1, Sigray, Inc., Concord, California, United States
During operation, the active materials within lithium ion batteries break down and degrade. This is one of the limiting factors of a promising high energy density, low cost, and low toxicity electrode material - lithium manganese oxide (LiMnO2). LiMnO2 degrades at a much faster rate than other popular electrodes and, consequently, its main use is restricted to primary (single use) lithium cells. This accelerated degradation is mostly due to dissolution of Mn into the electrolyte and subsequently depositing on the surface of the graphitic negative electrode, altering the composition of the solid electrolyte interphase (SEI) layer and accelerating the consumption of available lithium. An understanding of the influence of operating conditions on the spatial and temporal dynamics of Mn remains elusive. Developing a spatial high-resolution chemical map of positive and negative electrodes from LiMnO2 cells at different stages during their lifetime presents an important step towards uncovering the relationship between the rate of degradation and the transport of Mn. High-resolution X-ray fluorescence (XRF) holds the potential to elucidate this link by identifying and quantifying the presence of Mn on the surfaces of electrodes.
While X-ray imaging techniques, such as X-ray microscopy and computed tomography, have grown in popularity in recent years for spatial analysis of lithium-ion batteries, those techniques suffer from spatial resolutions that can be, at best, down to the tens to hundreds of nanometers. As the Mn migration results in very thin coatings of Mn on the negative electrode (tens to single nanometers), X-ray imaging techniques are not well-suited for this type of investigation. Recent developments in micro-XRF, however, have enabled trace element analysis down to the parts-per-billion (attogram) regime. This is due, in part, to a switchable X-ray target that allows the spectrometer to be optimized for the material under investigation, as well as a significantly brighter X-ray source that enhances weak detection signals. This state-of-the-art laboratory micro-XRF system has been employed to study the negative electrodes of fresh and cycled LiMnO2 batteries, with the system tuned to identify any regions of Mn in each specimen. In our comparison, we have found that this laboratory micro-XRF spectrometer is fully capable of detecting trace levels of Mn in the graphite negative electrode, and have shown that a significant quantity of Mn may be found in the aged electrode while not being detected in the fresh electrode. This technique has thus demonstrated substantial promise for studying the migration of transition metal oxides (e.g., manganese oxide) in lithium-ion batteries, paving the way for enhanced development of commercial battery devices.