Sodium Formate as a High-Capacity Cathode Presodiation Additive for Ultra-Stable Sodium-Ion Batteries

The initial Coulombic efficiency (ICE) of sodium-ion batteries (SIBs) is substantially lower than that of lithium-ion batteries, primarily owing to the intrinsic limitations of hard carbon anodes. Cathode sodium-compensation additives have emerged as a promising strategy to address this challenge, offering both safety and ready scalability for mass production. However, most reported additives rely on catalytic decomposition, which inevitably raises cost and introduces inert components into the battery. Here, catalyst-free sodium formate (HCOONa) is introduced as a cathode additive that delivers a sodium-compensation capacity of 398 mAh g^{- 1}. It is revealed that the HCOO^- possesses a higher highest occupied molecular orbital energy than conventional carbonate solvents, enabling preferential oxidation that suppresses detrimental electrolyte decomposition. The oxygen species derived from HCOONa facilitate the formation of a robust and thin cathode electrolyte interphase (CEI) on the NaNi_1/3Fe_1/3Mn_1/3O₂ cathode. This CEI comprises a Na₂O-rich inorganic phase and a COOR-rich organic phase, which enhance interfacial mechanical strength and electrochemical stability. Hard carbon||NaNi_1/3Fe_1/3Mn_1/3O₂ pouch cells containing 2.5 wt% HCOONa exhibit a 5.7% increased initial discharge capacity and superior cycling stability, retaining 88% capacity after 700 cycles. These findings establish an effective presodiation strategy that compensates for sodium loss while stabilizing electrode interfaces in practical SIBs.