Li-ion batteries (LIBs) have been widely used in the past 2 decades, due to their higher energy density and cycling ability, typically using carbon as an anode and layered LiCoO₂ as a cathode. The spinel, LiMn₂O₄ (LMO) is cheaper, safer, and more environmental friendly compared with widely-used LiCoO₂ cathodes. However, LMO is known to lose capacity during cycling, ultimately due to disproportionation reactions of Mn³⁺ transforming to Mn²⁺ and Mn⁴⁺, and the subsequent dissolution of Mn²⁺ into the electrolyte. Yet our understanding of this process remains rudimentary, and we expect that it can be effectively addressed through interfacial modification (e.g., coatings). This work specifically focuses on developing and using a well-defined model system to observe these interfacial instabilities, and to develop suitable approaches to prevent it.

Recently, we have successfully grown high quality epitaxial LMO film on multiple substrates by pulsed laser deposition (PLD). Since LMO film itself is a poor electrical conductor, we have designed systems with built-in current collectors that maintains the LMO-substrate epitaxy. Multiple approaches have been investigated to make the whole system conductive and suitable for cycling: using conductive substrates, a conductive buffer layer, or a conductive surface coating. Conductive substrate is the most straightforward method. High quality epitaxial LMO film has been grown on Nb:STO. For the conductive buffer layer approach, we have tried many possible material including TiN, Al:ZnO, ITO, SrRuO₃. It turns out that ITO and SrRuO₃ are robust and enable oriented LMO deposition. Graphene was adopted as conductive surface coating. We have successfully transferred graphene onto the surface of LMO film using an anhydrous method.

We focus on the in situ real-time surface X-ray studies of well orientated LMO films in the standard lithium ion battery electrolytes (e.g., LiPF6 in EC/DMC). Using the high quality model films grown by PLD, we have probed the mobility of ions and interfacial atomic- and molecular- structures across the electrolyte/electrode interface, including the surface/interface structure changes and reconstruction (using XRR and CTR), individual processes of cation adsorption (resonant XRR/CTR), insertion/extraction.