Benzenedisulfonic Acid as an ALD/MLD Building Block for Crystalline Metal-Organic Thin Films**

Article abstract of DOI:10.1002/chem.202100538

Two new atomic/molecular layer deposition processes for depositing crystalline metal-organic thin films, built from 1,4-benzenedisulfonate (BDS) as the organic linker and Cu or Li as the metal node, are reported. The processes yield in-situ crystalline but hydrated Cu-BDS and Li-BDS films; in the former case, the crystal structure is of a previously known metal-organic-framework-like structure, while in the latter case not known from previous studies. Both hydrated materials can be readily dried to obtain the crystalline unhydrated phases. The stability and the ionic conductivity of the unhydrated Li-BDS films were characterized to assess their applicability as a thin film solid polymer Li-ion conductor.

Full text of DOI:10.1002/chem.202100538

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Benzenedisulfonic Acid as an ALD/MLD Building Block for
Crystalline Metal-Organic Thin Films**
Juho Heiska,[a] Olli Sorsa,[a] Tanja Kallio,[a] and Maarit Karppinen*[a]
Abstract: Two new atomic/molecular layer deposition proc-
esses for depositing crystalline metal-organic thin films, built
from 1,4-benzenedisulfonate (BDS) as the organic linker and
Cu or Li as the metal node, are reported. The processes yield
in-situ crystalline but hydrated Cu-BDS and Li-BDS films; in
the former case, the crystal structure is of a previously known
metal-organic-framework-like structure, while in the latter
case not known from previous studies. Both hydrated
materials can be readily dried to obtain the crystalline
unhydrated phases. The stability and the ionic conductivity of
the unhydrated Li-BDS films were characterized to assess their
applicability as a thin film solid polymer Li-ion conductor.
Introduction
lithium transference number is usually larger than in SPEs and
closer to unity. These materials are also electrochemically stable,
and – according to theoretical calculations – can effectively
reduce the Li-ion concentration gradient in Li plating/
stripping.[21,22] However, the most significant drawback of
SPSLICs is still their low ionic conductivity compared to the ISEs
and SPEs. This is due to the strong association between the
sulfonate and the lithium ion. To overcome the problem,
SPSLICs are often polymerized e.g. in a polyethylene oxide
matrix or with other oligomers that can dissociate the Li+ ion to
make it more mobile.[23–25]
The only SPSLIC-type materials deposited with ALD/MLD are
aliphatic lithium compounds.[12,26,27] Here we like to propose
dilithium-1,4-benzenedisulfonate (Li-BDS) as a relatively simple
SPSLIC and possibly attainable through ALD/MLD synthesis.
Interestingly, the prospective precursor for BDS in ALD/MLD,
1,4-benzenedisulfonic acid (HBDS), shares many chemical
properties with terephthalic acid (TPA; 1,4-benzenedicarboxylic
acid), which is one of the most common MLD precursors and is
known to readily react with metal-bearing ALD precursors to
form stable crystalline metal-organic thin films, somewhat
similar to metal-organic framework (MOF) structures.[28–36]
Hence, we consider HBDS as an interesting analog to TPA in
ALD/MLD. It is significantly (approx. six orders of magnitude)[37]
more acidic than TPA, which could enable the use of less
reactive inorganic precursors. For some MOFs replacing the TPA
linker with BDS considerably enhances the thermal stability,
Atomic layer deposition (ALD) has been already for years the
fastest-growing thin-film deposition technology in micro-
electronics,[1,2] but it is gaining attention in energy applications
as well, including the lithium-ion battery (LIB) field.[3,4] Molecular
layer deposition (MLD) is a much less exploited counterpart of
ALD for organic thin films.[2,5] Combining these two methods
into ALD/MLD allows the deposition of metal-organic films. In
the LIB field, all these methods are used, especially in the
contexts of the micro-battery,[6] inorganic solid electrolyte (ISE),
solid polymer electrolyte (SPE), and electrode-electrolyte inter-
face design.[3,7–13] In particular, ALD/MLD has been exploited to
modify the electrode/electrolyte interfaces to improve the cycle
life of the battery.[14–17]
A prototype SPE is a polyethylene oxide mixed with lithium
salt. The lithium salt is needed to enhance the poor ionic
conductivity of the SPE, but the drawback is that it at the same
time tends to deteriorate the electrochemical stability of the
SPE material.[18] On the other hand, the SPEs win out in
processability and flexibility, suffer less from interfacial resist-
ance, and are cheaper to manufacture compared to the
ISEs.[19,20] Recently, a new type of SPE has regained research
interest, so-called solid polymeric single Li-ion conductor
(SPSLIC).[21] A SPSLIC material is based on immobilized anions
such as Li-sulfonates. Since the anions are immobilized, the
°
e.g. from ca. 200 to ca. 400 C for the Cu-TPA versus Cu-
BDS.[38,39] So far in the literature, considerably fewer BDS-based
MOFs have been reported compared to the extensive literature
on metal carboxylates.
