The delivery of cathodes with both high capacity and excellent cycling stability is a great challenge in the development of sodium-ion batteries (SIBs) for energy storage systems. Here, we exploited a novel potassium-ion-intercalated layered iron/manganese-based material (K0.7Fe0.5Mn0.5O2). On the basis of advanced in situ and ex situ X-ray diffraction analysis, we confirm that K0.7Fe0.5Mn0.5O2 can provide highly reversible layer spacing variations and an ultra-stable skeleton structure during the sodiation/desodiation processes. As a result, K0.7Fe0.5Mn0.5O2 displays superior performance, with both high capacity and superior cycling stability, as a cathode for SIBs. A high discharge capacity of 181mAhg−1 is achieved at 100mAg−1. Remarkably, even when cycled at high rate of 1000mAg−1, 85% of the initial discharge capacity is maintained after 1000 cycles. These results indicate that K0.7Fe0.5Mn0.5O2 is a promising candidate for high-capacity and long-life SIBs. Additionally, this work will provide a unique insight into the development of high-performance cathodes for energy storages.