Highly selective CO2 electrolysis in aqueous media by a water-soluble cobalt dimethyl-bipyridine complex.

A water-soluble Co complex with dimethyl-bipyridine ligands reduced CO2 to CO electrochemically with almost 100% selectivity at -0.80 V vs. NHE in an aqueous medium (pH 6.8) without an organic solvent. The reaction overpotential was 270 mV. A possible CO formation mechanism was discussed based on experiments and calculations.

Reversible CO2 Fixation and Release by a Trinuclear Zn(II) Cryptate Complex and Operando Analysis of the Complex Structure.

Metal complexes inspired by carbonic anhydrase (CA), which is a metalloenzyme containing Zn(II), have been investigated as alternatives for CO2 fixation systems operating under ambient temperature and pressure conditions. In this study, we designed a trinuclear Zn(II) cryptate complex (Zn3 L) and demonstrated rapid CO2 fixation with carbonation of CO2 using Zn3 L. The CO2 fixation performance of Zn3 L surpassed that of a standard CO2 absorbent, KOH(aq) solution, under conditions of the same solute concentration. In addition, the reaction achieved operation without support addition of base, which has been often required in systems of CA-inspired complexes. Fixed CO2 was released by protonating polyazacryptate ligand (L) and breaking the complex structure, and deprotonation of L induced the reconstruction of Zn3 L, allowing it to refix CO2 . This reaction mechanism was proposed based on the analysis of operando extended X-ray absorption fine structure spectroscopy. Zn3 L also demonstrated the ability to capture dilute CO2 from air, and the volume of CO2 captured by Zn3 L was approximately 2.6 times that captured by the KOH(aq) solution. Our Zn3 L exhibited three valuable properties: rapid CO2 fixation without a base, reversibility, and ability to capture dilute CO2 ; thus Zn3 L is a promising candidate as CO2 fixatives.

Toward Improving the Thermal Stability of Negative Electrode Materials: Differential Scanning Calorimetry and In Situ High-Temperature X-ray Diffraction/X-ray Absorption Spectroscopy Studies of Li2ZnTi3O8 and Related Compounds.

Negative electrode materials with high thermal stability are a key strategy for improving the safety of lithium-ion batteries for electric vehicles without requiring built-in safety devices. To search for crucial clues into increasing the thermal stability of these materials, we performed differential scanning calorimetry (DSC) and in situ high-temperature (HT)-X-ray diffraction (XRD)/X-ray absorption (XAS) up to 450 °C with respect to a solid-solution compound of Li4/3-2x/3ZnxTi5/3-x/3O4 with 0 ≤ x ≤ 0.5. The DSC profile of fully discharged x = 0.5 (Li2ZnTi3O8) with a LiPF6-based electrolyte could be divided into three temperature (T) regions: (i) T ≤ 250 °C for ΔHaccumi, (ii) 250 °C < T ≤ 350 °C for ΔHaccumii, and (iii) T > 350 °C for ΔHaccumiii, where ΔHaccumn is the accumulated change in enthalpy in region n. The HT-XRD/XAS analyses clarified that ΔHaccumi and ΔHaccumii originated from the decomposition of solid electrolyte interphase (SEI) films and the formation of a LiF phase, respectively. Comparison of the DSC profiles with x = 0 (Li[Li1/3Ti5/3]O4) and graphite revealed the operating voltage, i.e., amount of SEI films, and stability of the crystal lattice play significant roles in the thermal stability of negative electrode materials. Indeed, the highest thermal stability was attained at x = 0.25 using this approach.

Dynamic Structural Transformations in a Series of Zero-Strain Lithium-Ion Battery Materials: Almost Simultaneous Operando X-ray Diffraction and X-ray Absorption Spectroscopy on Li2ZnTi3O8 and Related Compounds.

