P/MoP2@C composite as a high rate capable anode for lithium ion batteries

Abstract

Phosphorus/molybdenum diphosphide (P/MoP₂) nanopowder is synthesized via a high energy mechanical milling (HEMM) and introduced as an anode for lithium ion batteries (LIBs). As-synthesized P/MoP₂ is composed of 10–20 nm-sized nanocrystallites, which are aggregated into a few hundred nanometer-sized secondary particles and MoP₂ nanocrystallites are embedded in the amorphous red P matrix. The P/MoP₂ electrode undergoes a conversion reaction of MoP₂ by forming molybdenum (Mo⁰) and lithium phosphide (Li₃P) and an alloying reaction of elemental P by forming Li₃P during lithiation. The recombination reaction from Mo⁰ and Li₃P to MoP₂ does not occur during charging. Consequently, the P/MoP₂ electrode shows high initial discharge and charge capacities of 1042.3 and 850.1 mAh g⁻¹, respectively. The additional HEMM process with P/MoP₂ and multiwall carbon nanotubes (MWCNTs) yields the P/MoP₂ nanopowder encapsulated with a 10-20 nm-thick carbon layer consisting of graphitic and non-graphitic carbons (P/MoP₂@C composite). The P/MoP₂@C electrode exhibits the improved cycle retention and excellent high rate capability, delivering the high reversible capacity of 572 mAh g⁻¹ after 250 cycles at 1000 mA g⁻¹. This improvement can be attributed to the graphitic/non-graphitic carbon coating layer, which reduces the charge transfer resistance and retains the stable solid electrolyte interphase (SEI) layer during cycling.