Na3V2(PO4)2F3 is one of the most studied polyanion type cathode materials
for
sodium-ion batteries (SIBs) and offers great promises. However, the
inferior rate capability induced by its sluggish diffusion of electrons
and ions greatly limits the practical application of electrode materials
in SIBs. Herein, we develop an efficient method to fabricate in situ
carbon-coated Na3V2(PO4)2F3 nanosheets by using cost-effective amylopectin. The
amylopectin not only could induce the nucleation of Na3V2(PO4)2F3 along its
backbone to form a 2D nanostructure, but also act as a source of amorphous
carbon for in situ coating on the active material surface. The composite
exhibits extraordinary rate capability (104 mA h g–1 at 40 C, 51 mA h g–1 at 150 C) and desirable cycling
stability. Such satisfactory achievements, especially the superior
rate performance, should be ascribed to its unique 2D nanostructure
which shortens the Na+ diffusion length, and the in situ
carbon coating endows the composites with effective electron transport.
Even applied to full cells, the obtained devices still display an
exceptionally high energy density (94.8 W h kg–1), high power density (7295 W kg–1), and excellent
cyclic stability.
The use of coal as a precursor for producing hard carbon is favored due to its abundance, low cost, and high carbon yield. To further optimize the sodium storage performance of hard carbon, the introduction of heteroatoms has been shown to be an effective approach. However, the inert structure in coal limits the development of heteroatom-doped coal-based hard carbon. Herein, coal-based P-doped hard carbon was synthesized using Ca3(PO4)2 to achieve homogeneous phosphorus doping and inhibit carbon microcrystal development during high-temperature carbonization. This involved a carbon dissolution reaction where Ca3(PO4)2 reacted with SiO2 and carbon in coal to form phosphorus and CO. The resulting hierarchical porous structure allowed for rapid diffusion of Na+ and resulted in a high reversible capacity of 200 mAh g−1 when used as an anode material for Na+ storage. Compared to unpretreated coal-based hard carbon, the P-doped hard carbon displayed a larger initial coulombic efficiency (64%) and proportion of plateau capacity (47%), whereas the unpretreated carbon only exhibited an initial coulombic efficiency of 43.1% and a proportion of plateau capacity of 29.8%. This work provides a green, scalable approach for effective microcrystalline regulation of hard carbon from low-cost and highly aromatic precursors.
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