A series of Li4/3-2x/3ZnxTi5/3-x/3O4 (LZTO) with 0 ≤ x ≤ 0.5 have received considerable interest as a negative electrode material for long-cycle-life lithium-ion batteries. However, their dynamic structural transformations under operating conditions have remained unknown, making an in-depth understanding essential for further improving the electrochemical performance. We, thus, performed almost simultaneous operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies on x = 0.125, 0.375, and 0.5. The x = 0.5 sample, Li2ZnTi3O8, indicated differences in the cubic lattice parameter between the discharge and charge reactions (δacs), corresponding to the reversible movement of Zn2+ ions between the tetrahedral and octahedral sites. δac was also observed for x = 0.125 and 0.375, although the capacity region exhibiting δac decreased with a decrease in x. For all of the samples, there is no significant difference in the nearest-neighbor distance of the Ti-O bond (dTi-O) between the discharge and charge reactions. We also demonstrated different structural transformations between the micro- (XRD) and atomic (XAS) scales. In the case for x = 0.5, for instance, the maximum microscale change in ac was within +0.29(3)%, whereas at the atomic scale, dTi-O changed by up to +4.8(3)%. Combined with our previous results for ex situ XRD and operando XRD/XAS measurements on other x compositions, the whole structural nature of LZTO, such as correspondence between ac and dTi-O, origins for voltage hysteresis, and zero-strain reaction mechanisms, has been unveiled.

Positive Feedback Mechanism to Increase the Charging Voltage of Li-O2 Batteries.

The large overpotential of nonaqueous Li-O2 batteries when charging causes low round-trip efficiency and decomposition of the electrode materials and electrolyte. The origins of this overpotential have been enthusiastically explored to date; however, a full understanding has not yet been reached because of the complexity of multistep reaction mechanisms. Here, we applied structural and electrochemical analysis techniques to investigate the reaction step that results in the increase of the overpotential when charging. Rietveld refinement of ex situ powder X-ray diffraction showed that a Li-deficient phase of Li2O2, Li2-xO2, formed when discharging and was present over the course of charging. The galvanostatic intermittent titration technique revealed that the rate-determining process in the first step of charging was a solid-solution type of delithiation. The chemical diffusion coefficient of Li+ ions in Li2-xO2, DLi, decreases as the cell voltage increases, which in turn leads to a decrease in the oxidation rate of Li2-xO2. Under galvanostatic conditions, the deceleration of oxidation induces further increase of the cell voltage; therefore, an intrinsic mechanism of positive feedback to increase the cell voltage occurs in the first step. The results demonstrate that the continuity of the first step can be extended by the suppression of changes in any of the elements of the positive feedback loop, i.e., the oxidation rate, cell voltage, or DLi.

Development of an in situ high-temperature X-ray diffraction technique for lithium-ion battery materials.

The development of an in situ high-temperature X-ray diffraction technique for lithium-ion battery materials is crucial for understanding the detailed mechanism of thermal runaway. We realized such a technique and employed it on a C6Lix electrode with an LiPF6-based electrolyte, thereby revealing multiple transformations through several intermediate stages, i.e., C6Li → C12Li → C18Li/C24Li → C36Li → C6, which could be helpful to improve the thermal stability.

Thermal Behavior of Li1+x[Li1/3Ti5/3]O4 and a Proof of Concept for Sustainable Batteries.

An in-depth understanding of the thermal behavior of lithium-ion battery materials is valuable for two reasons: one is to devise strategies for inhibiting the risk of catastrophic thermal runaway and the other is to respond to the increasing demand for sustainable batteries using a direct regeneration method. Li1+x[Li1/3Ti5/3]O4 (LTO) is regarded as a suitable negative electrode under the type of severe conditions that cause this thermal runaway, such as in ignition systems for automobiles. Thus, in this study, we used differential scanning calorimetry to systematically analyze lithiated LTO combined with ex situ and in situ high-temperature X-ray diffraction measurements. The observed thermal reactions with a LiPF6-based electrolyte were divided into three processes: (i) the decomposition of the initially formed solid electrolyte interphase below 200 °C, (ii) the formation of a LiF phase at 200 °C ≤ T ≤ 340 °C, and (iii) the formation of a TiO2 phase at T > 340 °C. Because the enthalpy change in process (ii) mainly contributed to the total heat generation, fluorine-free Li salts and/or stabilization of the LTO lattice may be effective in coping with the thermal runaway. Even in various lithiated states, a direct regeneration method returned the discharge capacity of LTO to ∼90% of its initial value, if we ignore the contributions from the electrochemically inactive LiF and TiO2 rutile phases. Hence, it can be concluded that the recycling performance of LTO is far superior to those of lithium transition metal oxides for a positive electrode, whose delithiated states easily convert into electrochemical-inactive phases at high temperatures.

