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Electrochemistry has extended from reactions at solid/liquid interfaces to those at solid/solid interfaces. However, photoelectrochemistry at solid/solid interfaces has been hardly reported. In this study, we achieve a stable photoelectrochemical reaction at the semiconductor-electrode/solid-electrolyte interface in a Nb-doped anatase-TiO2(a-TiO2:Nb)/Li3PO4(LPO)/Li all-solid-state cell. The oxidative currents of a-TiO2:Nb/LPO/Li increase upon light irradiation when a-TiO2:Nb is located at a potential that is more positive than its flat-band potential. This is because the photoexcited electrons migrate to the current collector due to the bending of the conduction band minimum toward the negative potential. The photoelectrochemical reaction at the semiconductor/solid-electrolyte interface is driven by the same principle as those at semiconductor/liquid-electrolyte interfaces. Moreover, oxidation under light irradiation exhibits reversibility with reduction in the dark. Thus, we extend photoelectrochemistry to all-solid-state systems composed of solid/solid interfaces. This extension would enable us to investigate photoelectrochemical phenomena uncleared at solid/liquid interfaces because of low stability and durability.

Sulfide-based solid electrolytes have attracted considerable attention for application in solid-state lithium batteries because of their high ionic conductivities, suitable mechanical properties, and successful operation with various active anode and cathode materials. However, sulfides react with traces of moisture to generate toxic H2S gas. This undesirable degradation reaction reduces lithium ionic conductivity, which is crucial to solid-state batteries. To understand the effect of moisture degradation on solid electrolyte particles, impedance spectroscopic measurements at high frequencies (up to 100 MHz) were performed at various temperatures. From the spectral analysis results, we separated the impedance components into grain boundaries and the internal bulk of the solid electrolyte particles before and after exposure to moist air. By calculating the activation energy of lithium ionic conduction from the temperature dependence of each impedance component (Arrhenius plot), we determined that the impedance and activation energy of the grain boundaries between the solid electrolyte particles increased. This indicates that lithium ionic conduction between the solid electrolyte particles was inhibited during the initial stage of exposure to moist air.

Oxyhydrides with multi-anions (O2–and H–) are a recently developed material family and have attracted attention as catalysts and hydride ion conductors. High-pressure and high-temperature reactions are effective in synthesizing oxyhydrides, but the reactions sometimes result in inhomogeneous products due to insufficient diffusion of the solid components. Here, we synthesized new perovskite oxyhydrides SrVO2.4H0.6and Sr3V2O6.2H0.8. We demonstrated that the addition of SrCl2flux promotes diffusion during high-pressure and high-temperature reactions, and can be used for selective synthesis of the oxyhydride phases. We conducted in-situ synchrotron X-ray diffraction measurements to reveal the role of this flux and reaction pathways. We also demonstrated the electronic and magnetic properties of the newly synthesized oxyhydrides and that they work as anode materials for Li-ion batteries with excellent reversibility and high-rate characteristics, the first case with an oxyhydride. Our synthesis approach would also be effective in synthesizing various types of multi-component systems.

Argyrodites, Li6PS5X(X=Cl, Br, I), have piqued the interest of researchers by offering promising lithium ionic conductivity for their application in all-solid-state batteries (ASSBs). However, other than Li6PS5Cl (651Cl) and Li6PS5Br (651Br), Li6PS5I (651I) shows poor ionic conductivity (10−7S cm−1at 298 K). Herein, we present Al-doped 651I with I−/S2−site disordering to lower activation energy (Ea) and improve ionic conductivity. They formed argyrodite-type solid solutions with a composition of (Li6−3xAlx)PS5I in 0⩽x⩽0.10, and structural analysis revealed that Al3+is located at Li sites. Also, the Al-doped samples contained anion I−/S2−site disorders in the crystal structures and smaller lattice parameters than the non-doped samples. Impedance spectroscopy measurements indicated that Al-doping reduced the ionic diffusion barrier, Ea, and increased the ionic conductivity to 10−5S cm−1; the (Li5.7Al0.1)PS5I had the highest ionic conductivity in the studied system, at 2.6×10−5S cm−1. In a lab-scale ASSB, with (Li5.7Al0.1)PS5I functioned as a solid electrolyte, demonstrating the characteristics of a pure ionic conductor with negligible electronic conductivity. The evaluated ionic conduction is due to decreased Li+content and I−/S2−disorder formation. Li-site cation doping enables an in-depth understanding of the structure and provides an additional approach to designing better-performing SEs in the argyrodite system.

All-solid-state Li-metal batteries require fast Li-ion conductors that are compatible with Li-metal electrodes. Herein, we aim to obtain such Li-ion conductors in Li2S–P2S5–LiX(X= Br, I) systems, where new tetragonal phases with P42/nmcsymmetry were formed at compositions of Li10P3S12Br (LPSBr) and Li10.25P3S12.25I0.75(LPSI). Rietveld refinement analyses indicated that both materials were structural analogues of a renowned superionic conductor, Li10GeP2S12(LGPS), with additional anions at 2aor 4csites within LPSBr or LPSI, respectively. The LPSBr and LPSI phases exhibited ionic conductivities of 5.8(1) and 9.1(2) mS cm–1at 300 K, respectively, with the latter having the highest conductivity among the reported Li–P–S–X(X= Br or I) systems. All-solid-state Li metal cells were prepared using LPSBr or LPSI as the separator to compare their charge–discharge cycle performances with those previously reported for Li cells using separator electrolytes with various chemical compositions. The most stable cyclability was observed for the Li cell using LPSBr, which exhibited a high ionic conductivity and phase purity, indicating that these two properties of the solid electrolyte are important for ensuring extended cycling of all-solid-state Li-metal batteries.

