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

ABSTRACT 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 Ti ₃ AlC _x O _{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 × 10 ⁶ dB cm ² 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.