Operando X-ray Diffraction and Double-Edge X-ray Absorption Spectroscopy Studies on a Perfect Zero-Strain Material.

"Zero-strain" insertion materials are essential for high-performance Li-ion batteries, but the experimental determination of changes in their local structures remains challenging. In this study, we successfully visualized the reaction scheme of a perfect zero-strain material, (Li0.75Zn0.25)[Li0.417Ti1.583]O4 with a spinel framework, using operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The operando XRD/XAS technique, which provided a series of XRD, Ti K-edge XAS, and Zn K-edge XAS data, can be employed owing to a recently developed tapered undulator and monochromator system. Although previous ex situ XRD measurements indicated the immutable cubic lattice parameter (ac) during the discharge process, these studies unveiled drastic structural variations occurring on the atomic scale between the charge and discharge reactions, such as differences in the ac, bond distances, and occupancies of the Zn2+ ions. This dynamic information obtained under operating conditions could be useful not only for understanding the zero-strain reaction scheme but also for designing advanced zero-strain insertion materials with enhanced energy density.

Revisiting LiCoO2 Using a State-of-the-Art In Operando Technique.

Lithium overstoichiometric cobalt oxide, Li(LiδCo1-δ)O2-δ, still occupies a privileged position as a positive electrode material for lithium-ion batteries. However, despite its widespread applications in commercial lithium-ion batteries, little is known about its reaction mechanisms and the effects of δ on cyclability at deep charge. We herein revisited this material through a recently developed in operando technique, i.e., rapid, alternating measurements of X-ray diffraction and X-ray absorption spectroscopy. The cyclability degraded when the charge cutoff voltage was >4.4 V versus Li+/Li, which corresponds to the Li composition exhibiting a minimum (maximum) lattice parameter along the ah (ch) axis. Differences in the structural parameters such as lattice parameters and bond distances clearly appeared between the charge and discharge reactions at a capacity below ∼220 mAh g-1. These changes occurred because deep charge and/or increasing the amount of δ induced a local distortion in the CoO6 octahedra. We found a critical Li extraction content that satisfied the need for both high capacity and cyclability for Li(LiδCo1-δ)O2-δ, which can be applied to other layered materials.

In situ X-ray Raman spectroscopy and magnetic susceptibility study on the Li[Li0.15Mn1.85]O4 oxygen anion redox reaction.

Li-rich compounds have received significant attention as electrode materials for lithium-ion batteries (LIBs) because of their large rechargeable capacities (qrecha). We have demonstrated a novel reaction scheme of one of the Li-rich compounds, Li[Li0.15Mn1.85]O4, where Mn4+ ions are reduced to lower valence states such as Mn3+ and Mn2+ ions during charging at voltages above 5.0 V.

High-pressure synthesis of ε-FeOOH from ß-FeOOH and its application to the water oxidation catalyst.

Research on materials under extreme conditions such as high pressures provides new insights into the evolution and dynamics of the earth and space sciences, but recently, this research has focused on applications as functional materials. In this contribution, we examined high-pressure/high-temperature phases of ß-FeO1-x (OH)1+x Cl x with x = 0.12 (ß-FeOOH) and their catalytic activities of water oxidation, i.e., oxygen evolution reaction (OER). Under pressures above 6 GPa and temperatures of 100-700 °C, ß-FeOOH transformed into ε-FeOOH, as in the case of α-FeOOH. However, the established pressure-temperature phase diagram of ß-FeOOH differs from that of α-FeOOH, probably owing to its open framework structure and partial occupation of Cl- ions. The OER activities of ε-FeOOH strongly depended on the FeOOH sources, synthesis conditions, and composite electrodes. Nevertheless, one of the ε-FeOOH samples exhibited a low OER overpotential compared with α-FeOOH and its parent ß-FeOOH, which are widely used as OER catalysts. Hence, ε-FeOOH is a potential candidate as a next-generation earth-abundant OER catalyst.

