Facile Method to Prepare for the Ni2P Nanostructures with Controlled Crystallinity and Morphology as Anode Materials of Lithium-Ion Batteries

Conversion reaction materials (transition metal oxides, sulfides, phosphides, etc.) are attractive in the field of lithium-ion batteries because of their high theoretical capacity and low cost. However, the realization of these materials in lithium-ion batteries is impeded by large voltage hysteresis, high polarization, inferior cycle stability, rate capability, irreversible capacity loss in first cycling, and dramatic volume change during redox reactions. One method to overcome these problems is the introduction of amorphous materials. This work introduces a facile method to synthesize amorphous and crystalline dinickel phosphide (Ni 2 P) nanoparticle clusters with identical morphology and presents a direct comparison of the two materials as anode materials for rechargeable lithium-ion batteries. To assess the effect of crystallinity and hierarchical structure of nanomaterials, it is crucial to conserve other factors including size, morphology, and ligand of nanoparticles. Although it is rarely studied about synthetic methods of well-controlled Ni 2 P nanomaterials to meet the above criteria, we synthesized amorphous, crystalline Ni 2 P, and self-assembled Ni 2 P nanoparticle clusters via thermal decomposition of nickel-surfactant complex. Interestingly, simple modulation of the quantity of nickel acetylacetonate produced amorphous, crystalline, and self-assembled Ni 2 P nanoparticles. A 0.357 M nickel-trioctylphosphine (TOP) solution leads to a reaction temperature limitation (∼315 °C) by the nickel precursor, and crystalline Ni 2 P (c-Ni 2 P) nanoparticles clusters are generated. On the contrary, a lower concentration (0.1 M) does not accompany a temperature limitation and hence high reaction temperature (330 °C) can be exploited for the self-assembly of Ni 2 P (s-Ni 2 P) nanoparticle clusters. Amorphous Ni 2 P (a-Ni 2 P) nanoparticle clusters are generated with a high concentration (0.714 M) of nickel-TOP solution and a temperature limitation (∼290 °C). The a-Ni 2 P nanoparticle cluster electrode exhibits higher capacities and Coulombic efficiency than the electrode based on c-Ni 2 P nanoparticle clusters. In addition, the amorphous structure of Ni 2 P can reduce irreversible capacity and voltage hysteresis upon cycling. The amorphous morphology of Ni 2 P also improves the rate capability, resulting in superior performance to those of c-Ni 2 P nanoparticle clusters in terms of electrode performance.