The mechanisms driving the thermo-electrochemical response of commercial lithium-ion cells under extreme overdischarge conditions (< 0.0 V) are investigated in the context of copper dissolution from the anodic current collector. A constant current discharge with no lower cutoff voltage was used to emulate the effects of forced overdischarge, as commonly experienced by serially connected cells in an unbalanced module. Cells were overdischarged to 200% DOD (depth of discharge) at C/10 and 1C rates to develop an understanding of the overdischarge extremes. Copper dissolution began when a cell reached its minimum voltage level (between −1.3 V and −1.5 V), where the anode potential reached a maximum value of ∼4.8 V vs. Li/Li + . Deposition of copper on the cathode, anode, and separator surfaces was observed in all overdischarged cells, verified with EDS/SEM results, which further suggests the formation of internal shorts, although the cell failures proved to be relatively benign. The maximum cell surface temperature during overdischarge was found to be highly rate-dependent, with the 1C-rate cell experiencing temperatures as high as 79 • C. Concentration polarization and solid electrolyte interphase (SEI) layer breakdown prior to the initiation of copper dissolution are proposed to be the main sources of heat generation during overdischarge.
The lithium-ion battery (LIB) electrode represents a complex porous composite, consisting of multiple phases including active material (AM), conductive additive, and polymeric binder. This study proposes a mesoscale model to probe the effects of the cathode composition, e.g., the ratio of active material, conductive additive, and binder content, on the electrochemical properties and performance. The results reveal a complex nonmonotonic behavior in the effective electrical conductivity as the amount of conductive additive is increased. Insufficient electronic conductivity of the electrode limits the cell operation to lower currents. Once sufficient electron conduction (i.e., percolation) is achieved, the rate performance can be a strong function of ion-blockage effect and pore phase transport resistance. Even for the same porosity, different arrangements of the solid phases may lead to notable difference in the cell performance, which highlights the need for accurate microstructural characterization and composite electrode preparation strategies.
Overdischarge is a potential problem in large battery packs since cells in a series string are discharged under the same load, despite having different capacities. Although a single overdischarge does not necessarily cause a safety hazard, it forces electrodes outside their safe potential range and adversely affects the integrity of cell components. This work aims to fill the knowledge gap about the combined effect of aging-induced and overdischarge–induced degradation mechanisms. Graphite/LCO pouch cells are cycled at a moderate rate using four lower cutoff voltages: 2.7 V, 1.5 V, 0.0 V, and −0.5 V. The cells aged above the onset of reverse potential have an extended cycle life with aging-induced solid electrolyte interphase (SEI) growth and electrolyte decomposition as the main degradation mechanisms. In contrast, the cells aged under reversal condition (Elower ≤ 0.0 V) exhibit fast degradation, dictated by the interplay among lithium plating, cathode particle cracking, and dissolution of Cu current collector. The analysis is complemented with a comparative study of various state of health (SoH) indicators, including an internal resistance based dimensionless SoH descriptor. The results prove that overdischarge-induced abuse although benign, may turn into a malignant condition when alternated with continuous charging.
Unlike conventional electrode processing for Li-ion batteries, which uses the expensive and highly toxic organic N-methyl-2-pyrrolidone (NMP) solvent, aqueous processing simply employs deionized water as the solvent. However, thick aqueous processed cathodes have been found to crack during drying. In this study, the influence of electrode drying temperature and thickness on cracking was investigated. LiNi1/3Mn1/3Co1/3O2 cathodes prepared with a hydrophilic binder, modified styrene–butadiene rubber (SBR), were coated at various thicknesses and dried at temperatures ranging from 20 to 70 °C. Experiments revealed cracking worsens with increased electrode thickness and elevated drying temperatures. Cracks were formed during the capillarity-driven phase during drying. Strong evaporation and weak diffusion played a critical role in the nonuniform distribution of the inactive phase. Images of electrode surfaces were processed to quantify crack dimensions and crack intensity factor (CIF). The average crack length and width, as well as CIF, increased with drying temperature and electrode thickness. Electrochemical performance revealed a strong and negative correlation between the crack density and performance in terms of specific capacity. Transport limitations associated with the presence of cracks adversely affect the advantage of high volume ratio of active materials in the thick electrodes.
In this study, a significant number of experimental tests to proton exchange membrane (PEM) fuel cells were conducted to investigate the effect of gas flow fields on fuel cell performance. Graphite plates with various flow field or flow channel designs, from literature survey and also novel designs by the authors, were used for the PEM fuel cell assembly. The fabricated fuel cells have an effective membrane area of 23.5 cm 2 . The results showed that the serpentine flow channel design is still favorable, giving the best single fuel cell performance amongst all the studied flow channel designs. A novel symmetric serpentine flow field was proposed for a relatively large sized fuel cell application. Four fuel cell stacks each including four cells were assembled using different designs of serpentine flow channels. The output power performances of fuel cell stacks were compared and the novel symmetric serpentine flow field design is recommended for its very good performance.
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