Researchers have begun focusing on the mineral argyrodite to usher in a new generation of 100% solid-state batteries, and a team at the Korea Institute of Science and Technology appears to have hit the nail on the head. Their new battery skips a costly high-heat, days-long synthesizing process in favor of a new formula that can be cooked up in one pot.
Why Solid-State Batteries Are Better
As the name suggests, solid-state batteries sport a solid electrolyte instead of the liquid electrolyte found in conventional lithium-ion batteries. Energy storage researchers in search of a better battery have been flocking to the solid-state field on account of their advantages over conventional batteries.
Batteries with a solid electrolyte provide for improved safety with the potential for lower costs and less environmental baggage over their lifecycle. They also offer higher energy density, which translates into longer range and lighter weight compared to conventional batteries.
CleanTechnica has been following the R&D trail towards commercially viable solid-state energy storage technology. The level of activity has ratcheted up over the past two years as leading EV makers catch wind of the opportunity to cut costs and improve performance. Practically everyone is piling on, from BMW, Vinfast, Ford, and Nissan to General Motors and Dongfeng Motor, among others.
The Argyrodite Solution For Sold-State Batteries
Despite all the activity, argyrodite has not come across the CleanTechnica radar yet. Our bad!
Argyrodite was first identified in 1886, in Germany. Our friends over at Wikipedia describe it as an “uncommon silver germanium sulfide material with formula Ag8GeS6. The color is iron-black with a purplish tinge, and the luster metallic.”
In recent years the name has also been applied to other minerals with a similar crystalline structure.
Researchers from The Netherlands and China took a look at argyrodite Li6PS5Br back in 2017, attracted by its potential for use in solid state batteries.
“However, more understanding is required on the relation between the solid electrolyte conductivity and the solid-state battery performance with the argyrodite structure, crystallinity and particle size that depend on the synthesis conditions,” they observed.
Their study indicated that solid-state battery performance can be improved by using larger particles within the electrolyte itself, and smaller particles in the cathode.
“Comparing the bulk and interfacial properties of the differently prepared Li6PS5Br materials, it is proposed that optimal solid-state battery performance requires a different particle size for the solid electrolyte only region and the solid electrolyte in the cathode mixture,” they concluded.
KIST Steps Up To The Argyrodite Plate
Much has happened since then. In 2020, the research team of Dr. Hyoungchul Kim at the Korea Institute of Science and Technology’s Center for Energy Materials Research reported that they developed a superionic conductor based on argyrodite, for use in solid-state batteries.
They explained that the high Li-ion concentration and the stability of argyrodite are two features that make it attractive as a solid-state electrolyte material. The problem is that the Li-ions are trapped in the cage-like, octahedral structure.
The KIST team resolved the problem by tweaking the structure at the atomic level with chlorine.
“The new solid electrolyte material…has an Li-ion conductivity of 10.2 mS/cm, which is equivalent to that of a conventional liquid electrolyte at room temperature, and still maintains electrochemical stability under various battery operating conditions,” they reported.
To ice the solid-state cake, they also developed a new method for synthesizing the electrolyte. The new process lopped about 30% off the time needed to synthesize a conventional solid-state electrolyte.
“While the conventional solid-state reaction process requires more than several days of processing time, this study proposed a simple synthesis method that combines the nanocrystalline nucleation process and an infrared rapid heat treatment technology to shorten the process time to within 10 hours,” the team reported.
Next Steps For Argyrodite Batteries: One Pot, Low Heat
Last month, Dr. Kim’s team was back with the latest results, published in the journal Advanced Functional Materials.
They noted that the main obstacle for argyrodite solid-state batteries is to maximize the structure, in order to meet or beat the ionic conductivity of liquid electrolytes. However, that typically requires a multi-step process. The material is first concocted, and then subjected to high heat of more than 500°C for a period of several days.
If that sounds cumbersome and expensive, it is. “It result[s] in high process costs and battery interface contact issues due to reduced mechanical deformability,” the KIST team notes.
The new research describes the successful synthesis of a high-performing solid electrolyte in a “one-pot” process, without the need for added heat, let alone high heat. The process can be carried out at room temperature, under normal pressure, and it can be completed in less than 15 hours.
You can get all the details in Advanced Functional Materials under the title, “Annealing-Free Thioantimonate Argyrodites with High Li-Ion Conductivity and Low Elastic Modulus,” in which the team describes how they fine-tuned their argyrodite electrolyte.
“The target thioantimonate, Li5.2Si0.2Sb0.8S4Br0.25I1.75, comprising bimetallic tetrahedra and bi-halogen anions is synthesized by two-step milling tuned for in situ crystallization, and exhibits excellent Li-ion conductivity (σion) of 13.23 mS cm−1 (averaged) and a low elastic modulus (E) of 12.51 GPa (averaged),” the team explains, adding that it has “a cubic argyrodite phase of ≈57.39% crystallinity with a halogen occupancy of ≈90.67% at the 4c Wyckoff site.”
Or, you can refer to Dr. Kim for a birds-eye view of the new research. “The new material will serve as a trigger for the commercialization of all-solid-state batteries suitable for electric vehicles and energy storage systems (ESS) because it has maximized material productivity by eliminating the high-temperature heat treatment and simultaneously possesses high deformability and superionic conductivity suitable for solving the problem of the electrode interface of all-solid-state batteries,” Dr. Kim explains.
Next Steps For The Electric Vehicle Of The Future
As for when you can get behind the wheel of a new electric vehicle sporting argyrodite solid-state batteries, that’s an open question. The emerging consensus among industry insiders is that solid-state batteries are still in the R&D phase, though semi-solid versions could break out of the lab earlier.
Lisa Drake, who is Vice President, EV Industrialization, at Ford Motor Company, sat down for an investor chat hosted by Bank of America in which she cited a commercialization timeline of 2030 for 100% solid-state batteries (follow the link for a transcript).
Meanwhile, why wait for the zero emission ride of the future? Improved versions of liquid electrolyte batteries are on the market already if you need four-wheeled transportation, and a decent e-bike can get you around town with a much smaller battery, solid-state or not.
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Image: Argyrodite formula for solid-state batteries courtesy of Korea Institute of Science and Technology via Eurekalert.
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