Vulnerable Solid Electrolyte Interphase Deposition in Sodium-Ion Batteries from Insufficient Overpotential Development during Formation

Formation of the solid electrolyte interphase (SEI) on hard carbon electrode significantly influences the performance of batteries, in terms of cycle performance, calendar life, and power characteristics. In sodium-ion batteries (SIBs), the energetically inferior SEI formation mechanism, compared with lithium-ion batteries (LIBs), results in the formation of a thin, thermally vulnerable, and less passivating SEI on the hard carbon electrode. Notably, electrolyte for SIBs have a higher lowest unoccupied molecular orbital (LUMO) energy level of Na-solvated ethylene carbonate and a upstream-shifted swing voltage range, compared with LIBs, which reduces the deposition of SEI on the hard carbon electrode from insufficient overpotential development. Additionally, the larger ionic radius of Na compared to that of Li-ion leads to a lower binding energy of Na ions to the anion in the SEI component, increasing the solubility of the SEI in the electrolytes. Consequently, the inferior thermal stability of the SEI in SIBs results in more-pronounced self-discharge of the hard carbon electrode during high-temperature storage, compared to LIBs.