Mitsunori Kitta

Study of Surface Reaction of Spinel Li4Ti5O12 during the First Lithium Insertion and Extraction Processes Using Atomic Force Microscopy and Analytical Transmission Electron Microscopy

Spinel lithium titanate (Li(4)Ti(5)O(12), 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 Li(4)Ti(5)O(12) surface during the charge-discharged (lithium insertion and extraction) processes. The AFM observation revealed that irreversible structural changes of an atomically flat Li(4)Ti(5)O(12) surface occurs at the early stage of the first lithium insertion process, which induces the reduction of charge transfer resistance at the electrolyte/Li(4)Ti(5)O(12) 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-Li(2)TiO(3). Although the transformation of Li(4)Ti(5)O(12) to Li(7)Ti(5)O(12) 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).

Atomic and electronic structures of Li4Ti5O12/Li7Ti5O12 (001) interfaces by first-principles calculations

We have performed density-functional theory calculations for Li4Ti5O12/Li7Ti5O12 (LTO/Li-LTO) interfaces and made a detailed analysis of the local atomic and electronic structures. In the bulk regions of the supercell, the atomic and electronic structures are well reproduced to be similar to those of the LTO and Li-LTO bulk crystals. The present (001) interface models show abrupt structural changes between the cubic spinel-based LTO and ordered rock-salt Li-LTO phases, while there occur no substantial strains around the interface due to the little volume change or lattice mismatch. Thus, the calculated interfacial energy is very small. The calculated O–K electron energy-loss near-edge structure/X-ray adsorption near-edge structure (ELNES/XANES) spectra in the bulk regions are similar to those of the bulk crystals, while the O–K edge spectra at the two kinds of interfaces have specific shapes, differently from the simple superposition of the bulk spectra. The preferential occurrence of the (001) interface can be understood from the preferential Li diffusion along the [110] direction in LTO and the small interfacial energy.

Synthesis of Highly Active Sub-Nanometer Pt@Rh Core–Shell Nanocatalyst via a Photochemical Route: Porous Titania Nanoplates as a Superior Photoactive Support

Sub-nanometer Pt@Rh nanoparticles highly dispersed on MIL-125-derived porous TiO2 nanoplates are successfully prepared for the first time by a photochemical route, where the porous TiO2 nanoplates with a relatively high specific surface area play a dual role as both effective photoreductant and catalyst support. The resulting Pt@Rh/p-TiO2 can be utilized as a highly active catalyst.

Encapsulating highly catalytically active metal nanoclusters inside porous organic cages

The creation of metal nanoclusters with dimensions ranging from subnanometre to ~2 nm for heterogeneous catalysis has received substantial attention. However, synthesizing these structures while retaining surface activity and avoiding aggregation is challenging. Here, we report a reverse double-solvents approach that enables encapsulation of highly catalytically active Pd nanoclusters inside the newly formed discrete organic molecular cage, RCC3. By encapsulating within the open cavities of soluble RCC3 cages, the obtained Pd nanocluster cores are produced with precisely controlled size (~0.72 nm) and show high solubility, excellent dispersibility and accessibility in solution, presenting significantly enhanced catalytic activities towards various liquid-phase catalytic reactions. Moreover, owing to the effective confinement of cage cavities, the as-prepared Pd nanoclusters possess excellent stability and durability. The strategy of encapsulation of metal nanoclusters within soluble porous organic cages is promising for developing stable and active catalysts.Small metal nanoclusters often display high catalytic activity, but also low stability due to aggregation. Here, Xu and co-workers show that subnanometre Pd clusters can be contained within porous organic cages. Not only do the particles retain high catalytic activity, they also show excellent solubility and stability.

