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

A sodium bis(fluoroallyl)malonato borate salt (NaBFMB) is synthesized. Using a Click thiol-ene reaction, NaBFMB can be photo-crosslinked with a tri-thiol (trimethylolpropane tris(3-mercapto propionate), TMPT) to create a single-ion conducting electrolyte (NaSIE), with all negative charges residing on the borate moieties and 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-1 at 30 C, and a high transference number (tNa+) of 0.91 as measured through the Bruce-Vincent method. The electrochemical stability of NaSIE polymer electrolyte is demonstrated via cyclic voltammetry (CV) to 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 cm-2. The stability of NaSIE is further demonstrated via a long cycling of the stripping/plating test with a current density of 0.1 mA cm-2 at five-minute intervals for over 10,000 min. Using the in-situ method, NaSIE is used as the electrolyte for a sodium metal battery using P2 (Na resides at prismatic sites with with ABBAAB stacking)-cathode of Na0.67Ni0.33Mn0.67O2 (NNMO) and is cycled between the cut-off voltages of 2.0-4.0 V. A high initial specific capacity (85.7 mAh g-1) with a capacity retention of 86.79% after 150 cycles is obtained.