Jodie Lutkenhaus

Here we report the synthesis of molten-salt etched vanadium carbide V₂CT_x MXenes guided by Gibbs free energy calculations for salt selection. The V₂AlC MAX phase is difficult to etch due to the high bond strength of V–Al. Most reports describe etching V₂AlC with HF, but this work utilizes acid-free molten-salt etching. Molten-salt etching of V₂AlC to V₂CT_x is not well understood, especially the elucidation of the mechanism of specific salts and synthesis conditions for successful etching. Etching affinity of the Lewis acid salts was predicted using Gibbs free energy and redox potential calculations (the reaction of each salt with Al) to determine promising salt etchants for molten-salt synthesis, which were then validated experimentally. Under the specific conditions, CuF₂ etched V₂AlC after etching for 12 hours. To observe gaseous etching products, we report using in situ thermogravimetric analysis, confirming no degradation of the MAX phase during etching. In addition, due to the lower crystallinity of the MXenes, we use wide-angle X-ray scattering to observe the hexagonal lamellar structure and (002) peak in V₂CT_x MXenes, facilitating the use of alternative X-ray techniques. We show that changing key variables in the synthesis process (etching time, aluminum in the MAX phase, and ratio of MAX : salt) affects the MXene structure: over-etching occurs at long etching times, excess aluminum in the MAX phase causes byproduct formation that hinders etching, and increasing the molar ratio of salt to MAX phase promotes over-etching and oxide formation. This work provides a template for molten-salt etching of new MAX phases in the future and guides the selection of etching parameters.

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Electrochemical energy storage systems experience performance degradation at low temperatures due to the strong temperature dependence of ionic transport, electronic conductivity, and interfacial reaction kinetics. Poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) redox-active polymer cathodes have shown the ability to mitigate some of these limitations through coupled electron-ion transport via rapid redox conversion through electron hopping. However, a physics-based understanding of how temperature governs transport and kinetic behavior remain limited. In this work, we extend a continuum electrochemical modeling framework for PTMA–lithium metal batteries to explicitly incorporate temperature-dependent transport and reaction kinetics, enabling prediction of the electrochemical response over a broad temperature range under both potentiostatic and galvanostatic operation. Simulation results reveal how temperature-induced changes in transport and kinetic parameters alter rate capability, polarization behavior, and accessible capacity, providing quantitative insight into performance-limiting mechanisms across operating temperatures. By capturing these temperature-dependent effects within a unified framework, the model offers predictive capability for assessing and optimizing PTMA-based battery operation under extremely low temperature conditions.

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Batteries capable of reliable low-temperature operation are critical for aerospace, space exploration, defense, and cold-climate electrification. Conventional lithium-based batteries suffer severe performance degradation under these conditions due to temperature-dependent transport limitations and sluggish interfacial kinetics, often requiring complex thermal management. Organic redox-active materials provide a promising alternative, as their charge-storage mechanisms rely on rapid, reversible redox conversion and ion-doping enabled by dual-ion transport that remains accessible at reduced temperatures. In hybrid organic–inorganic systems, redox mediation further facilitates charge transfer between dissimilar active phases, enabling complementary energy-storage mechanisms to be combined. This work develops a physics-based continuum electrochemical modeling framework for low-temperature organic and hybrid batteries. The framework captures coupled electron and multi-ionic transport, interfacial reaction kinetics, and temperature-dependent material properties, enabling systematic evaluation of electrochemical performance across operating temperatures. The model provides predictive insight into how composition, transport properties, and reaction mechanisms govern rate capability, polarization, and accessible capacity at low temperatures. Together, this modeling-driven approach enables rational design of next-generation organic and hybrid batteries optimized for high-rate operation in low-temperature environments.

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