Nickel-Yttria Stabilized Zirconia (Ni-YSZ) cermets are used as anodes in solid oxide fuel cells. These anodes are stable for tens of thousands of hours during operation and have low cost. In this work, Ni-YSZ anodes are infiltrated with nickel nanoparticles to increase the density of electrochemical reaction sites and improve their performance. However, infiltrated nickel nanoparticles are isolated from one another, so they are not electrochemically active. Two approaches have been utilized to activate infiltrated nickel nanoparticles: in-situ nickel spreading and simultaneous infiltration of nickel with Gd0.1Ce0.9O2-δ (GDC). In-situ nickel spreading, which occurs during exposure to anodic mass transfer limited currents, connects and activates nickel nanoparticles, improving anode performance but inherently causing nanoparticle coarsening. Simultaneous infiltration of Ni and GDC results in substantially improved anode performance, and the infiltrated nanostructures are more stable than infiltrated nickel. Detailed analysis of the electrochemical impedance by equivalent circuit modeling is used to separate the contributions of nickel and GDC infiltrants to the overall cell performance.

This study investigates the dependency of the internal processes of Li-ion batteries on operating conditions, cycling life, and mechanical damage. Li-ion Batteries are the preferred energy storage solution for many applications, including cell phones and electric vehicles. However, they can pose serious hazards if their safety is compromised, such as after sustaining mechanical damage. An example of such loadings is an electric vehicle crash or a drone's impact landing. Prior work has shown that mechanical damage to the battery may not affect its voltage, capacity, or other primary specifications. Currently, there is no reliable method to check the integrity of battery cells inside an electric vehicle battery pack once it has been subjected to a shock or impact. Here, we report a novel method to determine the time constants and polarizations of the main internal processes of Li-ion cells from their impedance spectra and investigate the effect of mechanical damage and aging on them. We formulate a distribution function of relaxation times to deconvolute the measured impedance spectra to achieve this goal. Our formulation is based on representing the battery dynamics via basis functions formed using a series of passive electrical elements consisting of inductors, resistors, and capacitors. We used a ridge regression optimization to determine the optimal number of elements and their values to represent the battery dynamics in the measured frequency range. We divided the samples into a control (intact) group and a test group, which went through controlled mechanical damage. We cycled the batteries and collected their impedance spectra at various temperatures and state-of-charge (SOC) levels. The experiments were conducted on LFP (Lithium Iron Phosphate) and NMC811 (80% nickel, 10% manganese, and 10% of cobalt in the active cathode material) cells, which are two main types of batteries used in commercial electric vehicles. After deconvoluting the impedance spectra using our formulation and criteria, we identified four peaks in the low- and medium-range frequencies related to diffusion, charge transfer, and solid electrolyte interface, as well as peaks in the high-frequency region related to contact resistances and ionic conductivity through the electrolyte and separator pores. We used the dependency of the peaks on the SOC level and temperatures to assign them to these processes. We represented each process with representative time constants defined as the local maxima of the peak and the area under the curve as the polarization of the process. We showed that the mechanically damaged cells have substantially different high-frequency time constant characteristics than the control group. Further, using our proposed approach, we studied the ability to identify degradation mechanisms during the aging process of a cell at different temperatures and states of charge. For LFP cells, the representative time constants remained almost unchanged during mechanical damage. However, the high-frequency peak height dropped by more than 36% during indentation, compared to less than 2.5% change in the control group. For NMC811 cells, the time constant of the high-frequency peak increased slightly with increased mechanical loading, and the associated peak height dropped by more than 12.9% during indentation and more than 17.8% during three-point bending. For the NMC811 cells, the average activation energy for charge transfer was 62 kJ/mol, while the activation energy for SEI was 49 kJ/mol. These values confirmed the physical relevance of the assigned peaks by verifying them with reported values in the literature. Finally, we analyzed the trend of changes in the impedance spectra (showed as EIS- Electrochemical Impedance Spectroscopy) collected during battery cycling at 0% and 100% SOC for NMC811 cells. The time constant of charge transfer increased significantly with aging, while the time constants of SEI and contact resistance increased slightly, and the high-frequency peaks remained almost constant. Polarization analyses showed significant increases with aging: the polarization of contact resistance, SEI, and charge transfer increased by 2.06, 2.36, and 2.24 times from cycle number 40 to 280 at 0% SOC, and by 1.86, 2.65, and 11.95 times at 100% SOC. Ohmic resistance increased slightly at both 0% and 100% SOC from cycle number 40 to 280. These results align with the observed linear degradation phase, where cells experienced a 4.8% capacity fade until cycle 280. We investigated the contribution of each degradation mode to changes in time constant and polarization of internal processes and degradation mechanisms based on aging stress factors, including large Depth of Discharge, low and high SOC, and a large number of cycles. This research demonstrated the effectiveness of our suggested DRT method in studying the effects of temperature, SOC, aging, and mechanical damage on the internal processes of LFP and NMC811 cells. This non-invasive method can detect hazardous mechanical damage in batteries, making it useful for applications such as electric vehicles after a crash or drones after impact landings. The aging results highlighted the potential of this approach for evaluating changes in the internal processes and degradation mechanisms caused by aging, which is essential for efficient battery management systems and estimating battery state of health. This method can also be used to diagnose battery safety in second-life applications, such as grid energy storage. / Mechanical Engineering

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