Unidimensional unit cell variation and Fe+3/Fe+4 redox activity of Li3FeN2 in Li-ion batteries

Li 3 FeN 2 displays rich and complex structural response upon electrochemical oxidation/reduction. During the first lithium deintercalation, 4 voltage plateaus corresponding to a total charge transfer of 1.14 e − per iron cation take place. Combining operando Mossbauer spectroscopy and X-ray diffraction, we evidence 3 biphasic reactions involving four orthorhombic phases. Despite a derived anti-fluorine type structure, Li 3 FeN 2 oxidation induces a unidirectional contraction along b axis. Mossbauer spectroscopy established a partial iron oxidation (∼90%). Therefore the participation of the nitrogen network as additional redox center is suggested to explain the observed extra capacity. Moreover, an unexpected low spin to high spin crossover took place for ∼10% of Fe 3+ during the oxidation of Li 3 FeN 2 . Based on the cationic mixing recently demonstrated and the anisotropic structural response, two possible explanations are discussed; (i) a significant deformation of 8 g lithium sites, which contain ∼10% of iron cations or (ii) migration of these cations into the neighboring octahedron 8 j . This High Spin Fe +3 contribution remains almost constant until the end of the oxidation.