Eric R. Logan

In recent years, the LiFePO ₄ (LFP) positive electrode chemistry has received increasing attention. Comprehensively understanding the high temperature degradation of LFP/graphite cells is still an active research topic. Here, the aging of LFP/graphite cells in cycling and storage experiments was compared, testing cells with three different artificial graphite (AG) negative electrodes, and two different Li salts in the electrolyte, LiPF ₆ , and Li bis(fluorosulfonyl)imide (LiFSI). Capacity retention performance was found to be worse for cells that were stored at 100% SOC compared to cycling, and for both aging conditions the trends were the same over the different artificial graphites and electrolytes tested. Parasitic reactions were probed using ex situ gas evolution measurements and isothermal microcalorimetry (IMC), highlighting the differences between parasitic reactions in full cells and separated charged electrodes for the different cell designs. Cycling and storage data were fit to a two-parameter lifetime model based on SEI growth and Fe deposition on the negative electrode, finding a good fit to the cycling data but a poor fit to the storage data. Finally, it was shown that measurements of parasitic reactions originating solely from the negative electrode correlate well to lifetime for cells under both cycling and storage aging.

With the growing adoption of LiFePO₄ (LFP)/Graphite (Gr) cell chemistry in electric vehicles and grid energy storage, understanding and enhancing its performance under high-temperature conditions has become increasingly critical. In this study, various concentrations of vinylene carbonate (VC) (1% to 5%) were introduced to LFP/Gr pouch cells cycled at 70 °C to evaluate their impact on cell lifetime. Additionally, two different lithium salts, LiFSI and LiPF₆, were investigated. Upon reaching the end-of-life (80% capacity retention), detailed post-mortem analyses were performed, including qNMR and GC-MS to determine changes in electrolyte composition, micro X-ray fluorescence (μXRF) to quantify Fe deposition on the negative electrode, and electrochemical impedance spectroscopy (EIS) to assess charge-transfer resistance. Various LFP/Gr pouch cells were evaluated, encompassing four distinct graphite types, two LFP surface area variations, and two cell form factors. The results demonstrate that higher VC concentrations significantly improve cell lifetime, reduce Fe dissolution, and suppress electrolyte degradation pathways, including the formation of ethyl methyl carbonate (EMC) and dimethyl 2,5-dioxahexane carboxylate (DMOHC). Furthermore, while LiFSI-based LFP/Gr cells exhibit enhanced performance in certain metrics, they suffer the production of gas at 70 °C, which can be mitigated by incorporating LiPF₆ salt.

