Introduction
Conclusions
The International Symposium on Inorganic and Environmental Materials, Ghent, June 17th-21th 2018
Figure 1. Scheme of a lithium ion microbattery with lithium anode, silica –based ionogel as solid electrolyte layer, and LiCoO2 cathode.
We succeeded to prepare silica-based ionogels with confined IL in an one-pot sol gel process catalyzed by TFA. Homogeneous and transparent gels were obtained in a gelation time lower than 12 h. Three ionogels were tested in Li/LiCoO2 microbatteries. From our very first results, it appears that the created silica network increases the ionic resistivity of the ionogel electrolyte layer, but does not hinder the growth of lithium dendrites.
Results and discussion
The gelation is significantly accelerated by the addition of TFA and by increasing the water content for a fixed molar ratio nIL/nTMOS of 3 (Fig.
3A). The added water amount is a crucial parameter since more than
two methoxy groups per TMOS molecule have to be hydrolysed and condensed to obtain a 3-dimensional silica network. The obtained ionogels are transparent (Fig. 3B). A phase separation is avoided by keeping the water amount under the miscibility threshold (here nH2O/nTMOS =2.3). With this water amount, gelation time of less than 12 h are achieved at high TFA amounts.
Ionogel preparation
Ionogels were prepared according to the literature [2] under ambient conditions with TFA. The lithium salt bis(trifluorosulfonyl)imide lithium salt (LiTFSI) was dissolved in the IL (N-Propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13-FSI)) to prepare the electrolyte solution. TMOS was used as a silicon precursor. The molar ratios of IL/TMOS was set to 3 or 5; the molar ratio of TFA/TMOS was between 0.1 and 0.5. Gelation and ageing were conducted for 48 h at RT. The gels were dried over 3 days with temperature increasing incrementally to 60 °C while stepping the pressure down to 1 mbar.
Characterization
The time of gelation was determined using the tilting method. Galvanostatic cycling of LiCoO2/ionogel/Li° cells was performed between 3.0 and 4.2 V at C/10.
Silica-based thin film solid-state electrolytes
for Lithium-ion microbatteries
K. Hartmann1, C. Calberg1, D. Liquet² and B. Heinrichs1
1. University of Liège, Department of Chemical Engineering – NCE, Belgium http://www.chemeng.uliege.be – khartmann@uliege.be
2. Prayon S.A., Rue Joseph Wauters 144, 4480 Engis, Belgium - http://www.prayon.com – liquetd@yahoo.fr
Experimental Section
Figure 3. (A) Gelation times in hours for the ionogel synthesis at RT. The molar ratio of IL and
TMOS was 3 . Two different water amounts (molar ration
H2O/TMOS= 2 (blue squares) and 2.3 (grey dots) and four
different TFA amounts were tested. The gelation time can be
significantly decreased by increasing water amount and increasing the catalyst amount. (B) Picture of the dried ionogel.
A B
Figure 2. Scheme of the steps of the ionogel preparation via a one-pot sol-gel process.
References [1] P.-E. Delannoy, . Power Sour., 2015, 274, 1085-1090.
0 0.1 0.2 0.3 0.4 0.5 0.6 0 20 40 60 80 100 H2O/TMOS=2.3 , T=25°C H2O/TMOS=2, T=25°C
molar ratio nTFA/nTMOS
tg e l [h ] H2O/TMOS= 2.3, T=25°C H2O/TMOS=2, T=25°C
The emerging market of the Internet of Things, smart objects, wearables and others increases the demand for micro energy sources. Rechargeable lithium-ion batteries are a well-known technology for energy storage. However, safety issues and high production costs constrain progress. Electrolyte solutions based on ionic liquids (ILs) with dissolved lithium salts can be confined into inorganic porous networks forming so-called ionogels, which are investigated as solid electrolytes [1]. Ionogels combine low hazard and good ionic conductivity. However, the growth of lithium dendrites may be observed during cycling, which reduce battery lifetime. In this project, we try to prepare a silica-ionogel to prevent dendritic growth by mechanical hindrance. The ionogel composition was studied to obtain a fast gelation and the correlation between the physical properties of the silica matrix and the electrochemical performances of the ionogel was evaluated.
0 50 100 150 200 250 2.8 3 3.2 3.4 3.6 3.8 4 4.2 Capacity [mAh/g] P o te n ti a l [V ] 0 50 100 150 200 250 2.8 3.3 3.8 Capacity [mA/g] P o te n ti a l [V ] 0 50 100 150 200 250 2.8 3 3.2 3.4 3.6 3.8 4 4.2 Capacity [mAh/g] P o te n ti a l [V
] Figure 4. First (blue) and fifth (grey)
dis-/charge curve from GCPL measurements at C/10 of the ionogel electrolyte with molar
ratios IL/TMOS=3 and TFA/TMOS=0.3 (A), IL/TMOS=5 and TFA/TMOS=0.3 (B), and
IL/TMOS=3 and TFA/TMOS=0.5 (C). Ionogel (B) was measured in an open cell. Dendrites grow
after a few cycles in all cases.
Figure 5. SEM image of the dried silica matrix after extraction of the IL-based solution obtained from the ionogel C.
The ionogel A (Fig.4A) reaches the theoretical dis-/charge capacity of 120 mAh/g in the first cycle, but dendrites appear after a few cycles and the discharge capacity slowly decreases. On the contrary, the discharge capacity of the ionogel C is very low. In this case, the ionic resistivity might be very high due to an enhanced TMOS condensation resulting from a higher catalyst amount. The increase of the IL amount (Fig. 4B) leads to soft gels, which require characterization in open cells. Looking at the charge capacities, these three gels do not inhibit dendritic growth.
B A
C
For ionogel C, a SEM image of the silica network after extraction supports the high resistivity of the ionogel. Indeed no macropores are visible at a magnification of 25000x.
