Finding suitable electrode materials for alkali-metal-ion storage is vital to the next-generation energy-storage technologies. Polyantimonic acid (PAA, H2Sb2O6 · nH2O), having pentavalent antimony species and an interconnected tunnel-like pyrochlore crystal framework, is a promising high-capacity energy-storage material. Fabricating electrochemically reversible PAA electrode materials for alkali-metal-ion storage is a challenge and has never been reported due to the extremely poor intrinsic electronic conductivity of PAA associated with the highest oxidation state Sb(V). Combining nanostructure engineering with a conductive-network construction strategy, here is reported a facile one-pot synthesis protocol for crafting uniform internal-void-containing PAA nano-octahedra in a composite with nitrogen-doped reduced graphene oxide nanosheets (PAA⊂N-RGO), and for the first time, realizing the reversible storage of both Li+ and K+ ions in PAA⊂N-RGO. Such an architecture, as validated by theoretical calculations and ex/in situ experiments, not only fully takes advantage of the large-sized tunnel transport pathways (0.37 nm2) of PAA for fast solid-phase ionic diffusion but also leads to exponentially increased electrical conductivity (3.3 S cm−1 in PAA⊂N-RGO vs 4.8 × 10−10 S cm−1 in bare-PAA) and yields an inside-out buffer function for accommodating volume expansion. Compared to electrochemically irreversible bare-PAA, PAA⊂N-RGO manifests reversible conversion-alloying of Sb(V) toward fast and durable Li+- and K+-ion storage.