Electrodes for Sodium-Ion Batteries with Excellent Rate Performance Using Porous Carbon Networks

Recently, sodium ion batteries (NIBs) gained increasing attention, because of its abundant reserves and relatively even geological distribution. However, the development of NIBs is greatly hampered because of the lack of appropriate active materials for both  cathodes and anodes. Since the Na ion is ca. 55% larger than the Li ion, finding suitable host materials with sufficiently large interstitial space to accommodate sodium ions and to allow reversible and rapid ion insertion and extraction is very difficult. NASICON-type Na₃V₂(PO₄)₃ has recently been considered as a prospective cathode material for NIBs, because of its highly covalent 3D framework crystalline structure, high energy density (~400 Wh/kg) and good thermal stability. For anode materials, disordered carbon is particularly attractive because of their large interlayer distance and disordered structure that could accommodate sodium ions.

Here, we briefly present porous carbon based electrodes for NIBs with excellent rate performance: (1) Carbon-coated nanosized Na₃V₂(PO₄)₃ embedded in the porous carbon matrix (abbreviated as CC-NVP@PCM); (2) Nitrogen-doped activated porous carbon fibers (ACFs); (3)Flexible free-standing porous carbon nanofibers (P-CNFs) with three-dimensional (3D) interconnected structure.

The CC-NVP@PCM was prepared by using a facile soft-chemistry based method with post heat-treatment. It shows high rate performance (44 mAhg^-1at 200C ). This ultrahigh rate performance is comparable to that of supercapacitor, but with much higher energy density.

The ACFs were synthesized by pyrolysis of Ppy nanofiber precursor. It shows excellent capacity retention and extraordinary rate performance. The reversible capacity at 0.05 Ag^-1 is 296 mAhg^-1 and could still maintain as 72 mAg^-1 at a current of 10 Ag^-1.

In case of the P-CNFs electrode prepared by electrospinning, it delivers a reversible capacity of 266 mAh.g^-1 after 100 cycles at 0.2C, corresponding to 80.3 % of the initial charge capacity. When cycled at a current density as high as 500 mAg^-1 (2C), it still delivers a reversible capacity of ~140 mAh.g^-1after 1000 cycles.

The improvement of electrochemical performance of these porous carbon based electrodes for NIBs is attributed to the special design and porous structure, which confined a variety of advantages: hierarchical porous channels enabling short transport length for ions and electrons, carbon coated structure resulting in low resistances, good mechanical properties leading to the excellent morphology stability.