Publications

This study describes a dip-coating method for applying an active material to commercially available intermediate modulus carbon fibers (CFs). A suite of tools were developed to assist with the handling and coating of CF tows to create disc electrodes. CF electrodes were fitted into 2025-type coin cells, for electrochemical analysis, first to determine their performance as anodes. Specimens of CFs were dip-coated with a slurry consisting of lithium iron phosphate, carbon black, and polyvinylidene fluoride, then dried, and fitted into half-cells to analyze the cathode performance. A cyclic voltammetry sweep was performed on each half-cell to determine suitable cycling potential limits, followed by galvanostatic cycling for a minimum of 30 cycles. Measured capacities for anode and cathode half-cells yielded 92 and 52.3 A h kg-1, respectively. A full-cell with CFs in both electrodes was assembled and tested, revealing a capacity of 24.7 A h kg-1 after 30 cycles. Finally, both the anode and cathode were examined with a scanning electron microscope to establish a benchmark (anode) to compare surface topologies and to analyze the quality of the active material applied via dip-coating (cathode).

Perovskite-type lithium-ion conductors are a potential class of solid-state electrolytes for solid-state batteries due to their excellent environmental stability, good mechanical strength, reasonably wide electrochemical stability window, and high ionic conductivity. A-site deficient Li6/16Sr7/16Ta3/4Hf1/4O3 is a promising perovskite composition identified, but it is prone to form impurity phases during synthesis, which introduces high grain bounary resistance to the total ionic conductivity. A systematic investigation is reported on the effect of the synthesis conditions (e.g., excess Li, sintering temperature, and mother powder protection) on the phase composition and properties of this perovskite. The results show that the mother powder bed protection and 1450 °C sintering temperature without Li compensation are the best conditions to achieve single phase. The single-phase sample exhibits 96% theoretical density, a bulk ionic conductivity of 0.408 mS cm−1, and an electronic conductivity of 3.6 × 10−9 S cm−1 at 25 °C with an activation energy of 0.352 eV, Young s modulus of 63.91 GPa, and shear modulus of 26.16 GPa. However, Li6/16Sr7/16Ta3/4Hf1/4O3 is unstable against lithium metal and can be reduced readily. Alternative anode materials or surface protection layers are needed if it is considered as an electrolyte for lithium-metal based solid-state lithium-ion batteries.

This paper presents a simplified thermal runaway model (FEM) used to guide the design of a novel battery pack designed to resist thermal runaway propagation passively. The model is based on the heat equation for a 2D geometry with a heat generation term based on the maximum amount of energy measured using a custom-made calorimeter. The model was validated against experimental data using a 48-cell subscale of a full-scale battery pack for three different runs with three trigger cells with Internal Short Circuit Devices (ISCD) implanted in the separators. One trigger cell was placed at the edge, one placed in the middle, surrounded by six cells, and one placed in one corner of the subscale pack. It was shown that by simplifying the geometry and looking at the complex thermal runaway propagation mechanism only from a thermal perspective (no electrochemical reactions or fluid flow), the model predicted the experimental data with good precision. Furthermore, such a model was used to validate some experimental observations, which indicated the practicality of such a simplified design tool.

Modeling heat distribution in Li-ion battery packs can be challenging, especially if the battery pack is large and the cells are operated at high C-rates, which usually requires high-order physics-based mathematical models. Reduced and simplifying models can, however, be used at lower rates. This paper presents a fast novel reduced lumped model (RLM) that can be used to calculate the temperature increase during the high-current discharge of cylindrical Li-ion cells in a subscale of a battery pack. By reducing the PDE utilized to calculate the state of charge (SoC) to ODE s and solving them analytically, the reduced model can be a very reliable and fast tool for calculating the temperature distribution in battery packs. The voltage was calculated by considering the charge overpotential, the ohmic overpotential, and the activation overpotential, while the properties of the parameters are dependent on the temperature following an Arrhenius-dependency. Comparing with experimental data, the model showed a good prediction of the temperature readings showing good potential in using the model for battery packs operating at high C-rates (\textgreater2 C).

