Sodium/potassium-ion　batteries　(SIBs/PIBs)　are　expected　to　replace　conventional　lithium-ion　batteries　(LIBs)　soon　in　view　of　their　lower　cost　and　the　abundant　reserves　of　sodium/potassium　resources.　However,　it　remains　a　challenge　to　explore　suitable　anode　materials　for　SIBs/PIBs　with　high　energy/power　density　and　long　lifespan　due　to　the　sluggish　kinetics　during　the　insertion/extraction　of　Na+/K+　ions　with　larger　ionic　sizes.　Herein,　ultra-small　SnS2　nanocrystals　encapsulated　in　sulphurized　polyacrylonitrile　(SPAN)　fibres　have　been　fabricated　via　electrospinning　paired　with　sulphuration　treatment.　Density　functional　theory　(DFT)　calculations　demonstrate　that　SPAN　fibre　can　concentrate　Na+　on　the　surface　and　offer　additional　storage　sites　with　enhanced　reaction　kinetics.　Consequently,　this　composite　electrode　manifests　superb　sodium/potassium-ion　storage　performance　with　high　capacities　(613　mA　h　g−1　after　50　cycles　under　0.1　A　g−1　for　SIBs;　565　mA　h　g−1　at　0.05　A　g−1　and　226　mA　h　g−1　at　5　A　g−1　after　50　and　2000　cycles　for　PIBs)　and　superior　long-life　cycling　capability　(261　mA　h　g−1　after　30　000　cycles　at　10　A　g−1　for　SIBs).　Additionally,　a　sodium　full　cell　assembled　with　Na3V2(PO4)3　as　the　cathode　and　SnS2-SPAN-470-1　as　the　anode　displays　excellent　cycling　performance.　This　work　can　provide　new　insights　into　the　construction　of　novel　transition　metal　dichalcogenide-based　nanostructures　and　nanocomposites　for　high-performance　electrochemical　energy　storage　and　conversion　devices.　©　2022　The　Royal　Society　of　Chemistry.