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Accepted Manuscript
Development of new pyrazole-based lithium salts for battery applications – Do
established basic design concepts really work?
Mariano Grünebaum, Annika Buchheit, Daniel Krause, Martin Manuel Hiller, Christina
Schmidt, Martin Winter, Hans-Dieter Wiemhöfer
PII:
S0013-4686(18)31842-5
DOI:
10.1016/j.electacta.2018.08.055
Reference:
EA 32489
To appear in:
Electrochimica Acta
Received Date: 16 April 2018
Revised Date:
25 July 2018
Accepted Date: 13 August 2018
Please cite this article as: M. Grünebaum, A. Buchheit, D. Krause, M.M. Hiller, C. Schmidt, M.
Winter, H.-D. Wiemhöfer, Development of new pyrazole-based lithium salts for battery applications
– Do established basic design concepts really work?, Electrochimica Acta (2018), doi: 10.1016/
j.electacta.2018.08.055.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
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ACCEPTED MANUSCRIPT
Corresponding author (and First author)
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Dr. rer. nat. Mariano Grünebaum
Helmholtz-Institute Münster, IEK-12
Forschungszentrum Jülich GmbH
Corrensstraße 46
48149 Münster
Germany
E-mail: m.gruenebaum@fz-juelich.de
Homepage: http://www.fz-juelich.de/iek/iek-12/EN/Home/home_node.html
2. author
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M.Sc. Annika Buchheit
Institute for Inorganic and Analytical Chemistry
University of Münster
Corrensstr. 28/30
48149 Münster
Germany
E-mail: annika.buchheit@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.ac/wiemho/index.html
3. author
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4. author
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M.Sc. Daniel Krause
Helmholtz-Institute Münster, IEK-12
Forschungszentrum Jülich GmbH
Corrensstraße 46
48149 Münster
Germany
E-mail: dan.krause@fz-juelich.de
Homepage: http://www.fz-juelich.de/iek/iek-12/EN/Home/home_node.html
Dr. rer. nat. Martin Manuel Hiller
Robert Bosch GmbH
Zentrum für Forschung und Vorausentwicklung
Robert-Bosch-Campus 1
71272 Renningen
Germany
E-mail: martin.manuel.hiller@gmail.com
Homepage: www.bosch-renningen.de
5. author
Dipl.-Chem. Christina Schmidt (last name changed, reason: marriage)
Institute for Inorganic and Analytical Chemistry
University of Münster
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Corrensstr. 28/30
48149 Münster
Germany
E-mail: schmidt.christina@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.ac/wiemho/index.html
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6. author
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Prof. Dr. Martin Winter
Helmholtz-Institute Münster, IEK-12
Forschungszentrum Jülich GmbH
Corrensstraße 46
48149 Münster
Germany
E-mail: m.winter@fz-juelich.de
Homepage: http://www.fz-juelich.de/iek/iek-12/EN/Home/home_node.html
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And:
7. author
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MEET Battery Research Center
University of Münster
Corrensstr. 46
48149 Münster
Germany
E-Mail: mwint_01@uni-muenster.de
Homepage; http://www.uni-muenster.de/MEET
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Prof. Dr. Hans-Dieter Wiemhöfer
Institute for Inorganic and Analytical Chemistry
University of Münster
Corrensstr. 28/30
48149 Münster
Germany
E-mail: hdw@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.ac/wiemho/index.html
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Development of new pyrazole-based lithium salts for battery
applications – Do established basic design concepts really work?
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Abstract
This work is focused on applying structural concepts and basic chemical principles to model
two N-heterocyclic lithium salts, based on trifluoromethyl substituted pyrazolide anions. An
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easily upscalable preparation method without difficult purification steps was also developed.
In a comparative study, the physicochemical properties of the two new lithium salts were
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investigated, particularly the effect of an additional BF3-group at the nitrogen atom. In
comparison to non-substituted lithium pyrazolide, the BF3-addition led to a strong
improvement of thermal and electrochemical stability, ionic conductivity, as well as better Crate and cycling performance. Furthermore, the anodic stability of Al current collectors was
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investigated and compared to commercial lithium salts, namely LiPF6 and lithium
bis((trifluoromethyl)sulfonyl)imide (LiTFSI). Possible mechanisms that lead to the presented
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Keywords
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improvements are discussed.
electrolyte salt anion
electrochemistry
heterocycles
lithium ion battery
basic concepts
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1. Introduction
Lithium ion batteries are featured by high power and energy densities, high efficiency,
longevity and environmental friendliness and thus have found wide application in the area of
consumer electronics and electromobility [1, 2]. A crucial point for next generation lithium
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metal batteries and dual-ion cells for stationary energy storage will be the development of
highly conducting and electrochemically stable lithium salts without anodic dissolution of Al
current collectors [3-8].
