Investigation of the Solid Electrolyte Interface on LiMn2O4 through Neutron Reflectometry.

Abstract

The Solid Electrolyte Interface (SEI) is of significant importance to successful operation of new high-energy-density lithium-ion batteries. The SEI governs chemical interaction between the electrode (cathode or anode) and the electrolyte in addition to modulating the diffusion of lithium between the mediums. SEI stability throughout the lifetime of the lithium-ion battery is essential for long-term stable operation. While both the anode and cathode exhibit an SEI growth, the anode growth is much thicker and more easily probed. In contrast, the SEI on the cathode is very thin on the order of nanometers and has proven to be much more difficult to probe in-situ.

This dissertation focuses on the application of Neutron Reflectometry (NR) to the study of the SEI on a Li_xMn_yO_z thin-film cathode. A cathode which meets the stringent morphological requirements for NR is developed with a roughness <2 nm rms. The thin-film cathode is produced by a sol-gel spinning technique on a silicon substrate with platinum and silicon dioxide thin-films which serve as charge collection and buffer layer. A method of electro-depositing lithium on a copper thin-film is developed to produce a dendrite-limited lithium thin-film anode to serve as the counter/reference electrode in an experimental cell. An electrochemical NR sample cell is built to produce a fully-functional full-cell battery for in-operando NR experimentation.

Two in-operando NR experiments are performed where one sample is allowed to sit at an open-circuit, and the other has the potential held slightly above the reported open-circuit potential for LiMn2O4 to prevent self-discharge. NR results indicate a SEI of thickness between 10-16 nm is formed immediately upon introduction of the electrolyte. The scattering length density of the SEI which forms during the open circuit experiment is observed to be lower than that of the SEI on the potential hold experiment. The thickness of the cathode layer is observed to increase throughout the experiment to an extent which is not explainable by known volume expansion of LiMn2O4 through a Jahn-Teller effect. The SEI formed by cycling the battery prior to allowing self-discharge to form LiMnO2 is observed to prevent significant degradation of the cathode layer.