Atsushi Sakuda

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.

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.

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.

LiCoO2 Electrode Particles Coated with Li2S – P2S5 Solid Electrolyte for All-Solid-State Batteries

Electrode/electrolyte composite materials for all-solid-state lithium secondary batteries were prepared by coating the 80Li 2 S·20P 2 S 5 (mol %) solid electrolyte onto LiCoO 2 electrode particles using the pulsed laser deposition method. Cross-sectional transmission electron microscopy images showed that the solid electrolyte layer was formed on the LiCo0 2 particles. The all-solid-state cell using the LiCoO 2 particles coated with the solid electrolyte was charged and discharged, and exhibited good cycle performance. This suggests that the electrolyte coatings provide a lithium-ion conduction path to LiCoO 2 , and the technique is effective in the development of all-solid-state cells.

All-Solid-State Lithium Secondary Batteries Using NiS-Carbon Fiber Composite Electrodes Coated with Li2S–P2S5 Solid Electrolytes by Pulsed Laser Deposition

Composite materials including NiS active materials, sulfide-based solid electrolytes (SE), and conductive additives (VGCF: vapor grown carbon fiber) were prepared by coating a highly conductive Li(2)S-P(2)S(5) solid electrolyte onto NiS-VGCF composite using pulsed laser deposition (PLD). From scanning electron microscopy, NiS nanoparticles were on VGCF surface after coating of solid electrolytes using PLD. All-solid-state cells using the SE-coated NiS-VGCF composite and the uncoated NiS-VGCF composite were fabricated, and then the coating effects on the electrochemical performance by forming the SE thin film onto the NiS-VGCF composite were investigated. At a high current density of 3.8 mA cm(-2) (corresponding to ca. 1 C), an all-solid-state cell fabricated using the SE-coated NiS-VGCF composite as a working electrode showed the initial discharge capacity of 300 mA h g(-1), and exhibited better cycle performance than the cell using the uncoated NiS-VGCF composite.

All-solid-state lithium secondary batteries with metal-sulfide-coated LiCoO2 prepared by thermal decomposition of dithiocarbamato complexes

LiCoO2 particles were coated with cobalt and nickel sulfides by thermal decomposition of their respective diethyldithiocarbamato complexes. All-solid-state lithium secondary batteries were fabricated using the coated LiCoO2 positive electrode and a Li2S–P2S5 solid electrolyte. The coatings reduced the interfacial resistance between LiCoO2 and the Li2S–P2S5 solid electrolyte of the all-solid-state batteries after the first charge, resulting in an improved cell performance. The all-solid-state cell with NiS-coated LiCoO2 was charged and discharged at a high rate of 10 C. The coatings reduced deterioration of the interface between LiCoO2 and the Li2S–P2S5 solid electrolyte, indicating that they function as an effective buffer layer at the interface.

Development of Sulfide Solid Electrolytes and Interface Formation Processes for Bulk-Type All-Solid-State Li and Na Batteries

All-solid-state batteries with inorganic solid electrolytes are recognized as an ultimate goal of rechargeable batteries because of their high safety, versatile geometry and good cycle life. Compared to thin-film batteries, increasing the reversible capacity of bulk-type all-solid-state batteries using electrode active material particles is difficult because contact areas at solid–solid interfaces between the electrode and electrolyte particles are limited. Sulfide solid electrolytes have several advantages of high conductivity, wide electrochemical window, and appropriate mechanical properties such as formability, processability, and elastic modulus. Sulfide electrolyte with Li7P3S11 crystal has the highest Li+ ion conductivity of 1.7 × 10-2 S cm-1 at 25 °C. It is far beyond the Li+ ion conductivity of conventional organic liquid electrolytes. The Na+ ion conductivity of 7.4 × 10-4 S cm-1 is achieved for Na3.06P0.94Si0.06S4 with cubic structure. Moreover, formation of favorable solid–solid interfaces between electrode and electrolyte is important for realizing solid-state batteries. Sulfide electrolytes have better formability than oxide electrolytes. Consequently, a dense electrolyte separator and closely attached interfaces with active material particles are achieved via “room-temperature sintering” of sulfides merely by cold pressing without heat treatment. Elastic moduli for sulfide electrolytes are smaller than that of oxide electrolytes, and Na2S-P2S5 glass electrolytes have smaller Young’s modulus than Li2S-P2S5 electrolytes. Cross-sectional SEM observations for a positive electrode layer reveal that sulfide electrolyte coating on active material particles increases interface areas even with a minimum volume of electrolyte, indicating that the energy density of bulk-type solid-state batteries is enhanced. Both surface coating of electrode particles and preparation of nanocomposite are effective for increasing the reversible capacity of the batteries. Our approaches to form solid–solid interfaces are demonstrated.