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Commercially and commonly used lithium salts still exhibit drawbacks. One of the first
investigated salts is LiClO4, which can violently react in organic solvents or gel-polymer
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electrolytes and eventually can lead to explosion at elevated temperatures. Additionally, it
should not be used at potentials >4.5 V vs. Li/Li+, as it degrades to ClO2, HCl and O2, which
is an explosive mixture [9-13]. The by far most often commercially used salt is LiPF6, which
decomposes in the presence of moisture and reacts with electrolytes at elevated temperatures
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resulting in the formation of POF3, PF5, LiF, HF and other decomposition products [14-16].
Another class are perfluoroalkyl sulfonyl-acid based lithium salts like LiTFSI, lithium
bis((perfluoroethyl)sulfonyl)imide (LiBETI) and LiC(SO2CF3)3 (LiTriTFSM), which show
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high electrochemical stability and solubility in the common organic solvents used in lithium
ion cells [17-19]. However, all these salts are unable to form passivation layers on Al current
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collectors and thus lead to Al degradation by anodic dissolution [20, 21]. Furthermore, they
are still too expensive for commercial applications. In summary, lithium salts with enhanced
electrochemical properties at affordable costs are still on demand.
2. Theory and Concepts
In this context, well-known structural concepts and basic chemical principles are used to
design new anions for lithium salts [22-29].
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2.1. Hard and soft acids and bases (HSAB)
To improve ionic conductivity and solubility of the anions, the principles of the HSAB theory
were used. To understand the effects of hard/hard and hard/soft interaction, Stenger et al.
showed at the example of simple lithium halides, that LiF (hard/hard cation-anion interaction)
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has a solubility of 0.0024 g LiF in 100 g of methanol, whereas the solubility increases from
LiF to LiI (hard/soft interaction) which shows a solubility of 298 g in 100 g methanol [30].
Due to the fact that Li+ ions are hard, the anion must be soft to ensure a high solubility and
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thus high conductivity. Introducing charge distributing and oxidation stable heteroaromatic
rings as substituents into an anion can be considered as an application of this concept. In our
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case, an - according to the HSAB theory - soft pyrazole-based hetero-aromatic structure was
investigated (Fig. 1).
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Place Fig. 1 here
2.2. Negative inductive effect (–I effect)
In addition to the application of HSAB-principles, the introduction of electronegative
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substituents or strong electron withdrawing groups like fluorine or perfluorinated alkyl chains
(Fig. 1) is of great interest for the design of new salts [31]. An induced –I effect on the
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heteroaromatic structure distributes the density of the negative charge over the molecule and
therefore should increase the anodic and thermal stability as well as lower the overall basicity
of the anions (Fig. 1). A comparison with two lithium salts, which were prepared and
investigated by Barthel et al. (Fig. 2), shows the effect of introducing fluorine to the structure.
The anodic stability increased by +0.5 V [32]. We decided to apply trifluoromethyl groups at
the 3- and 5-position of the pyrazole ring to enforce an even stronger -I effect on the ring.
Pyrazole-based salts with an additional substitution at the 4-position were not investigated
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because from a synthetic point of view they are difficult to access. 3,5-Disubstituted pyrazoles
are easily accessible from the corresponding β-diketones [33].
2.3. Modification group and sterically demanding effects
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Place Fig. 2 here
The most important concept is to apply an additional modification at the nitrogen atoms of the
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pyrazole-based ring to alter the electronic of the pyrazole ring, as well as to apply an
additional small sterical demanding effect at the nitrogen atoms (Fig. 1). Therefore, two salts
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with and without a modifying group are compared in this work. This modification should
affect the physicochemical properties in two ways: First, the group should hinder the
coordination of lithium ions direct to the nitrogen atoms and therefore reduce the
nucleophilicity of the pyrazole ring, see Fig. 3. Second, an additional small sterically
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shielding effect of the modification group could prevent direct interaction of the two nitrogen
atoms of the aromatic ring with the electrode surface, which should lead to increased
electrochemical stability. Additionally, the electron withdrawing effect of the BF3-group
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should lead to an increased anodic stability (see 2.2).
