In contrast to the large number of suitable electrode materials, thorough comprehension of the electrolyte remains lacking. Meanwhile, safety issues and side reactions in sodium-ion batteries caused by traditional organic liquid electrolytes are more severe than those in lithium-ion batteries due to the higher chemical reactivity of sodium than that of lithium which will result in a more drastic reaction with liquid electrolyte. Therefore, the discovery of an effective electrolyte remains a major challenge, which hinders the further application of SIBs. Solid-state sodiumion batteries (SSIBs) based on solid-state electrolytes (SSEs) have emerged as an attractive choice to solve these problems, avoiding safety concerns, and severe side reactions with the electrodes. [7] However, the insufficient ionic conductivity of SSEs is the main challenge for the further development of SSIBs. During the past years, many kinds of SSEs had been reported including ceramic-based, sulfide-based, and polymer electrolytes. [8][9][10][11] Although ceramic-based solid-state electrolyte have attracted much attention due to their high ionic conductivity at room temperature, harsh synthetic conditions (for instance, more than 1000 °C and 24 h) and poor contact capability with the electrodes are the main obstacles. Sulfide-based SSEs, featuring softness and superior ionic conductivity, are other promising alternatives with the merits of low-temperature process capability and good contact with the electrodes. Nevertheless, the chemical instability of the sulfide-based SSEs in ambient atmosphere is the biggest obstacle to make easily fabricated and low-cost solid-state sodium-ion batteries.Compared to the SSEs mentioned above, polymer-based SSEs demonstrate remarkable advantages such as excellent flexibility and excellent contact with electrodes. When employing polymer-based SSEs in large-scale industrial applications, the following parameters should be considered: 1) Effective ionic conductivity at room temperature or even at low temperature could guarantee the normal function of SSIBs in a wide temperature range and ensure a durable cycle life; 2) Excellent thermal stability could inhibit the incident caused by thermal runaway; 3) A large electrochemical window can ensure the compatibility between polymer-based SSEs and a high voltage cathode, thus increasing the energy density of the SSIBs but without electrochemical