Abstract
<jats:p> The defects in a carbon material control its behaviour, including electronic conductivity, catalytic activity and ability to store ions. These defects are almost always added after the carbon has been formed, using methods such as acid etching and oxidation, which are difficult to control at the molecular level. Here we show that the halogen counter-anions of a molecular precursor can act as an agent that controls how defects form in the carbon during a single heating step. A series of copper-based supramolecular networks, [Cu(PTA) <jats:sub>4</jats:sub> ]X·nH <jats:sub>2</jats:sub> O (X = NO <jats:sub>3</jats:sub> <jats:sup>–</jats:sup> , PF <jats:sub>6</jats:sub> <jats:sup>–</jats:sup> , Cl <jats:sup>–</jats:sup> , Br <jats:sup>–</jats:sup> and I <jats:sup>–</jats:sup> ; PTA = 1,3,5-triaza-7-phosphadamantane) was synthesised and used as precursors for the direct, phosphine-free synthesis of halogen-modulated N,P-co-doped carbon-confined copper phosphide nanocomposites (Cu <jats:sub>3</jats:sub> P@X‒C) through a single-step pyrolysis. Two fundamentally distinct and tunable halogen-mediated mechanisms were observed, governing structural evolution and electrochemical properties. Light halogens (F <jats:sup>–</jats:sup> , Cl <jats:sup>–</jats:sup> ) act as sacrificial etchants: upon thermal decomposition they transiently generate hydrogen halide gases (HF, HCl) that function as in-situ chemical etchants, carving out extensively defective and high surface-activity carbon matrices before evaporating entirely from the composite. Conversely, iodide <jats:sup/> heavy halogen, owing to its large atomic radius and high polarizability, become permanently entrapped as ionic species within the densifying carbon matrix. Bromide occupies the crossover between these regimes, as a minor fraction is retained within the matrix while the greater part is volatilised to produce a highly defective carbon similar to that observed for the lighter halogen counterparts. These divergent pathways yield different electrochemical outcomes. In lithium-ion cells, Cu <jats:sub>3</jats:sub> P@Cl‒C delivers a high reversible (first-cycle delithiation) capacity of 1223 mAh g <jats:sup>-1</jats:sup> and Cu <jats:sub>3</jats:sub> P@Br‒C the best retention (78% over 100 cycles). In sodium-ion cells, Cu <jats:sub>3</jats:sub> P@F‒C gives the highest reversible capacity (~247 mAh g <jats:sup>-1</jats:sup> after 100 cycles). Cu <jats:sub>3</jats:sub> P@I‒C exhibited the best high-rate capability in both lithium- and sodium-ion cells. This transient halogen-mediated, one-pot etching pyrolysis approach establishes a chemically rational, safer and versatile platform for tailoring defect-rich carbon nanocomposites as dual-functional anode materials for alkali-ion energy storage. </jats:p>