Name: Synthesis of Ionic Liquid Based Electrolytes, Assembly of Li-ion Batteries, and Measurements of Performance at High Temperature (Video) | JoVE Journal
Uploaded: 2016-12-20T13:00:00.000+00:00
Duration: 11 min 4 s
Description: Learn to synthesize phosphonium-based ionic liquid electrolytes and assemble non-flammable lithium-ion batteries for high-temperature applications. This method enables safe energy storage with thermally stable materials.

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This article was supported in part by BU and by the Advanced Energy Consortium:

1.Synthesis of Mono- and Di-phosphonium Ionic Liquids Paired with Chloride (Cl) and Bis(trifluoromethane)sulfonimide (TFSI) Anions
NOTE: The procedure for the mono-phosphonium ionic liquid possessing three hexyl and one decyl alkyl chain surrounding the phosphonium cation is described, and this ionic liquid is abbreviated as mono-HexC10Cl. The same procedure is repeated using 1,10-dichlorodecane to obtain the di-phosphonium ionic liquid in high yield, and this ionic liquid is abbreviated as di-HexC10Cl.
<ol>
	<li>While in a glove box under argon, weigh out trihexylphosphine (8.3 g, 29 mmol) using a glass pipet, and dispense into a heavy wall pressure vessel. Next, add 1-chlorodecane (5.22 g, 29.6 mmol) using a glass pipet to the same vessel. Cap the vessel containing the mixture with a PTFE bushing.</li>
	<li>Heat the resulting mixture under argon to 140 &#176;C for 24 hr while mixing to obtain the mono-HexC10Cl. The mixture will become viscous.</li>
	<li>Place the mixture under high vacuum at 140 &#176;C while stirring to remove any remaining volatile starting materials to obtain the crude mono-HexC10Cl.
	<ol>
		<li>To purify, extract the mono-HexC10Cl from the crude mixture using approximately 10 ml of a 1:1 dichloromethane (DCM) to saturated sodium chloride solution (brine) in a 250 ml separatory funnel. Collect the DCM phase. Repeat the extraction process three times.</li>
		<li>Combine the 15 ml of collected DCM solutions containing the product, and evaporate the solvent using a rotary evaporator to obtain the mono-HexC10Cl product.</li>
	</ol>
	</li>
	<li>Dissolve the mono-HexC10Cl (7.75 g, 16.74 mmol) in 10 ml of DCM and add LiTFSI (6.25 g, 21.76 mmol), pre-dissolved in 10 ml of deionized water. Cap the resulting mixture and stir it at room temperature for 24 hours.</li>
	<li>Extract the mono-HexC10TFSI from the mixture using a 250 ml separatory funnel filled with approximately 20 ml of DCM. Repeat the extraction process three times. Combine the DCM solutions.</li>
	<li>Add 1-2 drops of 1 N AgNO3 solution to 1 ml of&#160;the DCM phase to confirm the complete elimination of chloride anions from the organic phase. A white precipitate will be produced if chloride anions remain in solution. Repeat the extraction step until no white precipitate is produced.</li>
	<li>Add 1 g of anhydrous MgSO4 to the DCM solution, stir the mixture, and then decant the dried DCM solution. Next, evaporate the solvent by rotary evaporation. The yield is typically greater than 98%.</li>
	<li>Repeat the same procedure using 1,10-dichlorodecane to obtain the two di-phosphonium ionic liquids, di-HexC10Cl and di-HexC10TFSI, in high yield.</li>
	<li>Characterize the ionic liquids using 1H, 13C, and 19F NMR in deuterated-chloroform (shift 7.24) and submit the samples for elemental analysis and mass spectrometry analysis.</li>
</ol>
2. Characterization of the Ionic Liquids
<ol>
	<li>Differential scanning calorimetry (DSC)
	<ol>
		<li>Weigh out 5 to 10 mg of the ionic liquid (record the actual mass) and add it to the center of an aluminum sample pan, which is subsequently hermetically sealed. Be sure to complete this step efficiently as the ionic liquids are hydroscopic and the weight will change if left to stand.</li>
		<li>Load the sample pan and an unloaded (reference) pan into the differential scanning calorimeter. Be sure to place the sample and reference pan in the appropriate location as determined by the specific DSC used.</li>
		<li>Program a temperature ramp and cooling cycle: 1) heat from -70 &#176;C to 200 &#176;C at a heating rate of 10 &#176;C/min, 2) cool to -70 &#176;C at a cooling rate of 5 &#176;C/min, and 3) repeat the heat-cool cycling three times.</li>
		<li>By analyzing the thermal trace, determine the melting point (Tm), crystallization (Tc) and glass transition temperatures (Tg) (if applicable).</li>
	</ol>
	</li>
	<li>Thermal gravimetric analysis (TGA)
	<ol>
		<li>Clean and tare the platinum pan on the movable arm of the TGA. Add 5 to 10 mg of the ionic liquid on the pan. Only touch the pan using tweezers.</li>
		<li>Heat the sample from 20 to 500 &#176;C at a heating rate of 10 &#176;C/min.</li>
		<li>Identify the decomposition temperature where 10% of the original sample weight is lost. For long-term stability studies, heat the sample at a set temperature for a prolonged time and monitor the loss of weight.</li>
	</ol>
	</li>
	<li>Viscosity measurements
	<ol>
