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.

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.

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.

Reversible Movement of Zn2+ Ions with Zero-Strain Characteristic: Clarifying the Reaction Mechanism of Li2ZnTi3O8.

Lithium zinc titanate spinel, Li2ZnTi3O8, has received significant attention as a negative electrode material for lithium-ion batteries (LIBs). However, its reaction mechanism has not been fully clarified yet, particularly for the large voltage hysteresis between discharge and charge curves. We hence closely examined (Li1-xZnx)[Li1/3+x/3Ti5/3-x/3]O4 (LZTO) with 0 < x ≤ 0.5 by measuring its open-circuit voltage (OCV) and recording synchrotron radiation X-ray diffraction (XRD) patterns. Here, LZTO is a solid solution of Li[Li1/3Ti5/3]O4 (x = 0) and Li2ZnTi3O8 (x = 0.5), both of which have a spinel-framework structure. For the x = 0.5 sample, the OCV of the discharge reaction differed from that of the charge reaction, particularly at a capacity above 50 mAh·g-1. This difference was due to the migration of Zn2+ ions from tetrahedral sites to octahedral sites, and the Zn2+ ions moved back to tetrahedral sites during the charge reaction. Despite these drastic movements of Zn2+ ions, the cubic lattice parameter of the spinel was maintained during the whole reaction, i.e., zero strain. Perfect zero strain, which has never been reported for any LIB materials, was achieved with the x = 0.25 sample. The reaction mechanism with x = 0.5 and the contributions of the amount of Zn ions are discussed in detail.

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.

Magnetic anomalies and itinerant character of electrochemically Li-inserted Li[Li1/3Ti5/3]O4.

Spinel oxides of Li[LiyTi2-y]O4 with 0 ≤y≤ 1/3 exhibit two desirable features for solid state chemistry and condensed matter physics. One is a reversible lithium insertion/extraction reaction, in particular for Li[Li1/3Ti5/3]O4, and the other is a superconducting transition at Tc≃ 13 K for Li[Ti2]O4. To study the change in magnetic environments of the y = 1/3 compound with excess Li(x), we measured the magnetic susceptibility (χ) for the Li1+x[Li1/3Ti5/3]O4 samples with 0 ≤x≤ 0.95, which were prepared by an electrochemical Li insertion reaction into Li[Li1/3Ti5/3]O4. Even for the x = 0 sample, two magnetic anomalies were found at T (=63 K) and T (=21 K), despite the fact that all Ti ions should be in the 4+ state with S = 0. A comparative study of TiO2 and Li2TiO3 revealed that these magnetic anomalies are not impurity-induced effects but are caused by an intrinsic nature of Li[Li1/3Ti5/3]O4, probably due to either slight compositional deviation from stoichiometry or dislocations such as a Magnéli phase. For the x > 0 samples, the χ vs. temperature curve was found to consist of a temperature-independent Pauli-paramagnetic contribution and a Curie-Weiss contribution. Detailed analyses of χ clarified the systematic variations of the effective magnetic moment of Ti ions, effective mass of electrons in the conduction band, and density of states at the Fermi level with x, indicating that the Li(+) ions at the 16d site play a significant role in localizing d electrons of Ti(3+) ions.

Reactive surface area of the Li(x)(Co(1/3)Ni(1/3)Mn(1/3))O2 electrode determined by µ(+)SR and electrochemical measurements.

The self-diffusion coefficient of Li(+) ions (D(Li)) in the positive electrode material Li(x)(Co(1/3)Ni(1/3)Mn(1/3))O2 has been estimated by muon-spin relaxation (µ(+)SR) using powder samples with x = 1-0.49, which were prepared by an electrochemical reaction in a Li-ion battery. Here, since the implanted muons sense a slight change in the internal magnetic field due to Li-diffusion, µ(+)SR provides an intrinsic D(Li) through the temperature dependence of the nuclear field fluctuation rate (ν) [Sugiyama et al., Phys. Rev. Lett., 2009, 103, 147601]. Both D(Li) at 300 K and activation energy (E(a)) were estimated to be ∼2.9 × 10(-12) cm(2) s(-1) and 0.074 eV for the x = 1 sample, ∼11.0 × 10(-12) cm(2) s(-1) and 0.097 eV for x = 0.70, and ∼8.9 × 10(-12) cm(2) s(-1) and 0.062 eV for x = 0.49, assuming that the diffusing Li(+) ions mainly jump from a regular occupied site to a regular vacant site. The estimated D(Li) was smaller by roughly one order of magnitude than those for Li(x)CoO2 in the whole x range measured. Furthermore, by making comparison with D(Li) obtained by electrochemical measurements, the reactive surface area of the Li(x)(Co(1/3)Ni(1/3)Mn(1/3))O2 electrode in a liquid electrolyte was found to strongly depend on x particularly at x > 0.8.

