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Abstract All-solid-state lithium–sulfur batteries exhibit high energy densities, operate safely, and suppress polysulfide shuttling. However, their electrochemical performance is restricted by the insulating nature of sulfur and Li2S, and by severe cathode-related volumetric changes during cycling. Here, we prepare a Li2S-based cathode composite with high mixed-conductivity and stability, by infiltrating a Li2S–LiI active material solution into a mesoporous carbon replica with ~10-nm-sized pores, followed by mixing with a liquid-phase-synthesized Li6PS5Br solid electrolyte and vapour-grown carbon fibres. Benefiting from a mechanically reinforced, three-dimensional ion/electron-conducting structure, the cathode exhibits high discharge capacity (1009 mAh g−1, 20 cycles, 298 K, 0.05 C) and high reversible capacity (650 mAh g−1, 100 cycles, 298 K, 0.1 C). These findings underscore the feasibility of developing high-performance all-solid-state lithium-sulfur batteries by designing three-dimensional mixed-conducting mechanically robust cathodes.

Abstract Information on the cathode/organic–electrolyte interface structure provides clues regarding the rate and reversibility of lithium intercalation reactions in lithium‐ion batteries. Herein, structural changes within the LiCoO2 electrode, throughout the interphase region, and in the LiPF6/propylene carbonate electrolyte are observed concurrently by in situ neutron reflectometry. The formation of an electrical double layer (EDL) during the early stages of charging and discharging is investigated and compared with that at an intercalation‐inactive Nb:SrTiO3 electrode. At the intercalation‐inactive interface between Nb:SrTiO3 and the electrolyte, a voltage‐dependent ionic distribution corresponding to the EDL forms on the electrolyte side without the formation of a cathode/electrolyte interphase (CEI) layer. In contrast, at the intercalation‐active LiCoO2/electrolyte interface, a CEI layer forms immediately after cell construction, and the ionic distribution in the electrolyte formed outside the CEI layer scarcely changes upon voltage application. The CEI/electrolyte interface is shielded from potential changes by the electronically insulating CEI; therefore, structural changes in the EDL are restricted. This supports the prevailing understanding that the CEI layer defines the rates of solvation/de‐solvation and adsorption/desorption reactions of lithium ions.

A thin-film battery composed of a LiFePO4 cathode/Li3PO4 electrolyte/Li anode was fabricated on a Pt/Ti/Si (PTS) substrate via RF magnetron sputtering. The amorphous Li3PO4 film was densely stacked on a 60 nm-thick LiFePO4 film, which provided a suitable reaction field for understanding the electrochemical properties of LiFePO4 at the interface with the solid electrolyte. The LiFePO4 cathode film exhibited highly reversible lithium desertion/insertion at the interface at room temperature and 60 °C, without any side reactions. An irreversible oxidation reaction occurred during the initial charging process at 100 °C, leading to an increase in the charge-transfer resistance of the LiFePO4/Li3PO4 interface with no significant decrease in the lithium desertion/insertion capacity of LiFePO4. This result suggests the formation of a resistive interphase via the decomposition of Li3PO4 at 100 °C. A severe decrease in capacity is observed at 125 °C, which indicates the LiFePO4-side interface contributed to the side reactions. The film battery exhibits a severe decrease in capacity at 125 °C.

The development of new materials leads to the invention of new devices. The exploitation of high ionic conductivity materials has facilitated the emergence of a new category of energy storage devices, including the all-solid-state battery. This paper reviews the history of the development of lithium solid electrolytes and their application in all-solid-state batteries. Particular focus is given to the development process of Li10GeP2S12, which surpasses the conductivity characteristics of liquid-electrolyte systems targeted by lithium-ion conductors, and its application to solid-state batteries is described. Furthermore, this review describes new science that will be born when batteries become solid-state.

The electronic structure evolution within a battery during cycling can provide crucial cues for its optimization, but insights on operando band structures are extremely challenging to obtain. Here, the authors determine the overall band structure of a model thin-film solid-state lithium battery via operando hard X-ray photoelectron spectroscopy, considering the cathode and anode sides.

