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

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.


General considerations
All the chemicals used in this work were purchased from Aldrich and used without further purification. All the operations regarding the preparation of the quasi-solid electrolytes, the cell assembly for the electrochemical tests, as well as the packing of the NMR samples in HRMAS inserts, were carried out in an Ar-filled glove box (MBraum, H2O, O2 < 1ppm).

Synthesis of Mg-MOF-74 and preparation of the Li@Mg-MOF-74 quasi-solid electrolyte
Mg-MOF-74 was prepared according to the green synthetic procedure previously described in the literature. [1] An aqueous solution of Mg(NO3)2 . 6H2O (purity>99%) was rapidly added to an aqueous solution of dhtp (2,5-dihydroxyterephtalic acid) (purity>98%)and NaOH under stirring with a Mg/dhtp/NaOH molar ratio of 2:1:4 in a total volume of 10 mL. The reaction mixture was stirred for 4 hours at room temperature. A quick colour change from a transparent to yellow solution was clearly visible, indicating that the reaction is starting. The resulting yellow powders were collected by centrifugation at 6000 rpm for 20 minutes, washed three times with distilled water and methanol, and dried at 70°C overnight.
As-synthesised Mg-MOF-74 powders (200 mg) were activated by solvent exchange with methanol (10 mL) for 12 days (with daily methanol replacement) followed by a drying step in an alumina boat crucible under vacuum (10 -5 bar) in a tubular furnace to yield a light-yellow powder.
Using heating and cooling ramp rates of 5°C min -1 , the sample was either heated to 80°C, 100°C, 150°C, 200°C and held for 1 hour and then to 250 °C for 2 hours followed by cooling down to room temperature [1] or, in a second procedure tested in this work, heated to 200°C and held for 12 hours before cooling down.
The MOF-based quasi solid electrolytes were obtained by impregnation of Mg-MOF-74 with LiClO4 in liquid electrolytes using either propylene (PC) 2.0M or ethylene:dimethyl carbonates (EC:DMC) (1:1 v:v) 1.0M through capillary action. In a typical procedure, activated Mg-MOF-74 (75 mg) was soaked in a mortar with 45 μL of either liquid electrolyte solution and thoroughly mixed with a pestle until a colour change was observed from light yellow to a darker yellow. In order to ensure a better dispersion of the liquid electrolyte into the pores, the Li@Mg-MOF-74 sample was then heated at 40°C for 1 hour. The amount of liquids confined in the system was calculated in terms of volume uptake and determined to saturate the pores. Taking into account the BET analysis, the Mg-MOF-74 pore volume, Vp, results to be 22.5 L. The final loading amounts of the liquid electrolyte, Vl, were 24.6 L in case of LiClO4 2.0M (dsol = 1.42 g cm -3 ) and 27.5 L for LiClO4-EC/DEC 1.0M (dsol = 1.01 g cm -3 ). Since the final loading values are higher than pore volume for each sample, we assume all pores are saturated, while the excess is ex-pore.

Structural and morphological characterization
X-ray powder diffraction (XRPD) patterns of the as-prepared and activated Mg-MOF-74 samples were acquired at room temperature between 3° and 40° degrees inside an Ar-filled sample holder by using a Bruker diffractometer D8 Advance with a monochromatic Cu source (Kα1, λ = 1.5406 Å) in Debye-Scherrer geometry.
SEM analyses on MOF were performed using a Tescan Mira3XMU microscope operated at 20 kV and equipped with an EDAX EDS analysis system. The sample was coated with a carbon thin film using a Cressington 208 carbon coater.
N2 adsorption-desorption analysis was performed with a Coulter SA 3100 instrument. The surface area was calculated by Brunauer Emmet Teller (BET) method. The pore size distribution was determined by Barrett-Joyner-Halenda (BJH) method.