Spatially dependent low-temperature to room-temperature degradation mechanisms for Li(Ni0.5Mn0.3Co0.2)O2/LixC6 (NMC532/graphite) large format 50Ah Li-ion batteries were investigated. First, highly stressed regions of the cathode/anode are found to be exacerbated by extreme conditions (i.e., low-temperature cycling). The severe electrochemical polarization of large 50Ah electrodes at low temperature leads to substantial Li0 deposition and severe gassing at the regions of high stress (i.e., high curvature, edges, and electrode ripples). A series of analytical techniques (e.g., SEM, XPS, GC-MS, and Raman spectroscopy) found that Li0 plating (charge) or corrosion (storage) leads to severe gassing and decomposition products (including carbides). The expansion/contraction and extreme polarization during low-temperature cycling, was found to cause a ripple-type Li0 deposition on the electrode. Multilocation liquid nitrogen (N2) Raman spectroscopy of electrodes indicates significant quantities of Li0 deposition reside at ripple peaks (high-stress region) and are found negligible at ripple troughs. Postmortem analysis discovered two failure scenarios that originate from low-temperature cycling, either nonthermal runaway venting or an internally shorted thermal runaway. It was found in the first case (storage) that LiC6-Li0 undergoes severe corrosion and gassing during storage conditions (i.e., no movement, current, and temperature) and proceeds to trigger thermal runaway and ejection of materials (∼2 weeks). The second case (RT cycling after low temperature) resulted in nonthermal runaway overpressurized venting of the cell and release of detectable quantities of flammable/toxic gases (e.g., CO2, CO, CH4, and C2H2). The second event was found to be caused by competing reactions (i.e., Li0 stripping, Li0 corrosion, and severe gassing). This study finds that low-temperature Li0 plating and LiC6-Li0 corrosion results in severe gassing, which exacerbates highly stressed regions (i.e., electrode buckling) and greatly compromises safety of the application - via nonthermal runaway venting when cycled (e.g., stripping of Li0 and gassing) and catastrophic thermal runaway when resting under storage (e.g., larger quantities of LixC6-Li0 corrosion).

Rapid voltage and temperature estimations with a Reduced Order Lumped Electrochemical-Thermal Model (TLM) was developed by applying a State Space Approach to transform partial differential equations (PDEs) into ordinary differential equations (ODEs). The TLM is attractive for Battery Management Systems (BMS) because of model restrictions that result in only four parameters: exchange current (i0S), diffusion time constant ($\tau$), internal resistance (RIR), and the entropic heat coefficient (dUdT−1). The State Space approach is shown to be an effective method for reducing the computational time for the model by greater than 50% (\~2s to less than 1s). This study also shows that the required model parameters (i0S, $\tau$, RIR, dUdT−1) can be nondestructively extracted from real cells using the galvanostatic intermittent titration technique (GITT). This allows us to create cell-level temperature and state of charge (SOC) parameter surfaces that would be nearly impossible to develop experimentally. By confirming the extracted parameters with the model predicted parameters, future BMS models can further reduce computational time (approach millisecond predictions) by experimentally constraining the model. This means that the methodology reported in this paper can be ubiquitously implemented for other battery chemistries (e.g. cathodes, anodes), formats (e.g. 18650, pouch, prismatic), and properties (e.g. capacity ratios).

The Li(Ni 0.5 Mn 0.3 Co 0.2 )/Li x C 6 (NMC532/graphite) Li-ion battery chemistry has proven to be reliable in electric vehicle battery applications due to its high capacity, good energy density, and high cyclability. 1 Under normal operating conditions (at slightly above room temperature), the long-term battery stability and response to charge/discharge is very well understood. However, there are other possible applications for Li-ion batteries where they would be forced to operate under extreme cold such as high elevation/cold climate electric vehicles, satallites, electric aircraft, unmanned underwater vehicles. At these ultra-low temperatures, the degradation mechanisms are less understood and less predictable by current modeling approaches. What is known is that destructive events 2 like thermal runaway can occur even at very low temperatures (-30 o C, which is a rated temperature for Li-ion batteries). They are caused by Li plating events where stratified mossy-like Li 0 deposition occurs around highly stressed regions (e.g. edges, regions of high curvature) while deposition is more uniform around the electrode center. Morphological studies by SEM reveal considerable fusing (i.e. indistinguishable boundaries or complete collapse of particle-particle interfaces) of graphite particles, which leads to drasitic morphological changes to the graphite electrode structure. Moreover, severe gassing and displacement of anode material can lead to significant warping of the electrode material. Low temperature volumetric expansion and contraction can cause warping of the electrode in ripple-type formation. In addition, XPS and Raman spectroscopy revealed formation of lithium carbides from the corrosion of Li x C 6 -Li 0 on the peak of the ripple-type pattern and absent at the trough. The reasoning for this anisotropic distribution of Li is attributed to the preference for electron transfer at regions of high stress (e.g. edges, peaks, and regions of high curvature). In this study, a non-thermal runaway overpressurization of the cell was found to occur after a 50Ah NMC532/graphite cell was cycled at room temperature after repetitive low temperature experiments. In addition, a parallel cell was stored at room temperature (i.e. no movement, no current, no temperature) for 2 weeks before it spontaneously went into catastrophic thermal runaway. The presentation will provide a thorough study of the 50 Ah large format NMC532 /graphite cells 3 and will elucidate low temperature degradation processes that occur, leading to thermal runaway. References 1. Harlow, J. E., Ma, X., Li, J., Logan, E., Liu, Y., Zhang, N., Ma, L., Glazier, S. L., Cormier, M. M. E., Genovese, M., Buteau, S., Cameron, A., Stark, J. E. & Dahn, J. R. J. Electrochem. Soc. 166 , A3031–A3044 (2019). 2. Gao, S., Lu, L., Ouyang, M., Duan, Y., Zhu, X., Xu, C., Ng, B., Kamyab, N., White, R. E. & Coman, P. T. J. Electrochem. Soc. 166 , A2065–A2073 (2019). 3. Ng, B., Coman, P. T., Mustain, W. E. & White, R. E. J. Power Sources 445 , 227296 (2020).

