Monodispersed Carbon-Coated Cubic NiP Nanoparticles Anchored on Carbon Nanotubes as Ultra-Long-Life Anodes for Reversible Lithium Storage.

In search of new electrode materials for lithium-ion batteries, metal phosphides that exhibit desirable properties such as high theoretical capacity, moderate discharge plateau, and relatively low polarization recently have attracted a great deal of attention as anode materials. However, the large volume changes and thus resulting collapse of electrode structure during long-term cycling are still challenges for metal-phosphide-based anodes. Here we report an electrode design strategy to solve these problems. The key to this strategy is to confine the electroactive nanoparticles into flexible conductive hosts (like carbon materials) and meanwhile maintain a monodispersed nature of the electroactive particles within the hosts. Monodispersed carbon-coated cubic NiP nanoparticles anchored on carbon nanotubes (NiP@C-CNTs) as a proof-of-concept were designed and synthesized. Excellent cyclability (more than 1000 cycles) and capacity retention (high capacities of 816 mAh g after 1200 cycles at 1300 mA g and 654.5 mAh g after 1500 cycles at 5000 mA g) are characterized, which is among the best performance of the NiP anodes and even most of the phosphide-based anodes reported so far. The impressive performance is attributed to the superior structure stability and the enhanced reaction kinetics incurred by our design. Furthermore, a full cell consisting of a NiP@C-CNTs anode and a LiFePO cathode is investigated. It delivers an average discharge capacity of 827 mAh g based on the mass of the NiP anode and exhibits a capacity retention of 80.7% over 200 cycles, with an average output of ∼2.32 V. As a proof-of-concept, these results demonstrate the effectiveness of our strategy on improving the electrode performance. We believe that this strategy for construction of high-performance anodes can be extended to other phase-transformation-type materials, which suffer a large volume change upon lithium insertion/extraction.