The development of a nanoscale battery reaction in an electrode material associated with in situ microscopic observation is significant to an understanding of the solid-state mechanism of a battery reaction. With a Li4Ti5O12 (LTO) crystal as the negative electrode of a Li-ion battery (LIB), we show that a nanoscale-controlled Li-insertion reaction can be produced by electron beam irradiation with scanning transmission electron microscopy (STEM). A selected area in a Li2O-coated thin LTO crystal was irradiated by the electron probe of STEM with a high beam intensity of 2.5 × 107 (electrons per nm2). Electron energy-loss spectroscopy (EELS) revealed that significant changes in the chemical feature occurred only in the high-dose irradiation area in the LTO specimen. The features of Li-K, Ti-L and O-K spectra in that area were completely equal to those of a Li7Ti5O12 (Li-LTO) phase, as an electrochemically Li-inserted LTO phase, in contrast to usual LTO-like spectra in the region surrounding the specimen. For a pristine LTO specimen without Li2O coating, no Li-insertion reaction was observed under the same irradiation conditions. The high-dose electron beam seems to induce the dissociation of Li2O, providing Li ions and electrons, and the rapid and directional growth of a Li-LTO phase along the electron beam in the LTO specimen, forming a nanoscale steep interface with the surrounding LTO phase. The present phenomenon is a new type of electron beam assisted chemical reaction in a solid state, and could have a large impact on the science and technology of battery materials.

Nanoscale investigations of Li deposition on the surface of a Li electrode are crucial to understand the initial mechanism of dendrite growth in rechargeable Li-metal batteries during charging. Here, we studied the initial Li deposition and related protrusion growth processes at the surface of the Li electrode with atomic force microscopy (AFM) in a galvanostatic experiment under operand condition. A flat Li-metal surface prepared by precision cutting a Li-metal wire in electrolyte solution (100 mM LiPF6 in propylene carbonate) was observed with peak-force-tapping mode AFM under an inert atmosphere. During the electrochemical deposition process of Li, protrusions were observed to grow selectively. An adhesion image acquired with mechanical mapping showed a specifically small contrast on the surface of growing protrusions, suggesting that the heterogeneous condition of the surface of the Li electrode affects the growth of Li dendrites. We propose that a modification of the battery cell design resulting in a uniform solid–liquid interface can contribute to the homogeneous deposition of Li at the Li electrode during charging. Further, the mechanical mapping of Li surfaces with operand AFM has proven to play a significant role in the understanding of basic mechanisms of the behavior of the Li electrode.

Spinel lithium titanate (LTO; Li4Ti5O12) is one of the promising materials for negative electrodes of sodium-ion batteries (SIBs). The stable charge–discharge performance of SIB cells using LTO electrodes depends on the reversible Na insertion–extraction mechanism of LTO, where the spinel lattice is expanded with Na insertion, and two phases, Na-inserted LTO (Na-LTO) and Li-inserted LTO (Li-LTO) phases, are generated. These phases are confirmed using X-ray diffraction (XRD), while the mechanism of the two-phase coexistence with different lattice volumes is yet unclear. Here, we investigate the detailed morphology of the coexisting Na-LTO and Li-LTO phases using in situ XRD measurements and high-resolution transmission electron microscopy (TEM) observation. Na-LTO (a = 8.74 Å) and Li-LTO (a = 8.36 Å) phases are confirmed in both the electrochemically formed Na-inserted LTO electrode and the single-crystalline LTO thin specimen. We observed that the Na-LTO/Li-LTO interface is parallel to the (001) plane, and contains an inevitable lattice mismatch along the interface, while the expansion of the Na-LTO phase can be partially relaxed normal to the interface. We observed that the Na-LTO/Li-LTO interface has interface layers of lattice disordering with a 1–2 nm width, relaxing the lattice mismatch, as opposed to results from the previous scanning TEM observation. How the different lattice volumes at the two-phase interface are relaxed should be the key issue in investigation of the mechanism of Na insertion and extraction in LTO electrodes.

•Separation of Li4Ti5O12 and Li7Ti5O12 phases in a LTO crystal sample was identified by STEM–EELS.•Single crystalline LTO specimen for TEM analysis was prepared by Li-VIG method.•Two-phase boundary is formed parallel to the <011> direction by the preferential Li-ion diffusion.The coexistence state of two phases (Li4Ti5O12 and lithiated Li7Ti5O12) in spinel lithium titanium oxide (LTO; Li4Ti5O12) is examined using scanning transmission electron microscopy with electron energy-loss spectroscopy (STEM–EELS). A single crystalline LTO specimen is prepared from a TiO2 wafer and is then partially lithiated. STEM–EELS spectrum imaging of the lithiated specimen reveals that the two phases exist separately inside the specimen with phase boundaries, and no apparent misfit strains or misorientations are detected. The phase interface appears to be preferentially formed parallel to the (100) plane owing to preferential Li diffusion along the <011> and equivalent directions in the LTO crystal. The observed two-phase morphology can be explained by the fact that relative to solid solutions, the Li7Ti5O12 phase can accommodate inserted Li atoms with negligible lattice distortions or volume changes in the common Ti–O bond framework.

