Understanding the Air‐Exposure Degradation Chemistry of the Sacrificial Cathode Additive Li5FeO4 for Li‐Ion Batteries

Abstract Li5FeO4 (LFO) is emerging as a promising prelithiation additive due to the high lithium donor capacity (>700 mAh g−1), cheap raw materials, good environmental adaptability, and reliable synthesis approach. However, LFO suffers from extremely poor air stability and the underlying mechanism remains elusive. Herein, LFO is subjected to different atmospheres (the main component of air) of O2, CO2, N2, and 30% humidity air to unravel its reaction mechanism with air and the interfacial chemistry. It is found that Li+ ions will escape from the LFO crystal lattice to construct Li2O, Li2CO3, and LiOH impurity phases with an obvious change of LFO surface morphology in contact with O2, CO2, and humidity air. These insulating species remarkedly inhibit Li+ ion diffusion from the material and thus lead to an increment of interfacial impedance and an apparent capacity degradation. Consequently, the initial charging capacity decays rapidly from 681.5 to 169.4 mAh g−1 (under O2) and to 54.8 mAh g−1 (under CO2), respectively. In this regard, a facile coating strategy is proposed on the surface of LFO to effectively improve its air stability.