Understanding Li-ion conduction in superionic conductors accelerates the development of new solid electrolytes to enhance the charge–discharge performances of all-solid-state batteries. We performed a quasi-elastic neutron scattering study on a model superionic conductor (Li10+xGe1+xP2–xS12, LGPS), to reveal its ion dynamics on an angstrom-scale spatial range and a pico-to-nanosecond temporal range. The observation of spectra at 298 K confirmed the high lithium diffusivity. The obtained diffusion coefficient was in the order of 10–6cm2s–1at temperatures >338 K and was higher than the reported diffusion coefficient over a longer time scale, as determined by the pulse-field gradient nuclear magnetic resonance method. This difference indicates that there are impediments to ionic motion over a longer time scale. The dynamic behavior of the Li ions was compared with that observed for the Li9P3S9O3phase, which possesses the same crystal structure type, but a lower ionic conductivity. The LGPS phase possessed a high lithium mobility over a distance of ∼10 Å, as well as a larger fraction of mobile Li ions, thereby indicating that these features enhance lithium conduction over a longer spatial scale, which is important in all-solid-state batteries.

Complex compositional control is typically required in developing ionic conductors to tune their lattice size and the number of carriers. Moreover, compositional complexity may affect their ion-conducting properties. Therefore, in this study, lithium superionic conductors (LISICONs) with γ-Li3PO4-type structures were developed in quasi-ternary (Li-M-M′-M″-O) systems to elucidate the multicomponent effects. Among the compounds examined, Li3.68(Ge0.6V0.36Ga0.04)O4in the Li4GeO4–Li3VO4–Li5GaO4ternary system exhibited the highest ionic conductivity (1.5 × 10–4S cm–1) at 298 K, with extremely low activation energy (∼0.26 eV). Moreover, the ternary systems showed a trend in which those with framework cations with larger ionic radii exhibited enhanced conductive properties. Additionally, the changes in ionic conductivity associated with the ionic radius were larger in ternary systems than those in binary systems. A significant decrease in activation energy, in particular, was observed using multicomponent systems. The formation of a multicomponent framework facilitated compositional and structural optimization, yielding enhanced ion-conducting properties, and advanced the order of optimal ionic conductivities of existing LISICON systems from 10–5to 10–4S cm–1.

Li2MnO3is a promising cathode candidate for Li-ion batteries because of its high discharge capacity; however, its reaction mechanism during cycling has not been sufficiently explicated. Observations of Mn and O binding energy shifts in operando hard X-ray photoelectron spectroscopy measurements enabled us to determine the charge-compensation mechanism of Li2MnO3. The O 1speak splits at an early stage during the first charge, and the concentration of lower-valence O changes reversibly with cycling, indicating the formation of a low-valence O species that intrinsically participates in the redox reaction. The O 1speak-splitting behavior, which indicates the number of valences of O in Li2MnO3, is supported by the computational results for an O3 to O1 structural transition. This is in agreement with the results of our previous study, wherein we confirmed this O3 to O1 transition based on in situsurface X-ray diffraction analysis, X-ray photoelectron spectroscopy, and first-principles formation energy calculations.

Understanding the fast Li ionic conductors of oxygen-substituted thiophosphates is useful for developing all-solid-state batteries because these compounds possess a high electrochemical stability and thus may be applied as solid electrolytes. In this study, we synthesized the Li9+δP3+δ′S12–kOkseries of solid solution phases with the same structure as the Li10GeP2S12superionic conductor and characterized their crystallinity, solid solution range, and chemical stabilities. Two methods (mechanochemical and melt quenching) were used for sample synthesis. Mechanochemical synthesis was used to obtain samples within a wide range of sulfur/oxygen substitution degrees, and the solid solution range was determined to be 0 < k≤ 3.6 based on their lattice parameter variation. Meanwhile, the melt-quenched Li9P3S9O3phase exhibited a high degree of crystallinity up to its particle surface and was thus selected for neutron crystal structure analysis, which revealed the oxygen distribution related to the solubility limit. The highly crystalline melt-quenched Li9P3S9O3showed better stability in the air atmosphere compared to the mechanochemically synthesized counterpart with a low crystallinity, implying that sample crystallinity is an important parameter in evaluating the air stability of thiophosphates. The promising electrochemical properties of the solid solution series were demonstrated by the stable charge–discharge cycling of an all-solid-state lithium metal cell using the Li9+δP3+δ′S12–kOkelectrolyte with k= 0.9 and a conductivity of >1 × 10–3S cm–1at 300 K.

