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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

We report a new polymorph of lithium aluminum pyrophosphate, LiAlP2O7, discovered through a computationally guided synthetic exploration of the Li-Mg-Al-P-O phase field. The new polymorph formed at 973 K, and the crystal structure, solved by single-crystal X-ray diffraction, adopts the orthorhombic space group Cmcm with a = 5.1140(9) A, b = 8.2042(13) A, c = 11.565(3) A, and V = 485.22(17) A3. It has a three-dimensional framework structure that is different from that found in other LiMIIIP2O7 materials. It transforms to the known monoclinic form (space group P21) above ∼1023 K. Density functional theory (DFT) calculations show that the new polymorph is the most stable low-temperature structure for this composition among the seven known structure types in the AIMIIIP2O7 (A = alkali metal) families. Although the bulk Li-ion conductivity is low, as determined from alternating-current impedance spectroscopy and variable-temperature static 7Li NMR spectra, a detailed analysis of the topologies of all seven structure types through bond-valence-sum mapping suggests a potential avenue for enhancing the conductivity. The new polymorph exhibits long (>4 A) Li-Li distances, no Li vacancies, and an absence of Li pathways in the c direction, features that could contribute to the observed low Li-ion conductivity. In contrast, we found favorable Li-site topologies that could support long-range Li migration for two structure types with modest DFT total energies relative to the new polymorph. These promising structure types could possibly be accessed from innovative doping of the new polymorph.

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.

International audience; With the goal of finding new lithium solid electrolytes by a combined computational–experimental method, the exploration of the Li–Al–O–S phase field resulted in the discovery of a new sulfide Li3AlS3. The structure of the new phase was determined through an approach combining synchrotron X-ray and neutron diffraction with 6Li and 27Al magic-angle spinning nuclear magnetic resonance spectroscopy and revealed to be a highly ordered cationic polyhedral network within a sulfide anion hcp-type sublattice. The originality of the structure relies on the presence of Al2S6 repeating dimer units consisting of two edge-shared Al tetrahedra. We find that, in this structure type consisting of alternating tetrahedral layers with Li-only polyhedra layers, the formation of these dimers is constrained by the Al/S ratio of 1/3. Moreover, by comparing this structure to similar phases such as Li5AlS4 and Li4.4Al0.2Ge0.3S4 ((Al + Ge)/S = 1/4), we discovered that the AlS4 dimers not only influence atomic displacements and Li polyhedral distortions but also determine the overall Li polyhedral arrangement within the hcp lattice, leading to the presence of highly ordered vacancies in both the tetrahedral and Li-only layer. AC impedance measurements revealed a low lithium mobility, which is strongly impacted by the presence of ordered vacancies. Finally, a composition–structure–property relationship understanding was developed to explain the extent of lithium mobility in this structure type.