1.
Introduction
Society's long-standing energy demands have fuelled for centuries the quest for power-dense,
portable and economically viable energy carriers. Since the birth of the first rechargeable
battery in 1860 [1], emerging battery technologies have provided both answers to these
demands as well as additional obstacles. One ubiquitous energy storage device, the
metal or metal-ion battery, offers quintessential examples of both. The strongly reducing
nature of Group 1 and 2 metal ions qualifies these elements as viable energy-dense
anode materials: standard reduction potentials several volts below that of the standard
hydrogen electrode (SHE) allow a thermodynamically favourable oxidation of these metals
to readily release electrons that shuttle through an external circuit, generating
the electric current that serves as the power supply during battery discharge. Integration
of energy-dense materials into devices allows power sources to be compact and portable,
by maximizing energy output per unit mass of material. Further, the reversibility
of these oxidation events makes possible extensive battery cycling, thus providing
a rechargeable power source. Indeed, current Li-ion batteries boast an energy density
of 265 Wh kg−1, with the potential of a 20% improvement, and are operable for over
1000 charge–discharge cycles [2].
Although the chemical properties of metal-ion batteries offer impressive performance
and exciting possibilities, harnessing the power of such reactive workhorses in a
controlled manner comes with its own challenges. In Li-ion batteries, formation of
Li dendrites during charging can puncture the battery membrane separating the anode
from the cathode, causing a leakage of flammable electrolyte if the electrolyte is
liquid, or causing a short-circuit should the dendrites reach the cathode. Another
limitation of Li-ion batteries stems from the first charging cycle, which causes the
formation of a stable reduction product known as the solid–electrolyte interphase
(SEI) layer at the anode due to the fact that the anode and cathode lithiate at potentials
outside the stability window of common liquid electrolytes. Formation of the SEI layer
diminishes the cathode capacity, thus necessitating a larger amount of cathode material
to be incorporated into the battery relative to the anode mass. This additional material
lowers the battery's energy density. Another downfall of the charge–discharge process
in Li-ion batteries is that a poorly formed SEI will limit battery cycle life due
to continuous reactivity of the electrolyte at the anode and irreversible loss of
Li+ ions [2].
From an economic standpoint, the increasingly widespread adoption of rechargeable
batteries also highlights the difference in cost and geopolitical availability between
Li metal and more abundant metals such as Na, Mg, K, Ca or Al. These heavier metals
are indeed the focus of intense research in the context of electrical energy storage,
but present their own challenges. Na, an attractive candidate due to its high abundance,
relatively small ionic radius, high specific capacity and low reduction potential
(2.71 V versus SHE), has shown more problematic reactivity with organic liquid electrolytes
compared to Li and presents the same dendrite formation challenges as Li batteries
[3,4]. Mg, another viable candidate, is 5 orders of magnitude more abundant than Li,
does not form dendrites during charging, and offers almost double the volumetric capacity
of Li (3833 mAh ml−1 versus 2062 mAh ml−1 for Mg/Mg2+ versus Li/Li+, respectively).
However, the most common commercial electrolytes used in Li-ion batteries are not
appropriate choice for Mg-ion batteries because the SEI layer formed in the latter
is completely insulating for Mg2+, an obvious problem for battery cyclability [2].
Owing to its higher atomic weight, which inherently leads to lower energy density,
K has received comparatively less attention than Li and Na as a battery material.
However, its abundance and lower cost may offset this handicap, especially considering
that K also does not alloy with Al, a popular and cheap current collector that otherwise
needs further processing when used in Li-ion batteries. Additionally, the weaker Lewis
acidity of K+ ions relative to Li+ and Na+ ions accounts for lower desolvation energy
and enhanced transport kinetics across the electrolyte/electrode interface, which
ultimately increases ionic conductivity [5]. Lastly, Ca2+ features a small ionic radius
and a stable divalent oxidation state that would afford higher energy density. It
has high natural abundance, and a standard reduction potential close to that of Li+
which would allow a high potential window for electrolytes. However, one significant
obstacle preventing development of Ca batteries with organic liquid electrolytes is
that diffusion of Ca2+ ions through the SEI layer prevents re-plating of Ca on the
anode during charging [6]. Even further enhancement of capacity can be achieved by
taking advantage of trivalent ions such as Al3+, which features quadruple the volumetric
capacity of Li+ (8046 mAh cm−3). Although this is a promising feature for energy storage
advancement, challenges with Al-ion battery systems containing liquid electrolytes
stem from the formation of passive oxide films on the electrode surface and/or from
anode corrosion [7].
Efforts have also been made to improve metal and metal-ion battery performance by
further optimizing cell components beyond the metal anodes, and in particular the
electrolyte. The impedance of all metal-ion batteries is likely elevated due to mobile
species besides the active metal ions (e.g. charge-balancing anions, solvent molecules,
etc.) during cycling [2]. Pursuing various formulations of anode and cathode materials
[8–11], developing new supporting electrolytes, and new solvent or solvent mixtures
[12–14] have all been explored as potential solutions to these challenges. The focus
of this review is to present an argument for solid-state, rather than liquid, electrolytes
in such batteries and to discuss the potential utility of crystallographically ordered,
metal- and covalent-organic frameworks (MOFs and COFs), as solid-state electrolytes.
The review specifically covers reported MOFs and COFs as solid-state electrolytes,
distils metrics for vetting solid electrolyte candidates, and considers future directions
for this field.
2.
