Toyoki Okumura

Depth-resolved X-ray absorption spectroscopic study on nanoscale observation of the electrode–solid electrolyte interface for all solid state lithium ion batteries

Depth-resolved X-ray absorption spectroscopy (DR-XAS) measurements were performed for the direct observation of the chemical state and local structure at the LiCoO2 electrode–solid electrolyte model interface, which can contribute towards the enhancement of the power density in all solid-state lithium batteries. The charge transfer resistance, measured by AC impedance spectroscopy, of the LiCoO2 electrode–solid electrolyte interface decreased with the introduction of a NbO2 interlayer at the interface, while the resistance increased with ZrO2 and MoO2 interlayers. Using DR-XAS with a depth resolution of about 7 nm, the changes in electronic structure and local structure of the LiCoO2 electrode were clarified. The extended X-ray absorption fine structure of DR-XAS revealed that the introduction of the NbO2 layer is effective for restricting the large Co–O bond change at the interface during delithiation. This interlayer relieved the stress at the interface due to the volume change of LiCoO2 during delithiation and then decreased the activation energy for the charge transfer process.

Electronic and local structural changes with lithium-ion insertion in TiO2-B: X-ray absorption spectroscopy study

X-Ray absorption fine structure (XAFS) spectroscopy was carried out on submicron sized TiO2-B, which is one of the promising candidates for negative electrode materials, in order to clarify the electronic and local structural changes during its lithium-ion insertion process. From the extended X-ray absorption fine structure (EXAFS) results of lithiated LixTiO2-B, we propose the changes in lithium-ion insertion sites during electrochemical discharging. The lithium ions are inserted into the five-fold coordinated sites and/or distorted octahedral sites distributed at the vicinity of O layers parallel to the ab plane for x ≤ 0.5, while the lithium ions are accommodated into the five-fold coordinated site distributed at the vicinity of TiO2 layers parallel to the ab plane for x > 0.5. The interaction between the inserted lithium ion and TiO6 octahedra affects the lattice distortion of LixTiO2-B for x > 0.5, as well as the titanium reduction during discharging. It is suggested through the X-ray absorption near edge structure (XANES) spectroscopy of Ti K-, Ti L- and O K-edges that the distortion of TiO6 octahedra makes the hybridized state of O 2p and Ti 3d broad, and has a significant effect on the lithium-ion insertion properties of TiO2-B.

Effect of bulk and surface structural changes in Li5FeO4 positive electrodes during first charging on subsequent lithium-ion battery performance

Bulk and surface structural changes induced in a Li5FeO4 positive electrode with a defect anti-fluorite type structure are examined during its initial charge–discharge cycle by various synchrotron-radiation analysis techniques, with a view to determining the contribution of oxygen to its electrochemical properties. Bulk structural analyses including XRD, Fe K-edge XANES and EXAFS reveal that pseudo-cubic lithium iron oxides (PC-LFOs), in the form of LiαFe(4−α)+O2, are formed during the first charging process instead of the decomposition of pristine Li5FeO4. Moreover, the relative volume of this PC-LFO phase varies nonlinearly according to the charging depth. At the same time, the surface lithium compounds such as Li2O cover over the PC-LFO phase, which also contribute to the overall electrochemical reaction, as measured from the O K-edge XANES operating in a surface-sensitive total-electron yield mode. The ratio of these two different reaction mechanisms changes with the depth during the first charging process, with this tendency causing variation in the subsequent discharge capacity retention in relation to the depth of the charging electron and/or temperature of this “Li-rich” positive electrode. Indeed, such behaviour is noted to be very similar to the specific electrochemical properties of Li2MnO3.

Correlation of lithium ion distribution and X-ray absorption near-edge structure in O3- and O2-lithium cobalt oxides from first-principle calculation

First-principle calculation was employed to simulate X-ray absorption near-edge structure (XANES) spectroscopy of two typical types of lithium cobalt oxides to clarify the electronic and local structural changes during lithium-ion de-intercalation. The simulated Co K-edge XANES spectra agreed well with the observed spectra. The differences in the shape of the XANES spectra with structural symmetry and/or lithium contents in lithium cobalt oxides were analyzed based on the partial density of state (PDOS) of the excited energy level. First-principle calculation simulations revealed that the cobalt PDOS overlapped with nearby lithium PDOS via oxygen PDOS, and the overlap difference among various Li1−xCoO2 could be detected using both the experimental and theoretical Co K-edge XANES spectra.

Effect of average and local structures on lithium ion conductivity in La2/3−xLi3xTiO3

In order to investigate lithium ion conduction mechanism of the lithium ion–conducting perovskite oxides, La2/3−xLi3xTiO3 (x = 0.06–0.15) (LLT), their average structure, local structure and lithium ion conductivity were analyzed. We applied Rietveld analysis with X-ray diffraction for discussing the average structure of LLT with disordered (high temperature phase, HT) and ordered (low temperature phase, LT) A-site ion arrangements along the c-axis of its orthorhombic lattice. Synchrotron X-ray absorption fine structure (XAFS) analysis was also measured for considering local structures. Differences for lithium ion migration caused by A-site ion ordering or change of lithium ion concentration are well-explained by the atomic positions and Coulombic interactions among the lithium and surrounding ions. The local inter-atomic distances were estimated by extended XAFS (EXAFS), as well as Debye Waller factors that reflect local distortion around the ions of interest. Consideration on these local structure features and the activation energy for the lithium ion conduction indicates that the lithium ion conductivity is governed by the Coulombic repulsion force between the lithium ion and the titanium ion, and/or the bottleneck distortion in the lithium ion channels consisted of four oxygen ions.

