All-solid-state lithium battery will greatly increase the energy density and safety of the battery

All-solid-state lithium battery will greatly increase the energy density and safety of the battery

Most batteries consist of two solid electrochemically active layers called electrodes and are separated by a polymer membrane that is injected into a liquid or gel electrolyte. However, recent research has explored the possibility of an all-solid-state battery in which a liquid (potentially flammable) electrolyte will be replaced by a solid electrolyte, which enhances the energy density and safety of the battery.

Now, a team at the Massachusetts Institute of Technology has explored the mechanical properties of sulfide-based solid electrolyte materials for the first time to determine their mechanical properties when incorporated into batteries.

The new findings were published in this week's "Advanced Energy Materials" magazine by MIT graduate students Frank McGrogan and Tushar Swamy, materials science and engineering professor Krystyn Van Vliet, materials science and engineering professor Chen Mingqing, and Four undergraduate students, including the National Science Foundation's Undergraduate Research Experience (REU), managed by the MIT Materials Science and Engineering Center and its Materials Processing Center.

Lithium-ion batteries offer a lightweight energy storage solution that makes many of today's high-tech devices available, from smartphones to electric vehicles. However, in such a battery, the use of a solid electrolyte in place of a conventional liquid electrolyte can have significant advantages. At all weights, this all-solid-state lithium-ion battery can provide even greater energy storage at the battery level. They can also substantially eliminate the risk of tiny, finger-like metal protrusions called "dendrites" that can penetrate the electrolyte layer and cause short circuits.

“All-solid-state batteries are an attractive choice for performance and safety, but there are still some challenges,” says Van Vliet. In today's market-leading lithium-ion batteries, lithium ions pass through the liquid electrolyte from one electrode to the other while the battery is being charged, and then flow in the opposite direction during use. “These batteries are very effective, but liquid electrolytes are chemically unstable and even flammable,” says Van Vliet. “So, if the electrolyte is solid, it will be safer, and it will be smaller and lighter.”

However, the big problem with using such an all-solid battery is what kind of mechanical stress may occur in the electrolyte material inside the battery when the electrode is repeatedly charged and discharged. This cycle causes the electrode to expand and contract as lithium ions enter and leave their crystal structure. In rigid electrolytes, these dimensional changes can result in higher stresses. If the electrolyte is also brittle, a constant change in size can cause cracks and rapidly degrade battery performance, and may even create channels that facilitate densification of the dendrites formed by the battery, as in liquid electrolyte batteries. However, if the material resists fracture, those stresses can be absorbed before the material is rapidly cracked.

To date, the extreme sensitivity of sulfides to normal laboratory air has challenged the measurement of mechanical properties, including fracture toughness. To avoid this problem, the researchers performed mechanical tests in mineral oil baths to protect the samples from any chemical interaction with air or moisture. Using this technology, they were able to measure the mechanical properties of lithium sulfide in detail, and lithium sulfide is considered to be the most promising candidate for all-solid-state battery electrolytes.

"Solid electrolytes have many different candidates," McGrogan said. Other groups have studied the mechanical properties of lithium ion conductive oxides, but little research has been done on sulfides to date, even though they are capable of rapidly conducting lithium ions.

Previously, researchers used acoustic measurement techniques to pass sound waves through materials to detect their mechanical behavior, but the method did not quantify the resistance of the material to fracture. This new study uses fine-tip probes to enter the material and monitor its response, measuring the material's more important properties, including hardness, fracture toughness, and Young's modulus (measuring the material's ability to stretch under stress).

“The research team has measured the elastic properties of sulfide-based solid electrolytes, but did not measure fracture properties,” Van Vliet said. The fracture properties are critical to predict whether a material may break or break when used as an electrolyte in a battery.

The researchers found that the overall performance of the material is similar to the combination of plasticine or salt water toffee: it can be easily deformed when subjected to stress, but it can crack like a brittle glass sheet under sufficiently high stress.

“By understanding these properties in detail, you can calculate how much stress a material can withstand before breaking, and consider this information when designing a battery system,” says Van Vliet.

Sulfide materials have proven to be more brittle than the ideal materials used in batteries. "But as long as its properties are known and the system is properly designed, the material can still have the potential to be used as a solid electrolyte," McGrogan said. "You have to design around this knowledge."

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