Motivation for and evolution of solid-state battery electrolytes
Motivation for a solid-state electrolyte is several-fold. Firstly, solid-state electrolytes
would eliminate the hazard of housing a flammable liquid material inside of the battery,
enhancing safety. Secondly, a solid-state electrolyte may allow for immobilization
of charge-balancing anions, which would allow maximization of the cation transference
number. Thirdly, many liquid electrolytes are not stable in the required potential
window imposed by the battery electrodes. Solid electrolytes should aim to address
all of these challenges. In addition to minimizing the formation of reactive by-products,
a more stable electrolyte may prevent the formation of an SEI layer, consequently
improving the energy density of the cell by eliminating the need for excess sacrificial
cathode material. Elimination of the SEI layer would also increase the viability of
Mg and Ca-ion batteries, the development of which is currently limited by the inability
of these ions to travel through the SEI layer during charging. Finally, for metal
anode batteries, liquid electrolytes provide no morphological control over anodic
plating of the metal during battery charging; a solid electrolyte with sufficient
mechanical strength may encourage uniform plating, thereby preventing dendrite formation.
Several classes of materials have been evaluated as potential solid electrolytes for
metal or metal-ion batteries, including polymers and composites thereof [15], inorganic
solids [15] and, as will be discussed further, MOFs and COFs. Polymer electrolytes
can offer enhanced potential stability windows and cation transference numbers compared
to liquid electrolytes, due to the immobilized anionic hopping sites along the polymer
backbone. However, polymers are ineffective at preventing dendrite growth and typically
exhibit ambient temperature conductivity values that are too low for commercial applications
(10−8–10−5 S cm−1) [2,3]. Additives such as ceramics or ionic liquids have been doped
into polymer matrices to enhance ion mobility, creating more conductive polymer composites.
Dopants can increase the electrolyte conductivity by 2–3 orders of magnitude, but
optimization of the polymer/dopant blend and obtaining mechanistic understanding of
the transport pathways in such hybrids is not trivial [16]. Additionally, dopants
in the polymer matrix often compromise the electrode–electrolyte interface, and these
dopants can exhibit lower electrochemical or chemical stability and form themselves
a resistive layer at the electrode [3]. In terms of ionic conductivity and mechanical
robustness, inorganic solid electrolytes are among the most promising solid electrolytes
thus far. Li3OX-based antiperovskites (X = Cl−, Br−) exhibit Li+ activation energies
of 0.18–0.26 eV and conductivities of up to 2 × 10−3 S cm−1 at 25°C, exceeding the
conductivities of polymer electrolytes [17]. However, preparation of Li3OX antiperovskites
involves thermal treatment that inadvertently removes charge-balancing lithium, resulting
in decreased charge carrier density. Additionally, challenges exist regarding yield
and phase purity for these materials that contribute to poor interfacial contact between
the electrode and electrolyte. This, combined with formation of insulating SEI layers,
increases battery resistance [18]. Antiperovskites have also been shown to conduct
Na+ ions, albeit with modest conductivities of ca 10−5 S cm−1 at 160°C and activation
energies of 0.6–0.8 eV [19]. Much higher conductivities are observed in closo-borate
salts ACB9H10 (A = Li+ or Na+), which boast conductivities of 0.03 S cm−1 at temperatures
above an ordered–disordered phase transition temperature. Noteworthy activation energies
of 0.29 eV (Li+) and 0.20 eV (Na+) and potential stability windows of approximately
5 V were measured. A logistical barrier with these closo-borate materials is that
the phase transition is reversible, and thus the material must be kept above 127°C
(Li+) and 107°C (Na+) in order to maintain the conductive properties [20]. Another
archetypal Na+ electrolyte that has garnered attention is the Na Superionic Conductor
(NASICON), Na1+x
Zr2Si
x
P3−x
O12 (0 ≤ x ≤ 3) [21]. As its name suggests, phase-pure NASICON exhibits high Na+ conductivities
on the order of 10−3 S cm−1 at 25°C, the same order of magnitude conductivity as β-alumina
Na+ electrolytes [21]. However, this electrolyte exhibits instability to molten sodium
salts, limiting battery applications. Additionally, ionically insulating ZrO2 impurities
lower conductivity values. When contemplating other metals for energy storage applications,
Mg2+ conduction presents exciting opportunities as well as unique challenges due to
its highly polarizing nature. Mg2+ ion solid electrolytes include Mg(BH4)2 and MgZr4(PO4)6,
which feature relatively modest conductivities of 10−9 to 10−7 S cm−1, respectively,
even at greater than 100°C. Notably, the best Mg2+ ion solid conductor is in fact
a MOF, Mg2(dobpdc), impregnated with Mg(OPhCF3)2 (−OPhCF3 = 4-trifluoromethylphenolate)
and Mg(TFSI)2 (TFSI− = bis(trifluoromethanesulfonyl)imide) [22]. This material features
a Mg2+ conductivity of 10−4 S cm−1 at 25°C and will be discussed in greater detail
below.
