Near-Field IR Imaging and Spectroscopy of Interfaces and Interphases in Li-Ion Electrodes

Abstract

Most Li-ion battery chemistries involve the creation or transformation of materials under non-equilibrium environments. Because these chemical energy storage systems operate beyond the thermodynamic stability limits of the electrolytes, reactions between electrodes and electrolyte result in the formation of new phases and interphases, which hinder further electrolyte decomposition and assure battery operation over the lifespan of the application. The basic physico-chemical properties and long-term stability of these surface layers known as the solid electrolyte interphase (SEI) determine the performance and durability of the entire system [1,2]. However, our understanding of how and why some materials function, and, equally importantly, why some materials with many ideal characteristics actually fail, is mainly rudimentary and empirical. The requirements for long-term stability of Li-ion batteries are extremely stringent and necessitate control of the chemistry at a wide variety of temporal and structural length scales. However, the spatial resolution of standard optical spectroscopic and imaging techniques is limited by diffraction to light beam sizes on the order of the wavelength of the source i.e., ∼10 μm for IR and∼500 nm in the visible range. The advent of near-field optical methods during the past decades has led to the development of new advanced techniques for chemical analysis. This presentation provides an overview of novel imaging and spectroscopic optical methodologies, which exploit micro and nano-manipulation techniques and single crystal model electrodes to provide sufficient sample definition suitable for advanced multifunctional far- and near-field optical scanning probes and electrochemical characterization techniques. The SEI chemistry is known to be highly heterogeneous at the nanoscale, and although many compounds have been suggested as contributing, their distribution, relative concentration and functionality remain poorly understood [2]. To address this challenge we used the apertureless near-field scanning optical microscopy (aNSOM) (3-9), which combines the high resolution of AFM with the chemical specificity of IR spectroscopy, to resolve the SEI chemistry at high (∼20 nm) spatial resolution. Examples of detailed in situ molecular characterization of graphite Sn and Si electrodes surface and bulk processes at the nano-level scale exceeding the diffraction limit will be discussed [7-9]. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, under contract no. DE-AC02-05CH11231.