Thermally-responsive, nonflammable phosphonium ionic liquid electrolytes for lithium metal batteries: operating at 100 degrees celsius

Lithium metal battery cycling at 100 °C is enabled by thermally-responsive, nonflammable phosphonium ionic liquid electrolytes.


Anion Exchange Reactions
Mono-HexC10Cl (7.75 g, 16.74 mmol) and LiTFSI (6.25 g, 21.76 mmol) were mixed together in 20 mL DCM/H 2 O (1: 1) solvents. The mixture was stirred at room temperature overnight and washed by 3 × 15 mL of water. 1 N AgNO 3 solution was used to confirm the complete elimination of chloride anion. The organic layer was dried on anhydrous MgSO 4 and the solvent was removed under reduced pressure. The anion exchange reaction for other anions followed a similar procedure.

General Procedure for Thermal Measurements
Thermalgravimetric Analysis (TGA) measurements were performed with TGA Q50. All samples were heated from 20 to 500 o C at a heating rate of 20 o C/min. The decomposition point was marked as the 10% weight loss of the original sample weight. In addition, longterm thermal stability was determined. The phosphonium electrolyte was heated at 100 °C for 5 consecutive days, showing zero weight loss combined with no chemical changed indicating the suitable characteristics for a safe and stable electrolyte materials.
Samples were also tested with Differential Scanning Calorimetry (DSC) at a heating rate of 20 o C /min and a cooling rate of 10 o C /min from -70 to 200 o C. All samples were measured between 5 to 10 mg and scanned for three heat-cool cycles.

Conductivity Measurement
The conductivity measurements were performed using a Conductivity Meter (K912, Consort) that has a 4-electrode cell to prevent the polarization error and fouling of the electrode. The ionic liquid electrolytes were dried at 100 o C under high-vacuum S overnight to remove any trace amount of moisture before testing. Samples were loaded in test tubes sealed with septum stopper in order to maintain N 2 environment. A heating block was used to control the temperature and stirring was maintained during the measurement to maintain homogeneity. A 30-minute equilibration time was used at each temperature.

General Procedure for Rheological Measurement
About 1 mL of each sample was placed on an AR 1000 Controlled Strain Rheometer from TA Instruments equipped with a peltier temperature control using a 20 mm diameter parallel aluminum plate. The gap was set to be 1.0 -2.0 mm in all the runs. To minimize the effect of moisture in the air, the experiments were performed in a glove bag filled with nitrogen gas. Prior to each test, a pre-shear was done at shear rate 100 1/s for 10 s to S eliminate the physical memory of the sample, followed by a 15 minutes equilibrium step in order for the sample to reach a steady state condition.
Strain amplitude from 0.1 to 10 % was determined to lie within the linear viscoelastic region (LVR) via an oscillatory strain sweep at a fixed frequency (1 Hz). Dynamic shear measurements covering 0.628 -628 rad/s were conducted to obtain dynamic viscosity, storage modulus (G'), loss modulus (G'') and phase angle. Measurements were typically performed at 25 o C unless temperature effect was investigated. Oscillatory temperature sweep was conducted from 10 o C to 95 o C with increment of 5 o C and 1-minute equilibrium at each temperature. Strain and frequency were set to be 1.0 % and 1 Hz, respectively. Figure S1. Photographs of the flammability experiment where the Mono-(C 6 ) 3 PC 10 TFSI ionic liquid is exposed to an open flame.

S
To analyze the electrochemical stability window lithium/lithium/platinum three-electrode system was assembled and sealed in the glove box. The experiments were carried out outside the glove box. The cells were equilibrated at 25 or 100 o C in an oven. Then cyclic voltammetries at 1 mV/s between −0.2 V and 6.5 V versus Li + /Li were carried out using a Princeton Applied Research VersaStat.

General Procedure for Battery Cycling
A multichannel Princeton Applied Research VersaStat battery tester was used for S 7 Figure S4. Energy density per electrode calculation at every 10 cycles for a prototype battery working at 100 o C Figure S5. Galvanostatic charge-discharge cycling and minimum capacity decay of 1.6 M LiTFSI in Mono-(C6) 3 PC10TFSI at room temperature with a current rate at C/7.

General Procedure for Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) was performed using a Zeiss SUPRA 40VP field emission SEM. Samples were gently washed with dimethyl carbonate and were allowed to dry for 24h in the glove box. The samples were loaded with the protection of argon and were imaged at an accelerat 8 A) B) C) Figure S6. Surface growing at lithium anode at cycle 1 (left), 30 (middle) and 70 (right).