Stoichiometry‐Programmed MXenes via Precursor Engineering for High‐Performance EMI Shielding and Energy Storage

Device-level performance in MXenes is dictated by architecture-planar nanosheets are optimal for electromagnetic interference (EMI) shielding, while scrolled structures enhance ion transport for energy storage-particularly when morphology is programmed at synthesis. Whether such architectures can be deterministically encoded through precursor stoichiometry remains unresolved. Here, we demonstrate that precise carbon stoichiometry control in Ti₃AlC_xO_2- _x MAX phases tunes internal lattice strain and thereby directs the emergent MXene architecture. Carbon-rich precursors (x = 1.94) yield strain-relieved, high-crystalline nanosheets with metallic conductivity (∼23 300 S cm^-1), enabling ultrathin films with record-high EMI shielding performances across X- and W-bands (≥ 2.0 × 10⁶ dB cm² g^-1 at 8.2 GHz for 29 nm; 108 dB at 100 GHz for 8 µm) and robust W-band retention after 5,000 bending cycles (r = 2.5 mm). In contrast, carbon-deficient precursors (x = 1.71) introduce lattice compression and oxygen substitution, triggering spontaneous scrolling upon delamination. The resulting nanoscrolls offer exceptional ion accessibility, achieving 657 F g^-1 at 2 mV s^-1 with 99.4% retention over 12 000 cycles. This stoichiometry-programmed approach establishes a synthesis-stage lever linking MAX chemistry to MXene architecture and function, enabling application-specific architecture design within established MAX/MXene synthesis and solution-processing workflows for next-generation electronics and energy storage.