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Place Fig. 3 here
For a comparison of non-modified (1) and modified (2) pyrazolides (Fig. 4), we decided to
use a single BF3-group to achieve a simple electron withdrawn and sterical shielding. It can
conveniently be applied by adding BF3-etherate to the prepared lithium pyrazolide (1),
whereas
the
lithium
pyrazolide
itself
is
accessible
via
lithiation
of
3,5-bis(trifluoromethyl)pyrazole with n-butyllithium in diethyl ether at -78 °C. A detailed
preparation protocol of lithium salt 1 & 2 can be found in the experimental section.
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Place Fig. 4 here
3. Experimental
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3,5-Bis(trifluoromethyl)pyrazole was prepared from hexafluoroacetylacetone, hydrazine
hydrate, CHCl3 as solvent and P4O10 as drying agent according to literature [33]. The
corresponding NMR-data for all prepared lithium salts as well as all sample preparations and
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3.1. Synthesis of lithium pyrazolide (1)
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measurement setups can be found in detail the supplementary data.
1.00 eq. (25.0 mmol) of 3,5-bis(trifluoromethyl)pyrazole was introduced into a SCHLENK-type
vessel. The pyrazole was dissolved in 50 mL dry diethyl ether and the clear solution was
cooled to -78 °C in a dry ice acetone bath for 20 min. After cooling to -78 °C, 1.00 eq.
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(25.0 mmol) of 1.6 M n-butyllithium was added dropwise. The reaction mixture was stirred
for additional 30 min, then warmed to room temperature and dried under reduced pressure to
receive unmodified lithium pyrazolide 1 as white salt. Afterwards, the white salt was further
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dried under reduced pressure (turbomolecular pump at 10-7 mbar and room temperature for 5
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days) (Yield: 95%).
3.2. Synthesis of lithium pyrazolide (2)
For additional BF3-modification (lithium salt 2), 1.00 eq. (25.0 mmol) of BF3-etherate was
added to the solution of lithiated pyrazole 1. After warming to room temperature, the solution
was evaporated under vacuum to dryness to obtain BF3-modified lithium salt 2 as a white
powder. Afterwards, the lithium salt was dried according to 3.1. but at a temperature of 120
°C (Yield: 96%).
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4. Result and Discussion
4.1. Thermal properties
At first, the thermal properties of pure non-modified (1) and BF3-modified (2) lithium
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pyrazolide were investigated and compared via differential scanning calorimetry (DSC), see
Fig. 5. The measurements were performed starting at room temperature with a sweep rate of
10 K·min-1 and were stopped shortly after their exothermic decomposition (see supplementary
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data). The decomposition of non-modified lithium pyrazolide (1) starts at 50 °C, whereas the
decomposition of BF3-modified lithium pyrazolide (2) occurs at much higher temperature at
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≈240 °C. The gap between the two exothermic peaks is too large to only explain it with a
simple decomposition of the single molecules. In the case of non-modified pyrazolide (1), we
anticipated a thermally induced E1cB mechanism under elimination of LiF, followed by a
nucleophilic attack of further pyrazolide anions (Fig. 6 B). A comparable reaction with
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imidazolide anions (Fig. 6 A) was published by Kimoto et al. in 1979 [34-36]. In comparison
to that, the attached BF3-group suppressed the thermal induced elimination reaction and
prevented the aromatic rings of the pyrazolide 2 from early reactions and therefore increased
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the thermal stability (Fig. 6 C). Furthermore, the thermal stability of pyrazolide 2 is
considerably higher in comparison to literature known LiBF4 as conducting salt (ϑD = 162 °C)
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and LiPF6 (ϑD < 70 °C in carbonate-type solvents or ϑD = 91-106 °C as pure salt), which
allows applications at higher temperatures [37-40].
Place Fig. 5 here
Place Fig. 6 here
4.2. Electrochemical stability window
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Afterwards, the electrochemical stability was measured separately for the reduction reaction
(working electrode (WE) = Cu) and the oxidation process (WE = Pt) region in propylene
carbonate (PC) as solvent. The oxidation potential limits of the salts 1 & 2 were set by current
densities of < 0.1 mA·cm-2. The detailed measurement setup is explained in the supporting
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information.