LiMn₂O₄ (LMO) cells are known to suffer from the electrochemical oxidation and dissolution of manganese from the positive electrode into the electrolyte and subsequent transport to the negative electrode, where the metals are deposited on the solid electrolyte interphase (SEI)^1-3. Pouch cells of polycrystalline and single crystal spinel LMO / artificial graphite (AG) were tested under varying conditions of temperature and upper cutoff voltage to investigate degradation mechanisms. Mn dissolution from the positive electrode and deposition onto the graphite negative electrodes immediately after formation was found to be significantly suppressed by operating cells at -10°C during the formation cycle and reducing the upper cutoff voltage. A formation cycle at an elevated temperature of 70°C greatly increases the Mn deposition and gas generation from electrolyte reduction after just a single cycle and had long term effects at increasing lifetime gassing and Mn deposition. The cold formation advantage disappeared once all cells were cycled at 40°C, with similarly terrible cycle life (60 - 200 cycles) regardless of formation conditions. Studying fully lithiated graphite (LiC₆) extracted from LMO cells in an isothermal microcalorimeter (IMC), we found that the parasitic heat flow associated with electrolyte reduction and graphite delithiation increased for anodes with significant Mn deposition. Therefore, Mn deposition on the negative electrode causes cell failure by compromising the anode passivation, increasing lithiated graphite-electrolyte reactivity and thereby accelerating the lithium inventory loss due to electrolyte reduction on the Mn sites on the negative electrode. Figure 1: Correlating variables critical to the degradation mechanism of LMO/AG cells for polycrystalline (PC) and single crystal (SC) LMO cells. (a) Arrhenius plot of Mn deposition on the graphite electrode with exponential dependence on formation temperature. (b) Formation gas from electrolyte reduction vs Mn deposition. (c) Parasitic heat flow from lithiated graphite reacting with electrolyte in pouch bags measured by isothermal microcalorimetry as a function of Mn deposition. (d) Cell capacity loss and heat energy dissipation during voltage hold experiments as a function of Mn deposition. References: [1] Zhan, C.; Wu, T.; Lu, J.; Amine, K. Dissolution, Migration, and Deposition of Transition Metal Ions in Li-Ion Batteries Exemplified by Mn-Based Cathodes – a Critical Review. Energy Environ. Sci. 2018, 11 (2), 243–257. [2] Solchenbach, S.; Hong, G.; Freiberg, A. T. S.; Jung, R.; Gasteiger, H. A. Electrolyte and SEI Decomposition Reactions of Transition Metal Ions Investigated by On-Line Electrochemical Mass Spectrometry. J. Electrochem. Soc. 2018, 165 (14), A3304–A3312. [3] Eldesoky, A.; Kowalski, N.; Ni, H.; Logan, E. R.; Zsoldos, E.; Dutta, A.; Zhang, N.; Dahn, J. R. Studying The Impact of Electrolyte, Li Excess, NMC Blending, and Cycling Conditions on The Lifetime and Degradation of LMO/AG Cells Using UHPC Cycling, XRF, and Isothermal Microcalorimetry. J. Electrochem. Soc. 2023, 170 (9), 090530. Figure 1

In this work, pouch cells of spinel LiMn₂O₄ (LMO) / artificial graphite (AG) were tested under varying conditions to investigate degradation mechanisms. Mn dissolution from the positive electrode and deposition onto the graphite negative electrodes immediately after formation was found to be significantly suppressed by operating cells at −10 °C during the formation cycle and limiting the upper cutoff voltage. A formation cycle at an elevated temperature of 70 °C greatly increases the Mn deposition and gas generation from electrolyte reduction after just a single cycle and had long term effects at increasing lifetime gassing and Mn deposition. The cold formation advantage disappeared once all cells were cycled at 40 °C, with similarly terrible cycle life (60–200 cycles) regardless of formation conditions. Studying fully lithiated graphite (LiC₆) extracted from LMO cells in an isothermal microcalorimeter (IMC), we found that the parasitic heat flow associated with electrolyte reduction and graphite delithiation increased for anodes with significant Mn deposition. Therefore, Mn deposition on the negative electrode causes cell failure by compromising the anode passivation, increasing lithiated graphite-electrolyte reactivity and thereby accelerating the lithium inventory loss due to electrolyte reduction on the Mn sites on the negative electrode.

The impact of electrolyte, Li excess, NMC blending, and cycling conditions on the performance of Li_1+xMn_2-xO₄ (LMO)/Artificial Graphite (AG) cells was studied using ultra-high precision coulometry (UHPC), X-ray fluorescence (XRF), and isothermal microcalorimetry (IMC). Decreasing the Li excess resulted in severe capacity fade which was greatly improved by blending LMO with NMC622. The known synergy between NMC and LMO is electrolyte-dependant and was more significant at elevated temperatures. We showed with XRF that Mn deposition on the negative electrode occurs primarily during the early cycles and is reduced by increasing the Li excess in LMO or by blending with NMC622. IMC experiments demonstrates a correlation between parasitic heat flow and Mn loading on the negative electrode and gas generation. Finally, LiFSI co-salts were examined to suppress Al corrosion while retaining the beneficial role of LiFSI in improving cell performance.