Monodispersed Pt nanoparticles on reduced graphene oxide by a non-noble metal sacrificial approach for hydrolytic dehydrogenation of ammonia borane

Downsizing noble metal nanoparticles, such as Pt, is an essential goal for many catalytic reactions. A non-noble metal sacrificial approach was used to immobilize monodispersed Pt nanoparticles (NPs) with a mean size of 1.2 nm on reduced graphene oxide (RGO). ZnO co-precipitated with Pt NPs and subsequently sacrificed by acid etching impedes the diffusion of Pt atoms onto the primary Pt particles and also their aggregation during the reduction of precursors. The resulting ultrafine Pt nanoparticles exhibit high activity (a turnover frequency of 284 min−1 at 298 K) in the hydrolytic dehydrogenation of ammonia borane. The non-noble metal sacrificial approach is demonstrated as a general approach to synthesize well-dispersed noble metal NPs for catalysis.

First-principles calculations of O-K ELNES/XANES of lithium titanate

Electron energy-loss near-edge structure/x-ray absorption near-edge structure spectra of O-K edge for Li4Ti5O12 and Li7Ti5O12 have been calculated by the first-principles projector augmented wave method. The calculated spectra for various O sites in large Li32Ti40O96 and Li56Ti40O96 supercells with realistic site occupancies have two peaks in a lower energy region and three peaks in a higher energy region. The features of the averaged spectra of the two systems are consistent with recent experimental observations.

Real-Time Observation of Li Deposition on a Li Electrode with Operand Atomic Force Microscopy and Surface Mechanical Imaging

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.

True atomic-scale imaging of a spinel Li4Ti5O12(111) surface in aqueous solution by frequency-modulation atomic force microscopy

Spinel-type lithium titanium oxide (LTO; Li4Ti5O12) is a negative electrode material for lithium-ion batteries. Revealing the atomic-scale surface structure of LTO in liquid is highly necessary to investigate its surface properties in practical environments. Here, we reveal an atomic-scale image of the LTO(111) surface in LiCl aqueous solution using frequency-modulation atomic force microscopy. Atomically flat terraces and single steps having heights of multiples of 0.5 nm were observed in the aqueous solution. Hexagonal bright spots separated by 0.6 nm were also observed on the flat terrace part, corresponding to the atomistic contrast observed in the ultrahigh vacuum condition, which suggests that the basic atomic structure of the LTO(111) surface is retained without dramatic reconstruction even in the aqueous solution.

Stability of the LiMn2O4 surface in a LiPF6-based non-aqueous electrolyte studied by in-situ atomic force microscopy

Investigation of the surface stability of electrode materials in a liquid electrolyte is significantly important for understanding the deterioration of stored Li-ion battery cells. Here, we examined LiMn2O4 surfaces in a LiPF6-based non-aqueous electrolyte by in-situ atomic force microscopy. A LiMn2O4(111) surface sample with a well-defined atomically-flat structure was prepared from a MnO(111) wafer. Although the surfaces of non-exposed or dry-air-exposed samples did not change in a typical electrolyte such as LiPF6 dissolved in propylene carbonate, the surface morphology of an air-exposed sample greatly changed under the same condition. Transmission electron microscopy observation revealed that the surface roughness is increased by the dissolution of one or two atomic layers of LiMn2O4-crystal surfaces in the electrolyte. The adsorbed water on the air-exposed surface is the origin of this phenomenon.

Transmission electron microscopy investigation of the LiMn2O4/NaxMnO2 interface as a model study of a Na-ion battery electrode

The phase transformation from spinel LiMn2O4 to layered rock-salt NaxMnO2 via Na insertion-extraction cycles is crucial for a LiMn2O4 positive electrode in a Na-ion battery. To reveal the atomic-scale mechanism of the structural conversion, we applied advanced techniques of analytical electron microscopy to a Na-containing LiMn2O4 specimen, formed by lithiation of a thin MnO wafer containing Na impurity. Scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) analyses revealed that Na and Li are separately distributed in the two phases of the specimen, which are layered NaxMnO2 and spinel LiMn2O4 phases confirmed by annular bright field (ABF)-STEM observation. The large difference in the ionic radii of Na and Li is considered to be the reason for the clear phase separation without atomic-scale mixture. EDX analysis showed that the layered NaxMnO2 phase with P3 structure exhibits local variations in Na composition, with the ma...