UV Binders for Li-ion Batteries: Part 2

Figure 3. Conceptualizing nanogels 6

In the last edition of Professor’s Corner, the construction and components of the lithium-ion battery (LiB) were reviewed, with emphasis on the overall performance of the battery, including storage capacity, capacity retention during cycling and Electrochemical Impedance Spectroscopy (EIS) for cathode electrode binders used in lithium-ion batteries. 1-3

In this column we will focus on the effect of UV polymer backbone structure on battery performance by investigating the effect of ‘relative heterogeneity’ of the backbone chemical structure after UV polymerization that leads to ‘nanogel formation’ and effect on charge transfer resistance in Nyquist EIS plots. The physical properties of the UV backbone structure based on Dynamic Mechanical Analysis (DMA) and Tensile elongation and their effect in UV binder electrodes on battery specific capacity rate at a charge/discharge of 1 hr (abbreviated as 1C) will be reviewed.

Investigating the Nyquist Plot and Backbone Chemistry

Figure 1. Graph of Electrochemical Impedance Spectroscopy (EIS) for UV binders, showing variation by size of the semi-circle shape and x axes shift in resistance (Ret Z/Ohm). Black lines are PVdF cells, red and green are the two Miltec cathodes cells. The semicircle for PVdF (black) shows a smaller diameter semicircle vs. UV binders red and green, with larger semicircles indicating more resistance. 5
Figure 1. Graph of Electrochemical Impedance Spectroscopy (EIS) for UV binders, showing variation by size of the semi-circle shape and x axes shift in resistance (Ret Z/Ohm). Black lines are PVdF cells, red and green are the two Miltec cathodes cells. The semicircle for PVdF (black) shows a smaller diameter semicircle vs. UV binders red and green, with larger semicircles indicating more resistance. 5

In Part 1, an EIS plot was discussed as a method of characterizing resistance within the LiB as a means of distinguishing the effect of binder structure. EIS plots for PVdF and UV binder-based cells are illustrated in Figure 1, prior to rate testing. 4 Here, the x axis represents the ohmic resistance (RΩ) and y axis represents the negative imaginary component resistance. Features of the Nyquist plots are ohmic resistance (RΩ) of the cell, including electrolyte resistance and contact resistances within the cell, and a semi-circular curve in the high-frequency region representing the charge transfer resistance (RCT) at the electrode-electrolyte interface, followed by a sloping line in the low-frequency region indicative of lithium-ion diffusion within the electrode material. The diameter of the semicircles is the charge-transfer resistance, which translates to cell polarization when it starts to cycle. The cell with the UV binder exhibited higher charge-transfer resistance (7.5 ohms) when compared to the PVdF cell (5 ohms), due to the difference in chemical structure of the polymer backbone that affects Li-ion mobility. The upward sloped line at lower frequency following the semicircle is ion diffusion. Here, both UV binders and PVdF binders are similar as there does not seem to be any diffusion limitation of the lithium ions.

Figure 2. DMA of UV binder for Nyquist plot indicating a broad Tan Delta curve with single Tg at -6 ° C extending 100° C, indicating Relative Heterogeneity of polymer Tgs.
Figure 2. DMA of UV binder for Nyquist plot indicating a broad Tan Delta curve with single Tg at -6 ° C extending 100° C, indicating Relative Heterogeneity of polymer Tgs.

The chemistry underlying the higher observed resistance for the UV binder starts with the broad Tg in the DMA observed for this UV system (Figure 2). This is presumed to represent ‘relative heterogeneity’ at the molecular level, where the formation of nanogels occurs due to the presence of a highly functional dendritic acrylate. Hence, a broad Tg temperature range will be observed representing a range of Tgs stemming from the different chain-length structures being formed after UV polymerization. The DMA Tan Delta max for the acrylate portion of the UV binder was determined to be -6° C; that spans over 100° C with a shoulder at ca 30° C, the half-height being ca 75° C. This is graphically represented in Figure 3, where the left-hand side of the DMA thermogram would represent heterogeneous structure and the right-hand side represents uniformly amorphous structure, as noted by symmetry. Doubling the amount of dendritic material in this formulation resulted in an even broader Tan Delta Tg at 10° C, with a half-width greater than 100° C, indicating even greater ‘relative heterogeneity’ (Figure 4). Also noteworthy is the large difference in crosslink density (Storage Modulus) on going from 1x dendritic material to 2x dendritic material, as noted by increase from 14 MPa to 102 MPa. It is expected the ohmic resistivity RCT will be even higher. Note that the major Tan Delta peak has a shoulder on the right-hand side indicating a bimodal distribution comprised of the presence of both mono and difunctional acrylates, as well as dendritic polymeric acrylates >25 (Figure 2).

