<p>Reactive lithium metal renders it prone to corrosion, which severely limits calendar life and practicality in energy storage. Despite its importance, corrosion-induced degradation remains largely qualitative and lacks a clear mechanistic understanding. Here, we present a quantitatively integrated and experimentally validated framework that correlates lithium corrosion with interphase growth kinetics and interfacial morphological evolution. Guided by this model, a bi-layered anti-corrosive passivation layer composed of lithium polyacrylate embedded with lithium silver alloy-fluoride interphase is rationally designed. The outer polymer-rich layer resists swelling and blocks corrosion, while the underlying LiAg/LiF-rich interphase enhances interfacial transport kinetics. Operando X-ray microscopy reveals that calendar-aged lithium regions are particularly vulnerable to accelerated corrosion, which intensifies dendritic formation and is effectively suppressed by the passivation layer. Consequently, full-cells show a high-rate capacity of 133 mAh g⁻¹ at 10 C (6 min) and retain 74.6% capacity after 400 cycles at 0.5 C (120 min), with Coulombic efficiency above 99.9%. Under a four-hour rest protocol for calendar life evaluation, full-cells maintain 75.1% capacity after 200 cycles, and further pouch cell testing shows 85.5% capacity after 640 cycles. This study offers insights into corrosion dynamics and informs the design of passivation strategies for improving calendar life in lithium metal batteries.</p>