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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al quartz-crystal microbalances (EQCMs), electrochemical impedance spectroscopy (EIS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), and vibrational spectroscopies. However, innovative methods and advanced characterisation techniques are required to gain a deep understanding of the highly soluble SEI in NIBs. Electrochemical ‘pausing testing’ can be used to investigate dissolution [197, 198], whereby a cell is switched to its open-circuit voltage (OCV) for a fixed duration after a period of cycling to determine how much capacity is lost through self-discharge (figure 24). Such experiments have shown that the capacity loss is higher in NIBs than in LIBs. PES is very sensitive to surfaces and chemicals and is one of the techniques most commonly used to study SEI layers in all kinds of battery systems. Synchrotron hard x-ray photoelectron spectroscopy (HAXPES) measurements can be used to probe to a depth of 50 nm and have shown that the SEI thickness decreases with increasing pause durations (figure 24) [197, 198]. HAXPES measurements with large probing depths may allow for the elimination of electrode washing, an issue already discussed, although other parameters, including the pre-disassembly relaxation time, should also be considered. The future development of in situ and operando PES techniques may enable the direct measurement of SEI layer formation at an electrode during cycling. While infrared spectroscopic (FT-IR) analysis of the electrolyte composition from cycled cells has already been demonstrated [195], it is underutilised for detecting soluble SEI species. Complementary techniques, such as gas chromatography mass spectrometry (GC-MS) and inductively coupled plasma (ICP) methods may also provide valuable insights. However, the extraction of electrolytes from cycled cells and data interpretation can present challenges. The development of novel electrolyte components (salts and solvents) will almost certainly result in the formation of more stable SEIs and improved battery performance. However, given such development, one must bear in mind that cost-effective, non-toxic, and safe electrolytes will be required for the commercialisation of NIBs. Concluding remarks While relatively little is known about SEIs in NIBs and their formation mechanisms, particularly compared to their lithium counterparts, there is a growing body of knowledge on the topic as a result of increasing interest over recent years. Progress is inhibited, though, by the challenges posed by their complex nature. Their high solubility compared to the SEIs in LIBs causes difficulties in determining their composition and in preparing samples for ex situ characterisation. Furthermore, the lack of a suitably stable reference electrode and the issues of high resistance and crosstalk when employing sodium metal in half cells lead to complications in assessing the performance of materials. Novel electrolyte formulations will advance NIB technology towards commercialisation. However, a robust comprehension of the SEI achieved through the use of advanced characterisation techniques and innovative electrochemical testing methods will allow for the targeted design of electrolytes and a stable SEI. Acknowledgments The authors would like to acknowledge the financial support provided by the Å Forsk Foundation via Grant No. 19-638, by the Swedish Energy Agency via Project Number 48198-1 and by S T and U P for Energy. 49PDF Image | 2021 roadmap for sodium-ion batteries
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