Manganese Tetraphosphide (MnP4) as a High Capacity Anode for Lithium-Ion and Sodium-Ion Batteries

Phosphorus-rich 6-MnP4 nanoparticles are synthesized via high energy mechanical milling (HEMM) and their electrochemical properties as an anode for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) are investigated focusing on the electrochemical activity and reaction mechanism. The 6-MnP4 nanoparticles with a triclinic structure (P-1) are successfully synthesized by HEMM and they are composed of 5 to 20 nm-sized crystallites. During the lithiation process, the MnP4 phase undertakes the sequential alloying (MnP4 + 7 Li+ + 7 e(-) -> Li7MnP4) and conversion (Li7MnP4 + 5 Li+ + 5 e(-) -> Mn-0 + 4 Li3P) reactions. On the other hand, the MnP4 nanoparticles are directly converted to Mn-0 and Na3P without the formation of an intermediate Na-Mn-P alloy phase during sodiation process. The MnP4 electrode shows high initial discharge and charge capacity (1876 and 1615 mAh g(-1) for LIBs, and 1234 and 1028 mAh g(-1) for SIBs) and high initial Coulombic efficiency (86% for LIBs and 83% for SIBs), indicating a promising candidate for high capacity anodes. In addition, the long-term cyclability and high rate capability of MnP4 can be further improved through the formation of MnP4/graphene nanocomposites and vanadium substituted Mn0.75V0.25P4 solid solutions.