFSI/H₂O solution. MU interacts with water, Li⁺, and TFSI⁻ to disrupt the hydrogen-bond network, forming a solvation sheath dominated by MU and TFSI⁻ coordination. This novel solvation structure renders the formation of a stable, dual-layer solid electrolyte interphase (SEI) comprising an outer organic-rich and inner inorganic-rich configuration. As a result, the cathodic stability limit extends to 1.2 V vs. Li⁺/Li, enabling compatibility with high-capacity NbO₂ anode even at a low electrolyte concentration of 2.6 m. Under stringent testing conditions, including an areal capacity of 1.3 mAh/cm², P/N ratio of 1.2, a charge rate of 0.25 C, and the use of conventional aluminum current collector, NbO₂ | LiCoO₂ (184 Wh/kg) and NbO₂ | LiMn₂O₄ (161 Wh/kg) pouch cells demonstrated stable cycling over 150 cycles with an average coulombic efficiency >99.8% and 96% capacity retention after 24 hours’ storage at 100% state of charge (SoC). This straightforward and effective approach significantly enhances the energy density of aqueous lithium-ion batteries while preserving the benefits of low cost, non-flammability, and low toxicity, thereby advancing the practical development of high-voltage aqueous batteries.