All-solid-state battery resistance reduced through heating

Battery research is progressing with leaps and bounds over the course of last few years owing to proliferation of consumer electronics, electric transportation and space research. All-solid-state batteries are being pegged as powerhouse of next-generation electronics, but they are marred with issues that haven’t been solved yet. One of them is resistance of these batteries. Now a new study by researchers from Tokyo Tech, AIST, and Yamagata University have claimed that they have managed to reduced the all-solid-state battery resistance through heating.

Researchers have also explores the underlying reduction mechanism, paving the way for a more fundamental understanding of the workings of all-solid-state lithium batteries.

All-solid-state lithium batteries are being increasingly seen as the answer to our energy storage needs as the currently used conventional lithium-ion batteries are in no position to fulfil current high energy storage requirements for advanced technologies, such as electric vehicles, which demand high energy densities, fast charging, and long cycle lives.

All-solid-state batteries use solid electrolyte instead of a liquid electrolyte thereby making them compliant with current standards as well as offer a safer and more convenient means of energy storage as they have the possibility to charge in a short time.

However, the solid electrolyte comes with its own challenge. It turns out that the interface between the positive electrode and solid electrolyte shows 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 made to lower the resistance, none have managed to bring it down to 10 Ω cm2 (ohm centimeter-squared), the reported interface resistance value when not exposed to air.

Now, in a recent study published in ACS Applied Materials & Interfaces, a research team led by Prof. Taro Hitosugi from Tokyo Institute of Technology (Tokyo Tech), Japan, and Shigeru Kobayashi, a doctoral student at Tokyo Tech, may have finally solved this problem. By establishing a strategy for restoring the low interface resistance as well as unraveling 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 joint research by Tokyo Tech, National Institute of Advanced Industrial Science and Technology(AIST), and Yamagata University.

To start off, the team prepared thin film batteries comprising a lithium negative electrode, an LiCoOpositive electrode, and an Li3POsolid electrolyte. Before completing the fabrication of a battery, the team exposed the LiCoO2 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 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,” says Prof. 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 i.e. with the negative electrode deposited. Amazingly, this reduced the resistance down to 10.3 Ω cm2, 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.

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