Structure, Magnetism, and Electrochemistry of LiMg1-xZnxVO4 Spinels with 0 ≤ x ≤ 1.

Negative electrode materials with lower operating voltages are urgently required to increase the energy density of lithium-ion batteries. In this study, LiMgVO4 with a Na2CrO4-type structure, LiZnVO4 with a phenacite structure, and their mixture were treated under a high pressure of 12 GPa and a high temperature of 1273 K, and their electrochemical reactivities were examined in a nonaqueous lithium cell. Synchrotron X-ray diffraction (XRD) measurements and Raman spectroscopy revealed that the LiMg1-xZnxVO4 samples with 0 ≤ x ≤ 1 are in a single phase of the inverse spinel structure that forms a solid solution compound over the whole x range. All of the samples were brown or light black due to the presence of a small amount of V4+ ions with S = 1/2 and oxygen deficiencies. Since the majority of the vanadium ions are located at the route of the Li+ ion conduction pathway, no rechargeable capacity (Qrecha) would be expected. Nevertheless, all LiMg1-xZnxVO4 samples exhibited a Qrecha value of more than 200 mAh g-1 with an operating voltage of ∼0.8 V. This operating voltage is ∼1.6 V lower than that of LiV2O4 with a normal spinel structure. Furthermore, the x = 0.5 sample demonstrated an extremely stable cycle performance over 1 month. Ex situ XRD measurements clarified that the reversible electrochemical reaction can be attributed to the movement of vanadium ions from the tetrahedral 8a to octahedral 16c sites during the initial discharge reaction. Details regarding the crystal structure, magnetism, and electrochemistry of LiMg1-xZnxVO4 are presented.

Structural and Electrochemical Analyses on the Transformation of CaFe2O4-Type LiMn2O4 from Spinel-Type LiMn2O4.

Lithium manganese oxides have received much attention as positive electrode materials for lithium-ion batteries. In this study, a post-spinel material, CaFe2O4-type LiMn2O4 (CF-LMO), was synthesized at high pressures above 6 GPa, and its crystal structure and electrochemical properties were examined. CF-LMO exhibits a one-dimensional (1D) conduction pathway for Li ions, which is predicted to be superior to the three-dimensional conduction pathway for these ions. The stoichiometric LiMn2O4 spinel (SP-LMO) was decomposed into three phases of Li2MnO3, MnO2, and Mn2O3 at 600 °C and then started to transform into the CF-LMO structure above 800 °C. The rechargeable capacity (Q recha) of the sample synthesized at 1000 °C was limited to ∼40 mA h·g-1 in the voltage range between 1.5 and 5.3 V because of the presence of a small amount of Li2MnO3 phase in the sample (=9.1 wt %). In addition, the Li-rich spinels, Li[Li x Mn2-x ]O4 with x = 0.1, 0.2, and 0.333, were also employed for the synthesis of CF-LMO. The sample prepared from x = 0.2 exhibited a Q recha value exceeding 120 mA h·g-1 with a stable cycling performance, despite the presence of large amounts of the phases Li2MnO3, MnO2, and Mn2O3. Details of the structural transformation from SP-LMO to CF-LMO and the effect of Mn ions on the 1D conduction pathway are discussed.

Synthesis of Rhombohedral LiCo0.64Mn0.36O2 Using a High-Pressure Method.

