Akitoshi Hayashi

Superionic glass-ceramic electrolytes for room-temperature rechargeable sodium batteries

Innovative rechargeable batteries that can effectively store renewable energy, such as solar and wind power, urgently need to be developed to reduce greenhouse gas emissions. All-solid-state batteries with inorganic solid electrolytes and electrodes are promising power sources for a wide range of applications because of their safety, long-cycle lives and versatile geometries. Rechargeable sodium batteries are more suitable than lithium-ion batteries, because they use abundant and ubiquitous sodium sources. Solid electrolytes are critical for realizing all-solid-state sodium batteries. Here we show that stabilization of a high-temperature phase by crystallization from the glassy state dramatically enhances the Na(+) ion conductivity. An ambient temperature conductivity of over 10(-4) S cm(-1) was obtained in a glass-ceramic electrolyte, in which a cubic Na(3)PS(4) crystal with superionic conductivity was first realized. All-solid-state sodium batteries, with a powder-compressed Na(3)PS(4) electrolyte, functioned as a rechargeable battery at room temperature.

A sulphide lithium super ion conductor is superior to liquid ion conductors for use in rechargeable batteries

We report that a heat-treated Li2S–P2S5 glass-ceramic conductor has an extremely high ionic conductivity of 1.7 × 10−2 S cm−1 and the lowest conduction activation energy of 17 kJ mol−1 at room temperature among lithium-ion conductors reported to date. The optimum conditions of the heat treatment reduce the grain boundary resistance, and the influence of voids, to increase the Li+ ionic conductivity of the solid electrolyte so that it is greater than the conductivities of liquid electrolytes, when the transport number of lithium ions in the inorganic electrolyte is unity.

All-solid-state Li/S batteries with highly conductive glass–ceramic electrolytes

All-solid-state cells using sulfur-based cathode materials and Li2S–P2S5 glass–ceramic electrolytes were successfully prepared and exhibited excellent cycling performance at room temperature. The cathode materials consisting of sulfur and CuS were synthesized by mechanical milling using sulfur and copper crystals as starting materials. The cell performance was influenced by the milling time for the cathode materials and the cell with cathode materials obtained by milling for 15 min retained large capacities over 650 mA h g−1 for 20 cycles. Sulfur as well as CuS in cathode materials proved to be utilized as active materials on charge–discharge processes in the all-solid-state Li/S cells.

Formation of superionic crystals from mechanically milled Li2S–P2S5 glasses

A superionic crystal analogous to highly conductive thio-LISICON, which is a series of sulfide crystalline solid electrolytes such as Li4GeS4–Li3PS4, was successfully formed by the crystallization of mechanically milled Li2S–P2S5 glasses. The thio-LISICON phases have not been obtained by solid-phase reaction in the Li2S–P2S5 binary system so far, and this report is the first case of obtaining the thio-LISICON analogue in the binary system. The formation of this superionic crystal enhanced the conductivities of the glass, and the high ambient temperature conductivity of was achieved in the glass–ceramics derived from the Li-rich 80Li2S·20P2S5 (mol%) glass.

Sulfide Solid Electrolyte with Favorable Mechanical Property for All-Solid-State Lithium Battery

All-solid-state secondary batteries that employ inorganic solid electrolytes are desirable because they are potentially safer than conventional batteries. The ionic conductivities of solid electrolytes are currently attracting great attention. In addition to the conductivity, the mechanical properties of solid electrolytes are important for improving the energy density and cycle performance. However, the mechanical properties of sulfide electrolytes have not been clarified in detail. Here, we demonstrate the unique mechanical properties of sulfide electrolytes. Sulfide electrolytes show room temperature pressure sintering. Ionic materials with low bond energies and a highly covalent character, which is promising for achieving a high ionic conductivity, tend to be suitable for room-temperature processing. The Youngs moduli of sulfide electrolytes were measured to be about 20 GPa, which is an intermediate value between those of typical oxides and organic polymers.

Recent development of sulfide solid electrolytes and interfacial modification for all-solid-state rechargeable lithium batteries

Abstract Recent development of inorganic sulfide solid electrolytes and all-solid-state rechargeable lithium batteries with them is reviewed. Electrical conductivity, electrochemical stability and chemical stability of these sulfide electrolytes are reported. Formation of favorable solid–solid contacts between electrode and electrolyte is important in all-solid-state batteries. Useful techniques to achieving intimate electrode–electrolyte interfaces are proposed. Application of sulfur positive electrode and lithium metal negative electrode with large theoretical capacity to all-solid-state lithium batteries is demonstrated.

