114
mixture of PEO, Mg(ClO 4 ) 2 , nanometer-size silica fillers, and an ionic liquid
(1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide) shows high ionic conductivity of 5.4 × 10
−4
S cm
-1
at ambient temperature and exhibits a wide voltage stability range of 4 V vs. Mg/Mg
2+
.
5.3 MOFs
MOFs are naturally finding use in batteries and supercapacitors for similar reasons
as in proton conducting membranes [106]. MOFs were first employed in battery
technology as electrode materials. Notably, MOFs were used as porous metal
organic sources that could serve as sacrificial templates to yield porous metal oxides
for lithium ion intercalation [107]. Only recently have MOFs found use as lithium
ion electrolyte materials. In 2011, the first example was described by Long et al.
Here, it was demonstrated that the incorporation of a typical electrolyte solution
(containing lithium tetrafluoroborate in ethylene carbonate and diethyl carbonate
solvent) into a MOF with open metal cation sites can produce a solid with significant ionic conductivity. Furthermore, it was found that first anchoring lithium isopropoxide to the unsaturated magnesium ion sites in the MOF was necessary to yield
a higher concentration of lithium ions. Specifically, after soaking the original MOF,
Mg 2 (dobdc), (dobdc = 1,4-dioxido-2,5-benzenedicarboxylate) in the lithium isopropoxide, then filling with lithium tetrafluoroborate, a pellet of this composite material was found to have a conductivity of 3.1 × 10
−4
cm
- 1
at 300 K, with an activation
energy of just 0.15 eV [108]. In an interesting extension to this, the Long group also
demonstrated the first solid-state, room temperature magnesium ion conductor
[109]. A series of solid magnesium electrolytes were synthesized with the use of
magnesium phenolates to initially bind with coordinatively unsaturated metal sites
lining the pores of the two MOFs: Mg 2 (2,5-dioxidobenzene-1,4-dicarboxylate) and
Mg 2 (4,4′-dioxidobiphenyl-3,3′-dicarboxylate). Room temperature ionic conductivities up to 2.5 × 10
−4
S cm
− 1
were measured.
As ionic channels are essential features in biological cell membranes, selectively
transporting ions, some researchers were inspired by this idea. Similar to Hupp’s
report of HKUST-1 for proton transport, wherein open, acidic copper sites result in
an increase in acidity of coordinated waters [10], Shen et al. demonstrated Li
+
transport in HKUST-1. Here, LiClO 4 in propylene carbonate was introduced into the
MOFs, whereby ClO 4
−
ions spontaneously bind to the open metal sites, forming
negatively charged MOF channels and enabling transport of Li
+
ions with a low
activation energy of 0.21 eV. They also demonstrated this approach successfully
with Zr-containing MOFs having open metal sites, such as UiO-67 (Zr(IV) biphenyl
dicarboxylate MOF). The authors measured a low activation energy of 0.12 eV for
lithium ion conductivity, which reached ~10
−3
S cm
−1
with a lithium transference
number of 0.65. Notably, when the same procedure was utilized to introduce Li
+
into the non-coordinating IRMOF-1, having no open metal sites, activation energies
were significantly higher (0.4–0.5 eV), demonstrating the active role taken by the
C. A. Bauer
mixture of PEO, Mg(ClO 4 ) 2 , nanometer-size silica fillers, and an ionic liquid
(1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide) shows high ionic conductivity of 5.4 × 10
−4
S cm
-1
at ambient temperature and exhibits a wide voltage stability range of 4 V vs. Mg/Mg
2+
.
5.3 MOFs
MOFs are naturally finding use in batteries and supercapacitors for similar reasons
as in proton conducting membranes [106]. MOFs were first employed in battery
technology as electrode materials. Notably, MOFs were used as porous metal
organic sources that could serve as sacrificial templates to yield porous metal oxides
for lithium ion intercalation [107]. Only recently have MOFs found use as lithium
ion electrolyte materials. In 2011, the first example was described by Long et al.
Here, it was demonstrated that the incorporation of a typical electrolyte solution
(containing lithium tetrafluoroborate in ethylene carbonate and diethyl carbonate
solvent) into a MOF with open metal cation sites can produce a solid with significant ionic conductivity. Furthermore, it was found that first anchoring lithium isopropoxide to the unsaturated magnesium ion sites in the MOF was necessary to yield
a higher concentration of lithium ions. Specifically, after soaking the original MOF,
Mg 2 (dobdc), (dobdc = 1,4-dioxido-2,5-benzenedicarboxylate) in the lithium isopropoxide, then filling with lithium tetrafluoroborate, a pellet of this composite material was found to have a conductivity of 3.1 × 10
−4
cm
- 1
at 300 K, with an activation
energy of just 0.15 eV [108]. In an interesting extension to this, the Long group also
demonstrated the first solid-state, room temperature magnesium ion conductor
[109]. A series of solid magnesium electrolytes were synthesized with the use of
magnesium phenolates to initially bind with coordinatively unsaturated metal sites
lining the pores of the two MOFs: Mg 2 (2,5-dioxidobenzene-1,4-dicarboxylate) and
Mg 2 (4,4′-dioxidobiphenyl-3,3′-dicarboxylate). Room temperature ionic conductivities up to 2.5 × 10
−4
S cm
− 1
were measured.
As ionic channels are essential features in biological cell membranes, selectively
transporting ions, some researchers were inspired by this idea. Similar to Hupp’s
report of HKUST-1 for proton transport, wherein open, acidic copper sites result in
an increase in acidity of coordinated waters [10], Shen et al. demonstrated Li
+
transport in HKUST-1. Here, LiClO 4 in propylene carbonate was introduced into the
MOFs, whereby ClO 4
−
ions spontaneously bind to the open metal sites, forming
negatively charged MOF channels and enabling transport of Li
+
ions with a low
activation energy of 0.21 eV. They also demonstrated this approach successfully
with Zr-containing MOFs having open metal sites, such as UiO-67 (Zr(IV) biphenyl
dicarboxylate MOF). The authors measured a low activation energy of 0.12 eV for
lithium ion conductivity, which reached ~10
−3
S cm
−1
with a lithium transference
number of 0.65. Notably, when the same procedure was utilized to introduce Li
+
into the non-coordinating IRMOF-1, having no open metal sites, activation energies
were significantly higher (0.4–0.5 eV), demonstrating the active role taken by the
C. A. Bauer