Stacking Fault Formation in LiNi0.6Co0.2Mn0.2O2 during Cycling: Fundamental Insights into the Direct Recycling of Spent Lithium-Ion Batteries.

As the global marketplace for lithium-ion batteries (LIBs) proliferates, technologies for efficient and environmentally friendly recycling, i.e., direct recycling, of spent LIBs are urgently required. In this contribution, we elucidated the mechanisms underlying the degradation that occurs during the cycling of a Li/LiNi0.6Co0.2Mn0.2O2 (NCM622) cell. The results provided fundamental insights into the optimum procedures for direct recycling using a recently developed, state-of-the-art positive electrode material. Capacity fade in NCM622 was induced by cycling at high voltages above 4.6 V vs Li+/Li, during which the rhombohedral symmetry approached cubic symmetry. The selective line broadening and peak shifts that appeared in the X-ray diffraction patterns after cycling indicated the formation of stacking faults along the ch-axis. In addition, high-resolution transmission electron microscopy clarified that rock-salt domains were located on the NCM622 surface before and after cycling. These structural analyses confirmed that the NCM622 particles degrade not at their surfaces but rather in the bulk, contradicting previous reports where degradation during cycling is mainly caused by rock-salt domains on the surface. Material regeneration processes involving the restoration of the original stacking sequence are essential for effective direct recycling.

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.

Evidence of a reversible redox reaction in a liquid-electrolyte-type fluoride-ion battery.

Fluoride-ion batteries (FIBs) have received significant attention as promising alternatives to conventional lithium-ion batteries, but a reversible redox reaction has not been confirmed yet for liquid-electrolyte-type FIBs. We conducted ex situ X-ray diffraction and energy dispersive X-ray analyses for a conventional full-cell assembly of FIBs, in which BiF3, a Pb plate (or Pb powder), and tetraethylammonium fluoride dissolved in propylene carbonate were used as the positive electrode, negative electrode, and liquid electrolyte, respectively. A FIB using a Pb plate exhibited a flat operating voltage at ∼0.29 V during the discharge reaction with a discharge capacity of ∼105 mA h g-1. The reversible electrochemical reaction was, however, attained when the discharge and charge capacities were controlled to be less than 20 mA h g-1. In a such capacity-limited cycle test, Bi and PbF2 phases were formed during the discharge reaction, while BiF3 and Pb phases were generated during the charge reaction. Therefore, a reversible movement of F- ions between the BiF3 and Pb electrodes, i.e., reversible redox reaction was firstly confirmed for the liquid-electrolyte-type FIB. We also attempted to improve the reversibility at the first cycle by replacing the Pb plate with Pb powder electrodes, and consequently, the FIB using an annealed Pb powder indicated the best electrochemical performance.

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.

ϵ-FeOOH: A Novel Negative Electrode Material for Li- and Na-Ion Batteries.

The demand for eco-friendly materials for secondary batteries has stimulated the exploration of a wide variety of Fe oxides, but their potential as electrode materials remains unknown. In this contribution, ϵ-FeOOH was synthesized using a high-pressure/high-temperature method and examined for the first time in nonaqueous Li and Na cells. Under a pressure of 8 GPa, α-FeOOH transformed into ϵ-FeOOH at 400 °C and then decomposed into α-Fe2O3 and H2O above 500 °C. Here, FeO6 octahedra form [2 × 1] tunnels in α-FeOOH or [1 × 1] tunnels in ϵ-FeOOH. The ϵ-FeOOH/Li cell exhibited a rechargeable capacity (Q recha) of ∼700 mA h·g-1 at 0.02-3.0 V, whereas the ϵ-FeOOH/Na cell indicated a Q recha of less than 30 mA h·g-1 at 0.02-2.7 V. The discharge and charge profiles of ϵ-FeOOH and α-FeOOH were similar, but the rate capability of ϵ-FeOOH was superior to that of α-FeOOH.

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.

Li diffusion in LixCoO2 probed by muon-spin spectroscopy.

The diffusion coefficient of Li+ ions (D(Li)) in the battery material LixCoO2 has been investigated by muon-spin relaxation (mu+SR). Based on experiments in zero and weak longitudinal fields at temperatures up to 400 K, we determined the fluctuation rate (nu) of the fields on the muons due to their interaction with the nuclear moments. Combined with susceptibility data and electrostatic potential calculations, clear Li+ ion diffusion was detected above approximately 150 K. The D(Li) estimated from nu was in very good agreement with predictions from first-principles calculations, and we present the mu+SR technique as an optimal probe to detect D(Li) for materials containing magnetic ions.

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.

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.