[a] J. Heiska, Dr. O. Sorsa, Prof. T. Kallio, Prof. M. Karppinen
Department of Chemistry and Materials Science
Aalto University
00076 Espoo (Finland)
In this article, we report new ALD/MLD processes for two
metal 1,4-benzenedisulfonate materials, Cu-BDS and Li-BDS, see
Figure 1. The former compound was chosen as its crystal
structure was known from solution-synthesized bulk samples[39]
and it had shown some promise as a matrix for sulfur infiltration
in LiÀ S batteries, implying some degree of porosity.[40] The latter
Li-BDS compound, which we consider as the main candidate for
the new SPSLIC material, has been only briefly mentioned in
E-mail: maarit.karppinen@aalto.fi
[**] ALD=atomic layer deposition; MLD=molecular layer deposition.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100538
© 2021 The Authors. Chemistry - A European Journal published by Wiley-
VCH GmbH. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which permits use
and distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are made.
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somewhat longer pulse time of 10 s is required for HBDS, which
is typical for organic precursors in our reactor setup.[32,34,42] The
process yields well-crystalline Cu-BDS films for which the post-
deposition GIXRD analysis typically showed the presence of
water of hydration. However, in very dry ambient humidity
conditions it was possible to observe the unhydrated phase as
well in the as-deposited films, which was then quickly hydrated
unless properly protected. The water could also be afterwards
removed by annealing (see FTIR and GIXRD data in Figures S1
and S2). The GIXRD pattern shown in Figure 2 is for a typical as-
deposited Cu-BDS film and matches perfectly with the simu-
lated XRD pattern based on the structure data reported for
hydrated Cu-BDS,[39] evidencing the first ALD/MLD grown MOF-
type thin film based on the BDS linker. Annealing the film
removes the crystal water; the film remains crystalline, but the
GIXRD pattern consists of few broad peaks only (Figure S2).
Figure 1. Schematics of the present ALD/MLD processes and the resultant
products.
literature without any in-depth discussion or reported XRD
pattern.[41] The Li-BDS films turned out to be hygroscopic,
exhibiting different crystalline forms depending on whether
being hydrated or dehydrated. We investigate the thermal and
electrochemical stabilities of these films and assess their ionic
conductivity using AC impedance measurements.
Li-BDS ALD/MLD process
Inspired by the success with the Cu(thd)2 +HBDS process, we
proceeded with the Li(thd)+HBDS process. The ALD/MLD
parameters were found to be very similar for the two processes,
with somewhat lower GPC values for the latter process
(Figure 3). Very similar precursor pulse lengths were required
again for saturation: 6 s for Li(thd) and 10 s for HBDS. The
relatively large roughness of the films (e.g. 4 nm for a 30-nm
Results and Discussion
The experimental can be found in Supporting Information. All
the relevant process parameters, precursor synthesis, and
descriptions of electrochemical setups are found within.
thick film, typical for crystalline ALD/MLD Li-organic films[33,34]
)
made the fitting of the XRR data challenging. Therefore, the
thicknesses of the films grown with 300 and 400 cycles were
determined from cross-sectional SEM images (Figure S3). The
dependence of the growth rate on the deposition temperature
is depicted in Figure 3b, showing that with increasing temper-
Cu-BDS ALD/MLD process
°
The Cu(thd)2 +HBDS process experiments were carried out at
ature GPC remains essentially constant up to 260 C (2.3 to
°
°
210 C based on our preliminary tests. In the inset of Figure 2
2.0 Å/cycle). Above 260 C, the GPC rapidly decreases which is
we show the so-called growth-per-cycle (GPC) as a function of
the precursor pulse length, for both precursors separately. This
is the data needed to show that the surface reactions saturate.
From Figure 2, for Cu(thd)2 a pulse time of 4 s is enough while a
rather typical behavior for ALD/MLD processes.[28,29,32,33] We also
confirmed the linear dependence of the film thickness on the
number of ALD/MLD cycles (except the short “incubation
period” in the beginning; Figure 3c). This type of incubation
period is often seen with crystalline MOF-like structures,[30,32–34]
while it is much more imperceptible with amorphous MOF-like
materials that are deposited with ALD/MLD.[43] Finally, we
determined the density of our as-deposited (hydrated) Li-BDS
films to be 1.63 g/cm3 based on XRR.
