Study of Phase Changes in Lithium-Ion Battery Electrolytes via Spectroscopic Neutron Imaging

Abstract

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, 409 (2008).[4] S. Nowak and M. Winter, Journal of The Electrochemical Society 162, 2500 (2015).[5] A. Friesen et al., Journal of Power Sources 334, 1 (2016).[6] K. V. Tran et al., Materials Today Advances 9, 100132 (2021).[7] M. S. Ding et al., Journal of The Electrochemical Society 148, A299 (2001). Figure 1