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.