•Atomistic structure of Li4Ti5O12 (111) surface was investigated by STM and MEIS.•Spinel Li4Ti5O12 is a promising anode material for lithium-ion batteries.•Two kinds of Li-terminated structures with different stoichiometry were found by STM.•MEIS analysis revealed that the major part is an oxygen-rich surface.•The oxygen-rich surface should have electronic holes, affecting surface reactions.Spinel lithium titanate (Li4Ti5O12, LTO) is one of the promising anode materials for high-performance lithium-ion batteries (LIBs). It is crucial to investigate atomistic structures of LTO surfaces to understand the phenomena at LTO/electrolyte interfaces such as CO2-gas generation which greatly affects the performance and safety of LIBs. By applying scanning tunneling microscopy (STM) and medium energy ion scattering spectrometry (MEIS) to a LTO(111) film prepared from a TiO2 wafer, we found that there exist two kinds of Li-terminated (111) terraces bounded by steps with different heights. In the major terraces, the top hexagonal Li layer is stacked above the oxygen layer, while the top Li layer is stacked above the Ti–Li layer in the minor terraces. The relative stability between the two surface structures seems to depend on the atmosphere due to different stoichiometry. For the major terraces, the LTO surface should have electronic holes due to oxygen-rich stoichiometry, which is a possible origin of CO2 generation via redox interaction with electrolyte molecules.

Spinel LiMn2O4 is a very promising material for positive electrodes in a wide range of Li-ion battery applications due to its lower cost and more environmental friendliness than any other electrode materials. Although the bulk properties of LiMn2O4 have been studied intensively, there have been few reports about the structure and properties of LiMn2O4 surfaces in spite of the importance of solid/electrolyte interfaces. This is caused by the difficulty in preparing LiMn2O4 samples with accessible flat surfaces suitable for atomistic observations. To address this, we have successfully prepared a single crystalline LiMn2O4 film with an atomically flat surface by solid-state reaction from a MnO wafer with LiOH.H2O powder. X-ray diffraction (XRD) reveals the single crystalline growth of LiMn2O4 films depending on the orientation of a MnO wafer. Atomic force microscopy observations revealed that a LiMn2O4 (111) film has an atomically flat surface with steps of a {111} interlayer height. Electron energy-loss spectroscopy (EELS) study of the (111) film revealed that the sample consists of Li, Mn3+ and Mn4+ with a composition similar to LiMn2O4. The (111) film sample is also investigated by cyclic voltammetry and galvanostatic experiments, revealing that a crushed powder sample from the film has electrochemical activity as usual positive electrode material.Highlights► Single crystalline spinel LiMn2O4 film was prepared from MnO wafer. ► LiMn2O4(100), (110) and (111) films were formed on MnO(100), (110) and (111) wafers. ► Atomically flat terraces with steps of a minimum height of about 0.5 nm were confirmed on the (111) film surface. ► Prepared (111) film showed two-steps charge–discharge voltage profile that is typical electrochemical behavior of LiMn2O4.

•Distribution of Li4Ti5O12 and Li7Ti5O12 phases in a LTO secondary particle was clarified by STEM–EELS.•Li-inserted Li7Ti5O12 phases can be clearly identified by Li-K, Ti-L and O-K edge spectra.•Separated two-phase distribution was observed in a half Li-inserted secondary particle.•Li-insertion reaction should propagate between primary particles sequentially.Spinel lithium titanate, Li4Ti5O12 (LTO) is attracting much attention as an alternative material replacing carbon based anodes in lithium ion batteries, due to its high rate properties as well as its durability and safety. The coexistence of the two phases, spinel Li4Ti5O12 and rock salt Li7Ti5O12, during the charge–discharge reactions is the key issue for the understanding of reaction mechanism and the development of LTO electrodes with improved performance. However, the two-phase distribution morphology in real LTO electrodes has not yet been reported, due to the difficulty in identifying the Li-inserted Li7Ti5O12 phase with high resolutions. Thus we apply the scanning transmission electron microscopy with electron energy loss spectroscopy (STEM–EELS) using the latest equipment with high energy and spatial resolutions. We have successfully identified the presence of the Li7Ti5O12 phase in Li-inserted LTO secondary particles, by the EELS analysis of Li composition, Ti oxidation state, and local configurations around oxygen atoms, compared to the Li4Ti5O12 phase. We have also obtained the two-phase distribution image in 50% Li-inserted LTO secondary particles, where separate presence of both the phases is clearly revealed. This indicates the particle-by-particle reaction mechanism, where the rate determining process exists in the boundary between primary particles.Clear two-phase (Li4Ti5O12/Li7Ti5O12) distribution morphology in the electrochemical Li-inserted Li4Ti5O12 secondary particle suggested the particle-by-particle reaction mechanism.

