Over the past decade, there has been a revival of research activity on lithium metal batteries (LMBs) as these could be a solution for key challenges of electromobility and the energy revolution. While there is growing consensus in the scientific community that common reporting standards and testing conditions for LMBs have to be established, a vast majority of research activities on lithium metal use lab‐dependant testing protocols. For that reason, this publication aims to shed light on various, potentially neglected aspects in battery assembly and testing. Firstly, the long‐term cycling, regarding a range of experimental parameters, such as current density, capacity, electrolyte type and its quantity, as well as contribution of the electrode edges, is shown in both symmetric (Li||Li) and asymmetric (Cu||Li) configurations. The second part focuses on the reversibility of lithium thickness during cycling with and without protected electrode edges, investigated by operando dilatometry. By bringing the insights from this parameter study together, we aim to contribute to better experiment design for future LMB studies, as well as a better understanding for the failure mechanism of Li metal anodes.

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...

Most contemporary liquid Li‐ion battery chemistries are based on carbonate electrolytes, however, these typically perform poorly with metallic lithium. Especially, when only a small reservoir of metal is present, cells fail quickly due to the complete consumption of electrochemically active Li. Electrolyte modification is, therefore, a commonly chosen strategy to increase cycling stability and prolong lifetime of the cell. At the same time, complete redevelopment of an electrolyte usually creates a new set of problems, as conductivity, suitable electrochemical stability window and compatibility to all components of the cell needs to be ensured. It is, therefore, a reasonable strategy to alter existing electrolytes slightly, as has been done for the aforementioned carbonate electrolytes by incorporating lithium nitrate salt with triethyl phosphate as cosolvent into the mixture. As the role of the organophosphate is not fully clarified, our study aims to investigate its effect onto cycling stability, morphology, and reversibility of Li metal cycling both with and without LiNO3 salt. Using close‐to‐industrial conditions with limited excess of metallic Li at high current density, we are comparing cycling performance of the different electrolyte combinations. Contribution of the additive components to irreversible thickness growth due to the formation of inactive (“dead”) lithium and deposited lithium morphology is investigated by operando dilatometry and postmortem scanning electron microscopy. Finally, we are looking at modified solvent‐salt ratios to see whether further improvements of cell lifetime and morphology can be achieved.

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