Alkyl Dicarbonate-Based Electrolytes Can Enable Long-Lived Li-Ion Cells at High-Temperatures
Bibliographic record
Abstract
Alkyl dicarbonates are known electrolyte degradation products produced in Li-ion cells that use ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte components. Also referred to as dimerization compounds, dimethyl 2,5-dioxahexanedioate (DMOHC) and diethyl 2,5-dioxahexanedioate (DEOHC), were investigated as a possible sole electrolyte solvent or and one component of a blended solvent mixture, when mixed with linear carbonates. The viscosities of DMOHC and DEOHC were measured in this report and compared to the predictions of the Advanced Electrolyte Model1, along with the two alkyl dicarbonates mixed with lithium salts and other common electrolyte solvents. Electrolytes based on DMOHC or DEOHC alone exhibit much higher viscosity than conventional EC-based electrolytes at room temperature. Thus, for testing, LiNi0.5Mn0.3Co0.2O2/graphite (NMC532), LiNi0. 83Mn0. 6Co0. 11O2/graphite (Ni83) and LiFePO4(LFP)/graphite cells with DMOHC were tested at 70°C and 85°C using C/20 charge and discharge rates. DMOHC and DEOHC were mixed with two different electrolyte salts. First, the common LiPF6 salt and the second, lithium bis(fluorosulfonyl)imide (LiFSI), both with 2% vinylene carbonate (VC). These two electrolytes were tested in NMC532/graphite cells cycled to 4.3V at a C/20 charge/discharge rate and 70°C. Cells with DMOHC and LiFSI showed considerable improvements in capacity retention compared to those filled with an EC-based electrolyte with LiPF6 salt, also cycled at 70°C. The same conclusion was found with NMC532 cells filled with DEOHC with LiFSI salt or LiPF6. Figure 1a shows the fractional capacity versus time for NMC532/graphite, Ni83/graphite and LFP/graphite pouch cells with 1.0 M LiFSI in DMOHC with 2% VC (vinylene carbonate) and 1% DTD (ethylene sulfate) additives tested at C/20 and 85°C. These cells were tested to upper cut-off potentials of 3.8, 3.9 and 3.65 V respectively. In addition, Figure 1b shows the corresponding voltage polarization results. Results show that Ni-containing cells experience exceptional lifetimes and low impedance growth despite the high cycling temperature. DMOHC-containing cells have since been tested up to 100°C. DMOHC was most advantageous for mitigating severe gassing at high temperatures. Ex-situ gas experiments show DMOHC-containing Ni83 cells produce the least amount of gas even at high voltage (4.0 V) compared to cells using EC-based electrolytes. Ni-containing cells operating to low voltage limits show unprecedented cycling lifetimes when using DMOHC electrolytes containing LIFSI while also exhibiting limited gassing. To mitigate the high viscosity of DMOHC, various cells were filled with electrolytes that used DMOHC and various percentages of diethyl carbonate (DEC) and dimethyl carbonate (DMC) with LiFSI as the salt. DMC/DEC was chosen for its low viscosity and its ionic conductivity. Even with DMC, the cycling performance showed improvements over traditional EC-based electrolytes. There were slight decreases in performance compared to a cell with a pure DMOHC electrolyte. In addition, various additives were tested, including prop-1-ene-1,3-sultone (PES), to see if gassing could be further limited. We propose DMOHC and DEOHC as new solvents for Li-ion cell electrolytes, possibly replacing EC, to enable long-lasting, high-temperature tolerant Li-ion cells. REFERENCES E. R. Logan, E. M. Tonita, K. L. Gering, and J. R. Dahn, J Electrochem Soc, 165, A3350–A3359 (2018). Figure 1. (a) Normalized capacity versus time for LFP (blue), NMC532 (red) and Ni83 (black) artificial graphite cells, using 1M LIFSI DMOHC 2%VC 1%DTD electrolyte cycling at C/20 and at 85°C. (b) corresponding voltage polarization data versus time. Figure 1
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".