Demonstrating the highest Mg2+ ion conductivity among solids notwithstanding, MOFs
and COFs possess an arsenal of additional properties that identify them as attractive
candidates for solid-state electrolytes [23–27] (figure 1). Firstly, the high surface
area of MOFs and COFs, which is commonly thousands of m2 g−1 [28], enables a high
density of metal cations and hopping sites, contributing to a maximized power density
in a compact device. The long-range order and well-defined ion conductivity pathways
in MOFs and COFs provides affords efficient ion shuttling while reducing much of the
diffusion limitations associated with non-porous solids, especially for highly polarized
species. The crystallographic definition offers homogeneously dispersed hopping sites
while eliminating impedance stemming from electrolyte reorganization, as seen with
liquid and polymer electrolytes [13,16]. The electronic structure of MOFs and COFs
is also beneficial in that their composition rarely offers a high density of mobile
electrons or holes, with most materials in this class being excellent electrical insulators
[29]. This insulating character is an essential property of the electrolyte, so as
to separate the anode and cathode and prevent short circuiting. Porous solid-state
electrolytes can also aid in optimizing cation transference numbers; liquid electrolytes
often exhibit cation transference numbers of less than 0.4 because both the cations
and anions are mobile and thus both contribute to current passed [30,31]. Conversely,
anions can be coordinated to or integrated directly into the MOF/COF structure and
are therefore immobilized during battery charging and discharging, enhancing battery
efficiency. Not only can such materials be used to immobilize anions, but they can
also trap by-products that may be generated during battery cycling that otherwise
decrease battery lifetime upon contact with the electrodes [32]. Further, because
pores can host liquid electrolytes without leakage, porous solids offer the dielectric
benefits of liquid electrolytes without the safety concerns of the latter. Finally,
synthetic tunability of MOFs and COFs is a powerful feature: the ability to alter
the pore size, polarity, material density, metal (in the case of MOFs) and anion identity,
as well as the coordination environment enables the design of a host of electrolytes
featuring a wide range of properties that can meet a variety of device-specific criteria.
Figure 1.
Attractive features of MOFs and COFs as solid-state electrolytes. (Online version
in colour.)
Although beyond the scope of this review, it should be noted that the tunability of
MOFs and COFs in terms of their pK
a and water stability makes them effective proton conductive electrolytes for proton
exchange membranes [33–36]. Indeed, most studies of ion conduction in these materials
have focused on proton conduction, but emerging in the past decade have been pioneering
investigations of Groups 1 and 2 metal ion conduction. Whereas proton conductivity
often relies on the installation of acidic functional groups within the framework,
conduction of metal ions has different requirements, as will be discussed below.
3.
Metal ion conduction in MOF/COF composites
The utility of MOFs/COFs as solid electrolytes is highlighted both by their intrinsic
properties and by their role in composites with polymers and ionic liquids (ILs).
As part of composites, the ordered, crystalline nature of these materials aids in
controlling polymer and IL aggregation by housing the polymers or ILs within the pores,
while still retaining the ionically conductive and non-flammable properties of the
polymers and ILs themselves. In polymer composites, materials such as polyethylene
oxide (PEO) or polyethylene glycol (PEG) are commonly incorporated into the evacuated
MOF/COF pores by stirring the latter in organic solutions of Li-containing polymers
or by a solvent-free, hot press method. Access to an arsenal of composite formulation
techniques allows for tailoring the electrolyte preparation procedure to accommodate
limitations of a given host, e.g. mechanical instability, incompatibility to certain
solvents, etc. Such polymer composites exhibit ionic conductivity values between 10−6
and 10−4 S cm−1 [37–42], higher by up to two orders of magnitude compared to polymer-Li
salt composites alone [41]. Co-formulation of MOFs and COFs with polymers has been
thoroughly reviewed previously [23]. Although entrapping the polymers within the host
pores can prevent polymer crystallization and aggregation, which in turn enhances
conductivity, this approach to electrolyte development did introduce several challenges.
Filling the pores with a guest material significantly decreases surface area, which
in turn contributes to higher diffusion limitations for ion migration, effectively
nullifying one of the inherent advantages of porous materials as solid electrolytes.
Additionally, the reported alkali metal transference numbers for these electrolytes
are never higher than 0.55, and can be as low as 0.34, offering little to no improvement
over liquid electrolytes [38–40,42]. These modest transference numbers indicate that
although encaging the polymers within the MOFs or COFs does enhance conductivity,
this approach fails to immobilize charge-balancing anions and other mobile species.
Finally, the challenge with predicting the ultimate properties of the composites,
or understanding their interfacial structure, makes rational design of such electrolytes
difficult.
Incorporation of ILs into MOFs and COFs pores has also produced composites with some
notable properties as solid electrolytes. Isolating ILs within confined micropores
is particularly desirable because it can change the phase transition temperature of
certain ILs that otherwise solidify and are therefore not usable at ambient temperature
[43]. The four primary strategies for impregnating MOFs and COFs with ILs are soaking
the material in an IL with or without a co-solvent, allowing the IL to anchor to coordinatively
unsaturated sites within the porous host; the ‘ship in a bottle’ method wherein precursors
for the IL are introduced inside of the MOF/COF such that the final IL assembles within
the pores; capillary action-promoted diffusion of the IL into the pores [43,44]; and
one-pot assembly of the IL composite [45,46]. An appropriate method may be chosen
based on the presence or absence of coordinatively unsaturated sites in the MOF/COF,
the sizes of the aperture openings, and the molecular sizes of the ILs themselves.
An early example of a MOF-IL composite as a solid electrolyte was reported by Fujie,
Kitagawa and co-workers [47], who physically blended Zn(2-methylimidazole)2 (ZIF-8)
with EMI-TFSI (1-ethyl-3-methylimidazolium trifluoromethanesulfonimide)/LiTFSI to
obtain an electrolyte. A low activation energy of 0.16 eV and an ionic conductivity
of 10−5 S cm−1 at 25°C was reported for this composite, which nevertheless was lower
than that of the MOF-IL combination alone, measured in the absence of the Li salt.
