Valentine Riedo-Grimaudo, James Whitby, Lex Pillatsch
Modern lithium-ion batteries show high efficiency and capacity and have long cycle lives. These properties, together with the low self-discharge rate and negligible memory effect have made lithium-ion batteries a cost-effective solution for many applications, ranging from electronic devices to electric vehicles (EV) up to large stationary energy storage systems.  The rate of innovation in this field remains high, and a key driver for further developments (such as the development of new cathode materials), is expected continued growth in the market for electric vehicles. 
The remarkable gains in performance over the last thirty years since lithium-ion batteries were first introduced commercially have relied to a large extent on advances in materials technology, and these advances relied in part upon the ability to chemically characterize and image materials and devices. The TOFWERK fibTOF detector for secondary ion mass spectrometry is an ideal tool for imaging the spatial distribution of lithium in complex materials over a large range of concentrations.
Challenges in Lithium-Ion Battery Characterization
In lithium-ion rechargeable batteries, lithium plays an essential role as the charge carrying ion (Li+), which is intercalated in the host material of the electrode and electrolyte and which moves from the cathode to the anode during charging and vice versa during discharge.  Processes occurring during cell operation, which result in chemical or structural deterioration of electrode interfaces (aging effects), are gaining more and more attention as the key to improving cycle life . Identifying chemical changes over time and the capability to characterize these processes and their effects on the functionality of the device requires the ability to image the spatial distribution of lithium and other relevant constituents at high spatial resolution. Instrumentation enabling such chemical imaging can substantially support the further development of the technology [5,6].
Adding a mass analyzer to a FIB-SEM microscope allows for the analysis of light elements in battery research by enabling secondary ion mass spectrometric (SIMS) imaging. The energetic ion gun of the FIB is used as the primary ion source to generate secondary ions (positive and negative) via the sputtering process of the solid material. This measurement configuration is known as FIB-SIMS and allows for complementary measurements, e.g. the analysis of light elements. The fibTOF instrument developed by TOFWERK enables rapid visualization of the spatial distribution of all elements with high resolution (lateral <50 nm and depth <10 nm), with a mass resolving power of >700, and a sensitive detection of low-mass elements such as hydrogen, boron, lithium and fluorine down to the ppm range. It is thus well suited for lithium-ion battery research because sensitivity for lithium is high (lithium has one of the highest secondary ion yields of all elements), and the spatial resolution of the FIB-SIMS method is hard to match without resorting to time-consuming transmission electron microscopy techniques that require the preparation of thin lamellas with FIB.
The fibTOF is an orthogonal time-of-flight (TOF) mass analyzer designed to be used as add-on instrument for FIB-SEM microscopes (compatible with the major FIB-SEM manufacturers including Thermo Fischer Scientific, Tescan, and Zeiss).
Lithium-Ion Battery Characterization Examples
The figures below show data for various elements in a rechargeable lithium-ion pouch cell. A piece of the electrode stack was cut out and placed on the SEM stub for analysis. These figures highlight the ability of the fibTOF to image the distribution of light elements with high spatial resolution. For this work, a Ga FIB was used with the beam aligned normal to the sample surface. The FIB beam energy was 30 kV and the pixel dwell time was 10 µs (mass range from 1 to about 180 amu).
2D Imaging of Elements
Figure 1 shows chemical maps for the elements hydrogen, lithium, sodium and aluminum. The maps were generated by rastering 512 times the FIB beam line-by-line over an area of 100 µm x 100 µm with an ion current of 130 pA. Each scanned frame was recorded with 512 x 512 pixels, averaging (binning) over 4 by 4 blocks of pixels to give 128 effective pixels. Under these conditions good quality maps of the elements are obtained. The fibTOF does detect hydrogen, although the hydrogen observed in this example is largely due to water molecules from the residual gas in the microscope chamber.
In order to improve the spatial resolution, the acquisition conditions were modified compared to Figure 1: the field of view was 5 µm, the pixel dimensions were 1024 x 884, the ion current was set to 1.5 pA, and the binning was adjusted to 2 x 2. This smaller field of view and the less aggressive binning results in a smaller effective pixel size than for Figure 1 (now 11 nm) and the lower ion current results in a smaller FIB spot size. The image in Figure 2 is a map of the total lithium signal (both 6Li and 7Li, using a custom peak table entry in our user interface software) accumulated over 30 frames. With the modified acquisition conditions features much less than 100 nm can be seen. Be aware that topographical effects probably influence the intensity distribution due to the rough surface of the sample.
Imaging of Sublayers
Because FIB-SIMS intrinsically collects a 3D data set, the fibTOF can also image elemental distributions along the erosion direction (Z) of the FIB beam. This goes beyond a simple 1D depth profile — from the 3D dataset we can select arbitrary slices and regions of interest. The images presented in Figure 3a are front projections of the imaged cube, mapping the different chemical layers along the sample depth, e.g. in this case there is a thin layer of adsorbed hydrogen at the surface, the surface layer consisting of lithium, and an aluminum sublayer. The fibTOF cannot directly determine depths, but from the known fluence and the estimated sputter yield a local depth can be calculated. The plot in Figure 3b presents the same data set as depth profiles. Here, the counts per extractions of each pixel within a frame are summed to a value. To follow certain trends along the depth this representation may be more convenient, especially when investigating the interface between films. Because lithium-ion batteries are often constructed from layered films, FIB-SIMS allows the investigation of mass diffusion processes and troubleshooting of failures. The measurement in Figure 3 was recorded with an ion probe current of 50 pA, a field of view of 10 µm and a resolution of 512 x 442 pixel and 2 x 2 binning.
Measurement of Fluorine
By reversing the polarity of the applied voltages at the ion optics of the fibTOF it is possible to measure negatively charged secondary ions. The following example in Figure 4 shows top and side maps of a fluoride measurement recorded on a 10 x 10 µm area with 512 x 512 px and no binning applied using a 133 pA FIB current. The capability to image fluoride may become relevant for future fluoride ion batteries . Such batteries, first reported in 2011, also have the potential to become cheaper lightweight rechargeable energy sources, with the theoretical possibility of higher energy storage densities than lithium-ion batteries. A key area of research involves identifying and testing fluoride ion conducting materials.
Energy dispersive X-ray spectroscopy (EDS) in combination to a dual beam focused ion beam/ secondary electron microscopy (FIB-SEM) is a standard technique for element imaging. Though, this technique has some difficulties to detect light elements because of the low energy and the high absorption probability of the emitted X-ray which makes it not adequate for extensive analysis of e.g. Li ions. The following graph in Figure 5 is a resolution-sensitivity landscape showing the measurement capabilities of a multitude of analytical techniques. Improvements in the brightness of the FIB sources allows to address higher spatial resolutions than are historically associated with dynamic SIMS. FIB-SIMS is placed somewhere between STEM/EDS and dynamic SIMS, reaching detection limits down to the ppm and spatial resolution below 50 nm.
This application note demonstrates use of the fibTOF for the imaging of light elements relevant to lithium-ion batteries. These light elements are difficult or impossible to image with comparable spatial resolution by other techniques. It has been shown that lithium can readily be detected at spatial resolutions below 100 nm with potential to do better. To obtain a complete overview in research and development of rechargeable Li-ion batteries it is suggested to use fibTOF as complementary technique, e.g. to EDS.
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