Macromolecular Design Strategies for Preventing Active‐Material Crossover in Non‐Aqueous All‐Organic Redox‐Flow Batteries

Abstract

Intermittent energy sources, including solar and wind, require scalable, low‐cost, multi‐hour energy storage solutions in order to be effectively incorporated into the grid. All‐Organic non‐aqueous redox‐flow batteries offer a solution, but suffer from rapid capacity fade and low Coulombic efficiency due to the high permeability of redox‐active species across the battery's membrane. Here we show that active‐species crossover is arrested by scaling the membrane's pore size to molecular dimensions and in turn increasing the size of the active material above the membrane's pore‐size exclusion limit. When oligomeric redox‐active organics (RAOs) were paired with microporous polymer membranes, the rate of active‐material crossover was reduced more than 9000‐fold compared to traditional separators at minimal cost to ionic conductivity. This corresponds to an absolute rate of RAO crossover of less than 3 μmol cm−2 day−1 (for a 1.0 m concentration gradient), which exceeds performance targets recently set forth by the battery industry. This strategy was generalizable to both high and low‐potential RAOs in a variety of non‐aqueous electrolytes, highlighting the versatility of macromolecular design in implementing next‐generation redox‐flow batteries.

Towards Low Resistance Nonaqueous Redox Flow Batteries

“…Identifying thinner or more conductive separators can alleviate ohmic resistances; recent literature on size selective separators for NAqRFBs is providing a pathway toward separators that are sufficiently conductive and selective. [54][55][56][57] Interestingly, the ohmic loss through the porous electrode is negligible compared to the ohmic loss through the separator. Mass transfer losses are the second largest impediment to flow cell performance and can be alleviated by increasing electrolyte flow rate, increasing active species concentration, or decreasing electrolyte viscosity.…”

“…Simultaneously tailoring active species radius and separator pore size offers a promising strategy to enabling NAqRFBs with sufficiently low ASR, high selectivity, and good mass transport. 54 Further, while RFBs typically exhibit improved safety over other battery types (e.g., lithium-ion, sodium-sulfur), 4 the safety associated with the implementation of nonaqueous solvents in RFBs must be investigated as the technology continues to advance. Ultimately, the performance depicted in Figure 10 sets a benchmark for the electrochemical performance of nonaqueous flow cells, however, numerous other criteria must be simultaneously met (i.e., cell potential, stable actives) to achieve economically viable battery designs.…”

The Critical Role of Supporting Electrolyte Selection on Flow Battery Cost

“…From this realization emerged a different approach, first proposed in 2011, 62 of pairing SSS with larger active species where the separator allows smaller supporting ions to exchange between the two electrolyte streams while blocking crossover of the larger active species. [59][60][61][62][63][67][68][69] Since SSS are comprised of non-functionalized polymers, they promise higher conductivities and lower costs as compared to their functionalized IEM counterparts. However, to date, SSS have only been reported in a small number of literature studies, 60,62,63,[68][69][70][71] limiting the amount of available experimental data.…”

“…[59][60][61][62][63][67][68][69] Since SSS are comprised of non-functionalized polymers, they promise higher conductivities and lower costs as compared to their functionalized IEM counterparts. However, to date, SSS have only been reported in a small number of literature studies, 60,62,63,[68][69][70][71] limiting the amount of available experimental data. As such, we estimate their effective conductivities (κ eff , mS cm −1 ) using the Bruggeman relation (Equation 1), where κ is the electrolyte conductivity (mS cm −1 ) in the pore phase of the SSS, ε is the SSS porosity (-), and b is the Bruggeman coefficient (b = 1.5).…”

“…39 In the case of RFBs with SSS, we estimate that the active species equivalent weight must be ≈ 1.25 × that of a small active species, paired with an IEM, to successfully implement size exclusion; this equivalent weight increase is similar to that of the added molecular weight imposed by linkers between redox-active centers on oligomerized ROMs. 60 Finally, the active-species concentration is set to 1.5 M, for consistency with previous cell polarization calculations, which defines the total tank size required to store a particular amount of energy. Other electrolyte cost parameters are listed in Table III.…”