A monoclinic NaMnO2-y-δ(OH)2y with new polymorph consisting of H’3 and O’3 stackings has been prepared by a hydrothermal method. The H’3 stacking with 8-coordinate hexahedral sites was first time reported for the layered structure in the present work and it plays an important role in expanding the interlayer distance for the layered structure. With H’3 stacking, the monoclinic NaMnO2-y-δ(OH)2y without crystal water exhibits an expanded interlayer distance of about 7 Å, which is much larger than those of O3, P2 stackings. The expanded interlayer not only facilitates fast Na+ diffusion in the Na slab but also leads to greatly improved structural stability by solid solution reaction mechanism without phase transitions. With further introduction of O vacancies, the as-prepared NaMnO2-y-δ(OH)2y nanowall arrays exhibit large specific reversible capacity of 211.9 mAh g-1, excellent rate capability (156.0 mAh g-1 at 50 C), and ultrastable cycle performance (94.6% capacity retention after 1000 cycles), making them promising cathode candidate for practical application in SIBs. Importantly, the present study demonstrates successful example and provides important insights in layered structure design for improved metal ion storage.