Evaluation of Cyclooctatetraene‐Based Aliphatic Polymers as Battery Materials: Synthesis, Electrochemical, and Thermal Characterization Supported by DFT Calculations

Abstract

Organic electrode materials for rechargeable batteries are becoming a viable alternative for existing technologies. In particular, redox polymers have shown great performances. While many cathode‐active derivatives are known, the development of their anode‐active counterparts, required for the design of full‐organic batteries, lacks behind. Here we present investigation on the suitability of cyclooctatetraene (COT)‐based aliphatic polymers as anode‐active battery materials, inspired by the known reversible reduction chemistry of COT at low electrochemical potential. We found that both synthesized polystyrene derivatives, side‐group functionalized with COT, showed limited electrochemical reversibility of the reduction processes, whereas reductions proceeded reversibly in model compounds of these polymers. Differential scanning calorimetry measurements and density‐functional theory calculations showed that this incomplete reversibility was due to cross‐linking reactions occurring between COT units in the polymers. For the future of COT‐based redox polymers, we propose a molecular design that prevents these cross‐linking reactions.

Dibenzo[a,e]Cyclooctatetraene‐Functionalized Polymers as Potential Battery Electrode Materials

“…X‐P1 and X‐P2 showed high thermal stabilities with onsets for decomposition at 448 and 444 °C, respectively, from thermal gravimetric analysis (TGA) measurements. As opposed to the COT‐functionalized polymers reported by us before, [ 31 ] no exothermic events took place in differential scanning calorimetry (DSC) measurements, indicating no rearrangement processes of the DBCOT units to occur. As small molecule‐reference compound for P2 and X‐P2 para ‐tolyl‐functionalized DBCOT 5 was accessed from bromo‐DBCOT 3 by Suzuki–Miyaura coupling with 2‐( p ‐tolyl)‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane.…”

“…With bromo‐DBCOT 3 in hand, we proceeded to synthesize the vinyl‐ ( 20 ) and styryl‐ ( 21 ) DBCOT monomers ( Scheme ). Suzuki–Miyaura coupling of 3 with 2‐ethenyl‐ or 2‐(4‐ethenylphenyl)‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane [ 31 ] afforded 20 and 21 , respectively, in quantitative yield. Free radical polymerizations were effected using AIBN in toluene and furnished DBCOT‐based polymers P1 and P2 in good yields.…”

“…For poly(methacrylate) P3 , an irreversible reduction with a cathodic peak potential of E cp = −2.67 V was observed and a quasi‐reversible reduction at E 1/2 = −2.93 V (Figure 3). The first reduction likely stemmed from both the DBCOT units and the connecting ester groups, which can be reduced in this potential range, [ 31 ] however, in an irreversible way. The resulting anion radicals can undergo dimerization reactions in analogy to the Acyloin condensation.…”

“…[ 29,30 ] We recently investigated COT‐functionalized aliphatic polymers as potential battery electrode materials. [ 31 ] However, the reversibility of their redox processes was limited due to cross‐linking reactions occurring between COT units in the polymers. For this reason we turned our attention to its dibenzo‐derivative dibenzo[ a , e ]cyclooctatetraene (DBCOT, 2 in Figure 1).…”

Redox Polymers as Electrode-Active Materials for Batteries

“…Concerning n-type redox polymers, most redox-active groups are carbonyl compounds or derivatives thereof, with few exceptions. 57 Several review articles solely focus on organic carbonyl compounds (redox polymers and small molecules) as battery electrode materials. [58][59][60][61] The redox mechanisms of selected carbonyl-based redox-active groups are shown in Figure 5.…”