Structural polymorphism and carrier effects in sodium-ion conducting Prussian blue-type solid electrolytes

Abstract

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. By 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 the lowest activation energy. In terms of cell performance, the higher Na⁺ content of the monoclinic phase enhances cycling performance by reducing the chemical potential difference between manganese hexacyanoferrate and the anode. Monoclinic manganese hexacyanoferrate-based solid-state batteries enable stable cycling performance of the Na₂Mn[Mn(CN)₆] cathode, with discharge capacities of 60 mAh g⁻¹ using a Mn(I)/Mn(II) redox couple at room temperature and 120 mAh g⁻¹ using dual redox couples of Mn(I)/Mn(II) and Mn(II)/Mn(III) at 30 °C. This study underscores the critical role of Na⁺ and water content in optimizing Na⁺ conductivity and overall battery performance.