Unwanted parasitic reactions in lithium-ion cells lead to self-discharge and inefficiency, especially at high temperatures. To understand the nature of those reactions this study investigates the open circuit storage losses of LFP/graphite and NMC811/graphite pouch cells with common alkyl carbonate electrolytes. The cells perform a storage test at 40 °C with a 500 h open circuit period after formation at temperatures between 40 °C and 70 °C. Cells formed at elevated temperature showed a high reversible storage loss that could be assigned to a redox shuttle generated in the electrolyte during formation. A voltage hold after formation can reduce the shuttle-induced self-discharge as indicated by significantly lower reversible storage losses, the absence of shuttling currents in cyclic voltammetry and improved metrics in ultra-high precision cycling. The addition of two weight percent vinylene carbonate can prevent redox shuttle generation and leads to almost zero reversible self-discharge.

Unwanted self-discharge of LFP/AG and NMC811/AG cells can be caused by in situ generation of a redox shuttle molecule after formation at elevated temperature with common alkyl carbonate electrolyte. This study investigates the redox shuttle generation for several electrolyte additives, e.g., vinylene carbonate and lithium difluorophosphate, by measuring the additive reduction onset potential, first cycle inefficiency and gas evolution during formation at temperatures between 25 and 70 °C. After formation, electrolyte is extracted from pouch cells for visual inspection and quantification of redox shuttle activity in coin cells by cyclic voltammetry. The redox shuttle molecule is identified by GC-MS and NMR as dimethyl terephthalate. It is generated in the absence of an effective SEI-forming additive, according to a proposed formation mechanism that requires residual water in the electrolyte, catalytic quantities of lithium methoxide generated at the negative electrode and, surprisingly, polyethylene terephthalate tape within the cell.

Recent observations by our group show the creation of a reversible shuttle species in LFP/graphite and NMC811/graphite cells with 3:7 ethylene carbonate:dimethyl carbonate (EC:DMC) based electrolytes. This is indicated by a high reversible self-discharge of these cells in the absence of electrolyte additives. Electrolyte extraction from pouch cells after formation allowed to directly investigate the electrolytes for redox shuttle currents. For this purpose, the extracted electrolytes were inserted into coin cells with an Al foil as the working electrode (WE) and a Li foil as the counter electrode (CE). The measured cyclic voltammetry (CV) of the coin cells show a clear relationship between high formation temperature and high shuttle currents. Interestingly, the addition of vinylene carbonate (VC) to the electrolyte completely prevents the shuttle current, even at elevated formation temperatures. [1] In this study, we systematically investigate the effect of various electrolyte additives on the generation of shuttle molecules. LFP/graphite and NMC811/graphite pouch cells were filled with electrolyte consisting of 3:7 EC:DMC with 1.5 M lithium hexafluorophosphate (LiPF₆) and different additives. The pouch cells were formed at different temperatures, T_F. The electrolytes were then extracted and inserted into the aforementioned coin cell setup for CV measurements. We have found that additives such as VC, fluoroethylene carbonate (FEC), ethylene sulfate (DTD), prop-1-ene-1,3-sultone (PES), and triallyl phosphate (TAP), which are known to create a stable solid electrolyte interphase (SEI), [2-4] prevent shuttle current in the CV. On the other hand, additives such as succinonitrile (SN) and trimethylsilyl isothiocyanate (TMSNCS), which do not contribute to the formation of a better SEI, [5,6] cannot prevent the shuttle current. This suggests that the formation of the shuttles is due to a poor SEI and therefore occurs at the interface between electrolyte and graphite anode. Analogue experiments with DMC as only solvent instead of 3:7 EC:DMC show similar shuttle currents in CVs, which suggests that linear carbonates such as DMC are required to form the shuttle. Figure 1 shows CVs for 1.5 M LiPF₆ DMC electrolyte. The shuttle current appears to be the same for electrolyte extracted from LFP/graphite and NMC811/graphite cells ranging up to 6 μA in both cases. This indicates that the shuttle is formed independently of the cathode material, and therefore gives rise to the hypothesis that it is formed at the anode-electrolyte interface. Figure 1 also shows that the shuttle current increases with higher formation temperatures T_F. References: Boulanger, A. Eldesoky. S. Buechele, T. Taskovic, S. Azam, C. Aiken, E. Logan, M. Metzger, Investigation of redox shuttle generation in LFP/graphite and NMC811/graphite cells, Submitted (2022). Song, J. Harlow, E. Logan, H. Hebecker, M. Coon, L. Molino, M. Johnson, J. Dahn, M. Metzger, A Systematic Study of Electrolyte Additives in Single Crystal and Bimodal LiNi 0.8 Mn 0.1 Co 0.1 O 2 /Graphite Pouch Cells , J. Electrochem. Soc. 168 (2021) 090503. doi:10.1149/1945-7111/ac1e55. J. Nelson, J. Xia, J.R. Dahn, Studies of the Effect of Varying Prop-1-ene-1,3-sultone Content in Lithium Ion Pouch Cells, J. Electrochem. Soc. 161 (2014) A1884–A1889. doi:10.1149/2.0791412jes. Xia, L. Madec, L. Ma, L.D. Ellis, W. Qiu, K.J. Nelson, Z. Lu, J.R. Dahn, Study of triallyl phosphate as an electrolyte additive for high voltage lithium-ion cells, J. Power Sources. 295 (2015) 203–211. doi:10.1016/j.jpowsour.2015.06.151. Chen, F. Liu, Y. Chen, Y. Ye, Y. Huang, F. Wu, L. Li, An investigation of functionalized electrolyte using succinonitrile additive for high voltage lithium-ion batteries, J. Power Sources. 306 (2016) 70–77. doi:10.1016/j.jpowsour.2015.10.105. G. Han, M.Y. Jeong, K. Kim, C. Park, C.H. Sung, D.W. Bak, K.H. Kim, K.M. Jeong, N.S. Choi, An electrolyte additive capable of scavenging HF and PF5 enables fast charging of lithium-ion batteries in LiPF6-based electrolytes, J. Power Sources. 446 (2020) 227366. doi:10.1016/j.jpowsour.2019.227366. Acknowledgements This work was funded under the auspices of the NSERC/Tesla Canada Alliance Grant program. Figure 1

