Electrochemical lithiation and delithiation for control of magnetic properties of nanoscale transition metal oxides

Abstract

Transition metal oxides comprise a fascinating class of materials displaying a variety of magnetic and electronic properties, ranging from half-metallic ferromagnets like CrO2, ferrimagnetic semiconductors like Fey's, and antiferromagnetic insulators like rocksalt-structured FeO. The accessibility of multiple electronic configurations and coordination of cations in these oxides enables the control of magnetism by external stimuli. One such stimulus is the insertion of Li+, as occurs during the discharge cycle of a lithium battery. This can lead to the change in valence and locations of the metal cations within the structure therefore a change in magnetic moment. Fey's and CrO2 are of considerable interest, primarily because they demonstrate room-temperature magnetism and high spin polarization.Previous studies focussed on use of these materials as cathodes and characterization of lithiated compounds made through solid state chemical synthesis or via chemical lithiation. In this work, changes in magnetization and structure of pulsed laser deposition (PLD)-grown Fey's (magnetite) thin films, Fe3O4 nanoparticles, and CrO2 nanoparticles have been investigated upon electrochemical lithiation. The reasonable electrical conductivity of magnetite opens the possibility of modifying the saturation magnetization by inserting Li+ ions into thin films grown on conducting substrates. A substantial decrease in M8 (up to 30%) was observed in PLD-grown thin films. Significantly larger reduction in moment (up to 75%) was observed in commercially available nanoparticles upon addition of 2 moles of Li per formula unit, along with changes in remanence and coercivity. The smaller drop in M8 observed in thin films is attributed to a kinetic effect due to high density and greater diffusion lengths in PLD-grown films.

(cont.) The electrochemical lithiation process has also been applied to needle-shaped particles of chromium dioxide and a model has been proposed to explain the observations. The effects of cycling and discharge-charge rate on these CrO2 particles have been studied. It has been shown that the process may be partially reversible for low Li contents. The effects of increasing the temperature of cycling and decreasing the length of the CrO2 particles have been explored. These changes in magnetic moment may be rendered useful in magnetomechanical or magnetoelectronic applications.