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

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.