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

Li5FeO4 (LFO) is widely recognized as an attractive prelithiation additive due to its costeffective 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 (Li2CO3 /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 Li2CO3 coating on LFO particles. This coating effectively blocks moisture/oxygen penetration while preserving efficient Li + /e -transport. Therefore, the coated LFO@Li2CO3 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@Li2CO3 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 Li2CO3 coating significantly improves the air stability of LFO without compromising its prelithiation function, enabling its practical use in high-performance lithium-ion batteries.