| Summary: | Phosphorus is an attractive negative electrode material for sodium ion batteries due to its high theoretical specific capacity of 2,596 mAh g<sup>-1</sup> . However, it suffers poor conductivity (10<sup>-12</sup> S m<sup>-1</sup> ), slow reaction dynamics, and large volume expansion (~440 %) during the sodiation process, leading to rapid capacity decay upon cycling. Great attention has been devoted to improving the electrical conductivity via mixing phosphorus particles with conductive carbon materials, yet little emphasis has been placed on addressing the volume expansion issue, which may leads to the loss of electrical contact between the active material and the current collector, and the sequent deterioration of the overall electrochemical performance. Here, we demonstrate a carbothermic reduction method to fabricate ultrafine red phosphorus particles (~10 nm) embedded in a three-dimensional carbon framework, in which numerous interconnected nanopores are generated accompanied by the carbonization of polyethylene glycol. During discharge/charge processes, nanosized phosphorus particles accommodate the large stress without cracking, and decrease the diffusion length, as well as connect strongly with carbon framework, resulting in an improved conductivity, a reversible specific capacity of 1,027 mAh g<sup>-1</sup> (at 0.2 C) and high capacity retention of 88 % over 160 cycles.
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