Abstract
Development of highly efficient batteries with more rational understanding and precise control of the underlying microstructural features requires high resolution based characterization tools. Electron microscopy and spectroscopy offers information about the structure, morphology, chemistry and chemical composition of the battery materials on nano and atomic scale enabling us to establish the synthesis-structure-performance relationship and further direct the design of new battery materials with improved and high performance.
The key factors for a successful electrochemical system are the structure, morphology, chemistry and chemical information of the component materials. The improvement and further development of energy storage systems is based on detailed knowledge and interpretation of morphology, microstructure and phase composition of the individual components like electrodes, current collector, and separator. Additionally, the long-term stability of batteries is affected by the interaction of several components not only at their common interface but also in the volume of the whole device and in combination with intrinsic emerging mechanical loads. Battery materials require advanced skills and techniques to improve material combinations and synthesis strategies. High quality images, in situ measurements and chemical analysis can contribute to this.
The most important reason for the usage of electron microscopy instead of light microscopy is the diffraction limitation of resolution which is dependent of the wavelength. According to the Rayleigh criterion two points are regarded as just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other. The diameter of the corresponding Airy disc is direct proportional to the wavelength. With the Planck constant h = 6.6 · 10–34 Js, the electron mass me = 9.1 · 10–31 kg, e = 1.6 · 10–19 C, and the speed of light c = 3.0 · 108 m/s the wavelength of electrons are given trough the de-Broglie wavelength:
$${\lambda _{{\text{de Broglie}}}} = \frac{h}{{{p_e}}} = \frac{h}{{{m_e} \cdot {v_e}}} = \frac{h}{{\sqrt {2 \cdot {m_e} \cdot e \cdot U} }} \approx \frac{{1.22 \cdot {{10}^{ - 9}}{\text{ m}}}}{{\sqrt {\frac{U}{{\text{V}}}} }},$$
where
${v_e}$ is the velocity of the electrons and U the acceleration voltage for the electrons. For higher acceleration voltages U the relativistic correction is
$${\lambda _{{\text{de Broglie}}}} = \frac{h}{{{p_e}}} = \frac{h}{{{m_e} \cdot {v_e}}}\sqrt {1 - \frac{{v_e^2}}{{{c^2}}}} = \approx \frac{{1.22 \cdot {{10}^{ - 9}}{\text{ m}}}}{{\sqrt {\frac{U}{V}\left( {1 + 0.9788 \cdot {{10}^{ - 6}}{\text{ }}\frac{U}{{\text{V}}}} \right)} }}$$
Thus, the wavelength of electrons passed through 1 kV to 30 kV acceleration voltages is in the range from 3.9 · 10–11 m down to 7.0 · 10–12 m, which is the magnitude utilized for scanning electron microscopy (SEM). Transmission electron microscopes (TEM) require higher acceleration voltages up to 300 kV because the imaging electrons have to transmit the specimen. Therefore, the wavelength in TEMs is even smaller, i.e. 2.0 · 10–12 m. Hence, the smaller wavelength implies higher resolution for TEM than for SEM.
Another important difference between transmission and scanning electron microscopy is based on the type of electrons used for imaging. TEM is based on transmitted electrons and provides the details about morphology, internal composition, structure and crystallinity. SEM uses backscattered or secondary electrons and focuses on the sample’s surface and its composition. The sample for TEM has to be cut thinner whereas there is no such need for SEM sample.
Advanced techniques in state-of-the-art electron microscopy are always under development towards their wide applications in various aspects of materials research. Research in the field of advanced TEM techniques of battery systems is driven by the thirst towards energy storage systems in order to have better energy storage capabilities. Different groups worldwide contribute to a basic understanding of the processes that occur during the charging/discharging of a battery, as a basis for optimizing electrode, electrolyte materials and their interfaces.
For both, electron microscopy methods benefit from the multitude of interactions which take place after the electron beam hits the specimen surface or passed the specimen volume. The main aspects in respect to battery materials will be reviewed in the following sections.
Energy Storage: Battery Materials and Architectures at the Nanoscale