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

Summary

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.

Abstract

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 °C. We additionally describe the challenges in execution as well as the insights gained from performing these experiments.

Introduction

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 °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 °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 °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.

Protocol

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.

1. 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.
2. Heat the resulting mixture under argon to 140 °C for 24 hr while mixing to obtain the mono-HexC10Cl. The mixture will become viscous.
3. Place the mixture under high vacuum at 140 °C while stirring to remove any remaining volatile starting materials to obtain the crude mono-HexC10Cl.
   1. 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.
   2. 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.
4. 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.
5. 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.
6. Add 1-2 drops of 1 N AgNO₃ solution to 1 ml of 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.
7. 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%.
8. Repeat the same procedure using 1,10-dichlorodecane to obtain the two di-phosphonium ionic liquids, di-HexC10Cl and di-HexC10TFSI, in high yield.
9. 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.

2. Characterization of the Ionic Liquids

1. Differential scanning calorimetry (DSC)
   1. 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.
   2. 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.
   3. Program a temperature ramp and cooling cycle: 1) heat from -70 °C to 200 °C at a heating rate of 10 °C/min, 2) cool to -70 °C at a cooling rate of 5 °C/min, and 3) repeat the heat-cool cycling three times.
   4. By analyzing the thermal trace, determine the melting point (T_m), crystallization (T_c) and glass transition temperatures (T_g) (if applicable).
2. Thermal gravimetric analysis (TGA)
   1. 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.
   2. Heat the sample from 20 to 500 °C at a heating rate of 10 °C/min.
   3. 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.
3. Viscosity measurements
   1. 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.
   2. 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.
   3. To minimize the effect of moisture in the air, perform the measurements in a glove bag filled with nitrogen gas.
   4. 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.
   5. 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%.
   6. 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.
   7. Perform an oscillatory temperature sweep controlled by the instrument software from 10 °C to 95 °C with increments of 5 °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.
4. Conductivity measurements
   1. Dry the ionic liquid at 100 °C under high vacuum for 12 hr to remove trace amounts of moisture before testing.
   2. 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.
   3. Use a heating block to control the temperature and maintain the stirring during the measurement to maintain homogeneity.
   4. Read the conductivity at each temperature after a 30 min equilibration time.
5. Cyclic voltammetry (CV)
   1. Assemble a lithium/lithium/platinum three-electrode system in the glove box under an argon atmosphere.
   2. Charge the vessel with the ionic liquid and make sure all the electrodes are immersed in the ionic liquid. Seal the vessel under argon.
   3. Equilibrate the vessel at the desired temperature for 20 min. Sweep the potential rate at 1 mV/sec between −0.2 V and 6.5 V versus Li⁺/Li.

3. Preparation of the Electrolytes

1. Dry the ionic liquid under high vacuum at 80 °C overnight with rigorous stirring to ensure removal of trace amounts of water.
2. Dry the LiTFSI at 70 °C for three days in a vacuum oven.
3. Transfer the anhydrous ionic liquid and LiTFSI salt to the glove box.
4. 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.

4. Fabrication of the Lithium Ion Coin Cell Battery

1. 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.
2. Soak two pieces of the separators (porous polypropylene membranes) in the above-prepared ionic liquid electrolyte at 60 °C on a hot plate for 15 min.
3. Add the ionic liquid electrolyte to the surface of the LiCoO₂ cathode until the material is fully covered with electrolyte (≈ 0.5 ml).
4. 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.
5. 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.
6. Cap the coin cell and seal it with a crimper in the glove box.
7. Transfer the coin cell out of the glove box and rest the cell for 12 hr prior to initiating the battery/electrochemical tests.

5. Performance of the Battery at 100 °C

1. Place the coin cell in an oven operating at 100 °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.
2. Leave the cell at 100 °C for 30 min to equilibrate to the temperature.
3. Select galvanostatic charge-discharge cycling on the electrochemical testing station. Set the cycle number to 500.
4. Set the charge current to 500 µA and the voltage upper limit to 4.2 V. Set a rest time of 60 sec at 0 V after each charge.
5. Set the discharge current to 500 µA and the voltage lower limit to 3.0 V. Set a rest time of 60 sec at 0 V after each discharge.
6. Start the charge-discharge cycling at a current of 500 µA between 3.0 V to 4.2 V using the software. Evaluate the charge output against time.

Results

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...

Discussion

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...

Materials

Name	Company	Catalog Number	Comments
Silicone oil	Sigma-Aldrich	85409
Potassium hydroxide	Sigma-Aldrich	221473	Corrosive
Rotary evaporator	Buchi	R-124
High-vacuum pump	Welch	8907
Nitrogen, ultra high purity	Airgas	NI UHP300	Compressed gas
Tetrahydrofuran, stabilized with BHT	Pharmaco-Aaper	346000	Flammable. Dried through column of XXX
Dichloromethane	Pharmaco-Aaper	313000	Flammable, toxic.
Separatory funnel (1 L)	Fisher Scientific	13-678-606
Sodium sulfate	Sigma-Aldrich	239313
Ethanol, absolute	Pharmaco-Aaper	111USP200	Flammable, toxic.
Buchner funnel	Fisher Scientific	FB-966-F
Methanol	Pharmaco-Aaper	339000ACS	Flammable, toxic.
Triethylamine (anhydrous)	Sigma-Aldrich	471283	Toxic, flammable, harmful to environment
Glass syringe	Hamilton Company	1700-series
Deuterated chloroform	Cambridge Isotopes Laboratories, Inc.	DLM-29-10	Toxic
Nuclear magnetic resonance instrument	Varian	V400
Hydrogen	Airgas	HY HP300	Highly flammable.
Hexanes	Pharmaco-Aaper	359000ACS	Toxic, flammable.
Differential scanning calorimeter	TA Instruments	Q100
N,N-dimethylformamide	Sigma-Aldrich	227056	Toxic, flammable.
Trihexylphosphone	TCI America		Toxic, flammable.
1-Chlorodecane	Sigma-Aldrich		Toxic, flammable.
Bis(trifluoromethane)sulfonimide lithium salt	Sigma-Aldrich		Hydrophilic
1, 10-dichlorodecane	Sigma-Aldrich		Toxic, flammable.
Thermal Gravemetric Analysis (TGA)	TA Q50	TA instruments
Differential scanning calorimeter (DSC)	TA Q100	TA instruments
Controlled Strain Rheometer	AR 1000
Conductivity Meter	Consort	K912	4-electrode cell
Potentiostate/Galvanostat	Princeton Applied Research	VersaStat MC4	Electrochemical testing
Separators	Celgard	C480	polypropylene/polyethylene
CR2032 coin cells	MTI Corp.	EQ-CR2032-CASE
LiCoO2 electrode	MTI Corp.	EQ-CR2032	Cathode material
lithium metal	Alfa Aesar	10769	Anode Material
Stainless Steel Spacer	MTI Corp.	EQ-CR20-Spacer304-02	15.5 mm Dia x 0.2 mm
Wave Spring	MTI Corp.	EQ-CR20WS-Spring304
Electric Coin Cell Crimping Machine	MTI Corp.	MSK-160D
Glove box	Mbraun		Water free, oxygen free operation