Fundamental investigations at the electrode/electrolyte interface are essential for developing high-energy batteries. Here, the authors investigate the degradation mechanisms at the LGPS/NCM622 interface providing a quantitative model to interpret the interfacial resistance growth.

Oxygen reduction and evolution reactions (ORR and OER, respectively) of perovskite-type La0.8Sr0.2CoO3 were characterized using two-dimensional model electrodes with different reaction planes (001), (110), and (111). Synthesized by pulsed laser deposition, these thin (30 nm) and flat (roughness < 1 nm) electrodes can reveal the reaction plane dependence of the ORR activity. From steady-state polarization measurements in KOH (aq.), the ORR activity was the highest on the (001) film during the first ORR/OER cycle, and it decreased significantly during the second cycle. In-situ synchrotron X-ray diffraction clarified crystal structure changes in the bulk and surface regions of La0.8Sr0.2CoO3, and these changes are associated with forming oxygen defects during the initial electrochemical process. Furthermore, the La0.8Sr0.2CoO3 surface partially decomposed upon reacting with the aqueous solution, as clarified by hard X-ray photoemission spectroscopy. Therefore, the interfacial structures formed in the electrochemical reaction field is important for enhancing ORR and OER activities.

A combinatorial synthesis system consisting of co-sputtering multiple radio frequency (RF) cathodes was developed to investigate the presence of new amorphous solid electrolytes in a pseudo-ternary Li3PO4-Li4SiO4-LiAlO2 system. Oxynitride solid-electrolyte films were synthesized under an N2 atmosphere by reactive RF sputtering with cathodes made of different materials, such as Li3PO4, Li4SiO4, and LiAlO2 installed at different positions in the chamber. The formation of amorphous films with no grains and a continuous atomic distribution was confirmed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. In a single synthesis, we fabricated an oxynitride solid-electrolyte film, in which the composition range of the components was approximately one-eighth that in the corresponding pseudo-ternary system. The highest Li-ion conductivity of 3.1 × 10−6 S cm−1 was obtained for the LiPSiAlON film with a composition ratio of LiPON : LiSiON : LiAlON = 48.5 : 33.9 : 17.6. The different bonding states of O and N in the LiPSiAlON film doped with P, Si, and Al were examined using X-ray photoemission spectroscopy. The ionic conductivity was improved by the mixed anion effect. The combinatorial synthesis enables the efficient optimization of the chemical composition and facilitates the development of highly conductive amorphous solid electrolytes.

K2NiF4-type oxides are expected to be potential lithium-ion conductors because they have a structure similar to that of perovskites, which provide a reasonably flexible framework for accommodating defects such as charge carriers within the lattice. However, the K2NiF4-type structure as a framework for lithium conductors is scarcely reported. This article presents the preparation of Sr(La1−xLi3x)ScO4 with a K2NiF4 structure by a solid-state reaction at a high pressure of 2 GPa, and elucidates its lithium-ion-conducting properties. Sr(La1−xLi3x)ScO4 forms solid solutions in the x range of 0.05–0.20. Its orthorhombic lattice expands with lithium doping, indicating the incorporation of lithium ions as interstitial species in the structure. The highly doped samples exhibit high ionic conductivities (e.g., 4.66 × 10−6 S cm−1 at 250 °C for x = 0.15 and 4.29 × 10−2 S cm−1 at 375 °C for x = 0.2) with an activation energy of ∼100 kJ mol−1. The samples show an abnormal increase in the ionic conductivity at ∼300 °C, possibly due to the increase in the orthorhombicity of Sr(La1−xLi3x)ScO4. As the electronic conductivities of the developed oxide materials are a few orders of magnitude lower than their total conductivities, they can be used as solid electrolytes in all-solid-state lithium batteries. The study reveals that K2NiF4-type oxides are attractive candidates for developing novel lithium-ion conductors.