Chemical stability
Typically, the sulfonates are much more hygroscopic than their
acid analogs.[39,44] The hygroscopicity was quite apparent for our
Li-BDS films as well, which – without an exception – were
received from the deposition reactor in their hydrated form
(showing the characteristic À OH vibrations in FTIR spectra). The
source of hydration is very likely the ambient humidity.
Interestingly, the hydrated water simply disappeared when the
films were stored for few hours in a desiccator. Both the as-
deposited hydrated films and the post-deposition dehydrated
Figure 2. GIXRD pattern for as-deposited hydrated Cu-BDS film compared
with the calculated XRD pattern based on the structure reported for Cu-BDS
in bulk samples.[39] Inset shows the pulse saturation of the Cu(thd)2 +HBDS
°
ALD/MLD process at 210 C.
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°
Figure 3. ALD/MLD parameter optimization for the Li(thd)+HBDS process: (a) GPC dependency on the precursor pulse lengths (100 ALD/MLD cycles; 200 C),
the inset showing the XRR spectrumwith a fit, (b) GPC dependency on the deposition temperature (100 cycles), and (c) growth linearity at optimized
°
conditions (200 C, 6 s Li(thd) and 10 s HBDS). In (c) the points with 300 and 400 ALD/MLD cycles are estimations based on cross-sectional SEM images of the
films (Figure S1).
°
films were crystalline, but the crystal structures are different as
seen both from the GIXRD and FTIR data (Figure 4).
annealing at 450 C in the air for 1 h the films changed visually
as did the FTIR spectrum and GIXRD pattern and the structure
was certainly decomposed. Hence, in this case (opposite to the
Upon the dehydration, the film density (determined from
the critical angle in the XRR pattern) decreases, and the surface
roughness slightly increases. In SEM images (Figure 4c), we can
see that also the morphology changes upon the water release.
The as-deposited water-containing film shows small crystallites
typical to other crystalline ALD/MLD grown Li-organic films;[35]
the cracks seen in the image are most probably caused by
escaping water from the film while it is imaged under vacuum.
For the unhydrated film, rather unique surface morphology is
seen consisting of long crystal pillars.
The easy dehydration made the characterization somewhat
challenging, as it was impossible to verify exactly the degree of
(de)hydration. After the dehydration, the films were found
somewhat resistant towards rehydration, as sometimes a
mixture of both phases was seen in GIXRD (Figure S5). The Li-
Cu-BDS versus Cu-TPA case) the decomposition temperature is
somewhat lower for Li-BDS than for Li-TPA (500 C for 1 h),
underlining the fact that the thermal stability may depend also
on the metal node.
[33]
°
FTIR analysis
A detailed discussion of the FTIR spectrum is found in the
Supporting Information. The water of hydration is readily
detected in the FTIR from the À OH stretching and bending
modes around 3540 and 1640 cmÀ 1, respectively. The FTIR
spectrum of aromatic sulfonates is often very similar in the
range of 1300–1000 cmÀ 1 consisting of four intense bands
(roman numerals I–IV) due to νas(SO3), ν(CÀ S), νs(SO3), and
ν(ring),[45] see Figure 4b. The FTIR spectra of Li-BDS and Cu-BDS
are indeed very similar and the assignments for Li-BDS are
easily transferable to Cu-BDS. The most notable difference is
°
BDS films were stable at least up to 1 h anneal at 350 C in the
°
air or in a vacuum. At 400 C some decomposition began
(detected with FTIR), in a manner analogous to that of Cu-BDS,
presumably due to the evolution of sulfurdioxide.[44] When
°
°
°
Figure 4. (a) GIXRD patterns of hydrated and unhydrated Li-BDS films (800 cycles at 200 C). Inset shows a magnification between 18 and 24 to highlight the
°
differences in the pattern. (b) Representative FTIR spectra of hydrated (800 cycles at 200 C) and unhydrated Li-BDS spectra with inset highlighting the minor
differences in the 1500 to 400 cmÀ 1 range. Peak interpretation of the FTIR spectra is also included in the inset with blue (Li-BDS hydrated) and orange (Li-BDS
unhydrated) color coding listing the wavenumbers. The roman numerals I–IV highlight the most typical peaks of metal-BDS compounds. A detailed discussion
of the FTIR spectrum can be found in the Supporting Information. The symbols are ν for stretch, δ for bending, and τ for torsion. (c) SEM images of the Li-BDS
films. Hydrated film is imaged directly after deposition while the unhydrated sample was stored in desiccator for 24 h prior imaging.
