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Metal-organic frameworks (MOFs) are excellent platforms to design hybrid electrolytes for Li batteries with liquid-like transport and stability against lithium dendrites. We report on Li+ dynamics in quasi-solid electrolytes consisting in Mg-MOF-74 soaked with LiClO4-propylene carbonate (PC) and LiClO4-ethylene carbonate (EC)/dimethyl carbonate (DMC) solutions by combining studies of ion conductivity, nuclear magnetic resonance (NMR) characterization, and spin relaxometry. We investigate nanoconfinement of liquid inside MOFs to characterize the adsorption/solvation mechanism at the basis of Li+ migration in these materials. NMR supports that the liquid is nanoconfined in framework micropores, strongly interacting with their walls and that the nature of the solvent affects Li+ migration in MOFs. Contrary to the "free'' liquid electrolytes, faster ion dynamics and higher Li+ mobility take place in LiClO4-PC electrolytes when nanoconfined in MOFs demonstrating superionic conductor behavior (conductivity σrt > 0.1 mS cm-1, transport number tLi+ > 0.7). Such properties, including a more stable Li electrodeposition, make MOF-hybrid electrolytes promising for high-power and safer lithium-ion batteries.

Journal of the American Chemical Society 143(43), 18216 - 18232 (2021). doi:10.1021/jacs.1c07874, Extended anionic frameworks based on condensation of polyhedral main group non-metal anions offer a wide range of structure types. Despite the widespread chemistry and earth abundance of phosphates and silicates, there are no reports of extended ultraphosphate anions with lithium. We describe the lithium ultraphosphates Li$_3$P$_5$O$_{14}$ and Li$_4$P$_6$O$_{17}$ based on extended layers and chains of phosphate, respectively. Li$_3$P$_5$O$_{14}$ presents a complex structure containing infinite ultraphosphate layers with 12-membered rings that are stacked alternately with lithium polyhedral layers. Two distinct vacant tetrahedral sites were identified at the end of two distinct finite Li$_6$O_{16}$^{26���}$ chains. Li$_4$P$_6$O$_{17}$ features a new type of loop-branched chain defined by six PO43��� tetrahedra. The ionic conductivities and electrochemical properties of Li$_3$P$_5$O$_{14}$ were examined by impedance spectroscopy combined with DC polarization, NMR spectroscopy, and galvanostatic plating/stripping measurements. The structure of Li$_3$P$_5$O$_{14}$ enables three-dimensional lithium migration that affords the highest ionic conductivity (8.5(5) �� 10$^{���7}$ S cm$^{���1}$ at room temperature for bulk), comparable to that of commercialized LiPON glass thin film electrolytes, and lowest activation energy (0.43(7) eV) among all reported ternary Li���P���O phases. Both new lithium ultraphosphates are predicted to have high thermodynamic stability against oxidation, especially Li$_3$P$_5$O$_{14}$ which is predicted to be stable to 4.8 V, significantly higher than that of LiPON and other solid electrolytes. The condensed phosphate units defining these ultraphosphate structures offer a new route to optimize the interplay of conductivity and electrochemical stability required, for example, in cathode coatings for lithium ion batteries., Published by American Chemical Society, Washington, DC

Mixed anion materials and anion doping are very promising strategies to improve solid-state electrolyte properties by enabling an optimized balance between good electrochemical stability and high ionic conductivity. In this work, we present the discovery of a novel lithium aluminum sulfide-chloride phase. The structure is strongly affected by the presence of chloride anions on the sulfur site, as this stabilizes a higher symmetry phase presenting a large degree of cationic site disorder, as well as disordered octahedral lithium vacancies, in comparison with Li-Al-S ternaries. The effect of disorder on the lithium conductivity properties was assessed by a combined experimental-theoretical approach. In particular, the conductivity is increased by a factor 103 compared to the pure sulfide phases. Although it remains moderate (10−6 S·cm-1), Ab Initio Molecular Dynamics and Maximum Entropy (applied to neutron diffraction data) methods show that disorder leads to a 3D diffusion pathway, where Li atoms move thanks to a concerted mechanism. An understanding of the structure-property relationships is developed to determine the limiting factor governing lithium ion conductivity. This analysis, added to the strong step forward obtained in the determination of the dimensionality of diffusion paves the way for accessing even higher conductivity in materials comprising an hcp anion arrangement.