Filling Carbon Nanotubes with Co₉S₈ Nanowires through in Situ Catalyst Transition and Extrusion

We describe the synthesis of novel Co9S8-nanowire-filled carbon nanotubes (CNTs) by a simple method involving the pyrolysis of thiophene on cobalt catalyst in a conventional chemical vapor decomposition system. The encapsulated Co9S8 nanowires are single-crystalline, and their lengths are about 10 μm with their [110] direction parallel to the axis of the CNTs. Detailed investigation suggests that the filling of the Co9S8 nanowires results from the volume increase induced by a phase transition from cobalt to cobalt sulfide together with the in situ extruding action of CNTs as nanomolds. A new filling mechanism is thus found and proposed

<i>In Situ</i> Transmission Electron Microscopy Observation of the Conversion Mechanism of Fe₂O₃/Graphene Anode during Lithiation–Delithiation Processes

Transition metal oxides have attracted tremendous attention as anode materials for lithium ion batteries (LIBs) recently. However, their electrochemical processes and fundamental mechanisms remain unclear. Here we report the direct observation of the dynamic behaviors and the conversion mechanism of Fe₂O₃/graphene in LIBs by <i>in situ</i> transmission electron microscopy (TEM). Upon lithiation, the Fe₂O₃ nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li₂O layer. Single-crystalline Fe₂O₃ nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li₂O matrix. Surprisingly, the delithiated product was not Fe₂O₃ but FeO, accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. The charge–discharge processes of Fe₂O₃ in LIBs are different from previously recognized mechanism, and are found to be a fully reversible electrochemical phase conversion between Fe and FeO nanograins accompanying the formation and disappearance of the Li₂O layer. The macroscopic electrochemical performance of Fe₂O₃/graphene was further correlated with the microcosmic <i>in situ</i> TEM results

<i>In Situ</i> Transmission Electron Microscopy Observation of the Conversion Mechanism of Fe₂O₃/Graphene Anode during Lithiation–Delithiation Processes

Transition metal oxides have attracted tremendous attention as anode materials for lithium ion batteries (LIBs) recently. However, their electrochemical processes and fundamental mechanisms remain unclear. Here we report the direct observation of the dynamic behaviors and the conversion mechanism of Fe₂O₃/graphene in LIBs by <i>in situ</i> transmission electron microscopy (TEM). Upon lithiation, the Fe₂O₃ nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li₂O layer. Single-crystalline Fe₂O₃ nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li₂O matrix. Surprisingly, the delithiated product was not Fe₂O₃ but FeO, accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. The charge–discharge processes of Fe₂O₃ in LIBs are different from previously recognized mechanism, and are found to be a fully reversible electrochemical phase conversion between Fe and FeO nanograins accompanying the formation and disappearance of the Li₂O layer. The macroscopic electrochemical performance of Fe₂O₃/graphene was further correlated with the microcosmic <i>in situ</i> TEM results

<i>In Situ</i> Transmission Electron Microscopy Observation of the Conversion Mechanism of Fe₂O₃/Graphene Anode during Lithiation–Delithiation Processes

Transition metal oxides have attracted tremendous attention as anode materials for lithium ion batteries (LIBs) recently. However, their electrochemical processes and fundamental mechanisms remain unclear. Here we report the direct observation of the dynamic behaviors and the conversion mechanism of Fe₂O₃/graphene in LIBs by <i>in situ</i> transmission electron microscopy (TEM). Upon lithiation, the Fe₂O₃ nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li₂O layer. Single-crystalline Fe₂O₃ nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li₂O matrix. Surprisingly, the delithiated product was not Fe₂O₃ but FeO, accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. The charge–discharge processes of Fe₂O₃ in LIBs are different from previously recognized mechanism, and are found to be a fully reversible electrochemical phase conversion between Fe and FeO nanograins accompanying the formation and disappearance of the Li₂O layer. The macroscopic electrochemical performance of Fe₂O₃/graphene was further correlated with the microcosmic <i>in situ</i> TEM results