Figure 3. Conceptualizing nanogels 6
Figure 3. Conceptualizing nanogels 6

On a molecular level, the formation of three-dimensional nanogels after UV, due to cyclization of materials, is believed to tie up Li-ions due to ion-dipole, Li + OR2 , Li + SR2 or Li + O=C interactions within the high crosslinking of the dendritic material that could affect mobility of the Li-ions during high charge/discharge rates (high frequency) that lead to high resistivity of 7 ohms vs. PVdF at 5 ohms. 6,7 The reported Tg of the highly functionalized dendritic acrylate used in this study is on the order of 100° C, and film properties for this material when UV-cured are very brittle, with a tensile elongation of <2%. Within the polymer backbone structure, initially it was anticipated that UV-cured mono and difunctional acrylates, along with these dendritic acrylate materials, would improve Li+ ion mobility within the electrode during charge/discharge cycling due to the high concentration of crosslinked polar groups and thioether linkages in low volume, as shown in Figure 5. This was not the case.

Figure 4. DMA of UV binders for Nyquist plot formulation indicating a half width of ca 75° C, indicating a broad Tan Delta curve with single Tg at -6° C and the same UV binder having the same formulation, with exception to having twice the amount of the dendritic material. This formulation was determined to have a half-width of >100° C, indicating Relative Heterogeneity of polymer Tgs.
Figure 4. DMA of UV binders for Nyquist plot formulation indicating a half width of ca 75° C, indicating a broad Tan Delta curve with single Tg at -6° C and the same UV binder having the same formulation, with exception to having twice the amount of the dendritic material. This formulation was determined to have a half-width of >100° C, indicating Relative Heterogeneity of polymer Tgs.

Storage moduli behave as expected, with max at 1928 MPa at -33° C,
decreasing to the Rubber Plateau of 14 MPa at 80.5° C. The Loss Modulus shows a max of -21° C of 270 MPa that decreases with increasing temperature but did not show a flat regime based on experimental conditions. Tensile elongation was determined to be 26.0% avg., which is similar to that reported for PVdF at 30%. The average tensile strength at break of 8.4 MPa is, however, somewhat lower than that reported for PVdF at 30 Mpa. Despite the higher charge-transfer resistance of this 1x dendritic UV binder formulation, capacity rate data mAh/g vs. cycles indicate sound battery performance. At a load of 12 mg/cm2, the 1C and 2C (30 min) charge/discharge values are 163 mAh/g and 130 mAh/g, which are comparable to PVdF-prepared electrodes in the author’s lab.

Figure 5. Illustration of dendritic materials. Stick model and chemical structure example of thioether-derived acrylate material (LHS). 8,9 Illustration of proposed Li-ion-dipole interactions in cathode electrode within Lib.
Figure 5. Illustration of dendritic materials. Stick model and chemical structure example of thioether-derived acrylate material (LHS). 8,9 Illustration of proposed Li-ion-dipole interactions in cathode electrode within Lib.

Summary and Conclusions

To summarize these findings:

  • The observed higher resistivity, as noted from Nyquest plots of UV binder and PVdF binder used in cathode electrodes for LIB (Part I), is proposed to be due to the formation of three-dimensional nanogels (crosslinked particles 50-100 um in size) derived from high-functionality UV dendritic material and low-functionality UV-polymerized acrylates. The dendritic ether groups within the three-dimensional nanogels formed are proposed to limit lithium-ion diffusion through the high crosslinked binder, as shown by storage modulus and ionic interactions with polar functional groups and, hence, results in higher resistivity. The potential of nanogel materials as UV binders that exhibit host ion interactions to improve Li-ion conductivity is being explored further.
  • DMA analysis of UV binders indicated the Tan Delta (Tg) could be used to determine the extent of ‘relative heterogeneity’ within the UV binder matrix within a LiB electrode that influences critical properties such as charge-transfer resistance and 1C rate capacity.

References

  1. Sigel, G. “Professor’s Corner Part I.” UV+EB Technology, v12, No 1, 2025, p36.
  2. Sigel, G. RadTech 2024, A New Class of UV Curable Binders for Li-Ion Battery Applications, May 21, 2024
  3. Sigel, G. Bauman, D. Hamann, O, Abbott, P, Hensley, D,” UV+EB Technology, 9, No. 2, 2ndQuarter, 2023, pp. 20. “A New Class of UV Curable Binders for Li-Ion Battery Applications.”
  4. Nyquist plots/data analysis performed by Dr. Chariclea Scordilis-Kelly, Sion Power Corporation.
  5. Explanation of Nyquist plot of PVdF and NMC 811, Research Gate
  6. Christmas, B. K., “Professor’s Corner,” UV+EB Technology, 6, No. 2, 2ndQuarter, 2020, pp. 12-13, and useful discussions on DMA data.
  7. Christmas, B. K., “Professor’s Corner,” UV+EB Technology, 8, No. 3, 1st Quarter, 2021, pp. 14-15.
  8. Jeffery Klang, Radiation Curable Hyperbranched Polyester Acrylates, ©RadTech e|5 2006 Technical Proceedings
  9. Khuadyakov, I, New Self-Initiating Oligomers Based on Thiol-ene. Chemistry, Conference Proceedings, 2016

The author also acknowledges useful discussions with Dr. Byron Christmas and Dr. Dong Tian.

Gary SigelGary A. Sigel, Ph.D.
Senior Principal Scientist, Customer Lab Manager,
Miltec UV
gsigel@miltec.com

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