Selective Epitaxial Growth of Ca2NH and CaNH Thin Films by Reactive Magnetron Sputtering under Hydrogen Partial Pressure Control.

Calcium compounds with N and H are promising catalysts for NH3 conversion, and their epitaxial thin films provide a platform to quantitatively understand the catalytic activities. Here we report the selective epitaxial growth of Ca2NH and CaNH thin films by controlling the hydrogen partial pressure (PH2) during reactive magnetron sputtering. We find that the hydrogen charge states can be tuned by PH2: Ca2NH containing H- is formed at PH2 < 0.04 Pa, while CaNH containing H+ is formed at PH2 > 0.04 Pa. In situ plasma emission spectroscopy reveals that the intensity of the Ca atomic emission (∼422 nm) decreases as PH2 increases, suggesting that Ca reacts with H2 and N2 to form Ca2NH at lower PH2, whereas at higher PH2, CaHx is first formed on the target surface and then sputtered to produce CaNH. This study provides a novel route to control the hydrogen charge states in Ca-N-H epitaxial thin films.

Immense Reduction in Interfacial Resistance between Sulfide Electrolyte and Positive Electrode.

Low interfacial resistance between the solid sulfide electrolyte and the electrode is critical for developing all-solid-state Li batteries; however, the origin of interfacial resistance has not been quantitatively reported in the literature. This study reports the resistance values across the interface between an amorphous Li3PS4 solid electrolyte and a LiCoO2(001) epitaxial thin film electrode in a thin-film Li battery model. High interfacial resistance is observed, which is attributed to the spontaneous formation of an interfacial layer between the solid electrolyte and the positive electrode upon contact. That is, the interfacial resistance originates from an interphase mixed layer instead of a space charge layer. The introduction of a 10 nm thick Li3PO4 buffer layer between the solid electrolyte and positive electrode layers suppresses the formation of the interphase mixed layer, thereby leading to a 2800-fold decrease in the interfacial resistance. These results provide insight into reducing the interfacial resistance of all-solid-state Li batteries with sulfide electrolytes by utilizing buffer layers.

Drastic Reduction of the Solid Electrolyte-Electrode Interface Resistance via Annealing in Battery Form.

The origin of electrical resistance at the interface between the positive electrode and solid electrolyte of an all-solid-state Li battery has not been fully determined. It is well known that the interface resistance increases when the electrode surface is exposed to air. However, an effective method of reducing this resistance has not been developed. This report demonstrates that drastic reduction of the resistance is achievable by annealing the entire battery cell. Exposing the LiCoO2 positive electrode surface to H2O vapor increases the resistance by more than 10 times (to greater than 136 Ω cm2). The magnitude can be reduced to the initial value (10.3 Ω cm2) by annealing the sample in a battery form. First-principles calculations reveal that the protons incorporated into the LiCoO2 structure are spontaneously deintercalated during annealing to restore the low-resistance interface. These results provide fundamental insights into the fabrication of high-performance all-solid-state Li batteries.

Ionic Rectification across Ionic and Mixed Conductor Interfaces.

In electrochemical devices, it is important to control the ionic transport between the electrodes and solid electrolytes. However, it is difficult to tune the transport without applying an electric field. This paper presents a method to modulate the transport via tuning of the electrochemical potential difference by controlling the electronic states at the interfaces. We fabricated thin-film solid-state Li batteries using LiTi2O4 thin films as positive electrodes. The spontaneous Li-ion transport between the solid electrolyte and LiTi2O4 is controlled by tuning the electrochemical potential difference via use of an electrically conducting Nb-doped SrTiO3 substrate. This study establishes the foundation for rectifying the ionic transport via electronic energy band alignment.

Relaxation of the Interface Resistance between Solid Electrolyte and 5 V-Class Positive Electrode.

Solid-state Li batteries using 5 V-class positive electrode materials display a higher energy density. However, the high resistance at the interface of the electrolyte and positive electrode (interface resistance, Ri) hinders their practical applications. Here, we report the relaxation of Ri between a solid electrolyte (Li3PO4) and a 5 V-class electrode (LiCo0.5Mn1.5O4). Although Ri is small at the Mn3+/4+ redox voltage of 4.0 V vs Li/Li+ (11 Ω cm2), it rapidly increases by more than 2 orders of magnitude as the voltage increases above the Co3+/4+ redox voltage of 5.2 V vs Li/Li+. After the applied voltage is reduced to 4.0 V vs Li/Li+, Ri decays to the original value after 3 h. The relaxation of Ri after exposure to high voltages suggests that the increase in Ri above 5 V vs Li/Li+ is attributable to the formation of an interfacial layer at the LPO/LCMO interface.

