MgP₄/CNT-Graphene Embedded in Hard Carbon Matrix as a High-Capacity Anode for Next-Generation Sodium-Ion Batteries

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 (MgP₄) 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 MgP₄/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.% MgP₄/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.