Understanding the degradation complexity of ultrahigh-energy lithium metal batteries

The combination of Li-rich layered oxide cathodes and lithium metal anodes enables lithium metal batteries (LMBs) to achieve specific energies exceeding 600 Wh kg^-1, which is a crucial threshold requiring the activation of anionic oxygen-redox of cathode. The specific energy is attained owing to oxygen-redox reactions at the cathode and reversible Li plating-stripping at the anode, but these processes also induce distinct failure mechanisms. Structural destabilization at the cathode and anodic dendrite growth cause cell-level failures that impact the lifespan of LMBs more profoundly than material degradation alone. Moreover, the presence of lithium metal anodes obscures the detection of active Li loss, often leading to misinterpretations related to capacity fading and cycle life. This Review examines the progress in realizing 600 Wh kg^-1 LMBs and understanding their lifespan failure mechanisms. We discuss the challenges in accurately assessing the lifespan and Li loss pathways of LMBs, and we elucidate the fundamental chemistry mechanisms driving both material-level and cell-level failures. In particular, the electrochemical implications of cell parameters, cell assembly and operating conditions on the lifespan are highlighted. We also outline the gaps in knowledge and advanced techniques required to decipher detailed failure modes for LMBs with oxygen-redox reactions and Li plating-stripping.