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 ultrahigh energy densities beyond 500 Wh kg^-1, yet prior work mainly studied on <350 Wh kg^-1 AFLMBs, which lack 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 (CE) and examined their degradation across cell, electrode, and atomic dimensions. Under high-energy-density conditions (5.6 mAh cm^-2 cathode, 2.0 g Ah^-1 electrolyte), the battery degradation behavior is dictated by the dominant inactive lithium component. Two typical modes of "sudden death" and "gradual decay" are revealed, which are associated with "truffle-shaped" dead lithium caused by brittle solid electrolyte interphase (SEI) and "mushroom-shaped" active lithium interwoven with flexible SEI, respectively. The gradual-decay mode, with slower degradation and minimized dead lithium, exhibits a superior lifespan and safety. In addition, the SEI's elastic deformation emerges as a more reliable parameter than Young's modulus for lithium metal stabilization. These findings are different from previous lower-energy-density studies, wherein the batteries could run many cycles even under a relatively low CE and abundant dead lithium, thus establishing a more reliable composition-structure-performance correlation for battery failure and facilitating the development of high-energy-density practical AFLMBs.