<i>In Situ</i> Transmission Electron Microscopy Observation of the Conversion Mechanism of Fe₂O₃/Graphene Anode during Lithiation–Delithiation Processes

Transition metal oxides have attracted tremendous attention as anode materials for lithium ion batteries (LIBs) recently. However, their electrochemical processes and fundamental mechanisms remain unclear. Here we report the direct observation of the dynamic behaviors and the conversion mechanism of Fe₂O₃/graphene in LIBs by <i>in situ</i> transmission electron microscopy (TEM). Upon lithiation, the Fe₂O₃ nanoparticles showed obvious volume expansion and morphological changes, and the surfaces of the electrode were covered by a nanocrystalline Li₂O layer. Single-crystalline Fe₂O₃ nanoparticles were found to transform to multicrystalline nanoparticles consisting of many Fe nanograins embedded in Li₂O matrix. Surprisingly, the delithiated product was not Fe₂O₃ but FeO, accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. The charge–discharge processes of Fe₂O₃ in LIBs are different from previously recognized mechanism, and are found to be a fully reversible electrochemical phase conversion between Fe and FeO nanograins accompanying the formation and disappearance of the Li₂O layer. The macroscopic electrochemical performance of Fe₂O₃/graphene was further correlated with the microcosmic <i>in situ</i> TEM results

In Situ Transmission Electron Microscopy Observation of the Lithiation–Delithiation Conversion Behavior of CuO/Graphene Anode

The electrochemical conversion behavior of metal oxides as well as its influence on the lithium-storage performance remains unclear. In this paper, we studied the dynamic electrochemical conversion process of CuO/graphene as anode by in situ transmission electron microscopy. The microscopic conversion behavior of the electrode was further correlated with its macroscopic lithium-storage properties. During the first lithiation, the porous CuO nanoparticles transformed to numerous Cu nanograins (2–3 nm) embedded in Li₂O matrix. The porous spaces were found to be favorable for accommodating the volume expansion during lithium insertion. Two types of irreversible processes were revealed during the lithiation–delithiation cycles. First, the nature of the charge–discharge process of CuO anode is a reversible phase conversion between Cu₂O and Cu nanograins. The delithiation reaction cannot recover the electrode to its pristine structure (CuO), which is responsible for about ∼55% of the capacity fading in the first cycle. Second, there is a severe nanograin aggregation during the initial conversion cycles, which leads to low Coulombic efficiency. This finding could also account for the electrochemical behaviors of other transition metal oxide anodes that operate with similar electrochemical conversion mechanism

In Situ Transmission Electron Microscopy Observation of Electrochemical Behavior of CoS₂ in Lithium-Ion Battery

Metal sulfides are a type of potential anode materials for lithium-ion batteries (LIBs). However, their electrochemical behaviors and mechanism during the charge and discharge process remain unclear. In the present paper, we use CoS₂ as a model material to investigate their electrochemical process using in situ transmission electron microscopy (TEM). Two kinds of reaction behaviors are revealed. The pure CoS₂ particles show a side-to-side conversion process, in which large and anisotropic size expansion (47.1%) occurs that results in the formation of cracks and fractures in CoS₂ particles. In contrast, the CoS₂ particles anchored on reduced graphene oxide (rGO) sheets exhibit a core–shell conversion process involving small and homogeneous size expansion (28.6%) and few fractures, which attributes to the excellent Li⁺ conductivity of rGO sheets and accounts for the improved cyclability. Single-crystalline CoS₂ particle converts to Co nanocrystals of 1–2 nm embedded within Li₂S matrix after the first lithiation. The subsequent electrochemical reaction is a reversible phase conversion between Co/Li₂S and CoS₂ nanocrystals. Our direct observations provide important mechanistic insight for developing high-performance conversion electrodes for LIBs

Synthesis of Porous NiO-Wrapped Graphene Nanosheets and Their Improved Lithium Storage Properties

