Fast charging is a key requirement for lithium-ion battery (LIBs) technology in a wide range of applications from portable devices to electric vehicles. However, fast charging impose high C-rates and temperature gradients to the system, which cause electrolyte degradation and polymerization, resulting in reduced performance, cycle life, and capacity [1,2]. Therefore, for a safe and efficient implementation of fast charging, it is critical to understand its effect on LIBs components, particularly in the electrolyte.There is a lack of non-invasive methods to elucidate changes in the electrolyte during LIBs operation, and it is commonly studied via post-mortem analysis or ex-situ degradation [3–5]. Neutron imaging (NI) is suitable for studying electrolyte distribution in LIBs, since hydrogen provides high contrast when interacting with the neutron beam, while casing materials like stainless steel or aluminum provide low contrast [6]. Furthermore, the neutron attenuation spectrum of organic molecules depend on the motion of an atom due to molecular vibrations or diffusion, making neutron spectroscopy a suitable tool to identify the chemical composition and aggregation state in batteries. Here, we introduce spectroscopic neutron imaging (SNI) as the new method to study these phenomena in a spatially resolved way.Imaging of electrolyte and solvent samples, performed at the V20 beamline of HZB in Berlin and the IMAT beamline of ISIS in UK (Figure 1-a), show that a liquid binary mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) exhibit similar attenuation spectra – though the different chemical composition and diffusivities result in small variations. On the other hand, solidified species (red region) present a noticeable contrast change due to the reduced molecular diffusion. At 17°C, the organic binary mixture, EC:DEC (1:1 volume ratio), exhibits liquid (EC and DEC) and solid (EC) phases. A similar behavior for solids is observed in the short wavelength (λ<3Ȧ) region of the normalized 1H cross-section spectra, while the curves in the diffusion region (λ>3Ȧ) are bounded to the mobility properties of each molecule (figure 1-b).Additionally, we will present measurements of different electrolytes and organic binary mixtures exposed to temperature-dependent phase changes, obtained via SNI at the ICON beam line of PSI. Setting a lower wavelength resolution requirement allows faster measurements, in order to capture the moment when the sample experiences a phase change. This novel method paves the way for in situ electrolyte behavior analysis, as it allows the detection of fine variations in the electrolyte linked to charge/discharge schemes that negatively affect LIBs performance.Understanding the electrolyte behavior will contribute to the improvement of battery materials to avoid issues in fast charging mechanisms.[1] Y. Liu, Y. Zhu, and Y. Cui, Nature Energy 4, (2019).[2] A. Tomaszewska et al., eTransportation 1, 100011 (2019).[3] G. Gachot et al., Journal of Power Sources 178, 40...

One of the critical requirements for automotive PEFC stack is unassisted cold-start at -30°C [1]. Sub-zero operation of polymer electrolyte fuel cells (PEFCs) can induce ice formation that causes performance failure and irreversible damages to membrane electrode assembly (MEA). It is challenging to predict the location and the moment of freezing event, because it is a stochastic process. The proposed solutions include material optimization, load control, and assisted start-up [2]. mechanisms in PEFCs needs to better understood for further developments.Our previous investigations demonstrate that water is produced in super-cooled (SC) state, and the transition from liquid to solid is initiated when a SC water cluster encounters a nucleation seed, such as broken carbon fiber. Due to the unstable state of SC water, the transition spontaneously propagates to neighboring water cluster, and a single freezing event may affect a significant portion of the active area as seen in Figure 1 (a). In this work, we investigate an operating PEFC under sub-zero temperature and introduce a assembly to prevent ice propagation.The segmentation was implemented by creating non-active lines in the 3-layer MEA, where electrodes were partially removed by laser beam. The energy level of the laser was tuned to selectively remove carbon without affecting the membrane. The pieces of GDL, matching the size of the segmented active areas, were hot-pressed on the modified MEA. A Teflon gasket was placed among GDLs to complete the MEA fabrication. The segmented 5-layer MEA was mounted on the advanced PEFC hardware with thermoelectric and heat flux sensing modules for monitoring heat release upon freezing. Furthermore, we applied time-of-flight (ToF) neutron imaging to detect freezing in the operating PEFC at ICON beamline of Swiss Spallation Neutron Source (SINQ) [4]. This technique is based on the principle that the neutron cross-sections of ice and SC water are similar at short wavelength, but different at long wavelength. Using these methods, we confirmed that the operation time of the PEFC under isothermal sub-zero condition was significantly increased with the segmentation. The improvement was attributed to localization of freezing events as in Figure 1 (b).The findings of this work will contribute to the development of cold-start technologies in PEFCs. Besides, ToF neutron imaging method will be a valuable tool for studying phase-changes of other electrochemical device materials. References [1] Hydrogen and Fuel Cell Technologies Office Multi-Year Research, Development, and Demonstration Plan, Energy.Gov. https://www.energy.gov/eere/fuelcells/downloads/hydrogen-and-fuel-cell-technologies-office-multi-year-research-development (accessed April 20, 2021).[2] A.A. Amamou, A Comprehensive Review of Solutions and Strategies for Cold Start of Automotive Proton Exchange Membrane Fuel Cells, 4 (2016) 14.[3] J. Biesdorf, A. Forner-Cuenca, M. Siegwart, T.J. Schmidt, P. Boillat, Statistical Analysis of Isothermal Cold Starts of PEFCs: Impact of Gas ...