A research group in the doctoral program of Toyohashi University of Technology’s Department of Electrical and Electronic Information Engineering that includes a doctoral student Hirotada Gamo and specially appointed assistant professor Jin Nishida, specially appointed associate professor Atsushi Nagai, assistant professor Kazuhiro Hikima, professor Atsunori Matsuda and others, developed a large-scale manufacturing technology of Li7P3S11 solid electrolytes for all-solid-state lithium-ion secondary batteries.
This method involves the addition of an excessive amount of sulfur (S) along with Li2S and P2S5, the starting materials of Li7P3S11, to a solvent containing a mixture of acetonitrile (ACN), tetrahydrofuran (THF) and a slight amount of ethanol (EtOH). This helped to shorten the reaction time from 24 hours or longer to only two minutes. The final product obtained using this method is highly pure Li7P3S11 without an impurity phase that showed high ionic conductivity of 1.2 mS cm-1 at 25 degC. These results enable us to produce a large quantity of sulfide solid electrolytes for all-solid-state batteries at low cost. The results of the research were published online by Advanced Energy and Sustainability Research on April 28, 2022.
All-solid-state batteries are expected to be the next generation of batteries for electric vehicles (EVs) because they are very safe and enable a transition to high energy density and high output power. With a view to the potential applications of all-solid-state batteries for electric vehicles (EVs), sulfide solid electrolytes have been actively researched. Unfortunately, commercialization of large-scale manufacturing technology has not been possible for sulfide liquid electrolytes. This is because sulfide electrolytes are inherently unstable in the environment and require atmospheric control. It is urgent that a liquid-phase manufacturing technology for sulfide-solid electrolytes be developed. This will allow for low-cost, high-scalability.
Li7P3S11 solid electrolytes exhibit high ionic conductivity and thus are one candidate solid electrolyte for all-solid-state batteries. The liquid-phase synthesis of Li7P3S11 generally occurs in an acetonitrile (ACN) reaction solvent via precursors including insoluble compounds. These conventional reaction processes take a lot of time because they undergo a kinetically undesirable reaction from an insoluble start material to an insoluble intermediary. It is possible for the insoluble intermediate to cause non-uniformity due to a complex phase formation. This can lead to an increase of large-scale manufacturing cost.
Against this background, the research group worked on the development of a technology for liquid-phase production of highly ion conductive Li7P3S11 solid electrolytes via uniform precursor solutions. It has been shown that the recently developed method can obtain a uniform precursor solution containing soluble lithium polysulfide (Li2Sx) in just two minutes, by adding Li2S and P2S5, the starting materials of Li7P3S11, and an excessive amount of S to a solvent containing a mixture of ACN, THF and a small amount of EtOH. This method allows for rapid synthesis by adding a small amount or excessive amount of EtOH to form lithium polysulfide.
To elucidate the mechanism of the reaction in this method, ultraviolet-visible (UV-Vis) spectroscopy was used to investigate the chemical stability of Li2Sx with and without the added EtOH. The study showed that the presence of EtOH made Li2Sx more chemically stable. The following steps would be required to achieve the reaction described in this method. First, lithium ions can be strongly coordinated with EtOH (a highly polar solvent). Next, shielding polysulfide ions against lithium ions stabilizes highly reactive S3*– radical anions which are a kind of polysulfide. The generated S3*– attacks the P2S5, breaking the cage structure of P2S5 and causing the reaction to progress. This reaction produces lithium thiophosphate, which is dissolved into a highly soluble mixture solvent containing ACN or THF solvents. This may have helped to