Facile Method to Prepare for the NiP 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 (NiP) 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 NiP nanomaterials to meet the above criteria, we synthesized amorphous, crystalline NiP, and self-assembled NiP nanoparticle clusters via thermal decomposition of nickel-surfactant complex. Interestingly, simple modulation of the quantity of nickel acetylacetonate produced amorphous, crystalline, and self-assembled NiP nanoparticles. A 0.357 M nickel-trioctylphosphine (TOP) solution leads to a reaction temperature limitation (∼315 °C) by the nickel precursor, and crystalline NiP (c-NiP) 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 NiP (s-NiP) nanoparticle clusters. Amorphous NiP (a-NiP) nanoparticle clusters are generated with a high concentration (0.714 M) of nickel-TOP solution and a temperature limitation (∼290 °C). The a-NiP nanoparticle cluster electrode exhibits higher capacities and Coulombic efficiency than the electrode based on c-NiP nanoparticle clusters. In addition, the amorphous structure of NiP can reduce irreversible capacity and voltage hysteresis upon cycling. The amorphous morphology of NiP also improves the rate capability, resulting in superior performance to those of c-NiP nanoparticle clusters in terms of electrode performance.