Utilising angle-dependent critical current data in the electromagnetic modelling of HTS coils

A detailed methodology is presented for modelling the electromagnetic characteristics of HTS coils using angle-dependent critical current data obtained from experimental measurements of real wire samples. The results of such an analysis are contrasted with those obtained using more prevalent approaches such as a global minimum critical current method or an elliptical field magnitude-dependent functional approximation. Several interesting design consequences of significance to real-world devices that emerge only when the full anisotropy of real wires is taken into account are outlined and discussed. These include the beneficial impact on various performance metrics of the adoption of mixed conductor windings, the importance of coil orientation in optimising device performance, and the potential opportunity to derive a specific design benefit from the targeted use of conductors possessing inclined planarity.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain 1 , 2 H , 6 , 7 Li , 19 F , 23 Na NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that Li + ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides ( Ba, Ca) F 2 , Li-bearing garnets ( Li 7 La 3 Zr 2 O 12 ) and thiophosphates such as LiTi 2 ( PS 4 ) 3 . In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in ( Me, Ca ) F 2 ( Me = Ba , Pb ) cation mixing influences F anion dynamics, in Li 6 PS 5 X ( X = Br , Cl , I ) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor Li 4 + x Ti 5 O 12 cation-cation repulsions immediately lead to a boost in Li + diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured RbSn 2 F 5 or MeSnF 4 . Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Operando NMR Visualization of Ion Dynamics in PEDOT:PSS

While organic mixed ionic/electronic conductors are widely studied for various applications in bioelectronics, energy generation/storage, and neuromorphic computing, a fundamental understanding of the interactions between the ionic and electronic carriers remains unclear, particularly in the wet state and on electrochemical cycling. Here, we show that operando NMR spectroscopy can selectively probe and quantify ion and water movement during the doping/dedoping of poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) films, the most widely used organic mixed conductor. Na+ ions near or within the PEDOT-rich domains experience an anisotropic environment resulting from the underlying partial PSS chain orientation in the polymer films, giving rise to a distinct quadrupolar splitting in the 23Na NMR spectrum. Operando 23Na NMR studies reveal a linear correlation between the quadrupolar splitting and the charge stored in the film, which is interpreted in terms of the roles that the Na+ ions at the PEDOT/PSS interfaces play in charge balance and electric double layer formation. The observed correlation is quantitatively explained by a competitive binding model, in which holes on the PEDOT backbone are bound to PSS, the hole concentration changes during doping/dedoping inducing variations in the Na+ binding percentage at the PEDOT/PSS interfaces. The Na+-to-electron coupling efficiency, measured via 23Na NMR intensity changes, varies noticeably depending on the cycling history of the film. Operando 1H NMR spectroscopy confirms that water molecules accompany the ions that are injected into/extracted from the films. These findings shed light on the working principles of organic mixed conductors and demonstrate the utility of operando NMR spectroscopy in revealing structure-property relationships in electroactive polymers.

Top-Down Coarse-Grained Framework for Characterizing Mixed Conducting Polymers

<div> <div> <div> <p>Organic polymers that exhibit both ionic and electronic conduction are of interest for energy storage devices and emerging bioelectronic applications. Nevertheless, organic mixed conductors are at an early stage of development with nascent design rules and relatively few material chemistries having been experimentally characterized. Here we report a coarse-grained modeling framework that is sufficiently flexible to represent a range of mixed conducting chemistries while retaining the molecular physics necessary to interrogate structure-function relationships. A detailed overview of the framework is presented, accompanied by an applied study of the effect of hydration and oxidation levels on a representative mixed conductor. The model recapitulates experimental trends related to the macroscopic ionic and electronic conductivities, including the non-linear suppression of the electronic mobility with respect to oxidation level and the direct relationship between ionic mobility and hydration level, while revealing the complex interplay of polymer morphology, ionic-electronic coupling, and electrolyte distribution that govern these relationships. These results provide a validation of this framework for future applications in establishing structure-function relationships in this important materials class, and suggests several near-term opportunities for tailoring mixed conduction via side-chain design.</p> </div> </div> </div>

Top-Down Coarse-Grained Framework for Characterizing Mixed Conducting Polymers

<div> <div> <div> <p>Organic polymers that exhibit both ionic and electronic conduction are of interest for energy storage devices and emerging bioelectronic applications. Nevertheless, organic mixed conductors are at an early stage of development with nascent design rules and relatively few material chemistries having been experimentally characterized. Here we report a coarse-grained modeling framework that is sufficiently flexible to represent a range of mixed conducting chemistries while retaining the molecular physics necessary to interrogate structure-function relationships. A detailed overview of the framework is presented, accompanied by an applied study of the effect of hydration and oxidation levels on a representative mixed conductor. The model recapitulates experimental trends related to the macroscopic ionic and electronic conductivities, including the non-linear suppression of the electronic mobility with respect to oxidation level and the direct relationship between ionic mobility and hydration level, while revealing the complex interplay of polymer morphology, ionic-electronic coupling, and electrolyte distribution that govern these relationships. These results provide a validation of this framework for future applications in establishing structure-function relationships in this important materials class, and suggests several near-term opportunities for tailoring mixed conduction via side-chain design.</p> </div> </div> </div>