Polyethylene Oxide/Na_3.2Ca_0.1Zr_1.9Si₂PO₁₂ as Gel Electrolyte for Quasi-solid-state Sodium-ion Batteries

Recently, solid-state sodium-ion batteries (SSSIBs) have attracted wide attention as a choice for next-generation energy storage systems owing to several advantages. Firstly, the low cost of sodium metal and aluminum current collectors makes SIBs a prospective alternative for large-scale industrial use. Secondly, solid electrolytes possess high thermal stability, wide electrochemical window, and high mechanical strength which can suppress dendrite growth. However, high interface resistance between the solid electrolyte and electrode and low ionic conductivity limit the performance of SSSIBs. Hence, developing a gel electrolyte with low contact resistance and high ionic conductivity is the key to enabling the practical application of next-generation SIBs.<br/>In this work, a calcium-doped NASICON-type solid electrolyte, Na_3.2Ca_0.1Zr_1.9Si₂PO₁₂ (NCZSP) was synthesized by a sol-gel method. Then, solvent casting was used to prepare a polyethylene oxide (PEO)/NCZSP film. The PEO/NCZSP gel electrolyte achieved high ionic conductivity of approximately 6 × 10⁻⁴ S cm⁻¹ (n = 10) at room temperature. After, quasi-solid-state SIBs were assembled and tested with a high-entropy layered oxide (HEO) as cathode and sodium metal as anode under room temperature. The HEO||PEO/NCZSP||Na cells exhibited excellent electrochemical performance with a high discharge capacity of approximately 120 mAh g⁻¹ under the current density of 0.1C and good retention of 78% after 100 cycles, demonstrating that PEO/NCZSP has a good potential as a gel electrolyte for SIBs.