Stress–Strain Engineering with Lithium Solid Solution/Lithium Alloy Composite Anodes for Stable Lithium‐Metal Batteries

High-performance thin lithium (Li) metal anodes are indispensable for high-energy-density Li metal batteries, yet their electrochemical-mechanical instability precipitates rapid failure. Uncontrolled lithium plating, accompanied by severe side reactions and the formation of a porous, non-compact deposit, swells the electrode and generates destructive stress. An in situ investigation of stress, strain, and their spatial distributions during charge/discharge cycling identifies the coupled volume variation and stress-strain evolution as an operando descriptor of degradation and underpins the rational design of a dual-phase Li-Ag solid-solution/Li₂₂Sn₅ alloy composite anode. Ag in Li-Ag solid solution enhances the affinity with Li⁺ for uniform Li plating, whereas a rigid, ion-permeable Li₂₂Sn₅ scaffold confines volume expansion. LiNi_0.6Co_0.2MnO_0.2 (NCM622)||Li-Ag/Li₂₂Sn₅ pouch cell exhibits a markedly lower internal-pressure rise during cycling than NCM622||Li counterpart under identical conditions. Specifically, the latter shows a 58% higher pressure increase than the NCM622||Li-Ag/Li₂₂Sn₅ pouch cell after charging from 0% to 100% SOC in the 5th cycle. The 1.3 Ah laminated NCM622||Li-Ag/Li₂₂Sn₅ (40 µm-thick anode) pouch cell retains 87% capacity over 100 cycles when operating at an industry-relevant N/P ratio of 1.5 and electrolyte budget of 2.6 g Ah^-1. Mechanical-electrochemical coupling of Li metal anode design, therefore, offers a practical route to long-lifespan Li metal batteries.