Blending EMI-TFSI/Li-TFSI with Zr6O4(OH)4(H2TCPP)3 (MOF-525, H2TCPP = tetracarboxyphenylporphyrin)
gave an electrolyte with a conductivity of 10−4 S cm−1 and a Li+ transference number
of 0.36 [48]. Although still only in the same range as liquid electrolytes, the transference
number for the MOF-IL composite was a marked improvement upon the transference number
measured for EMI-TFSI/LiTFSI itself, and was attributed to confinement of the EMI+
and TFSI− ions within the MOF pores. These early studies of MOF/ILs composites highlighted
certain potential benefits of confining the ILs to micropores, but also revealed unexpected
results such as diminished conductivity upon addition of Li+. A similar trend was
observed in a composite of ZnO4(BDC)3 (MOF-5, BDC2− = 1,4-benzenedicarboxylate) with
AMImTFSI (1-allyl-3-methylimidazolium TFSI) [45]. Doping this composite with increasing
amounts of LiTFSI afforded electrolytes with gel-like consistencies with good ionic
conductivities of 10−3–10−2 S cm−1 at 51°C, which showed inverse dependence with the
amount of Li+. Although the authors attributed this unexpected observation to a change
to a more tortuous Li+ conduction pathway in the more highly loaded samples, experiments
to substantiate such mechanistic implications are difficult and often not pursued
in the MOF/COF literature thus far. Regardless, these rather complex composites exhibit
impressively low activation energies of less than 0.1 eV and working potential windows
greater than 5.2 V, warranting additional future studies. The wide potential windows
of the above MOF composites highlight the resilience against reduction or oxidation
that solid electrolytes may feature even if the structures contain metal ions.
One word of caution is that both the anion and the cation in an IL have non-zero mobilities
within the framework, and both can contribute to overall ionic conductivity, such
that the metal cations are not the only charged mobile species within these electrolytes
[49]. Measuring the Li+ transference numbers of the composites is an important step
in identifying the Li+ contribution to the conductivity. Additionally, as with polymer
composite electrolytes discussed above, understanding the interfacial interaction
between the MOF/COF and the IL is difficult, making the discovery of new IL-based
composites squarely an empirical challenge with little hope of rational design [46].
4.
Metal ion conduction in neat MOFs and COFs
(a)
Coordinating anions to open metal or other cationic sites
The structural and compositional tunability of MOFs and COFs is one of the attributes
that encourages their exploration as neat solid electrolytes. Although the ability
of these materials to intercalate ions has resulted in numerous works detailing their
use as battery electrode materials [24,27,50–53], employing them as solid-state electrolytes
has emerged only recently. One of the pioneering studies in this context was published
in 2011 by Wiers, Long and co-workers [54]. This study reported soaking Zn4O(BTB)2
(MOF-177, BTB3− = 1,3,5-benzenetribenzoate), H3[(Cu4Cl)3(BTTri)]8 (Cu-BTTri, BTTri3− = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene),
and Mg2(dobdc) (dobdc4− = 5-dioxido-1,4-benzene-dicarboxylate) in 1 : 1 ethylene carbonate:diethyl
carbonate solutions of LiBF4 and conducting electrical impedance spectroscopy (EIS)
on the pressed pellet samples. The Li+-doped MOFs yielded ionic conductivities ranging
from 10−9 to 10−6 S cm−1, with the most promising host being Mg2(dobdc). Although
an intriguing early result, the ionic conductivity of 1.8 × 10−6 S cm−1 observed in
Mg2(dobdc) was still at least two orders of magnitude lower than the technological
benchmark for battery applications [13]. Taking advantage of the coordinatively unsaturated
Mg2+ sites in this framework, the authors added Li
i
OPr and showed that coordination of ‒i
OPr to these sites immobilized the anions and allowed the cations to move more freely,
further increasing the conductivity by a factor of 10. The optimized electrolyte,
Mg2(dobdc)·0.35Li
i
OPr·0.25LiBF4·EC·DEC (EC = ethylene carbonate, DEC = diethyl carbonate) (figure 2)
exhibited a conductivity of 3.1 × 10−4 S cm−1 and an activation energy of 0.15 eV,
meeting superionic conductor qualifications [55]. The need for LiBF4 in this optimized
formulation was justified by implicating it in inter-particle conductivity, with EC
and DEC solvating the Li+ ions in the pores and improving inter-particle contacts.
Figure 2.
A portion of the structure of Mg2(dobdc)·0.35Li
i
OPr·0.25LiBF4·EC·DEC. The cross-sectional view depicts the envisioned migration path
for Li+ ions through the electrolyte. H atoms are omitted for clarity.
The Long group later expanded upon the notion of immobilizing charge-balancing anions
on open metal sites in MOFs in order to maximize exclusively Li+ mobility. The framework
UiO-66 (Zr6O4(OH)4(BDC)6) can undergo thermal dehydration to afford coordinatively-unsaturated
Zr4+ sites [56,57] (figure 3
a). Ameloot, Long and co-workers capitalized on this feature by soaking the dehydrated
UiO-66 in a tetrahydrofuran solution of Li-O
t
Bu, consequently saturating the Zr4+ coordination sphere with alkoxide anions and
incorporating charge-balancing Li+ cations [58]. The resulting Li+ ionic conductivity
was reported to be 1.8 × 10−5 S cm−1, one order of magnitude lower than the reported
Mg2(dobdc)·0.35Li
i
OPr·0.25LiBF4·EC·DEC [54] but still competitive with solid polymer electrolytes [14,59].
