Playing with the Redox Potentials in Ludwigite Oxyborates: Fe₃BO₅ and Cu₂MBO₅ (M = Fe, Mn, and Cr)

Ludwigite oxyborates with general formula M₂²⁺M′³⁺BO₅ (where M and M′ are metals) represent an interesting class of conversion-type electrode materials for lithium-ion batteries. The homometallic Fe₃BO₅ shows a first lithiation capacity of 678 mA h g^–1 (∼6.5 Li). Very low voltage polarizations for conversion-type reactions (300 and 440 mV) are observed for the two reversible redox couples at ∼1.3 and ∼1.8 V, which give rise to a stable capacity of 345 mA h g^–1 between 0.75 and 3.0 V versus Li/Li⁺. Ex situ X-ray diffraction and operando X-ray absorption spectroscopy show that Fe₃BO₅ is almost completely converted to iron metal nanograins embedded in a lithia matrix during the initial lithiation and that subsequent cycling takes place between amorphous or nanocrystalline Fe-based phases. In Cu₂MBO₅ (M = Fe, Mn, and Cr), the trivalent transition metals are found to be electrochemically active in addition to copper, but at lower voltages, causing a large spread in redox potentials. When limiting the voltage range to the Cu²⁺/Cu⁰ redox couple (at ∼2.4 V), the best performance in terms of voltage polarization and reversible capacity is obtained for Cu₂FeBO₅. Annealing of Cu₂FeBO₅ in a reducing atmosphere at low temperatures (∼250 °C) is identified as a means to improve the first cycle reversibility.