Orthorhombic and monoclinic modifications of FeNb11O29, as promising anode materials for lithium batteries: Relationships between pseudocapacitive behaviour and structure

An intriguing and promising anode material for Li ion batteries, FeNb11O29, is emerging in the last few years. Its main strength lies into the possibility to exchange up to 23 electrons per formula unit, thanks to the redox reactions involving Fe3þ and Nb5þ ions, reaching a theoretical capacity value of 400 mAh/g. In this paper, a systematic study to determine the effect of carbon type and polymorph (monoclinic or orthorhombic) used to prepare the anode ink on the electrochemical performances was carried out. The best high rate capacity values (250 mAh/g at 10C after 100 cycles) and structural cycling stability (about 96.4%) were obtained for the orthorhombic polymorph mixed with Super C65 carbon. For the intercalation/ deintercalation process, a prevailing of pseudocapacitive behaviour with respect to the diffusive one was evidenced, especially at higher C rate, that is related to the peculiar structural aspects of FeNb11O29 shear structure.