MgP ₄ /CNT‐Graphene Embedded in Hard Carbon Matrix as a High‐Capacity Anode for Next‐Generation Sodium‐Ion Batteries

ABSTRACT Sodium‐ion batteries (SIBs) are promising low‐cost alternatives to lithium‐ion batteries (LIBs), but their energy density remains limited. Hard carbon (HC) offers only modest capacity, while high‐capacity LIB anode materials, notably silicon (Si), are ineffective in SIBs due to the unfavorable thermodynamics of Na‐rich alloy formation. Here, we report a scalable mechanochemical synthesis of magnesium tetraphosphide (MgP 4 ) and demonstrate that hybrid carbon matrix engineering enables durable and high‐capacity anodes. A two‐step multi‐walled carbon nanotube (MWCNT, T)/graphene(G) assembly, denoted as T2G1 (2:1 T:G by weight), where CNTs are introduced prior to graphene, constructs a continuous conductive and mechanically robust network. In contrast, reversing the assembly order (G2T1) leads to fragmented conductive pathways and inferior structural stability. Multiscale analyses, including cross‐sectional resistance mapping and structural characterization, reveal that the optimized matrix promotes uniform charge transport and suppresses structural degradation during cycling. As a result, the MgP 4 /T2G1 anode delivers stable high‐rate performance, retaining 468.5 mAh g −1 over 500 cycles at 1000 mA g −1 (85.3% retention). Furthermore, integrating 30 wt.% MgP 4 /T2G1 into commercial HC yields a practical composite with a reversible capacity of 146.5 mAh g −1 after 2000 cycles at 1000 mA g −1 , corresponding to ∼2.6 times higher capacity than that of pristine HC electrode.