Filled Carbon Nanotubes as Anode Materials for Lithium-Ion Batteries.

Downsizing well-established materials to the nanoscale is a key route to novel functionalities, in particular if different functionalities are merged in hybrid nanomaterials. Hybrid carbon-based hierarchical nanostructures are particularly promising for electrochemical energy storage since they combine benefits of nanosize effects, enhanced electrical conductivity and integrity of bulk materials. We show that endohedral multiwalled carbon nanotubes (CNT) encapsulating high-capacity (here: conversion and alloying) electrode materials have a high potential for use in anode materials for lithium-ion batteries (LIB). There are two essential characteristics of filled CNT relevant for application in electrochemical energy storage: (1) rigid hollow cavities of the CNT provide upper limits for nanoparticles in their inner cavities which are both separated from the fillings of other CNT and protected against degradation. In particular, the CNT shells resist strong volume changes of encapsulates in response to electrochemical cycling, which in conventional conversion and alloying materials hinders application in energy storage devices. (2) Carbon mantles ensure electrical contact to the active material as they are unaffected by potential cracks of the encapsulate and form a stable conductive network in the electrode compound. Our studies confirm that encapsulates are electrochemically active and can achieve full theoretical reversible capacity. The results imply that encapsulating nanostructures inside CNT can provide a route to new high-performance nanocomposite anode materials for LIB.

Enhanced magnetism and time-stable remanence at the interface of hematite and carbon nanotubes.

The interface of two dissimilar materials is well known for surprises in condensed matter, and provides avenues for rich physics as well as seeds for future technological advancements. We present some exciting magnetization (M) and remanence (µ) results, which conclusively arise at the interface of two highly functional materials, namely the graphitic shells of a carbon nanotube (CNT) and α-Fe2O3, a Dzyaloshinskii-Moriya interaction driven weak ferromagnet (WFM) and piezomagnet (PzM). We show that the encapsulation inside a CNT leads to a significant enhancement in M and correspondingly in µ, a time-stable part of the remanence, exclusive to the WFM phase. Up to 70% of in-field magnetization is retained in the form of µ at room temperature. The lattice parameter of the CNT around the Morin transition of the encapsulate exhibits a clear anomaly, confirming the novel interface effects. Control experiments on bare α-Fe2O3 nanowires bring into the fore that the weak ferromagnets such as α-Fe2O3 are not as weak, as far as their remanence and its stability with time is concerned, and encapsulation inside a CNT leads to a substantial enhancement in these functionalities.

Weak ferromagnetism and time-stable remanence in hematite: effect of shape, size and morphology.

A number of Dzyaloshinskii-Moriya interaction (DMI) driven canted antiferromagnets or weak ferromagnets (WFM) including hematite exhibit two distinct time scales in magnetization relaxation measurements, one of which is ultra-slow. This leads to the observation of a part of remanence that is time-stable in character. In this work, our endeavor is to optimize the magnitude of this time-stable remanence for the hematite, a room temperature WFM, as a function of shape size and morphology. A substantial enhancement in the magnitude of this unique remanence is observed in porous hematite, consisting of ultra-small nano particles, as compared to crystallites grown in regular morphology, such as cuboids or hexagonal plates. This time-stable remanence exhibits a peak-like pattern with magnetic field, which is significantly sharper in porous sample. Experimental data suggest that the extent and the magnitude of the spin canting associated with the WFM phase can be best gauged by the presence of this remanence and its unusual magnetic field dependence. Temperature variation of lattice parameters bring out correlations between strain effects that alter the bond length and bond angle associated with primary super exchange paths, which in-turn systematically alter the magnitude of the time-stable remanence. This study provides insights regarding a long standing problems of anomalies in the magnitude of magnetization on repeated cooling in case of hematite. Our data caps on these anomalies, which we argue, arise due to spontaneous spin canting associated with WFM phase. Our results also elucidate on why thermal cycling protocols during bulk magnetization measurements are even more crucial for hematite which exhibits both canted as well as pure antiferromgnetic phase.