Both lithium pyrazolides, non-modified (1) and BF3-modified (2) (Fig. 7 A & B), showed
only two large peaks in the reductive region around 0 V due to the plating/stripping process of
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metallic lithium on the copper working electrode. The coulombic efficiency of the lithium
stripping/plating process was 55.9% for the non-modified salt (1) in PC and 67.5% for the
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BF3-modified salt (2) in PC. Both electrolyte compositions were stable against reduction
under the here shown reductive conditions down to the plating/stripping range of metallic
lithium at ≈0 V. A reason of the here shown behavior may be the formation of stable SEI
layers, which prevent the investigated electrolytes from further reduction. In contrast to that,
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cycling date (see supporting information) against graphite anodes without any additive show
after 20 cycles a continuous capacity fading, which can be explained by the reductive
instability of the perfluorinated side chains on lithiated graphite of the here shown five
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membered heterocyclic anions, which was already discussed by Shkrob et al. [41, 42]. In the
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anodic region, the non-modified pyrazolide (1) showed an oxidation potential of ≈ 4.54 V vs.
Li/Li+ and in the case of BF3-modified pyrazolide (2) the oxidation shifts roughly by +0.6 V
to a higher oxidation potential of 5.19 V vs. Li/Li+(cf. Fig. 7 B). This can be rationalized by
the above mentioned steric shielding effect of the BF3-group as illustrated in Fig. 3 B which
impedes a close contact of the heteroatoms to the electrode and thus increases the anodic
stability. Additionally, the BF3-group withdraws electrons from the aromatic heterocycle
(-I effect) and therefore amplifies the anodic stability of the lithium salt 2 even more.
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Place Fig. 7 here
4.3. Anodic dissolution of aluminum current collectors
To complete the analysis of electrochemical stability, the anodic dissolution of Al current
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collectors in the presence of the lithium pyrazolides was investigated. Furthermore, the results
were compared with those of LiTFSI and LiPF6 under the same conditions. LiTFSI is a
typical non-Al-passivating lithium salt in carbonate type solvents, whereas LiPF6, for
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instance, quickly forms a stable passivation layer on Al current collectors [5, 43, 44].
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For LiTFSI, oxidation processes start at 3.7 V vs. Li/Li+ in Fig. 8 A for all three cycles, with a
steeply increasing current density up to ≈30 mA·cm-2 from the 1st to the 3rd cycle (cf. “plateau
region” Fig. 8 A). For LiPF6 (Fig. 8 B), however, a low current density up to 7 µA·cm-2
(nearly 4300 times lower as with LiTFSI) was observed within the plateau region only in the
3st cycle. No further oxidation was observed up to 5 V in the following cycles (cf. 1nd and 2rd
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cycle in Fig. 8 B). Interestingly, both lithium pyrazolides showed a similar electrochemical
behavior vs. the Al collector as LiPF6 except an additional sharp oxidation peak at 4.5 V for
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pyrazolide 1 (Fig. 8 C). For the subsequent cycles, the current densities below 5 V quickly
became small and almost vanished indicating the growth of the stabilizing passivation layer
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on aluminum (cf. results in Fig. 8 C and D).
Place Fig. 8 here
For the passivation reaction of Al current collector surfaces in the presence of the lithium
pyrazolides, two different mechanisms are suggested. In case of lithium salt 2, the
BF3-modification may act just like the PF6--anion as a fluoride ion source forming a stable
AlF3 passivation layer on the Al current collector, see Fig. 9 A, whereas lithium salt 1
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probably forms hardly soluble poly(pyrazolyl)aluminate complexes with Al3+ on the Al
surface under anodic conditions protecting the surface from further anodic dissolution (Fig.
9 B).
Evidences for the suggested formation of insoluble complexes comes from
prepared several poly(pyrazolyl)aluminate complexes[45-49].
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Place Fig. 9 here
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investigations of Cortés et al., Zheng et al., Snyder et al. and Sambade et al. (Fig. 9 C). They
4.4. Ionic conductivity
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Furthermore, the ionic conductivity of both salts (1 & 2) was measured in six different
concentrations (0.2-1.2 mol·kg-1) via electrochemical impedance spectroscopy (EIS) in a
solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 weight
ratio. This solvent mixture is comparable to the well-known LP30 standard electrolyte
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(1 M LiPF6 in EC/DMC 1:1, σ = 10.4 mS·cm-1 at 25 °C) [37, 50]. The detailed measurement
setup can be found in the supporting information.