Hydrogen transport in solids, applied in electrochemical devices such as fuel cells and electrolysis cells, is key to sustainable energy societies. Although using proton (H+) conductors is an attractive choice, practical conductivity at intermediate temperatures (200–400 °C), which would be ideal for most energy and chemical conversion applications, remains a challenge. Alternatively, hydride ions (H–), that is, monovalent anions with high polarizability, can be considered a promising charge carrier that facilitates fast ionic conduction in solids. Here, we report a K2NiF4-type Ba–Li oxyhydride with an appreciable amount of hydrogen vacancies that presents long-range order at room temperature. Increasing the temperature results in the disappearance of the vacancy ordering, triggering a high and essentially temperature-independent H–conductivity of more than 0.01 S cm–1above 315 °C. Such a remarkable H–conducting nature at intermediate temperatures is anticipated to be important for energy and chemical conversion devices.

Li4OBr2, an n= 1 member of the inverse Ruddlesden–Popper (iRP) phase Li3n+1OnBrn+1, was prepared for the first time by high-pressure (HP) synthesis. The changes in the stability and the ionic conductivity induced by F or I substitution for Br were also investigated by density functional theory (DFT) calculations and were experimentally attempted as well. The experimentally observed activation energy of Li4OBr2was 0.63 eV, which was further lowered by F or I substitution for Br or the introduction of Li–Br defects.

We evaluated the structural change of the cathode material Li2MnO3that was deposited as an epitaxial film with an (001) orientation in an all-solid-state battery. We developed an in situsurface X-ray diffraction (XRD) technique, where X-rays are incident at a very low grazing angle of 0.1°. An X-ray with wavelength of 0.82518 Å penetrated an ∼2 μm-thick amorphous Li3PO4solid-state electrolyte and ∼1 μm-thick metal Li anode on the Li2MnO3cathode. Experiments revealed a structural change to a high-capacity (activated) phase that proceeded gradually and continuously with cycling. The activated phase barely showed any capacity fading. First-principles calculations suggested that the activated phase has O1 stacking, which is attained by first delithiating to an intermediate phase with O3 stacking and tetrahedral Li. This intermediate phase has a low Li migration barrier path in the [001] direction, but further delithiation causes an energetically favorable and irreversible transition to the O1 phase. We propose a mechanism of structural change with cycling: charging to a high voltage at a sufficiently low Li concentration typically induces irreversible transition to a phase detrimental to cycling that could, but not necessarily, be accompanied by the dissolution of Mn and/or the release of O into the electrolyte, while a gradual irreversible transition to an activated phase happens at a similar Li concentration under a lower voltage.

Li9.54Si1.74P1.44S11.7Cl0.3(LSiPSCl), which exhibits a Li10GeP2S12(LGPS)-type structure, presents the highest reported Li-ion conductivity for solid electrolytes, but the formation of a secondary phase and a limited electrochemical stability restricts its performance in all-solid-state cells. Herein, oxygen atoms were substituted into LSiPSCl, and a monophasic LGPS-type solid solution was obtained (Li9.54Si1.74P1.44S11.7–zCl0.3Oz, LSiPSClOz; 0 < z≤ 0.6). Compared with LSiPSCl, the oxygen-substituted sample showed an improved ionic conductivity (7.4 ± 0.2 mS cm–1) for its cold-pressed powder pellet containing both bulk and grain-boundary resistances. This trend is consistent with the bulk conductivities at 298 K (28 ± 3 mS cm–1) estimated from impedance measurements at low temperatures. The electrochemical stabilization effect of oxygen substitution was confirmed by the charge–discharge measurement for an all-solid-state cell using the oxygen-substituted material as a separator electrolyte, which exhibited improved cycling compared to the cell using the nonsubstituted phase. Oxygen substitution in the LGPS-type phase of the Li–Si–P–S–Cl system therefore provides a higher purity and enhances the electrochemical stability of all-solid-state batteries, suggesting that oxygen substitution could lead to stable LGPS-type superionic conductors in the halogen-substituted Li–M–P–S (M= Ge, Si, and Sn) system.

X-ray computed tomography (CT) measurements with high-pressure compression and performance measurements were conducted for anode half-cells with a sulfide solid electrolyte and different volume ratios of graphite as the active material, in order to elucidate the relationship between the three-dimensional structure and performance of the anodes. The X-ray CT images were captured by a laboratory-scale CT imager and battery performance was measured using charge-discharge cycles and the current interrupt method. It was found that a high volume ratio of active material led to a decrease in the battery charging speed, because there was a decrease in connections between solid electrolyte and active material due to the aggregation of the active materials.

Understanding the electrochemical reactions at the interface between a Si anode and a solid sulfide electrolyte is essential in improving the cycle stabilities of Si anodes in all-solid-state batteries (ASSBs). Highly dense Si films with very low roughnesses of <1 nm were fabricated at room temperature via cathodic arc plasma deposition, which led to the formation of a Si/sulfide electrolyte model interface. Li (de)alloying through the model interface hardly occurred during the first cycle, whereas it proceeded stably in subsequent cycles. Hard X-ray photoelectron spectroscopy and neutron reflectometry directly revealed that the reduction or oxidation of the interfacial component or Li3PS4electrolyte occurred during the first cycle. Consequently, an interfacial layer with a thickness of 13 nm and primarily composed of Li2S, SiS2, and P2S5glasses was formed during the first cycle. The interfacial layer acted as a Li-conductive, electron-insulating solid electrolyte interphase (SEI) that provided reversible (de)lithiation. Our model interface directly demonstrates the electrochemical reaction processes at the Si/Li3PS4interface and provides insights into the structures and electrochemical properties of SEIs to activate the (de)lithiation of Si anodes using a sulfide electrolyte.