High-capacity Li2S–nanocarbon composite electrode for all-solid-state rechargeable lithium batteries

All-solid-state lithium secondary batteries with Li2S active materials and sulfide-based solid electrolytes (SEs) were fabricated and the Li2S electrodes were characterized. Li2S–nanocarbon composite electrodes were prepared by hand grinding and mechanical milling. Mechanical milling of a mixture of Li2S, acetylene black (AB), and Li2S–P2S5 SE enhanced the reversible capacity of the all-solid-state cells. Cross-sectional transmission electron microscopy images and electron-energy-loss spectroscopy maps revealed the formation of favorable contacts among Li2S, AB, and SE. Nanocomposites consisting of approximately 500 nm Li2S particles and AB and SE particles were well-dispersed both before and after charge–discharge cycles. The effect of reducing the particle size of the Li2S active material in the composite electrodes on cell performance was investigated. Cells containing milled Li2S with small particle sizes exhibited a charge capacity of about 1000 mA h g−1 under 0.064 mA cm−2 at 25 °C and they were charged and discharged at a high current density of 6.4 mA cm−2 (3.5 C). To improve the reversible capacity and the rate performance of all-solid-state cells, it is important to realize intimate contact among electrode components and reduce the particle size of active materials and composite electrodes by mechanical milling.

Improvement of High-Rate Performance of All-Solid-State Lithium Secondary Batteries Using LiCoO2 Coated with Li2O – SiO2 Glasses

Development of rate capability is one of the most important issues to be solved in all-solid-state lithium secondary batteries. The electrochemical performance of these batteries has been highly improved by coating LiCoO 2 particles with Li 2 O-SiO 2 thin film. The interfacial resistance between LiCoO 2 and the glass-ceramic electrolyte was decreased by those coatings. The rate capabilities of the cells using the coated LiCoO 2 particles were superior to that of the cell using noncoated LiCoO 2 . The Li 2 Si 3 coating was more effective than the SiO 2 coating in enhancing rate performance, suggesting that lithium-ion conductivity of the coating materials is important for high-rate performance.

Fabrication of electrode–electrolyte interfaces in all-solid-state rechargeable lithium batteries by using a supercooled liquid state of the glassy electrolytes

The softening behavior of a 80Li2S·20P2S5 (mol%) glass electrolyte was investigated and a favorable electrode–electrolyte interface was fabricated by sticking the supercooled liquid state of the 80Li2S·20P2S5 electrolyte on active material particles. A dense pellet of the glass electrolyte without an obvious grain boundary or any voids was prepared by softening the 80Li2S·20P2S5 glass by means of a hot press. The electrical conductivity of the pellet was 8.8 × 10−4 S cm−1 at room temperature. Sticking the solid electrolyte on the Li4Ti5O12 active material particles increased the contact area at the electrode–electrolyte interface and the utilization of the active material was increased in the all-solid-state cells. However, LiCoO2 reacted with the solid electrolyte during the hot press and the electrochemical performance of the cells using hot-pressed LiCoO2 with the glass electrolyte degraded. LiNbO3 coating suppressed the reaction of LiCoO2 with the solid electrolyte. The all-solid-state full-cell Li4Ti5O12/80Li2S·20P2S5 glass/LiNbO3-coated LiCoO2 prepared by hot press showed a larger reversible capacity of 120 mAh g−1 at 0.064 mA cm−2 compared with the full-cell prepared by cold press. The softening of the 80Li2S·20P2S5 glass electrolyte is an effective way for increasing the contact area between the active materials and solid electrolyte.

New lithium ion conducting glass-ceramics prepared from mechanochemical Li2S-P2S5 glasses

Amorphous solid electrolytes in the system Li 2 S-P 2 S 5 were prepared from a mixture of crystalline Li 2 S and P 2 S 5 using a mechanical milling technique at room temperature. In the composition range x≤87.5 of xLi 2 S(100-x)P 2 S 5 , conductivities of the glassy powders mechanically milled for 20 h were as high as 10 -4 S cm -1 at room temperature. The heat treatment of the 80Li 2 S.20P 2 S 5 glassy powders at around 220 °C produced dense glass-ceramics with high conductivity around 10 -3 S cm -1 at room temperature. The crystallization of conductive phases of Li 7 PS 6 , Li 3 PS 4 and unknown crystals from the glass matrix and the decrease of grain boundaries by the softening of the glassy powders are simultaneously achieved at relatively low temperatures, around 220 °C.