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.
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 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.
Exposure of Prussian White to moisture results in chemical changes due to the formation of surface contaminants, as well as structural changes due to the absorption of water into the bulk crystal structure. Here we report an analysis of the formation rate of surface contaminants and bulk water absorption by weight tracking, infrared spectroscopy, and X-ray diffraction over extended periods of storage in high relative humidity air for fully sodiated Na2Mn0.8Fe0.15[Fe(CN)6]0.95 and partially sodiated Na1.32Mn0.8Fe0.15[Fe(CN)6]0.95. Fully sodiated Prussian White gains almost 20% in mass due to the formation of interstitial water during 20 h of storage in 100% relative humidity at 25°C. Surface hydroxides and carbonates are found after storage and a structural change from the rhombohedral to a monoclinic crystal structure is observed. It is found that vacuum drying of Prussian White powder or electrodes at 150°C can remove the interstitial water and restore the rhombohedral crystal structure, but not remove surface contaminants. Prussian White immersed in water during aqueous electrode processing also shows interstitial water and a monoclinic crystal structure, but no surface contaminants. This suggests that aqueous electrode processing of Prussian White is feasible when effective drying strategies are employed.
Prussian whites, such as the here tested NaxMn0.8Fe1.2(CN)6 (PW), are promising candidates as Na-Ion cathode active materials for cost-effective and safe battery technologies. The use of abundant elements such as sodium, iron or manganese obtained in a sustainable way makes this technology extremely attractive, especially in grid storage applications where high energy densities are not of major concern.1,2 However, PWs still suffer from low electronic conductivity, limited capacity retention, and a high sensitivity towards moisture which results in structural changes and loss of cyclable Na.3,4 Different approaches can be utilized to mitigate these issues, such as surface coatings or strict handling of the materials under inert conditions, starting from the storage of PWs powders to slurry preparations and to the handling of electrodes.3,5 In this study, we investigated the changes in surface chemistry during ambient storage (here called wet storage) and during subsequent heating of stored material. ATR-IR and TGA-MS data of materials stored for different times ranging from 1 h to up to a week showed a sharp increase in moisture-content in the material. The increase plateaued after 24 h of storage. XRD analysis of stored material showed a clear trend of structural changes upon even small storage times (<1h). Furthermore, surface hydroxides and carbonates were found via ATR-IR and a reannealing at low temperatures (<200 °C) showed the reversible release of adsorbed and interstitial water, but surface hydroxides and carbonates were not easily removed and stayed on the surface. By conducting electrochemical testing in coin-cells we analyzed the difference in cycling performance of as-received and ambient stored materials, and materials washed in water. As seen in earlier studies we also saw a drastic change in the charge profile after storage, due to changes in crystal structure.4 Interestingly, we found that this detrimental effect is reversible if the electrodes are dried at elevated temperatures. The pH of water after washing PW materials was neutral indicating the absence of major ion exchange.6 This finding, combined with the effective drying of hydrated electrodes resulted in the formulation of a water-based slurry process. As seen in Figure 1, a cell built with electrodes made from a 1 week wet-stored CAM which were dried at 120 °C (blue lines) showed a drastic different charge profile, compared to a cell built with an as-received electrode (black lines, panel a). Interestingly a cell built with CAM stored for 1 week and heated to 150 °C (orange lines, panel b) and a cell built with a cathode from a H2O-based coating which was dried at 150 °C (green lines, panel b), looks almost identical to the NMP-cells. Therefore, we assume that most water can be removed easily, and a water-based slurry process is possible. Cycling tests of water-based and environmentally more sustainable hard-carbon/PW full-cells reveal a similar cycling stability in comparison to cells with electrodes made from a traditional NMP process. References: N. Tapia-Ruiz et al., JPhys Energy, 3, 031503 (2021). Q. Liu et al., Adv. Funct. Mater., 30, 1–15 (2020). D. O. Ojwang et al., ACS Appl. Mater. Interfaces, 13, 10054–10063 (2021). J. Song et al., J. Am. Chem. Soc., 137, 2658–2664 (2015). L. Yang et al., J. Power Sources, 448, 227421 (2020). D. Pritzl et al., J. Electrochem. Soc., 166, A4056–A4066 (2019). Acknowledgements The authors acknowledge the financial support of NSERC and Tesla Canada under the auspices of the Industrial Research Chair program. Figure 1: panel a: First cycle capacity of Na/PW half-cells with cathodes either made from an as-received CAM in a NMP-based slurry (black lines), or 1 week wet-stored material made with the same slurry process (blue lines) dried at low temperatures (120 °C). First cycle capacity of Na/PW half-cells with cathodes made from either 1 week stored material heated to 150 °C (orange lines) or of an as-received material, but the slurry was done in a water-based slurry and the electrodes were dried at 150 °C (green lines)). The half-cells were cycled at 30 °C (CC mode, C/20), using an FEC/DMC (2:8) electrolyte with 1.5 m NaPF6. Figure 1
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, TF. 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 TF. 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
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