Hollandite-type potassium titanium oxide with exceptionally stable cycling performance as a new cathode material for potassium-ion batteries

•K0.17TiO2 has a (2 × 2) tunnel structure with K+ ions presenting in the center of tunnel.•Unexpectedly high operation voltage is observed at ∼2.5 V due to the binding of K with 8 oxygens.•K0.17TiO2 undergoes a single-phase reaction accompanied by a Ti4+/3+ redox pair.•K0.17TiO2 cathode delivers the specific capacity of 60 mAh g–1 at 5 C-rates with 98% for 1000 cycles.