An iron formate framework with mixed-valence metal active sites as a promising anode material for stable Li-storage

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Metal organic frameworks (MOFs) with metal active sites of mixed-valence are still less studied for Li-storage, especially for the anode. We present the Li-storage performance of a formate framework with both Fe(II) and Fe(III) as active sites. It delivered an impressive reversible capacity of ≈920 mAh/g at 50 mA g−1, good rate performance of 683 mAh/g at 500 mA g−1, and long-term cycling stability of 500 cycles with 73% capacity retention. Reversible structure conversion accompanying self-nano-crystallization at certain crystal directions accounts for the good cycling stability and progressively improved rate performance.