Solution Synthesis of Iodine-Doped Red Phosphorus Nanoparticles for Lithium-Ion Battery Anodes

Red phosphorus (RP) is a promising anode material for lithium-ion batteries due to its earth abundance and a high theoretical capacity of 2596 mA h g^-1. Although RP-based anodes for lithium-ion batteries have been reported, they were all in the form of carbon-P composites, including P-graphene, P-graphite, P-carbon nanotubes (CNTs), and P-carbon black, to improve P's extremely low conductivity and large volume change during cycling process. Here, we report the large-scale synthesis of red phosphorus nanoparticles (RPNPs) with sizes ranging from 100 to 200 nm by reacting PI₃ with ethylene glycol in the presence of cetyltrimethylammonium bromide (CTAB) in ambient environment. Unlike the insulator behavior of commercial RP (conductivity of <10 ^-12 S m^-1), the conductivity of RPNPs is between 2.62 × 10^-3 and 1.81 × 10^-2 S m^-1, which is close to that of semiconductor germanium (1.02 × 10^-2 S m^-1), and 2 orders of magnitude higher than silicon (5.35 × 10^-4 S m^-1). Around 3-5 wt % of iodine-doping was found in RPNPs, which was speculated as the key to significantly improve the conductivity of RPNPs. The significantly improved conductivity of RPNPs and their uniform colloidal nanostructures enable them to be used solely as active materials for LIBs anodes. The RPNPs electrodes exhibit a high specific capacity of 1700 mA h g^-1 (0.2 C after 100 cycles, 1 C = 2000 mA g^-1), long cycling life (∼900 mA h g^-1 after 500 cycles at 1 C), and outstanding rate capability (175 mA h g^-1 at the charge current density of 120 A g^-1, 60 C). Moreover, as a proof-of-concept example, pouch-type full cells using RPNPs anodes and Li(Ni_0.5Co_0.3Mn_0.2)O₂ (NCM-532) cathodes were assembled to show their practical uses.