All-solid-state lithium batteries could address a number of the shortcomings of conventional lithium-ion batteries in advanced applications such as in electric vehicles, which demand high energy densities, fast charging, and long cycle lives. However, the solid electrolyte comes with its own challenge; 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—the reported interface resistance value when not exposed to air.

Now, in an open-access paper published in ACS Applied Materials & Interfaces, a research team led by Prof. Taro Hitosugi from Tokyo Institute of Technology (Tokyo Tech) and Shigeru Kobayashi demonstrates that drastic reduction of the resistance is achievable by annealing the entire battery cell.

Exposing the LiCoO₂ positive electrode surface to H₂O vapor increases the resistance by more than 10 times (to greater than 136 Ω cm²). The magnitude can be reduced to the initial value (10.3 Ω cm²) by annealing the sample in a battery form. First-principles calculations reveal that the protons incorporated into the LiCoO₂ structure are spontaneously deintercalated during annealing to restore the low-resistance interface. These results provide fundamental insights into the fabrication of high-performance all-solid-state Li batteries.

—Kobayashi et al.

The team prepared thin film batteries comprising a lithium negative electrode, an LiCoO₂ positive electrode, and an Li₃PO₄ solid 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.

The researchers 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, said Prof. Hitosugi.

The team next heated the sample at 150°C for an hour in battery form i.e. with the negative electrode deposited. This reduced the resistance down to 10.3 Ω cm₂, comparable to that of the unexposed battery.

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

The study was the result of a joint research by Tokyo Tech, National Institute of Advanced Industrial Science and Technology (AIST), and Yamagata University.

Resources

• Shigeru Kobayashi, Elvis F. Arguelles, Tetsuroh Shirasawa, Shusuke Kasamatsu, Koji Shimizu, Kazunori Nishio, Yuki Watanabe, Yusuke Kubota, Ryota Shimizu, Satoshi Watanabe, and Taro Hitosugi (2022) “Drastic Reduction of the Solid Electrolyte–Electrode Interface Resistance via Annealing in Battery Form” ACS Applied Materials & Interfaces doi: 10.1021/acsami.1c17945