A novel electrochemical setup for operando hard X-ray photoelectron spectroscopy (HAXPES) was proposed to investigate the bulk-sensitive electronic structure of cations and anions in the active materials of an all-solid-state Li-ion battery. A model all-solid-state thin film cell was prepared by depositing a LiCoO2 positive electrode and an Al current collector on the front of the solid electrolyte substrate, and a Li3PO4 buffer solid electrolyte and a Li negative electrode on the back side, to give a structure of Al (10 nm)/LiCoO2 (35 nm)/Li1+x+yAlx(Ti, Ge)2−xSiyP3−yO12 (180 μm)/Li3PO4 (500 nm)/Li (1000 nm). The reversible changes of the core levels (Al 1s, P 1s, Li 1s, Co 2p3/2, O 1s) and the valence bands in the charged (4.2 V)/discharged (3.0 V) states were observed without disassembling the LiCoO2 thin film cell inside the vacuum chamber. The operando HAXPES spectra acquired during the charge–discharge process revealed that the reversible changes in the Co 2p3/2 and O 1s spectra were attributed to the participation of cobalt cations and oxygen anions from LiCoO2 in the redox mechanism.

The establishment of three-electrode cells for all-solid-state lithium-ion batteries is an important issue to clarify the electrochemical behavior of the battery components and solve problems for the practical applications. Here we prepared an all-solid-state three-electrode cell with a chemically-reduced Li4Ti5O12 reference electrode and investigated electrochemical properties of a LiCoO2 composite electrode in a Li-In|Li10GeP2S12|LiCoO2 cell. Plateau potentials in charge-discharge curves were coincident with reported values. Cell impedance measured with two-electrode configuration agreed well with the sum of the impedances of the positive and the negative electrodes measured with the reference electrode. These results show that chemically-reduced Li4Ti5O12 reference electrode was worked as a Li7Ti5O12|Li4Ti5O12 reference electrode and electrochemical properties of the LiCoO2 electrode and the Li-In electrode were separated. From the temperature and the state of charge dependences of LiCoO2 impedance, resistances related to interface or interphase and charge-transfer reaction were identified. These results show that all-solid-state cells with chemically-reduced Li4Ti5O12 reference electrodes are promising candidates for three-electrode cells for all-solid-state lithium-ion battery systems.

Abstract In general, the ionic conductivity of sulfide glasses decreases with their crystallization, although it increases for a few sulphide glasses owing to the crystallization of a highly conductive new phase (e.g., Li7P3S11: 70Li2S-30P2S5). We found that the ionic conductivity of 75Li2S-25P2S5 sulfide glass, which consists of glassy and crystalline phases, is improved by optimizing the conditions of the heat treatment, i.e., annealing. A different mechanism of high ionic conductivity from the conventional mechanism is expected in the glassy phase. Here, we report the glassy structure of 75Li2S-25P2S5 immediately before the crystallization by using the differential pair distribution function (d-PDF) analysis of high-energy X-ray diffraction. Even though the ionic conductivity increases during the optimum annealing, the d-PDF analysis indicated that the glassy structure undergoes no structural change in the sulfide glass-ceramic electrolyte at a crystallinity of 33.1%. We observed the formation of a nanocrystalline phase in the X-ray and electron diffraction patterns before the crystallization, which means that Bragg peaks were deformed. Thus, the ionic conductivity in the mixture of glassy and crystalline phases is improved by the coexistence of the nanocrystalline phase.

Perovskite-type lithium ionic conductors were explored in the (LixLa1−x/3)ScO3 system following their syntheses via a high-pressure solid-state reaction. Phase identification indicated that a solid solution with a perovskite-type structure was formed in the range 0 ≤ x < 0.6. When x = 0.45, (Li0.45La0.85)ScO3 exhibited the highest ionic conductivity and a low activation energy. Increasing the loading of lithium as an ionic diffusion carrier expanded the unit cell volume and contributed to the higher ionic conductivity and lower activation energy. Cations with higher oxidation numbers were introduced into the A/B sites to improve the ionic conductivity. Ce4+ and Zr4+ or Nb5+ dopants partially substituted the A-site (La/Li) and B-site Sc, respectively. Although B-site doping produced a lower ionic conductivity, A-site Ce4+ doping improved the conductive properties. A perovskite-type single phase was obtained for (Li0.45La0.78Ce0.05)ScO3 upon Ce4+ doping, providing a higher ionic conductivity than (Li0.45La0.85)ScO3. Compositional analysis and crystal-structure refinement of (Li0.45La0.85)ScO3 and (Li0.45La0.78Ce0.05)ScO3 revealed increased lithium contents and expansion of the unit cell upon Ce4+ co-doping. The highest ionic conductivity of 1.1 × 10−3 S cm−1 at 623 K was confirmed for (Li0.4Ce0.15La0.67)ScO3, which is more than one order of magnitude higher than that of the (LixLa1−x/3)ScO3 system.

