Assessment of the Electrochemical Stability of Carbonate-Based Electrolytes in Na-Ion Batteries

The large abundance of Na combined with the feasibility of Na-based insertion compounds, such as Na3V2(PO4)2F3, makes the Na-ion battery an attractive technology compared with Li-ion battery for a few applications. Nonetheless, one identified limitation of the Na3V2(PO4)2F3/HC system is its poor long-term cycling performance at elevated temperature, hence the sorely need to screen the proper electrolyte formulation. Here, we report a thorough survey aiming to assess the pros and cons of cyclic vs. linear carbonates with respect to the performances of Na3V2(PO4)2F3/HC cells. Through complementary in-situ UV and CV analytical techniques we reveal the formation of soluble species stemming from the partial decomposition of linear carbonates (DMC and EMC) in reduction into soluble products, hence stressing their failure to provide a stable solid electrolyte interphase (SEI) at the hard carbon electrode and by the same token accounting for the poor performance of the overall cell. We discuss the decomposition reaction paths, and propose a shuttle mechanism to account for the cell deterioration. Our results, which underscores the detrimental effects of linear carbonates in full Na-ion Na3V2(PO4)2F3/HC cells, should serve as an impetus to identify superior electrolyte formulation for increasing the high temperature robustness of this technology.