		<li>With a glass pipette, place 1 ml of ionic liquid on the Peltier stage of a controlled strain rheometer. Make sure the aluminum plate is fully covered with the ionic liquid.</li>
		<li>Use a 20 mm diameter parallel aluminum plate (or cone) and set the gap between the aluminum plate and the top surface of the sample to be 1.0 - 2.0 mm in all of the runs.</li>
		<li>To minimize the effect of moisture in the air, perform the measurements in a glove bag filled with nitrogen gas.</li>
		<li>Prior to each test, pre-shear the sample at a shear rate of 100 Hz for 10 sec to eliminate any physical memory of the sample, follow with a 15 min equilibrium step in order for the sample to reach a steady state condition.</li>
		<li>Determine the linear viscoelastic region (LVR) via an oscillatory strain sweep at a fixed frequency (1 Hz) with a strain amplitude from 0.1 to 10%.</li>
		<li>Select a strain that lies in the LVR and perform the oscillatory frequency sweep from 0.1 to 10 Hz. Determine the complex viscosity at a particular frequency and strain.</li>
		<li>Perform an oscillatory temperature sweep controlled by the instrument software from 10 &#176;C to 95 &#176;C with increments of 5 &#176;C and a 1 min equilibrium at each temperature. Define strain and frequency, for example, to be 1.0% and 1 Hz, respectively. Complex viscosities at different temperatures are read out.</li>
	</ol>
	</li>
	<li>Conductivity measurements
	<ol>
		<li>Dry the ionic liquid at 100 &#176;C under high vacuum for 12 hr to remove trace amounts of moisture before testing.</li>
		<li>In a glove box under an argon atmosphere, load approximately 4 ml of the sample in a test tube, be sure to add enough sample to immerse the sensing tape of the conductivity probe completely.</li>
		<li>Use a heating block to control the temperature and maintain the stirring during the measurement to maintain homogeneity.</li>
		<li>Read the conductivity at each temperature after a 30 min equilibration time.</li>
	</ol>
	</li>
	<li>Cyclic voltammetry (CV)
	<ol>
		<li>Assemble a lithium/lithium/platinum three-electrode system in the glove box under an argon atmosphere.</li>
		<li>Charge the vessel with the ionic liquid and make sure all the electrodes are immersed in the ionic liquid. Seal the vessel under argon.</li>
		<li>Equilibrate the vessel at the desired temperature for 20 min. Sweep the potential rate at 1 mV/sec between &#8722;0.2 V and 6.5 V versus Li+/Li.</li>
	</ol>
	</li>
</ol>
3. Preparation of the Electrolytes
<ol>
	<li>Dry the ionic liquid under high vacuum at 80 &#176;C overnight with rigorous stirring to ensure removal of trace amounts of water.</li>
	<li>Dry the LiTFSI at 70 &#176;C for three days in a vacuum oven.</li>
	<li>Transfer the anhydrous ionic liquid and LiTFSI salt to the glove box.</li>
	<li>Add the ionic liquid (e.g., mono-HexC10TFSI, 4.50 g, 6.4 mmol) and LiTFSI (1.83 g, 6.4 mmol) to an oven-dried flask containing a stir bar. Stir the mixture overnight until it is homogeneous to obtain a concentration of 1.6 M for the electrolyte.</li>
</ol>
4. Fabrication of the Lithium Ion Coin Cell Battery
<ol>
	<li>In the glove box under argon atmosphere, place one spring and one stainless steel disc in the bottom cap of the coin cell. Place a 12.7 mm diameter LiCoO2 electrode (24 mg) in the stainless steel disc.</li>
	<li>Soak two pieces of the separators (porous polypropylene membranes) in the above-prepared ionic liquid electrolyte at 60 &#176;C on a hot plate for 15 min.</li>
	<li>Add the ionic liquid electrolyte to the surface of the LiCoO2 cathode until the material is fully covered with electrolyte (&#8776; 0.5 ml).</li>
	<li>Place the separators soaked in the electrolyte in the center of the coin cell. Then add a few more drops of ionic liquid electrolyte (a few microliters) onto the separators.</li>
	<li>Cut a piece of lithium metal with a diameter of 12.7 mm in the glove box. Place the lithium metal on top of the separators.</li>
	<li>Cap the coin cell and seal it with a crimper in the glove box.</li>
	<li>Transfer the coin cell out of the glove box and rest the cell for 12 hr prior to initiating the battery/electrochemical tests.</li>
</ol>
5. Performance of the Battery at 100 &#176;C
<ol>
	<li>Place the coin cell in an oven operating at 100 &#176;C, which has a small hole in the back wall where the cables from the electrochemical testing station have been threaded. Connect the coin cell to the electrochemical testing station.</li>
	<li>Leave the cell at 100 &#176;C for 30 min to equilibrate to the temperature.</li>
	<li>Select galvanostatic charge-discharge cycling on the electrochemical testing station. Set the cycle number to 500.</li>
	<li>Set the charge current to 500 &#181;A and the voltage upper limit to 4.2 V. Set a rest time of 60 sec at 0 V after each charge.</li>
	<li>Set the discharge current to 500 &#181;A and the voltage lower limit to 3.0 V. Set a rest time of 60 sec at 0 V after each discharge.</li>
	<li>Start the charge-discharge cycling at a current of 500 &#181;A between 3.0 V to 4.2 V using the software. Evaluate the charge output against time.</li>
</ol>

