Full-Cell Cycling of a Self-Supporting Aluminum Foil Anode with a Phosphate Conversion Coating

Aluminum foil is a promising candidate anode material for lithium-ion batteries (LIBs), due to its high theoretical capacity, low lithiation voltage, and abundance. However, as a matter of fact, it has been a great challenge to make Al foil cycle in full cells at industrially acceptable areal capacities of 2-4 mAh/cm² for commercial 18650 LIBs and some high-power LIBs. In this study, we defined the concepts of electrochemical true contact area (ECA) (areas with perfect electrolyte/electrode contact) and electrochemical noncontact area (ENA) (referred to regions without electrolyte spread on) for the metal foil anode. An initial ECA/ENA partition would cause severe inhomogeneity of the alloying reaction, cause localized electrode pulverization, and exacerbate ECA/ENA inequality even more. Through a phosphate conversion coating on aluminum foil, we killed two birds with one stone: first, the Al foil with a phosphate conversion coating has improved wettability (characterized by the contact angle that decreased from 35.2 to 15.9°) and favors the elimination of ENA, thus guaranteeing uniform electrochemical contact; also, the coating functions as an artificial solid electrolyte interface, which stabilizes the fragile naturally formed solid electrolyte interface and a "steady-state" electrolyte/electrode interface. Therefore, when pairing the phosphated Al foil anode against a commercial LiFePO₄ (LFP) cathode (with ∼2.65 mAh/cm²), it can cycle 120 times without Li excess and stabilizes at 1.27 mAh/cm².