A simple synthesis method to produce hard carbon decorated with silicon nitride or silicon oxycarbide nanoparticles was developed. Silicon tetrachloride is reacted with the hydroxide groups on cellulose (cotton wool) before carbonisation to form hard carbon. Use of a nitrogen or argon carbonisation atmosphere enables production of silicon nitride or oxycarbide coatings by carbothermal nitridation or reduction. This is the first time that a silicon nitride has been used in sodium-ion batteries, and it has a very high capacity. The incorporation of 7.9 wt% of silicon nitride produces an increase in the reversible (desodiation) capacity from 284 mA h g⁻¹ for pure hard carbon to 351 mA h g⁻¹ for the silicon nitride – hard carbon composite, at 50 mA g⁻¹ in sodium half-cells. The associated silicon nitride capacity is estimated as 848 mA h g⁻¹ when normalised by the mass of silicon nitride, ascribed to the good dispersion of silicon nitride nanoparticles on the hard carbon, and facile electrochemical reactions in the amorphous and non-stoichiometric silicon nitride. X-ray photoelectron spectroscopy (XPS) of the electrodes before and after cycling is used to elucidate the mechanism of sodium storage, involving formation of amorphous silicon, which then reacts with the electrolyte forming SiO_x surface species.