Multimodal investigation of electronic transport in PTMA: Impact on organic radical battery performance

Abstract

Organic radical batteries (ORBs) represent a viable pathway to a more sustainable energy storage technology compared to conventional Li-ion batteries. For further materials and cell development towards competitive energy and power densities, a deeper understanding of electron transport and conductivity in organic radical polymer cathodes is required. Such electron transport is characterised by electron hopping processes, which depend on the presence of closely spaced hopping sites. Using a combination of electrochemical, electron paramagnetic resonance (EPR) spectroscopic, and theoretical molecular dynamics (MD) as well as density functional theory (DFT) modelling techniques, we explored how compositional characteristics of cross-linked poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) polymers govern electron hopping and rationalise their impact on ORB performance. Electrochemistry and EPR spectroscopy not only show a correlation between capacity and the total number of radicals in an ORB using a PTMA cathode, but also indicates that the state-of-health degrades about twice as fast if the amount of radical is reduced by 15%. The presence of up to 3% free monomer radicals did not improve fast charging capabilities. Pulsed EPR indicated that these radicals readily dissolve into the electrolyte but a direct effect on battery degradation could not be shown. However, a qualitative impact cannot be excluded either. The work further illustrates that nitroxide units have a high affinity to the carbon black conductive additive, indicating the possibility of its participation in electron hopping. At the same time, the polymers attempt to adopt a compact conformation to increase radical–radical contact. Hence, a kinetic competition exists, which might gradually be altered towards a thermodynamically more stable configuration by repeated cycling, yet further investigations are required for its characterisation.

Rescaling Classical Electron Transfer Energy as a Reaction Coordinate and Understanding the Influence of the Electrolyte Environment on the Reorganization Energy

“…This will especially be the case for multicomponent systems employed in ORBs. 2 Therefore, these considerations will be important for the directed optimization of ORB cathode materials.…”

“…Emerging research in the field of energy storage has been highlighting the pivotal role of organic radical batteries (ORBs) 1,2 due to their potential for high power density and fast charge/discharge rates. Uniquely, these ORBs present a promising alternative to conventional lithium-ion batteries, given their cost-effective and environmentally friendly aspects, derived primarily from the use of abundant and non-toxic organic materials.…”