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</ol>

The authors declare that they have no competing financial interests.

Our approach to develop non-flammable and high temperature functional Li-ion batteries involves the synthesis of new ionic liquid electrolytes and their subsequent evaluation in prototypical coin cells. Specifically, mono-HexC10TFSI and di-HexC10TFSI based electrolytes were tested in a coin cell possessing a Li metal anode and LiCoO2 cathode. The critical steps within this approach are to: 1) identify the lead electrolyte according to a set of design specifications; 2) maintain dryness and ensure water does no...

Li-ion batteries are devices that transform energy between electrical energy and chemical energy and provide a convenient means to store and to deliver energy on demand and on-the-go. Today, Li-ion batteries dominate the portable electronics market because of their high energy density and re-chargeability, and are of interest for large-scale and specialty applications, such as down-hole drilling and automotive.1-5 Batteries are composed of four primary components: cathode, anode, separator, and electrolyte. While the chemistry of the two electrodes dictates the theoretical energy density of the battery, the safety and working temperature are mainly limited by the electrolyte material.6-9 Carbonate based organic solvent electrolytes (e.g., dimethyl carbonate (DMC) and ethylene carbonate (EC)) are widely used in Li-ion batteries due to their low viscosity, high conductivity, and high lithium salt solubility. Moreover, certain combinations of the carbonate solvents (DMC/EC) also form a stable solid electrolyte interface (SEI), thereby preventing degradation reactions between the electrolyte and the electrode, and extending battery life. However, carbonate solvents suffer from low boiling points and flash points, limiting the operation temperature of Li-ion batteries to below 55 &#176;C, with potentially severe safety issues when there is a short-circuit.10,11
Ionic liquids are a class of salts that have melting temperatures below 100 &#176;C.12 In contrast to typical inorganic salts, ionic liquids possess a wide liquid range and can be liquid at room temperature. Ionic liquids are composed of one or multiple organic cationic centers, such as imidazolium, phosphonium, pyridinium, or ammonium and paired with an inorganic or organic anion, such as methansulfonate, hexafluorophosphate, or halide.13,14 The wide variety of possible cation and anion combinations allows for a large number of compositions with tunable properties. In addition, the strong ionic interactions within ionic liquids result in negligible vapor pressure, non-flammability, and high thermal and electrochemical stability.15,16
Replacing conventional electrolytes with ionic liquids is one solution that addresses the inherent safety issues in current Li-ion batteries, and could enable high temperature applications.17-27 To illustrate the general synthetic and material processing methods utilized to construct lithium ion batteries containing ionic liquids for high temperature applications, we describe the synthesis, thermal properties, and electrochemical characterization of mono- and di-phosphonium ionic liquids paired with either the chloride (Cl) or bis(trifluoromethane)sulfonimide (TFSI) anion. Different concentrations of lithium bis(trifluoromethane)sulfonimide (LiTFSI) are subsequently added to the phosphonium ionic liquids to give electrolytes. Based on the performance of the phosphonium TFSI electrolytes with added LiTFSI compared to the chloride analogs, a coin cell is constructed with either the mono- or di-phosphonium TFSI electrolytes along with a Li metal anode and LiCoO2 cathode. Finally, battery performance is evaluated at 100 &#176;C for the two different coin cell batteries. The detailed procedures, the challenges in execution, and the insights gained from performing these experiments are described below.