Further, the bulky aliphatic groups on the alkoxide shield the negative charge of
the anion, thus weakening the interaction between the anion and the Li+ cations and
enabling a low Li+ activation energy of 0.18 eV. Unfortunately, a symmetric Li cell
with this electrolyte could only be cycled three times before shorting due to Li dendrite
formation. It may be possible that altering the pore shapes/channel orientations may
allow better control over the uniformity of Li plating, which could aid in decreasing
dendrite formation. If dendrites formed along grain boundaries, forming larger host
crystals and thus decreasing grain boundary density, or adding a polymeric binder
to mitigate the effects of grain boundaries, may also help eliminate dendrite formation.
Figure 3.
Structures of MOFs and COFs that have served as pioneers in the porous solid electrolyte
field: (a) UiO-66 (isomorphic with UiO-67), (b) MIT-20, (c) HKUST-1, (d) MIL-100 (M = Cr3+
Fe3+, or Al3+), (e) [ScX(μ4-pmdc)2(H2O)2]·5H2O, (f) COF-5, and (g) TpPa-1. Water molecules
and H atoms are omitted. (Online version in colour.)
In addition to Li+ conduction, another promising application of porous material-based
electrolytes is conduction of more charge-dense Mg2+ ions. One consideration when
designing materials for Mg2+ conduction is the larger size of Mg2+ ions compared to
that of Li+, particularly when solvated, which necessitates larger pore size to allow
Mg2+ transport. Aubrey, Long and co-workers explored both Mg2(dobdc) (structure shown
in figure 2) and its expanded analogue Mg2(dobpdc) (dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate)
as Mg2+ ion conductors [22]. In line with the expanded pore size of Mg2(dobpdc) compared
to that of Mg2(dobdc) (diameter = 21 Å versus 13 Å), Mg2(dobpdc) could accommodate
more than three times the mole equivalents of free Mg2+ ions than the Mg2(dobdc) host
and more than two times the mole equivalents of the dielectric triglyme, which was
added to all samples. This is accompanied by a more than 100-fold increase in conductivity,
with conductivity values of approximately 10−4 S cm−1 observed in Mg2(dobpdc)-Mg(TFSI)2
and approximately 10−6 S cm−1 observed in Mg2(dobdc)-Mg(TFSI)2. Champion devices made
from soaking the MOFs in Mg(TFSI)2 and Mg(OPhCF3)2 afforded conductivities of 10−4 S cm−1
with Mg2(dobdc) and slightly higher with Mg2(dobpdc). These conductivity values are
higher than those reported for any solid Mg2+ electrolytes and, combined with low
activation energies of 0.11–0.19 eV, render the materials relevant for commercial
applications. Studies of the stability of these materials to prolonged cycling and
to Mg metal or other electrode materials would be useful for exploring the potential
of these MOFs in a battery assembly.
The ability to coordinate a variety of anions to open metal sites in MOFs and impart
conductivity of various cations introduces the opportunity to establish material-specific
trends in performance. For example, Park, Tulchinsky and Dincă reported an anionic
CuII-azolate MOF, (CH3)2NH2 [Cu2Cl3BTDD]·(DMF)4(H2O)4.5 (MIT-20, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo-[1,4]dioxin)
that featured charge-balancing dimethylammonium cations (figure 3
b) [60]. Presence of free dimethylammonium cations in the parent structure suggested
that the MOF could accommodate and potentially conduct metal cations. Removal of residual
DMF and water molecules, as well as one equivalent of dimethylammonium chloride afforded
a neutral framework, Cu2Cl2BTDD. The thermodynamic favouring of the anionic framework
during synthesis allowed a quantitative yield of the anionic MIT-20 charge-balanced
by free Group 1 and 2 metal cations when soaking in the respective metal salts. Because
this quantitative transformation of MIT-20 affords isostructural materials regardless
of the nature of the anion, soaking the MOF in LiCl, LiBr, and LiBF4 salts with addition
of the dielectric propylene carbonate (PC) enabled the exploration of the effect of
anion identity on electrolyte performance. Gratifyingly, an increasing softness of
the anion correlated well with increasing Li+ conductivity (10−5 S cm−1 to 10−4 S cm−1)
and decreasing activation energy (0.32–0.16 eV). A Li+ transference number of 0.66
was measured for MIT-20-LiCl, confirming that the primary contributor to the conductivity
was mobile Li+, and the Cl− anions were conversely immobilized on the open metal sites
in the framework. MIT-20 also exhibited good Na+ and Mg2+ conductivity (σ
Na = 1.8 × 10−5 S cm−1 and σ
Mg = 8.8 × 10−7 S cm−1) and activation energies of 0.39 eV and 0.37 eV upon soaking
in solutions of NaSCN and MgBr2, respectively. The ability to install both different
anions and different cations within the MIT-20 structure highlights the versatility
of this material as a solid electrolyte. This material also exemplifies the generalization
that MOFs and COFs which have isostructural phases that are isolable in multiple states
of formal charge could be promising candidates for ionically conductive solid electrolytes.