State-of-the-art Li-ion cells can have decades of lifetime (>40 years) and tremendous cycle life greater than 10000 cycles¹. Such incredible cells greatly exceed the goal of 80% capacity retention after 800 cycles, promoted by some as sufficient for electric vehicles (EVs). However, by 2030 it is projected that more than 90% of all Li-ion batteries will be used to power vehicles², with very few remaining for energy storage from intermittent renewables which must displace fossil fuels for power generation. The batteries in electric vehicles will represent a vast amount of energy storage capacity, which can be harnessed using vehicle to grid (V2G) technology. Batteries with that can maintain 80% of their capacity after a mere 800 cycles are unsustainable since most of the charge-discharge cycles in V2G will occur when the EV is stationary. For this reason, ultra-long-lived cells are a critical component of a sustainable future. In this talk, we will discuss the important role of graphite material and upper cut-off voltage (UCV) on the lifetime of NMC811 cells, and how avoiding volume contraction in Ni-rich materials yields cells that should be eminently suitable V2G applications^3,4. Additionally, we will discuss the results of a two-year study on the impact of C-rate, depth of discharge (DoD), UCV, and temperature on the lifetime of NMC811 cells. Finally, we will demonstrate that NMC cells with a low UCV, with an emphasis on Ni-rich materials, yield greater energy throughput compared to their higher energy counterparts due to their prolonged lifetime. The $/kWh metric is surely important when selecting a battery technology, but the levelized cost of energy over the battery lifetime is more important when batteries last for many decades. References Harlow, J. E. et al. A Wide Range of Testing Results on an Excellent Lithium-Ion Cell Chemistry to be used as Benchmarks for New Battery Technologies. Journal of The Electrochemical Society 166, A3031–A3044 (2019). Pillot, C. The Rechargeable Battery Market and Main Trends 2018-2030. https://rechargebatteries.org/wp-content/uploads/2019/02/Keynote_2_AVICENNE_Christophe-Pillot.pdf (2019). Eldesoky, A. et al. Impact of Graphite Materials on the Lifetime of NMC811/Graphite Pouch Cells: Part I. Material Properties, ARC Safety Tests, Gas Generation, and Room Temperature Cycling. Journal of The Electrochemical Society 168, 110543 (2021). Eldesoky, A. et al. Impact of Graphite Materials on the Lifetime of NMC811/Graphite Pouch Cells: Part II. Long-Term Cycling, Stack Pressure Growth, Isothermal Microcalorimetry, and Lifetime Projection. Journal of The Electrochemical Society 169, 010501 (2022).

