Application of high-spatial-resolution secondary ion mass spectrometry for nanoscale chemical mapping of lithium in an Al-Li alloy

High-spatial-resolution secondary ion mass spectrometry offers a method for mapping lithium at nanoscale lateral resolution. Practical implementation of this technique offers significant potential for revealing the distribution of Li in many materials with exceptional lateral resolution and elemental sensitivity. Here, two state-of-the-art methods are demonstrated on an aluminium-lithium alloy to visualise nanoscale Li-rich phases by mapping the 7Li+ secondary ion. NanoSIMS 50L analysis with a radio frequency O- plasma ion source enabled visualisation of needle-shaped T1 (Al2CuLi) phases as small as 75 nm in width. A compact time-of-flight secondary ion mass spectrometry detector added to a focused ion beam scanning electron microscope facilitated mapping of the T1 phases down to 45 nm in width using a Ga+ ion beam. Correlation with high resolution electron microscopy confirms the identification of T1 precipitates, their sizes and distribution observed during SIMS mapping.


Main text
The characterisation of lithium (Li) has received considerable attention from multiple disciplines in materials science in particular researchers working on the development of Li-ion battery materials and high-strength, low-density Al-Li alloys [1][2][3][4][5][6][7][8][9] . High-spatialresolution chemical mapping of Li is of particular interest in the case of Li-ion battery materials for investigating Li ion transport on the electrodes with nanoscale structures 1,8,9 . It also offers excellent potential for revealing the distribution of nanoscale Li-rich precipitates in Al-Li alloys to understand their influence on mechanical and corrosion performance [4][5][6][7] . However, the analysis of Li is challenging as many techniques, such as energy dispersive X-ray spectroscopy (EDX or EDS) and X-ray photoelectron spectroscopy (XPS), have low elemental sensitivity for Li and/or insufficient spatial resolution 3,10 . While the use of electron energy loss spectroscopy (EELS) and atom probe tomography (APT) enables the characterisation of Li with high spatial resolution if carried out with great care, these techniques are only able to cover extremely small areas due to the complexity of sample preparation and small sampling volume (e.g. ~ 1 × 10 -2 μm 2 and ~ 5 × 10 -3 μm 3 for EELS mapping and APT, respectively) 11,12 .
Secondary ion mass spectrometry (SIMS) is a surface analysis technique that provides detailed chemical analysis and has been used widely on solid materials 13 . Elemental, isotopic and molecular information can be obtained from the top surface of a solid sample after bombardment by incident primary ions. The sputtered secondary ions are then extracted, transported and separated by a mass spectrometer according to their mass-to-charge ratio. SIMS offers excellent elemental sensitivity, in the parts per million to parts per billion range, in combination with the ability to detect light elements, including hydrogen, as well as isotopes from areas measuring 10's -100's µm 14,15 . This technique offers excellent potential for obtaining statistically meaningful information with a very high elemental sensitivity of Li, as it is an electropositive element with a high ionisation probability. However, the application of SIMS instruments is typically limited to a lateral resolution of ~ 1 μm 1-7 , which is insufficient for revealing the details of Li distribution at the nanoscale.
The development of high-spatial-resolution SIMS instruments in recent years has facilitated imaging of Li distribution with a lateral resolution in the sub-micron range. 3 For instance, researchers using the NanoSIMS 50L (Cameca, France), using a Cs + primary ion beam with an impact energy of 16 keV and a lateral resolution of ~ 100 nm, were able to perform nanoscale analysis of Li in LiCoO2-based battery cathode materials by mapping 7  The material investigated in this study was an Al-Li-Cu-Mg alloy. The as-cast material 4 was hot-rolled to a thickness of 50 mm and then solution treated, followed by water quenching to room temperature. The material was then pre-stretched at room temperature, followed by a peak ageing treatment (T8) to facilitate the formation of nanoscale strengthening precipitates including  (Al3Li) and T1 (Al2CuLi) phases.
Samples for SIMS analysis were extracted from the rolling plane at the mid-thickness  The SIMS maps collected using NanoSIMS and FIB ToF-SIMS (Figures 1a and 1c) both reveal a clear 7 Li + signal from the needle-shaped T1 phases. Whilst as expected the majority of the constituent intermetallic phases show a lower level of 7 Li + signal than the surrounding matrix. The observation of T1 and constituent intermetallic phases is validated by BSE imaging in the same regions, Figures 1b and 1d. This is also consistent with our prior knowledge of the expected phases based on alloy composition and thermomechanical processing. These phases are clearly visualised with distinctive contrast differential to the surrounding matrix due to the abundance of heavier elements such as Cu 21 .      Figure S4, the intragranular T1 precipitates that are < 50 nm in length and 1 -2 nm in width are too small to detect using either SIMS instrument. However, the grain boundary T1 precipitates are formed as clusters that are continuously distributed on the grain boundary, Figure S4a. The clusters measure ~ 90 nm in width, which is consistent with the measured values of widths for the segregated regions showing a strong intensity of 7 Li + signal as shown in Figure 2a. In addition, the T1 phase particles measure 200 − 300 nm in length, which is similar to the lengths of Li-rich particles in the FIB ToF-SIMS 7 Li + map as shown in Figure 3e. This confirms that the segregation of Li as observed using FIB ToF-SIMS corresponds to the clusters of Lirich T1 precipitates on the grain boundary.
In summary, the potential of advanced high-spatial-resolution SIMS instruments to 9 achieve chemical mapping with nanoscale lateral resolution has been shown by mapping 7 Li + in Al-Li alloys. Li is challenging to detect with many characterisation techniques, but the results presented here show that it is possible to not only map Li but localise it with very high lateral resolution. NanoSIMS analysis was able to resolve features as small as 75 nm in size, whilst FIB ToF-SIMS could visualise the Li in T1 phases as small as 45 nm in size, although neither technique could detect the intragranular precipitates that are smaller in size. The implementation of high-lateralresolution SIMS instruments for characterising the distribution of Li in modern Al-Li alloys is of significant importance for understanding the influence of microstructure on mechanical and corrosion behaviour. It also offers significant potential for the highspatial-resolution mapping of Li distribution in Li-ion battery materials.