To develop inorganic Li conductors for all-solid-state Li-ion batteries, the present study clarified temperature-dependent structural changes in the Li2MS3(M= Si, Si0.5Ge0.5, Ge) phases, where new crystal structures favoring ionic conduction are potentially formed. Structural analysis was performed on powder X-ray diffraction data obtained at high temperatures. At 600 °C, tetragonal phases were formed in all three cases. Meanwhile, structural changes on cooling from 600 °C were different according to the Mspecies, leading to various crystal systems at room temperature (monoclinic for M= Si, orthorhombic for Si0.5Ge0.5, and hexagonal for Ge). Their ionic conductivities at 25 °C were 2.3 × 10–5, 1.7 × 10–7, and 8.6 × 10–9S cm–1, respectively, indicating that the conductivity depends significantly on the framework of the crystal structure. These phase identification results were further utilized to search the Li2S–GeS2pseudobinary system, revealing that the structure of Li16Ge5S18could not be assigned to any polymorphs identified for Li2MS3. This new crystalline phase showed an ionic conductivity of 7.2 × 10–5S cm–1at room temperature, which is 4 orders of magnitude higher than that of hexagonal Li2GeS3. This study highlights a drastic increase in the lithium conductivity achieved by controlling the crystal structure of the Li2MS3-related materials.

We have investigated the initial-cycle battery behavior of epitaxial thin films of Li2MnO3cathodes by employing resonant inelastic X-ray scattering (RIXS) at the O K-edge and Mn L3-edge. Thin films (25 nm thickness) with Li/Mn ratios of 2.06 (stoichiometric) and 2.27 (overstoichiometric) were epitaxially grown by pulsed laser deposition and electrochemically cycled as battery cathodes in a half-cell setup, stopped at potentials for full charge (delithiation) and complete discharge (relithiation) for X-ray analysis. No signatures of localized oxygen holes are found in O K-RIXS of the Li2MnO3regardless of the Li/Mn ratio. The prepeak region of the O K-XAS shows a net increase (a net decrease) after delithiation (relithiation) for both material compositions. Moreover, signatures of additional phases are detected only for the overstoichiometric material upon relithiation. Mn L-RIXS of the overstoichiometric cathode material shows a progressive loss of charge transfer state intensity during the first battery cycle, revealing a more rapid loss of Mn–O covalency in the overstoichiometric material.

Single crystals of the lithium-ion conductor Li10GeP2S12have been successfully grown by the self-flux method and are studied by means of X-ray diffraction and impedance spectroscopy. The weak anisotropic ionic conductivity is observed to be 27 and 7 mS cm–1in the [001] and [110] directions, respectively, at room temperature. Markedly, however, the activation energies are nearly equal, approximately 0.3 eV, in the two directions, which means that common diffusion paths along [110] dominate the long-range diffusion in Li10GeP2S12.

Crystal structures of Li10GeP2S12(LGPS)-type Li10+δ[SnySi1–y]1+δP2−δS12(Li4–x[SnySi1–y]1–xPxS4) solid electrolytes were analyzed by Rietveld refinement using neutron diffraction data. Maximum entropy method analysis was performed to visualize the distribution of lithium along the c-axis via the Li1–Li3 sites, which indicated one-dimensional (1D) lithium diffusion for all the examined compositions. The Li10.35Sn0.27Si1.08P1.65S12(Li3.45Sn0.09Si0.36P0.55S4) (δ = 0.35, x= 0.55, y= 0.2) system, which had the highest ionic conductivity in Li–Sn–Si–P–S system, exhibited an additional lithium diffusion pathway in the ab-plane through the Li1 and Li4 sites. High ionic conductivity (>10 mS cm–1) was achieved in the Sn–Si derivatives owing to the formation of three-dimensional (3D) ion diffusion channels. Comparison of the conductivity and related crystal structural parameters revealed the requirements for fast lithium diffusion along the c-axis and 3D lithium diffusion in the LGPS-type crystal structure. Large atomic displacement of the Li1 site, a large S3–S3 distance, a large bottleneck size, and small differences in Li1–Li4 distances are important for 1D and 3D lithium diffusion, respectively.

Hydride (H–) conduction is a new frontier related to hydrogen transport in solids. Here, a new H–conductive oxyhydride Ba2ScHO3was successfully synthesized using a high-pressure technique. Powder X-ray and neutron diffraction experiments investigated the fact that Ba2ScHO3adopts a K2NiF4-type structure with H–ions preferentially occupying the apical sites, as supported by theoretical calculations. Electrochemical impedance spectra showed that Ba2ScHO3exhibited H–conduction and a conductivity of 5.2 × 10–6S cm–1at 300 °C. This value is much higher than that of BaScO2H, which has an ideal perovskite structure, suggesting the advantage of layered structures for H–conduction. Tuning site selectivity of H–ions in layered oxyhydrides might be a promising strategy for designing fast H–conductors applicable for novel electrochemical devices.

