Résumé
Upon electrochemical reduction in a lithium cell, crystalline FeBO3 and Fe3BO6 give amorphous compounds, which can be cycled reversibly at low potential (Rowsell, J. Power Sources2001, 97−98, 254). We report here capacities as high as 300 Ah/kg at the 14th cycle. The mechanism of reaction was investigated by step-potential electrochemical spectroscopy and in situ Mössbauer and X-ray absorption spectroscopies on electrochemical cells encapsulated in thin sealed plastic bags. These techniques confirm the reduction mechanism consisting of the early destruction of the crystallized borates and a direct FeIII−Fe0 multiphase reduction on first discharge. On recharge and subsequent cyclings, a reversible redox reaction occurs between Fe0 and FeII/III in an amorphous or nanocrystalline matrix.