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Despite the promises in high‐energy‐density batteries, Li‐metal anodes (LMAs) have suffered from extensive electrolyte decomposition and unlimited volume expansion owing to thick, porous layer buildup during cycling. It mainly originates from a ceaseless reiteration of the formation and collapse of solid‐electrolyte interphase (SEI). This study reveals the structural and chemical evolutions of the reacted Li layer after different cycles and investigates its detrimental effects on the cycling stability under practical conditions. Instead of the immediately deactivated top surface of the reacted Li layer, the chemical nature underneath the reacted Li layer can be an important indicator of the electrolyte compositional changes. It is found that cycling of LMAs with a lean electrolyte (≈3 g Ah−1) causes fast depletion of salt anions, leading to the dynamic evolution of the reacted Li layer structure and composition. Increasing the salt‐solvent complex while reducing the non‐solvating diluent retards the rate of depletion in a localized high‐concentration electrolyte, thereby demonstrating prolonged cycling of Li||NMC622 cells without compromising the Li Coulombic efficiencies and high‐voltage stability.

Development of practical lithium (Li) metal batteries (LMBs) remains challenging despite promises of Li metal anodes (LMAs), owing to Li dendrite formation and highly reactive surface nature. Polyolefin separators used in LMBs may undergo severe mechanical and chemical deterioration when contacting with LMAs. To identify the best polyolefin separator for LMBs, this study investigated the separator‐deterministic cycling stability of LMBs under practical conditions, and redefined the key influencing factors, including pore structure, mechanical stability, and chemical affinity, using 12 different commercial separators, including polyethylene (PE), polypropylene (PP), and coated separators. At extreme compression triggered by LMA swelling, isotropic stress release by balancing the machine direction and transverse direction tensile strengths was found to be crucial for mitigating cell short‐circuiting. Instead of PP separators, a PE separator that possesses a high elastic modulus and a highly connected pore structure can uniformly regulate LMA swelling. The ceramic coating reinforced short‐circuiting resistance, while the cycling efficiency degraded rapidly owing to the detrimental interactions between ceramics and LMAs. This study identified the design principle of separators for practical LMBs with respect to mechanical stability and chemical affinity toward LMAs by elucidating the impacts of separator modification on the cycling performance.

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