Structural Properties of Manganese Hexacyanoferrates against Li Concentration

Manganese hexacyanoferrate, La x Mn[Fe(CN) 6 ] y z H 2 O, is a promising candidate for cathode materials for lithium ion secondary batteries, because the compound exhibits an ideal two-electron reaction without structural phase transition at y =0.83. We found that its capacity ( Q ) increases with Fe concentration ( y ) from Q = 115 mAh/g at y = 0.83 to 130 mAh/g at 0.87, and 143 mAh/g at 0.93. We further investigated the structural properties of La x Mn[Fe(CN) 6 ] y z H 2 O against the Li concentration ( x ). In the low- x region, the y = 0.87 compound exhibits a phase separation into the two cubic phases, whereas the y = 0.93 compound separates into the cubic and tetragonal phases. We ascribed these phase separations to the Jahn–Teller (JT) instability as well as the smaller ionic radius of Mn 3+ .