Sodium-ion batteries have recently aroused the interest of industries as possible replacements for lithium-ion batteries in some areas. With their high theoretical capacities and competitive prices, P2-type layered oxides (Na_xTMO₂) are among the obvious choices in terms of cathode materials. On the other hand, many of these materials are unstable in air due to their reactivity toward water and carbon dioxide. Here, Na_0.67Mn_0.9Ni_0.1O₂ (NMNO), one of such materials, has been synthesized by a classic sol-gel method and then exposed to air for several weeks as a way to allow a simple and reproducible transition toward a Na-rich birnessite phase. The transition between the anhydrous P2 to the hydrated birnessite structure has been followed via periodic XRD analyses, as well as neutron diffraction ones. Extensive electrochemical characterizations of both pristine NMNO and the air-exposed one vs sodium in organic medium showed comparable performances, with capacities fading from 140 to 60 mAh g^-1 in around 100 cycles. Structural evolution of the air-exposed NMNO has been investigated both with ex situ synchrotron XRD and Raman. Finally, DFT analyses showed similar charge compensation mechanisms between P2 and birnessite phases, providing a reason for the similarities between the electrochemical properties of both materials.