Effect of Temperature and Tfsi Concentration on Lithium-Ion Diffusion in (PVDF-HFP)-PYR14 TFSI-LiTFSI Gel Electrolytes

Lithium batteries have been considered as the most probably next generation of power sources for portable electronic devices and electric vehicles.  However, safety risks related to the use of highly flammable liquid electrolytes in combination with high voltage electrodes are the main obstacle to their introduction into the market. Recently ionic liquids have emerged as potential materials to substitute the conventional organic carbonate-based liquid electrolytes.  In this work we report on the effect of temperature and lithium salt molar concentration on lithium ion diffusion in (PVDF-HFP)-PYR₁₄TFSI-LiTFSI gel electrolytes. TFSI molar ratios range from 0,05 to 1 mol%. The self-diffusion coefficient of lithium ions (D ⁷Li) was measured at temperatures from 20 to 100°C using the pulsed-field gradient NMR (PFG-NMR) method by using a Bruker 400MHz Avance NMR spectrometer with a magnetic field of 9.4 T. A stimulated echo pulse sequence with gradient pulses of duration (δ) of 1-2 ms and an observation time (Δ) equal to 20 ms was applied. The decay of the signal in dependence on the gradient strength g results in the diffusion coefficient by fitting the exponential decay function.  A parallel analysis was made for PYR₁₄TFSI/TFSI ionic liquid electrolytes.

The results obtained indicate that the value of D ⁷Li at high temperature for both liquid and gel electrolytes is practically independent on the LiTFSI molar concentration, while larger differences have been detected at low temperatures, in accordance with ionic conductivity data. The temperature dependence of the calculated self-diffusion coefficients fit the Arrhenius equation. The values of the activation energy E_ac, calculated by plotting the shelf diffusion coefficient vs. 1/T, depends on the relative concentration of the lithium salt in the gel, increasing when TFSI molar ratio goes from 0,05 to 1 mol% for both liquid and gel electrolytes.