Lithium superionic conductors with the Li10GeP2S12 (LGPS)-type structure are promising materials for use as solid electrolytes in next-generation lithium batteries. A novel member of the LGPS family, Li9.42Si1.02P2.1S9.96O2.04, and its solid solutions were synthesised by quenching from 1273 K in the Li2S–P2S5–SiO2 pseudoternary system. The material exhibited an ionic conductivity as high as 3.2×10−4 S cm−1 at 298 K, as well as the high electrochemical stability to lithium metal, which was improved by the introduction of oxygen into the LGPS-type structure. An all-solid-state cell with a lithium metal anode and Li9.42Si1.02P2.1S9.96O2.04 as the separator showed excellent performance with a high coulomb efficiency of 100%. Thus, oxygen doping is an effective way of improving the electrochemical stability of LGPS-type structure.

Mechanical properties of Li10.35Ge1.35P1.65S12 (LGPS) solid electrolytes with different grain sizes were investigated via indentation tests. Hand-milled LGPS (HM LGPS, d50: 1.32 μm) and wet-milled LGPS (WM LGPS, d50: 0.46 μm) powders were uniaxially pressed to obtain pellet samples. The HM LGPS and WM LGPS pellets had a similar bulk density of 1.63 g cm-3. At the initial loading/unloading, the WM LPGS pellet showed a large deformation owing to a decrease in cavities compared with the HM LGPS. In the subsequent cycles, both pellets exhibited elastic deformation behavior in the pressure range up to 20 N. The elastic modulus and relative residual depth of HM LGPS and WM LGPS were 21.7 GPa and 15.0 GPa and 0.75 and 0.71, respectively. This result revealed that WM LGPS is more elastically deformable and less plastically deformable than HM LGPS, which could be associated with the grain boundary strengthening interpreted by the Hall-Petch relation. Based on these mechanical properties, the superior cycle stability at the In-Li electrode/WM LGPS electrolyte interface during charging/discharging was discussed. Controlling the mechanical properties of sulfide solid electrolytes by the grain size is important for suppressing physical degradation in all-solid-state batteries. [ABSTRACT FROM AUTHOR]

A machine learning method was developed, which predicts ionic conductivity based on chemical composition alone, aiming to develop an efficient method to search for lithium conductive oxides. Under the obtained guideline, the material search was focused on the Li2O-SiO2-MoO3 pseudo-ternary phase diagram, which is predicted to have high ionic conductivity (> 10-4 S·m-1). We investigated the formation range, ionic conductivity, and crystal structure of the lithium superionic conductor (LISICON) solid solution on the Li4SiO4-Li2MoO4 tie line. The ionic conductivity of the LISICON phases is about 10-7S·cm-1, which is higher than that of the end members; however, two orders of magnitude lower than that of the analogous LISICON materials. In addition, the experimental values were two or three orders of magnitude lower than the predicted conductivity values by machine learning. However, the developed prediction model can be used as an initial guideline for material exploration since the predicted values follow the trend of practical conductivity in the phase diagram. The crystal structure analysis indicated that the distance between the lithium sites and the occupancy of each lithium site in the crystal structure contributed to the decrease in ionic conductivity. This strong correlation between crystal structure and ionic conductivity was one of the reasons for the discrepancy between the predicted ionic conductivity based on chemical composition alone and the experimental value. [ABSTRACT FROM AUTHOR]