Spinel lithium titanate (Li4Ti5O12, LTO) is a promising anode material for a lithium ion battery because of its excellent properties such as high rate charge–discharge capability and life cycle stability, which were understood from the viewpoint of bulk properties such as small lattice volume changes by lithium insertion. However, the detailed surface reaction of lithium insertion and extraction has not yet been studied despite its importance to understand the mechanism of an electrochemical reaction. In this paper, we apply both atomic force microscopy (AFM) and transmission electron microscopy (TEM) to investigate the changes in the atomic and electronic structures of the Li4Ti5O12 surface during the charge–discharged (lithium insertion and extraction) processes. The AFM observation revealed that irreversible structural changes of an atomically flat Li4Ti5O12 surface occurs at the early stage of the first lithium insertion process, which induces the reduction of charge transfer resistance at the electrolyte/Li4Ti5O12 interface. The TEM observation clarified that cubic rock-salt crystal layers with a half lattice size of the original spinel structure are epitaxially formed after the first charge–discharge cycle. Electron energy loss spectroscopy (EELS) observation revealed that the formed surface layer should be α-Li2TiO3. Although the transformation of Li4Ti5O12 to Li7Ti5O12 is well-known as the lithium insertion reaction of the bulk phase, the generation of surface product layers should be inevitable in real charge–discharge processes and may play an effective role in the stable electrode performance as a solid-electrolyte interphase (SEI).

The development of a nanoscale battery reaction in an electrode material associated with in situ microscopic observation is significant to an understanding of the solid-state mechanism of a battery reaction. With a Li4Ti5O12 (LTO) crystal as the negative electrode of a Li-ion battery (LIB), we show that a nanoscale-controlled Li-insertion reaction can be produced by electron beam irradiation with scanning transmission electron microscopy (STEM). A selected area in a Li2O-coated thin LTO crystal was irradiated by the electron probe of STEM with a high beam intensity of 2.5 × 107 (electrons per nm2). Electron energy-loss spectroscopy (EELS) revealed that significant changes in the chemical feature occurred only in the high-dose irradiation area in the LTO specimen. The features of Li-K, Ti-L and O-K spectra in that area were completely equal to those of a Li7Ti5O12 (Li-LTO) phase, as an electrochemically Li-inserted LTO phase, in contrast to usual LTO-like spectra in the region surrounding the specimen. For a pristine LTO specimen without Li2O coating, no Li-insertion reaction was observed under the same irradiation conditions. The high-dose electron beam seems to induce the dissociation of Li2O, providing Li ions and electrons, and the rapid and directional growth of a Li-LTO phase along the electron beam in the LTO specimen, forming a nanoscale steep interface with the surrounding LTO phase. The present phenomenon is a new type of electron beam assisted chemical reaction in a solid state, and could have a large impact on the science and technology of battery materials.

Spinel lithium titanate (LTO; Li4Ti5O12) is one of the promising materials for negative electrodes of sodium-ion batteries (SIBs). The stable charge–discharge performance of SIB cells using LTO electrodes depends on the reversible Na insertion–extraction mechanism of LTO, where the spinel lattice is expanded with Na insertion, and two phases, Na-inserted LTO (Na-LTO) and Li-inserted LTO (Li-LTO) phases, are generated. These phases are confirmed using X-ray diffraction (XRD), while the mechanism of the two-phase coexistence with different lattice volumes is yet unclear. Here, we investigate the detailed morphology of the coexisting Na-LTO and Li-LTO phases using in situ XRD measurements and high-resolution transmission electron microscopy (TEM) observation. Na-LTO (a = 8.74 Å) and Li-LTO (a = 8.36 Å) phases are confirmed in both the electrochemically formed Na-inserted LTO electrode and the single-crystalline LTO thin specimen. We observed that the Na-LTO/Li-LTO interface is parallel to the (001) plane, and contains an inevitable lattice mismatch along the interface, while the expansion of the Na-LTO phase can be partially relaxed normal to the interface. We observed that the Na-LTO/Li-LTO interface has interface layers of lattice disordering with a 1–2 nm width, relaxing the lattice mismatch, as opposed to results from the previous scanning TEM observation. How the different lattice volumes at the two-phase interface are relaxed should be the key issue in investigation of the mechanism of Na insertion and extraction in LTO electrodes.