<table><tbody><tr><td>Silicone oil</td><td>Sigma-Aldrich</td><td>85409</td><td></td></tr><tr><td>Potassium hydroxide</td><td>Sigma-Aldrich</td><td>221473</td><td>Corrosive</td></tr><tr><td>Rotary evaporator</td><td>Buchi</td><td>R-124</td><td></td></tr><tr><td>High-vacuum pump</td><td>Welch</td><td>8907</td><td></td></tr><tr><td>Nitrogen, ultra high purity</td><td>Airgas</td><td>NI UHP300</td><td>Compressed gas</td></tr><tr><td>Tetrahydrofuran, stabilized with BHT</td><td>Pharmco-Aaper</td><td>346000</td><td>Flammable. Dried before use.</td></tr><tr><td>Dichloromethane</td><td>Pharmco-Aaper</td><td>313000</td><td>Flammable, toxic.</td></tr><tr><td>Separatory funnel (1 L)</td><td>Fisher Scientific</td><td>13-678-606</td><td></td></tr><tr><td>Sodium sulfate</td><td>Sigma-Aldrich</td><td>239313</td><td></td></tr><tr><td>Ethanol, absolute</td><td>Pharmco-Aaper</td><td>111USP200</td><td>Flammable, toxic.</td></tr><tr><td>Buchner funnel</td><td>Fisher Scientific</td><td>FB-966-F</td><td></td></tr><tr><td>Methanol</td><td>Pharmco-Aaper</td><td>339000ACS</td><td>Flammable, toxic.</td></tr><tr><td>Triethylamine (anhydrous)</td><td>Sigma-Aldrich</td><td>471283</td><td>Toxic, flammable, harmful to environment</td></tr><tr><td>Glass syringe</td><td>Hamilton Company</td><td>1700-series</td><td></td></tr><tr><td>Deuterated chloroform</td><td>Cambridge Isotopes Laboratories, Inc.</td><td>DLM-29-10</td><td>Toxic</td></tr><tr><td>Nuclear magnetic resonance instrument</td><td>Varian</td><td>V400</td><td></td></tr><tr><td>Hydrogen</td><td>Airgas</td><td>HY HP300</td><td>Highly flammable.</td></tr><tr><td>Hexanes</td><td>Pharmco-Aaper</td><td>359000ACS</td><td>Toxic, flammable.</td></tr><tr><td>Differential scanning calorimeter</td><td>TA Instruments</td><td>Q100</td><td></td></tr><tr><td>&lt;em&gt;N,N&lt;/em&gt;-dimethylformamide</td><td>Sigma-Aldrich</td><td>227056</td><td>Toxic, flammable.</td></tr><tr><td>Trihexylphosphone</td><td>TCI America</td><td></td><td>Toxic, flammable.</td></tr><tr><td>1-Chlorodecane</td><td>Sigma-Aldrich</td><td></td><td>Toxic, flammable.</td></tr><tr><td>Bis(trifluoromethane)sulfonimide lithium salt</td><td>Sigma-Aldrich</td><td></td><td>Hydrophilic</td></tr><tr><td>1,10-dichlorodecane</td><td>Sigma-Aldrich</td><td></td><td>Toxic, flammable.</td></tr><tr><td>Thermal Gravemetric Analysis (TGA)</td><td>TA Q50</td><td>TA instruments</td><td></td></tr><tr><td>Differential scanning calorimeter (DSC)</td><td>TA Q100</td><td>TA instruments</td><td></td></tr><tr><td>Controlled Strain Rheometer</td><td>AR 1000&amp;nbsp;</td><td></td><td></td></tr><tr><td>Conductivity Meter&amp;nbsp;</td><td>Consort</td><td>K912</td><td>4-electrode cell</td></tr><tr><td>Potentiostat/Galvanostat</td><td>Princeton Applied Research&amp;nbsp;</td><td>VersaStat MC4&amp;nbsp;</td><td>Electrochemical testing</td></tr><tr><td>Separators</td><td>Celgard&amp;nbsp;</td><td>C480&amp;nbsp;</td><td>polypropylene/polyethylene</td></tr><tr><td>CR2032 coin cells</td><td>MTI Corp.</td><td>EQ-CR2032-CASE</td><td></td></tr><tr><td>LiCoO&lt;sub&gt;2&lt;/sub&gt; electrode&amp;nbsp;</td><td>MTI Corp.</td><td>EQ-CR2032</td><td>Cathode material</td></tr><tr><td>lithium metal&amp;nbsp;</td><td>Alfa Aesar</td><td>10769</td><td>Anode Material</td></tr><tr><td>Stainless Steel Spacer</td><td>MTI Corp.</td><td>EQ-CR20-Spacer304-02</td><td>15.5 mm Dia x 0.2 mm</td></tr><tr><td>Wave Spring</td><td>MTI Corp.</td><td>EQ-CR20WS-Spring304</td><td></td></tr><tr><td>Electric Coin Cell Crimping Machine</td><td>MTI Corp.</td><td>MSK-160D</td><td></td></tr><tr><td>Glove box</td><td>Mbraun</td><td></td><td>Water free, oxygen free operation</td></tr></tbody></table>