An epitaxial Li2MnO3(001) thin film electrode with layered rock-salt structure was tested in an all-solid-state battery configuration for the first time. Using amorphous Li3PO4solid electrolyte, good discharge capacity after the 5th cycle, excellent reversibility for 100 cycles, and high rate capability at room temperature were observed. The same electrode showed significant capacity fading when cycled in liquid electrolyte. The all-solid configuration could prevent electrolyte decomposition, Mn dissolution, and oxygen extraction from Li2MnO3electrode.Epitaxial Li2MnO3(001) thin film electrode with layered rock-salt structure was tested in an all-solid-state battery configuration. Using amorphous Li3PO4solid electrolyte, good discharge capacity after the 5th cycle, excellent reversibility for 100 cycles, and high rate capability at room temperature were observed. The same electrode showed significant capacity fading when cycled in liquid electrolyte. The all-solid configuration could prevent electrolyte decomposition, Mn dissolution, and oxygen extraction from Li2MnO3electrode.

The Li10GeP2S12-type solid electrolyte was obtained by a liquid-phase process. A small amount of solids (1–3 wt %) in MeOH solvent formed a homogeneous solution. Stirring for 2 h and subsequent drying provided an amorphous Li-Ge-P-S. Sintering of the powder at 550 °C for 8 h crystallized the Li10GeP2S12providing a superionic conductivity of over 10−3S cm−1. The further study of this liquid-phase synthesis could enable the development of a battery fabrication process using the liquid-phase.A liquid-phase process for the Li10GeP2S12-type solid electrolyte was examined. A small amount of solids (1–3 wt %) in MeOH solvent provided a homogeneous solution. Stirring for 2 h and subsequent drying provided an amorphous Li-Ge-P-S. Sintering of the amorphous powder at 550 °C for 8 h crystallized the Li10GeP2S12providing a superionic conductivity of over 10−3S cm−1.

Lithium-rich layered rocksalt oxides are promising cathode materials for lithium-ion batteries, owing to their high charge–discharge capacities of over 250 mA h g–1. However, their poor rate capability remains to be addressed. Here, we investigate the effects of surface modification by amorphous Li3PO4on the structures and electrochemical reactions in the surface region of an epitaxial Li2RuO3(010) film electrode fabricated by pulsed laser deposition. Structural characterization using surface X-ray diffraction (XRD), hard X-ray photoemission spectroscopy, and neutron reflectometry shows that surface modification by 3 nm thick Li3PO4resulted in the partial substitution of P for Li in the surface region of Li2RuO3. The modified (010) surface exhibits better rate capability at 20 C (41% of the discharge capacity at 0.3 C) compared to the unmodified surface (5% of that at 0.3 C). In situsurface XRD confirmed that highly reversible structural changes occurred at the modified surface during lithium (de)intercalation, whereas the unmodified surface showed irreversible structural changes. These results demonstrate that this surface modification stabilizes the crystal structure in the surface region, and it can improve the rate capability of lithium-rich layered rocksalt oxide cathodes.

Li2MnO3 film synthesized on SrTiO3 substrate with SrRuO3 buffer layer using pulsed laser deposition has demonstrated a high initial charge-discharge capacity. It has been found that the Li2MnO3 crystals were composed of nano-domains with 5 nm in size, which followed a dominant orientation relationship (OR) with respect to the SrRuO3 crystals at the Li2MnO3/SrRuO3 interface. The domain size is in agreement with the calculated critical size, which is determined by the elastic strain originated from the lattice mismatch at the Li2MnO3/SrRuO3 interface. We proposed that the high initial charge-discharge capacity of the Li2MnO3 film is due to the combined effects of (1) being able to release the lattice strain at the Li2MnO3 domain boundaries during (de)intercalation of Li-ions that causes lattice expansion and shrinkage, and (2) crystal structure of the nano-domains and their ORs with respect to the SrRuO3 crystals being well-maintained due to the above strain release. In addition, our observation confirmed the existence of anti-phase domain boundaries and stacking faults in the Li2MnO3 crystals. However, it is considered that their influences on the (de)intercalation of Li-ions are not significant, because these defects do not obstruct the conduction paths of Li atoms that are orderly arranged in the c-planes.

Phase stability of LiMn2O4 and LiMn1.8Al0.2O4 was investigated to clarify the effects of the lithium content and substitution of aluminum on phase stability. Thermal stability and crystal structures were measured by differential thermal analysis (DTA) and high-temperature X-ray diffraction (XRD). DTA spectra indicated that an exothermic reaction was observed for Li0.79Mn2O4 at around 125-200degC. High-temperature XRD spectra indicated that both the lattice expansion caused by thermal expansion and the lattice shrinkage caused by formation of a manganese-deficient spinel structure occurred in this temperature range. These phenomena were not found for Li0.82Mn1.8Al0.2O4, which might be due to the effect of aluminum substitution. The low durability of Li0.79Mn2O4 was probably related to low phase stability at the specific lithium contents. In addition, when the temperature was raised to 400degC, Li0.79Mn2O4 remained in the spinel structure, but tetragonal Mn3O4 formed and the spinel structure almost disappeared at other lithium contents. This indicates the high phase stability of the manganese-deficient spinel structure formed at 125-200degC.

