Imaging of cathode, anode, binder, separator at low accelerating voltages with ZEISS FE-SEM instruments
Rechargeable Li-ion batteries are the power source of choice for portable electronic devices such as cellphones, laptops, cameras, etc. and are becoming increasingly important in the automotive industry and for stationary storage applications. They are complex electrochemical energy storage devices incorporating many material types, ranging from high-density metals to low-density polymers, and require a flexible suite of instrumentation for proper characterization & analysis. Especially challenging are charging effects and the danger of damage on some sensitive components when using higher accelerating voltages in the electron microscope. In this application note, very low voltage imaging was used to investigate the usability of a scanning electron microscope (SEM) on battery materials. The surface and binder of cathode and anode as well as the separator were analyzed.
Structure of a Li-ion Battery Cell
A lithium ion battery cell comprises an electrochemically coupled positive (anode) and negative (cathode) electrode, electrically isolated from each other by a polymer separator. Only ions can pass the separator, so the direct short circuit of electrons between the electrodes is prevented . Thin aluminum and copper foils serve as current collectors for the electrodes. The electrode layers consist to a large extent of active materials (lithium ion storage particles) which can reversibly host lithium ions. During charge, the lithium ions are transported and intercalated in the anode. During discharge, the lithium ions are accepted by the lattice structure of the Li-transition metal-oxide in the cathode. The amount of lithium ions that can be stored by the active materials determines cell capacity. In commercial cells, graphite is a common active material for the anode . On the cathode side, lithium metal oxides in various compositions are used. In this study, cobalt oxide LiCoO2 (LCO) was used as cathodic material. Besides the active materials, the electrode coating also comprises a polymer binder and carbon. The polymer binder (for example polyvinylidendifluorid PVDF) provides the cohesion of the active materials as well as their adhesion to the metallic current collector. The conductive additives (carbon black and/or graphite) form an electrically conductive network that connects the active material particles with the current collector. The better the electrical connectivity, the lower the internal resistance of the electrode layer . Owing to their small size – often only a few nanometers – carbon black particles are embedded in the binder matrix. The binder material itself has a low conductivity, thus oversupply of binder should be avoided. [4, 5]
Materials and Methods
Two commercial prismatic lithium ion battery cells (3.7 V / 1500 mAh) intended for consumer electronics applications have been used for the investigation. Both cells utilized LiCoO2 (LCO) in the positive electrode and graphite in the negative electrode. One cell was investigated in the pristine state, the other cell was subject to a ageing procedure comparable to several years of use in a mobile device. The cells were discharged and then opened in a glove box under protective argon atmosphere. Subsequently, the electrode foils were separated, washed in dimethyl carbonate (DMC), and dried. Small samples were cut from the electrodes and the separator. The samples were placed as obtained on a microscopy stub and secured with a carbon tape as well as molded into an epoxy resin for cross section analysis. High resolution scanning electron microscope images were collected with ZEISS GeminiSEM 500 (1.1 nm resolution @ 1kV without Tandem decel) and ZEISS Merlin (1.4 nm resolution @ 1 kV).
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 D. Kehrwald et al., Journal of the Electrochemical Society 158 (2010), A1393-A1399.
 S. Freitag, C. Berger et al., Integrated SEM and Raman imaging of Lithium Ion Battery, Application Note, ZEISS Microscopy, Oct. 2015