Abstract
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.
Highlights
All-solid-state batteries using inorganic solid electrolytes (SEs), used in place of conventional organic liquid electrolytes, have been studied because of their high safety, long cycle life, and versatile geometries (Takada, 2013; Tatsumisago et al, 2013; Tatsumisago and Hayashi, 2014)
We have reviewed recent developments related to sulfide SEs and interface formation processes for all-solid-state rechargeable batteries
Sulfide glass electrolytes with high alkali content were prepared by high-energy ball milling process
Summary
All-solid-state batteries using inorganic solid electrolytes (SEs), used in place of conventional organic liquid electrolytes, have been studied because of their high safety (non-flammability with no liquid leakage), long cycle life, and versatile geometries (Takada, 2013; Tatsumisago et al, 2013; Tatsumisago and Hayashi, 2014) These features are important for large rechargeable lithium batteries with high energy density for application to eco-cars, such as electric vehicles and plug-in hybrid vehicles. Li9.54Si1.74P1.44S11.7Cl0.3 has been reported to show the highest conductivity of 2.5 × 10−2 S cm−1 Using this superior electrolyte, high power competing with that of supercapacitors can be achieved in all-solid-state rechargeable lithium batteries (Kato et al, 2016b). To realize bulk-type all-solid-state batteries, the formation of favorable solid–solid interfaces between electrode and electrolyte is important in addition to the development of superior sulfide electrolytes. Formation of a solid–solid interface for using lithium metal negative electrodes is discussed
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