From Cell to Atomic Level: Understanding the Degradation in 99% Coulombic Efficiency and 450 Wh kg –1 Anode-Free Pouch Cells

Anode-Free Lithium Metal Batteries (AFLMBs) promise ultra-high energy densities beyond 500 Wh kg–1, yet a significant gap from prior studies mainly limited to <350 Wh kg–1, which lacks advantages over commercial Li-ion batteries and mechanistic understanding of battery degradation at high-energy levels. Here we developed Ah-level 450 Wh kg–1 AFLMBs with >99% coulombic efficiency and examined their degradation comprehensively. Under high-energy conditions, battery degradation behavior is dictated by the major inactive lithium component, and the flexibility of solid electrolyte interphase (SEI) proves more vital than Young’s modulus for lithium metal stabilization. Two typical modes of “sudden death” and “gradual decay” are revealed, which associate with “truffle-shaped” dead lithium caused by brittle SEI and “mushroom-shaped” active lithium interwoven with flexible SEI, respectively. The SEI-dominated gradual-decay mode, with slower degradation and minimized dead lithium, exhibits superior lifespan and safety. These insights, contrasting with previous lower-energy-density battery studies, provide crucial guidance for developing practical, high-energy-density, and safe AFLMBs.