Disentangling Lithium Failure Mechanisms in Liquid Electrolytes by 2D Exchange Solid-State NMR

High Coulombic efficiency (CE, >99.9%) for Li plating/stripping is essential for thin-Li and anode-free Li metal batteries, yet understanding Li failure mechanisms remains challenging. Here, we propose a failure analysis method employing two-dimensional exchange solid-state NMR spectroscopy (2D EXSY ssNMR) to define the relative dead Li0/solid electrolyte interphase (SEI) contribution (R(A/B)) and the ratio of SEI-dead Li0 exchange intensity to SEI (R(C/B)). Together, these descriptors link inactive-Li composition with SEI-Li0 exchange, enabling ionic contact failure to be distinguished from electronic contact failure. In ester-based electrolytes, a large R(A/B) indicates dead-Li0-dominated inactive Li loss, while the observable exchange peak suggests that ionic exchange pathways are preserved. The failure is therefore mainly associated with electronic isolation of whisker-like Li deposits. Ether-based (localized) high-concentration electrolytes effectively suppress dead Li0 formation, while decreasing R(C/B) reveals progressively weakened Li exchange across inorganic-rich SEI during extended cycling. Thus, long-term failure in high-CE electrolytes is dominated by ionic contact loss rather than electronically isolated dead-Li0 accumulation, because inorganic-rich SEI lacks sufficient deformability to maintain interfacial contact during repeated volume changes. This process may be further intensified by gradual liquid-electrolyte depletion. Overall, these results establish 2D EXSY ssNMR as a comparative diagnostic tool for Li failure modes and highlight the need for SEI designs that combine chemical stability with mechanical adaptability.