Solid Electrolyte Promises All-Solid-State Batteries with Lithium Anodes
Researchers from Tohoku University and the High Energy Accelerator Research Organization have developed a new complex hydride lithium superionic conductor that could lead to all-solid-state batteries with the highest energy density so far. (See diagram above).
The scientists observed that the new material, which they designed with structures of hydrogen clusters (complex anions), demonstrates high stability against lithium metal, making it a potential anode material for all-solid-state batteries. All-solid-state batteries incorporating a lithium metal anode could address the energy density issues of conventional lithium-ion batteries.
However, the instability of the electrolyte against lithium metal is the primary cause of the lithium-ion transfer resistance that has limited the use of lithium metal anodes. For this reason, the researchers say that this new solid electrolyte that exhibits both high ionic conductivity and high stability against lithium metal could be used for all-solid-state batteries that utilize a lithium metal anode.
"We expect that this development will not only inspire future efforts to find lithium superionic conductors based on complex hydrides, but also open up a new trend in the field of solid electrolyte materials that may lead to the development of high-energy-density electrochemical devices," said Sangryun Kim of Shin-ichi Orimo's research group at Tohoku University.
Lithium-ion batteries have intrinsic drawbacks including limited energy density, electrolyte leakage, and flammability. All solid-state batteries hold promise for resolving these inherent issues.
Among known anode materials, lithium metal is widely considered to be the ultimate anode material for all-solid-state batteries because it has the highest theoretical capacity (3860 mAh/g) and the lowest potential (-3.04V compared to a standard hydrogen electrode)
Lithium-ion-conducting solid electrolytes are a vital component of all-solid-state batteries because of the combination of the ionic conductivity and the stability of the solid electrolyte determine battery performance.
Most existing solid electrolytes have chemical or electrochemical instability and/or poor physical contact against lithium metal, unavoidably causing undesired side reactions at the interface. These side reactions lead to an increase in interfacial resistance, significantly degrading battery performance during repeated cycling.
As demonstrated in previous studies, which proposed strategies including interface modification and alloying the lithium metal, this degradation process is challenging to address because its source is the high thermodynamic reactivity of the lithium metal anode with the electrolyte.
The main obstacles to utilizing the lithium metal anode are high stability and high lithium ion conductivity of the solid electrolyte.
Sangryun Kim from Tohoku University's Institute of Material Research (IMR) and Shin-ichi Orimo from the University's Advanced Institute for Materials Research (AIMR) led the Tohoku University team.
"Complex hydrides have received a lot of attention in addressing the problems associated with the lithium metal anode because of their outstanding chemical and electrochemical stability against the lithium metal anode," said Kim. "But because of their low ionic conductivity, using complex hydrides with the lithium metal anode have never been attempted in practical batteries. So we were very motivated to see if developing complex hydride that exhibit lithium superionic conductivity at room temperature can enable the use of lithium metal anode. And it worked."