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

Abstract

Li3FeN2 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 Mössbauer spectroscopy and X-ray diffraction, we evidence 3 biphasic reactions involving four orthorhombic phases. Despite a derived anti-fluorine type structure, Li3FeN2 oxidation induces a unidirectional contraction along b axis. Mössbauer 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 Fe3+ during the oxidation of Li3FeN2. Based on the cationic mixing recently demonstrated and the anisotropic structural response, two possible explanations are discussed; (i) a significant deformation of 8g lithium sites, which contain ∼10% of iron cations or (ii) migration of these cations into the neighboring octahedron 8j. This High Spin Fe+3 contribution remains almost constant until the end of the oxidation.