Surface-coated/uncoated epitaxial LiCoO2 film electrodes with (104) orientations were fabricated on SrRuO3(100)/Nb:SrTiO3(100) using pulsed laser deposition. Films with thicknesses of [?]18 nm and flat surfaces with roughnesses of less than 1 nm were model systems for clarifying the kinetics of Li-ion transfer at the electrode/electrolyte interface. The Li-ion transfer characteristics at the interface between the LiCoO2 electrode and the electrolyte (LiPF6, ethylene carbonate + diethyl carbonate) were investigated by electrochemical impedance spectrometry. The charge-transfer resistance (Rct) of uncoated LiCoO2 increased from the early cycles when charged/discharged at 3.0-4.2 V. When charged/discharged at 3.0-4.5 V, the Rct of the uncoated LiCoO2 rapidly increased from the first charge. In contrast, the Rct of Li2ZrO3-coated LiCoO2 remained almost constant during the early cycles when charged/discharged either at 3.0-4.2 or at 3.0-4.5 V. The interfacial resistances of the coated and uncoated LiCoO2 electrodes were almost equal ([?]100 O cm2). The activation energy for charge transfer was lower for the coated LiCoO2 electrode compared to that for the uncoated electrode. The current-rate capability was significantly improved by surface coating even at high-voltage charge/discharge at 3.0-4.5 V. The charge transfer process is the rate-determining step of the charge/discharge reaction.

Local crystalline structures of LiCoO2nanothin film cathodes in a lithium ion battery have been spectroscopically elucidated through confocal Raman imaging analysis at high spatial resolution of several hundred nanometers. A significant difference in the crystalline structure is found between the nanometric thin films and bulk powders. Thermally induced local decomposition of LiCoO2into an impurity phase on the films has also been revealed along with the mechanism of the temperature-triggered decomposition process. Moreover, frequency-based Raman imaging enables us to locally probe spatial separation between stoichiometric (LiCoO2) and non-stoichiometric (Li1-xCoO2, 0 < x< 1) crystal phases on the thin films. Such local crystalline analysis is a promising approach to provide new insights into the degradation mechanism of lithium-ion batteries, which would result in improving the performance of thin film-based lithium ion batteries.

We report the formation of La2LiHO3epitaxial thin films and the high H–conductivities in the ab-planes of the crystals. Single-phase La2LiHO3(001) epitaxial thin films are fabricated on LaSrAlO4(001) single-crystal substrates via pulsed laser deposition. The H–conductivity in the ab-plane (1.3 × 10–8S cm–1) at room temperature (22 °C) is four orders of magnitude higher than those of bulk polycrystals, and furthermore, the activation energy (0.46 eV) is less than 2/3 of those of bulk polycrystals. This study reveals the intrinsic intragrain H–conductivity of La2LiHO3. Moreover, the results suggest that the H–conductivities of bulk polycrystals can be substantially enhanced by controlling the properties of the grains and aligning the orientations of the crystallites.

Considering the compositional complexity concept, novel solid electrolytes based on quaternary and quinary lithium superionic conductor (LISICON) systems were investigated for the development of highly ion-conducting materials. Phase identification indicated that even in highly complex compositions, relatively large Ti- and Ga-doping contents were difficult to achieve in the γ-Li3PO4framework. Relatively low ionic conductivities (10–7to 10–5S cm–1) and high activation energies (0.44–0.53 eV) were confirmed compared to ternary LISICON systems, indicating a negligible positive effect of the compositional complexity. In addition, undesirable anti-site disorder occurred in the LISICON crystal, which was considered to limit fast ion conduction in the quaternary and quinary LISICON systems. A further systematic and detailed study is therefore required to accurately determine the effects of compositional complexity in the design of fast ion conductors.

Abstract

Clarification of the interaction between the electrode and the electrolyte is crucial for further improvement of the performance of lithium-ion batteries. We have investigated the structural change at the interface between the surface of a 104-oriented epitaxial thin film of LiCoO2(LiCoO2(104)), which is one of the stable surfaces of LiCoO2, and an electrolyte prepared using a carbonate solvent (1 M LiClO4in ethylene carbonate and dimethyl carbonate) by in situneutron reflectivity measurements. Owing to the decomposition of the organic solvent, a new interface layer was formed after contact of LiCoO2(104) with the electrolyte. The composition and thickness of the interface layer changed during Li+extraction/insertion. During Li+extraction, the thickness of the interface layer increased and the addition of an inorganic species is suggested. The thickness of the interface layer decreased during Li+insertion. We discuss the relationship between battery performance and the dynamic behavior at the interface.

Capacity degradation of the LiMn2O4 cathode during high-temperature storage was investigated for various lithium contents. The capacity recovery rate was determined after the storage test, and the phases and valence state of manganese ions were investigated by X-ray diffraction and X-ray absorption fine structure. The storage characteristics depend on the state of charge (SOC); a significantly low recovery rate was found for the shallow SOC region of LiMn2O4. The phase changes determined after the deterioration were as follows. (i) The lattice parameters decrease with an increase in manganese valence, caused by the formation of defects at the 16d site due to manganese dissolution. (ii) A new phase forms as a second phase. This phase has higher manganese valence and a larger amount of manganese vacancies. The same type of phase was produced by a high-temperature treatment of the shallow SOC phase at 150degC, which indicates that the phase was stable at high temperatures. (iii) A lower manganese valence state was observed at the spinel surface, which is related to manganese dissolution. Deterioration in the shallow SOC region during high-temperature storage is related to the phase stability of the de-intercalated spinel phases.

