Unraveling the delithiation mechanism of air-stabilized fluorinated lithium iron oxide pre-lithiation material

Integrating pre-lithiation materials into the cathode to compensate for the irreversible loss of active lithium ions due to the anodes is one of the essential technologies for achieving high-energy lithium-ion batteries. Li5FeO4 (LFO) has emerged as an appealing candidate for its high initial charge specific capacity, cost-effective, and suitability for the solvents/binders. Unfortunately, suffering from notorious degradation while in storage hinders its large-scale application. Furthermore, the delithiation mechanism and the impacts of oxygen species during charging remain elusive. Herein, a fluorinated LFO, featuring the F-doped carbon layer on the surface and gradient fluorination in the bulk, is synthesized to overcome the above obstacles. Surface modification enhances the air stability in storage and prevents oxygen species from attacking the electrolyte upon charging. The introduction of fluorine is proposed to lower the migration barrier of Fe from the tetrahedral interstitial site to the octahedral one and narrow the band gap, thus maintaining a stable phase transition at high charge density. As a result, the fluorinated LFO possesses excellent air stability (541.9 mAh g−1 after humid air exposure of 24H) and exhibits attractive initial charge specific capacity (703.4/580.5 mAh g−1 at 0.05/1.0C rate) as well as slight electrolyte decomposition during the charging process. This work opens up an alternative avenue to developing industry-applicable pre-lithiation materials for high-energy batteries.