115
framework in the conductivity mechanism. The authors also note that smaller pore
sizes in the MOFs tend to improve conductivity [28].
A recent article demonstrates the first use of a MOF that functions as a matrix to
hold ionic liquids for lithium ion transport. Kitagawa et al. showed that ZIF-8 could
be doped with a mixture of 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imideand LiTFSI (lithium bis (trifluoromethylsulfonyl) imide). It was suggested that the Li
+
cations diffuse through the micropores via the exchange of the
solvating counter anions, which is comparable to the Grotthuss mechanism used to
describe proton conductivity [110]. This was followed by another report in 2017,
whereby an ionic-liquid-impregnated MOF demonstrated a high room temperature
ionic conductivity of 3.0 × 10
−4
S cm
−1
, a Li
+
transference number of 0.36, and good
compatibilities against both Li metal and active electrodes with low interfacial
resistances [111]. In related work involving doping a MOF with ion-conductive
guests, Chen et al. reported a polystyrene sulfonate and lithium salt could be
threaded through a HKUST-1 membrane for fast and selective lithium ion separation (Li
+
vs. Na
+
, K
+
, and Mg
2+
) [112]. The resulting composite exhibited very high
Li
+
conductivity of 5.53 × 10
−4
S cm
−1
at 25 °C, and 1.89 × 10
−3
S cm
−1
at 70 °C,
which are five orders of magnitude higher than that of the pristine HKUST-1 membrane. The selectivity was attributed to the different size-sieving effects and the
affinity differences of the Li
+
, Na
+
, K
+
, and Mg
2+
ions to the sulfonate groups.
In 2017, A Cu(II)–azolate MOF that contains tubular pores was shown to support
record-high single-ion conductivities for Li
+
-, Na
+
-, and Mg
2+
-loaded MOFs. The
framework was shown to undergo a reversible single crystal-to-single crystal
(SCSC) transition between neutral and anionic phases upon reaction with stoichiometric amounts of salts. The stoichiometric transformation between the two phases
allows loading of record amounts of charge-balancing Li
+
, Na
+
, and Mg
2+
ions for
MOFs. Whereas the counter anions are bound to the metal centers and thus stationary, the cations move freely within the one-dimensional pores, giving rise to singleion solid electrolytes. The respective Li
+
-, Na
+
-, and Mg
2+
-loaded materials exhibit
ionic conductivity values of 4.4 × 10
−5
, 1.8 × 10
−5
, and 8.8 × 10
−7
S cm
−1
. With
addition of LiBF 4 , the Li
+
conductivity improves further to 4.8 × 10
−4
S cm
−1
. These
are the highest values yet observed for MOF solid electrolytes (See Fig. 6, bottom) [90].
Novel scandium-based alkaline MOFs were prepared and tested for ionic conductivity. The newly reported structures, {[ScM(μ4-pmdc) 2 (H 2 O) 2 ]·solv}n, where
M = Li, Na; pmdc = pyrimidine-4,6-dicarboxylate; solv = corresponding solvent,
compounds were made such that they contained ions in the matrix. However, the
conductivity was low in the pristine MOFs, but it was further enhanced by doping
with divalent transition metal ions and soaking the samples in alkaline salts, resulting in an increase in the number of alkaline ions. This resulted in high conductivities
for MOFs at room temperature, measuring 4.2 × 10
−4
and 9.2 × 10
−5
S cm
−1
for
M = Li and M = Na, respectively [113].
Polymer Nanocomposites for Ion Transport
framework in the conductivity mechanism. The authors also note that smaller pore
sizes in the MOFs tend to improve conductivity [28].
A recent article demonstrates the first use of a MOF that functions as a matrix to
hold ionic liquids for lithium ion transport. Kitagawa et al. showed that ZIF-8 could
be doped with a mixture of 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imideand LiTFSI (lithium bis (trifluoromethylsulfonyl) imide). It was suggested that the Li
+
cations diffuse through the micropores via the exchange of the
solvating counter anions, which is comparable to the Grotthuss mechanism used to
describe proton conductivity [110]. This was followed by another report in 2017,
whereby an ionic-liquid-impregnated MOF demonstrated a high room temperature
ionic conductivity of 3.0 × 10
−4
S cm
−1
, a Li
+
transference number of 0.36, and good
compatibilities against both Li metal and active electrodes with low interfacial
resistances [111]. In related work involving doping a MOF with ion-conductive
guests, Chen et al. reported a polystyrene sulfonate and lithium salt could be
threaded through a HKUST-1 membrane for fast and selective lithium ion separation (Li
+
vs. Na
+
, K
+
, and Mg
2+
) [112]. The resulting composite exhibited very high
Li
+
conductivity of 5.53 × 10
−4
S cm
−1
at 25 °C, and 1.89 × 10
−3
S cm
−1
at 70 °C,
which are five orders of magnitude higher than that of the pristine HKUST-1 membrane. The selectivity was attributed to the different size-sieving effects and the
affinity differences of the Li
+
, Na
+
, K
+
, and Mg
2+
ions to the sulfonate groups.
In 2017, A Cu(II)–azolate MOF that contains tubular pores was shown to support
record-high single-ion conductivities for Li
+
-, Na
+
-, and Mg
2+
-loaded MOFs. The
framework was shown to undergo a reversible single crystal-to-single crystal
(SCSC) transition between neutral and anionic phases upon reaction with stoichiometric amounts of salts. The stoichiometric transformation between the two phases
allows loading of record amounts of charge-balancing Li
+
, Na
+
, and Mg
2+
ions for
MOFs. Whereas the counter anions are bound to the metal centers and thus stationary, the cations move freely within the one-dimensional pores, giving rise to singleion solid electrolytes. The respective Li
+
-, Na
+
-, and Mg
2+
-loaded materials exhibit
ionic conductivity values of 4.4 × 10
−5
, 1.8 × 10
−5
, and 8.8 × 10
−7
S cm
−1
. With
addition of LiBF 4 , the Li
+
conductivity improves further to 4.8 × 10
−4
S cm
−1
. These
are the highest values yet observed for MOF solid electrolytes (See Fig. 6, bottom) [90].
Novel scandium-based alkaline MOFs were prepared and tested for ionic conductivity. The newly reported structures, {[ScM(μ4-pmdc) 2 (H 2 O) 2 ]·solv}n, where
M = Li, Na; pmdc = pyrimidine-4,6-dicarboxylate; solv = corresponding solvent,
compounds were made such that they contained ions in the matrix. However, the
conductivity was low in the pristine MOFs, but it was further enhanced by doping
with divalent transition metal ions and soaking the samples in alkaline salts, resulting in an increase in the number of alkaline ions. This resulted in high conductivities
for MOFs at room temperature, measuring 4.2 × 10
−4
and 9.2 × 10
−5
S cm
−1
for
M = Li and M = Na, respectively [113].
Polymer Nanocomposites for Ion Transport