In situ microscopy techniques for understanding Li plating and stripping in solid-state batteries.

Solid-state batteries have potential to realize a rechargeable Li-metal anode. However, several challenges persist in the charging and discharging processes of the Li-metal anode, which require a fundamental understanding of Li plating and stripping across the interface of solid-state electrolytes (SEs) to address. This review overviews studies on Li-metal anodes in solid-state batteries using in situ observation techniques with an emphasis on Li electrodeposition and dissolution using scanning electron microscopy and SEs such as lithium phosphorus oxynitride and garnet-type compounds such as Li7La3Zr2O12. The previous research is categorized into three topics: (i) Li nucleation, growth and dissolution at the anode-free interface, (ii) electrochemical reduction of SE and (iii) short-circuit phenomena in SE. The current trends of each topic are summarized.

Electrochemical Li+ Insertion/Extraction Reactions at LiPON/Epitaxial Graphene Interfaces.

Redox reactions of the Li+ insertion/extraction from one to two interlayers of graphene (Gr) on area-defined single-crystalline SiC substrates are investigated using lithium phosphorus oxynitride glass (LiPON) as the solid-state electrolyte. Unlike an organic liquid electrolyte, this glassy electrolyte does not induce a reduction current and excludes the desolvation reaction of Li+. Gr electrodes with less than two Gr layers show a single reduction peak and one or two oxidation peaks below +0.21 V (vs Li+/Li), differing distinctly from those of graphite and multilayer Gr, which display multiple peaks (multiple stage transitions). However, this finding aligns with the conventional understanding that graphite stage structure transitions proceed with stepwise increases or decreases in the number of Gr layers between adjacent Li-inserted interlayers. Cyclic voltammetry measurements indicate the presence of surface capacity due to Li+ adsorption/desorption at the LiPON/Gr interface. Moreover, Li+ insertion and extraction induce different charge transfer resistances at the level of a single interlayer. These sensitive measurements are achieved using high-quality epitaxial Gr and LiPON electrolyte, which prevent the formation of a solid electrolyte interphase and the desolvation reaction of Li+. Similar measurements using bilayer Gr produced by chemical vapor deposition coupled with a Gr transfer method and an ethylene carbonate/dimethyl carbonate liquid electrolyte are not reliable. Thus, the proposed method is effective for electrochemical measurement of Gr electrodes with a controlled number of layers.

Preparation of Li-Excess and Li-Deficient Antiperovskite Structured Li2+xOH1-xBr and Their Effects on Total Ionic Conductivity.

This paper describes about the effect of Li-H exchange amount on total lithium-ion (Li+) conductivity of Li2+xOH1-xBr (x = -0.5 to +0.4). These samples are systematically prepared at room temperature by a dry ball-milling process using LiOH, LiOH·H2O, Li2O, and LiBr as starting materials. Synchrotron X-ray diffraction analysis reveals that single-phase Li2+xOH1-xBr samples are formed within x = -0.5 to +0.35. For improving total Li+ conductivity (σt), a larger x value increases both the Li carrier density and lattice constant as positive factors, while that decreases both the crystallite size and OH rotational unit possibly assisting Li+ conduction as negative factors. This trade-off provides an optimized σt of 3.6 × 10-6 S cm-1 at the Li-excess Li2.2OH0.8Br composition, which is ca. 3 times higher than pristine Li2OHBr (1.1 × 10-6 S cm-1). The hydrogen incorporation into the lattice is confirmed by neutron diffraction analysis, and the refined composition is almost consistent with the prepared composition.

Synthesis of the Metastable Cubic Phase of Li2OHCl by a Mechanochemical Method.

The oxyhalide-based solid electrolyte Li2OHCl usually forms the thermodynamically stable orthorhombic phase at room temperature and shows poor lithium ionic conductivity. Above 35 °C, a structural phase transition into the cubic phase occurs and ionic conductivity is enhanced. In this work, mechanochemical synthesis of Li2OHCl is reported. The as-prepared Li2OHCl formed a cubic Pm3̅m structure and showed an ionic conductivity of 2.6 × 10-6 S cm-1 at 25 °C. Once the cubic phase was treated at 200 °C, the orthorhombic Pmc21 structure appeared at 25 °C and the ionic conductivity decreased down to 1.4 × 10-7 S cm-1. Formation of the metastable cubic phase could be explained in terms of low crystallinity of Li2OHCl derived from mechanochemical synthesis.

Temperature Effects on Li Nucleation at Cu/LiPON Interfaces.

This study reports the effect of temperature on Li nucleation at the Cu/LiPON interface. Galvanostatic Li plating is performed on LiPON glass electrolytes at different temperatures ranging from 25 to 100 °C. At any temperature, the negative voltage peak appears, indicating Li nucleation, immediately after starting Li plating. The nucleation overpotential and nucleation number density decrease with increasing temperature. This is because the diffusivity of Li adatoms/ions along the Cu/LiPON interface increases with temperature, resulting in an increase in the amount of Li atoms incorporated into a single Li nucleus. The critical nucleation area also extends with increasing temperature. It is found that the activation energy for the interfacial diffusion of Li adatoms/ions along the Cu/LiPON interface is 51 kJ mol-1 (0.53 eV), which is close to the activation energy for Li+ conduction in LiPON.

Electrodeposition and behavior of single metal nanowire probes.

This paper describes the fabrication of scanning probes with single metal nanowires (NWs) at the probe tip. The porous-template technique can produce NWs of various kinds of metals, with diameters down to 10-20 nm, which compete with multiwall carbon nanotube diameters. Metal NWs are grown by electrodeposition on the scanning probe tip. One NW can be selected to remain by focused ion beam technique. A variety of metals can be chosen as the tip material. Electric potentials of NWs at the probe tip can be measured. Single NW probes can measure surface topographies, electrode potentials, and their mechanical bending properties.

Nanotubular array solid oxide fuel cell.

This report presents a demonstration and characterization of a nanotubular array of solid oxide fuel cells (SOFCs) made of one-end-closed hollow tube Ni/yttria-stabilized zirconia/Pt membrane electrode assemblies (MEAs). The tubular MEAs are nominally ∼5 µm long and have <500 nm outside diameter with total MEA thickness of nearly 50 nm. Open circuit voltages up to 660 mV (vs air) and power densities up to 1.3 µW cm(-2) were measured at 550 °C using H2 as fuel. The paper also introduces a fabrication methodology primarily based on a template process involving atomic layer deposition and electrodeposition for building the nanotubular MEA architecture as an important step toward achieving high surface area ultrathin SOFCs operating in the intermediate to low-temperature regime. A fabricated nanotubular SOFC theoretically attains a 20-fold increase in the effective surface, while projections indicate the possibility of achieving up to 40-fold.