Fig. 10 shows salts 1 & 2 at concentrations with the highest ionic conductivities. At 20 °C the
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BF3-modified pyrazolide (2) has an ionic conductivity of 4.8 mS·cm-1 which is only by a
The non-modified
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factor of two lower compared to the LP30 standard with LiPF6.
pyrazolide (1), however, shows a conductivity as low as 0.05 mS·cm-1) and hence two orders
less than the modified pyrazolide. Therefore, the steric blocking effect illustrated in Fig. 3 A
increases the dissociation and the ionic conductivity as predicted. Rolland et al. used a similar
principle for single ion conducting block-co-polymer systems to decouple the lithium ions
from the polymer by sterically demanding BF3-groups [51]. This basic concept inspired us in
the design of pyrazole-based lithium salts.
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Place Fig. 10 here
Due to the high ionic conductivity of 4.8 mS·cm-1, which is close to that of 1 M LiBF4 in
EC/DMC 1:1 (LB30 electrolyte)[52], we decided to investigate “salt species in solution” via
B-NMR- and
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F-NMR-spectroscopy, because fluoroborates tend to undergo ligand
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exchanges, which is literature known from other borate-based lithium salts like the wellknown LiBF4 + LiBOB → 2 LiDFOB equilibrium.[53, 54] According to that, we anticipated
Place Fig. 11 here.
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comparable three successive equilibria in the following Fig. 11 A)-C):
In each subsequent equilibrium, LiBF4 was formed as byproduct and across all three
equilibria, four BF3-modified lithium pyrazolide molecules will be consumed under formation
of tetrakis(pyrazolato)borate (c.f. Fig. 11 C)) and three molecules of LiBF4. Comparison of
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B-NMR- and
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F-NMR spectra of BF3-modified lithium pyrazolide (2)
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the corresponding
and LiBF4 in EC/CMC 1:1 solution (Fig. 12) and integration of the related signal areas yield,
that LiBF4 was formed in a molar ratio of 14 : 86 mol% (LiBF4 to BF3-modified lithium
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pyrazolide (2)). Taking into account, that only a small amount of LiBF4 was formed in
EC/DMC 1:1 solution (Fig. 12) and additionally, that the ionic conductivity of salt 2 in
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EC/DMC 1:1 (Fig. 10) reaches its maximum at lower concentration in comparison to LB30,
the major part of the ionic conductivity can be referred to the BF3-modified lithium pyrazolide
(2).
Place Fig. 12 here.
4.5. Cycling tests
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Additionally, first cycling tests were performed in half cells against LiFePO4 (LFP), a
commonly used cathode material as working electrode; the counter electrode (CE) and a
reference electrode (RE) both consisted of lithium metal (cf. details in Fig. 13, 14 and in the
supporting information).
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An effect of the large difference in ionic conductivity on cell performance can clearly be seen
in the C-rate tests of salts 1 & 2 (Fig. 13 A and B). For the non-modified pyrazolide (Fig.
13 A), the initial specific capacity is 115 mAh·g-1 at a charge rate of 0.1C and decreases
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dramatically with increasing C-rate, reaching 0 mAh·g-1 at 3C. The initial specific capacity is
relatively low for LFP-based cathode material and can be explained with the two order of
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magnitude lower ionic conductivity of the non-modified pyrazolide (Fig. 10). In comparison,
the modified pyrazolide (Fig. 13 B) has an initial specific capacity of 155 mAh·g-1 (also at a
charge rate of 0.1C), which is close to the theoretical value of LFP (≈ 170 mAh·g-1) [55]. For
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Place Fig. 13 here
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a high charge rate of 5C, the capacity is still high with 126 mAh·g-1 [56, 57].
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The low capacity drop of the BF3-modified lithium pyrazolide-based electrolyte (Fig. 13 B)
prompted us to repeat the C-rate investigations with higher C-rates up to 100C (Fig. 14 A).
Even at 75C, a specific capacity of ≈ 45 mAh·g-1 was retained. It is also noteworthy to say,
that the BF3-modified lithium pyrazolide containing electrolyte can even be cycled up to a Crate of 100C against the here shown high power LFP in the half cell configuration.
