Air-stable Li ₅ FeO ₄ additive enabled by ultra-thin Li ₂ CO ₃ coating for advanced Li-ion batteries

Abstract Li ₅ FeO ₄ (LFO) is widely recognized as an attractive prelithiation additive due to its cost-effective synthesis and high theoretical lithium-donor capacity (867 mAh g ⁻¹ ). However, its practical application is severely hindered by the air sensitivity. When exposed to air, LFO undergoes rapid Li ⁺ extraction, resulting in the spontaneous growth of insulating surface species (Li ₂ CO ₃ /LiOH) and significant irreversible capacity decay. Herein, we propose a surface-engineering strategy that employs high-pressure CO ₂ treatment to construct a dense and conformal Li ₂ CO ₃ coating on LFO particles. This coating effectively blocks moisture/oxygen penetration while preserving efficient Li ⁺ /e ^- transport. Therefore, the coated LFO@Li ₂ CO ₃ has a high initial charge capacity of 708.6 mAh g ⁻¹ after 1 h of air exposure, which is much higher than 483.6 mAh g ⁻¹ of the uncoated LFO. Applied as a prelithiation additive in graphite||LiFePO ₄ full cells, LFO@Li ₂ CO ₃ compensates active Li loss effectively, boosting the first-cycle discharge capacities by 8.9% at 1 C. The cells also exhibit enhanced rate capability (130.88 mAh g ⁻¹ at 5 C) and cycling stability along with a 12.7% increase in energy density and 96.78% capacity retention over 200 cycles. This work demonstrates that a rationally designed Li ₂ CO ₃ coating significantly improves the air stability of LFO without compromising its prelithiation function, enabling its practical use in high-performance lithium-ion batteries.