Lithium transition metal (M) oxides with a rhombohedral structure, r-LiMO2, have attracted a great deal of attention as a positive electrode material for lithium-ion batteries. Despite intensive studies thus far, Mn-rich r-LiMO2 compounds have remained unattainable, due to a cooperative Jahn-Teller distortion of Mn3+ ions in the MnO6 octahedra. We employed a high-pressure method for synthesizing r-LiCo xMn1- xO2 ( r-LCMO) with x = 0.5 and examined its electrochemical properties in a nonaqueous lithium cell. The high-pressure method successfully suppressed the Jahn-Teller distortion of Mn3+ ions, and the r-LCMO phase was observed in a wide temperature-pressure region when using a LiOH·H2O precursor. The rechargeable capacity of the sample synthesized at 600 °C and 12 GPa reached 126 mAh g-1, although the r-LCMO phase was contaminated with electrochemically inactive rock-salt LCMO and hexagonal LCMO phases. Compositional and structural analyses clarified that the actual Co/Mn ratio of the r-LCMO phase was 64/36, which deviated slightly from the initial composition (50/50). The high-pressure method was found to be effective for synthesizing Mn-rich r-LiMO2 compounds, although their electrochemical properties should be improved.

High-pressure synthesis and electrochemical properties of tetragonal LiMnO2.

Tetragonal structured LiMnO2 (t-LiMnO2) samples were synthesized under pressures above 8 GPa and investigated as a positive electrode material for lithium-ion batteries. Rietveld analyses based on X-ray diffraction measurements indicated that t-LiMnO2 belongs to a γ-LiFeO2-type crystal structure with the I41/amd space group. The charge capacity during the initial cycle was 37 mA h g-1 at 25 °C, but improved to 185 mA h g-1 at 40 °C with an average voltage of 4.56 V vs. Li+/Li. This demonstrated the superiority of t-LiMnO2 over other lithium manganese oxides in terms of energy density. The X-ray diffraction measurements and Raman spectroscopy of cycled t-LiMnO2 indicated an irreversible transformation from the γ-LiFeO2-type structure into a Li x Mn2O4 spinel structure by the displacement of 25% of the Mn ions to vacant octahedral sites through adjacent octahedral sites.

Toward Positive Electrode Materials with High-Energy Density: Electrochemical and Structural Studies on LiCo x Mn2-x O4 with 0 ≤ x ≤ 1.

To obtain positive electrode materials with higher energy densities (Ws), we performed systematic structural and electrochemical analyses for LiCo x Mn2-x O4 (LCMO) with 0 ≤ x ≤ 1. X-ray diffraction measurements and Raman spectroscopy clarified that the samples with x ≤ 0.5 are in the single-phase of a spinel structure with the Fd3̅m space group, whereas the samples with x ≥ 0.75 are in a mixture of the spinel-phase and Li2MnO3 phase with the C2/m space group. The x-dependence of the discharge capacity (Q dis) indicated a broad maximum at x = 0.5, although the average operating voltage (E ave) monotonically increased with x. Thus, the W value obtained by Q dis × E ave reached the maximum (=627 mW h·g-1) at x = 0.5, which is greater than that for Li[Ni1/2Mn3/2]O4. Furthermore, the change in the lattice volume (ΔV) during charge and discharge reactions approached 0%, that is, zero-strain, at x = 1. Because ΔV for x = 0.5 was smaller than that for Li[Ni1/2Mn3/2]O4, the x = 0.5 sample is found to be an alternative positive electrode material for Li[Ni1/2Mn3/2]O4 with a high W.

Chemical solution deposition of the highly c-axis oriented apatite type lanthanum silicate thin films.

Highly c-axis oriented apatite-type lanthanum silicate (LSO) thin films were fabricated by a simple solution coating method. In the solution coating method, LSO thin films are obtained by crystallization of initially deposited amorphous LSO precursor thin films. The degree of orientation was influenced by the precursor morphologies and a dense LSO precursor led to a high c-axis orientation perpendicular to the substrate. The oriented LSO thin films were composed of columnar grains with a single crystal orientation over the entire film thickness. In-plane orientation was not detected, which indicates that the c-axis orientation of the LSO thin films can be attributed to self-orientation.