LFP/Graphite cells are attractive because they are cheaper^[1], safer^[2,3] and can achieve acceptable energy density for most applications. A limitation of the LFP/Graphite cells is their inferior capacity retention at elevated temperature when compared to NMC/Graphite cells especially in the absence of electrolyte additives, e.g., VC (Vinylene carbonate), as observed by our group in a recent study^[4].The time it takes for a LFP graphite cell to completely self-discharge at 60^oC is around 500 hours with a base electrolyte of 1.5 M LiPF₆ (Lithium hexafluorophosphate) dissolved in 7:3 DMC:EC (Dimethyl carbonate/ethylene carbonate)^[4]. It also has been observed that during cycling some Fe will accumulate on the anode which can be explained by dissolution of Fe from the cathode and subsequent deposition on the anode. With the same electrolyte as mentioned before we can observe up to 0.2 μg/cm² on the anode after 60 cycles at 40⁰C^[4]. The goal of this research is to understand what is happening inside the LFP/Graphite cells by analyzing the electrolyte from cells that only did a formation procedure. We were able to extract different electrolytes from LFP/Graphite and NMC811/Graphite pouch cells. We expected the electrolyte to stay clear as it was just after preparation, but we observed yellow and red colors depending on the temperature of formation. If the electrolyte contained 2%wt VC, no color change was observed. The different electrolytes used for the experiments were LiPF₆, LiFSI (Lithium bis(fluorosulfonyl)imide) and LiPF₆+2%wt VC all dissolved in 7:3 DMC:EC. The extracted electrolytes were put inside coin cells with an Al foil working-electrode (WE) and a Li foil counter-electrode (CE) and cyclic voltammetry (CV) was done on them from 2.6 V - 3.75 V vs. Li⁺/Li. The CV traces show the presence of current in the μA range for electrolyte without VC extracted from LFP cells, indicating the presence of a reversible shuttle species. The electrolyte with no VC expected from LFP cells showed more current than the corresponding electrolyte extracted from NMC811 cells. There was almost no current in the coin cells using electrolytes with 2% VC extracted from the LFP and NMC811 cells. We also made systematic experiments at different formation temperature and different wait times before extraction. Figure 1: Observation of the different electrolytes extracted from LFP/Graphite cells with a) 1.5 M LiPF₆ EC:DMC 3:7 and b) 1.5 M LiPF₆ +2%wt VC EC:DMC 3:7 that did formation at 25, 40 ,55 and 70⁰C (left to right). References W. Li, Y. Cho, W. Yao, Y. Li, A. Cronk, R. Shimizu, M. A. Schroeder, Y. Fu, F. Zou, V. Battaglia, A. Manthiram, M. Zhang, and Y. S. Meng. “Enabling high areal capacity for Co-free high voltage spinel materials in next-generation Li-ion batteries”, Journal of Power Sources, 473 (2020). D. Jian, T. Xuan, D. Haifeng, Y. Ying, W. Wangyan, W. Xuezhe, and H. Yunhui. “Building Safe Lithium-Ion Batteries for Electric Vehicles: A Review”, Electrochem. Energ. Rev. 3, 1–42 (2020). W. Li, H. Wang, Y. Zhang, and M. Ouyang. “Flammability characteristics of the battery vent gas: A case of NCA and LFP lithium-ion batteries during external heating abuse”, Journal of Energy Storage, 24 (2019) E. R. Logan, H. Hebecker, A. Eldesoky, A. Luscombe, M. B. Johnson, and J. R. Dahn. “Performance and Degradation of LiFePO4/Graphite Cells: The Impact of Water Contamination and an Evaluation of Common Electrolyte Additives”, Journal of The Electrochemical Society, 167, 13 (2020) Figure 1