All-solid-state batteries usher in a major breakthrough and take a solid step towards commercialization

A research team from Tokyo Institute of Technology, AIST, and Yamagata University has recently invented a strategy to restore low resistance, thus taking a solid step on the road to commercializing all-solid-state batteries. They also explored the underlying reduction mechanism, paving the way for a fundamental understanding of how all-solid-state lithium batteries work.

All-solid-state lithium batteries have become a new craze in materials science and engineering because traditional lithium-ion batteries can no longer meet the standards of advanced technologies, such as electric vehicles requiring high energy density, fast charging, and long cycle life. All-solid-state batteries, which use a solid electrolyte instead of the liquid electrolyte found in conventional batteries, not only meet these standards, but are also relatively safer and more convenient because they have the potential to be charged in a short period of time.

However, solid electrolytes also have their own challenges. One of the important challenges is that the interface between the cathode and the solid electrolyte shows a large resistance, the source of which is not well understood. In addition, when the electrode surface is exposed to air, the resistance increases, degrading the capacity and performance of the battery. Although some attempts have been made to reduce the resistance, no one has been able to reduce it to 10Ω cm2 (ohm-centimeter-square), the reported interfacial resistance value when not exposed to air.

In a recent study published in ACS Applied Materials & Interfaces, a research team led by Professor Taro Hitosugi of Tokyo Institute of Technology (Tokyo Tech) in Japan and Shigeru Kobayashi, a doctoral student at Tokyo Institute of Technology, may have finally solved the problem.

By establishing a strategy to restore low interfacial resistance, and unraveling the mechanism of this reduction, the team provided valuable insights into the fabrication of high-performance all-solid-state batteries. The research is the result of a joint study by Tokyo Institute of Technology, Japan's National Institute of Advanced Industrial Technology (AIST) and Yamagata University.

First, the team prepared a thin-film battery consisting of a lithium anode, a lithium cobalt oxide cathode, and a 3PO4 solid electrolyte. Before completing the fabrication of the battery, the team exposed the lithium cobalt oxide surface to air, nitrogen (N2), oxygen (O2), carbon dioxide (CO2), hydrogen (H2) and water vapor (H2O) for 30 minutes.

To their surprise, they found that exposure to N2, O2, CO2 and H2 did not degrade the performance of the cells compared to unexposed cells. "Only the H2O vapor strongly degrades the Li3PO4-LiCoO2 interface and dramatically increases its resistance value, which is more than 10 times higher than that of the unexposed interface," said Prof. Hitosugi.

The team next performed a process called "annealing," in which the sample was subjected to a battery-style heat treatment at 150°C for an hour, where the negative electrode was deposited. Surprisingly, this brought the resistance down to 10.3Ω cm2, which is comparable to the resistance of an unexposed cell. By performing numerical simulations and cutting-edge measurements, the team then found that this reduction could be attributed to the spontaneous removal of protons from the lithium dioxide structure during "annealing."

Prof. Hitosugi concludes: "Our study shows that protons in the lithium cobaltate structure play an important role in the recovery process. We hope that the elucidation of these interfacial microscopic processes will help to broaden the application potential of all-solid-state batteries".