On the Feasibility of Sodium Metal as Pseudo-Reference Electrode in Solid State Electrochemical Cells

A set-up of a sodium metal anode vs. a solid polymer electrolyte (SPE) comprising poly(ethylene oxide) (PEO) and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) has been evaluated in detail for the feasibility to use sodium metal as a pseudo-reference electrode (pseudo-RE). To evaluate the stability and reproducibility, we monitored the half-wave potential (E-1/2) of added decamethylferrocene (Me(10)Fc) and the stability of the interface by electrochemical impedance spectroscopy (EIS). The sodium/SPE interface resistance (R-Na/SPE) increases with time, up to 2.8k Omega cm(-2), and causes the E-1/2 of the Me(10)Fc(+/0) reference redox couple to drift up to 15mV during 88hours. Moreover, the sodium potential is very irreproducible, even initially after cell assembling the values can differ by 60mV, likely due to extreme sensitivity of the metal surface even to an "inert and dry" glove box environment. Indeed, freshly cut sodium readily reacts with water, forming NaOH, and adsorbs impurities that can be present even in a glove box atmosphere. The oxidation layer and the amount of adsorbed impurities increase with the exposure to the glove box atmosphere, as revealed by ATR-FTIR spectroscopy. Altogether, this calls for attention when evaluating any battery materials in half-cell configurations using sodium metal as the pseudo-RE.