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Researchers Discuss Ion Mobility Through Solid Polymeric Electrolytes

In a paper recently published in the journal ACS Polymers Au, researchers reviewed the physical origins of typical liquid transport mechanisms, matrix dynamics, and ionic transport decoupling and explored the conditions for decoupled ionic mobilities.

Study: Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes. Image Credit: vectorwin/Shutterstock.com

From simple fluids to polymers and inorganic systems, ion transport is a crucial subject in a variety of material systems and applications. However, cross-fertilization throughout material classes may result in potential breakthroughs.

Host Material Properties

Electrolytes that support substantial fluxes of redox reactive species—particularly metal cations—are required by the workings of electrochemical devices. However, the intricacy of the devices, which necessitate high voltage stability, strong ionic conductivity, as well as mechanical rigidity, is not adequately captured by ionic conductivity. Notably, ion concentration and mobility are two interrelated parameters that, in general, control ionic conductivity.

The production of dissociated ions often happens through the solvation of salts that have been introduced, wherein the mobile ionic species are usually doped with salt and added to the electrolyte. An efficient electrolyte must enable the movement of mobile ionic species while also generating them. The frictional coefficient is most frequently treated theoretically by making it a constant proportional to the material's segmental relaxation time scale. However, this method does not consider the intricate specialized relationships and dynamics.

The main components of conventional approaches to enhancing SPE conductivity are either enhancing salt solubility in the electrolyte or enhancing salt mobility by lowering the electrolyte's glass transition temperature. Since it has been demonstrated that the coordination of ether-oxygens with lithium results in high solubilities of lithium salts, linear poly(ethylene oxide) (PEO) has emerged as a model polymer host for lithium conduction. PEO's performance enhancement efforts are mostly focused on lowering its crystallinity, further decreasing the transition temperature (Tg), and enhancing salt solvation ability.

Ion transport can be strongly decoupled from matrix motion in various materials, from precisely constructed ceramics to ice under extreme temperature and pressure conditions. Walden construction is a helpful experimental technique for a practical examination of ion transport because it can help identify the transport mechanism and reveal the function of ion aggregation. A non-ideal transport system may also be revealed by observing deviations from the ideal Walden behavior. 

Alternate Ion Conduction Mechanisms

Due to their charge neutrality, superior dielectric qualities, and distinctive self-assembly behaviors, crystalline zwitterionic (ZI) compounds with diffused and bulky ion groups hold tremendous potential as solid-state ion conductors. Some polymeric zwitterions exhibit superionic ion transport while avoiding the drawbacks of pure polycrystalline electrolytes, including poor electrode contact, cracking, and brittleness, as well as limited chemical and electrochemical durability against a high-energy cathode or a metal anode.

Further research on these substances has been motivated by the outstanding performance of polymeric zwitterions in the form of solid and Li+ selective hosts for ion transport. Further knowledge is still needed regarding the thermodynamics of zwitterions and if related alloys can be formed because this alloying may have significant effects on the ionic transport of these structures. 

The tight packing of stiff polymers can encourage decoupled transport by supplying free volume for ions to hop through opportunistically. Despite being hindered, fragile chains are often less able to generate free volume for ion motion through a relaxation mechanism, while the static free volume in these systems can be enough to allow hopping-like ion transport. Furthermore, selection rules that are ion size-sensitive can result from transport in restricted channels. As alkali metal ions are often smaller than their counterions, this presents the potential to enhance selectivity for these ions.

Multivalent and Large Ions

The limited mobility of multivalent ions in conventional electrolytes presents a hurdle for such battery chemistries. Interestingly, liquid electrolytes also exhibit low mobilities for similar cations. Since frustrated chain packing produces a variety of exploitable void sizes as opposed to the organized and discrete voids produced by other techniques, it may be beneficial in conducting large ions.

Larger cations are known to coordinate with cyclic crown ethers. This could offer the potential to create nanochannels. Larger multivalent ions reduce the effectiveness of size-selectivity methods. To facilitate easy hopping between solvation sites, controlling ion-matrix interactions could prove to be of even greater importance when designing multivalent ion conductors.

Conclusions

To summarize, the researchers examined the optimum liquid transport mechanism in SPEs. The team noted that decoupling between polymer segmental mobility and ion transport is practically beneficial by decreasing the dependency of ionic conductivity on temperature. This dependence has been found to have critical significance to the operation of electrochemical devices over a wide temperature range and the ability to independently tune mechanical and ion transport properties.

Decoupling requires the satisfaction of three characteristics: (1) an inhomogeneity that facilitates ion motion that is different from segmental motion, (2) labile ion-matrix interactions to encourage activated transport between sites, and (3) small-length scale motion inside the channels generated by inhomogeneity.

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Source:

Jones, Seamus D. and Bamford, James and Fredrickson, Glenn H. and Segalman, Rachel A., Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes, ACS Polymers Au, 2022, DOI: 10.1021/acspolymersau.2c00024.

Chinmay Chari

Written by

Chinmay Chari

Chinmay Chari is a technical writer based in Goa, India. His academic background is in Earth Sciences and he holds a Master's degree in Applied Geology from Goa University. His academic research involved the petrological studies of Mesoarchean komatiites in the Banasandra Greenstone belt in Karnataka, India. He has also had exposure to geological fieldwork in Dharwad, Vadodara, in India, as well as the coastal and western ghat regions of Goa, India. As part of an internship, he has been trained in geological mapping and assessment of the Cudnem mine, mapping of a virgin area for mineral exploration, as well understanding the beneficiation and shipping processes of iron ore.

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