Afterwards, the cells were cycled at a constant rate of 40C for 120 cycles (Fig. 14 B). The
cells showed a specific capacity of ≈ 70 mAh·g-1 with a coulombic efficiency of 98% with the
exception of some interruptions during cycles 53-57, 63-84 and 103-109. These can be
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explained by formation of lithium metal dendrites under the applied high current rates leading
to sudden short-circuits during the measurements [58-61]. After cycling grey metallic
“lithium sponges” were observed on the anode surfaces and in the separator.
Finally, a new set of cells with identical electrolyte composition was cycled for 200 cycles at
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a C-rate of 1C. Fig. 12 C shows stable charge and discharge capacities with an average of
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147 mAh·g-1 and an average capacity retention of 99.6% for the 200 cycles in Fig. 14 C.
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Place Fig. 14 here
Additional cycling data for BF3-modified lithium pyrazolide containing electrolytes against
graphite, and
Li4Ti5O12 (LTO) as anode materials and against LiNi0.33Mn0.33Co0.33O2
(NMC111) as cathode materials as well as the cycling of a full-cell setup (graphite as anode
4.6. Hydrolytic stability
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LFP as cathode material) can be found in the supplementary data.
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Another important point is the hydrolytic stability of the BF3-modified lithium pyrazolide (2).
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Comparable borate-based lithium salts like LiBF4, LiDFOB and LiBOB, as well as the
commercially used LiPF6 readily hydrolyse in the presence of moisture, whereas the rate
constant of the hydrolytic processes strongly depend on the water concentration.[62-64]
In this context, a provocation experiment was performed. A freshly prepared electrolyte
solution of lithium salt (2) in dry EC/DMC 1:1 was prepared and
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B-NMR and
19
F-NMR
spectra where measured. Subsequently, deionized water was added in a concentration of 5.5
mol·L-1 to allow complete hydrolysis of all borate substituents and after three hours, an
additional set of NMR spectra was taken, see Fig. 15 and Fig. 16.
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Place Fig. 15 here.
Place Fig. 16 here.
It can be observed from the 11B-NMR spectra, that the strong BF3-signal of lithium pyrazolide
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(2) completely disappeared after three hours of the initial water addition, whereas the BF4signal has grown during this time (e.g. Fig. 15), which on the one hand also appeared in the
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F-NMR spectra (see Fig. 16). On the other hand, no signal was observed, which indicates
pyrazolide
(2)
To sum it up, under these conditions, the BF3-modified lithium
completely
hydrolyzed
after
three
hours
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should appear.[64]
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the formation of HF during this provocation experiment. Otherwise, a signal at δ = 153.7 ppm
under
formation
of
3.5-bis(trifluoromethyl)pyrazole and LiBF4 in equilibrium with boric acid (see Fig. 15 middle
and Fig. 16 middle). Furthermore, no HF was formed during this hydrolysis experiment.
Additionally, the formed LiBF4 could further hydrolyse in the presence of excess water under
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formation of HF but under a significant lower rate constant.[62]
However, it should be taken into account that under completely dry conditions, the BF3modified lithium pyrazolide (2) is stable and suitable for lithium ion battery applications,
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performences.
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because all shown cycling tests (e.g. Fig. 13, 14 and supporting information) exhibit good
5. Conclusions
In summary, the combined deliberated application of established design concepts led to a
novel class of modified lithium salts with high chemical, thermal and electrochemical
stability, as well as with high conductivities in standard electrolyte solvents of lithium ion
cells, verified by application in cell with various standard lithium cell materials.
Acknowledgements
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The authors acknowledge the support of the Deutsche Forschungsgemeinschaft (DFG) within
the research initiative “Functional materials and material analysis for high power lithium
batteries (PAK 177)”, as well as the Bundesministerium für Bildung und Forschung (BMBF)
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within the project BenchBatt (03XP0047A).
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[49]
Parkin,
tris(pyrazolyl)hydroaluminate
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Synthesis
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Fig. 1. Structural design of soft pyrazolide anions.
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Fig. 2. Anodic stability vs. Li/Li+ of A) non-fluorinated B) fluorinated lithium
bis-[1,2-benzoldiolato(2-)-O,O’]borate.