Fast charge–discharge process has been reported to give a high capacity loss. A nanobattery consisting of a single LiMn2O4nanowire cathode, ionic liquid electrolyte and lithium titanium oxide anode was developed for in situtransmission electron microscopy. When it was fully charged or discharged within a range of 4 V in less than half an hour (corresponding average C rate: 2.5C), Li-rich and Li-poor phases were observed to be separated by a transition region, and coexisted during whole process. The phase transition region moved reversibly along the nanowire axis which corresponds to the [011] direction, allowing the volume fraction of both phases to change. In the electron diffraction patterns, the Li-rich phase was seen to have the (100) orientation with respect to the incident electron beam, while the Li-poor phase had the (111̅) orientation. The orientation was changed as the transition region moved. However, the nanowire did not fracture. This suggests that a LiMn2O4nanowire has the advantage of preventing capacity fading at high charge rates.

The interfacial reactions, especially the gas evolution, between carbon conductive agents and the electrolyte at the positive electrode in high-voltage batteries (potentials over 4.5 V) have been investigated. The amount of gas generated was quantified for various conductive agents: acetylene black (AB), furnace black, specially customized AB, and graphite (GR). The experiments revealed that in the high-voltage system, the specific gas evolution was induced by both the cathode active material and the conductive agent, with the carbon conductive agents resulting in the generation of 8 to 15 times more gas than the cathode active material LiNi0.5Mn1.5O4 (LNMO) itself. The high-voltage properties of the carbon conductive agents, such as the anion intercalation and self-discharging properties, were evaluated for each carbon electrode. The results implied the existence of a local battery composed of the conductive agent and LNMO; this redox couple appears to play a key role in the gas evolution.

The effects of surface coatings on LiMn2O4 were investigated using LiMn2O4 epitaxial thin films with a thickness of 30 nm. Bare and surface-coated LiMn2O4 epitaxial thin films were synthesized on SrTiO3 (111) substrates using a pulsed laser deposition method. The surface coating, which was formed using the solid electrolyte Li3PO4 and had a thickness of 3 nm, improved the reversibility of the electrochemical reactions undergone by the LiMn2O4 epitaxial thin films. The modified surface structure provided a stable field for reversible reactions at the electrode surface, resulting in superior battery performance. The changes induced in the surface structure were maintained during battery operation; in contrast, the bare LiMn2O4 thin film exhibited structural degradation and Mn dissolution. The structural changes induced in the coated electrode and the increase in its surface stability were intrinsic effects of the Li3PO4 coating and improved the electrochemical performance of the LiMn2O4 thin-film electrode.

Abstract

Amorphous Fe3+-based oxide nanoparticles produced by Leptothrix ochracea, aquatic bacteria living worldwide, show a potential as an Fe3+/Fe0conversion anode material for lithium-ion batteries. The presence of minor components, Si and P, in the original nanoparticles leads to a specific electrode architecture with Fe-based electrochemical centers embedded in a Si, P-based amorphous matrix.

We developed a “nanobattery” consisting of LiMn2O4nanowires, ionic liquid electrolyte, and Li4Ti5O12crystals and performed in situ transmission electron microscope observation during charge and discharge cycles. The LiMn2O4nanowire was found to be changed into the tetragonal phase at the interface region with electrolyte during the discharge process of the 4 V reaction (vs Li/Li+). This result is explained by the lithium accumulation due to the different local lithium diffusion rates: the lithium ions were inserted into the nanowire at a higher rate than they diffused toward the bulk side of the nanowire. We also found that the LiMn2O4nanowire cathode was restored to be cubic phase without any fracture in the charge process. No fracture during the cubic-tetragonal transition is promising for the long-lifetime battery with LiMn2O4nanowire cathode.

The evolution of lithium-containing species on the surface of grains of layered material during the aging process in (ethylene carbonate/dimethyl carbonate, ) electrolyte has been followed using magic angle spinning NMR spectroscopy. Materials displaying different surface areas have been investigated in order to study the influence of the surface/volume ratio. The evolution of the NMR signal shows that the reaction of the active material with the electrolyte is extremely fast during the first moments of exposure and tends to slow down for longer exposure times. Coupled NMR, electrochemical impedance spectroscopy, and transmission electron microscopy experiments showed that the surface of the material grains is not covered by a homogeneous layer, indicating that the reaction with electrolyte cannot be considered as a real passivation reaction. The aging process performed on a sample initially stored in ambient atmosphere clearly demonstrates the dissolution of a pristine surface layer and the growth of an interphase made primarily of fluorinated compounds.

