A new class of materials for lithium-ion batteries: iron(III) borates

The first electrochemical investigation of transition metal borate structures are reported, focusing on FeBO3 and Fe3BO6. In Fe3BO6, uptake of lithium initiates a two-phase process that results in framework disintegration, as expected from structural considerations and formation of an amorphous material that reversibly cycles lithium with a specific capacity of 450 mhA/g at an average potential of 1.6 V, in the regime of anode applications. Substitution of Fe(III) with other metals in the Fe3BO6 lattice is found to be detrimental to cycling behavior. Reduction of the particle size results in a reversible capacity up to 700 mAh/g, albeit with some fading on cycling.