In this edition of Professor’s Corner, we will focus on properties of one-plot synthesis of UV urethane acrylate copolymers from simple building blocks and review final physical properties based on DMA and Tensile Strength, as well as their effect in UV binder electrodes on battery specific capacity rate at 1C. Herein, we report on our first series of copolymers synthesized to better understand the chemical backbone structure effects on mechanical properties and on final battery rate capacity performance when used as UV binders in LiB.
In the last edition of Professor’s Corner, Part 3, four copolymers were prepared in a similar fashion, where different ratios of OH terminated Polyglycol (Poly G), OH terminated Polybutadiene (PBD), and OH terminated polyester acrylate (AOH) were used. 1
The end-product backbone urethane structure, comprised of diisocyanate D, theoretically could be comprised of different chains of random P and B groups endcapped by an acrylate moiety A and copolymers made up of ADBDPDA units and many others (see Table 1). Chemical structures are discussed below for each of the block copolymers.
Investigation of DMA Analysis of Block Copolymers
In a previous Professor’s Corner, we learned about how the backbone structure affects the glass transition temperature by using Dynamic Mechanical Analysis. Here, the effect of pendent side groups, polarity, shape and length of polymer is reviewed relative to Tg as determined by DMA for UV-cured films. 3
Figure 1 provides the DMA thermograms for each cured UV binder material. The green curves represent the Loss Modulus with temperature, where the maximum temperature represents where frictional forces reach a maximum and fall off as chains move further apart. The blue curves represent the storage modulus change over the temperature range. The red curves represent the ratio of the loss modulus and storage modulus, that is termed “Tan Delta,” where the maximum value represents Tg.
Tan d, Storage Modulus and Loss Modulus for the free films of these four block copolymers – UAO, UA1, UA2, and UA3 – using DMA analysis are summarized in Table 2. All Tan d curves are broad, with the most abundant portion of polymer represented by max Tan d Tg based on the ratio of the loss modulus to the storage modulus, which is defined as the damping factor or loss factor and denoted as Tan δ.
The broadening of the Tan d curve is due to non-homogeneity of crosslink density as a result of differing polymer chain lengths, as in the case of broad polydispersity that gives high values Mn/Mw. Urethane possesses H-bonding due to the rigid segments that form hydrogen bonds by the interaction on the -NH groups with the urethane carbonyl functional groups. This can lead to a microphase separation.
The differences in Tg are attributed to the different ratios of OH PolyG: PBD: OH terminated acrylate where two Tgs are observed as bimodal distributions, as summarized in Table 2. The two different Tan d maximums represent two different soft and hard segments. The soft segment is assigned to the polybutadiene portion, and the hard segment is assigned to the branched polyglycol urethane portion that contains a side group that restricts rotation of the carbon bond.
The first block co-polymer prepared was UA0, having a 1:2:4.8 PBD:PolyG:OH Acrylate ratio with a polyol:OH-acrylate ratio of 1.6 with an observed single Tan d at -5.27° C. The second blocked polymer prepared was UA1, having a 1:1:6.4 equivalent of Polybutadiene:polyol:OH Acrylate with an OH polyG to OH acrylate ratio of 3.2 and a final NCO/OH ratio of 1.3. Here, a bimodal distribution was observed where the major Tan d was observed at 15.6° C with a shoulder at -26° C.
The third UA2 block co-polymer was prepared having a 1:1:3.2 equivalent Polybutadiene:PolyG:OH Acrylate ratio with a OH polyG to OH acrylate ratio of 1.6 and NCO/OH of 1.1. The major Tan d Tg went from 15.6 ° C for U1 to 1.85° C due to dramatically cutting the crosslinking in half, thereby giving rise to a lower Tan d Tg. The last blocked co-polymer, UA3, has a 3:1:1 equivalent of Polybutadiene:polyG:OH Acrylate with an OH PolyG to OH acrylate ratio of 1.6 and a final NCO/OH ratio of 1.0. This resulted in a decrease in Tan d to -17 ° C with a shoulder at 5° C. This is due to increasing the ratio of soft to hard of PBD: branched PolyG to 3:1 OH equivalents thereby lowering the Tg.
Storage modulus values look normal with values around 2000 MPa that drop to 100 MPa into the rubbery plateau region above the glass transition temperature becoming independent of temperature at approximately 25° C. Loss modulus (LM) temperatures at max peak MPA, which have been used as Tg, were found to correlate well to Tan d Tg values, Tg (shoulder) = -23: LM temperature -20° C, Tg = 15.59: LM temperature 18° C, indicating no abnormalities due to phase separation.
The DMA thermograms for each co-polymer illustrated in Figure 1 show Loss and Storage Modulus, along with Tan d curves. The most notable difference in Tan d curves is UA3 in comparison to UA1 and UA2. Both UA1 and UA2 display similar bimodal distributions where the distinct Tan d shoulder on the left-hand side (LHS) of the major peak are near identical in temperature at -26° C and -28° C, respectively, with the major Tan d peaks being assigned to 15.6° C and 1.85° C for UA1 and UA2, respectively. In this case, decreasing the end-capped acrylate amount from OH/OH-AC 3.2 to 1.6 reduced the Tan d Tg from 15.6° C to 1.85° C as fewer crosslinks are formed after UV polymerization. In the case of UA3, changing the ratio of PBD: OH PolyG from 1:1 to 3:1 resulted in the expected decrease in Tan d Tg from UA2 at 1.85° C to -17° C for UA3 due to the flexible PBD backbone. No Beta transitions below the Tan d Tg’s were observed for any of the copolymers. Re-running samples at an initial start temperature of -100° C did not show a transition indicative of side chain movement. For UA0, the ratio of PBD: OH PolyG is 1:2, having the highest proportion of PolyG, results in a broad Tan d Tg of -5.17° C.