Another example of capitalizing on the modularity of MOFs to establish structure–function
relationships was Shen, Dunn and co-workers' exploration of the MIL-100 and UiO series
of MOFs as tunable solid electrolytes [61]. A proof-of-principle was demonstrated
by targeting the installation of
Cl
O
4
−
ions from a PC solution of LiClO4 onto the coordinatively unsaturated Cu2+ sites of
activated Cu3(BTC)2 (HKUST-1, BTC3− = benzene-1,3,5-tricarboxylate) (figure 3
c), allowing Li+ ions to move freely upon polarization for a Li+ conductivity of 3.8 × 10−4 S cm−1
and an activation energy of 0.18 eV. A similar PC-LiClO4 treatment of materials in
the activated MIL-100 series (M3O(BTC)2OH, M = Cr3+ Fe3+, or Al3+) (figure 3
d) produced solids whose conductivity ranged from 10−3 S cm−1 to 10−2 S cm−1. The
highest ionic conductivity of the MIL-100 MOFs, observed with MIL-100-Al3+, was consistent
with the assertion that the increased Lewis acidity of Al3+ compared to Fe3+ and Cr3+
led to decreased ion pairing strength between the
Cl
O
4
−
and the Li+, thus enhancing Li+ mobility. The effect of MOF pore size on ionic conductivity
was also explored using activated UiO-66 and the larger-pore Zr6O4(OH)4(BPDC)6 (UiO-67,
BPDC2− = biphenyl-4,4′-dicarboxylate). Soaking these MOFs in PC solutions of LiClO4
gave Li+ conductivities of 1.8 × 10−4 S cm−1 and 6.5 × 10−4 S cm−1 for UiO-66 and
UiO-67, respectively. The higher Li+ conductivity observed in UiO-67 was attributed
to the larger pore size being able to accommodate a higher extent of solvation around
the Li+ ions, which enhances mobility. This trend was consistent with that observed
in Mg2(dobdc) versus its expanded analogue Mg2(dobpdc), as discussed earlier. Lower
activation energy was measured in UiO-67 versus UiO-66 as well (E
a = 0.12 eV versus 0.21 eV for UiO-67 versus UiO-66, respectively).
In addition to coordinating anions to open metal sites as cation hopping sites within
MOFs, anions have also been incorporated into positively charged MOF/COF structures
simply through weaker Coulombic interactions. Recently, Chen and co-workers reported
a cationic COF comprising alternatively linked triaminoguanidinium and 1,3,5-triformylphloroglucinol
ligands which was proposed to feature π−π stacking, forming channels from the aligned
pores [62]. Stirring the COF in an aqueous solution of LiTFSI replaced the parent
chloride ions with TFSI− ions. One equivalent of TFSI− was charge-balancing the triaminoguanidinium
within the framework and one equivalent was charge-balancing Li+ ions which remained
in the electrolyte matrix. This electrolyte exhibited a conductivity of 5.74 × 10−5 S cm−1
at 30°C and an activation energy of 0.34 eV. The Li+ transference number of 0.61 was
consistent with at least a portion of the TFSI− ions being immobilized through interaction
with the cationic triaminoguanidinium groups. Additionally, a respectable operating
potential window of 3.8 V was measured. Studies suggested that the TFSI− ion existed
within the framework both as a ‘free’ anion stabilized within the COF channel through
Coulombic interactions, and as an ion pair. The ion-paired TFSI− likely decreases
the Li+ transference number, given that the equivalent of TFSI− present within the
framework to charge-balance the Li+ is likely not coordinated to the COF. Although
this example showcases post-synthetic alteration of the anion identity that is not
feasible in borate-based COFs (see below), the challenge with using a cationic COF
rather than a coordinatively unsaturated charge-neutral MOF is that addition of alkali
metal salts such as LiTFSI introduces equivalents of monoanions both to charge-balance
the cationic framework and to ion-pair with the metal cations. Such electrolytes still
possess an advantageously high density of anionic hopping sites and the safety features
of solid electrolytes, but obtaining higher metal cation transference numbers will
likely be a challenge due to the large percentage of mobile anions. An interesting
extension upon this work could involve soaking the COF in a polylithium salt [63–67].
This could yield the COF with equal equivalents of the polyanion immobilized within
the channels, triaminoguanidinium groups within the framework itself and mobile Li+.
One consideration with this approach would be careful selection of the anion, particularly
in terms of size; Chen et al. reported diminished π−π stacking within the COF upon
replacing the Cl− ions with larger TFSI− ions. This partial collapse of the stacked
structure may obstruct metal transport pathways within the framework.
(b)
Incorporating anions directly into the structure
An alternative to introducing stoichiometric equivalents of anions concomitant with
mobile cations is to target inherently negative frameworks, where the negative charges
are built into the MOF/COF building blocks themselves. In 2015, Van Humbeck, Long
and co-workers reported cross-linked tetraarylborate moieties that form a negatively
charged porous polymer wherein the anionic borates serve as immobile Li+ hopping sites
[68]. This approach was inspired by early reports of linear polymers containing ionic
groups such as anionic perfluoroalkyl carboxylates [69] or cationic diallyldimethylammonium
units [70] within the polymer structure. Such polymers were also used as solid electrolytes
with the goal of achieving single-ion conductivity, and exhibited mobile ion transference
numbers nearing unity. The observed conductivities for such polymers fell in the 10−6
to 10−5 S cm−1 range, possibly due to undesirably large distances between the hopping
sites along the polymer backbone. In contrast, the material designed by Van Humbeck,
Long et al. features an interpenetrated network that provides a high density of ion
hopping sites. Measurements gave a Li+ conductivity of 3.6 × 10−5 S cm−1, which increased
further by one order of magnitude upon perfluorination of the aryl groups in the tetraarylborate
network (σ
Li = 2.7 × 10−4 S cm−1). Installation of electron-withdrawing fluorine atoms on the
aryl rings was thought to weaken the borate–Li+ interaction and thus encourage Li+
mobility through the electrolyte. As expected, immobilization of the anions within
the framework afforded strong single-ion conducting character with a high Li+ transference
number (t
Li+ = 0.9). Interestingly, altering synthetic conditions afforded a permanently porous
fluorinated tetraarylborate material (BET surface area = 480 m2 g−1) that exhibited
10-fold lower conductivity than its dense-phase congener. Although this difference
in conductivity between the porous and dense phase is consistent with the need for
closely packed hopping sites, the activation energies of the two phases were identical,
0.25 eV. The phase-independent activation energy data may point to dominating surface
conduction pathways as proposed by the authors. However, the distinct differences
in conductivity as a function of phase density and the fact that the identities of
the hopping sites do remain constant in both phases highlights the importance of hopping
site density on ionic conductivity. Most tellingly, it emphasizes that three-dimensionally
connected pores become detrimental to ion transport beyond a certain diameter.
