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High frequency resistance (HFR) has become the most important input signal for the on-board water management of polymer electrolyte membrane fuel cell (PEMFC) vehicles in practical applications, due to its close physical relation to the membrane water content. However, according to our latest experimental result, the paradox arises that HFR reaches a steady state regardless of continuous reduction in membrane water content, in other words, the pseudo-steady state of HFR is achieved. To investigate the triggering mechanism, for the first time, we combine current mapping technique with image difference technique to observe dynamic evolution of current in-plane distribution, and, furthermore, to analyze membrane water distribution. We can demonstrate that, the heterogeneity of membrane water in-plane distribution triggers the pseudo-steady HFR, and it can be distinguished by the distributed monitoring, to avoid the potential failure of water management. [ABSTRACT FROM AUTHOR]

Sub-zero startup ability remains a key barrier for commercial application of polymer electrolyte fuel cells (PEMFC), especially for automotive applications. In order to improve the startup ability and durability of fuel cells, understanding of the characteristics and mechanisms of cold start is essential, and here modeling of fuel cell cold start plays an important role. In this study, a onedimensional model is developed to simulate the fuel cell cold start. The model includes mass transport and phase change, heat transfer and electrochemical reaction. Key features such as membrane water and local current distributions are analyzed. Based on the one-dimensional model and simulation results, a spatial reduced simplified model is developed that distinguished only n states across the cell. The simplified model inherits the key features of the one-dimensional model, while the computational cost is significantly reduced to 10% (from 216 s to 20.88 s). The one-dimensional model and simplified model are both validated by the cold start experiment and the voltage error and temperature error are within 15% and 1.2 K respectively. Thus, the proposed simplified model could be used in dynamic simulation and in further multi-scale modeling study to build a stack model. [ABSTRACT FROM AUTHOR]

Thermal runaway is always a troublesome problem that hinders the safe application of high energy lithium-ion batteries. There is an urgent need to interpret the voltage and temperature changes and their underlying mechanisms during thermal runaway, in order to guide the safe design of a battery system. This paper is dedicated to building a coupled electrochemical-thermal model that can well predict the voltage drop and temperature increase during thermal runaway. The model can capture the underlying mechanism of 1 ① the capacity degradation under high temperature; 2② the internal short circuit caused by the thermal failure of the separator; and 3③ the chemical reactions of the cell components that release heat under extreme temperature. The model is validated using by experimental data, therefore the modeling analysis has high fidelity. We employ the model to analyze 1) the capacity degradation under extreme temperature; 2) the influence of the SEI decomposition and regeneration on the thermal runaway behavior; 3) the heat generation by internal short circuit in the thermal runaway process. The discussions presented here help extend the usage of lithium-ion batteries at extreme high temperature (>80°C), and guide the safe design of lithium-ion batteries with less hazard level during thermal runaway. [ABSTRACT FROM AUTHOR]

Long-term stability of reference electrode is a major issue in lithium-ion batteries. A composite Ni-Li reference electrode (RE) with a longer lifetime than traditional Cu-wire Li reference electrodes is proposed in this study. However, the larger size of the Ni-Li RE creates a blocking effect when positioned internally in the battery. The blocking effect manifests as an abnormal feature in the measured electrode potential. A two-dimensional (2D) finite element model of the cell with an internal RE is constructed to investigate themechanism of the blocking effect. The internal RE blocks the path of local current flow in the electrolyte, and decreases the electrochemical reaction rate of particles in the anode and cathode, leading to measurement errors. Characteristic lengths of RE and applied currents were examined as parameters in the finite element model to determine their effect on the magnitude of error. A useful guide is provided for proper selection of size of RE and applied current in dynamic tests of lithium-ion batteries. [ABSTRACT FROM AUTHOR]

Lithium plating leads to severe capacity fading and possible safety problems in lithium-ion batteries. Thus, non-destructive detection methods for lithium plating are critical for safe and reliable operation of lithium-ion batteries. In this paper, an electrochemical model incorporated with lithium plating and stripping reactions is established to investigate the lithium plating-stripping process at low temperature. The model is validated at different current rates and temperatures and can successfully predict the characteristic voltage plateau during the rest period after low temperature charging. Modeling analysis helps to develop detectionmethod for lithium plating via differential analysis on the voltage plateau. Distinct local minima can be found in the differential voltage curves of the batteries with plated lithium. The time when the local minima occurs in the differential voltage curves exhibits a linear relationship with the amount of reversible lithium, and thus can be applied as a quantitative indicator for detection of lithium plating. The detection of lithium plating based on differential voltage analysis is further extended to the discharge voltage profiles of the batteries after low temperature charging, considering the effects of rest time. [ABSTRACT FROM AUTHOR]

Internal short circuit (ISCr) is one of the major safety issues of lithium batteries and would lead to thermal runaway of batteries. Repeating ISCr in laboratory requires to create small-scale short circuit inside integrated batteries, which is very hard for existed safety test methods. In this paper, a novel ISCr trigger method based on shape memory effect of shape memory alloy (SMA) is proposed, which is easy to be implanted into batteries while has minor influences on batteries normal performance. The proposed SMA ISCr trigger method is employed to conduct ISCr experiments, including the severest type of ISCr, the Aluminum-Anode ISCr, and the most common type of ISCr, the Cathode-Anode ISCr. The experiments results show that the Aluminum-Anode ISCr leads to explosion while the Cathode-Anode ISCr only leads to mild self-discharging. Compared with the nail penetration test, the proposed SMA ISCr trigger method in the Aluminum-Anode ISCr experiment has 1) better consistency, given that all of the 4 tested batteries have their maximum temperature in the range of 383~393°C; 2) better reliability in evaluating battery safety properties, given that all of the 4 tested batteries have the repeatable explosion behavior. The relative ease of inserting this controllable SMA ISCr and the repeatability of the produced data can lead to better modeling and detection techniques of battery internal shorts such as plating and dendrite formation. [ABSTRACT FROM AUTHOR]

Internal short circuit (ISCr) is one of the major reasons for lithium-ion battery thermal runaway. A new phenomenon, named as the Fusing Phenomenon, is observed during the ISCr experiments. During the Fusing Phenomenon, the ISCr current path will melt down due to the Joule heat of the short current and the ISCr process will be interrupted. The Fusing Phenomenon raises the re-ISCr problem, which means that the battery may have an ISCr again after the end of the former ISCr process. The Fusing Phenomenon's life cycle is given by the hypothesis, which includes the heating-melting period, the ion discharging period and the implosion period. The heating-melting period is analyzed using an axisymmetric local ISCr model. When the ISCr area radius increases, the ISCr current path melting position will change from the ISCr area to the ISCr area edge of the aluminum current collector. If the ISCr area radius is large enough, the battery will run into the thermal runaway instead of the Fusing Phenomenon. The influence factors of the Fusing Phenomenon, including the ISCr area radius and the ISCr material, are analyzed. This research provides new insights on the battery ISCr and enriches our understanding of the ISCr process. [ABSTRACT FROM AUTHOR]