Associative pyridinium electrolytes for air-tolerant redox flow batteries.

Pyridinium electrolytes are promising candidates for flow-battery-based energy storage1-4. However, the mechanisms underlying both their charge-discharge processes and overall cycling stability remain poorly understood. Here we probe the redox behaviour of pyridinium electrolytes under representative flow battery conditions, offering insights into air tolerance of batteries containing these electrolytes while providing a universal physico-chemical descriptor of their reversibility. Leveraging a synthetic library of extended bispyridinium compounds, we track their performance over a wide range of potentials and identify the singlet-triplet free energy gap as a descriptor that successfully predicts the onset of previously unidentified capacity fade mechanisms. Using coupled operando nuclear magnetic resonance and electron paramagnetic resonance spectroscopies5,6, we explain the redox behaviour of these electrolytes and determine the presence of two distinct regimes (narrow and wide energy gaps) of electrochemical performance. In both regimes, we tie capacity fade to the formation of free radical species, and further show that π-dimerization plays a decisive role in suppressing reactivity between these radicals and trace impurities such as dissolved oxygen. Our findings stand in direct contrast to prevailing views surrounding the role of π-dimers in redox flow batteries1,4,7-11 and enable us to efficiently mitigate capacity fade from oxygen even on prolonged (days) exposure to air. These insights pave the way to new electrolyte systems, in which reactivity of reduced species is controlled by their propensity for intra- and intermolecular pairing of free radicals, enabling operation in air.

Quantum-Acoustical Drude Peak Shift.

Quantum acoustics-a recently developed framework parallel to quantum optics-establishes a nonperturbative and coherent treatment of the electron-phonon interaction in real space. The quantum-acoustical representation reveals a displaced Drude peak hiding in plain sight within the venerable Fröhlich model: the optical conductivity exhibits a finite frequency maximum in the far-infrared range and the dc conductivity is suppressed. Our results elucidate the origin of the high-temperature absorption peaks in strange or bad metals, revealing that dynamical lattice disorder steers the system towards a non-Drude behavior.

Propagation of waves in high Brillouin zones: Chaotic branched flow and stable superwires.

We report unexpected classical and quantum dynamics of a wave propagating in a periodic potential in high Brillouin zones. Branched flow appears at wavelengths shorter than the typical length scale of the ordered periodic structure and for energies above the potential barrier. The strongest branches remain stable indefinitely and may create linear dynamical channels, wherein waves are not confined directly by potential walls as electrons in ordinary wires but rather, indirectly and more subtly by dynamical stability. We term these superwires since they are associated with a superlattice.

Rise and Fall of Anderson Localization by Lattice Vibrations: A Time-Dependent Machine Learning Approach.

The intricate relationship between electrons and the crystal lattice is a linchpin in condensed matter, traditionally described by the Fröhlich model encompassing the lowest-order lattice-electron coupling. Recently developed quantum acoustics, emphasizing the wave nature of lattice vibrations, has enabled the exploration of previously uncharted territories of electron-lattice interaction not accessible with conventional tools such as perturbation theory. In this context, our agenda here is two-fold. First, we showcase the application of machine learning methods to categorize various interaction regimes within the subtle interplay of electrons and the dynamical lattice landscape. Second, we shed light on a nebulous region of electron dynamics identified by the machine learning approach and then attribute it to transient localization, where strong lattice vibrations result in a momentary Anderson prison for electronic wavepackets, which are later released by the evolution of the lattice. Overall, our research illuminates the spectrum of dynamics within the Fröhlich model, such as transient localization, which has been suggested as a pivotal factor contributing to the mysteries surrounding strange metals. Furthermore, this paves the way for utilizing time-dependent perspectives in machine learning techniques for designing materials with tailored electron-lattice properties.

Chaos-Assisted Dynamical Tunneling in Flat Band Superwires.

Recent theoretical investigations have revealed unconventional transport mechanisms within high Brillouin zones of two-dimensional superlattices. Electrons can navigate along channels we call superwires, gently guided without brute force confinement. Such dynamical confinement is caused by weak superlattice deflections, markedly different from the static or energetic confinement observed in traditional wave guides or one-dimensional electron wires. The quantum properties of superwires give rise to elastic dynamical tunneling, linking disjoint regions of the corresponding classical phase space, and enabling the emergence of several parallel channels. This paper provides the underlying theory and mechanisms that facilitate dynamical tunneling assisted by chaos in periodic lattices. Moreover, we show that the mechanism of dynamical tunneling can be effectively conceptualized through the lens of a paraxial approximation. Our results further reveal that superwires predominantly exist within flat bands, emerging from eigenstates that represent linear combinations of conventional degenerate Bloch states. Finally, we quantify tunneling rates across various lattice configurations and demonstrate that tunneling can be suppressed in a controlled fashion, illustrating potential implications in future nanodevices.