Rock-salt-type lithium metal sulphides as novel positive-electrode materials

One way of increasing the energy density of lithium-ion batteries is to use electrode materials that exhibit high capacities owing to multielectron processes. Here, we report two novel materials, Li2TiS3 and Li3NbS4, which were mechanochemically synthesised at room temperature. When used as positive-electrode materials, Li2TiS3 and Li3NbS4 charged and discharged with high capacities of 425 mA h g−1 and 386 mA h g−1, respectively. These capacities correspond to those resulting from 2.5- and 3.5-electron processes. The average discharge voltage was approximately 2.2 V. It should be possible to prepare a number of high-capacity materials on the basis of the concept used to prepare Li2TiS3 and Li3NbS4.

Evaluation of mechanical properties of Na2S–P2S5 sulfide glass electrolytes

Mechanical properties such as formability and elastic moduli of solid electrolytes are important for the fabrication of all-solid-state batteries and retention of their charge–discharge capacities. In this paper, xNa2S·(100 − x)P2S5 (mol%) sulfide glass electrolytes (x = 50, 67, and 75) were prepared and their formability and elastic moduli were evaluated by an ultrasonic pulse-echo technique and a compression test under uniaxial pressing. The glasses were better densified and showed lower Youngs moduli (15–19 GPa) than the Li2S–P2S5 glasses.

High Reversibility of “Soft” Electrode Materials in All-Solid-State Batteries

All-solid-state batteries using inorganic solid electrolytes (SEs) are considered to be ideal batteries for electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) because they are potentially safer than conventional lithium-ion batteries (LIBs). In addition, all-solid-state batteries are expected to have long battery lives owing to the inhibition of chemical side reactions because only lithium ions move through the typically used inorganic SEs. The development of high-energy (more than 300 Wh kg-1) secondary batteries has been eagerly anticipated for years. The application of high-capacity electrode active materials is essential for fabricating such batteries. Recently, we proposed metal polysulfides as new electrode materials. These materials show higher conductivity and density than sulfur, which is advantageous for fabricating batteries with relatively higher energy density. Lithium niobium sulfides, such as Li3NbS4, have relatively high density, conductivity, and rate capability among metal polysulfide materials, and batteries with these materials have capacities high enough to potentially exceed the gravimetric energy density of conventional LIBs. Favorable solid-solid contact between the electrode and electrolyte particles is a key factor for fabricating high performance all-solid-state batteries. Conventional oxide-based positive electrode materials tend to be given rise to cracks during fabrication and/or charge-discharge processes. Here we report all-solid-state cells using lithium niobium sulfide as a positive electrode material, where favorable solid-solid contact was established by using lithium sulfide electrode materials because of their high processability. Cracks were barely observed in the electrode particles in the all-solid-state cells before or after charging and discharging with a high capacity of approx. 400 mAh g-1, suggesting that the lithium niobium sulfide electrode charged and discharged without experiencing substantial mechanical damage. As a result, the all-solid-state cells retained more than 90% of their initial capacity after 200 cycles of charging and discharging at 0.5 mA cm-2.