Another example of anionic borates being featured in ionically conductive MOFs and
COFs was a spiroborate-based COF synthesized by base-promoted transesterification
of diols and trimethylborate using LiOH as the base. The latter served the roles of
both deprotonating the diol during the transesterification and providing the Li+ ions
for the electrolyte [71], thus allowing a one-pot synthesis of a Li+-loaded solid
electrolyte (figure 4). Incorporating the spiroborate structure into the COF was motivated
by previous reports of Li borate salts used as Li+ electrolytes [72]. The spiroborate
COF/polyvinylidene fluoride (PVDF) formulation exhibited a Li+ conductivity of 3.05 × 10−5 S cm−1
and an activation energy of 0.24 eV. In addition to favourable conductivity and activation
energy values, a high Li+ transference number of 0.8 was measured. Finally, a respectable
potential window of ca 4.5 V was reported, further highlighting the utility of solid-state
electrolytes over liquid electrolytes that decompose at lower potentials. The formation
of inherently negatively charged frameworks provides a host matrix with a homogeneous
distribution of cation hopping sites that contributes nothing to increasing the anion
transference number. It provides a promising blueprint for very efficient cation conductors,
but has only rarely been used thus far.
Figure 4.
Spiroborate COF featuring anionic sites integrated into structure, and charge-balancing
Li+ ions provided by the base during synthesis. (Online version in colour.)
One instance where this strategy proved effective with MOFs involved the substitution
of trivalent Sc3+ ions in [ScX(μ4-pmdc)2(H2O)2]·5H2O (pmdc2− = pyrimidine-4,6-dicarboxylate;
X = Li+ or Na+) (figure 3
e) by divalent Cd2+ or Mn2+ [73]. Low conductivity in the parent samples presumably
stems from the alkali metal cations being affixed in the framework, thus hindering
their mobility. As such, aliovalent substitution of Sc3+ with Cd2+ or Mn2+ was pursued
with the goal of installing additional free alkali metal cations for charge balance.
Indeed, free Li+ and Na+ ions compensated for the charge imbalance created by this
aliovalent substitution. However, these ions still contributed low Li+ and Na+ conductivity
values (10−7 to 10−6 S cm−1 for the Cd2+ and Mn2+-doped MOFs). The authors reported
enhanced Li+ and Na+ conductivity values from simply soaking the parent Sc3+-MOFs
in solutions of LiBF4 or NaPF6. The resulting electrolytes exhibited ionic conductivities
of 10−5 S cm−1 (Na+) and 10−4 S cm−1 (Li+). Even though the soaking procedure afforded
enhanced conductivity, this treatment also caused cracking of the crystals, while
peak broadening in the 1H and 7Li nuclear magnetic resonance (NMR) spectra revealed
increased structural heterogeneity. Together, these observations complicate the direct
correlation between conductivity and the mobile charge density and prevent detailed
studies probing potentially new conduction mechanisms in the metal-exchanged samples.
Such information could have been useful in explaining the large discrepancy in activation
energies for the Li samples versus the Na samples (0.25 eV versus 0.64 eV, respectively).
Despite the low conductivity observed in the aliovalently doped samples described
above, the idea that aliovalent substitution can increase the mobile cation density
in a MOF is potentially quite general and could in principle be applied to MOFs made
from tri- or higher-valent metal ions, with cation substitution in general established
as a versatile synthetic technique in this class [74].
(c)
Neutral host frameworks
Although neutral host frameworks that do not easily accommodate anions may not seem
like ideal candidates for solid electrolytes, some notable examples that highlight
the importance of processing porous solid electrolytes do use such hosts. For instance,
uniaxial pressure applied to C9H4BO2 (COF-5) (figure 3
f) and TpPa-1 (Tp = triformylphloroglucinol, Pa = paraphenylenediamine) (figure 3
g) promoted preferred orientation of platelet crystallites, thereby forcing alignments
of the COF pores and the formation of long-range channels for more efficient ion transport
[75]. Soaking these materials in solutions of LiClO4 followed by evaporation and uniaxial
pressing afforded solid electrolytes with conductivities of 2.6 × 10−4 S cm−1 and
1.5 × 10−4 S cm−1 for COF-5 and TpPa-1, respectively. Although 7Li NMR experiments
confirmed the highly mobile nature of Li+, the mobility of the charge-balancing
Cl
O
4
−
anions which can also contribute to the conductivity was not measured. As before,
obtaining the Li+ transference numbers for these COFs would be critical for assessing
the Li+ contribution to the total ionic conductivity.
Recently, the notion of forming true hybrids between porous materials and polymers
for ion conduction has been pursued in the form of the ‘polyelectrolyte’ COFs such
as TPB-DMTP-COF (TPB = 1,3,5-tri(4-aminophenyl)benzene, DMTP = 2,5-dimethoxyterephthalaldehyde)
and TPB-BMTP-COF (BMTP = 2,5-bis((2-methoxyethoxy)methoxy)terephthalaldehyde) [76].