synthesis of ionic liquid based electrolytes, assembly of li-ion batteries, and measurements of performance at high temperature

Synthesis of Ionic Liquid Based Electrolytes, Assembly of Li-ion Batteries, and Measurements of Performance at High Temperature

The chemical instability of the traditional electrolyte remains a safety issue in widely used energy storage devices such as Li-ion batteries. Li-ion batteries for use in devices operating at elevated temperatures require thermally stable and non-flammable electrolytes. Ionic liquids (ILs), which are non-flammable, non-volatile, thermally stable molten salts, are an ideal replacement for flammable and low boiling point organic solvent electrolytes currently used today. We herein describe the procedures to: 1) synthesize mono- and di-phosphonium ionic liquids paired with chloride or bis(trifluoromethane)sulfonimide (TFSI) anions; 2) measure the thermal properties and stability of these ionic liquids by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA); 3) measure the electrochemical properties of the ionic liquids by cyclic voltammetry (CV); 4) prepare electrolytes containing lithium bis(trifluoromethane)sulfonamide; 5) measure the conductivity of the electrolytes as a function of temperature; 6) assemble a coin cell battery with two of the electrolytes along with a Li metal anode and LiCoO2 cathode; and 7) evaluate battery performance at 100 &#176;C. We additionally describe the challenges in execution as well as the insights gained from performing these experiments.

The ionic liquids, mono-HexC10Cl and di-HexC10Cl, were prepared via a nucleophilic reaction, and a subsequent halide exchange reaction gave the mono-HexC10TFSI and di-HexC10TFSI ionic liquids, respectively (Figure 1A).14 All four ionic liquids were colorless and slightly viscous liquids (Figure 1B). A representative 1H NMR of the mono-HexC10TFSI ionic liquid is shown in Figure 1C, and along with mass spectrometry and...