Electrochemical measurements on Li@Mg-MOF-74
The ionic conductivity was measured between 20°C and 70°C by means of Electrochemical The Li transference number, t + , was calculated by coupling EIS and chronoamperometry experiments on a Li/Li@MOF/Li symmetrical cell, as defined by the following equation: [2] t + = I SS (∆V − I 0 R 0 ) where ΔV is the applied voltage (15 mV), I 0 and I ss are the current densities at the beginning of the polarization and at the steady state, respectively, and R 0 and R ss are the interfacial resistances before and after polarization, respectively. High Li ion transference number, ideally close to 1, is a much desired property for electrolytes, as a low transference number implies large movement of the anions, which would cause concentration polarization during battery operation. Solid State NMR 6 Li magic angle spinning (MAS) NMR spectra were recorded at a Larmor frequency of 0/2 = 60 MHz using a 4 mm HXY MAS probe (in double resonance mode) on a 9.4 T Bruker DSX solidstate NMR spectrometer. All data acquisitions were quantitative using recycle delays longer than five-times the spin-lattice relaxation times T1 (measured using a standard saturation recovery sequence). 6 Li NMR data were obtained with a pulse of length 3 µs at a radio frequency (rf) field amplitude of 1/2 = 83 kHz and at a MAS rate of r/2 = 8 kHz. The 6 Li NMR spectra were referenced to 10 M LiCl in D2O at 0 ppm. 7 Li static NMR spectra were recorded at 0/2 = 156 MHz using a 4 mm HXY MAS probe (in double resonance mode) below room temperature and a 4 mm HX High Temperature MAS probe above room temperature on a 9.4 T Bruker Avance III HD spectrometer. All 7 Li NMR spectra were obtained with a pulse length of 1.5 µs at a rf amplitude of 1/2 = 83 kHz. Spin-lattice relaxation rates in the laboratory frame (T1 -1 ) were obtained using a saturation recovery pulse sequence and the data fitted to the stretch exponential function 1-exp[-(/T1)  ] where  is the variable delay and  the stretch exponent factors (fitted  values between 0.6 and 1 were extracted). These were used in order to account for a distribution of correlation times as well as temperature gradients across the sample.
Spin-lattice relaxation rates in the rotating frame (T1 -1 ) were obtained using a standard spin-lock pulse sequence at 7 Li frequencies 1/2 of 20, 33 and 50 kHz, fitting the data to a stretch exponential function of the form exp[-(/T1 -1 )  ] (fitted  values ranging from 0.3 and 1 were obtained). The temperature calibration of the probes were performed using the 207 Pb NMR chemical shift thermometer of Pb(NO3)2. [3,4] The standard error associated with this method arises from temperature gradients across the sample which ranged from 2-7 K and were determined experimentally through the peak width of the static powder pattern of Pb(NO3)2. All 7 Li NMR spectra were referenced to 10 M LiCl in D2O at 0 ppm. No 1 H decoupling was applied during any 6,7 Li NMR experiments.
All 1 H and 13 C MAS NMR spectra were recorded at Larmor frequencies of 0/2 = 400 and 100 MHz, respectively, using a 4 mm HXY MAS probe (in double resonance mode) on Bruker Avance III HD NMR spectrometer under MAS condition at a frequency r/2π = 10 kHz at room temperature. All 1 H pulses were performed at a rf amplitude of ω1/2 = 83 kHz. The 1 H-13 C cross polarization (CP) and heteronuclear correlation (HETCOR) spectra were acquired using 13 C rf field amplitude of 1/2 = 50 kHz matched to obtain maximal signal at a 1 H rf field of 1/2 = 60 kHz, with a SPINAL-64 heteronuclear decoupling [5] rf amplitude during 13 C detection of 1/2 = 83 kHz.
Contact times for the CP step ranged from 5 s to 5 ms (up to 1 ms for the HETCOR). In directly excited 13 C MAS NMR experiments a 13 C rf pulse amplitude of 1/2 = 60 kHz was applied, and SPINAL-64 heteronuclear decoupling at a rf field amplitude 1/2 = 10 kHz was used in high power decoupling experiments (HPDEC) All 1 H and 13 C spectra were referenced to water at 4.8 ppm and to the tertiary carbon of adamantane at 29.45 ppm, [6] respectively.
Li + ion jump rates  -1 were extracted from the temperature dependency of the line width data in Figure 2b through fitting the data to a Boltzmann sigmoid regression curve of the form: where (T) is the line width of the central transition at temperature T, ∞ is the residual line with caused by non-dipolar interactions when motional narrowing is completed, R is the line width of the rigid lattice, Tpoint is the temperature of the inflection point and a is a fitting parameter. At the inflection point of the narrowing curve, the mean Li + jump rate is expected to be in the order of 2R.             LiClO4-EC-DMC@MgMOF74 240(5) 0.85(9) 4.5(2) -5.4(8) Tpoint: the temperature of the inflection point, ∞: residual line with caused by non-dipolar interactions when motional narrowing is completed, r line width of the rigidi lattice, a: fitting parameter.  (8) LT: low temperature, HT: high temperature, JR: jump rate ( Figure 4)