Search for a space charge layer in thin film battery materials with low-energy muons

In an all solid state Li-ion battery, it is crucial to reduce ionic resistivity at the interface between the electrode and the electrolyte in order to enhance Li⁺ mobility across the interface. Recent first principles calculations predict the presence of a space-charge layer (SCL) at the interface due to the difference in the Li⁺ chemical potential at the interface between two different materials, as in the metal-semiconductor junction in electronic devices. However, the presence of SCL has never been experimentally observed. Our first attempt in a fresh multilayer sample, Cu(10 nm)/Li₃PO₄(50 nm)/LiCoO₂(100 nm) on a sapphire substrate, with low-energy µ⁺SR (LE µ⁺SR) revealed a gradual change in the nuclear magnetic field distribution width as a function of implantation depth even across the interface between Li₃PO₄ and LiCoO₂. This implies that the change in the field distribution width at SCL of the sample is too small to be detected by LE µ⁺SR.