Characterization and control of lithium-ion battery interfaces

Energy production from any source, be it fossil fuels, coal, natural gas, solar, wind brings with it the question on how to distribute, and or how to store the energy produced. At the large scale, technologies such as pump-hydro and thermal energy storage are usually the best available options. When it comes to small scale, day-to-day use of energy, the lithium-ion battery has become the mode of choice for most applications, be it electronics, powered tools, and, more recently, the advent of the electric car. In this thesis we present many approaches to the study of a key component of the lithium-ion battery: the solid-electrolyte interphase (SEI). The SEI provides mechanical and chemical stability to the battery and forms on the surface of the electrodes upon initial battery cycling. Chapter 4 will show an operando approach using vibrational sum-frequency (SFG) spectroscopy (a surface sensitive technique) to observe the real time growth of one of the main components of the SEI, lithium ethylenedicarbonate. The observations made provided insight over the dynamic nature at the SEI formation over many battery cycles, which was at the time still debated in literature. Chapter 5 details the attempt to apply matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) to identify materials formed at cathode surfaces after cyclic voltammetry experiments. Although the method provided insight into organic components of the SEI on anodic surfaces in the past, the work here showed that MALDI-MS is less effective than previously thought. Ultimately, it was shown that desorption electrospray ionization mass spectrometry (DESI-MS) is a much better method to identify the polymeric decomposition product, shown to be poly(ethylene glycol) dimethyl ether (PEG). Lastly, Chapter 6 details the attempt to control the electrochemical activity as well as the formation of a more functional interfacial layer than the naturally formed SEI on lithium manganese oxide (LMO) cathodes utilizing alkylphosphonic acids to decorate the metal oxide surface with self-assembled monolayers (SAM). In this work we found that the electrochemical behavior of LMO half-cells could be tuned by choosing different alkyl moieties for the SAMs. A combination of materials characterization techniques and computational approaches indicates that this tunability is mainly related to the wettability of these “artificial SEI”. The alkylphosphonate layer was shown to protect the LMO particles from chemical attack by HF impurities in the electrolyte when decorated with a 16 carbon alkylphosphonate, decreasing manganese dissolution by about 90%. Upon galvanostatic cycling, however, protection from Mn dissolution proved to be more modest, yielding an improvement of about 5% in capacity retention versus uncoated LMO particles