Radical Polymer-based Positive Electrodes for Dual-Ion Batteries: Enhancing Performance with γ-Butyrolactone-based Electrolytes.

Dual-ion batteries (DIBs) represent a promising alternative for lithium ion batteries (LIBs) for various niche applications. DIBs with polymer-based active materials, here poly(2,2,6,6-tetramethylpiperidinyl-N-oxyl methacrylate) (PTMA), are of particular interest for high power applications, though they require appropriate electrolyte formulations. As the anion mobility plays a crucial role in transport kinetics, Li salts are varied using the well-dissociating solvent γ-butyrolactone (GBL). Lithium difluoro(oxalate)borate (LiDFOB) and lithium bis(oxalate)borate (LiBOB) improve cycle life in PTMA||Li metal cells compared to other Li salts and a LiPF6- and carbonate-based reference electrolyte, even at specific currents of 1.0 A g-1 (≈10C), whereas LiDFOB reveals a superior rate performance, i. e., ≈90 % capacity even at 5.0 A g-1 (≈50C). This is attributed to faster charge-transfer/mass transport, enhanced pseudo-capacitive contributions during the de-/insertion of the anions into the PTMA electrode and to lower overpotentials at the Li metal electrode.

Multimodal investigation of electronic transport in PTMA and its impact on organic radical battery performance.

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 as well as density functional theory 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.