Heat treatment to reduce resistance in solid-state batteries

Researchers introduce a strategy to restore solid-state batteries low electrical resistance, exploring their underlying reduction mechanism.

All-solid-state lithium batteries have become the new requirement in materials science and engineering. This is because conventional lithium-ion batteries can no longer achieve the standards for advanced technologies, such as electric vehicles, which demand high energy densities, fast charging, and long cycle lives. All-solid-state batteries, which use a solid electrolyte as a replacement for the liquid electrolyte traditionally found in batteries, not only meet these standards but are comparatively safer and more convenient as they have the capability to charge in a short time.

However, the solid electrolyte comes with its own challenge. The interface between the positive electrode and solid electrolyte highlights a large electrical resistance whose origin is not well understood. Furthermore, the resistance increases when the electrode surface is exposed to air, degrading the battery capacity and performance. While several attempts have been conducted to lower the resistance, none have yet succeeded in reducing it to to 10 Ω cm2 (ohm centimetre-squared), which is the reported interface resistance value when not exposed to air.

Although now, in a recent study published in ACS Applied Materials & Interfaces, a research team led by Professor Taro Hitosugi from the Institute of Technology (Tokyo Tech), Japan, and Shigeru Kobayashi, a doctoral student at Tokyo Tech, have attempted to solve this problem.

Solid-state battery analysis

By establishing a strategy for restoring the low interface resistance as well as analysing the mechanism underlying this reduction, the team has provided valuable insights into the manufacturing of high-performance all-solid-state batteries. The study was the result of a research collaboration by Tokyo Tech, National Institute of Advanced Industrial Science and Technology (AIST), and Yamagata University.

To begin, the team prepared thin film batteries comprising of a lithium negative electrode, an LiCoOpositive electrode, and an Li3POsolid electrolyte. Then, the researchers completed the fabrication of the battery. The team then exposed the LiCoO2 surface to air, nitrogen (N2), oxygen (O2), carbon dioxide (CO2), hydrogen (H2), and water vapor (H2O) for 30 minutes.

The results

They discovered that exposure to N2, O2, CO2, and H2, did not degrade the battery performance compared to a non-exposed battery. “Only H2O vapor strongly degrades the Li3PO4 – LiCoO2 interface and increases its resistance drastically to a value more than 10 times higher than that of the unexposed interface,” explained Professor Hitosugi.

The team next performed a process called ‘annealing’, in which the sample underwent a heat treatment at 150°C for an hour in battery form, with the negative electrode deposited. As a result, this reduced the resistance down to 10.3 Ω cm2; this is comparable to that of the unexposed battery.

By performing numerical simulations and cutting-edge measurements, the team then revealed that the reduction could be attributed to the spontaneous removal of protons from within the LiCoO2 structure during annealing.

“Our study shows that protons in the LiCoO2 structure play an important role in the recovery process,” concluded Hitosugi. “We hope that the elucidation of these interfacial microscopic processes would help widen the application potential of all-solid-state batteries.”

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