UC San Diego

Lithium ion batteries have become one of the most important rechargeable energy storage devices used in our modern society today. As the demand for such devices shift from portable electronics to electric vehicles and large scale storage in order to utilize energy sustainably, ever increasing energy densities both in terms of weight and volume are needed. To satisfy this demand, lithium ion batteries utilizing solid state electrolytes show promise of a new paradigm shift in energy storage technologies. The introduction of solid state electrolyte could, in principle, yield many advantages over conventional lithium ion batteries. Foremost, lithium metal can be used as the anode along with a high voltage cathode to boost energy density. Secondly, removal of flammable liquid electrolyte greatly improves the inherent safety of the battery.

We focused on using Focused Ion Beam (FIB) nano-fabrication technique to prepare Transmission Electron Microscopy (TEM) samples of all-solid-state batteries produced through physical vapor deposition techniques. The particular full cell chemistry of lithium cobalt oxide (LiCoO2) as cathode, amorphous silicon (a-Si) as anode, and lithium phosphorus oxynitrdie (LiPON) as electrolyte was used for investigations. Through analysis of TEM images and electron energy loss spectroscopy (EELS), important interfacial phenomena were observed at the anode-electrolyte interface and the cathode-electrolyte interface. Overcharging of the anode resulted in accumulation of lithium at the anode-current collector interface and interdiffusion of phosphorus and silicon atoms at the anode-electrolyte interface.

Furthermore, we developed a unique methodology using FIB fabrication techniques to prepare electrochemically active TEM samples of all-solid-state nanobatteries that can be galvanostatically charged in the FIB or TEM. This new methodology enabled in situ TEM observations of a previously undiscovered interfacial layer between the LiCoO2 cathode and LiPON electrolyte. This interfacial layer is composed of a highly disordered rocksalt like cobalt oxide phase that is oxidized and forms lithium oxide species during in situ charge. Additionally, electrochemically cycling at elevated temperatures (80 °C) causes further decomposition of the cathode layer decreasing the overall capacity and increasing interfacial impedance of the cell. These results indicate that proper engineering of electrode-electrolyte interface is essential for the performance of all-solid-state batteries.