Reversible Nature of Impedance on LiCoO<sub>2</sub>/Graphite and Li[Ni<sub>0.42</sub>Mn<sub>0.42</sub>Co<sub>0.16</sub>]O<sub>2</sub>/Graphite Pouch Cells Cycled to High Voltages
Bibliographic record
Abstract
Introduction Development of high voltage Li-ion cells is critical for the improvement of energy density of Li-ion cells [1]. Electrolyte additives are used to improve the properties and performance of Li-ion cells [2]. However, the way that electrolyte additives function and the impedance changed that occurs with voltage in Li-ion cells have not been well-explained in the literature. A Maccor 4000 series charger, combined with a frequency response analyzer (Maccor FRA 0356), was used to investigate the effects of electrolyte additives on cell impedance changes with voltage in Li[Ni0.42Mn0.42Co0.16]O2 (NMC)/graphite and LiCoO2(LCO)/graphite pouch cells. Experimental Machine-made Li[Ni1/3Mn1/3Co1/3]O2/graphite and LiCoO2/graphite dry pouch cells (402030 size, 220 mAh) were supplied by reputable manufacturers and were filled and sealed at Dalhousie University. Cells were filled with 1 M LiPF6in ethylene carbonate (EC):ethylmethyl carbonate (EMC) (3:7 by weigh, BASF) as control electrolyte. Vinylene carbonate (VC, BASF, 99.97%) was used as an electrolyte additive. After electrolyte filling and vacuum sealing (MTI Corporation, MSK-115A) in an argon-filled glove box, a 24 h hold at 40.0 ± 0.1°C and 1.5 V was used to ensure complete wetting of the cell coil. The first charge cycle (called the formation process here) consisted of charging at 11 mA (corresponding to C/20 current) to 3.5 V. Then cells were degassed in the glove box and vacuum sealed again. Subsequently, cells were charged to 4.5V using the same current, followed by degassing and vacuum sealing in the glove box. The cells were cycled using a Maccor 4000 series charger between 2.8 and 4.7 V at 40.0 ± 0.1°C using currents corresponding to C/20 while the cell impedance was measured at every 0.1 V interval between 3.6 and 4.7 V with a Maccor frequency response analyzer (FRA 0356). The FRA unit and cells during testing were in temperature controlled environments (21°C for NMC/graphite pouch cells, and 30°C for LCO/graphite pouch cells) with variations in temperature of an amplitude less than 2°C. AC impedance spectra were collected from 10 kHz – 10 mHz with an amplitude of 2 mV. Ten data points per decade were measured. Results and discussion Figure 1 shows selected Nyquist plots for NMC cells containing 2 % VC during the first charge and the first discharge. The charge transfer resistance (Rct) was marked in Figure 1a and taken to be the diameters of overlapping semicircles from the Nyquist plots [3]. Cells with a small Rct are much more desirable. Figure 2 shows summary of Rctversus voltage for NMC/graphite and LCO/graphite pouch cells with 2 % VC. The impedance of NMC/graphite cells increases a lot as the cells are charged above 4.3 V. This impedance change is almost reversible over one cycle but the impedance slowly increases cycle by cycle. By contrast the LCO materials do not show any real impedance increase after charging to 4.4V. There are many interesting things to note which will be discussed in the lecture. References [1] M. Hu, X. Pang and Z. Zhou, J. Power Sources, 237, 229 (2013). [2] S.S. Zhang, J. Power Sources, 162,1379 (2006). [3] D.Y. Wang, N.N. Sinha, R. Petibon, J.C. Burns, and J.R. Dahn, J. Power Sources, 251, 311 (2014).
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".