This article reports a facile preparation of NiO–graphene composite by the combination of a solution-based method and subsequent annealing. X-ray diffraction and electron microscopy reveals that the graphene nanosheets are uniformly wrapped by porous NiO nanosheets in the product. The composite shows highly improved electrochemical performance as anode for Li–ion batteries (LIBs). The NiO–graphene nanosheets deliver a first discharge capacity of 2169.6 mAh g–1 and remain a reversible capacity up to 704.8 mAh g–1 after 50 cycles at a current of 200 mA g–1 in half cells. Contrarily, the pristine NiO nanosheets show only a reversible capacity of 134 mA g–1 after 50 cycles. The NiO–graphene composite also exhibits ameliorative rate capacity of 402.6 mAh g–1 at the current of 1600 mA g–1. In particular, these novel nanostructured composites show exceptional capacity retention in the assembled NiO–graphene/LiNi1/3Mn1/3Co1/3O2 full cell at different current density. The enhanced electrochemical performances are ascribed to the stable sheet-on-sheet architectures and the synergistic effects between the conductive graphene and thin porous NiO nanosheets

Ultrathin SnO₂ Nanosheets: Oriented Attachment Mechanism, Nonstoichiometric Defects, and Enhanced Lithium-Ion Battery Performances

We successfully synthesized large-scale and highly pure ultrathin SnO₂ nanosheets (NSs), with a minimum thickness in the regime of ca. 2.1 nm as determined by HRTEM and in good agreement with XRD refinements and AFM height profiles. Through TEM and HRTEM observations on time-dependent samples, we found that the as-prepared SnO₂ NSs were assembled by “oriented attachment” of preformed SnO₂ nanoparticles (NPs). Systematic trials showed that well-defined ultrathin SnO₂ NSs could only be obtained under appropriate reaction time, solvent, additive, precursor concentration, and cooling rate. A certain degree of nonstoichiometry appears inevitable in the well-defined SnO₂ NSs sample. However, deviations from the optimal synthetic parameters give rise to severe nonstoichiometry in the products, resulting in the formation of Sn₃O₄ or SnO. This finding may open new accesses to the fundamental investigations of tin oxides as well as their intertransition processes. Finally, we investigated the lithium-ion storage of the SnO₂ NSs as compared to SnO₂ hollow spheres and NPs. The results showed superior performance of SnO₂ NSs sample over its two counterparts. This greatly enhanced Li-ion storage capability of SnO₂ NSs is probably resulting from the ultrathin thicknesses and the unique porous structures: the nanometer-sized networks provide negligible diffusion times of ions thus faster phase transitions, while the “breathable” interior porous structure can effectively buffer the drastic volume changes during lithiation and delithiation reactions

Lithiation Behavior of Individual Carbon-Coated Fe₃O₄ Nanowire Observed by in Situ TEM

Fe3O4 nanowires, as a typical transition-metal oxide (TMO), are being considered as promising anodes for lithium ion batteries (LIBs) due to their 1D structure and high specific capacity. However, their underlying mechanism and electrochemical behaviors are still poorly understood. Here, the dynamic behavior and the electrochemical reaction of the carbon-coated Fe3O4 (Fe3O4@C) nanowire have been investigated directly through assembling a nanoscale LIBs inside transmission electron microscope (TEM). The in situ TEM results reveal that the Fe3O4 nanowires undergo cracking and fracturing upon the first lithiation, but the carbon coatings still embrace the oxide cores well after lithiation and play a role in maintaining the mechanical and electrical integrity. Meanwhile the lithiation process involves the conversion of Fe3O4 nanowires to Fe nanograins and the formation of Li2O along the lithium ions diffusion direction. The delithiated product is FeO rather than the original phase of Fe3O4 after the first delithiation process. This irreversible phase conversion may be a major cause of capacity fading of the electrode in the first cycle. As for the Fe3O4 electrode, about 78% of the capacity loss can be attributed to the irreversible phase reaction in the first cycle. During the subsequent lithiation-delithiation cycles, the Fe3O4 electrode shows a reversible conversion between Fe and FeO nanograins, accounting for the good reversibility of Fe3O4 anodes for LIBs. Our in situ results provide important insights into the electrochemical behavior and conversion mechanism of TMO-based anodes in LIBs and are helpful for designing LIBs with outstanding performance