<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

<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 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

Microstructure Evolution and Conversion Mechanism of Mn₃O₄ under Electrochemical Cyclings

Probing the microstructure evolution, phase change, and fundamental conversion mechanism of anodes for lithium ion batteries (LIBs) during lithiation–delithiation cycles is important to gain insights into understanding how the electrode works and thus how it can be improved. The electrochemical reaction and phase evolution of Mn₃O₄ during lithiation–delithiation cycles remain unknown. To observe the real-time electrochemical behaviors of Mn₃O₄ during lithiation–delithiation cycles, a nanosized LIB was constructed inside a transmission electron microscope (TEM) using an individual Mn₃O₄/graphene moiety as the anode. Upon the first lithiation, Mn₃O₄ nanoparticles are lithiated into the crystallized Mn nanograins embedded within the Li₂O matrix. However, Mn and Li₂O cannot be recovered to the original Mn₃O₄ phase but to MnO after the first full delithiation, which results in an irreversible phase transformation. Such incomplete conversion reaction accounts for the huge capacity fading during the first cycle of Mn₃O₄-based LIBs. Excellent cyclability between Mn and MnO is also established during the subsequent lithiation–delithiation cycles, which is beneficial to the capacity retention in real battery. It provides an in-depth understanding of the phase evolution and conversion mechanism of Mn₃O₄ during lithiation–delithiation and holds the promise of improving the capacity for the development of durable, high-capacity, and high-rate anodes for LIBs

Microstructure Evolution and Conversion Mechanism of Mn₃O₄ under Electrochemical Cyclings

Probing the microstructure evolution, phase change, and fundamental conversion mechanism of anodes for lithium ion batteries (LIBs) during lithiation–delithiation cycles is important to gain insights into understanding how the electrode works and thus how it can be improved. The electrochemical reaction and phase evolution of Mn₃O₄ during lithiation–delithiation cycles remain unknown. To observe the real-time electrochemical behaviors of Mn₃O₄ during lithiation–delithiation cycles, a nanosized LIB was constructed inside a transmission electron microscope (TEM) using an individual Mn₃O₄/graphene moiety as the anode. Upon the first lithiation, Mn₃O₄ nanoparticles are lithiated into the crystallized Mn nanograins embedded within the Li₂O matrix. However, Mn and Li₂O cannot be recovered to the original Mn₃O₄ phase but to MnO after the first full delithiation, which results in an irreversible phase transformation. Such incomplete conversion reaction accounts for the huge capacity fading during the first cycle of Mn₃O₄-based LIBs. Excellent cyclability between Mn and MnO is also established during the subsequent lithiation–delithiation cycles, which is beneficial to the capacity retention in real battery. It provides an in-depth understanding of the phase evolution and conversion mechanism of Mn₃O₄ during lithiation–delithiation and holds the promise of improving the capacity for the development of durable, high-capacity, and high-rate anodes for LIBs

Microstructure Evolution and Conversion Mechanism of Mn₃O₄ under Electrochemical Cyclings

Probing the microstructure evolution, phase change, and fundamental conversion mechanism of anodes for lithium ion batteries (LIBs) during lithiation–delithiation cycles is important to gain insights into understanding how the electrode works and thus how it can be improved. The electrochemical reaction and phase evolution of Mn₃O₄ during lithiation–delithiation cycles remain unknown. To observe the real-time electrochemical behaviors of Mn₃O₄ during lithiation–delithiation cycles, a nanosized LIB was constructed inside a transmission electron microscope (TEM) using an individual Mn₃O₄/graphene moiety as the anode. Upon the first lithiation, Mn₃O₄ nanoparticles are lithiated into the crystallized Mn nanograins embedded within the Li₂O matrix. However, Mn and Li₂O cannot be recovered to the original Mn₃O₄ phase but to MnO after the first full delithiation, which results in an irreversible phase transformation. Such incomplete conversion reaction accounts for the huge capacity fading during the first cycle of Mn₃O₄-based LIBs. Excellent cyclability between Mn and MnO is also established during the subsequent lithiation–delithiation cycles, which is beneficial to the capacity retention in real battery. It provides an in-depth understanding of the phase evolution and conversion mechanism of Mn₃O₄ during lithiation–delithiation and holds the promise of improving the capacity for the development of durable, high-capacity, and high-rate anodes for LIBs

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