ABSTRACT
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.
Subject(s)
Electrochemical Techniques/methods , Ions/chemistry , Lithium/chemistry , Nanotubes, Carbon/chemistry , Cobalt/chemistry , Electric Conductivity , Electric Power Supplies , Electrodes , Ferric Compounds/chemistry , Manganese Compounds/chemistry , Microscopy, Electron, Scanning , Microscopy, Electron, Transmission , Nanocomposites/chemistry , Nanocomposites/ultrastructure , Nanotubes, Carbon/ultrastructure , Oxides/chemistry , Tin/chemistryABSTRACT
The ferrimagnetic and high-capacity electrode material Mn3O4 is encapsulated inside multi-walled carbon nanotubes (CNT). We show that the rigid hollow cavities of the CNT enforce size-controlled nanoparticles which are electrochemically active inside the CNT. The ferrimagnetic Mn3O4 filling is switched by electrochemical conversion reaction to antiferromagnetic MnO. The conversion reaction is further exploited for electrochemical energy storage. Our studies confirm that the theoretical reversible capacity of the Mn3O4 filling is fully accessible. Upon reversible cycling, the Mn3O4@CNT nanocomposite reaches a maximum discharge capacity of 461 mA h g-1 at 100 mA g-1 with a capacity retention of 90% after 50 cycles. We attribute the good cycling stability to the hybrid nature of the nanocomposite: (1) Carbon encasements ensure electrical contact to the active material by forming a stable conductive network which is unaffected by potential cracks of the encapsulate. (2) The CNT shells resist strong volume changes of the encapsulate in response to electrochemical cycling, which in conventional (i.e., non-nanocomposite) Mn3O4 hinders the application in energy storage devices. Our results demonstrate that Mn3O4 nanostructures can be successfully grown inside CNT and the resulting nanocomposite can be reversibly converted and exploited for lithium-ion batteries.
ABSTRACT
The transport length ltr and the mean free path le are determined for bulk and surface states in a Bi2Se3 nanoribbon by quantum transport and transconductance measurements. We show that the anisotropic scattering of spin-helical Dirac fermions results in a strong enhancement of ltr (≈ 200 nm) and of the related mobility µtr (≈ 4000 cm2 V-1 s-1), which confirms theoretical predictions.1 Despite strong disorder, the long-range nature of the scattering potential gives a large ratio ltr/le ≈ 8, likely limited by bulk/surface coupling. This suggests that the spin-flip length lsf ≈ ltr could reach the micron size in materials with a reduced bulk doping and paves the way for building functionalized spintronic and ballistic electronic devices out of disordered 3D topological insulators.
ABSTRACT
Shubnikov-de Haas oscillations were studied under high magnetic field in Bi2Se3 nanostructures grown by chemical vapor transport, for different bulk carrier densities ranging from 3 × 10(19) cm(-3) to 6 × 10(17) cm(-3). The contribution of topological surface states to electrical transport can be identified and separated from bulk carriers and massive two-dimensional electron gas. Band bending is investigated, and a crossover from upward to downward band bending is found at low bulk density as a result of a competition between bulk and interface doping. These results highlight the need to control electrical doping both in the bulk and at interfaces in order to study only topological surface states.