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Fig. 3. A) Reduction of nucleophilicity by a sterically demanding BF3-group and
B) anticipated sterical shielding and electron withdrawing effect preventing early oxidation on
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electrode surfaces.
Fig. 4. Sterically non-modified (1) and BF3-modified (2) pyrazole-based lithium salt.
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lithium salt.
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Fig. 5. Thermal stability of neat non-modified (1) and BF3-modified (2) pyrazole-based
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Fig. 6. A) E1cB mechanism of imidazole-based anions, B) mechanism A transferred to a
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pyrazolide anion 1 and C) E1cB mechanism suppressed by a BF3-group.
Fig. 7. Cyclic voltammograms of A) 1.0 mol·kg-1 non-modified pyrazolide in PC and
B) 1.0 mol·kg-1 BF3-modified pyrazolide in PC (v = 1 mV·s-1, ϑ = 22 °C).
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Fig. 8. Cyclic voltammograms of A) 0.5 mol·kg-1 LiTFSI in PC, B) 0.5 mol·kg-1 LiPF6 in PC,
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C) 0.5 mol·kg-1 non-modified lithium pyrazolide (1) in PC and D) 0.5 mol·kg-1 BF3-modified
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lithium pyrazolide (2) in PC (v = 1 mV·s-1, ϑ = 22 °C).
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induced
by the
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Fig. 9. Illustration of A) AlF3 passivation layer formation on the current collector surface
attached
BF3-group
of
pyrazolide 2,
B) formation
of
insoluble
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poly(pyrazolyl)aluminate complexes of pyrazolide 1 on the aluminum surface, anticipated
after Cortés et al., Zheng et al., Snyder et al. and Sambade et al. (examples shown in C) [4549].
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EC/DMC 1:1.
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Fig. 10. Ionic conductivities of non-modified (1) and BF3-modified lithium pyrazolide (2) in
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Fig. 11. Anticipated ligand exchange equilibria between BF3-substituted pyrazolide anions.
A) Formation of Difluorobis(pyrazolato)borate, B) fluorotris(pyrazolato)borate and C)
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tetrakis(pyrazolato)borate. In each equilibrium, LiBF4 was formed as byproduct.
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Fig. 12. Comparison of corresponding
11
B-NMR (upper two spectra) and
19
F-NMR spectra
(lower two spectra) of BF3-modified lithium pyrazolide and LiBF4 in EC/DMC 1:1 solution.
Peak integration results, that LiBF4 was formed in a molar ratio of 14 : 86 mol% (LiBF4 to
BF3-modified lithium pyrazolide (2)).
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Fig. 13. C-rate test of A) 0.8 mol·kg-1 non-modified pyrazolide (1) in EC/DMC 1:1 against
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LFP as working electrode and B) 0.8 mol·kg-1 BF3-modified pyrazolide (2) in EC/DMC 1:1
against LFP as working electrode (separator: 1 layer FS2190, WE = 85% LFP, 10% Carbon,
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5% NaCMC, active material load = 3.5 mg, RE & CE = Li).
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Fig. 14. A) C-rate performance of 0.8 mol·kg-1 BF3-modified pyrazolide (2) in EC/DMC 1:1
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against LFP as cathode material, B) constant current cycling of BF3-modified salt (2) in
EC/DMC 1:1 at a C-rate of 40C for 120 cycles and C) constant current cycling of
salt (2)
in
EC/DMC 1:1
at
AC
C
BF3-modified
a
C-rate
of
1C
for
200 cycles
(separator: 1 layer FS2190, WE = 85% LFP, 10% Carbon black, 5% NaCMC, active material
load = 3.5 mg, RE & CE = Li).
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Fig. 15. 11B-NMR spectra of freshly prepared and dry solution of 1 M salt (2) in EC/DMC 1:1
(top), solution of 1 M salt (2) in EC/DMC 1:1 with 5.5 M H2O after three hours (middle) and
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solution of 1 M LiBF4 in EC/DMC 1:1 (bottom).
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Fig. 16. 19F-NMR spectra of freshly prepared and dry solution of 1 M salt (2) in EC/DMC 1:1
(top), solution of 1 M salt (2) in EC/DMC 1:1 with 5.5 M H2O after three hours (middle) and
AC
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solution of 1 M LiBF4 in EC/DMC 1:1 (bottom).
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