Tuning the Schottky Barrier Height at the Interfaces of Metals and Mixed Conductors.

Understanding electronic and ionic transport across interfaces is crucial for designing high-performance electric devices. The adjustment of work functions is critical for band alignment at the interfaces of metals and semiconductors. However, the electronic structures at the interfaces of metals and mixed conductors, which conduct both electrons and ions, remain poorly understood. This study reveals that a Schottky barrier is present at the interface of the Nb-doped SrTiO3 metal and a LiCoO2 mixed conductor and that the interfacial resistance can be tuned by inserting an electric dipole layer. The interfacial resistance significantly decreased (by more than 5 orders of magnitude) upon the insertion of a 1 nm thick insulating LaAlO3 layer at the interface. We apply these techniques to solid-state lithium batteries and demonstrate that tuning the electronic energy band alignment by interfacial engineering is applicable to the interfaces of metals and mixed conductors. These results highlight the importance of designing positive electrode and current collector interfaces for solid-state lithium batteries with high power density.

Clean Solid-Electrolyte/Electrode Interfaces Double the Capacity of Solid-State Lithium Batteries.

Solid-state lithium (Li) batteries using spinel-oxide electrode materials such as LiNi0.5Mn1.5O4 are promising power supplies for mobile devices and electric vehicles. Here, we demonstrate stable battery cycling between the Li0Ni0.5Mn1.5O4 and Li2Ni0.5Mn1.5O4 phases with working voltages of approximately 2.9 and 4.7 V versus Li/Li+ in solid-state Li batteries with contamination-free clean Li3PO4/LiNi0.5Mn1.5O4 interfaces. This clean interface has the effect of doubling the capacity of conventional battery cycling between the Li0Ni0.5Mn1.5O4 and Li1Ni0.5Mn1.5O4 phases. We also investigated the structural changes between the Li0Ni0.5Mn1.5O4 and Li2Ni0.5Mn1.5O4 phases during battery cycling. Furthermore, we found an inhomogeneous distribution of the Li2Ni0.5Mn1.5O4 phase in the LiNi0.5Mn1.5O4 electrode, induced by spontaneous Li migration after the formation of the Li3PO4/LiNi0.5Mn1.5O4 interface. These results indicate that the formation of a contamination-free clean Li3PO4/LiNi0.5Mn1.5O4 interface is key to increase the battery capacity.

High Li-Ion Conductivity in Li{N(SO2F)2}(NCCH2CH2CN)2 Molecular Crystal.

There is an urgent need to develop solid electrolytes based on organic molecular crystals for application in energy devices. However, the quest for molecular crystals with high Li-ion conductivity is still in its infancy. In this study, the high Li-ion conductivity of a Li{N(SO2F)2}(NCCH2CH2CN)2 molecular crystal is reported. The crystal shows a Li-ion conductivity of 1 × 10-4 S cm-1 at 30 °C and 1 × 10-5 S cm-1 at -20 °C, with a low activation energy of 28 kJ mol-1. The conductivity at 30 °C is one of the highest values attainable by molecular crystals, whereas that at -20 °C is approximately 2 orders of magnitude higher than previously reported values. Furthermore, the all-solid-state Li-battery fabricated using this solid electrolyte demonstrates stable cycling, thereby maintaining 90% of the initial capacity after 100 charge-discharge cycles. The finding of high Li-ion conductivity in molecular crystals paves the way for their application in all-solid-state Li-batteries.

Low-Energy-Consumption Three-Valued Memory Device Inspired by Solid-State Batteries.

We report the creation of a low-energy-consumption three-valued memory device based on the switching of open-circuit voltages. This device consists of a stack of Li, Li3PO4 solid electrolyte, and Ni electrode films. We observed reversible voltage switching between high, intermediate, and low open-circuit voltages. According to the scaling law, the energy required to switch a device is estimated to be 8.8 × 10-11 J/µm2 and this value is almost 1/50 of that of a typical dynamic random access memory. Both the high- and low-voltage states converged to the intermediate-voltage state, indicating that the intermediate-voltage state is the most stable metastable state.

A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction.

Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3 This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.