Yasuaki Matsuda

A reversible dendrite-free high-areal-capacity lithium metal electrode

Reversible dendrite-free low-areal-capacity lithium metal electrodes have recently been revived, because of their pivotal role in developing beyond lithium ion batteries. However, there have been no reports of reversible dendrite-free high-areal-capacity lithium metal electrodes. Here we report on a strategy to realize unprecedented stable cycling of lithium electrodeposition/stripping with a highly desirable areal-capacity (12u2009mAhu2009cm−2) and exceptional Coulombic efficiency (>99.98%) at high current densities (>5u2009mAu2009cm−2) and ambient temperature using a diluted solvate ionic liquid. The essence of this strategy, that can drastically improve lithium electrodeposition kinetics by cyclic voltammetry premodulation, lies in the tailoring of the top solid-electrolyte interphase layer in a diluted solvate ionic liquid to facilitate a two-dimensional growth mode. We anticipate that this discovery could pave the way for developing reversible dendrite-free metal anodes for sustainable battery chemistries.

Lithium ion diffusion measurements on a garnet-type solid conductor Li6.6La3Zr1.6Ta0.4O12 by using a pulsed-gradient spin-echo NMR method.

The garnet-type solid conductor Li7-xLa3Zr2-xTaxO12 is known to have high ionic conductivity. We synthesized a series of compositions of this conductor and found that cubic Li6.6La3Zr1.6Ta0.4O12 (LLZO-Ta) has a high ionic conductivity of 3.7×10(-4)Scm(-1) at room temperature. The (7)Li NMR spectrum of LLZO-Ta was composed of narrow and broad components, and the linewidth of the narrow component varied from 0.69kHz (300K) to 0.32kHz (400K). We carried out lithium ion diffusion measurements using pulsed-field spin-echo (PGSE) NMR spectroscopy and found that echo signals were observed at T≥313K with reasonable sensitivity. The lithium diffusion behavior was measured by varying the observation time and pulsed-field gradient (PFG) strength between 313 and 384K. We found that lithium diffusion depended significantly on the observation time and strength of the PFG, which is quite different from lithium ion diffusion in liquids. It was shown that lithium ion migration in the solid conductor was distributed widely in both time and space.

Phase relation, structure and ionic conductivity of Li7−x−3yAlyLa3Zr2−xTaxO12

The phase relation, structure and ionic conductivity of garnet-like Li7−x−3yAlyLa3Zr2−xTaxO12 were investigated. The improved sample dissolution process in the ICP measurements enabled the exclusion of the ambiguity of the atomic composition in the samples. The tetragonal phase formed at x = 0–0.375 and the cubic phase appeared with x = 0.4–2.0 in the Li7−xLa3Zr2−xTaxO12 system. In the Al-doped system of Li7−x−3yAlyLa3Zr2−xTaxO12, the tetragonal phase was formed at x + 3y < 0.4. The border between the tetragonal and the cubic phases exists at Li6.6−z/2Alz/2□0.4La3Zr1.6+zTa0.4−zO12. The tetragonal/cubic structure change corresponds to the order/disorder of lithium ions and is dependent on the cation content at the lithium sites. The ionic conductivity of the cubic compounds has a positive tendency with respect to the lithium content, whereas that of the tetragonal compounds is opposite. A high total ionic conductivity exceeding 5.0 × 10−4 S cm−1 at 25 °C was observed for Al-doped Li6.6−z/2Alz/2La3Zr1.6+zTa0.4−zO12. The highest total conductivity of 1.03 × 10−3 S cm−1 at 25 °C with an activation energy of 0.35 eV was obtained at z = 0.275. Nuclear magnetic resonance spectroscopy revealed that Al3+ substitution decreases the diffusion of lithium ions in the structure. The high total conductivity of Al-doped Li6.6−z/2Alz/2La3Zr1.6+zTa0.4−zO12 may be due to the enhancement of lithium diffusion at the grain boundaries.

Synthesis, crystal structure, and ionic conductivity of tunnel structure phosphates, RbMg1−xH2x(PO3)3·y(H2O)

A new proton conductor, RbMg1−xH2x(PO3)3·yH2O, was synthesized by the coprecipitation method followed by sintering at 540 K. The range of solid solution for RbMg1−xH2x(PO3)3·yH2O was 0.00 < x < 0.18 and the highest conductivity of 5.5 × 10−3 S cm−1 was observed at 443 K for the composition of x = 0.11. The structure of RbMg1−xH2x(PO3)3·yH2O was determined from 298 to 553 K by Rietveld refinement with multi-profile analysis using X-ray and neutron diffraction data. The framework structure is composed of PO4 tetrahedra connected with each other by corner-sharing to form spiral PO4 chains along the c-direction. One-dimensional tunnels are formed between these spiral chains, in which water molecules are located. The water molecules form one-dimensional spiral chains and are connected to the spiral PO4 chain by hydrogen bonding. The introduction of acidic, hydrophilic head groups (–PO3H) into the PO4 framework by the formation of a solid solution provides binding sites for water and an environment for the efficient diffusion of protons. One-dimensional proton diffusion, similar to proton channels in biological systems, could be explained by the vehicle mechanism of H3O+ along the one-dimensional water chain.

Synthesis and crystal structure of novel proton conductor, RbMg(PO3)3·3(H2O)

Proton conductors have been studied for applications in electrochemical devices such as fuel cells. Phosphate and sulfate based materials are one of the categories of proton conductors suitable for medium temperature range. Among these materials, proton-conductive solid acid salts, CsH2PO4 and CsHSO4, are well known as high proton conductors. The ionic conductivity exceeds 10 Scm at 200°C. However, the temperature range where the material shows high proton conduction is rather narrow, and the search for new proton conductors is still necessary. In the present study, novel proton conduction materials were synthesized in the solid acid salt systems. The structure of RbxMg1-x(PO3)3 were examined by x-ray and neutron diffraction measurements. The proton conductivities were also determined by ac impedance methods. The relationship between the structure and proton conductivity mechanism will be discussed.