Nitrides or carbides coated on hard carbon for sodium ion batteries

Sodium ion batteries (SIBs) are a promising substitute for lithium ion batteries (LIBs) because of the natural abundance and lower price of sodium. Hard carbon (HC), known as “non-graphitizable” carbon, is the most popular negative electrode material in SIBs. In this thesis, we investigate the development of composite materials in which the hard carbon is combined with a sodium conversion material with the aim of improving the capacity and cycling stability. Hard carbon obtained from cotton wool at 1400 °C shows a best reversible capacity of 319 mA h g-1 at current of 20 mA g-1 . The sodium storage analysis is consistent with the traditional insertion/absorption mechanism in the hard carbon. The slope region is more related with interlayer distance and degree of graphitization while the plateau part is more related to the micropores size and volume. An effective route to synthesis composites of metal nitrides and carbides with carbon was reported. Titanium tetrachloride is reacted with hydroxide groups on cellulose (cotton wool) before firing to convert the cellulose to hard carbon. Hard carbon-nanocrystalline titanium nitride composites with a good distribution of the titanium across the fibrous hard carbon structure were obtained by firing the treated cellulose under nitrogen. Hard carbon-nanocrystalline titanium carbide composites were obtained by carbonized under argon. Both composites show similar first cycle capacities to hard carbon, but the titanium nitride composite delivers a better capacity retention (85.2%) after 50 cycles than that of hard carbon (74.3 %). Ex situ grazing incidence XRD patterns of the HC-TiN composites suggest the reactions occurring only on the surface region of TiN. VN-HC composites have been synthesised using the same pyrolysis process after reacting VOCl3 with cellulose. The introduction of VN produces an increased capacity: with addition of 8.6 wt% VN, the hard carbon-based electrode achieves a first cycle reversible (oxidation, de-sodiation) capacity of 354 mA h g-1 at 50 mA g-1 , while with pure hard carbon it is 302 mA h g-1 . The additional specific capacity achieved upon addition of VN, compared with the pure hard carbon, is 605 mA h g-1 when referred to the mass of VN only, which is the highest capacity of VN materials in sodium-ion batteries reported to date. In addition, VN also improves the capacity retention with cycling: after 50 cycles the reversible capacity of hard carbon electrodes with 8.6 wt% VN is 294 mA h g-1 , while with pure hard carbon it is 239 mA h g-1 . Insights into the reaction mechanism are obtained by ex situ characterization of the discharged and charged electrodes. Amorphous silicon nitride and silicon oxycarbides were obtained at 1200 °C. The as-prepared silicon nitride coated on hard carbon shows a reversible capacity of 351 mA h g-1 at 50 mA g-1 , better than that of 284 mA h g-1 from pure hard carbon. Furthermore, hard carbon coated with silicon nitride delivers a capacity retention of 85.4% in 50 cycles. The surface evolution of electrode before and after reduction cycling has been investigated by XPS measurement