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the splitting of νas in Cu-BDS Δνas is much larger (62 versus
39 cmÀ 1) indicating that Cu-BDS is more degenerate. Overall,
our FTIR peaks correspond well to the expected vibrations for
Li-BDS and Cu-BDS. Additional comparison of the ATR-FTIR
spectra of the HBDS precursor and the hydrated Li-BDS film
(Figure S4) confirms that the precursors have indeed fully
reacted.
derives from the low degree of dissociation of the Li-ions in the
structure. Typically, the sulfonate-based solid ion conductors
exist as side groups in other polymer matrices where for
example the polyethylene glycol can dissociate the Li-ions and
act as a conductive medium rather than the strictly bound Li-
sulfonate groups. However, increasing the conductivity might
decrease the stability of the material. This underlines the
importance to further optimize the morphologies and (crystal)
structures of the conductive SPSLICs to support the higher
degree of Li-ion dissociation, without sacrificing the stability,
and then challenge the ALD/MLD fabrication of these materials.
Electrochemical measurements
The ionic conductivity was measured in-plane for a 100 nm
thick Li-BDS film deposited on interdigitated platinum electro-
des (DropSens); a precise description of this setup can be found Conclusion
in Ref.[46] and the calculations for the conductivity in Supporting
Information. The fitted impedance spectra are shown in Fig-
ure 5a with the relevant equivalent circuit,[47] and the resultant
Arrhenius plot of conductivity and calculated activation energy
We developed two new ALD/MLD processes for Li- and Cu-
based 1,4-benzenedisulfonate coordinate compounds. Both
processes yielded crystalline films with hydration water. The
crystal structure of the latter films matched with previously
reported bulk-synthesized hydrated Cu-BDS. The Li-BDS films
were crystalline and hydrated as well, but the crystal structure
could not be identified as there were no previous reports for
the crystal structure of Li-BDS. The as-deposited Li-BDS films
readily released the hydrated water, and remained crystalline
and stable, allowing us to characterize them for the ionic
°
in Figure 5b. At low temperature (RT-60 C), no EIS resulting
from ionic conductivity was produced but at elevated temper-
°
atures of 80 and 118 C, decent ionic conductivity values of
4.1·10À 9 and 6.4·10À 8 S/cm, respectively, were determined.
These values are low but comparable to those reported for the
closest analog, i.e. 2.1·10À 8 S/cm at 70 C for amorphous poly(4-
°
lithium styrene sulfonate).[24] The high activation energy value
of 0.9 eV found for Li-BDS is a clear indication that conduction
is intrinsic where defects are induced due to an increase in
temperature.
conductivity; the value of 6.4·10À 8 S/cm reached at 118 C is
°
low but comparable to related materials in bulk form. We
foresee that with the clever tuning of the structure, mainly
adding groups to dissociate the Li-ions, it could be possible to
increase the ionic conductivity of these materials without
obliterating the stability. This would be important to facilitate
the ALD/MLD fabrication of solid polymer ionic conductors of
true application relevance.
Cyclic voltammetry experiments were carried out in cells
with liquid electrolyte (Supporting Information). The results
revealed little (to no) electroactivity compared to the blank
reference. This is expected since sulfonates are known to be
electrochemically quite stable.[21] During the 1st cycle there is a
small reduction peak at 0.75 V, which could simply be due to a
different kind of SEI formation compared to the reference. The
large peak at 4.25 V is due to the oxidation of the liquid Acknowledgements
electrolyte. Therefore, as the oxidation stability could be even
higher than the limit of a wet cell, the electrochemical stability
of Li-BDS seems very good and the biggest problem still lies in
the low ionic conductivity. The low conductivity presumably
The authors would like to thank the Academy of Finland (Profi 3
and 5 projects) and the European Union’s Horizon 2020
research and innovation program under grant agreement
Figure 5. (a) Measured EIS spectra at different temperatures with fits and equivalent circuit, (b) Arrhenius plot of conductivity and calculated activation energy
according to the linearization. (c) Cyclic voltammetry of a reference cell without a film and with 100 nm of Li-BDS film.
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Manuscript received: February 10, 2021
Accepted manuscript online: March 29, 2021
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A new organic linker for atomic/
molecular layer depositions (ALD/
MLD) was demonstrated with copper
and lithium metal nodes. The films are
crystalline with and without water of
crystallization. The applicability of
dilithium-1,4-benzenedisulfonate as a
solid polymer ion conductor is
assessed.
J. Heiska, Dr. O. Sorsa, Prof. T. Kallio,
Prof. M. Karppinen*
1 – 6
Benzenedisulfonic Acid as an ALD/
MLD Building Block for Crystalline
Metal-Organic Thin Films