Li+ Dynamics of Liquid Electrolytes Nanoconfined in Metal-Organic Frameworks

Abstract

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 σ_rt > 0.1 mS cm^-1, transport number t_Li⁺ > 0.7). Such properties, including a more stable Li electrodeposition, make MOF-hybrid electrolytes promising for high-power and safer lithium-ion batteries.

Keywords: dendrites; lithium-ion batteries; metal−organic frameworks; nanoconfinement; quasi-solid electrolytes.

Figure 1

(a) Crystal structure of Mg-MOF-74, showing the hexagonal channels formed by Mg (blue) and the dhtp linker (carbons in gray, oxygens in red, and hydrogens omitted for clarity), running parallel to the crystallographic c axis. (b) Arrhenius plots of the ionic conductivity for the LiClO₄-PC and LiClO₄-EC-DMC@MgMOF74 quasi-solid electrolytes. (c) Li plating/stripping behavior of Li//Li symmetric cells with different electrolytes; black line: liquid LiClO₄–PC 2.0M and red line: quasi-solid LiClO₄-PC@MgMOF74.

Figure 2

(a) ⁶Li MAS NMR spectra for LiClO₄-PC@MgMOF74 (black) and LiClO₄-EC-DMC@MgMOF74 (red). Experimental spectra, spectral deconvolution, and simulated spectra are shown with full, dashed, and dotted lines, respectively. (b) Temperature dependence of ⁷Li static NMR line width for LiClO₄-PC@MgMOF74 (black) and LiClO₄-EC-DMC@MgMOF74 (red). The Li⁺ jump rates τ^–1 were determined by the dashed lines, which identify the temperature of the inflection point of the sigmoidal regression curve and NMR line width. A summary of the output of these fits is shown in Table S2.

Figure 3

Arrhenius plots of the ⁷Li NMR SLR in the laboratory frame (T₁^–1) at ω₀/2π = 156 MHz and in the rotating frame (T_1ρ^–1) at ω₁/2π = 20, 33, and 50 kHz for (a) LiClO₄-PC@MgMOF74 (blue) and (b) LiClO₄-EC-DMC@MgMOF74 (red). The temperature range used to determine activation barriers E_a is represented by black dotted lines.

Figure 4

Arrhenius plot of Li⁺ jump rates τ ^–1 obtained from ⁷Li NMR line width plot and NMR SLR measurements for LiClO₄-PC@MgMOF74 (black) and LiClO₄-EC-DMC@MgMOF74 (red). The jump rates were extracted from the line narrowing experiments in Figure 2b and the position of the maxima in the temperature dependency of the SLR rates T₁^–1 and T_1ρ^–1 data in Figure 3.

Figure 5

Chemical structures of (a) linker and (b) PC, EC, and DMC electrolytes. 2D ¹H–¹³C HETCOR NMR spectra for (c) LiClO₄-PC@MgMOF74 and (d) LiClO₄-EC-DMC@MgMOF74 using a long CP contact time of 1 ms. Signals for the linker, PC, EC, and DMC are labeled in black, orange, red, and blue, respectively. The spectra for LiClO₄-EC-DMC@MgMOF74 and LiClO₄-PC@MgMOF74 show typical correlations for EC (in red), DMC (in blue), PC (in orange), and the MOF organic linker (in black). The extra correlations observed in these spectra versus the one obtained with a shorter CP contact time of 50 μs (Figure S12) are given in green to emphasize close proximity between the solvent and MOF linker (Figure S13), suggesting absorption into the pores. In the ¹³C direct dimension of the HETCOR, ¹³C CP MAS spectra of Mg-MOF-74, LiClO4-PC@MgMOF74 [(in panel (c)], and LiClO4-EC-DMC@MgMOF74 [in panel (d)], simulated (dashed gray lines) and deconvoluted spectra (gray lines) are given and show the shifts in the NMR signals of most carbons in the organic linker versus pure, thermally activated Mg-MOF-74. In the ¹H indirect dimension, a comparison of the ¹H one pulse spectra of Mg-MOF-74, the quasi-solid electrolytes (full lines), and the internal projections of the HETCOR (dashed gray lines) is provided from left to right. The asterisk (*) denotes the spinning sideband.