Design of a Single-Ion Conducting Polymer Electrolyte for Sodium-Ion Batteries

A sodium bis(fluoroallyl)malonato borate salt (NaBFMB) was synthesized. NaBFMB can be photo-crosslinked to create a single-ion conducting electrolyte (NaSIE), with anions immobilized through the 3-D crosslinked network. The NaSIE can be prepared either as a free-standing film or through a drop-cast method followed by a photo crosslinking method for an in-situ formation on top of the electrodes. The free-standing film of NaSIE has a high ionic conductivity of 2×10-3 S/cm at 30 oC, and a high transference number (tNa+) of 0.91 . The electrochemical stability of NaSIE polymer electrolyte was demonstrated be stable up to 5 V vs Na/Na+. When tested inside a symmetrical Na//Na cell, the NaSIE shows a critical current density (CCD) of 0.4 mA/cm2. The stability of NaSIE was further demonstrated via a long cycling of the stripping/plating test with a current density of 0.1 mA/cm2 at five-minute intervals for over 10,000 minutes. Using the in-situ method, NaSIE was used as the electrolyte for a sodium metal battery using P2cathode of Na0.67Ni0.33Mn0.67O2 (NNMO) and was cycled between the cut-off voltages of 2.0 – 4.0 V. A high initial specific capacity (85.7 mAh/g) with a capacity retention of 86.79% after 150 cycles was obtained.