Reducing interface resistance in solid-state lithium batteries via annealing

Though promising in terms of energy density and charge-discharge cyclability, the performance of all-solid-state lithium batteries is undermined by significant resistance at the interface between the solid electrolyte and positive electrode. While the origin of the electrical resistance has not been fully determined, it is well known it increases more than tenfold when the electrode surface is exposed to air.

Several attempts have been made to lower such resistance but none have managed to bring it down to the 10Ωcm² (ohm centimeter-squared) interface resistance value of devices which are not exposed to air. Now, researchers from Japanese bodies the Tokyo Institute of Technology, the National Institute of Advanced Industrial Science and Technology, and Yamagata University have reported promising results and demonstrated a drastic reduction of electrical resistance is achievable by annealing entire battery cells.

The team prepared thin film batteries comprising a lithium negative electrode, an LiCoO₂ positive electrode, and an Li₃PO₄ solid electrolyte. Before completing fabrication of a battery, they 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 exposure to N₂, O₂, CO₂, and H₂, did not degrade 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 Taro Hitosugi, from Tokyo Tech.

The team then performed annealing, with a sample undergoing a heat treatment at 150 degrees Celsius for an hour in battery form: with the negative electrode deposited. As a result, resistance was reduced to the initial 10.3Ωcm². Simulations and measurements revealed 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. We hope that the elucidation of these interfacial microscopic processes would help widen the application potential of all-solid-state batteries,” said Hitosugi.

The results are described in the paper “Drastic Reduction of the Solid Electrolyte–Electrode Interface Resistance via Annealing in Battery Form,” published in ACS Publications.