Here, we describe protocols to prepare phosphonium-based ionic liquid and lithium bis(trifluoromethane)sulfonimide salt electrolytes, and assemble a non-flammable and high temperature functioning lithium-ion coin cell battery.

The chemical instability of the traditional electrolyte remains a safety issue in widely used energy storage devices such as Li-ion batteries. Li-ion ...

The overall goal of this procedure is to assemble a high-temperature functioning lithium ion battery that utilizes a non-flammable, thermally stable phosphonium-based ion liquid electrolyte containing lithium bis trifluoromethane sulfonamide. This method addresses key challenges in a high temperature energy storage field, specifically the safety issues with the use of conventional electrolytes in lithium ion batteries. The main advantage of this technique is that it provides an approach to synthesize and characterize thermally stable electrolytes and to construct high temperature functioning lithium ion batteries.Though this method provides insight into rational design bionic liquid electrolytes optimized for lithium ion batteries, it can also be applied to super capacitors and other energy storage devices. Generally, individuals new to this method struggle because the measurements will be taken under inert atmosphere with extremely dry materials. Visual demonstration of this method is critical as introduction of bionic liquids in the batteries is challenging due to their high viscosities.In an argon-filled glove box, use glass pipettes to dispense 8.3 grams of trihexyl phosphene and 5.22 grams of 1-chlorodecane into an oven-dried heavy wall pressure vessel containing a stir bar. Cap the vessel with a PTFE bushing and stir the mixture under argon at 140 degrees Celsius for 24 hours. Remove volatiles under vacuum at 140 degrees to obtain crude mono-hexe-10-chloride.Extract the mono-hexe-10-chloride into DCM three times with ten milliliter portions of a one to one mixture of DCM and brine. Remove volatiles from the DCM phase to isolate the mono-hexe-10-chloride product. Next, dissolve 6.25 grams of lithium TFSI in ten milliliters of deionized water.Then, dissolve 7.75 grams of mono-hexe-10-chloride in ten milliliters of DCM. Add the TFSI solution. Cap the container and stir the mixture at room temperature for 24 hours to obtain crude mono-hexe-10-TFSI.Extract the mono-hexe-10-TFSI into 20 milliliters of DCM three times. Test for the presence of chloride by adding one to two drops of one normal silver nitrate solution to one milliliter of the DCM phase. Repeat the extraction if silver chloride is observed as a white precipitate.Then, dry the mono-hexe-10-TFSI solution with one gram of anhydrous magnesium sulfate. Decant the dried solution and remove the solvent with a rotary evaporator to isolate the product. To synthesize the di-phosphonium analogues follow this procedure using 1, 10-dichlorodecane and appropriately adjust its molar ratios.To perform viscosity measurements, set up a controlled strain rheometer in a nitrogen gas-filled glove bag. Place one milliliter of the ionic liquid onto the Peltier stage of the rheometer ensuring that the aluminium plate is fully covered. Set the gap between the sample and a parallel 20 millimeter diameter aluminium plate to be one to two millimeters in all of the test runs.Pre-shear the sample at a shear rate of 100 Hertz for ten seconds and then perform a 15 minute equilibrium step. Then, determine the linear visco-elastic region or LVR with an oscillatory strain sweep at a fixed frequency of one Hertz with a strain amplitude from 0.1 to ten percent. Select a strain within the LVR and perform an oscillatory frequency sweep from 0.1 to ten Hertz.Determine the complex viscosity at a chosen frequency and strain. Then, at that frequency and strain, perform an oscillatory temperature sweep from ten to 95 degrees Celsius at five degree increments with one minute of equilibrium at each temperature. To measure the sample conductivity, first dry the ionic liquid at 100 degrees Celsius under vacuum for 12 hours.Place about four milliliters of the ionic liquid in a test tube. Insert the conductivity probe and add more sample as needed to cover the probe sensor completely. Heat the sample to the first measurement temperature while stirring.Equilibrate the sample at that temperature for 30 minutes and then take the conductivity reading. Repeat this process for each temperature point. Next, set up a lithium-lithium platinum three electrode cyclic voltammetry assembly in the glove box.Place the ionic liquid in the CV chamber, immerse the electrodes in the liquid and ensure that the chamber is sealed. Equilibrate the sample at the first measurement