Views: 3 Author: Linda Romano Publish Time: 2017-08-09 Origin: EAG Laboratories
In a recent blog post, we explained what can go wrong with lithium-ion batteries. How, then, can we identify which potential problems plague a specific battery and use that knowledge to make batteries safer and more reliable? The secret is characterization. Many tools exist to evaluate batteries during design and selection or to analyze failed batteries to understand why they failed.
Selecting an appropriate characterization technique depends on what information is needed, at what level of accuracy, and the budget for qualification and testing. The most precise techniques tend to use the most expensive instruments and take the most time, but they are sometimes needed to understand failure mechanisms and improve battery design.
Characterization techniques fall into one of two primary categories: imaging or chemical composition analysis. Imaging can be done either in situ – inspecting without damaging the battery – or destructively, often during failure analysis. Discharging battery before disassembly is necessary for analyzing chemical composition.
Optical, X-ray or electron microscopy techniques are useful for imaging the various battery layers. Optical microscopy may be sufficient to observe cracks and hot spots on the battery layers as well as external damage that may occur on the battery casing. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) is used to measure the various layer thicknesses and observe changes in the microstructure, such as microscopic holes (voids) or defects.
SEM and TEM are inherently destructive techniques, but ion milling can preserve the integrity of the sample so that it accurately represents the original state of the battery materials before testing.
X-ray imaging can be helpful when disassembling a battery prior to further analysis, allowing a technician to see exactly where to cut. This minimizes the risk of cutting in the wrong place and creating an electrical short that wasn’t present in the failed battery.
As batteries age, the chemicals inside the cells are depleted with each charging cycle, resulting in reduced capacity over time. Solid-electrolyte interphase (SEI) layers build up on the electrodes, limiting further ion transport. Several chemical analysis techniques help engineers understand the chemical changes that occur over the life of a battery to differentiate between normal aging and something that indicates improper design or manufacturing defects.
X-ray photoelectron spectroscopy (XPS) is a valuable tool for analyzing the structure and composition of the various battery layers to show migration of lithium and other elements within the cathode, anode and separator. XPS provides detailed quantitative information that can aid in failure analysis or can help understand how changes in materials or design affect the rate and extent of SEI formation.
Glow discharge mass spectrometry (GDMS) is useful for detecting trace quantities of elements. In battery applications, this technique can be used to identify impurities and contaminants that may adversely affect battery performance.
Inductively coupled plasma optical emission spectrometry (ICP-OES) detects the presence of trace metals by heating and ionizing the species in a sample. This technique can measure levels of lithium and other metals in the cathode, measuring small changes that correlate with decreased battery performance.
Gas chromatography–mass spectrometry (GCMS) for battery applications requires siphoning gases from a hole drilled into a battery cell to analyze the gases released during breakdown of the electrolyte. This technique is especially helpful in cases where the battery has swelled or experienced thermal runaway.
The above techniques are only a few examples of possible methods to evaluate lithium-ion batteries. For more information on lithium-ion batteries and how proper characterization can improve safety and performance, please download our free white paper.