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.
A three-electrode cell can be a useful tool for measuring electrode-level and cell-level electrochemical characteristics, such as the impedance response and potential variations in lithium-ion cells. In this paper, a reliable three-electrode coin cell setup is introduced, which improves the stability and accuracy of electrochemical measurements by modifying the electrode alignment and employing Li 4 Ti 5 O 12 as a reference electrode. An important highlight is the ability to obtain impedance evolution characteristics at different depth of discharge (DOD) for an individual electrode and the full cell based on both the frequency response analysis and the carrier function Laplace transform characteristics. The reliability of the proposed modified three-electrode coin cell setup has been validated by analyzing the impedance response of symmetric and full cells, and the voltage profiles of the full cell along with the positive/negative electrode contributions. The importance of the resistance contributions from the negative and positive electrodes to the full cell impedance evolution at different DOD is highlighted. Lithium-ion batteries (LIBs) are popular candidates for use in electric vehicles and in the application of portable electronic devices due to their favorable energy density and power capability.1-4 The wide usage of LIBs, in recent years, has raised interest in the observation of their performance and degradation phenomena. The typical configuration of battery cells, which only have anode and cathode, is suitable when the whole cell is the objective of the analysis. However, the typical cell configuration is limited when the interest is in separately studying the electrochemical characteristic of each electrode. To study the electrochemical characteristics of anode and cathode separately during the charging and discharging processes, a three-electrode cell, which includes a reference electrode (RE), working electrode (WE), and counter electrode (CE), has been introduced. By investigating the electrochemical impedance spectroscopy (EIS) and potential from three-electrode cell, the influence of the cathode and anode on the cell performance and degradation phenomena can be separately analyzed and characterized. 5-9The three-electrode cell can be setup with the configuration of plastic pouch cell, 10-12 steel cell housing, 13 or Swagelok cell. 14,15 These previous setups have the disadvantage of having a large size and high cost. To overcome these disadvantages, the three-electrode coin cell setup was selected due to its advantage of being well-sealed, portable, and low cost. To the best of the authors' knowledge, few of these three-electrode coin cell setups have been implemented to study the EIS and full cell performance (electrode potential). Delacourt et al. 16 developed a T-cell-like three-electrode coin cell setup. In this T-celllike setup, the distortion in the EIS measurement, especially in the low frequency region, can be attributed primarily to the alignment and uneven separation of the RE, CE, and ...
Increasing intercalation of Li-ions brings about distortive structural transformations in several canonical intercalation hosts. Such phase transformations require the energy dissipative creation and motion of dislocations at the interface between the parent lattice and the nucleated Li-rich phase. Phase inhomogeneities within particles and across electrodes give rise to pronounced stress gradients, which can result in capacity fading. How such transformations alter Li-ion diffusivities remains much less explored. In this article, we use layered VO as an intercalation host and examine the structural origins of the evolution of Li-ion diffusivities with phase progression upon electrochemical lithiation. Galvanostatic intermittent titration measurements show a greater than 4 orders of magnitude alteration of Li-ion diffusivity in VO as a function of the extent of lithiation. Pronounced dips in Li-ion diffusivities are correlated with the presence of phase mixtures as determined by Raman spectroscopy and X-ray diffraction, whereas monophasic regimes correspond to the highest Li-ion diffusivity values measured within this range. First-principles density functional theory calculations confirm that the variations in Li-ion diffusivity do not stem from intrinsic differences in diffusion pathways across the different lithiated VO phases, which despite differences in the local coordination environments of Li-ions show comparable migration barriers. Scanning transmission X-ray microscopy measurements indicate the stabilization of distinct domains reflecting the phase coexistence of multiple lithiated phases within individual actively intercalating particles. The results thus provide fundamental insight into the considerable ion transport penalties incurred as a result of phase boundaries formed within actively intercalating particles. The combination of electrochemical studies with ensemble structural characterization and single-particle X-ray imaging of phase boundaries demonstrates the profound impact of interfacial phenomena on macroscopic electrode properties.
Overcharge presents a serious safety concern for large scale applications of Li-ion batteries. Despite the availability of several studies of aging-induced and overcharge-induced degradation, there still exists a knowledge gap of what would happen if both degradation mechanisms simultaneously occur. In this work, commercial graphite/LCO pouch cells (5 Ah) are continuously cycled at different upper cutoff voltages, 4.2 through 4.8 V, to elucidate the cumulative effect of the overcharge process on the long-term cycling. As the upper cutoff voltage is extended, the cell gains a higher initial capacity but the cycle life diminishes significantly. Cells overcharged beyond 4.5 V experience significant volume expansion and a high rate of capacity fade, as well as a considerable increase in the temperature and internal resistance. Lithium plating and electrolyte decomposition are observed in cells charged beyond 4.5 V, with SEM-EDS verifying their presence. Electrochemical evidence of both degradation modes appears as a voltage undershoot in the discharge curves. A comparative study of various State of Health (SoH) estimation parameters is presented with the introduction of a new dimensionless SoH indicator, ΦR, based on internal resistance measurement. The proposed degradation number is found to be a good indicator of aggravated degradation in Li-ion cells.
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