A Self-Forming Composite Electrolyte for Solid-State Sodium Battery with Ultralong Cycle Life

Replacing organic liquid electrolyte with inorganic solid electrolytes (SE) can potentially address the inherent safety problems in conventional rechargeable batteries. However, solid-state batteries (SSBs) have been plagued by the relatively low ionic conductivity of SEs and large charge-transfer resistance between electrode and SE. Here, a new design strategy is reported for improving the ionic conductivity of SE by self-forming a composite material. An optimized Na+ ion conducting composite electrolyte derived from the Na1+ n Zr2Si n P3− n O12 NASICON (Na Super Ionic Conductor) structure is successfully synthesized, yielding ultrahigh ionic conductivity of 3.4 mS cm−1 at 25 °C and 14 mS cm−1 at 80 °C. On the other hand, in order to enhance the charge-transfer rate at the electrode/electrolyte interface, an interface modification strategy is demonstrated by utilization of a small amount of nonflammable and nonvolatile ionic liquid (IL) at the cathode side in SSBs. The IL acts as a wetting agent, enabling a favorable interface kinetic in SSBs. The Na3V2(PO4)3/IL/SE/Na SSB exhibits excellent cycle performance and rate capability. A specific capacity of ≈90 mA h g−1 is maintained after 10 000 cycles without capacity decay under 10 C rate at room temperature. This provides a new perspective to design fast ion conductors and fabricate long life SSBs.