TiO₂-engineered polyurethane gel electrolyte enabling dendrite-free Na metal cycling and high-fidelity state estimation

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries due to their low cost and abundant sodium resources; however, conventional liquid electrolytes suffer from safety issues and interfacial instability. Herein, a gel polymer electrolyte (TPU–TiO₂ GPE) was developed by incorporating 5 wt% TiO₂ nanoparticles into a thermoplastic polyurethane (TPU) matrix. The Lewis acidic centers of TiO₂ interact synergistically with polyurethane chain segments, reducing polymer crystallinity and enhancing chain mobility. As a result, the TPU–TiO₂ GPE exhibits higher room-temperature ionic conductivity (2.91 × 10−4 S·cm−1) and tensile strength (32.7 MPa) than pristine TPU.Moreover, the TPU–TiO₂ GPE forms a stable and uniform solid–electrolyte interphase (SEI) on sodium metal, effectively suppressing dendrite growth. Na||Na symmetric cells demonstrate stable cycling for over 250 h at 0.2 mA·cm−2, while Na||Na₃V₂(PO₄)₃ full cells retain 93.84% of their capacity after 280 cycles at 0.5C. The state of charge (SOC) of the full cells was estimated using a second-order RC equivalent circuit model combined with an extended Kalman filter (EKF). Compared with the pristine TPU system, the TPU–TiO₂ GPE exhibits smoother OCV–SOC characteristics and reduced polarization, leading to significantly improved SOC estimation accuracy. The voltage-model fitting RMSE is reduced from approximately 2.4 mV to 1.3 mV. This improvement enables more reliable SOC tracking across the entire SOC range. These results indicate that the TPU–TiO₂ GPE enhances both electrochemical performance and SOC predictability, providing a viable strategy for safe and intelligently managed sodium-ion batteries.