temperature for 20 minutes and then sweep the potential at a rate of one millivolt per second between negative 0.2 volts and 6.5 volts. First is the lithium ion lithium reference potential.Repeat this measurement for each desired temperature. Prior to beginning the battery fabrication, dry the lithium TFSI for three days in a vacuum oven at 70 degrees Celsius. Dry the ionic liquid to be tested under high vacuum at 100 degrees Celsius for 24 hours while stirring vigorously.Bring both dry liquids and an oven-dried flask and stir bar into a glove box. Place 6.4 millimoles each of the ionic liquid and lithium TFSI in the flask. Stir the mixture overnight to obtain a homogenous mixture with a 1.6 molar electrolyte concentration.Dry the coin cell hardware components and a 12.7 millimeter diameter lithium cobalt oxide electrode at 70 degrees Celsius overnight before transferring them to the glove box. Place one spring and stainless steel disk in the coin cell bottom cap. Place the electrode in the steel disk.Heat the electrolyte mixture to 60 degrees Celsius on a hot plate and soak two polypropylene membrane separators in the mixture while stirring for 15 minutes. Cover the surface of the lithium cobalt oxide cathode with about 0.5 milliliters of the electrolyte mixture and then place both separators in the center of the coin cell. Place a few drops of the electrolyte mixture on top of the separators, ensuring that all crevices in the cell are filled.Heat the electrolyte-covered cathode to 60 degrees Celsius for 15 minutes. Cut out a 12.7 millimeter diameter piece of 0.75 millimeter thick lithium metal and place the metal onto the electrolyte-soaked separators. Cap the coin cell and seal it with a crimper.Remove the cell from the glove box and allow the cell to sit for 12 hours. To test the battery performance at high temperatures, thread cables from an electrochemical testing station to the back wall of an oven. Connect the coin cell to the testing station in the oven and leave the cell in the oven for 30 minutes to equilibrate to the oven temperature.At the testing station, select galvanostatic charge-discharge cycling and set the cycle number to 500. Set the charge and discharge parameters and rest times and then start the charge-discharge cycling to evaluate charge output over time. Four phosphonium based ionic liquids were evaluated for use in lithium ion batteries at high temperatures.Thermogravimetric analysis showed that the decomposition temperatures ranged from 340 to 385 degrees Celsius. The TFSI-based ionic liquids were less viscous making them better suited for completely filling pores and crevices in the battery components. Cyclic voltammetry measurements of mono and di-hexC10TFSI with lithium-TFSI in the presence of the lithium cobalt oxide cathode showed the expected lithium cobalt oxide redox peaks.For both liquids, the current increased significantly with temperature. The mono-hexC10TFSI mixture showed a higher current response to all temperatures. Charge-discharge cycling tests of coin cell batteries containing mono-hexC10TFSI at 100 degrees Celsius showed the battery capacity decay was rapid at low concentrations of lithium TFSI.At concentrations nearer the saturation point, the capacity decay was reduced to about ten percent after 20 cycles. An extended galvanostatic charge-discharge cycling test of a coin cell battery containing 1.6 molar TFSI and mono-hexC10TFSI was performed at 100 degrees Celsius. The battery capacity was initially about 135 milliampere hours per gram.After one month, the capacity had decreased to 70 milliampere hours per gram. While attempting this procedure, it's important to thoroughly dry all components of the battery such as the electrolyte and electrodes prior to cell assembly. Moreover, when assembling the coin cell, care must be taken to coat the cathode and separator with the electrolytes.After watching this video, you should be able to synthesize ionic liquid electrolytes and fabricate lithium ion batteries. Care must be taken when handling the ionic liquids since their high viscosities pose additional handling challenges compared to standard electrolytes.

synthesis-ionic-liquid-based-electrolytes-assembly-li-ion-batteries

Research

JoVE Journal

Engineering

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Learn to synthesize phosphonium-based ionic liquid electrolytes and assemble non-flammable lithium-ion batteries for high-temperature applications. This method enables safe energy storage with thermally stable materials.

Synthesis of Ionic Liquid Based Electrolytes, Assembly of Li-ion Batteries, and Measurements of Performance at High Temperature

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