Prussian Blue analogues are renowned for their open-framework structures composed of CN- ligands, which distinguish them from conventional solid electrolyte materials. This study explores the effects of mobile carrier concentrations, water content, and structural integrity on the performance of solid electrolytes in all-solid-state batteries. Analyzing different phases of manganese hexacyanoferrate: cubic, monoclinic, and rhombohedral, we correlate Na+ and water content with lattice distortions and Na+ conductivity. Computational simulations corroborate experimental findings on activation energies and Coulombic interactions between Na+ and CN- ligands, taking into account carrier concentrations and structural polymorphism. The cubic phase, exhibiting lower Na+ content and comparable water content to the monoclinic phase, demonstrates the fastest Na+ migration and lowest activation energy. In terms of cell performance, the higher Na+ content of the monoclinic phase enhances cycle performance by reducing the chemical potential difference between manganese hexacyanoferrate and the anode. Monoclinic manganese hexacyanoferrate-based solid-state batteries enable stable cycle performance of a Na2Mn[Mn(CN)6] cathode, with discharge capacities of 60 mAh g-1 using a Mn(I)/Mn(II) redox couple and 120 mAh g-1 using dual redox couples of Mn(I)/Mn(II) and Mn(II)/Mn(III). This study underscores the critical role of Na+ and water content in optimizing Na+ conductivity and overall battery performance.