Impedance study of protecting film formation on lithium and lithiated graphite induced by bis(fluorosulfonyl) imide anion

The process of Li⁺ reduction from room temperature ionic liquids consisting of N-methyl-N-propylpyrrolidinium cation (MPPyr⁺) and bis(fluorosulfonyl) imide (FSI⁻) or bis(trifluoromethanesulfonyl) imide (TFSI⁻) anions was studied with the use of impedance spectroscopy. Reduction was carried out on both metallic lithium (Li) and graphite (G) electrodes. It has been found that the FSI anion in high amounts is able to form a protective film on both graphite and metallic lithium. The Li⁺/Li couple should rather be represented by a Li⁺/SEI/Li system. The SEI structure depends on the manner of its formation (chemical or electrochemical) and is not stable with time. The rate constant for the Li⁺ + e⁻ → Li process at the Li/SEI/Li⁺ (in MPPyrFSI) interface is k_o = 4.2 × 10⁻⁵ cm/s. In the case of carbon electrodes (G/SEI/Li⁺ interface), lithium diffusion in solid graphite is the rate determining step, reducing current by ca. two orders of magnitude, from ca. 10⁻⁴ A/cm², characteristic of the Li/SEI/Li⁺ electrode, to ca. 10⁻⁶ A/cm².