Discrete Li-occupation versus pseudo-continuous Na-occupation and their relationship with structural change behaviors in Fe ₂ (MoO ₄) ₃

The key factors governing the single-phase or multi-phase structural change behaviors during the intercalation/deintercalation of guest ions have not been well studied and understood yet. Through systematic studies of orthorhombic Fe 2 (MoO 4) 3 electrode, two distinct guest ion occupation paths, namely discrete one for Li and pseudo-continuous one for Na, as well as their relationship with single-phase and two-phase modes for Na + and Li +, respectively during the intercalation/deintercalation process have been demonstrated. For the first time, the direct atomic-scale observation of biphasic domains (discrete occupation) in partially lithiated Fe 2 (MoO 4) 3 and the one by one Na occupation (pseudo-continuous occupation) at 8d sites in partially sodiated Fe 2 (MoO 4) 3 are obtained during the discharge processes of Li/Fe 2 (MoO 4) 3 and Na/Fe 2 (MoO 4) 3 cells respectively. Our combined experimental and theoretical studies bring the new insights for the research and development of intercalation compounds as electrode materials for secondary batteries.