In this work, a plain glassy carbon electrode has been investigated as a base platform to build a superoxide-ion-involved, 2-dimensional, multi-physics model to describe its oxygen reduction mechanism in caustic media. A rotating ring disk technique has been used to quantify the peroxide content and to compare the results predicted by a general multiphysics model, which was further used to extract the influencing kinetic parameters. There are three proposed models involving different mechanism combinations made up of: a sequential, single electron reduction of oxygen to superoxide, then to peroxide; a sequential two electron reduction of oxygen to peroxide followed by the final reduction to hydroxide; and a direct four electron reduction of oxygen straight to hydroxide. One model stands out to be the best description for the multistep oxygen reduction behavior of the glassy carbon electrode in 0.1 M KOH with very satisfactory results, which yields a series of important electrode kinetic transfer coefficients and exchange current densities for the elementary electrochemical reactions considered.

© 2020 The Electrochemical Society (“ECS”). Published on behalf of ECS by IOP Publishing Limited Lithium sulfur (Li-S) batteries have the potential to outperform the current lithium ion batteries and transform the technology of the future. However, dissolution, diffusion, and shuttling of the dissolved polysulfides result in parasitic reactions and substantial capacity loss. To provide a better understanding of the shuttling process, a 1D porous electrode mathematical model has been developed in this paper. An approximation method is used to account for the shuttling-induced capacity loss by adding an extra source/sink term in the material balance equations for the species involved in the parasitic reactions. Shuttling constants used in the source terms can be determined by fitting the model predictions to the experimental measurements. The results showed that by including the approximation method, the model was able to predict the active material loss and the continuous decrease of volume fractions of Li2S on the cathode surface. The model sheds light on the capacity loss mechanism occurring inside the cell as a result of the shuttling of polysulfides.

In this work, the role of carbon in alkaline oxygen electrocatalysis was investigated using an oxygen-deficient, perovskite-structured oxide catalyst, La0.6Sr0.4CoO3−$δ$ (LSCO), mixed in different mass ratios with Vulcan carbon (XC-72R, referred to as XC-72) to observe the effect of the LSCO-to-XC-72 ratio (LSCO/XC-72) on the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics. Six different LSCO/XC-72 mass ratios were designed for the study as follows: 1:0 (pure LSCO), 10:1, 7:1, 5:1, 2:1 and 1:1. A thin-film oxygen electrode consisting of the LSCO/XC-72 catalysts was characterized in 0.1 M KOH solution using cyclic voltammetry (CV), rotating disk electrode (RDE) based linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). A physics-based, generalized electrochemical model was used to establish the charge-transfer mechanism and to obtain the electrode kinetic transfer coefficients and exchange current densities for the electrochemical reactions considered. The results show that the oxygen electrocatalysis process depends quite heavily on the carbon content in the catalyst, and there appears to be a synergistic effect between the perovskite-oxide and the XC-72, with a transition from the two-step, 2e− pathway to the direct 4e− transfer pathway as carbon content increases.