Improvement of lithium ion conductivity for A-site disordered lithium lanthanum titanate perovskite oxides by fluoride ion substitution

Lithium ion conducting perovskite oxides, namely La0.56−yLi0.33+3yTiO3 (y = 0, 0.01, 0.02, 0.04) with various La3+ ion concentrations and La0.56−yLi0.33TiO3−3yF3y (y = 0.01, 0.017, 0.033, 0.05) with various F− ion concentrations, were prepared. When y = 0.017, the Li-ion conductivity of La0.56−yLi0.33TiO3−3yF3y was 2.30 × 10−3 S cm−1 at 30 °C, which is one of the highest Li-ion conductivities in these systems. The F− substitution was effective for decreasing the activation energy for lithium ion migration. To understand the effect of F− ion substitution on lithium ion conduction, synchrotron X-ray absorption fine structure (XAFS) techniques were carried out. It was found with the XAFS analysis that the local distances around Ti4+ ions increased and the Debye–Waller factors decreased with F− substitution. These local structural changes were expected to expand the bottleneck for lithium ion hopping and to decrease the activation energy.

Further findings of X-ray absorption near-edge structure in lithium manganese spinel oxide using first-principles calculations

X-ray absorption near-edge structure (XANES) spectroscopy, which reveals the features of the electronic and local structure, of lithium manganese oxides LixMn2O4 (x = 0–2) was examined using first-principles calculations. Both the easily observable parts and the tiny peaks of the theoretical Mn K-edge XANES spectra agreed with the experimental spectra. From the theoretical results of two anti-ferromagnetic LiMn2O4 models, the contributions of the Mn3+ ion and Mn4+ ion centers to the XANES spectra differ due to the difference in the overlap between the Mn 4p partial density of state (PDOS) and the O 2p PDOS. Similar results can be also seen by comparing the theoretical XANES spectra and the PDOS between Li(Mn3+Mn4+)O4 and de-intercalated Li0.5(Mn3+0.5Mn4+1.5)O4 and Mn4+2O4 (λ-MnO2). The XANES spectral changes with the lithium ion displacement (six- to four-coordination) due to the phase transition (cubic Fdm LiMn2O4 to tetragonal I41/amd Li2Mn2O4) can be determined by the indirect contribution of the Li 2p PDOS to the Mn 4p PDOS via the O 2p PDOS.

Computational Simulations of Li Ion Conduction in (Li,La)TiO3

The locations and local environments of the Li ions in La0.56Li0.33TiO3 have been investigated by classical molecular dynamics (MD) simulations and first-principles (FP) calculations. The pair correlation functions of Li-O and Li-Ti indicate that the Li ions are located somewhat broadly mainly in the vicinity of the midpoint between the center of the A-site and the center of the bottleneck formed by four O2-. This is consistent well with that suggested from previous neutron diffraction and 6Li-NMR studies. The FP calculations suggest a different location of the Li ion in the vicinity of the midpoint between the centers of two adjcent bottlenecks; however it coincides with one of the locations shown by the trajectories simulated with the MD calculations.

Classical Molecular Dynamics Simulations on Fast Li Ion Conduction in (Li,La)TiO3

In order to reproduce the observed ionic conductivities and activation energies computationally, the potential parameters (PMs) were optimized for classical molecular dynamic simulations on Li ion conduction in the A-site deficient perovskite solid solution La056Li0.33TiO3 with disordered A-site ion arrangement. By the use of the optimized PMs, the conductivities and the activation energies were improved considerably from 4.1×10-3 Scm-1 to 4.4×10-2 Scm-1 at 800 K and 0.02 eV to 0.2 eV, respectively. The pair correlation functions calculated with the optimized PMs reveal that the Li-ions are located somewhat broadly mainly in the vicinity of the midpoint between the center of the A-site and the center of the bottleneck formed by four O2-, and that the simulated Li location is significantly related to the conductivity.

Rechargeable potassium-ion batteries with honeycomb-layered tellurates as high voltage cathodes and fast potassium-ion conductors

Rechargeable potassium-ion batteries have been gaining traction as not only promising low-cost alternatives to lithium-ion technology, but also as high-voltage energy storage systems. However, their development and sustainability are plagued by the lack of suitable electrode materials capable of allowing the reversible insertion of the large potassium ions. Here, exploration of the database for potassium-based materials has led us to discover potassium ion conducting layered honeycomb frameworks. They show the capability of reversible insertion of potassium ions at high voltages (~4 V for K2Ni2TeO6) in stable ionic liquids based on potassium bis(trifluorosulfonyl) imide, and exhibit remarkable ionic conductivities e.g. ~0.01 mS cm−1 at 298 K and ~40 mS cm–1 at 573 K for K2Mg2TeO6. In addition to enlisting fast potassium ion conductors that can be utilised as solid electrolytes, these layered honeycomb frameworks deliver the highest voltages amongst layered cathodes, becoming prime candidates for the advancement of high-energy density potassium-ion batteries.The development of potassium-ion batteries requires cathode materials that can maintain the structural stability during cycling. Here the authors have developed honeycomb-layered tellurates K2M2TeO6 that afford high ionic conductivity and reversible intercalation of large K ions at high voltages.