Three of the four thermograms for UA0, UA1 and UA2 show a convergence of the storage modulus at approximately 50° C, representing the beginning of the rubbery plateau regime. UA1 however shows a different profile curve. This same behavior is observed for the Loss Modulus cures.
Investigation of Charge/Discharge Capacity
Block copolymers were used to prepare cathode electrode slurries by combining these materials with additives, dispersants, NMC811, Carbon and propyl acetate, as described previously. 4 The slurry was applied to an aluminum carrier, briefly dried to remove IPA, UV cured and calendered.
Coin cells were assembled in an inert Argon glove box by first stamping out UV binder/cathode electrode coin disc (see Figure 2) using a 200 mm circular coupon the diameter of a 2032 coin cell from each electrode sample for further assembly. Coin cells were tested using Neware software as illustrated in Figure 3, where the output generated is plotted as Specific Capacity mAh/g vs. Cycles. Coin cells were subjected to cyclic charge and discharge starting at a rate of C/10 (discharge over 10 hours) with a voltage window of 3.0-4.3 V.
All coin cells containing the copolymers UA0, UA2 and UA3 as part of the cathode electrode UV binder formulation performed quite well, as evidenced by the mirror in values from C/10 up to C/5 rate where UA0 and UA2 began to fade. By C/5, there is a distinct difference in coin cell behavior between UA3 and UA2 and UA0 where UA3 has a lower rate capacity of 116 mAh/g and UA2 and UA0 are similar at 148 mAh/g and 141 mAh/g, respectively. The final rate capacity at ‘C’ follows the order UA2 > UA0 >UA 3 where U2 1:1 PBD:PolyG has the highest value at 93 mAh/g and UA3 3:1 PBD:PolyG has the lowest value at 62 mAh/g (the rate capacity of UA0 being between UA2 and UA3 at 79.6 mAh/g).
Table 2 summarizes the mechanical and battery properties of these copolymers. Most notably is 1:1 ratio of PBD:PolyG; OH/OHAC = 1.6 and the lower elongation of 25% and lower tensile strength of 109 MPA vs. the other UAs in this series, indicating optimum performance. Higher and lower ratios of PBD:PolyG gave lower rate capacities at 1C, which is somewhat surprising as higher PBD levels would have been expected to give higher rate capacities due to the inherent added flexibility of the PBD backbone and lower Tg that would prevent adhesive failure of the electrode to the carrier during charge/discharge cycling. The adhesion of the active materials, conductive carbons to the current collector, is quantified by using the peel strength test. 4 Qin has covered this topic, indicating “the loss of adhesive force, which leads to the detachment of electrodes from current collectors and the delamination of active materials from electrode surfaces. This structural destruction contributes to the disruption of ion and electron pathways and the loss of active materials, which is closely associated with the capacity decay.” 5 Peel strength test values indicate for the series UA2>UA0>UA3, 17.8 gf/20 mm, 12.68 gf/20 mm and 10.1of 8 gf/20 mm, respectively.
The design of this initial series of UV-curable copolymers demonstrates that these UV binders can be used successfully in LiB applications.
Summary and Conclusions
The author’s company has found that single-pot synthesis of block UV binders for LiB can be designed to influence critical properties, such as DMA and Tensile and Peel properties that ultimately affect battery performance as determined by medium-high specific capacity (mAh/g) vs. cycles at 1C rate capacity from 62-94 mAh/g at high loadings 19-21 mg/m2. The highest specific capacity tested within this first series was found to be a 1:1 ratio of PBD:PolyG where the DMA of the UV-cured polymer displayed a bimodal distribution with the major Tg at 1.85° C and a shoulder at -28.5° C. Further one pot synthesis efforts are underway that exhibit higher specific capacity values at 1C that will be the subject of a future publication.
Gary A. Sigel, Ph.D.
Senior Principal Scientist, Lab Manager, Miltec UV
gsigel@miltec.com
References
- Sigel, G. “Professor’s Corner Part III.” UV + EB Technology, v11, No 3, 2025, p10.
- Igor V. Khudyakov, Kenneth W. Swiderski, ‘Structure‐property relations in UV‐curable urethane acrylate oligomers,’ Journal of Applied Polymer Science, January 2006, 99(2) p489.
- Christmas, B. K., “Professor’s Corner,” UV+EB Technology, 6, No. 3, 2nd Quarter, 2020, pp. 12-’Understanding Glass Transition Temperature: Part 3.
- Sigel, G. Bauman, D. Hamann, O, Abbott, P, Hensley, D,” UV+EB Technology, 9, No. 2, 2nd Quarter, 2023, pp. 20. “A New Class of UV Curable Binders for Li-Ion Battery Applications.”
- Qin, T. Yang, H. Li, Q, Li, H. Design of functional binders for high-specific-energy lithium-ion batteries: from molecular structure to electrode properties.
- Useful discussions with Dr. Byron Christmas and Kristi English in preparation of manuscripts.