Condensation of TPB with either DMTP or BMTP resulted in porous, stacked two-dimensional
COFs with either methoxy groups (TPB-DMTP-COF) or oligo(ethylene oxide) chains (TPB-BMTP-COF)
branching off of the phenyl rings and lining the pore walls. This approach aimed to
combine the ion transport benefits of polymer electrolytes with the mechanical and
thermal stability of MOF/COF electrolytes. Soaking these porous, crystalline structures
in solutions of LiClO4 afforded materials with conductivities of 10−7 S cm−1 (TPB-DMTP-COF)
and 10−6 S cm−1 (TPB-BMTP-COF) at 40°C, both improvements upon that of the PEO-Li+
complex, which has a Li+ conductivity of 10−8 S cm−1 at 40°C. It should be noted that
both materials exhibit high activation energies for ionic transport, 0.96 eV for TPB-DMTP-COF
and 0.87 eV for TPB-BMTP-COF, which suggests that improvements are likely for a class
of materials that allows for considerable combinatorial potential. Once again, more
systematic improvements would be facilitated by 7Li NMR studies and measurements of
the Li+ transference numbers to parse out the mobility of free Li+ ions versus ion-paired
LiClO4. The approach of implementing polymeric building blocks that have proven ion
conductivity into crystallographically well-defined and mechanically and thermally
robust COF structures is intriguing. Further structural characterization of these
analogues after incorporation of Li salts would aid in determining whether the COF
structure is retained in the final electrolyte matrix.
5.
Scouting criteria: what makes a MOF/COF ionically conductive?
The several examples discussed herein were chosen to showcase the multiple approaches
available for achieving ionic conductivity in MOFs and COFs. When evaluating these
porous materials as potential candidates for ion conduction, the following considerations
may prove useful:
— Does the MOF feature metal sites with coordination environments that include removable
solvent molecules, or other anion docking sites?
— Are anions incorporated into the MOF/COF structure, e.g. as part of the building
blocks or by Coulombic forces?
— Is the material isolable in multiple states of formal charge? Is isolation of these
states reversible?
— Is the MOF/COF electrically insulating?
— Is there a high density of hopping sites within the structure?
— What are the sizes of metal ions that could be accommodated within the pores?
— Would the mobile metal ions be solvated in the pores? How many equivalents of solvated
metal ions can the structure accommodate?
When testing MOFs and COFs for ion conduction, the following criteria can serve as
reference benchmarks for evaluating performance:
— Ionic conductivity ≥10−4 S cm−1, ideally when T ∼ 25°C [55]
— Electrical conductivity ≤10−10 S cm−1, to avoid cell shorting [13]
— Activation energy ≤0.4 eV [55]
— Working potential window of ≥4 V for commercial applications [77]
— Transference number of ≥0.5, to avoid polarization effects [60]
— Structurally stable to the desired mobile metal salts, dielectric additives, and
the electrode materials
— No or nominal increase in resistance during cycling
6.
Conclusions/future directions
The continuously growing energy demand is being addressed with innovative, sustainable
technology and metal and metal-ion batteries remain leaders for electrical energy
storage in terms of combined energy density, portability, longevity and cost. Continued
optimization of these devices requires enhanced safety and even greater operating
efficiency, both of which can be greatly improved by an optimized solid-state electrolyte.
MOFs and COFs have gained attention as promising candidates for solid-state electrolyte
technology due to their crystallographic definition which contributes immobilized
and homogeneously distributed ion hopping paths, enhanced thermal and mechanical stability,
and a morphology that in principle could prevent hazardous dendrite formation. The
high surface area of these materials allows an abundance of cation hopping sites,
which aids in minimizing battery resistance. Reports detailing installation of hopping
sites into MOFs and COFs both by coordinating anions to cationic sites within the
frameworks and by installing anionic sites directly as components of the frameworks
highlight the versatility of this class of materials for battery electrolyte applications.
Several examples have shown great promise in this arena by exhibiting ionic conductivities
of 10−6 to 10−4 S cm−1 under ambient conditions, activation energies of 0.1–0.4 eV,
cation transference numbers of 0.6–0.9, and potential windows exceeding 4.0 V.
Looking forward, exploring the effect of MOF/COF crystal size on conductivity could
aid in elucidating whether ion mobility is an inter- or intra-crystal phenomenon.
Such studies could also aid in optimizing conductivity versus dendrite formation,
which may occur along grain boundaries. Further, many of the reported MOFs and COFs
have shown promising properties when combined with monolithium salts. Expansion of
these studies could involve use of a polylithium salt, to achieve higher Li+ loading.
Another underexplored area is the utilization of inherently anionic materials balanced
by potentially mobile cations residing in the pores. This may encourage homogeneous
distribution of the charge balancing metal ions throughout the host matrix while minimizing
incorporation of mobile, exogenous species that are typically introduced using more
iterative electrolyte preparation methods. Additionally, hybridizing porous materials
with traditional polymer electrolytes may allow for retention of the ionically conductive
properties of the polymers while adding benefits associated with porous solid electrolytes,
e.g. minimized electrolyte reorganization, maximized hopping site density, and potentially
no dendrite growth. Finally, targeting good ion conductors for K+, Ca2+ or Al3+ transport
in the MOF/COF context could prove fruitful given that these larger/higher-valent
ions might require larger pores than typically available with denser materials. Employing
MOFs and COFs as solid electrolytes for K-ion, Ca-ion or Al-ion batteries would combine
the benefits of porous material-based electrolytes with the advantages of using energy-dense,
earth-abundant ions. The wealth of metal and ligand combinations that may engender
a host of pore shapes, sizes, and local electronic environments that may accommodate
any number of metal ions lays an expansive foundation for a bright future of MOF/COF-based
solid electrolytes.