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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al Figure 8. (a) Key reversible redox-active organic systems at the molecular level, together with representative examples exhibiting redox rocking from the aromatic to the quinoid form. X/Y can be N, O, S, P, π-systems, carboxylate, anhydride, or amide functional groups; and R, R′ are potentially integrated within the same cyclic structure. Note that p- and n-type structures correspond to systems A and B, respectively, according to Hünig’s classification [56]. (b) Corresponding cell configurations obtained by experimenting with both n- and p-type systems, shown during the discharge process. Reprinted with permission from [52]. Copyright (2020) American Chemical Society. Advances in science and technology to meet challenges Efforts must be focused on molecular design to obtain high-potential sodiated cathode materials. This would probably entail the identification of innovative n-type organic functional groups possibly guided by simulation and modelling. Furthermore, potential gain could still be possible by considering electronic effects acting on a redox centre of interest. Thus, the redox potential is classically increased at the molecular level (discrete entities) by introducing electron-withdrawing groups. More recently, it was demonstrated for a lithiated organic cathode that the electronic effects acting on a redox-active organic centre can be very efficiently mitigated in the solid state thanks to structural effects [66]. Alternatively, other OEMs can operate as cathodes thanks to p-type functionality with anionic compensation (figure 8). The latter, which is a redox system rarely encountered in inorganic materials, typically works at a higher potential than n-type redox centres, making these organic systems very appealing for Na-based batteries. For instance, several organic p-type polymer families are known to be stable in organic liquid electrolytes. Poly(2,2,6,6- tetramethylpiperidinyloxy methacrylate) or PTMA represents a relevant example in this field which exhibits bipolar (n/p) electroactivity. From its initial state (centre of the half-reaction at the positive electrode in table 1, entry 5), PTMA can electrochemically accommodate a cation at 2.3 V vs. Na+/Na (in reduction) or an anion at 3.6 V vs. Na+/Na (in oxidation) [63]. However, in the case of the closely related poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl vinyl ether) or PTVE, the cation insertion mechanism was shown to be unstable, and steady capacity was obtained with anion insertion alone (table 1, entry 6) [64]. Such p-type functionality gives access to high-potential materials which can be paired with n-type negative OEMs in a dual-ion cell configuration (table 1, entry 7) [65]. P-type OEMs are usually synthesised in their reduced state, making them suitable for a positive electrode without requiring an electrochemical pre-treatment. As monomers, they do not possess permanent negative charges to prevent dissolution in aprotic electrolytes, but polymeric p-type OEMs seem to be the most appropriate choice of positive electrode for Na-based batteries to achieve a combination of high potential and cycling stability. However, as underlined in figure 8(b), the electrolyte is a reservoir of ions for charge compensation within electrode materials which causes a depletion of ionic carriers during operation, requiring a larger volume of electrolyte. Strictly speaking, such a cell configuration cannot be considered to be a NIB, although promising electrochemical data upon cycling have been reported (table 1, entry 7). 19PDF Image | 2021 roadmap for sodium-ion batteries
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