A great deal of research is being carried out in the battery industry to develop future energy storage systems. This task is one of the most important modern technological challenges. First marketed in 1991, the lithium-ion battery has a relatively low self-discharge rate compared with other batteries. The principle of this accumulator is the displacement of the lithium ion between two electrodes, one positive (cobalt or manganese dioxide) and the other negative (graphite).

It is essential to carry out a series of analyses and characterise the effect of the reaction of the various elements present on the electrodes, particularly on their surface and in the interface zones.

Different types of Li-ion batteries exist, some designed for small products and others as high-power industrial accumulators for hybrid vehicles or aeronautics. This requires analytical techniques capable of differentiating chemical states with great sensitivity and high spatial resolution.

This has led to extensive research into Li-ion batteries, as they are envisaged as the basis for high power density secondary sources for electric vehicles and storage devices for the smart grid.

The vital performance of Li-ion batteries, such as lifetime, internal resistance and capacity, is closely linked to the microstructure of the electrodes. The solid electrolyte interphase (SEI) has proven to be crucial for increasing the performance and lifetime of lithium-ion batteries. It is therefore necessary to understand the mechanisms and reactions that lead to the formation of SEI layers on the electrodes of Li-ion batteries.

High-resolution imaging reveals microstructural information about the composite structure made up of

spherical microparticles of the active ingredient held together by the polymer matrix. 

Scanning Electron Microscopy (SEM) is a technique capable of producing high-resolution images of the surface of a sample. SEM is used in many fields, from biology to materials science and microelectronics, and on all types of sample, even insulators can be observed after metallisation. Analysis takes place in a controlled inert atmosphere or at low voltage (close to kV).

SEM is generally used to study the 3D morphology of a surface or object with nanometric resolution.
Chemical and elemental composition can also be obtained using X-ray microanalysis.

 

The principle of this technique is based on the use of a beam of incident electrons of a few tens of kilovolts sweeping across the surface of the sample, which then re-emits a whole spectrum of particles and radiation: secondary electrons, backscattered electrons, Auger electrons and X-rays.

Detection of the various particles or radiation emitted provides information about the sample: its morphology, topography, crystalline structure, elemental chemical composition (qualitative and semi-quantitative analysis), etc.

Identification of chemical contaminants

The presence of contaminants can partially affect the life and performance of batteries. These contaminants can cause parasitic chemical reactions, corrosion, degradation of battery materials and loss of lithium stocks.

SEM observation and EDX and/or ToF-SIMS elemental chemical mapping are used to identify and analyse these contaminants and their effect on the degradation of battery materials.

In these non-exhaustive examples, it has been demonstrated that scanning electron microscopy (SEM), with or without FIB and EDX, is an ultra-powerful microscopy tool for the structural and chemical study of Li-ion batteries.

With an excellent depth of field (~ 100 x that of optical microscopy), the SEM provides high-resolution images of all materials.

For more applications of SEM analysis or our other analytical and microscopy techniques, click here.

The combination of SEM/EDX and ToF-SIMS techniques facilitates a complete analysis of material composition. X-ray tomography enables internal features such as porosity, cracks and phase distribution to be viewed non-destructively. In dynamic mode, it is possible to visualise 3D changes in internal structures when they undergo modifications such as loading or absorption of liquids.