Naoto Tanibata

Preparation and characterization of highly sodium ion conducting Na3PS4–Na4SiS4 solid electrolytes

This study investigated electrical and electrochemical properties of (100 − x)Na3PS4·xNa4SiS4 (mol%) glass–ceramics prepared using mechanical milling and consecutive heat treatment. Glass–ceramics at the compositions of 0 ≤ x ≤ 10 exhibited higher conductivity than 10−4 S cm−1 at room temperature, which was achieved by precipitation of cubic Na3PS4 crystal with high sodium ion conductivity. The conductivity increased concomitantly with increasing Na4SiS4 contents at compositions of 0 ≤ x ≤ 6. The glass–ceramic with 6 mol% Na4SiS4 exhibited conductivity of 7.4 × 10−4 S cm−1 at room temperature, which is the highest value reported for Na+ ion conducting sulfides to date. The conductivities of the glass–ceramics in the composition range of 25 ≤ x ≤ 100, where unknown phases are crystallized, were 10−7–10−5 S cm−1. These were lower than the conductivities of the corresponding glasses before heat treatment. The 90Na3PS4·10Na4SiS4 electrolyte showed a wide electrochemical window of 5 V.

A novel discharge–charge mechanism of a S–P2S5 composite electrode without electrolytes in all-solid-state Li/S batteries

All-solid-state Li/S cells with high safety and high capacity were fabricated using a sulfur composite electrode prepared by mechanically milling S, P2S5 and a conductive additive (Ketjen black). The cells with 50 wt% sulfur content in the composite electrode showed a high reversible capacity of 942 mA h (g-sulfur)−1 at a constant current density of 0.64 mA cm−2 (0.1C). The discharge–charge mechanism of the high-capacity sulfur composite electrode without electrolytes was investigated. XRD and NMR measurements showed that amorphous P2S5+x species, where sulfur chains bridged phosphorus atoms, were produced in the as-milled composite electrode. Mixing of the amorphous P2S5+x and Ketjen black in the submicron order was indicated from the FE-SEM observation and EDX mapping of the electrode. XRD and TEM measurements of the sulfur electrodes before and after the discharge–charge processes indicated that the compounds in the electrodes remained in the amorphous state during these processes. XPS measurement showed that cleavages and associations of the disulfide bonds occurred in the amorphous compounds during the discharge–charge processes. A novel discharge–charge mechanism with an atomic-level dispersion of a sulfur redox part in an ion conductive part was proposed for the high-capacity sulfur electrode.

The crystal structure and sodium disorder of high-temperature polymorph β-Na3PS4

Solid sodium conductors are of great interest as electrolytes for all-solid sodium batteries and also for sodium–sulphur batteries. Here we provide the dynamic structures of crystalline Na3PS4 by high-temperature powder X-ray diffraction (XRD) and molecular dynamics (MD) simulations. At room temperature, Na3PS4 adopts a tetragonal structure: α-Na3PS4. [Jansen and Henseler, J. Solid State Chem., 1992, 99(1), 110] α-Na3PS4 transforms to a cubic superionic phase β-Na3PS4 at ca. 530 K, but its detailed structure has not been solved so far. The overall structure of β-Na3PS4 is understood as Tl3VS4-type with characteristic spreading of Na distribution, reflecting the dynamic motion of Na. The α to β structural transition involves expansion of bottlenecks of Na migration especially along the tetragonal c axis, and enables three-dimensional ionic transport.

Preparation and characterization of Na3BO3–Na2SO4 glass electrolytes with Na+ ion conductivity prepared by a mechanical milling technique

Abstract The (100 − x)Na3BO3·xNa2SO4 (0 ≤ x (mol%) ≤ 50) glasses were prepared by mechanical milling. Halo patterns were observed in the compositions 0 ≤ x ≤ 50 by XRD measurements. The Raman spectra indicated that all the glasses were composed of BO33− anions, SO42− anions and Na+ cations. The (100 − x)Na3BO3·xNa2SO4 glasses showed good deformation properties and a dense pellet was prepared by cold-press. The conductivities of the glasses increased with increasing Na2SO4 content, and the 50Na3BO3·50Na2SO4 glass showed the highest conductivity of 5.9 × 10−8 S cm−1 at 25 °C.