State‐of‐the‐art LiFePO4technology has now opened the door for lithium ion batteries to take their place in large‐scale applications such as plug‐in hybrid vehicles. A high level of safety, significant cost reduction, and huge power generation are on the verge of being guaranteed for the most advanced energy storage system. The room‐temperature phase diagram is essential to understand the facile electrode reaction of LixFePO4(0 < x< 1), but it has not been fully understood. Here, intermediate solid solution phases close to x= 0 and x= 1 have been isolated at room temperature. Size‐dependent modification of the phase diagram, as well as the systematic variation of lattice parameters inside the solid‐solution compositional domain closely related to the electrochemical redox potential, are demonstrated. These experimental results reveal that the excess capacity that has been observed above and below the two‐phase equilibrium potential is largely due to the bulk solid solution, and thus support the size‐dependent miscibility gap model.

Solid-state electrolytes that exhibit high ionic conductivities at room temperature are key materials for obtaining the next generation of safer, higher-specific-energy solid-state batteries. However, the number of currently available crystal structures for use as superionic conductors remains limited. Here, we report a lithium superionic conductor, Li2SiS3, with tetragonal crystal symmetry, which possesses a new three-dimensional framework structure consisting of isolated edge-sharing tetrahedral dimers. This species exhibits an anomalously high ionic conductivity of 2.4 mS cm–1at 298 K, which is 3 orders of magnitude higher than the reported ionic conductivity for its orthorhombic polymorph. The framework of this conductor consists mainly of silicon, which is abundant in natural resources, and its further optimization may lead to the development of new solid-state electrolytes for large-scale applications.

The impact of ambient air exposure on nanocomposites has been investigated. A pristine sample was prepared without any exposure to ambient air through the whole process of synthesis and characterization and compared to the exposed samples. A small amount of lithium deintercalates from the olivine structure during exposure, a majority of which can be electrochemically reintercalated. This phenomenon changes the initial surface and bulk properties and should be taken into account when diminishing the particle size of . Keeping nanocomposites away from oxidative moisture atmosphere could be a solution to minimize these side reactions.

Epitaxial film electrodes of La0.8Sr0.2MnO2 (LSM) and Y0.5Ca0.5MnO3 (YCM) with the perovskite structure were obtained by annealing films deposited on the yttria- stabilized zirconia (YSZ) substrates by pulsed laser deposition method. These films were characterized by thin-film X-ray diffraction measurement and ac impedance spectroscopy. The film orientations depended on the substrate plane of YSZ. The electrode reactions proceeded by successive several processes. The activation energies of charge transfer process of both the LSM and YCM films depended on the crystal plane of the electrode surface; the film on YSZ (100) showed smaller activation energy than the film on YSZ (110). Charge transfer process of the LSM films was also affected by the lattice misfit. The crystal plane of the electrode surface and structural distortion of the film were indicated to be important parameters for the electrode reaction.

A series of synthetic heterosite−purpurite, (MnyAB1-y)PO4 (y < 0.8), with negligible disorder and impurities, was obtained by chemical oxidation of the well-crystallized isotypic tryphillite−lithiophilite series, Li(MnyAB1-y)PO4 (ordered olivine structure, space group Pnma). Comparative magnetic and X-ray/neutron powder diffraction investigations of the two solid−solution lines were performed as a function of Mn content to increase understanding of the electrochemical activity loss of Mn3+/Mn2+ in the Lix(MnyAB1-y)PO4 electrode system. Introducing Mn ions into the 4c site did not cause significant change in the local geometry of M2+O6 and PO4 polyhedra, while the M3+O6 octahedra became severely distorted with an increase in the number of Jahn−Teller active Mn3+ ions. The edge-sharing geometry of M3+O6 and PO4 polyhedra fixed the shared O3‘−O3‘ interatomic distance, causing selective strong elongation of the M3+−O3‘ distance with small shrinkage of other M3+−O1, M3+−O2, and M3+−O3 bond lengths. The overall distortion of the MO6 octahedra with M = Mn3+ was much larger than the corresponding change in the unit-cell orthorombicity and significantly increased asymmetry in the M−O−M superexchange interaction. All samples exhibited antiferromagnetism; however, the trivalent series had more than a sevenfold larger decrease in Neel temperature TN (from ca. 130 K at y = 0 to ca. 50 K at y = 0.8) compared to the divalent series (from ca. 52 K at y = 0 to ca. 35 K at y =1) as a function of the Mn content y.

Phase relationships, structures, magnetic properties, and phase transitions in the lithium manganese oxide spinel were studied based on a wide variety of samples synthesized at temperatures over a range of 750 ≤ t≤ 900 °C with LiMn ratios between 0.5 and 0.55 and various starting materials. The system was divided into three categories: oxygen deficient spinels, LiMn2O4−δ; lithium-substituted spinels, Li1xMn2−xO4−δ; and the stoichiometric spinel, LiMn2O4. Their structures were discussed based on the structure data determined by TOF neutron and synchrotron X-ray Rietveld analysis. The amount of oxygen vacancy was determined to be δ 0.143 in LiMn2O4−δ, for example, for the spinel synthesized at 900 °C, and the nearly stoichiometric composition was obtained at 750 °C followed by heating at 470 °C for several times from lithium hydroxide and manganese oxides as starting materials. The oxygen vacancy was dependent upon the synthesis conditions and starting materials. It decreased as the LiMn ratio varied from 0.5 to 0.55, and as the synthesis temperature decreased from 900 to 750 °C. Based on the synthesis and structure results, the phase relationship in the ternary diagram, Li–Mn–O, is discussed.